We characterize and compare creep preceding and following
the complex 2011 Pampahasi landslide (∼40 Mm3±50 %) in the city of La Paz, Bolivia, using spaceborne radar
interferometry (InSAR) that combines displacement records from both
distributed and point scatterers. The failure remobilized deposits of an
ancient complex landslide in weakly cemented, predominantly fine-grained
sediments and affected ∼1.5 km2 of suburban development.
During the 30 months preceding failure, about half of the toe area was
creeping at 3–8 cm a-1
and localized parts of the scarp area showed
displacements of up to 14 cm a-1. Changes in deformation in the 10 months
following the landslide demonstrate an increase in slope activity and
indicate that stress redistribution resulting from the discrete failure
decreased stability of parts of the slope. During that period, most of the
landslide toe and areas near the head scarp accelerated, respectively, to
4–14 and 14 cm a-1. The extent of deformation increased to cover most, or
probably all, of the 2011 landslide as well as adjacent parts of the slope
and plateau above. The InSAR-measured displacement patterns, supplemented by
field observations and optical satellite images, reveal complex slope
activity; kinematically complex, steady-state creep along pre-existing
sliding surfaces accelerated in response to heavy rainfall, after which
slightly faster and expanded steady creeping was re-established. This case
study demonstrates that high-quality ground-surface motion fields derived
using spaceborne InSAR can help to characterize creep mechanisms, quantify
spatial and temporal patterns of slope activity, and identify isolated
small-scale instabilities; such details are especially useful where
knowledge of landslide extent and activity is limited. Characterizing slope
activity before, during, and after the 2011 Pampahasi landslide is
particularly important for understanding landslide hazard in La Paz, half of
which is underlain by similar large paleolandslides.
Introduction
Creep – steady, imperceptibly slow movement under the influence of gravity
– precedes many large landslides in both bedrock (e.g. Chigira and Kiho,
1994; Kilburn and Petley, 2003) and soil (e.g. Kalaugher et al., 2000;
Petley et al., 2002). Its temporal patterns can improve understanding of
slope-deformation processes (Petley et al., 2002) and the possible timing of
impending catastrophic failure (Voight, 1989; Crosta and Agliardi, 2003;
Federico et al., 2015). Its spatial patterns indicate the style (Gischig et
al., 2011; Schlögel et al., 2016) and approximate magnitude of
instability. Consequently, characterization of temporal and spatial patterns
of slow deformation is a focus of many landslide hazard assessments,
monitoring systems, and warning programs (Eberhardt, 2012; Baron and Supper,
2013; Hermanns et al., 2013a). In many cases, however, pre-failure creep is
not quantified, because either it is not recognized prior to catastrophic
failure or failure carries a relatively low risk to people and
property.
Creep is rarely monitored following a landslide, except at sites where
renewed catastrophic failure is expected (e.g. Gischig et al., 2011).
Limited post-event monitoring sometimes stems from an assumption that stress
release during catastrophic failure enhances slope stability. In addition,
instrumentation that may have been present on or within the slope is
typically destroyed during failure (e.g. Crosta and Agliardi, 2003).
Consequently, activity soon after failure is generally unclear and can
rarely be compared with pre-failure deformation. Such records are, however,
of utmost importance in light of observed spatiotemporal clustering of
failure events (e.g. Hermanns et al., 2006, and references therein; Crosta
et al., 2017; Hilger et al., 2018).
Spaceborne radar interferometry (InSAR) has long been recognized for its
promise in characterizing the spatiotemporal evolution of slopes (e.g.
Petley et al., 2002; Colesanti and Wasowski, 2006; Wasowski and Bovenga,
2014). Continued technological advancements and data availability offer the
potential for further improvements to the characterization of slope
behaviour (Wasowski and Bovenga, 2014), including prediction of future
failure (Moretto et al., 2017). Improvement of radar systems is increasing
ground resolution and phase quality, and the establishment of satellite
constellations is improving temporal resolution and interferometric
coherence. Due to such improvements, pre-failure accelerated creep has
recently been detected and measured, for both natural (Intrieri et al.,
2018; Handwerger et al., 2019) and cut (Carlà et al., 2018) slopes. Such
advancements similarly improve opportunities to analyze post-failure creep.
Our quantification of creep during the 30 months preceding and 10 months
following the largest modern landslide in the city of La Paz, Bolivia (2011
Pampahasi landslide, ∼40 Mm3), using InSAR demonstrates
complex slope behaviour. Contrary to the sometimes-invoked view that a major
failure reduces large-scale instability, the affected slope shows enhanced
post-failure activity, highlighting the complexity of slope stability in La
Paz. The Pampahasi case study illustrates the necessity for land-use
planning views that better align with the complexity and commonly recurrent
nature of large landslides. The InSAR technique we employ is optimized to
maximize displacement record density and distinguish abrupt changes in
motion, which together improve the characterization of spatially complex
deformation patterns typical of landslides. The resulting displacement
fields provide nearly spatially continuous quantification of motion over
large portions of the landslide and surrounding terrain that enable
discrimination of localized motion and aid interpretation of failure
mechanisms.
Setting and material propertiesLa Paz region
A tributary of the Amazon River system penetrates the Cordillera Real
∼60 km southeast of La Paz and drains the eastern margin of
the otherwise internally drained Altiplano plateau (∼4000 m a.s.l.) (Fig. 1a). The city of La Paz (population ∼0.8 million)
is located within the upper part of this watershed where the valley
system has incised into up to 800 m of weakly lithified Neogene and
Pleistocene sediments shed from the Cordillera Real (Dobrovolny, 1962) (Fig. 1b). South of the city, incision has exposed folded and faulted Paleozoic
to Mesozoic metasedimentary basement rocks forming the root of the range
(Fig. 1b). The overlying Cenozoic fill sequence comprises over 600 m of
predominantly fine-grained lacustrine and fluvial sediments of the weakly
lithified Miocene–Pliocene La Paz Formation. It coarsens upward and toward the
cordillera into more competent late Pliocene and Early Pleistocene glacial
and proglacial sediments (Roberts et al., 2018) that are up to 400 m thick.
The fill has been offset by numerous faults trending northwest and west
(Dobrovlny, 1962; Lavenu, 1978; Roberts et al., 2018) (Fig. 1b).
Setting of La Paz, Bolivia. (a) Location and physiographic context.
(b) Generalized geology after Dobrovolny (1962) and Anzoleaga et al. (1977),
with faults from Dobrovolny (1962) and Lavenu (1978). Terrain is from the
ASTER GDEM v2 produced by METI and NASA.
The slopes and floors of the La Paz valley system are mantled by colluvium,
including deposits of large (up to 50 km2; Dobrovolny, 1968)
paleolandslides derived predominantly from the La Paz Formation (Dobrovolny,
1962; Anzoleaga et al., 1977) (Fig. 1b). Most of the paleolandslides date to
the Late Pleistocene and Holocene (Dobrovolny, 1962; Hermanns et al., 2012).
Historic instability is concentrated within paleolandslide deposits (e.g.
