# Precise relocation of the 14 August 2021 Mw 7.2 Nippes, Haiti, earthquake sequence using broadband and citizen-hosted short-period seismometers

Sylvert Paul, Tony Monfret, Eric Calais, Françoise Courboulex, Bertrand Delouis, Anthony Lomax, Bernard Mercier de Lépinay, Steeve J Symithe, Anne Deschamps, David Ambrois, et al.

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Sylvert Paul, Tony Monfret, Eric Calais, Françoise Courboulex, Bertrand Delouis, et al. Precise relocation of the 14 August 2021 Mw 7.2 Nippes, Haiti, earthquake sequence using broadband and citizen-hosted short-period seismometers. Tectonophysics, 2026, 918, pp.230982. 10.1016/j.tecto.2025.230982. hal-05438311

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# Precise relocation of the 14 August 2021 M<sub>w</sub> 7.2 Nippes, Haiti, earthquake sequence using broadband and citizen-hosted short-period seismometers

Sylvert Paul<sup>a,b,c ID</sup>, Tony Monfret<sup>b,c ID</sup>, Eric Calais<sup>d ID,*</sup>, Françoise Courboulex<sup>b</sup>, Bertrand Delouis<sup>b</sup>, Anthony Lomax<sup>e ID</sup>, Bernard M. de Lépinay<sup>b</sup>, Steeve J. Symithe<sup>a ID</sup>, Anne Deschamps<sup>b</sup>, David Ambrois<sup>b ID</sup>, Sadrac St Fleur<sup>a</sup>, Dominique Boisson<sup>a</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

Dataset link: https://ds.iris.edu/ds/nodes/dmc/, https://doi.org/10.7914/sn/hy, https://ayit.i.unice.fr/ayiti-seismses/fdsnws/, https://zenodo.org/record/17423384

**Keywords:**

Caribbean
Citizen seismology
Nippes earthquake
Aftershocks
Transpression

## ABSTRACT

On 14 August 2021, the Southern Peninsula of Haiti experienced a M<sub>w</sub> 7.2 earthquake, 15 years after the devastating M<sub>w</sub> 7.0 event that struck the capital city of Port-au-Prince on 12 January 2010. We use the data from a local temporary broadband seismic network, a national network of low-cost seismometers, and regional seismic networks, together with a probabilistic, global-search, non-linear location method (NLL-SSST-coherence), to obtain a catalog of 5341 precisely relocated events spanning 20 August 2021 to 6 February 2022, with local magnitudes ranging from 0.5 to 5.6. We compute focal mechanisms for a subset of 73 events through waveform inversion. The catalog can be split into aftershocks directly related to the Nippes earthquake rupture process, and two off-rupture clusters. A first one concerns the Anse-à-Veau–Miragoâne area and corresponds mostly to the aftershock sequence of two M<sub>w</sub> 5.3 and 4.9 earthquakes that likely activated a segment of the offshore, south-dipping, Jérémie–Malpasse reverse fault system. A second sequence, offshore Jérémie and clustered close to the offshore trace of that same fault, started immediately after the Nippes mainshock and continued during the entire time interval of the present study. The swarm–like temporal distribution of this sequence, as well as evidence for directional propagation of the epicenters, indicate that it was likely driven by fluid migration. We interpret this seismicity as the result of oblique sub-crustal slip on a south-dipping fault which accounts for oblique convergence between the Gonâve and Caribbean plates in southern Hispaniola. Strain in the crust then partitions between reverse faulting on the Jérémie–Malpasse fault system, strike-slip on the Enriquillo fault, and hybrid faulting in between. Seismic hazard assessment for the region should therefore account for faults other than the Enriquillo fault as potential sources for future earthquakes.

## 1. Introduction

Haiti occupies the western part of the island of Hispaniola (Fig. 1), which straddles the boundary between the Caribbean and North American plates (Mann and Burke, 1984). Their current relative motion at 20 mm/yr, slightly oblique to the overall east–west trend of the plate boundary zone (DeMets et al., 2000), is accommodated in a large part by two major left-lateral strike-slip faults that cross-cut the island (Mann et al., 1995), the Septentrional Fault (SF) to the north, with a slip rate of 9–10 mm/yr, and the Enriquillo Plantain Garden Fault (EPGF) to the south, with a slip rate of 7–9 mm/yr (Manaker et al., 2008; Symithe et al., 2015). In addition, a significant component

of plate boundary-normal shortening is present, as evidenced by diffuse off-shore reverse faulting in the Gulf of Gonave identified in marine data and earthquake focal solutions (Corbeau et al., 2019; Possee et al., 2019), and predominantly northward thrusting along the Jérémie–Malpasse fault identified via a combination of GNSS, marine geology, and seismological data (Symithe and Calais, 2016; Rodriguez et al., 2018; Calais et al., 2023). GNSS measurements indicate that shortening concentrates in the southern part of the island, encompassing the southern Peninsula of Haiti, coincident with the highest topography of the island, with peaks reaching elevations >2000 m (Macaya, La Selle) bounded offshore by >3000 m–deep oceanic basins (Jérémie and Southern Haiti basins).

<footnote><sup>a</sup> Université d'Etat d'Haiti, Faculté des Sciences, URGéo, Port-au-Prince, Haiti</footnote>

<footnote><sup>b</sup> Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, IRD, Géoazur, Valbonne, France</footnote>

<footnote><sup>c</sup> Barcelona Center for Subsurface Imaging, Institut de Ciències del Mar (ICM), CSIC, Barcelona, Spain</footnote>

<footnote><sup>d</sup> Université PSL, École Normale Supérieure, CNRS, Paris, France</footnote>

<footnote><sup>e</sup> ALomax Scientific, Mouans-Sartoux, France</footnote>

<footnote>* Corresponding author.
E-mail address: eric.calais@ens.fr (E. Calais).</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Seismotectonic context of the Caribbean–North American plate boundary around Hispaniola. Main active fault systems are shown with black lines. Stars indicate major destructive historical (white) and instrumental (dark orange) earthquakes (Scherer, 1912; McCann, 2006; ten Brink et al., 2011). Large arrow shows the 2 cm/yr motion of the Caribbean plate with respect to North America (DeMets et al., 2000). The three highest points of Hispaniola are indicated, with their elevation. JMF: Jérémie–Malpasse Fault, PFTB: Peralta Fold and Thrust Belt, SDB: Santiago Deformed Belt, HB: Haiti Basin, M: Matheux Range, N: Neiba Range. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

