The frequency–time distribution of earthquakes between 25 August 2019 and 18 August 2021 shows the immediate seismicity related to the 18 August 2020 Mw 6.6 event (Fig. ). The foreshocks, mainshock, and aftershocks are identifiable in the probability density subplot (Fig. a) as a distinct bell-shaped curve relative to the generally quiescent background activity. The island of Masbate typically experiences less than 10 earthquakes per day, corresponding to a moderate background seismicity. An increase in earthquake frequency defines the foreshock sequence at around 32 d before the mainshock, as shown by the distinct trend increase in the probability density subplot. The peaks of the frequency plots (Fig. a and b) correspond to 141 earthquakes which were mostly aftershocks on 18 August 2020. A negative slope in the probability density subplot depicts the aftershock sequence. The decaying earthquake frequency is initially symmetrical to the foreshock trend with an inflection point around 30 d after the mainshock, which transitioned to a more gradual decrease. The aftershocks persisted for 166 d or up to 31 January 2021. The succeeding return to background levels saw a similar seismic activity to the previous background activity from 2019 to early 2020.

In general, the background seismicity (M≤4) is well distributed throughout the overall length of the Masbate segment of the PF traversing the islands of Ticao and Burias, as well as the provincial mainland (Fig. a). However, an absence of seismic activity is noticeable in the northern portion of the fault in mainland Masbate during this period. On the contrary, an earthquake cluster occurred near the location of the mainshock. Foreshock activity is concentrated in southeastern Masbate adjacent to the mainshock and is composed of mostly weak (M 2–3) and shallow hypocenters peaking at M 4.9 (Fig. b).

Three moment tensor solutions from GlobalCMT, PHIVOLCS, and USGS highlight the mainshock's dominant left-lateral strike-slip movement (Fig. c). The solutions of PHIVOLCS and USGS show a noticeable, albeit minimal, dip-slip component toward the NNE. The epicentral location of the mainshock from GlobalCMT and PHIVOLCS coincided with the fault trace, while USGS' focal mechanism solution deviated to the northeast of the fault.

Aftershocks that transpired on the same day as the mainshock (Fig. d) are shallow (1–30 km) and densely grouped in the southeastern offshore extension of the segment and deepen (10–50 km) as they taper toward the onshore portion. The aftershocks have weak magnitudes ranging from M 2.0 to M 4.4. Succeeding first-month aftershocks are weak to moderate, with a wide range of magnitudes between M 1.7 and M 5.1, mostly occurring at depths above 30 km. A monthly progressive dispersal of hypocenters in space is also observed, with the first month showing a dense cluster in the southeastern section that tapers to the northwest (Fig. e). The pattern and geographic extent of the first-month aftershocks appear to be an expansion of the same-day aftershocks. The succession of the aftershock patterns in the following months continued to scatter along the Masbate segment and the SSF while retaining a perceptible concentration in the offshore southeastern extension of the causative fault segment (Fig. f–i). The last 2 months of aftershock activity (Fig. h and i) feature the least number of hypocenters throughout the aftershock sequence.

Background levels returned on 2 February 2021 defined by the well-distributed hypocenters on the fault segments with moderate magnitudes (Fig. j) similar to the pre-event seismicity in 2019 (Fig. a). The frequency and distribution of earthquakes in time and space enabled the distinction of various stages of the earthquake sequence relative to the 2020 Masbate event.

The acquired pixel offset maps (Fig. ) illustrate the fault rupture and the known left-lateral movement of the Masbate segment, despite the presence of noise that obscures portions of the rupture. Only one of the eight image pairs from the short time span spaced months apart (Fig. a) produced outputs with perceptible rupture due to the low SNR. Annual time-span pairs resolved the rupture more clearly compared with the short time-span image pairs. The north–south slip components show better representations of the rupture, whereas the east–west components tend to be noisier. Nevertheless, the valid image pairs (Sect. S1.1) with resolvable slip and subjected to stack profiling exhibit reliable estimates of the slip in the near-field region.