Fig. S1 in the Supplement), particularly at their margins, likely reflecting low residual
strengths and possibly differential stresses related to suspected ongoing
creep. Failures < 1 Mm3 in size occur yearly and happen mainly
during the rainy season (December–March) (O'Hare and Rivas, 2005; Roberts,
2016). In contrast, of the seven historic landslides larger than 1 Mm3,
four happened during the dry season (April–November; Table S1 in the Supplement). Although
general geomorphic and geologic characterization has been undertaken for
some of the larger landslides (Dobrovolny, 1962; Anzoleaga et al., 1977;
Quenta et al., 2007, 2008; Hermanns et al., 2012), detailed site
investigations are lacking. Land-use decisions in La Paz have historically
overlooked recurrent slope failures, with large landslide complexes being
either unrecognized or repeatedly resettled after reactivations.
Ground surface conditions in the Pampahasi area. (a) Surficial
geology prior to the start of the twenty-first century; adapted from
Anzoleaga et al. (1977). (b) Slope morphology and ground disturbance showing
the extent and morphologic features of landslides that occurred during the
period of radar acquisition (September 2008 to December 2011). Horizontal
displacements during the 2011 Pampahasi landslide are based on comparison of
pre-failure and post-failure WorldView-2 scenes. Contours are from NASA's
Shuttle Radar Topographic Mission v3.0.
Pampahasi area
Pampahasi Plateau is 2.2 km long, 0.2–0.7 km wide, and forms part of the
interfluve between ríos Orkojahuira and Irpavi (Fig. 1b). Its margins
are generally scalloped (Fig. 2) and in several places terminate in nearly
vertical cliffs dropping 20–80 m to irregular, locally steep valley slopes
below. The plateau surface dips slightly southward and lies at an elevation
several hundred metres below the local southwest-dipping Altiplano and its
erosional remnants (Fig. 3a).
The Río Irpavi valley is cut into well-stratified, fine-grained facies
of the La Paz Formation, which comprises horizontal to sub-horizontal silt
and sand beds interlensing with mainly granitic pebble–cobble gravel (Fig. 3b). The variable texture and limited (< 1 km) lateral continuity of
these beds are typical of the unit, particularly its middle part (see
Ahlfeld, 1945a; Dobrovolny, 1962; Bles et al., 1977). The age of the La Paz
Formation beneath Pampahasi Plateau (Fig. 3b) is constrained by the 5.4 Ma
Cota Cota Tuff (Lavenu et al., 1989; Servant et al., 1989), which crops out at
∼3520 m a.s.l. in the southern part of the city (Lavenu et
al., 1989), and the 2.74 Ma Chijini Tuff (Roberts et al., 2017), which crops
out along the west margin of the plateau (Dobrovolny, 1962) between 3800 and
3850 m a.s.l. The local Altiplano surface formed on top of the thick
sequence of Pliocene–Pleistocene glacial sediments by 1.8 Ma or possibly as
recently as 1.0 Ma (Roberts et al., 2018), attaining an elevation of
approximately 4200 m a.s.l. in the vicinity of Pampahasi (Fig. 3a).
Stratigraphic sequence. (a) Cross-section oblique to the trend of
the Cordillera Real through the fill underlying the Altiplano (location in
Fig. 1b). (b) Lithostratigraphy beneath Pampahasi Plateau, including
principle units involved in the Pampahasi paleolandslide and 2011 Pampahasi
landslide. Modified from Anzoleaga et al. (1977) based on field mapping
throughout the La Paz area.
About 400 m of incision of the Altiplano surface by ancestral Río
Irpavi after 1.8–1.0 Ma formed a south-sloping (Dobrovolny, 1962)
valley-bottom unconformity onto which the Middle Pleistocene Pampahasi
gravel (Fig. 3b) was deposited, as either fluvial channel deposits (Ahlfeld,
1945b) or an ancient landslide (Bles et al., 1977). This multilithic
silty sandy pebble–cobble gravel is commonly more than 20 m thick
(Dobrovolny, 1962) and may be up to 50 m thick along the plateau's southern
edge where the unit overlies a locally steep erosional contact (Dobrovolny,
1955).
Many slopes descending from Pampahasi Plateau are mantled by ancient
landslide deposits derived from the La Paz Formation (Dobrovolny, 1962;
Anzoleaga et al., 1977). The deposits consist of largely intact brittle
blocks of the La Paz Formation within a matrix of softer, ductile, and completely
disaggregated La Paz Formation. Small (< 1 to 20 ha) failures
evident in aerial photographs have occurred since at least the early
twentieth century in many places within large (∼20–200 ha)
prehistoric failures. The slope descending ∼380 m to Río
Irpavi directly east of the southern half of Pampahasi Plateau comprises the
undated Pampahasi paleolandslide (Fig. 2a).
Details of the prehistoric Pampahasi paleolandslide, which substantially
predates historic records, must be inferred from its surface expression
because no subsurface investigations have been conducted on this slope. Its
morphology indicates a complex failure, but whether it represents one or
multiple events is uncertain. Rotational displacement of the upper quarter
of the slope transitions downward to eastward sliding and localized flow.
The landslide displaced Río Ipravi eastward from its alignment farther
upstream and downstream by at least 300 m. The substantial height of the
head scarp (up to 80 m) suggests that the failure zone is many tens of metres
below surface in the upper part of the landslide. The failure depth lower on
the landslide is likely much shallower.
Surface incision of the Pampahasi paleolandslide by several gullies suggests
that it is of substantial antiquity. The adjacent and similarly incised
Villa Salomé paleolandslide is likely a separate landslide complex
because its failure surface is higher and is separated from the Pampahasi
paleolandslide along Río Jankopampa by a > 120 m
thick
sequence of intact La Paz Formation (Fig. 2a). In the area of the Pampahasi
paleolandslide, the Río Irpavi channel is modified with check dams and
concrete armouring. Before the 2011 failure, western tributaries of Río
Irpavi draining the Pampahasi and Villa Salomé paleolandslides were
little modified and had incised into weak loose material. The main channels
draining the area (ríos Chujilluncani and Jankopampa, Fig. 2) have
since been entombed in concrete culverts. In light of the geomorphic
evidence of recurrent instability at the sites, Scanvic and Girault (1989)
recommended that this area not be developed. However, the initially sparse
development greatly expanded during the last decade of the twentieth century
and first decade of the twenty-first century, resulting in the establishment of
several large neighbourhoods.
Geomechanical properties
The different characteristics of the Pampahasi gravel and La Paz Formation
give rise to contrasting material properties, which have been documented by
Anzoleaga et al. (1977; summarized in Table S2). The Pampahasi gravel
(50 %–75 % clasts, 19 %–50 % sand and silt, < 6 % clay) is
permeable (k∼1×10-3 m s-1), has low to medium
plasticity (plasticity index ≤10), and exhibits high internal friction
(> 30∘), particularly when dry. Fine-grained zones are
weaker but uncommon.
The properties of the La Paz Formation are variable due to the unit's
heterogeneity. Coarse sand and gravel zones (35 %–75 % clasts, 13 %–65 %
sand and silt, < 12 % clay) have permeability similar to that of the
Pampahasi gravel but are commonly cemented by carbonate (≤15 %) or
iron oxide. Fine-grained zones (≤10 % clasts, 40 %–90 % sand and
silt, 10 %–40 % clay) are much weaker and, as the main facies, dominate the
unit's behaviour. They are weakly cemented by carbonate (< 5 %)
and locally abundant clay. The clay is largely non-expansive, but
montmorillonite (smectite) is common in some beds and lenses. Due to low
permeability (k<1×10-5 m s-1), water circulates mainly
through fractures in interstratified gravel lenses or along joints.