The two most recent large earthquakes in the area exemplify this coexistence of left-lateral shear and plate boundary-normal shortening. The 10 January 2010 M<sub>w</sub>7.0 earthquake that devastated Haiti's capital region started as a reverse faulting event on a blind fault distinct from the EPGF, then propagated westward on a strike-slip fault segment (Calais et al., 2010; Hayes et al., 2010; Douilly et al., 2013; Symithe et al., 2013). In the end, 60% of the moment release was strike-slip and 40% reverse, although the two components were separated in space and time. The 14 August 2021 M<sub>w</sub>7.2 Nippes earthquake showed a similar pattern, with pure reverse faulting on a north-dipping fault followed by strike-slip on a separate fault segment and a similar moment release ratio (Calais et al., 2022; Maurer et al., 2022; Okuwaki and Fan, 2022; Li and Wang, 2023; Raimbault et al., 2023). These two transpressional events ruptured fault segments located close to the EPGF but to the north of its main surface trace, while that surface trace shows geological evidence of cumulative displacement that is purely left-lateral strike-slip (Mann et al., 1995; Fleur et al., 2020, 2024).

Douilly et al. (2023) used local and regional seismic data from 20 August 2021 to 31 December 2021, to define a 1D seismic velocity model of the epicentral area of the August 2021 Nippes earthquake and to precisely locate 2528 aftershocks with a double-difference method. The resulting aftershock distribution broadly delineates north-dipping structures with slightly different azimuths, subparallel to the direction of the EPGF but located to the north of it. They coincide well with the coseismic fault segments inferred from geodetic and teleseismic seismological data (Calais et al., 2022; Maurer et al., 2022; Okuwaki and Fan, 2022; Li and Wang, 2023; Raimbault et al., 2023).

In this study, we use data from a local temporary broadband network, low-cost stations from a national, citizen-hosted seismic network (Paul et al., 2023), and other regional seismic stations to expand the aftershock catalog of Douilly et al. (2023) to a total of 5341 precisely relocated events spanning 20 August 2021 to 6 February 2022. We compute focal mechanisms for a subset of 73 events through waveform inversion, which we then use to determine the direction of principal stress tensor axes in the study area. We include in our analysis two off-rupture earthquake clusters of specific interest for the regional seismotectonics. A first one is located about 50 km to the northeast of the Nippes mainshock, in the Anse-à-Veau–Miragoâne area, a second one is located off the western tip of the Southern Peninsula north of Jérémie. We use our results, together with additional regional geological and geodetic information, to propose a seismotectonic interpretation of the Southern Peninsula of Haiti.

## 2. Seismic networks and event detections

Although no seismic stations were in place in Haiti at the time of the 2010 earthquake, the 2021 Nippes event benefited from the progressive installation, since 2019, of low-cost Raspberry Shake seismometers (Anthony et al., 2019) hosted by volunteer Haitian citizens (Calais et al., 2020). This “seismo-citizen” network, combined with conventional regional stations, currently provides near-real-time location of all earthquakes greater than magnitude 2.4 on the Haitian territory (Paul et al., 2023; Ayiti-Séismes, 2025). These citizen-hosted, low-cost, seismic stations were key to a rapid understanding of the mainshock mechanism and to the real-time monitoring of its aftershock sequence (Calais et al., 2022). To increase the number of aftershocks detected and improve their location, a Haitian team installed a temporary network of twelve 3-component broadband stations in the epicentral area within a week of the mainshock. Four more temporary broadband stations were added on 21 January 2022, between the cities of Anse-à-Veau and Miragoâne (Fig. 2).

In this study, we analyze the recordings of these 3-component temporary broadband stations, the permanent Raspberry Shake stations from the citizen network, the available permanent broadband stations in Haiti, and the regional broadband stations in Cuba and the Dominican Republic (Fig. 2). We include all available seismic recordings from 20 August 2021 to 6 February 2022. All the stations used here recorded in continuous mode at a sampling rate of 100 samples/s, except stations UR01, UR02, UR03 and UR04, where the sampling rate was 200 samples/s. The Raspberry Shake stations are model RS4D, with one vertical component short-period velocimeter, and 3-component MEMs accelerometers. Further information on stations characteristics is provided in Supplementary Table 1. Data was stored in standard miniSEED format, in a SeisComP Data Structure (SDS) organized by 24 hour-long continuous files. This structure provides direct access to the data with the processing software, and enables data availability to be checked via web service tools, described in Paul et al. (2023).

We analyze the data using the SeisComP3 software (SC3, Helmholtz-Centre Potsdam, 2008), a tool originally built for real-time seismic monitoring. The real-time stream is simulated with records collected by injecting time-ordered multiplexed data, called playback, in blocks of 24 h. We process the data in two steps. First, we run a phase detection based on a standard short-term average/long-term average (STA/LTA) analysis between 0.5 and 60 s (Allen, 1982) on data that is bandpass
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Seismological networks used in this study. Blue hexagons, code Z2: temporary network composed of 16 broadband stations. Red triangles, code HY: permanent network of citizen-hosted Raspberry Shake stations. Orange squares: Cuban network, code CW (CENAIS). Steelblue square: Canadian network, code CN (NRCAN). Green squares: Dominican network, code LO (Instituto Politecnico Loyola). Purple squares: U.S. Geological Survey network, code CU. A broadband station operated at site LBOR from 2021/08/27 to 2021/09/02, and was then moved to site SALO from 2021/02/10 onward. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

filtered between 4 and 20 Hz. These parameters have been determined so that the algorithm is sensitive to rapid increase in amplitude over time, a characteristics of local earthquakes. For *S* phases, we use the Akaike Information Criterion detection algorithm implemented in SC3 (Maeda, 1985). The second step is the association of the automatic picks, consists of linking seismic arrivals (picks) detected at multiple stations to a single event. This is performed by the SC3 “*scanloc*” module, which uses Density-Based Spatial Clustering of Applications with Noise (Ester et al., 1996), a machine learning algorithm for clustering tasks. In SC3, phase association is closely linked to location, as the epicenter estimate allows certain associations to be confirmed or rejected. The “*scanloc*” module handles these two aspects simultaneously (SC3, Helmholtz-Centre Potsdam, 2008). To limit false detections, we require a minimum of six associated phases to create an event. In total, the automatic catalog contains 6345 events between August 2021 and February 2022 (Fig. 3). We calculate $M_{lv}$ magnitudes using the SC3 “*scmag*” module from pre-computed amplitudes. We did not apply local empirical calibration factors to the magnitude calculation. Determining these factors is an avenue for future research. At this point we therefore have a playback catalog with automatically detected *P* and *S* phases.

