Interactive comment on “ Revision of the geological context of the Port-au-Prince , Haiti , metropolitan area : implications for seismic microzonation ”

4.1: Method, âĂć we have add complementary descriptions on the main steps of the process of assessment and on the ground movement inventory âĂć the reference of Mora-Castro and al. is deleted because it concerns map at the regional scale. And so, in fact we think that is not the subject of this article. âĂć we think that is not the place to make " a complete overview of the state of the art about the landslide hazard estimation" and that the details in the text and in the table and figures are enough to show the general principle of the mapping process. We think that it’s necessary to keep an equilibrium between the description of the geological analysis and the ground movements study. Paragraph 5 Implications for preventive seismic recommendations Complementary information are given about the comparison between the map of damages in 2010 and the geological map. But the assessment of the site effects is the next step of the microzonation as indicated in the text.


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Abstract
A geological study has been conducted in the framework of the microzonation of Portau-Prince, Haiti.It reveals the deposit of Miocene and Pliocene formations in a marine environment and the impact on these deposits of the Enriquillo-Plantain Garden N80 • E fault system and of N110 • E faults.The tectonic and morphological analysis indicates motion during the Quaternary along several mapped reverse left-lateral N110 • E faults affecting the capital.Assessing ground-movement hazards represents an integral component of seismic microzonation.The geological results have provided essential groundwork for this assessment.Seismic microzonation aims to take seismic risk more fully into account in the city's urbanization and development policies.To this end, assumptions are made as to risks induced by surface rupture and ground movement from active faults.

Context
On 12 January 2010, Haiti was struck by one of the most violent earthquakes to have affected it in recent centuries.In its wake, the earthquake left 230 000 dead, 300 000 injured, and over a million homeless.Port-au-Prince, capital city of Haiti, and its metropolitan area suffered very massive losses.
To better integrate seismic risk into the reconstruction and development planning of the capital, a seismic microzonation study devoted to Port-au-Prince was undertaken, involving the PNUD, the Haitian government (LNBTP and BME) and BRGM.The selected zoning scale is 1 : 25 000 over the metropolitan area as a whole and 1 : 10 000 for sectors of particular interest.The zonation takes into account the seismic ground motion in the bedrock (regional hazard) and modifications produced by this motion due to local topographic and sedimentary conditions (location and characterization of site effects and induced effects).The techniques implemented for this mapping initiative rely on available knowledge, enriched by on-site investigations (geological, geophys-Introduction

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Full ical and geotechnical) and/or modeling.Intended for decision-makers, development planners, structural engineers and architects, but also for the public at large, this seismic microzonation is usefull both in rebuilding those zones of the Haitian territory that were devastated and, preventively, in the framework of a development planning policy for the territory that will take natural risks into proper account.

The geodynamic background
The island of Hispaniola consists of Haiti to the west and the Dominican Republic to the east.With the islands of Jamaica and Porto-Rico it goes to make of the archipelago of the Greater Antilles.These mark the boundary between the North-American and Caribbean Plates.The main motion between the two plates amounts to a relative mean rate of 2 cm yr −1 , with a converging direction of these two plates of N70 • E that is oblique with respect to the E-W plate margins (Mann et al., 2002).At Hispaniola, two great fault systems form the boundary between the two plates: the Septentrional-Oriente fault zone (SOFZ), running along the north of Hispaniola, and the Enriquillo-Plantain Garden fault zone (EPGFZ), to the south (see Fig. 1).
The SOFZ cuts across the north of the Dominican Republic and pursues its course westward beyond southern Cuba.The mean slip measured on it is 12 mm yr −1 (Calais et al., 2010; website http://web.ics.purdue.edu).The strongest known earthquake associated with this fault system, having a magnitude estimated at 8, is that of 7 May 1842.
Responsible for a very large number of fatalities, this event also caused considerable damage in several towns in northern Haiti.
The EPGFZ runs obliquely along Haiti's entire Southern Peninsula.To the west, this fault system reaches the island of Jamaica (the Plantain Garden plain).East of the Haitian/Dominican Republic border, the EPGFZ abuts against the Muertos fault system, which marks in this spot the subduction of the Caribbean plate beneath the Greater Antilles.The mean slip measured on the EPGFZ is 7(±2) mm yr −1 (Calais et al., 2010; website http://web.ics.purdue.edu).In addition to the 12 January 2010 Introduction