Plasticity is medium to high (plasticity index of 7–32), and shear strength
ranges from 18 to 37∘. The unit is generally strong
when dry and undisturbed (cohesion 0.2–0.7 MPa) and stands in near-vertical
slopes. However, wet silty sediments have much lower cohesion (0.01–0.1 MPa)
and shear strength. Smectite-rich clay lenses have greater cohesion (≥0.1 MPa) but high plasticity (plasticity index of 60–80) and low shear
strength (≤14∘). Parts of the La Paz Formation that have
failed in the past are even weaker due to loss of compaction and localized
reduction of shear strength to residual levels (as low as 13∘ in
silty and sandy zones and less than 8∘ in clay zones).
MethodsFailure event characterization
Our characterization of the 2011 landslide is based on field visits in the
years before and after the event and on comparison of high-resolution
optical satellite images acquired shortly before and after the event. Field
observations by municipal staff and eyewitness accounts of residents provide
details on the location, type, and magnitude of damage shortly before,
during, and after the 2011 failure. We mapped features of the 2011 (26 February to 1 March) landslide from the first cloud-free, post-failure
imagery (WorldView-2 acquired on 23 March 2011), and we quantified horizontal
displacement vectors during the failure by comparing it with a pre-failure
(5 January 2011) image from the same sensor, which we coregistered using
points outside the landslide-affected area. Due to errors resulting from the
lack of orthorectification and from elevation differences cause by the
landslide, the estimated horizontal vectors are approximate (rounded to the
nearest 5 m increment).
HDS-InSAR
We applied Homogeneous Distributed Scatterer InSAR (HDS-InSAR; Eppler and
Rabus, 2011; Rabus et al., 2012) to a stack of 44 fine-beam-mode (5.1 m
azimuth × 8.0 m ground-range pixel spacing) RADARSAT-2 ascending scenes (look
direction: 36.3∘ incidence angle from vertical at scene centre,
76.0∘ aspect angle counter clockwise from north) acquired between
September 2008 and December 2011 (Tables S5
and S6). We processed the scenes separately as a
pre-failure stack (32 scenes) and a post-failure stack (12 scenes) to compare
deformation patterns before and after the landslide and to reduce
decorrelation caused by landslide-induced terrain changes. Although a range
of displacement directions is possible, ground-surface displacement vectors
due to mass movements are most probable in the downslope direction; we thus
estimated true displacement rates from the angular deviation between
downslope vectors and the satellite's line of sight (LOS).
The HDS approach combines the strengths of Persistent Scatterer InSAR
(PS-InSAR; Ferretti et al., 2001) and Small Baseline Subset InSAR
(SBAS-InSAR; Berardino et al., 2002) for considering, respectively, point
and distributed targets. A continuously weighted, spatially adaptive filter
defines spectrally similar pixel clusters (HDS neighbourhoods) within a
rectangular search area based on similarity of their backscatter amplitude
time series. The filter is applied to interferograms to generate
differential phase and coherence for each HDS neighbourhood (Eppler and
Rabus, 2011), which comprises either a single-pixel cluster (a persistent
point target) or the weighted mean of a multi-pixel cluster (interpreted as
the area covered by a homogenous distributed target). To more efficiently
represent the high density of displacement records, we interpolated linear
deformation maps from HDS to provide base images of long-term average LOS
deformation over the entire area.
Like PS-InSAR, HDS-InSAR generates high-quality displacement time histories
attributable to single pixels, each containing a dominating point target
with near-linear displacement characteristics (Colesanti and Wasowski,
2006). Additionally, it provides time histories for multi-pixel clusters
representing homogeneous distributed scatterers. Like SBAS-InSAR, HDS-InSAR
characterizes strongly non-linear motion (Necsoiu et al., 2014) using the
higher continuity coverage (Lauknes et al., 2010) of distributed targets.
Unlike SBAS-InSAR, however, it does not significantly diminish phase quality
or reduce spatial resolution. Two key differences set HDS-InSAR and some
other advanced InSAR algorithms (e.g. Ferretti et al., 2011) apart from the
PS-InSAR and SBAS-InSAR techniques: the use of adaptive filtering to
preserve, as much as possible, spatial resolution while suppressing the
noise from surface decorrelation of nearby incoherent pixels, and
optimization to characterize spatially uncorrelated ground motion. These
features make HDS-InSAR particularly well suited for characterizing
landslides, which commonly display ground motion variability over short
distances or across abrupt transitions. Although HDS-InSAR and
SqueeSAR (Ferretti et al., 2011) share similar adaptive filtering methods, the latter
uses wrapped phase triangulation to invert the interferometric network. In
contrast, HDS-InSAR relies on prior unwrapping of the network
interferograms, with potential unwrapping errors being corrected iteratively
using the network redundancy a posteriori. Despite the differences in their
algorithms, the final accuracy and spatial detail of both advanced InSAR
methods are similar.
2011 Pampahasi landslide
The Pampahasi landslide
The 2011 failure is also locally called
the Callapa landslide after one of the neighbourhoods it destroyed.
occurred at the end of February 2011 following a week of variable
precipitation (0–39.2 mm daily, Fig. 4), which included one of the wettest
days on record (25 February, Table S3). The ∼1.5 km2
failure remobilized a large part of the Pampahasi paleolandslide deposit
between the east side of Pampahasi Plateau and Río Irpavi (Fig. 2). In
addition to previously failed material, it included small volumes of in
situ weakly cemented La Paz Formation and, in the head-scarp area, the
overlying uncemented Pampahasi gravel (Fig. 2a). None of the sediments
involved were substantially weathered.
The precipitation record for the Laykacota meteorological station
(see Fig. S1 for station location). (a) Comparison of monthly average
precipitation (1945–2016) with monthly precipitation for the second half of
2010 and first half of 2011. (b) Daily precipitation and 7-day cumulative
precipitation during February and early March 2011.
Several locations in the upper part of the slope showed field evidence of
slow ground deformation, largely visible as offset walls and road surfaces
(Fig. 5, locations in Fig. 2a) in the years prior to catastrophic failure
(Quenta and Calle, 2005; Hermanns et al., 2012). These features were
recognized as evidence of an impending landslide (Quenta and Calle, 2005),
although not of the magnitude of the 2011 failure. In response, the
municipality of La Paz increased risk communication and installed concrete
pillars to remediate what became the 2011 landslide head scarp (Hermanns et
al., 2012). However, due to their localized and shallow nature, the
stabilization efforts probably had little if any influence on the stability
of the slope or behaviour of the subsequent failure.
Field evidence of pre-failure creep in what would become the head
region of the 2011 Pampahasi landslide. (a) Small recent slumps (arrows)
along Río Chujilluncani, ∼400 m downslope from the east
margin of Pampahasi Plateau. (b) Minor cracking and centimetre-scale vertical
offset of a paved road about 200 m east of, and 75 m below, the east margin
of the plateau. (c) Unpaved road with ∼1 m displacement near
the 2011 head scarp. (d) Vertical offset along a gully ∼75 m
from the future south lateral margin of the 2011 landslide. The side of the
gully nearer the middle of the 2011 landslide (right of the photo) has
dropped ∼0.5 m relative to the adjacent slope, causing
deformation of the gabions and curb in the foreground. (e) Cracks in an adobe
wall (right foreground) and brick house (left middle ground) ∼50 m downslope of the east margin of the plateau, corresponding to the
middle of the 2011 landslide head scarp. See Figs. 2a and 8a, b for
locations of photographs. Photos taken by Reginald L. Hermanns in October 2005.