# 3. Hypocenter relocation

## 3.1. NLL-SSST-coherence methodology

We locate hypocenters using the NLL-SSST-Coherence (NLL-SC) procedure proposed by Lomax and Savvaidis (2022), which extends the NonLinLoc location algorithm (NLL) with source-specific station travel time corrections (SSST), to improve the location precision on large and intermediate scales, and inter-event waveform similarity (coherence), to improve the fine-scale precision. This procedure was also used in Calais et al. (2022) to locate the early Nippes aftershocks from

14 August to 9 September 2021, when the data from the temporary broadband stations were not yet available.

We first apply the NLL algorithm to obtain an initial estimate of the posterior probability density function (PDF) for hypocenter location. We use a 1-dimensional seismic velocity model that combines the 1D model of *P* and *S* arrivals of Douilly et al. (2023) between 0 and 30 km depth, and the IASP91 model (Kennett and Engdahl, 1991) at larger depths (Supplementary Figure 1). A PDF provides the maximum likelihood hypocenter and a complete description of location uncertainty. In the NLL procedure, we use an equal differential time likelihood (EDT) function (H.-W., 1994; Lomax, 2005, 2008; Lomax et al., 2014), which remains robust in the presence of outlier picks of data or travel-times.

Next, we generate NLL-SSST corrections which improve the location precision on large and intermediate scales by iteratively generating travel-time corrections for each station and phase over collapsing smoothing distances. For each smoothing distance, SSST corrections are accumulated throughout a 3D grid on the study volume from the sum of residuals after each is Gaussian-weighted by the corresponding event distance from each grid point. The earthquakes are relocated with NLL using the playback catalog of events, arrival data, and travel-times from the previous iteration (starting with static corrections using the initial model travel-times) with SSST smoothing distances of 64, 32, and 16 km. The result is a set of 3D travel-time tables for each station and phase which vary over a spatial scale length of ~16 km. The location of an event and the phase of a station to be included in the SSST correction calculations must meet quality criteria that we established after several tests: root mean square of residuals (RMS) ≤ 0.15 s, number of phases ≥ 8, azimuthal gap ≤ 270, *P*-wave residual ≤ 1.0 s, *S*-wave residual ≤ 2.0 s, and semi-major axis of the 68% confidence ellipsoid ≤ 7.0 km.

The last step in the procedure, called waveform coherency (NLL-SC), uses the final NLL-SSST travel-times and location probability density functions for waveform-based cross-correlation coherence analysis. The
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Number of events per day (blue bars) and magnitude $M_{Iv}$ of each event (red circles) for the 3 sub-regions described in the text from August 20, 2021 to February 6, 2022 in the NLL-SC catalog. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

coherence is based on the normalized maximum cross-correlation between the waveforms of different stations for event pairs at a specified distance from each event after the NLL-SSST relocation. NLL-SC aims to reduce random location error by consolidating information between locations of seismic events with similar waveforms. Similar waveforms are considered to indicate that events are located close together and on a portion of a fault with similar characteristics (Geller and Mueller, 1980). To quantify this proximity in NLL-SC, we use a waveform window including *P* and *S* waves, the *P* coda, and part of the *S* coda. We use the maximum coherence between event-pair waveforms as a weight to stack NLL-SSST relocation PDF's and improve the hypocenter location of each target event. This procedure effectively combines the arrival time data between nearby events and reduces the effects of random error in the data.

We measure the coherence between waveforms using the vertical components of BFIN, CHARD, HASL, PBEAU, TROU, five neighboring broad-band stations that we selected for their location and reliability of operation (Fig. 2). The waveforms are filtered from 1 to 5 Hz in a 4 s window before and after the arrival of the *P* and *S* waves respectively. A cross-correlation is applied between sliding waveform windows of 2.0 s, and a weighting of 0 to 1 is set on coherence values for minimum coherence of 0.5 to 1.0. This procedure is applied to the final NLL-SSST relocations, with final SSST smoothing width D=16 km, for all event pairs with a maximum NLL-SSST hypocenter separation of 10 km (Figs. 4 and 5).

## 3.2. Precision of relocations

The precision and accuracy of hypocenter locations are crucial to correctly image the geometry of faults at depth, which is key to better

understanding earthquake sources (Fig. 4). Thus, in most seismically active regions, when data permits, advanced localization techniques are employed to obtain the finest possible determination of the seismic activity. The NLL-SC method has some advantages over others, in particular the classic double-difference method used by Douilly et al. (2023) to relocated the Nippes aftershock sequence. For instance, Lo-max and Henry (2023) showed, in the Parkfield region, that the NLL-SC method was effective at refining relative locations at scales ranging from sub-kilometer to approximately 50 km. As a consequence, while double difference relocations exhibit a fault geometry with bends and small-scale segments, NLL-SC results in a smoother, nearly planar, fault surface. It also outperforms HypoDD in the number of events relocated, as HypoDD only relocates events with a minimum of high cross-correlation connections to neighboring events (e.g., Uccio et al., 2024).