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Full event, other major historical earthquakes in recent centuries (1701, 1751, 1770, and 1860) are ascribed to this structure.According to Bakun et al. (2012), the 1751 event had a magnitude of at least 7.4, and for the one in 1770, the magnitude is estimated at 7.5.
In the center of the island of Hispaniola, a number of large active thrust faults trending NW-SE are visible in the present-day morphology (Calais et al., 1992;Pubellier et al., 2000).These faults structure the Transhaitian Belt.To the north, they connect into the SOFZ and, to the south, into the Muertos fault system.The thrust of the Matheux range corresponds to the northern boundary of the Cul-de-Sac Plain (see Fig. 1).This is the main structure of the Transhaitian Belt.According to GPS data, the mean measured slip on this thrust is approximately 1 mm yr −1 (Calais et al., 2010; website http://web.ics.purdue.edu).Seismic profiles implemented in the Gulf of Gonâve (Mann et al., 1995) reveal the presence of folds and reverse faults dating in age from the Miocene to the Quaternary, which signal the active frontal edge of the Transhaitian Belt.The reverse motion of the Transhaitian fault system partially compensates the oblique component of the convergence of the North-American and Caribbean Plates with respect to their edges.
The 12 January 2010 earthquake had a magnitude M w of 7.1; its focal depth was 13 km.The epicenter was located between Gonâve Island and the eastern end of Haiti's Southern Peninsula (see Fig. 1).The focal mechanisms computed for the main shock and its aftershocks, the radar interferometry measurements, and those from GPS indicate motion along a fault striking ENE-WSW (N71 • E), that is north-dipping with a reverse left-lateral component (Calais et al., 2010).Thus, this fault is slightly oblique with respect to the trend of the EPGFZ (N80 • E) and lies to the north of this zone.Designated the "Léogâne fault" (Calais et al., 2010), it is interpreted as being a blind thrust.
According to the analysis of aftershocks of the 12 January 2010 event, the main rupture on the Léogâne fault appears to have triggered a reverse movement on a N120 Full  et al., 2011), and the reactivation at depth in a left-lateral strike of the main fault plane of the EPGFZ (Hayes et al., 2010;Douilly et al., 2012).

The geological study of the Port-au-Prince metropolitan area
The geological facies of the Port-au-Prince Metropolitan area constitute the essential input data controlling both the soil responses to sismic vibrations and the susceptibility to ground movements.The geological information available for the Port-au-Prince region was relatively succinct, mainly consisting in the geological map of Haiti at 1 : 250 000 scale by Momplaisir and Boisson (1987) and the recent geomorphological sketch by Bachhuber et al. (2010), at a 1/50 000 scale.
For achieving the seismic microzonation, it was necessary to improve the knowledge of geological context.The geological study consisted in observing several dozen outcrops, supplemented by the analysis of geotechnical boreholes drilled earlier, of aerial photographs from 2002, of the digital terrain model with pixels 2 m to a side calculated from 2012 Pléiades satellite images (KalHaiti, SERTIT/CNES), SPOT 2010 ortho-images, and 3-D-view satellite images of various dates under Google Earth.

Mapping lithological facies
The Port-au-Prince metropolitan area lies in the Cul-de-Sac alluvial plain (see Fig. 2).Eastwards, the urban area ends near the Grise Rivière and compared to recent work on similar lateral facies (Hernaiz Huerta et al., 2007), while to the west, the city is limited by the sea.In a southerly direction, the city abuts on the mornes (mounts) of Saint-Laurent and Hôpital in the La Selle range.These mornes are bounded to the south by the Rivière Froide, with its course, in turn, determined by the EPGFZ.
The various facies encountered have been classified into ten geological types or units ranging from the oldest to the most recent: F1 (Upper Eocene) through F10 (Plio-Quaternary to Present) (cf.Fig. 3).The estimated ages of these units are derived Introduction