The 2011 Pampahasi landslide occurred in several phases from the evening of
26 February to 1 March (Fig. 6, locations in Fig. 2b). Eyewitness accounts
suggest that failure initiated in an area of earlier small slumps along
Río Chujilluncani
This stream is sometimes also referred to
as Arroyo Pampahasi.
(Hermanns et al., 2012; Fig. 5a) ∼400 m
downslope of the east margin of Pampahasi Plateau. Motion was fastest (up to
several metres per second) and largely vertical in the head region, which
failed shortly after, lasting 1 to 2 h and forming a ∼60∘ and 80 m high head scarp (Fig. 6a). Movement in the central zone
and at the toe, which began soon after the initial formation of the
head scarp, was slower (metres per minute to metres per day) and largely
horizontal. The degree of ground disturbance and the magnitude of
displacement generally decreased to the east and north (Fig. 2b). The
northern part of the landslide's toe experienced relatively limited, but
economically costly, ground disturbance (e.g. Fig. 6b–d) and moved as a
more or less coherent mass ∼15 m southeastward (Fig. 2b).
Deformation of the lower part of the landslide continued for days. A bridge
across Río Irpavi (Fig. 6e) was shifted 10 m and uplifted on the
morning of 27 February; uplift of several metres at the east end of the
bridge suggests a rotational failure zone passing, at least locally, under
Río Irpavi. Buildings near the south toe of the landslide (Fig. 6f)
collapsed on 28 February and 1 March due to delayed relatively minor
translation.
Impacts of the 2011 Pampahasi landslide. (a) Head scarp (80 m
height) along the east margin of Pampahasi Plateau (note excavator and
trucks in lower middle ground for scale). (b) Ground fissures in the northern
part of the lower landslide mass. The wall in the background is
∼2 m high. (c) Back-tilted houses and apartment buildings on
the northern part of the lower landslide mass (movement was from left to
right). (d) Community building with collapsed wall on the northern part of
the lower landslide mass. (e) Bridge across Río Irpavi that was deformed
and shifted from its footings on 27 February near the centre of the
landslide toe. (f) Buildings on the southern part of the toe of the landslide
that collapsed on 28 February and 1 March (dates of collapse shown; photo by
Marco-Antonio Guzmán, 2 March 2011). (g) Water service line ruptured by the
landslide (upper pipe) and replacement water service line (lower pipe). See
Figs. 2b and 8c, d for locations of photographs. Photos taken by Nicholas J. Roberts in May 2011 unless otherwise indicated.
The surface morphology of the landslide likely reflects a complex failure
zone with both rotational and translational components. Assuming a minimum
average thickness of the landslide deposit of 15 m, its volume is at least
20 Mm3. However, the volume is more likely about 40 Mm3, given
deeper failure in both the upper rotational part (40 m average depth over
32 ha) and the main predominantly translational body (25 m average depth over
110 ha).
Due to prompt evacuation of the head-scarp area and the dominantly moderate
to low movement velocities farther downslope, no lives were lost during the
landslide. However, about 1000 homes were destroyed and 6000 people were
displaced. The rupture of a water line crossing the upper part of the landslide
(Fig. 6g) left between 200 000 and 300 000 people in southern La Paz without
potable water for several months (Hermanns et al., 2012; Aguilar, 2013).
Continued episodic collapse of the steep head scarp in the years since the
landslide has resulted in additional loss of homes and repeated expansion of
an evacuation zone on Pampahasi Plateau. Large portions of the landslide
complex have been recently resettled or are being prepared for reoccupation,
with limited control of slope infiltration and runoff.
Deformation before and after failure
Due to the high spatial density of displacement records, particularly in
high-coherence areas (Fig. 7b, e), interpolated linear deformation maps are
preferable to HDS point data for interpreting the spatial variability of
slope creep (see Fig. 7c, f). Furthermore, the interpolation weighting
suppresses very localized HDS clusters that differ greatly from the average
(Supplement, Sect. S4.7). Consequently, these small-scale variations, whether
representing surficial movement or noise from uncorrected phase unwrapping
errors, are insignificant to large-scale patterns described below. The
displacement maps record deep, spatially regular slope movements as well as
shallower, more variable movements, but their differentiation requires
consideration of displacement patterns and may not always be clear.
Typical density and spacing of HDS-InSAR results for an area of
mixed suburban, agricultural, and commercial development. Both the
pre-failure stack (a, b, c) and post-failure stack (d, e, f) are shown.
Panels for each stack depict satellite images showing land cover without
InSAR data (a, d); point data representing HDS neighbourhoods (pixel
neighbourhoods) (b, e), each recording a displacement time history; and
linear displacement map interpolated from HDS points (c, f). Localized
variability in HDS results is suppressed by linear deformation
interpolation. Displacement rates are in the satellite line of sight. The
deforming area is the southern boundary of the active part of the landslide
toe. Base image is a 4 January 2011 QuickBird
image viewed in GoogleEarth™. Base images are from GoogleEarth
(a, b, c 4 January 2011 and d, e, f 25 July 2011). See Fig. 8a, c for
location.
InSAR-measured line-of-sight ground displacements in the Pampahasi
area. Average linear deformation rates during (a, b) the 30-month period
before and (c, d) the 10-month period after the Pampahasi landslide.
Compressed (-1 to 1 cm a-1; a and c) and expanded (-5 to 5 cm a-1; b and d)
displacement scales emphasize, respectively, the spatial limits and spatial
variability of slope activity. Dashed lines are the limits of landslides
that occurred in 2009 and 2011 during the period of radar scene acquisition.
Roman numerals show locations of displacement histories in Fig. 9. Letters
show locations of photos taken, respectively, before (Fig. 5) and after
(Fig. 6) the 2011 landslide. Base images are from GoogleEarth (a, b 4 January 2011 and c, d 25 July 2011).
InSAR-measured ground deformation in the Pampahasi area is almost entirely
restricted to mapped prehistoric landslide deposits, namely the Pampahasi
and Villa Salomé paleolandslides (Fig. 2a), and the terrain directly
behind their head scarps (Fig. 8). Their characteristic displacement
histories are shown in Fig. 9 (locations i–x). Data gaps in each stack
are locations of low coherence that mainly result from decorrelation related
to remedial earthworks performed after a 10 ha landslide in 2009 (Fig. 8a,
b) and the 2011 landslide (Fig. 8c, d) or from aliasing where displacement
rates are greater than the detection threshold of RADARSAT-2
(∼2.8 cm LOS – equivalent to a two-way travel distance
comparable to RADARSAT-2's 5.6 cm wavelength – over 24 days). Motion toward
the satellite suggested by the data in the east-facing slopes (Fig. 8) is
improbable, given that it implies uplift or motion upslope. Such issues are
almost entirely restricted to the post-failure records (Fig. 8c), suggesting
that the small size and limited redundancy of the interferometric network in
the thinner stack has resulted in incomplete network correction of either
phase unwrapping errors or atmospheric effects.