To illustrate the improvements that coherence brings to hypocenter relocation, we collected statistics from the dataset used here at the intermediate NLL-SSST step (Supplementary Figure 2a). We find that events with uncertainties in the horizontal and vertical coordinates less than 3 km account for 88% and 94% of the NLL-SC catalog (Supplementary Figure 2b), and 69% and 80% of the NLL-SSST catalog, respectively (Supplementary Figure 2a). With this uncertainty cutoff, we found that the final catalog obtained with NLL-SC was enriched by 308 events ($1.6 < M_{Iv} < 2.6$), primarily between 14 and 18 km depth, compared to the NLL-SSST catalog. Thus, the improvement at the coherence step of the analysis is significant. We then ask whether these uncertainties are consistent across all areas, including in zone 3, which is outside of the footprint of the local seismic stations used here. To investigate this, we excluded the events from zone 3 from the
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Top left: epicenter locations from August 20, 2021 to February 6, 2022 in the NLL-SC catalog. Events are color-coded by hypocenter depth and sized with respect to their magnitude. The blue circle marks the mainshock of August 14, 2021 ($M_w$ 7.2) from Calais et al. (2022), shifted in location on the map to avoid cluttering the aftershock distribution. The black boxes delimit zones 1, 2, and 3, separately described in the text. Black lines show the main active faults as in Calais et al. (2023): EPGF, Enriquillo Plantain Garden Fault; RF, Ravine du sud Fault; JMF, Jérémie-Malpasse Fault, as well some cities: J, Jérémie; A, Anse-à-Veau; M, Miragoâne. Bottom left and top right: hypocenter depths projected along east-west and north-south cross-sections. Bottom right: histogram of hypocenter depth with 2 km interval bins. The dashed red rectangle shows the area encompassed on Fig. 8. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

statistics and found that the overall uncertainties remained unchanged. The uncertainties in the horizontal coordinates and depths are below 3 km in 88% and 94% of cases, respectively, for the NLL-SC events compared to 70% and 81% for the NLL-SSST events.

The relocation of events in zone 3 (Fig. 5) highlights the importance of coherence in improving the precision of hypocenter locations. Seismic activity is concentrated between 10 and 12 km depth, with significant gains in precision in the final NLL-SC catalog. The NLL-SSST model indeed relocates 269 events, of which 55% have horizontal uncertainties less than 3 km, and 52% have depth uncertainties below 3 km. In contrast, the NLL-SC model relocates 304 events, with 91% of horizontal uncertainties and 93% of depth uncertainties less than 3 km (Supplementary Figure 2b).

Although the precision of the hypocenter determination is improved thanks to the NLL-SC algorithm, their accuracy depends for a significant part on the quality of the seismic velocity model used. A limitation of the present analysis is the fact that we are using a combined, one-dimensional velocity model (Supplementary Figure 1) that does not account for lateral variations in crustal structures. The Southern Peninsula is a broad anticlinal system that involves a crustal basement made of Cretaceous oceanic plateau material (Caribbean Large Igneous Province or CLIP, (Duncan and Hargraves, 1984)), capped by up to 2 km of late Cretaceous to Miocene marine limestone (Momplaisir, 1986). The CLIP terrane overthrusts island arc terranes to the north,

which are part of a Cretaceous–Paleocene orogenic belt (Great Arc of the Caribbean, (Burke, 1988)), along the Jérémie–Malpasse fault system (Calais et al., 2023). As a result, and to first-order, the crustal structure of the Southern Peninsula and surroundings, though it may not vary significantly in an east-west direction, likely exhibits significant variations in a north-south direction. This is also evidenced by the topography contrast between its > 2,000 m-high peaks and the 3500 m-deep Jérémie basin to the north (Fig. 1), with sedimentary layers that are not accounting for here. Quantifying 2-dimensional effects would require using crustal models that are not yet available for the study area. However, since this study is largely based on rays between local earthquakes and local short-period and temporary broadband stations, the effect of a north-south variability in crustal structures, in particular Moho depth, is likely to be limited.

## 4. Focal mechanisms

We perform a waveform inversion to determine the focal mechanism, hypocentral depth, and moment magnitude $M_w$ for 73 selected events with magnitude $M_{tv} \ge 3.3$ (Supplementary Table 2). We use the semi-automatic FMNEAR method of Delouis (2014) to obtain double-couple focal mechanisms and moment magnitude based on a waveform inversion of data from near-source seismic records. We model the source as a point if the event magnitude is lower than 5.5, or by a
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Cross-sections of the depth-distribution of hypocenters from August 20, 2021 to February 6, 2022 in the NLL-SC catalog. Hypocenters are grouped in boxes and projected on the corresponding sections shown on the map at the bottom right. Focal mechanisms (Fig. 6) are shown in cross section. Topographic profiles are shown by a brown curve above each section. The surface trace of the Enriquillo and Ravine du Sud are marked by a red and blue circle, respectively. The possible geometry of the Jérémie-Malpasse fault is shown, given its offshore trace and assuming a constant dip of 34° to the southeast. EPGF = Enriquillo Plantain Garden Fault; RF = Ravine du Sud Fault; NF = Nippes Fault(s); JMF = Jérémie-Malpasse fault. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

linear extended source for larger magnitudes. For the best recorded earthquakes of magnitude greater than 3, we process the data in three successive steps: (1) pre-processing of the signals, (2) inversion of the waveforms, and (3) scanning of the source depth.

Pre-processing consists of converting the pre-recorded waveforms in miniSEED into the Seismic Analysis Code (SAC) format and initializing the parameters used into the inversion from the hypocenter and a first estimate of the magnitude of each earthquake. This parameter initialization consists of fixing the station list, manually picking the arrival of the P-wave, identifying any tilted or saturated signals, and specifying the optimal filter band on integrated displacement traces for FMNEAR. The filter width can vary from one component to another at the same seismic station for a given earthquake, as the background noise level can differ across the three components. This implies a trade-off between optimizing the signal-to-noise ratio and keeping the frequency as low as possible. The minimum frequency used in all cases is 0.01 Hz, and the maximum one 0.40 Hz. However, the effective frequency range used, considering low-frequency noise and the complexity of the signal at high frequencies, is narrower. Supplementary Table 3 provides the frequency bands used at each station for the example of the 28 August 2021, M$_{Iv}$ 4.2 (M$_{w}$ 3.9) earthquake. At this step, any station with all components saturated is excluded from the inversion. Those with only

one or two saturated components can be retained, but the saturated component(s) are removed during inversion. Signals with baseline drift are used after high-pass filtering or are simply suppressed. To use the vertical velocity component of RS stations in the inversion, we triplicated it to create two pseudo-horizontal components that will be eliminated in the inversion. For the vertical component of RS stations, we retain the 0.15–0.3 Hz frequency band after deconvolution from the instrumental response, in accordance with the short-period sensor.