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Full from the work of Butterlin (1960).The rocky substratum consists of limestones, silts, marls or conglomerates (or breccia).Sandy facies are rarely encountered.The marls are frequently associated with limestones to form "marly-limestone" alternations or to concentrate the few marine fossils encountered within the studied zone.The presentday topography is largely the result of the Pliocene orogeny and of the activation of the faults that delimit the blocks, atop which Quaternary and recent erosion has occurred.
At the Saint-Laurent and Hôpital mornes, Eocene to Oligocene limestones outcrop to the south and Miocene conglomerates to the north.The main orographic axis is N80 • E, parallel to the EPGFZ.Between the Cul-de-Sac alluvial plain to the north and the northern slope of Morne Hôpital, small secondary ranges trending globally N110 • E mark the topography.The marly limestones and Mio-Pliocene marls or silts form the foundation of these small mounts.Here, the stratigraphic surfaces of these formations trend WNW-ESE and dip steeply (angle greater than 45 • ) towards the north or south (see Fig. 2).These reliefs are bounded, to the east and north, by the alluvial formations of the Grise River, and to the west by marine alluvia.To the south, colluvial deposits or dejection cones cover these mornes in spots.The colluvia in place are mostly fairly shallow, with thicknesses generally ranging between 0.1 and 0.5 m on more rugged relief and from 0.5 to 1 m on average in less hilly areas.These remobilized superficial deposits may attain thicknesses of several meters in narrow valley bottoms.On Fig. 3, the analysis of Miocene to Pliocene terrains in the Cul-de-Sac Plain shows: (1) deepwater to sublittoral marine deposits (marly limestones, mudstones, fine-grained sandstones and silts) characteristic of low-energy environments; (2) sharp intercalation of detritic formations related to sequences of sandy and gravelly to silty turbidities (conglomerates).These formations correspond to marine deposits with a proportionally variable limestone component, overlain by flysch sequences that become increasingly regular and frequent at the end of the Miocene and beginning of the Pliocene.These sharp detritic intercalations, considered to be tectonic in origin, are ascribed to the continual activation during the Mio-Pliocene of the faults bounding the Port-au-Prince plain to the south.Introduction

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Full These Miocene and Pliocene formations are several hundreds of meters thick, possible exceeding a thousand meters, on the strength of the marine seismic profiles in the Gulf of Gonâve (Mann et al., 1995) as well as the borehole drilled in the Cul-du-Sac Plain by ATLANTIC REFINING in 1947 (Momplaisir, 1987).

Fault mapping
As to fault mapping, this was performed by observing outcropping fault planes (Fig. 2), supplemented by an interpretation of morphological data.A majority of the fault planes measured trend E-W to NW-SE.Most present reverse left-lateral activation (cf.Fig. 4).In several locations, the contact via the fault between quaternary alluvia and Miocene to Plioene formations is visible (Fig. 5).
In the center of the metropolitan area, a conglomerate formation composed of uncemented, graded, polymetric limestone elements embedded in a marly sandy matrix overlies the Mio-Pliocene calcareous marl and silt formations.This formation corresponds to a piedmont alluvial cone probably deposited between the end of the Pliocene and the early Quaternary.The hydrographic system responsible for this deposit is to be sought in the limestone relief to the south, quite probably in the upstream portion of the Bristout River catchment area (Fig. 6).The debris cone is characterized by three distinct assemblages, interpreted as successive deposits from the drainage basin.The oldest lies furthest north, and the most recent, southernmost one is still fed by the present-day Bristout ravine.Based on the central axis of the north and center portions of the cone, the hypothesis of a 1000 m offset cannot be excluded (cf.Fig. 6).This would indicate a horizontal displacement rate of 0.6 (±0.2) mm yr 4 Evaluation of ground movement hazard