Pre-failure creep
Prior to the end of February 2011, about 180 ha of the lower part of the
slope within the limits of the 2011 landslide was moving at rates as high as
14 cm a-1 (9 cm a-1 LOS; Fig. 8a, b). Motion was most widespread over the
northern part of the toe of the Pampahasi paleolandslide (2–5 cm a-1 LOS,
i in Fig. 8), where it extended into Río Jankopampa alluvium mantling the
paleolandslide deposit. There it terminated abruptly ∼50 m
north of the river (Fig. 8a) at a locally stable slope comprising
undisturbed La Paz Formation (Fig. 2a). A 1 ha zone directly west of Río
Irpavi and halfway along the toe of the paleolandslide (ii in Fig. 8) crept even
faster (5–8 cm a-1 LOS, Fig. 8b). In contrast, activity on the southern part
of the paleolandslide toe (iii in Fig. 8) was limited to a few small areas creeping
at less than 0.5 cm a-1 LOS.
Pre-failure movement in the upper half of the Pampahasi paleolandslide was
restricted to an area of no more than ∼250 m along the slope by
∼300 m downslope. Given the presence of stationary ground in
some parts of the upper half of the landslide (north and south of B in
Fig. 8a, b), creep may have been localized to a few smaller areas there.
Minor isolated movement (< 1 cm a-1 LOS) is apparent along what became
the 2011 landslide's upper lateral margins (e.g. iv in Fig. 8). Creep within the
Pampahasi paleolandslide deposit beyond the limits of the 2011 failure was also
localized and generally slower than 1 cm a-1 LOS (Fig. 8a). The largest of
these areas involved 12 ha of paleolandslide material between the 2009 and
2011 landslides (v in Fig. 8) directly upslope (northwest) of the most active part
of the slope toe and crept at 0.5 cm a-1 LOS. The most rapid pre-failure
motion (9 cm a-1 LOS) was restricted to the upper portion of the future
landslide where infrastructure damage had been documented in prior years
(Fig. 5b, c). Aside from a small, slow-moving (0.5 cm a-1 LOS; vi in Fig. 8) area, no
motion is evident along the future landslide head scarp, including sites of
localized deformation observed in 2005 (Fig. 5d, e), or on the plateau behind
it (vii–x in Fig. 8).
Post-failure creep
During the 10 months following the landslide, the area of movement on
Pampahasi Plateau and the valley slope between it and Río Irpavi
increased by about two-thirds to nearly 300 ha and included the entire area
of the 2011 landslide where coherence was maintained (Fig. 8c). The maximum
inferred downslope displacement rates following the landslide (14, 9 cm a-1 LOS; Fig. 8d) were similar to those before it but occurred in a region
of the toe that was previously stable (iii in Fig. 8) and along the new head scarp
(vi in Fig. 8) where creep had previously been slow (0.5 cm a-1 LOS). Displacement
rates also increased in the northern part of the toe (ii in Fig. 8, from 2–5 to 3–6 cm a-1 LOS) and were more spatially variable than in the pre-failure period
(Fig. 8).
Detection of movement is not possible over much of the middle and upper
parts of the 2011 landslide due to decorrelation resulting from earthworks
that continued for years after the event. However, because this zone is
bordered on all sides by moving terrain (> 1 cm a-1 LOS, Fig. 8c),
much of it also was likely creeping throughout the period of the
post-failure stack. The rate and extent of creep increased in a zone
measuring nearly 500 m by 500 m (including v) in the paleolandslide
deposit between the 2009 and 2011 landslides. Given the limited transport,
and thus bulking, across most of the area of the 2011 landslide (Fig. 2b) as
well as the occurrence of abundant post-failure surface displacement beyond
its limits (Fig. 8c), ground motions following the event should largely
represent mass movements as opposed to soil settlement or compaction.
The most striking change in ground motion following the 2011 failure is the
development of a new area of creep several hundred metres wide fringing the
head region of the landslide (Fig. 8c). This zone extends up to nearly 400 m
beyond the head scarp on Pampahasi Plateau, in one place reaching the
opposite margin of the plateau surface. The magnitude of creep decreases
steadily away from the head scarp (vi–x in Figs. 8c, d, and 9b). This
zone of new deformation extends hundreds of metres down both lateral
margins.
Displacement time series from the Pampahasi area (locations i to
x in Fig. 8). (a) Movement of the west slope of the Río Irpavi valley
within the area affected by the 2011 landslide (i–iii) and adjacent
paleolandslide deposits (iv–v). The right axis is approximate, given that
additional error is introduced from conversion from LOS to true motion. (b) Movement on Pampahasi Plateau behind the head scarp of the 2011 landslide
(vi–x). Variability in the data, which is most pronounced in parts of the
time series with low displacement rates, indicates phase noise resulting
from factors unrelated to ground motion. Displacements between the end of
the pre-failure stack and the start of the post-failure stack are not
precisely constrained within the limits of the 2011 landslide (i–iii). For
the Pampahasi slope adjacent to the 2011 failure and for the Pampahasi
Plateau, displacements between the stacks are based on results of full-stack
processing (see Sect. S4.8).
All portions within the 2011 landslide area where coherence was maintained
in both InSAR stacks and most areas within several hundred metres of the
landslide experienced post-failure changes in slow ground motion (Fig. 10).
The changes largely involved increased motion away from the satellite of
between 2 and at least 5 cm a-1 LOS. Only a few small areas (totaling
< 4 ha) in the northeast part of the toe, which were creeping at 4 cm a-1 LOS or more prior to February 2011, slowed following failure.
Conceptual failure model
Combining details of the 2011 failure event with patterns of long-term
pre-failure and post-failure creep, we identify several types of instability
that inform the geomechanical evolution of the Pampahasi slope.
Components of failure
The failing slope has many interconnected components that are largely
within, but also extend locally beyond, the Pampahasi paleolandslide (Fig. 11a). Slope morphology before and after the 2011 landslide shows that
long-term failure occurs predominantly by complex rotational–translational
sliding (Fig. 11b). Differing displacement behaviour of several zones within
the slope indicate separate, although likely interacting, kinematic
components. The northern part of the Pampahasi slope toe consists of an
apparently competent mass that creeped uniformly at rates of 2–5 cm a-1 LOS
prior to late February 2011 (Fig. 8b) and moved more or less coherently
during the 2011 landslide (Fig. 2b). This block-like mass accelerated
slightly to 3–6 cm a-1 LOS following the 2011 failure and displayed a more
spatially variable pattern of deformation (Fig. 8d) suggesting partial
break-up during the failure event. The resulting removal of toe support –
rapidly during the 2011 landslide but otherwise gradually – likely drove
creep of the 12 ha zone of paleolandslide material (v) directly upslope
(to the northwest). Although not part of a historic landslide event (Fig. 2), this zone was creeping slowly (∼0.5 cm a-1 LOS, Fig. 8a)
prior to the 2011 failure and accelerated thereafter (> 1 cm a-1
LOS, Fig. 8c).