The waveform modeling inversion is performed using the discrete wavenumber method of Bouchon (1981), generating the synthetic signals with our 1D seismic velocity model. We use the FASTHYPO program (Herrmann, 1979) to calculate theoretical arrival times and to allow time alignment between observed and synthetic signals. The inversion process is carried out as described in Delouis (2014) in two successive steps combining grid-search and simulated annealing. The depth range varied between 1 and 25 km in this study, with a step of 1 km. At the depth that minimizes the waveform misfit function, we obtain the strike, dip, and rake parameters of the double-couple focal mechanism and the moment magnitude associated with the event. The results are evaluated on the basis of quality criteria, namely the root mean square error (RMS) of adjustment of the waveforms and a confidence index reflecting the uniqueness of the solution (0%–100%).
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Focal mechanisms obtained by waveform inversion of near-source seismic records using the FMNEAR method of Delouis (2014), except for the ones shown in black, which are from global analysis centers. The data used here allow for the confident estimation of 73 moment tensor solutions. The Anse-à-Veau–Miragoâne cluster around longitude –73.3°, which consist of reverse or reverse – strike-slip focal mechanisms, is displayed with more detail on Fig. 8. Focal mechanisms are classified according to the geological conceptual classification of faults and the projection technique of Kaverina et al. (1996), with SS = Strike-Slip; SSN = Strike-Slip – Normal; NSS= Normal – Strike-Slip; N = Normal; R = Reverse; RSS = Reverse – Strike-Slip; SSR = Strike-Slip – Reverse. The focal mechanisms shown in black are for two M<sub>w</sub> 5.1 and 4.7 events outside of our catalog time window whose epicenter however coincides with the Jérémie cluster, or zone 3, described in this paper. It was reported by several global observatories, among which the USGS, GFZ, and OCA also calculated a moment tensor solution (EMSC (2022), 2022). It is plotted at the epicenter location from Ayiti-Séismes (2025), which is the most compatible with the catalog described in this paper.</figcaption></figure>

We generate a final rating from A for highly constrained mechanisms, to F for unconstrained ones based on the two criteria described above, plus consideration of the number of modeled components and the azimuthal gap in the station distribution (last column of Supplementary Table 2). Quality A (67 events) and B (4 events) results are considered reliable, quality C results (2 events) can be validated after visual inspection of the model fit to the waveforms.

The 73 resulting focal mechanisms are displayed on Fig. 6, color-coded into seven categories according to the plunge angle of their P, T and B axes (Álvarez-Gómez, 2019) and classified according to the projection technique of Kaverina et al. (1996). We do not compute focal mechanisms for events located offshore north of Jérémie (zone 3, Fig. 4) due to the insufficient number of available station components after waveform pre-processing, as this area is located outside of the footprint of the temporary broadband network (Fig. 2). The hypocentral depths calculated with FMNEAR are similar to those determined using the NLL-SC procedure (Fig. 7a), while M<sub>w</sub> values follow M<sub>w</sub> = 0.854 M<sub>Iv</sub> +0.182 (Fig. 7b), valid for events with 3.3≤M<sub>Iv</sub>≤5.5. M<sub>Iv</sub> values therefore tend to overestimate M<sub>w</sub>. Overall, focal mechanisms show a majority of pure reverse and reverse strike-slip events. They will be interpreted in more detail in the next section.

**5. Final earthquake catalog**

In the end, we precisely relocated with NLL-SC 5341 of the 6345 initially detected events. We determined moment tensor solutions for 73 of them. The local magnitude M<sub>Iv</sub> of the final catalog ranges from 0.5 to 5.6. As shown in map view on Fig. 4, most of the epicenters are located north of the EPGF, consistent with previous studies based

on shorter time intervals and fewer seismic stations (Calais et al., 2022; Douilly et al., 2023). We shall split this catalog into 3 groups that are separated in epicentral locations (Figs. 4 and 5) and also show distinct temporal distributions (Fig. 3). We find completeness magnitudes of 3.6, 2.6, and 3.4, respectively, for the Anse-à-Veau–Miragoâne, main aftershock sequence, and offshore Jérémie zones described below (Supplementary Figure 4). The b-value for the main aftershock sequence and the Anse-à-Veau–Miragoâne areas are close to 1, as for classic earthquake catalogs. The b-value is 1.4 for the offshore Jérémie sequence, a relatively high value typically associated with fluid pressure buildup (e.g., Stanton-Yonge et al., 2025) or magma movement (e.g., Carlino et al., 2025).

**5.1. Zone 1: Anse-à-Veau – Miragoâne**

Zone 1 shows a seismic cluster of 357 events concentrated between 4 and 12 km depth, located between the EPGF and the northern coast of the Southern Peninsula. Supplementary Figure 5 shows that 329 of them have a depth uncertainty < 2 km. This cluster is separated from the main aftershock sequence by a gap between longitudes 73.4°W and 73.3°W, which corresponds to the eastern end of the mainshock rupture (e.g., Calais et al., 2022). Their temporal distribution (Fig. 3 top) shows a first sequence of events lasting until early October 2021, followed by a background seismicity level, then by a second burst starting with two M<sub>w</sub>5.3 and 4.9 earthquakes on 24 January 2022. Their epicenters are located between the localities of Anse-à-Veau and Miragoâne, about 50 km to the northeast of the Nippes mainshock epicenter. The temporal separation between these two sequences is also illustrated by the Ayiti-séismes catalog, as shown on Supplementary
<figure><img src="image_10.png" /><figcaption>Fig. 7. (A) Comparison between the hypocentral depth determined for the 73 moment tensor solutions of Fig. 6 and the hypocentral depth derived from the NLL-SC relocation. (B) Local magnitude $M_{lv}$ vs moment magnitude $M_{w}$ from FMNEAR. Black line shows the least square regression corresponding to equation $M_{w} = 0.854 M_{lv} + 0.182$.</figcaption></figure>

Figure 6. A first increase in the daily number of events occurs right after the Nippes mainshock, until ~day 50, followed by a plateau, then by a sharp increase 163 days after the mainshock. One also notices on Supplementary Figure 6 that some seismicity was present in the Anse-à-Veau - Miragoâne before the Nippes mainshock. The spatial and temporal relationships between these two sequences are further highlighted on Fig. 8, showing that early events (blue circles) are distributed around EPGF around longitude 73.3°W and extend about 6 km north of it, while post-January 24 events (cyan circles), in particular the two $M_{w}$ 5.3 and 4.9 ones, are located well to the north of the EPGF surface trace.