A generalized fault trace (yellow line in Fig. ) was delineated from the valid pixel offset rasters and the known fault trace, which served as the baseline for slip profiling. The fault trace is mostly straight, except for an apparent northeasterly strike rotation toward the north, where it follows the piedmont of a topographic high in Dimasalang. No measurements were acquired in the area due to anomalously high values, which are probably terrain residuals. Topographic artifacts and decreasing slip away from the epicenter posed difficulties in delineating the northern rupture section near Naro Bay. In the southwestern section,the offset rasters show a new splay striking NW–SE that bisects the previously recognized bifurcation in Cataingan. The rupture traverses the deltaic area and possibly continues southward across Cataingan Bay, then resurfaces on the opposite side of the bay. Smaller fractures and minute rupture components are not visible due to ubiquitous noise.

Left-lateral offsets were quantified along the ∼25 km onshore length of the fault from Dimasalang to Cataingan, given the inland spatial extents and continuity where optical correlation was feasible. Edge artifacts and the obscured western block in the south of the bay hindered the acquisition of slip profiles.

A total of 51 wide swath profile boxes were generated throughout the fault rupture, with the first box in Dimasalang and the last in Cataingan to determine the along-strike-slip distribution (Fig. ). The optical correlation results generated a total of 126 left-lateral measurements over six image pairs (Sect. S1.2). Using a 325° general strike azimuth of the Masbate segment, the east–west and north–south displacement components (Fig. a and b) were transposed into the fault-parallel direction (Fig. c).

The 9 April 2020 to 2 April  2021 pair was selected as the representative dataset due to its wide spatial extent and reliable representation of the fault rupture. The north–south component (Fig. a) shows a continuous linear rupture trace from Dimasalang to the deltaic area in Cataingan, although the Dimasalang area and its border with Palanas exhibited unusually high values due to topographic artifacts. The east–west component (Fig. b) is less coherent, likely due to smaller ground movements in this direction or overprinted data from the noise. The outputs show noise from both artificial and environmental sources, represented by topographic artifacts in the southwestern block, an apparent seamline, and reduced coherence due to differences in land cover between the images. Nevertheless, the fault-parallel component (Fig. c) shows good estimates of the left-lateral slip, further supported by the near-field offset models.

Box 32 (Fig. d), located ∼6 km north of the GlobalCMT surface projection, shows a 43 cm slip in the near field (Fig. e). Pixel offset values increase in the far field for the western block, which may either represent noise or distributed deformation. Box 47, situated ∼1.5 km south of the GlobalCMT epicenter, exhibits a 59.2 cm left-lateral offset. The box represents the peak offset in this image pair and displays a clear contrast between the eastern and western blocks.

The plot of the slip distribution along strike (Fig. ) displays the 126 left-lateral offsets compiled from the valid offset rasters to characterize the displacement field. The slip minima ranges from 6 to 9 cm, located 4.5 km from the northern shoreline in Dimasalang. The displacement increased to ∼50 cm around the vicinity of the GlobalCMT projection of the centroid. Slip values remained constant within a ∼5 km span in Cataingan and increased to a peak ∼60 cm at around ∼1 km southeast of the centroid. South of the peak value, the displacement declines toward the shoreline. The skewed semi-elliptical weighted moving mean curve (solid gray line in Fig. ) shows the average left-lateral offset of 37.7±10 cm, while the individual measurements indicate a peak 60.6±9.8 cm offset adjacent to the surface projection of the centroid in Cataingan.

The Sentinel-1 coseismic wrapped interferograms in the ascending and descending tracks (Fig. a and c) show the butterfly-shaped fringe patterns and oppositely verging LOS displacements, both of which are typical of strike-slip events. The fringe lobe patterns illustrate a range increase from the ascending track and a range decrease from the descending track for the western side of the fault. Since the orbit of the ascending track is subparallel to the regional fault strike, the left-lateral movement is indicated with a possible subsidence component. This is consistent with the moment tensor solutions (Fig. c) and agrees with the sense of movement shown by the optical correlation outputs and field observations.

Low phase coherence persists in the near-fault region. Widespread decorrelation leads to mostly unusable information upon phase unwrapping due to the likelihood of creating unwrapping errors. Noise is similarly dominant and overprints most of the deformation signals in an approximately 18 km-wide region across the fault trace, hence, the lack of near-fault information in preliminary published interferograms . In addition, the rupture is unresolved upon phase unwrapping (Fig. b and d) due to decorrelation. This loss of resolvable information in the near field ultimately hinders slip measurements in the coseismic interferograms.