Change in line-of-sight slope creep of the Pampahasi area
following the 2011 failure relative to pre-failure creep. Positive and
negative values indicate, respectively, decreased and increased post-failure
displacement rates away from the satellite. Green areas experienced no
change in displacement and were largely stable both before and after the
2011 landslide. White areas lacked coherence phase in pre-failure scenes,
post-failure scenes, or both. Roman numerals show locations of displacement
histories in Fig. 9. Pre-failure and post-failure imagery spans,
respectively, 8 September 2008 to 13 February 2011 and 9 March to 22 December 2011.
Proposed geomechanical model for the Pampahasi landslide and
adjacent terrain. (a) Components of failure, which differ in kinematics and
activity history. (b) Generalized section through the failing slope
illustrating the approximate depth and attitude of the failure zone. The
topographic surface is based on NASA's Shuttle Radar Topographic Mission
v3.0, which depicts the generalized pre-failure terrain. Dashed and solid
lines in the source area (0 to ∼150 m x-axis distance)
represent, respectively, pre-failure topography and post-failure topography
based on terrain changes measured in the field using a handheld laser
rangefinder. Terrain lower on the failed slope changed comparatively little
during the landslide; the solid line beyond ∼150 m x-axis
distance is thus generally representative of both the pre-failure and
post-failure topography. (c) Schematic temporal displacement history
generalized from InSAR measurements bracketing the 2011 Pampahasi landslide
and eyewitness accounts. Due to the revisit frequency of RADARSAT-2 (24 days), the lack of acceleration leading up to the 2011 Pampahasi landslide
is based on the lack of such effects reported by local residents. Velocity
classes are from Cruden and Varnes (1996), spanning all classes except for
“extremely rapid” (> 5 m s-1). The inset shows schematic
stages of slope movement during first-time failure by fracture propagation
(after Hermanns and Longva, 2012).
A 1 ha zone near the midline of the landslide toe (ii) is one of the few
locations of decreased post-failure activity (Fig. 10). Its high, localized
activity prior to the 2011 landslide (5–8 cm a-1 LOS, Fig. 8b) suggests a
shallow failure driven by incision at a location where Río Irpavi
impinges on the toe of the slope (Fig. 2). This localized activity is
probably similar to the east-bank landslide mapped by Anzoleaga et al. (1977) ∼400 m farther upstream, opposite the Río
Jankopampa confluence (Fig. 2a), which was creeping throughout the entire
period of radar monitoring (Fig. 8). Decreased post-failure activity of the
small west-bank instability may reflect local modification of the Río
Irpavi channel by the 2011 landslide and subsequent channel remediation
efforts, or it may result from post-failure stress reorganization. Several
similar linear areas of decreased post-failure creep along the toe of the
2011 landslide as far north as Río Jankopampa (Fig. 10) may represent
similar conditions.
The southern part of the toe of the paleolandslide (iii in Fig. 8), which was
stationary prior to the 2011 Pampahasi landslide (Fig. 8a), reactivated
following the failure (Fig. 8c). Although the exact cause of the
reactivation is uncertain, it is clearly a result of forces imposed on the
toe of the paleolandslide by the 2011 event and may be related to consequent
adjustment of stream erosion along the east margin of the deposit. Creep in
this area was greatest directly upslope of the place where several buildings
collapsed (Fig. 6f) in the final days (28 February and 1 March) of the
landslide event, suggesting that delayed infrastructure damage there relates
to the transition from the failure event to the new post-failure instability
regime.
The height and steepness of the 2011 landslide head scarp indicate a
deep-seated basal failure zone in the upper slope (Fig. 11b). Neither the
paleolandslide nor the 2011 failure aligns with documented faults (Dobrovlny,
1962; Lavenu, 1978; Fig. 1b) that might indicate a structural control. A
lack of displacement records for the head region following the 2011
landslide limits inferences about post-failure behaviour. However, pre-failure
records, although incomplete, provide some insight into localized activity.
Displacements within the head region prior to the landslide and the lack of
movement in adjacent steep paleolandslide scarps suggest that pre-failure
creep was part of the ongoing slope failure rather than localized
instability resulting only from steep slopes. Localized small-scale
slumping occurred lower in the head region along Río Chujilluncani
(A
in Fig. 8a, b) adjacent to, or potentially within, a region of pre-failure
creep (∼1 cm a-1 LOS), supporting eyewitness accounts
suggesting that the 2011 landslide initiated there.
The new displacements behind the head scarp and along the upper lateral
margins of the 2011 landslide (Fig. 8c) gradually expanded over time into,
respectively, the Pampahasi gravel (Fig. 9b) and paleolandslide deposits
(iv and v in Fig. 9a). Movement in the latter suggests post-failure
reactivation of paleolandslide material as a result of the 2011 landslide.
Movements on the plateau are likely the result of dilation of the Pampahasi
gravel in response to removal of material to a depth of up to 80 m along the
2011 head scarp (Fig. 6a).
Mechanisms of failure
Progressive failure is increasingly being recognized as a precursor of
large, rapid landslides (e.g. Petley et al., 2002, 2005; Kilburn and Petley,
2003; Intrieri et al., 2018). In first-time brittle failures, creep reflects
damage accumulation during the reduction of intact material strength,
discontinuity strength, or both (Brideau and Roberts, 2015, and references
therein) and consequent stress concentration (Cornelius and Scott, 1993). In
such slopes, acceleration increases over time (Petley et al., 2002; Kilburn
and Petley, 2003). Eventual development of a continuous failure surface
(Petley et al., 2005) enables catastrophic release, commonly resulting in
debris fragmentation, high run-out, and velocities exceeding 5 m s-1 (Hermanns
and Longva, 2012). Sudden exceedance of resisting forces – for instance by
seismic loading, increased pore pressure, or slope undercutting – may
prematurely complete the failure surface, rapidly terminating the
displacement trend (Hermanns and Longva, 2012). In slopes failing along a
ductile shear zone or an existing plane of weakness, creep acceleration
decreases over time, trending toward zero (Petley et al., 2002). Because
displacement typically accelerates in each of the aforementioned failure
types (Fig. 11c inset), it provides an opportunity for failure forecasting
(Voight, 1989; Crosta and Agliardi, 2003; Eberhardt, 2012; Baron and Supper,
2013; Hermanns et al., 2013a; Federico et al., 2015; Moretto et al., 2017).
In contrast, over the 40-month period we monitored the Pampahasi slope (Fig. 11c), long-term steady-state creep was punctuated by temporary acceleration.
Pre-2011 displacement rates are for the most part extremely slow (after
Cruden and Varnes, 1996) but constant. No progressive acceleration was
evident at the temporal resolution of RADARSAT-2 or from eyewitness accounts
during the days leading up to failure. Displacement rates during the
4-day landslide event ranged from moderate to very rapid but did not
reach the extremely rapid range (> 5 m s-1: Cruden and Varnes,
1996) typical of catastrophic failures. Creep rates following the 2011
landslide were also constant but were greater than those prior to failure,
indicating a new state of instability following the landslide, which may
reflect temporary post-failure adjustment. Constant creep rates suggest that
the slope had, prior to failure and quickly afterward, reached steady-state
behaviour, consistent with the presence of an existing failure surface (see
Petley et al., 2002). Consequentially, prediction of large-scale failure
events on the Pampahasi slope or at similar sites in La Paz should not rely
exclusively on long-term creep time series.