The first sequence is included in cross section 14 on Fig. 5 and was also mapped by Douilly et al. (2023, their Figure 10 and cross-section AA'). Part of these events likely occurred on the EPGF, assuming it is vertical, as shown in Douilly et al. (2023). They coincide in time and location with the postseismic afterslip episode reported by Yin et al. (2023), with ~2 cm of slip on the mapped EPGF to the east of the main rupture in the two weeks following the earthquake. This afterslip and associated seismicity occurs on a segment of the EPGF where Coulomb failure stress increased as a result of the 2010, $M_{w}$ 7.0, and 2021, $M_{w}$ 7.2 earthquake ruptures to the west and east, respectively (Calais et al., 2022).

The second sequence, which follows the $M_{w}$ 5.3 and 4.9 earthquakes of 24 January 2022, has the time decay characteristics of a mainshock– aftershocks sequence (Fig. 3 top). It extends through the Anse-à-Veau – Miragoâne area, between the rupture of the 2010 and 2021 earthquakes, north of an approximately 40 km–long segment of the EPGF that did not slip co-seismically in either of those two events (Calais et al., 2022; Yin et al., 2023). Aftershocks delineate a well-defined plane dipping about 34° to the southwest, as shown on cross-sections 14 on Fig. 5. Both the $M_{w}$ 5.3 and 4.9 earthquakes of 24 January 2022 had reverse faulting focal mechanisms with one nodal plane consistent with the fault plane delineated by the spatial distribution of their aftershocks (Fig. 8). In addition, the fault plane orientations of 38 aftershocks for which we determined focal mechanisms align in a northwest–southeast direction, with a southwestward dip consistent with the hypocenter distribution. As for the $M_{w}$ 5.3 and 4.9 earthquakes, all these events have reverse focal mechanisms oblique to the strike of the EPGF.

These seismological observations are supported by independent InSAR data, that show that the Anse-à-Veau–Miragoâne earthquake sequence was associated with up to 3 cm of ground uplift, indicative

of pure reverse faulting on a south-dipping fault consistent in location, dip, and orientation with the one imaged here (Raimbault et al., 2025). The surface projection of this fault (Fig. 8b) coincides with the surface trace of the Jérémie-Malpasse reverse fault system imaged offshore the northern coast of the Southern Peninsula of Haiti, (Corbeau et al., 2016), which absorbs 3–6 mm/yr of shortening between the Southern Peninsula and central Haiti (Calais et al., 2023). It is likely that the Anse-à-Veau–Miragoâne seismic crisis activated a segment of the Jérémie-Malpasse fault system. Fig. 9 shows the Coulomb failure stress (CFS) imparted by the Nippes rupture of 14 August 2021 on faults with the average strike, dip, and rake of the main 24 January 2022 earthquakes. These events and the subsequent aftershock sequence fall into an area of increased CFS caused by the 2021 event, an indication that their occurrence was promoted by the associated stress change.

In map view, the epicenters of the Anse-à-Veau–Miragoâne earthquake sequence and the coseismic uplift area detected by InSAR coincide with a ~1000 m elevation topographic high locally termed “Rochelois Plateau” (Figs. 4 and 8) capped with middle to upper Eocene limestone (Mompaisir, 1986; Fleur et al., 2019) that may be actively rising as a result of reverse faulting at depth on the Jérémie-Malpasse reverse fault. This area, although rural and not densely populated, has been the locus of repeated felt earthquakes for a long time. It was struck on 8 April 1860, by the so-called “Anse-à-Veau earthquake sequence”, which included a foreshock and mainshock of respective intensity magnitude of 6.2 and 6.8 (Sykes and Ewing, 1965; Martin and Hough, 2022). It was also the locus of a $M_{s}$5.9 earthquake on 27 October 1952 (Sykes and Ewing, 1965; Bakun et al., 2012), followed by felt aftershocks until March 1953, which severely affected the built infrastructure of Anse-à-Veau (Neumann, 1953; Bettembourg et al., 1955). More recently, it experienced a series of felt earthquakes in October–December 2015 (Prépetit, 2016). According to this latter report, the 2015 epicenters were located just offshore Anse-à-Veau–Miragoâne, but in the absence of local seismic stations the accuracy of these determinations is unknown.

## 5.2. Zone 2: Main aftershock sequence

Zone 2, which contains the majority of the seismic events, consists of aftershocks directly related to the Nippes earthquake rupture, with a time decay typical of aftershock sequences (Fig. 3 middle). As in previous studies, epicenters delineate a ~80 km–long narrow cluster
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Distribution of seismicity and focal mechanisms in the Anse-à-Veau (A) – Miragoâne (M) area. The blue and aquamarine hypocenters represent earthquakes recorded before and after the M<sub>w</sub> 5.3 and M<sub>w</sub> 4.9 earthquakes of 24 January 2022, whose focal mechanisms are shown in cross-section on the lower panel, with hypocenter depth from the NLL-SC solution for consistency with the other mechanisms. Note the coincidence of the epicenters with an oval-shape topographic high. The AA' profile shows the depth distribution of the hypocenters of earthquakes located within the black dashed box on the map. The post-24 January events define a plane dipping 34° to the southwest, indicated by a thick gray dashed line. Its surface projection is located offshore, coincident with the mapped trace of the Jérémie-Malpasse Fault system. The pre-24 January events occur within a triangle bounded by this SW-dipping fault and the vertical projection of the surface trace of the EPGF. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

roughly parallel to, and north of the surface trace of the EPGF. As shown on Supplementary Figure 3, shallower events with hypocenter depth less than 12 km parallel the EPGF while the deeper ones aligns in a ~110°–120°E direction, oblique to the EPGF. Hypocenters reach a depth of 30 km, with the deepest one located close to the northern coastline of the Southern Peninsula (Supplementary Figure 3b). The details of their depth distribution, illustrated on Fig. 5, highlights a series of north-dipping fault segments, which we describe hereafter from west to east.