Post-seismic interferograms capture an improved overall manifestation of the fault rupture compared with the coseismic ones (Fig. ). This is a consequence of the expected smaller afterslip values relative to the coseismic slip, which decreases the number of fringes and increases the coherence. The temporal coverage of the interferogram stack allows us to quantify afterslip to complement the estimated coseismic slip from optical correlation. However, the first post-earthquake Sentinel-1 revisit occurred 2 d after the mainshock, resulting in all measurements to be lower-bound estimates of the afterslip.

Unwrapping errors are less frequent on the ascending and descending track interferograms and are further minimized upon time-series stacking. The remaining phase unwrapping errors upon stacking and filtering were manually excluded in profiling. These unwrapping errors occur as significant high amplitude phase shifts and polarity reversals, noticeable in the Dimasalang and Palanas areas.

The fault-parallel afterslip deformation field (Fig. c) shows a strongly linear and simple fault trace that accommodated the post-seismic deformation. Deviations from the regional fault strike are observed around 5–13 km from Naro Bay in the north, represented by multiple short NNE–SSW oriented (∼290–300° azimuth) bends which are adjacent to and coincide with the narrow stepover in the Palanas–Dimasalang area. However, the noticeable unwrapping errors and low ground resolution (80 m) precludes the validity and recognition of these rupture complexities.

The afterslip is resolved from Naro Bay in the north toward Cataingan Bay in the south along the ∼25 km simplified fault trace (Sect. S4.1). A total of 49 swath profile boxes were generated along this trace. Individual measurements reveal centimetric fault-parallel afterslip with minimal across-fault variability and minor 1σ uncertainties, except where the boxes overlap with the phase unwrapping error patches. The deformation was detected up to the far-field extents of the profiles from most swath boxes.

Boxes 4, 13, 25, and 39 (Fig. d–g) illustrate select wide swath profiles roughly equidistant from each other. Box 4, situated in Dimasalang, shows a 5.4 cm left-lateral fault-parallel afterslip and reasonably low variability across the fault. Unwrapping errors in the far field at 1.5 km from the fault on both sides caused irregularities in the profile. Box 13, located around the boundary of Palanas and Dimasalang, depicts an 8.4 cm fault-parallel slip. Both sides of the fault exhibit exceedingly even slip across the fault, except for a noticeable dip in the eastern edge. The dip is likely due to data loss since the box overlapped with the water body to the east. The profile also appears to be offset by about ∼100 m to the west, which corresponds to one of the abovementioned bends that may be an unwrapping artifact. Still, the measured slip from the arctangent model remains valid. Box 25 shows a 6.9 cm afterslip in Palanas, located at the southern edge of the fault complexity. Multiple across-fault deviations are visible around 200 and 500 m east of the fault, as well as in the western far field. The eastern anomalies could either be unwrapping or topography related, while the western patch comes from unwrapping issues. Box 39 in Cataingan displays a 12.0 cm left-lateral afterslip with good symmetry and even across-fault distribution.

The along-strike afterslip distribution graph (Fig. ) depicts the computed fault-parallel offsets from north to south on the mainland. The smallest value is measured in Dimasalang, adjacent to Naro Bay, amounting to 4.3 cm of left-lateral afterslip. The ground offsets exhibit a generally increasing trend to around ∼5 km from the north. Fluctuating values are observed between 6.1 to 13.8 km from the north in Dimasalang and Palanas. The measured displacements oscillate between 5.0 and 9.6 cm in this span. The increasing trend continued to a 14.3±0.9 cm peak value in Cataingan, located 24 km from Naro Bay and ∼3 km southwest of the GlobalCMT surface projection. The displacements decrease from the largest value to 11.5 cm on the shoreline of Cataingan Bay. The weighted moving mean curve (solid blue line in Fig. ) indicates a mean left-lateral afterslip of at least 8.8±0.7 cm.

Remnant effects after about 2 years post-earthquake were observed on laterally offset cultural and geomorphic features from Dimasalang to Cataingan following the known fault trace (Sect. S2). Assisted by the remote sensing data, the observed ground manifestation of the earthquake from 57 stations (Fig. a and b) include fractures, reported earthquake-induced landslides (EIL), indications of liquefaction features and lateral spreading, secondary gravitational features, and ruptures with quantifiable displacements. Earthquake effects recorded during the field activity include reported ground shaking from interviews with locals.