Controlling factors
Geomechanical behaviour of the Pampahasi slope indicates failure nearer to
that of sediment than to rock. Both the La Paz Formation and the Pampahasi
gravel have compressive strengths well below 1 MPa, the threshold typically
used to differentiate between rock and sediment (Brideau and Roberts, 2015).
Parts of the main failure zone, as well as minor rupture zones within the
landslide body, likely follow weak, fine-grained seams sheared during the
Pampahasi paleolandslide or other past failures. Residual shear strength
will be especially low in clay-rich zones. The lower half of the valley
slope and the inferred failure zone beneath it dip at a similar angle
(∼8∘, Fig. 11b) to the residual shear strength of
the weakest (clay) zones within the failed sequence (Table S2), further
suggesting that failure occurred along an existing failure surface.
Geotechnical properties of the slope, particularly its reduced strength
resulting from extensive previous failure, enable its continual activity.
However, external factors contribute to the long-term instability of this
slope and many others in the La Paz basin. Ongoing fluvial downcutting and
toe erosion by Río Irpavi help to maintain the slope's metastable
state; spatiotemporal changes in fluvial erosion may have contributed to the
differing pre-failure activity in the northern and southern parts of the
landslide, as well as the increase in post-failure activity in both areas.
This driver is a response to the 4 km drop in base level since the Early
Pleistocene – from the time when the Altiplano plateau was internally
draining to the present drainage to the Atlantic Ocean (Roberts et al.,
2018) – and operates along other trunk streams in the basin.
Fewer than half of the largest historic landslides in the La Paz area follow
intense precipitation events (Table S1), suggesting that
enhanced pore-water pressure may play a role in initiating only some large
failures. The sudden onset of the 2011 Pampahasi landslide (Fig. 11c) is
typical of triggered landslides (Fig. 11c inset; Hermanns and Longva, 2012).
Heavy precipitation on the previous day – the 10th wettest on record – is
the only apparent trigger for the failure. Slope wetting was likely enhanced
by effluent from homes and concentrated overland flow from impervious
surfaces. We thus postulate that rainwater infiltrating the slope and
plateau above it on 25 February reached the basal failure zone by the
following evening; a consequent rapid pore-pressure increase and
shear-strength reduction appear to have triggered the landslide. Rupture of
the water line crossing the slope added much more water and likely
contributed to the mobility of the 2011 landslide directly downslope, where
displacement rates were greatest.
Sources of uncertainty
Possible acceleration in the days or weeks prior to the 2011 landslide
cannot be evaluated with the temporal resolution of the displacement
histories used here. RADARSAT-2's 24-day revisit period provides low
sampling frequency compared to some other radar satellites (Moretto et al.,
2017) and especially to in situ sensors or ground-based remote sensors (e.g.
Kalaugher et al., 2000; Crosta and Agliardi, 2003; Gischig et al., 2011;
Eberhardt, 2012; Federico et al., 2015; Confuorto et al., 2017; Carlà et
al., 2018). Additionally, the temporal filter used in processing
(Sect. S4.6) attenuates records of non-linear displacement
trends. The resulting temporal details reported here are sufficient to
characterize gradual acceleration over periods of months (Fig. 9), as has
been documented for pre-failure creep using InSAR elsewhere (Carlà et
al., 2018; Intrieri et al., 2018), or more abrupt acceleration over just a
few radar acquisitions but not shorter-term changes. Sporadic displacement
activity that may signal impending acceleration (see Kalaugher et al., 2000)
could similarly have gone undocumented.
Ground motion represented in displacement maps is independent of the
structural behaviour of the built environment. Isolated building instability
is likely in light of some local construction practices in La Paz but will
be extremely localized and thus is removed during spatial interpolation of
the maps. Phase change due to thermal expansion will be minimal given
limited seasonal temperature differences in the study area. Due to their
cyclic nature, any such phase component will not influence long-term
displacement trends.
The HDS-InSAR processing chain is complex and includes many non-linear
steps, which greatly complicates development of an accurate
error-approximation model. Both the pre-failure and post-failure stacks have
good baseline diversity, allowing relative errors between them to be
approximated by first-order estimates from the square root of the number of
scenes (32 vs. 12). In the absence of a rigorous model, we assume that error
for the thinner stack is conservatively twice as large as that of the
thicker stack. We approximate the errors as 3 and 6 mm a-1, respectively,
for the pre-failure and post-failure stacks. Because the average
displacement rates across much of the Pampahasi area exceed the error
estimates, the displacement patterns documented here are deemed to be
reliable.
Due to the structure of the HDS-InSAR processing chain, differing
environmental conditions between the two stacks, namely topography and
moisture (see Wasowski and Bovenga, 2014, 2015), have minimal effects. The
reference digital terrain model (DTM) is used only for an initial
topographic correction; stack processing solves for height error relative to
the reference DTM and provides a new elevation solution for each of the two
stacks (Sect. S4.6), which improves terrain representation. Topographic
correction of the post-failure stack thus accounts for landslide-induced
terrain changes, which were greatest in the source area (Fig. 11b). Temporal
soil moisture variability is unlikely to affect phase by more than
100∘ (Rabus et al., 2010), which equates to approximately
one-third of an interferometric fringe or 0.9 cm for the sensor used here.
Comparison of precipitation records in the 30 months before and 10 months
after the 2011 Pampahasi landslide indicates that long-term precipitation
amounts during the pre-failure and post-failure stacks were comparable (Fig. S2; Table S4). Spatial moisture gradients are a more substantial error
source (Rabus et al., 2010), but major differences present in a single scene
are removed during stack processing.
The assumption of slope-parallel motion is likely incorrect in some parts of
the Pampahasi area. The complex nature of the 2011 failure, particularly the
variability in the displacement directions (Fig. 2b), suggests that separate
masses may have moved obliquely relative to one another during pre-failure
and post-failure creep. Localized compression or extension within the
landslide would additionally impart vertical components of movement.
Additional error is thus introduced in the conversion from measured
one-dimensional (LOS) displacement to approximate three-dimensional (true)
motion. Provided that the sense of movement at any given location on the
slope has not substantially changed following the 2011 failure, LOS motion
vectors record similarly scaled changes in the true rate of motion and thus
reliably represent the degree of change in slope activity. Additionally, the
close alignment between the radar LOS and the fall line of the slope ensures
that the InSAR-measured displacement record is particularly sensitive to
expected gravity-driven slope motion between Pampahasi Plateau and Río
Irpavi.
Because RADARSAT-2 acquisitions over the site ended 10 months after the
failure, the current activity of the slope is poorly constrained and it is
possible that increased post-failure slope creep was short-lived. Additional
InSAR acquisitions planned for this and other sites in La Paz, including
both ascending and descending orbits, will provide additional insight into
more recent temporal and spatial complexity of slope activity.
Advances in InSAR monitoring of landslidesComparison with other studies
Our detailed documentation of slope activity preceding and following a large
landslide using spaceborne InSAR is, to our knowledge, unique. Numerous
studies show evidence of long-term, generally sustained creep of slow-moving
landslides (e.g. Colesanti and Wasowski, 2006; Lauknes et al., 2010;
Hermanns et al., 2013b; Necsoiu et al., 2014; Schlögel et al., 2016)
and in many cases document temporal variability in displacements. A much
smaller number of studies identify short-term deformation between successive
scenes (Moro et al., 2011) or creep leading up to catastrophic failure
(Carlà et al., 2018; Intrieri et al., 2018; Handwerger et al., 2019).