Method
Ground movements may be triggered by seismic vibrations.This is why these must be analyzed in the seismic microzonation process, as potential site effects.In addition to local topographic conditions, the lithologic nature of the terrains constitutes one of the main environmental factors governing ground stability.
The geological map prepared in the framework of the microzonation operation, the digital terrain model calculated using Pléiades (KalHaiti, SERTIT/CNES) images, and the inventory of ground movements constitute the reference mapped data for assessing the ground-movement related hazard.
As attested in the past ground movements listed on Table 1, Port-au-Prince is often concerned by slope instabilities.These events fall into two main categories: landslides, potentially large-scale, and rockfall, more local in nature.Among the forty or so events recorded for Port-au-Prince, 19 dispose of precise informations, as the location, the geological facies involved and the instability's geometry.For the remainder, they are eyewitness accounts indicating some events triggered by the 2010 earthquake.
Landslides generally involve calcareous marl formations (F4, F5 and F6, cf.Fig. 3), which may be overlain by more recent layers (F7 and F8), while rockfall events concern mainly consolidated rocky layers (such as F3 and F2).In a few instances, calcretizations may be at issue (these may be up to 1 m thick) observed at the roof of the Morne Delmas formation (F6 to F8) or the shelly marlstone banks (F6) in the calcareous marl alternations.
While the lithological nature largely conditions the type and the exent of the potential ground movement, the hill slope takes also a significant part in the ground instabil-Introduction

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Full ity assessment.Other factors influence the susceptibility of hillsides instabilities, but these are often difficult to integrate at this study scale, among them, for example, the structural data (as dip of stratigraphic or fracture planes), lithological variations within geological formations, or hydrogeological circulations.
For each geological formation, slope thresholds have been defined, mostly calibrated by means of the feedback given by the inventory (cf.Table 1).Thus, a preliminary map was produced thanks to a data digital cross-comparison, with a 5 m-resolution.At each spot, a value is assigned according to the geological nature and the value of the slope.This digital processing yields a systematic preliminary assessment of the level of susceptibility at each point of the study area.The approach is comparable to that of Mora-Castro et al. ( 2012), but reposes on more precise mapped data and a more complete inventory of ground movements.Precise analysis was carried out based on the interpretation of orthophotos, aerial photos, DTM and its derivatives (slope, exposure, shape of topography), as well as onsite investigations.The feedback was used to justify the limit of the assigned hazard levels.

The ground movement hazard map
The finalized hazard map for ground movements is at 1 : 10 000 scale.It preserves the distinction between the two types of ground movements, i.e. landslide and rockfall (Fig. 7).Virtually half of the studied territory is concerned by landslide.The rockfall phenomenon, involving less than 10 % of the studied area, is concentrated along the southern boundary of the studied zone as well as on the foothills of the La Selle mountain chain.Landslides remain by far the most frequent and most damaging phenomenon.
Four levels of susceptibility of increasing severity are defined: 1. Zones assigned a hazard that is weak or non-existent, applying to areas that are flat or gently sloping (< 5 • ).These zones are little affected, if at all, by landslide or rockfall.The few events that may occur are minor.Introduction

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Full 2. Zones assigned a moderate hazard may be subject to landslides of limited (a few hundred m 3 ) or moderate scale (a few thousand m 3 ).The areas concerned present gentle slopes (between 5 and 10 4. Zones assigned a high level of hazard are proned either to frequent superficial landslides or to large-scale events (a few to several hundred thousand m 3 , active or potential).These are characterized by intermediate to steep slopes with unfavorable geology or land-use factors (typical configurations of the historical well-known landslides like Vivy Mitchell 1 and 2, Musseau), or they are subject to massive landslide propagation from higher altitude.Thus, included under this category, we find ravines and foot slopes, receptacles for materials that have slipped off slopes and may be remobilized in the ravines as mudflows.

Implications for preventive seismic recommandations
The geological study of Port-au-Prince brought the main information to assess the soil responses to seismic vibrations, i.e. extent and the nature of the geological ground formations.