The observed rupture zone of the 2020 Masbate earthquake persisted from Dimasalang to Cataingan and crossed Cataingan Bay. Measured surface ruptures strike at a low angle with respect to the regional fault trace (Fig. c) with a mean strike azimuth of 313°. Our northernmost resolvable ground offset was observed on a road in Dimasalang, around ∼3 km from Naro Bay. An apparent 0.5 cm offset fracture oriented parallel to the fault trace was recorded in this point, possibly representing the minimum northward constraint of the field rupture. A 2–5 cm displacement oriented 325° NW occurring on a residential structure on top of the fault was observed on the border between Dimasalang and Palanas. The rupture displacement increased further south ∼7 km from the northern shoreline, where a 7 cm left-lateral offset was seen coupled with spreading of the road pavement. The fault bisected a coconut tree at the southern limit of the fault complexity in Palanas (Fig. d) , resulting in a cumulative 87 cm opening from the 2003 and 2020 events. The newer slip is estimated at 27±10 cm on a 275° W-oriented rupture. Right-stepping en echelon fractures occurred adjacent to the coconut tree, most of which were eroded at the time of visit.

The rupture is continuous southward, up to 4 km from the GlobalCMT centroid projection. The fault rupture traversed an abandoned house and deflected its wall by roughly ∼50 cm. A soil mound at the base of the wall (Fig. e) shows a 297° W-oriented rupture with a 48±3 cm left-lateral offset. Residents reported a ∼200 m-long rupture in the area, which was already mostly eroded. Nevertheless, the remnant right-stepping en echelon fractures formed a linear rupture zone connected to the displaced wall. A 30 cm offset oriented 279° E on a road in Cataingan likely represents the northern indication of the rupture bisecting the bifurcation in Cataingan. The rupture continued to an agricultural area (Fig. f), where the embankments are displaced by 60±15 cm, representing the peak field offset measurements. Long, continuous ruptures were observed on the rice fields immediately after the event but were erased by the time of the field investigation. Another southerly continuation was identified, where a 341° NW-oriented rupture displaced fishpond levees (Fig. g) by about 34±5 cm. Locals stated that the visible rupture appeared lengthy and continuous in the deltaic areas immediately after the earthquake. The rupture was evident again south of Cataingan Bay, manifested on a collinear set of points through several residential structures. A septic tank diagonally cut by a 279° NW-oriented rupture shows a 30 cm left-lateral displacement (Fig. h). Offset measurements adjacent to the septic tank vary from 5 to 30 cm.

The plot of offset features along the fault strike (Fig. ) and shows the minimum 0.5 cm left-lateral displacement ∼3 km from Naro Bay in Dimasalang. This point corresponds to the northernmost resolvable ground offset. The measurements continually increased to 50 cm beginning at 17 km from Naro Bay, around the boundary of Palanas and Cataingan. The offset measurements remained constant for a ∼4 km span followed by an abrupt dip to 30 cm, which increased again to the peak 60 cm offset. The local 30 cm minimum was measured on a ruptured road. This single discordant measurement could be due to off-fault displacement, physical reduction due to post-earthquake repairs, or an inaccurate measurement from a cultural feature. The displacement tapered to 30 cm near the shoreline of Cataingan Bay. Spatially coincident offsets ranging from 5 to 30 cm resulted in a wide range of uncertainties at the southeastern limitation of the plot. The weighted moving mean curve of the slip distribution (broken red line in Fig. ) for the 2020 Masbate event indicates a 25.3 cm mean left-lateral displacement, while the individual observations denote the maximum of 60 cm displacement near the GlobalCMT centroid projection.

Independently applying optical correlation and interferometry to analyze the 2020 Masbate event provided a basis to assess the technical capabilities and shortcomings of the methods in a tropical setting. Peak displacements of the Masbate earthquake amount to ∼60 cm, denoting a high interferometric phase gradient for Sentinel-1. SAR interferograms lose coherence when offsets exceed half the interferometric fringe per pixel , highlighting the effectiveness of the method for small deformations on millimeter and centimeter scales . The LOS measurements of and from coseismic interferograms are likely underestimated due to data loss because of the high phase gradients and near-field decorrelation on the coseismic interferograms (Fig. ).