However, we are aware of only one other spaceborne InSAR investigation that
compares pre-failure and post-failure activity. Confuorto et al. (2017)
report displacement rates during a 1-year period starting ∼20 months following the 2012 Via Piave landslide in southern Italy that were
marginally faster than rates during a ca. 2-year period ending
∼16 months before failure (see Sect. S5 for
comparison with the dataset presented here). In that investigation, the
limited number of point reflectors, their concentration in a narrow band of
the head scarp, and the 3-year data gap bracketing the landslide limit
insight into potential post-failure changes in the slope.
Expanding utility
Advanced InSAR approaches such as HDS-InSAR increase ground-target density
and discriminate spatially uncorrelated deformation, improving the
characterization of landslides with spaceborne InSAR. Even though applied
here to radar data of moderate spatial and temporal resolution, HDS-InSAR
has provided detailed characterization of a large, dynamic urban slope.
Applying the technique to datasets with higher spatial resolution will allow
even more detailed characterization of landslide creep, and sensor
constellations will help increase the temporal resolution of displacement
records. The HDS technique also provides insight into the precise locations
of landslide margins and is thus particularly useful in dense urban settings
where development and remedial works remove evidence that is useful in
landslide investigations.
Furthermore, such techniques enable more comprehensive detection and
characterization of landslides of different depth and size. Preferential
detection of deep landslides by InSAR (Wasowski and Bovenga, 2014, 2015)
reflects the typically high spatial regularity of their displacements.
Shallow instability, especially in areas of variable microtopography,
generates spatially irregular ground motion that is more difficult to
detect. HDS-InSAR's optimization for locally variable ground motion
particularly improves the characterization of shallow landslides and thus
reduces biasing toward deep-seated instability.
Conclusions
The 2011 Pampahasi landslide is only one event in the long-term, and likely
ongoing, activity of a metastable slope. High-spatial-density InSAR
results demonstrate that several parts of the slope were active over the
years prior to the landslide and that nearly all regions with coherent
ground-motion records within and adjacent to it experienced post-failure
increases in the extent and rate of creep. This change in ground deformation
counters any expectation that a complex landslide might stabilize, at least
temporarily, following a discrete failure and highlights that such an
assumption, at least for short-term stability, is imprudent for
multigenerational failures. Slope dynamics documented here support
observations and theory from the scientific literature that spatiotemporal
clustering of landslides are responses to stress redistribution, which is
likely to exceed stress changes due to background erosion.
InSAR-derived deformation patterns provide new insight into failure
mechanisms of the Pampahasi slope. The complex failure comprises multiple
unstable parts, including many areas of shallow instability and at least one
large creeping block. Accelerating creep, a typical feature of first-time
and reactivated failures, was not apparent leading up to the disastrous
landslide on 26 February 2011. Any increase in displacement during the
13-day window following the last pre-failure radar scene (13 February)
apparently was not perceived by residents on the slope. The absence of
detectable acceleration may instead reflect the achievement of steady-state
creep along an existing surface, which is supported by the presence of
previously failed sediments and the similarity of the gradient of lower
slopes to the residual strength of the weakest geologic units. Increased
pore pressures resulting from particularly heavy rainfall triggered a large
failure that temporarily perturbed the dynamic equilibrium of the slope. The
failure reorganized stresses in the slope as well as beneath the plateau
above it, leading to a new, although possibly temporary, state of dynamic
equilibrium affecting even more of the slope.
Improved understanding of instability of the Pampahasi slope is instructive
in evaluating and reducing risk from landslides in La Paz, especially given
the large extent of paleolandslides in the city, their close association
with recent landslides, and incomplete knowledge of slope activity. The
Pampahasi slope is similar to numerous other slopes formed on large ancient
landslides in the city, including slopes where many of the largest historic
landslides have occurred (Fig. 1b). These slopes are underlain by generally
weak, fine-grained sediments of the La Paz Formation. Such slopes,
particularly those descending from the west margin of Pampahasi Plateau, are
key sites for future stability and risk assessments. Additionally, stress
redistribution suggested by enhanced creep of the Pampahasi
paleolandslide between the margins of the 2009 and 2011 failures and
possibly in adjacent parts of the Villa Salomé paleolandslide may
increase their susceptibility to large-scale rapid failure in the future.
Evaluating the possibility of future large-scale reactivations of complex
landslides should not, however, rely solely on creep acceleration,
especially given the lack of such a signal leading up to the 2011 landslide.
Furthermore, although fewer than half of the historic failures exceeding 1 Mm3 happened during the rainy season, coincidence of the 2011
reactivation of the Pampahasi paleolandslide with particularly wet
conditions indicates that consideration of high-rainfall scenarios is
advisable. In light of the evidence presented here of enhanced post-failure
activity, creeping slopes in La Paz are generally not appropriate for new
settlements or resettlement of residents displaced by disasters.
Optimizing InSAR characterization of landslides requires processing
methodologies that afford high ground-target density and discriminate
spatially uncorrelated deformation. The improved understanding of the
activity of the Pampahasi area reflects the detailed spatial
characterization made possible by the HDS-InSAR technique. Applying advanced
InSAR processing techniques such as this to modern and next-generation radar
datasets that, due to continued technological improvement, have higher
ground resolution and revisit frequency will improve understanding of
complex unstable slopes.
Data availability
The data are not publicly available as HDS-InSAR is part of a commercial product line.
The processed data contain intellectual property developed through research and development at MacDonald, Dettwiler and Associates (MDA).
The supplement related to this article is available online at: https://doi.org/10.5194/nhess-19-679-2019-supplement.
Author contributions
NJR, JJC, RLH, and BTR conceived the investigation. NJR processed radar
datasets with guidance from BTR. All authors participated in fieldwork. MAG
and EM arranged field logistics and provided crucial details on the history
of development and instability. NJR, JJC, BTR, and RLH interpreted
observations and results. NJR prepared the paper with contributions
from all co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
Nicholas J. Roberts used facilities and software at MacDonald, Dettwiler and Associates
(MDA) courtesy of Harold Zwick and Christian Nadeau and was assisted in house by Jayanti Sharma and
Jayson Eppler. He was partially funded by the Natural Science and
Engineering Research Council (NSERC) of Canada (postgraduate scholarship)
and the American Society for Photogrammetry and Remote Sensing (ASPRS;
Robert C. Colwell Fellowship). MDA Geospatial Services provided RADARSAT-2
imagery. Field investigation was supported by NSERC (Discovery Grant 24595
to John J. Clague), APSRS (Ta Liang Memorial Award to Nicholas J. Roberts), and the International
Centre for Geohazards (ICG contribution no. 377 to Reginald L. Hermanns). Victor Ramírez
and Eddy Baldellón (municipality of La Paz) relayed eyewitness accounts
of the 2011 Pampahasi landslide reported by residents of the affected area.
Detailed comments and suggestions from the anonymous reviewers significantly
improved the final paper.
Edited by: Thomas Glade
Reviewed by: two anonymous referees
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