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Full In the framework of the seismic microzonation, the next step is to complete these geological informations with geotechnical data and geophysical investigation, to map the soil responses capacity.
The geological study of Port-au-Prince reveals also that the capital is located near or directly over two active fault systems (Fig. 2), which have been taken into account in the seismic risk assessments: (1) the EPGFZ, which bounds the southern limit of the Portau-Prince urban area, and (2) the N110 • E faults, very probably corresponding to the south-easterly extension of the reverse faults located in the Gulf of Gonâve and which connect into the EPGFZ to the east.The system of N110 • E faults mapped on land has a maximum length of 15 km but, as indicated by offshore geophysical data, this system almost certainly extends WNW into the Gulf of Léogâne.Tectonic deformations in terrains dated presumably to the middle or recent Quaternary is observed at several locations along these faults.Taken in conjunction with the morphostructural data, these indicate a reverse left-lateral displacement, associated with a mean horizontal slip on the order of 0.6 (±0.2) mm yr −1 .
Thus, in the context of the seismic zonation of the Port-au-Prince metropolitan area, the construction of such high-stake buildings as schools, hospitals, disaster relief centers or structures of strategic importance in the event of emergency is prohibited in the vicinity of active faults.
Concerning the ground movement hazard, each specific zone identified on the hazard map (Fig. 7) is subject to specific recommendations (measures) designed to take the ground movement risk into account in existing or future urban land-use development.The zones associated with reduced or no hazard, i.e., little affected by landslides or rockfalls, are not obligated to respect any specific recommendations, other than the landscaping and building state of the art.The zones associated with a moderate, intermediate or high hazard categories are governed by specific recommendations, entailing geotechnical studies more or less onerous that could go to prohibit any new residential construction or, at least, high-risk structures.

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Conclusion
The geological study performed in the framework of the seismic microzonation of Portau-Prince has enabled the environmental context of the deposit of the Miocene and Pliocene formations to be established.These are marine deposits, either deep or sublittoral, which are characteristic of low-energy environments liable to sudden inputs of detrital materials, inputs associated with turbidities and with a tectonic origin probably related to the continual activation of the southern boundary faults of the Port-au-Prince Plain during the Miocene and Pliocene.Moreover, the study confirms the presence and active nature of several faults inside Port-au-Prince.Concerned are not only the EPGFZ bounding the Port-au-Prince metropolitan area to the south, but also faults trending N110 • E that are directly visible within it.
The geological study identifies several lithological units for which an age is estimated from the work of Butterlin (1960).These ages should be confirmed by chronostratigraphic dating.Moreover, it would be necessary to determine the relationships and contours for units F6 through F8, which are characterized by specific depositional environments.Furthermore, the F9 deposits needs to be determined with greater precision, notably its relationship with the upstream portion of the catchment area.Also, concerning the risk of surface rupture of active faults, recommendations are made; however, it would be preferable to reduce uncertainties on the fault traces and to better understand the seismogenic potential of these structures.To this end, the analysis could be followed up by local geophysical reconnaissance missions and subsequent paleoseismic trenching and dating of the geological formations.
Mainly on the basis of geological map and inventory of past events, this work has yielded a ground movement hazard map at 1 : 10 000 scale which takes into account both the initial predisposition and the propagation of instabilities, while also distinguishing between landslide and rockfall phenomena.The results will be directly integrated into the seismic microzonation of Port-au-Prince.Full  Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | −1 for the WNW-ESE fault systems mapped at this location.This estimate is compatible with the values from GPS measurements on the compressive deformations of the Transhaitian Belt (Fig. 1).Furthermore, between the Musseau and Chavannes quarters, the hydrographic network presents a number of inflections in hydrographic drains following a WNW-ESE trend.These inflections may locally have sizes up to 300 m.This mean value is coherent with the hypothesized horizontal displacement rate indicated above, bearing in 1619 Discussion Paper | Discussion Paper | Discussion Paper |mind, in this instance, that these are markers of more recent date (Middle to Upper Quaternary).
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | KalHaiti, SERTIT, CNES: Production d'un Modèle Numérique de Surface -Acquisition Pléiades tristéréoscopique, Port-au-Prince, Haïti, 2012.Mann, P., Taylor, F, Edwards, R., and Ku, T.: Actively evolving microplate formation by oblique collision and sideways motion along strike-slip faults: An example from the northeastern Caribbean plate margin, Tectonophysics, 246, 1Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |
, composed of marl, calcareous marl, silt or alluvia, or zones subject to the propagation of landslides initiated on steeper inclines upslope.Zones with intermediate hazard may also be at risk for rockfall; in this case, the terrains involved are rocky, with slopes exceeding 10 • , or zones liable to the propagation of rockfall from above. •

Table 1 .
Characteristics of the most important ground movements inventories over the Port-au-Prince metropolitan area (L: landslide; R: Rockfall; B: Bank Erosion).