The ability of InSAR to detect centimetric displacements was evident when comparing the post-seismic and coseismic interferograms. The rupture trace is conspicuous and the offset is observable in both the near field and far field of the post-seismic interferograms. Left-lateral afterslip measurements are at least 4–14 cm with an average 1σ uncertainty of 1.5 cm. Millimeter-scale offsets were not detected given the coarse 80 m pixel resolution and noise of the resultant interferograms.

Optical correlation is useful to assess the near-field region and larger displacements equivalent to at least one-tenth of the pixel resolution . Results show that near-field displacements are resolved (Figs.  and ) despite noticeable high noise levels. OIC, using PlanetScope data with 3.125 m pixel resolution, was able to detect left-lateral offsets between 7 and 60 cm in the near field but with a larger 19.9 cm 1σ average uncertainty than InSAR due to noise. Thus, careful interpretation of swath profiles is crucial to identify valid measurements because noise easily overprints tectonic signals. Noise signals are amplified by misaligned acquisition geometry, unequal solar illumination angle, and insufficient geometric corrections . The representative 9 April 2020 to 2 April 2021 (satellite ID 1105) images had decent illumination and acquisition geometry agreement (Table ), and support the validity of the measurements.

The pair classes of PlanetScope orthoimages used in the study showed that images roughly 1 year apart resolved the rupture far better than those imaged months apart primarily due to the contrasting vegetative cover. Satellite images from the same seasons are critical for optical correlation because feature similarity helps the algorithm match features more accurately . Feature matching in agricultural and developing areas can be difficult in the subpixel domain given the difference in crop patterns and rapid anthropogenic development, leading to differing pixel intensity values in the pre-event and post-event images .

Images that are 1 d apart generate deformation fields with minimal seasonal and anthropogenic artifacts. However, this may have exceptions such that correlations for the 2016 Fukushima earthquake using 1 d interval drone images resulted in noisy deformation fields with low correlation (Sotiris Valkaniotis, personal communication, 2021). Single-day intervals were not tested in Masbate due to the lack of usable orthoimagery in terms of study area coverage, compatible acquisition parameters, and cloud cover.

DInSAR relies on coherence to resolve the deformation along the LOS direction, which is controlled by the similarity of point scatterers between the acquisitions. Good coherence in urban areas and dry deserts that remain constant throughout several years produce quality interferograms , which indicate the capability of interferometry to function for longer periods. However, in other types of surface cover, such as vegetation and snow, coherence has an inverse relationship with temporal 6 d revisit of Sentinel-1 conveniently dictating the pre-event and post-event radar data pairing in areas affected by sub-optimal land cover. The baseline can be increased for L-band satellites, such as ALOS-2, for they can detect the bare ground irrespective of surface cover. Interferograms with severe unwrapping errors are manually excluded in the time-series analysis , effectively limiting the temporal baseline range to 6–12 d.

The temporal range of the pixel offset rasters and field investigation date implies the presence of both coseismic and post-seismic slip components in the optical correlation measurements and field measurements. To estimate the peak coseismic offsets, we subtracted the largest afterslip value from the peak pixel offsets and field measures. The average values were derived from the mean of residuals of the moving mean curves of the two total slip measures with InSAR (Fig. b). From this, the estimated peak coseismic slip amounts to 46.3±10.7 cm from the optical correlation and 45.7±15.9 cm from the field measure residuals. The average values are 27.7 and 24.3 cm from pixel offsets and field measures, respectively.

The amount of coseismic slip is a key factor in the assessment of seismic moment, rupture mechanism, and degree of seismic hazard. The results of this study show a unimodal curve with a characteristic long wavelength and isolated short wavelength variability for the regional slip distribution (Fig. ). The pattern suggests a low generalized resolution due to temporally influenced smoothing and noise. In addition, the long wavelength trend can also indicate fault maturity , which is a credible interpretation for the Masbate segment of the PF given its Middle Miocene initiation and rate of activity . Moreover, the known fault trace in Masbate is morphologically linear, supporting the general maturity estimate .