On cross-section 1, aftershocks delineate a broad north-dipping cluster between 5 and 15 km depth, with a dip angle of ~70°, which projects at the surface on the trace of either the EPGF or Ravine du Sud fault. To the east, on cross section 2, the main aftershock cluster has the same dip angle and depth range but projects on the Ravine du Sud trace, while a distinct group of aftershocks, more spread out, is observed just to the south, possibly projecting on the EPGF trace. Cross-section 3 shows a triangle-shape cluster with the same dip angle and depth range as on sections 1 and 2 which partly projects on the EPGF trace, with a smaller and distinct cluster between 15 and 20 km
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Coulomb failure stress (CFS) imparted by the 14 August 14 2021, M<sub>w</sub> 7.2 Nippes earthquake on receiver faults with strike, dip, and rake of 110°, 35°, and 90° respectively, using the finite fault source model of Raimbault et al. (2023). The CFS is calculated at 5-km depth with a friction coefficient of 0.8. J, Jérémie; A, Anse-à-Veau; M, Miragoâne. Gray circles are aftershocks from the catalog obtained in this work. White circles: post January 24, 2022, aftershocks of the Anse-à-Veau–Miragoâne sequence (see also Fig. 8).</figcaption></figure>

depth. Sections 4, 5, 6, drawn perpendicular to the western half of the main aftershock cluster, as well as sections 7, 8, 9, drawn perpendicular to the eastern half of that cluster, show these clusters in a clearer manner. The southernmost one, limited to 15 km depth, again projects to the EPGF trace, while the cluster just to the north is clearly distinct, with deeper hypocenters ranging from 10 to 20 km depth. This deeper cluster seems to include several lineaments.

The main aftershock sequence therefore appears to define two separate faults. An east-west trending one, steeply north-dipping and limited to about 15 km depth, coincides at the surface with the trace of the EPGF. All focal mechanisms on that structure are strike-slip. We interpret it as revealing the geometry of the EPGF at depth. A ~N110-trending fault, also north-dipping, is disconnected from the previous one. All focal mechanisms on that segment show reverse faulting. As shown on Fig. 4, the mainshock of the Nippes earthquake sequence occurred on this fault, which we name “Nippes Fault”. Its projection to the surface – see cross-sections 7, 8, 9 – does not correspond to a well-defined fault in the geology.

The aftershock distribution determined here matches the one described in Douilly et al. (2023) based on a shorter catalog and a double-difference relocation technique. These authors however interpret the whole sequence as occurring on a fault distinct from the EPGF, and assume the EPGF to be vertical in the study area, although no direct evidence for this exists. The aftershock distribution also matches the rupture segments inferred from teleseismic and geodetic finite fault inversions of Calais et al. (2022), Okuwaki and Fan (2022), and Raimbault et al. (2023) with a mostly reverse eastern segment and a steeper dipping western segment with mostly strike-slip. This dual behavior is visible in the distribution of focal mechanism on Fig. 6, with mostly reverse and reverse-strike-slip events in the eastern segment, and mostly strike-slip and strike-slip-reverse in the western one. It therefore appears that the Nippes earthquakes initiated on a north-dipping reverse fault roughly parallel to the Jérémie-Malpasse fault system, along which it released the largest part of the seismic

moment (Calais et al., 2022; Raimbault et al., 2023; Yin et al., 2023), before jumping to the steeply north-dipping EPGF.

### 5.3. Zone 3: Offshore jérémie

Zone 3 includes a cluster located about 50 km to the north of the main EPGF, offshore the locality of Jérémie. The sequence started immediately after the Nippes mainshock (Figure 3), with a first $M_{lv}$ event detected five hours after the mainshock (Ayiti-Séismes, 2025). It continued during the entire time interval of the present study (Fig. 3 bottom), with several bursts of activity. In map view, aftershock epicenters cluster just to the south of the offshore trace of the Jérémie-Malpasse reverse fault, along which the Southern Peninsula overthrusts the 3500 m-deep Jérémie basin (Mompraisir, 1986; Calais et al., 2023). Their depth distribution delineates a flat cluster at about 12 km depth, as visible on Fig. 4, top right. Although these events occurred outside of the footprint of the local temporary broadband network, they locate about 10 km north of the northernmost seismic stations – thus not far outside the network – and their hypocenter determinations benefit from broadband stations to the north in Cuba. As a result, the quality of their NLL-SC location in terms of RMS, number of phases, or uncertainties is similar to events in zones 1 and 2.

This sequence culminated with a $M_w$ 5.1 event on 23 March 2022 (Ayiti-Séismes, 2025), outside of the time interval of the present catalog, but whose epicenter plots within the seismic cluster described here (Fig. 6). It was reported by several global observatories, among which the USGS, GFZ, and OCA also calculated a moment tensor solution (EMSC (2022), 2022). The hypocentral depth is however not constrained in any of these solutions. The three focal mechanism solutions for this event show reverse faulting on ~east-west trending fault planes. Another event on 8 April 2022, of $M_w$ 4.7, was reported by the OCA global center, also with a reverse faulting mechanism (EMSC (2022), 2022).
The Jérémie earthquake sequence started just after the Nippes mainshock in an area devoid of previous seismic activity (Ayiti-Séismes, 2025). It does not show any dominant, large event, or an Omori-type decaying rate of aftershocks, but shows a b-value of 1.4 (Supplementary Figure 4) and a temporal distribution more common to earthquake swarms associated for instance with fluid-driven seismicity (e.g., Noir et al., 1997; Antonioli et al., 2005; Shapiro and Dinske, 2009). We tried to identify a spatio-temporal migration pattern, noting that epicenters of the Jérémie sequence concentrate in a narrow band trending NW-SE (Fig. 10 top). We selected this group of events and retained those determined with at least 12 phases and whose locations have a standard ellipsoid semi-major axis length less than 2.4 km. These values correspond to the mean number of phases and mean semi-major axis length for the entire earthquake catalog. This selection reduces the number of events from 284 for the whole sequence to 93. In map view, Fig. 10 (top) shows that early events plot at the southeastern termination of the cluster, while later ones plot toward its northwestern end. This is better seen when plotting event distances from the southeastern cluster end as a function of time, as shown on Fig. 10 (bottom). The number of events is limited, but a spatio-temporal migration pattern emerges, with early events at the southeastern end of the cluster and the progression of a seismicity front to the northwest with time. There is an indication that this progression occurs in the form a series of subclusters around 50, 70, and 125 days, although this may be biased by the detection limitations of the catalog.