Slip distributions can give clues into the fault complexity and mechanical properties of the fault plane . This suggests that the observed unimodal slip distribution with a singular peak implies a relatively simple fault zone at depth, subject to validation through geodetic inversion. Such peaks usually define the location of the ruptured asperity where the highest stress drop occurs due to peak moment release from the accumulated potential energy .

Our seismotectonic interpretations (Sect. 5.4) imply a variable slip distribution on the submerged sections of the Masbate segment of the PF since it represents the primary depression of the identified transtensional basin. This inferred slip trend may portray a complex shallow fault structure in this section as the slip is distributed on multiple fault segments . Moreover, recent marine geophysical surveys in the vicinity of Cataingan Bay revealed the geometric complexity of the fault, which supports our interpretation.

Offset values gradually decay to zero toward the northwest. The smooth slip trend, devoid of short wavelength variation, suggests a systematic rupture on a relatively simple sub-vertical fault plane . Tapering and deepening aftershock patterns along with the focal mechanism solutions could support this observation.

Our interferometry results denote a 14 cm largest afterslip, which accounts for at least 30 % of the estimated peak coseismic displacement over the rapid aftershock decay interval. This lower-limit estimate is a consequence of the 6 d revisit cycle of Sentinel-1, resulting in incomplete data since the first radar image in the time-series stack was from 20 August 2020, and does not include the earliest post-seismic deformation immediately after the 18 August 2020 mainshock.

The distribution of afterslip (Fig. ) along the fault demonstrates a general slip increase southwards, which can be divided into two distinct sections. The northern ∼15 km section is characterized by short wavelength variations and average left-lateral offsets of 5–6 cm, whereas the southern section includes the peak afterslip without distinctive variability. The northern trend may indicate the occurrence of diffuse afterslip on a wide zone, which may or may not have breached the surface. suggested an increased relative friction such that the released post-seismic moment is typically less compared with the mainshock, resulting in heterogeneous loading. However, we cannot exclude the possibility that variability may stem from surface complexities or data processing artifacts, as indicated by the observed unwrapping errors and the lack of data from the earliest post-seismic period.

Post-seismic deformation usually occurs outside the regions of peak coseismic slip as the shear stress is transferred by the mainshock and causes stress loading at the locations of incomplete rupture and unruptured sections . Our results reflect the coincidence of the post-seismic and coseismic slip distributions on the surface, whose maxima are aligned and equidistant from the centroid, accompanied by a gradual slip recession northwards (Fig. ). This is similar to the 1999 Izmit , 1996 Biak , 2011 Maduo , and 2014 South Napa earthquakes, which also exhibit spatially overlapping coseismic and post-seismic slip distributions. We interpret this overlap of the slip distributions to indicate a vertical stress migration along the fault surface to either deeper or shallower sections of the fault.

Aseismic afterslip, viscoelastic relaxation, and poroelastic rebound are the common mechanisms of post-seismic deformation . The existence of resolvable afterslip is a clear indication of post-seismic activity as the stress is released by the mainshock. Our observed coincidence of the coseismic and post-seismic slip distributions probably indicates a localized stress transfer to the adjacent velocity strengthening vicinity of the primary asperity in accordance with the rate-and-state friction model . This would suggest that the post-seismic deformation is likely stress driven and operated as aseismic afterslip to accommodate the excess coseismic stress. This also agrees with the hypothesized vertical migration of stress along the fault plane.

We recognize that the data are limited to surface measurements and more information is required to accurately describe the occurrence of the afterslip along the fault plane. Determining the mechanics of the superposed slip character of the 2020 Masbate earthquake event is of interest and warrants investigation into potential factors such as non-steady-state friction , residual stress heterogeneity , stress reorganization, or shallow material redistribution . Additionally, the contribution of known interseismic creep associated with the Masbate segment to the slip distributions merits further investigation.

Surface rupture length estimates from remote sensing data were assumed to be symmetrical about the peak displacement zone (Table ). The ground displacement distributions from optical correlation and field survey (Fig. ) imply that the northern rupture terminus is located approximately 3.2 km from the northern shoreline, with peak slip values occurring between 17.3 and 23.5 km, resulting in rupture length estimates ranging from 28.2 to 41 km. The absence of offset measurements in the submerged portions of the fault necessitated the use of interpolation. Given the asymmetrical nature of strike-slip distributions, which can take on either elliptical or triangular forms , caution is critical when interpreting the interpolated values. The rupture lengths are also measured from the total length of the field survey and its extension toward the stepover in Cataingan Bay resulting in 25 and 35.6 km rupture lengths, respectively.

Moment magnitude (Mw) and seismic moment (Mo) estimates for the 2020 Masbate event were calculated from the surface rupture parameters (Table ). Mw calculations used the corresponding empirical equations from regression of rupture parameters and moment magnitudes . The Mo was derived as a function of Mw from scaling relationships . This approach was preferred over directly calculating the Mo from the rupture area and material rigidity due to the lack of reliable estimates of rupture width and subsurface fault rupture parameters.

The Mw estimates from the maximum and average displacements range between Mw 6.5 and Mw 6.7. The estimated maximum and average coseismic offsets all equate to Mw 6.5. The values closely fit the instrumental 6.6 moment magnitude. All rupture length estimates returned high Mw values between Mw 6.7 and Mw 7.0 which agrees with the 2003 earthquake observations, wherein the fault rupture is longer than expected relative to the magnitude. However, a downward counterfactual approach , given the fault uncertainties at depth, suggests the probability of the larger Mw estimates. Nevertheless, the estimated coseismic displacements reasonably fit with the instrumental seismic moment of the mainshock. The calculated Mw 6.5 value from the estimated coseismic slip parameter is consistent with a seismic moment of 8.4×1018 N⋅m.

The lower limit of the post-seismic moment was assessed using the largest and average afterslip values (Table ). The largest afterslip, which measured 0.14 m, corresponds to Mw 6.2, while the average afterslip of 0.09 m is consistent with Mw 6.1. By comparing the largest afterslip value (Mo=2.1×1018N⋅m) with the peak coseismic moment, we estimate that the afterslip resulted in the release of energy equal to at least 25 % of the mainshock.

The fault geomorphology (Fig. c) and geodetic observations (Figs.  and ) highlight the linearity of the Masbate segment. However, the fault complexity around 12°07'N was not clearly defined due to unwrapping errors in interferometry and high-amplitude noise in optical correlations.

The ruptures in Palanas form a right-stepping Riedel shear zone, with the ruptures occurring at a low angle with respect to the regional fault strike. Furthermore, a possible new splay was observed to the south, which potentially represents the primary fault plane, cutting across the bifurcating trace and traversing Cataingan Bay. Antithetic shears were observed at the bifurcation point and along the cross-cutting splay.

Topography and bedrock geology may also control the rupture such that the southwestern rupture termination is observed on an alluvial plain, succeeded by rugged topography northwards. The fault complexity in Palanas and the bifurcation point in Cataingan coincide with the stratigraphic contacts of the Late Oligocene Nabangig and Miocene–Pliocene Buyag formations. The previously identified fault complexities in this area may reflect the rheological contrast between the clastic limestones of the Nabangig Formation and the conglomeratic Buyag Formation.

The morphology of mainland Masbate following the 2020 event (Fig. ) was the basis for assigning a late relative development stage in a modeled transtensional basin development sequence . The basin boundary is characterized by floodplains surrounded by topographic highs. Apparent asymmetrical depocenters were recognized, with the shallower depocenter occurring on the floodplain and the nearshore section of Cataingan Bay, while the second one extends further southwest.

A separate transtensional basin is discernible in Dimasalang given the topography, rupture occurrence, and extensional structures in north Masbate . The northern and southern basins appear to be linked by the Riedel shear zone in Palanas. Fault complexities in the principal deformation zone of a pull-apart basin impede rupture propagation in line with the implications of the observed decreasing coseismic and post-seismic slip toward the north. However, the scarcity of observed ruptures inhibits further analysis of the northern transtensional basin.

Fitting the development stage of the southern basin is constrained by the absence of information regarding local strain partitioning. Nonetheless, the expressed fault maturity and identified negative flower structures in the northern and southern bays highlight the presence of oblique extensional stress bounding the Masbate mainland. The orientation of the regional fault implies a WNW–ESE-oriented 1σ. However, real-world conditions are more complex compared with the simplified parameters in analog models.