Earthquake migration is commonly observed as a result of hydraulic injections or natural fluid circulation, or associated with slow aseismic slip on faults. In the former case, migration of the propagation front occurs at velocities of the order of meters per day (Pacchiani and Lyon-Caen, 2010; Miller et al., 2004; De Barros et al., 2020), as observed here with about 6 km in 160 days. This propagation can be modeled as pore-pressure diffusion of fluids in a poroelastic medium such that the migration velocity $V$ scales with the sequence duration $T$ according to $V \propto T^{-0.5}$. The distance of the propagation front $r$ as a function of time $t$ is then of the form $r = \sqrt{4\pi Dt}$, where $D$ is the hydraulic diffusivity (Shapiro et al., 1997), typically on the order of $0.1–0.4 \text{ m}^2\text{s}^{-1}$ (e.g., Talwani et al., 1999; Pacchiani and Lyon-Caen, 2010; Yang et al., 2023). Fitting the propagation front, indicated with gray squares on Fig. 10 (bottom), with this diffusion law gives $D = 0.24 \text{ m}^2\text{s}^{-1}$. More recently, experiments have shown that seismicity migration was actually likely driven by the interplay between fluid pressure and aseismic fault slip (Guglielmi et al., 2015; Cappa et al., 2019). In that case, Danré et al. (2024) show that the migration velocity scales with the sequence duration such that $V \propto T^{-0.8}$. A linear fit to the propagation front gives a velocity of 35 m/day for a duration of 160 days, which falls in the middle of the empiric law of Danré et al. (2024, their figure 2) for fluid induced seismicity.

The quasi-constant depth of this earthquake sequence is intriguing. It may have occurred on a sub-horizontal section of the Jérémie-Malpasse reverse fault system, as perhaps indicated by the shallow south-dipping focal plane, about 20°, in OCA's focal mechanisms on Fig. 6. Assuming a ramp-and-flat fault geometry at depth (e.g., Suppe, 1983), a ~12 km depth would imply a very steep ramp to connect with the fault surface trace (Fig. 5, cross-section 1), or the sequence may highlight a blind flat propagator of the reverse fault system. Alternately, this sequence may take place within a sub-horizontal, high-permeability lithological unit within the crust that favors the propagation of fluids, as observed in some instances of reservoir-induced seismicity (e.g., Johann et al., 2018). Finally, one cannot entirely dismiss an artifact in the event depth of this cluster, close to a 12 km-deep interface of the crustal seismic model (Supplementary Figure 1) and outside the overall network geometry. It is therefore possible that absolute depths in the SSST relocations are affected by this interface, which may artificially clusters hypocenter depths, although the events actually locate slightly above the model interface.

## 6. Discussion

We used the focal mechanisms obtained in this study and an inversion procedure based on a simulated annealing algorithm combined with a one-dimensional grid search approach (Delouis et al., 2002) to determine the direction and dip of the principal axes, $\sigma_1, \sigma_2, \sigma_3$, of the stress tensor for the local region around the seismic events considered. A goodness of fit score is calculated as the average of the scalar products between theoretical and observed slip vectors for all focal mechanisms. If slip vectors observed and predicted by the stress tensor are identical, the scalar product is equal to 1. It is equal to 0 when they are perpendicular, and becomes negative when the angular difference is greater than 90°. The highest score over all tested stress tensors, that is, the closest to 1, corresponds to the best-fitting model, and then two other decreasing confidence levels define the second and third best solution groups, respectively. The stress tensor shape factor $R = (\sigma_z - \sigma_x)/(\sigma_y - \sigma_x)$ where $\sigma_y > \sigma_x$ and $\sigma_z$ is the principal stress axis nearest to the vertical, reflects the relative amplitude of the intermediate principal stress $\sigma_2$ with respect to the other two, $\sigma_1$ and $\sigma_3$ (Rivera and Cisternas, 1990). The value of the shape factor $R$ determines whether the stress regime is compressional ($R < 0$), strike-slip ($0 < R < 1$) or extensional ($R > 1$).

The inversion of the 73 focal mechanisms determined here (Fig. 6) for the orientation of the principal stress axes leads to a maximum compression ($\sigma_1$) that is quasi-horizontal (5° plunge) and oriented N44°E, while $\sigma_3$ dips steeply with a 74° plunge angle (Fig. 11). The resulting shape factor $R$ has a value of -0.59. This implies that the stress released immediately after the 2021 Nippes earthquake, that had been building up interseismically, was purely compressional. This is at odds with the supposedly strike-slip regime of the Southern Peninsula of Haiti, where current tectonics is thought to be dominated by the Enriquillo Plantain garden left-lateral strike-slip fault (Mann et al., 1995; Wessel et al., 2013; Fleur et al., 2020).

While left-lateral strike-slip on the EPGF undoubtedly plays a major role in the regional kinematics, recent GNSS and offshore geology results indicate that a significant amount of plate boundary-normal shortening is accommodated on the Jérémie-Malpasse fault system (Calais et al., 2023) and farther north within the Gulf of Gonâve (Corbeau et al., 2019). The occurrence of the January 2010 $M_w$ 7.0 and August 2021 $M_w$ 7.2 earthquakes, which both initiated on blind, north-dipping, oblique-reverse fault segments, then propagated westward onto strike-slip fault segments, are also indicative of the release of compressional elastic stress within the Southern Peninsula. This is corroborated by the Anse-à-Veau-Miragoâne seismic cluster that followed the 2021 Nippes earthquake, with pure reverse slip on a southwest-dipping fault. A similar scenario followed the 2010 Haiti earthquake, with the activation of the Trois Baies fault, part of the Jérémie-Malpasse fault system, with pure reverse slip (Nettles and Hjörleifsdóttir, 2010; Douilly et al., 2013).

In addition, the geometry of the EPGF through the Southern Peninsula is not that of a restraining bend as observed, for instance, in the case of the Transverse Ranges of Southern California along a major bend of the San Andreas fault (e.g., Luyendyk, 1991), or in the Lebanon restraining bend along the Dead Sea fault (e.g., Gomez et al., 2007). The EPGF trace is rather linear, with the two local exceptions of the Macaya peak restraining bend and the Miragoâne pull-apart basin. The built up of stress within the crustal sliver bounded to the south by the EPGF and to the north by the Jérémie-Malpasse fault system, is therefore likely to result from the oblique convergence between the Gonâve microplate and the Southern Peninsula observed in GNSS data (Calais et al., 2023) rather than from the strike-slip kinematics of a transpressional fault system.

The partitioning of stress and strain observed in Southern Haiti is indeed well explained by the model of Bowman et al. (2003), where the distribution of strain is driven by localized oblique slip at depth, i.e., with both convergence and strike-slip as shown on their Fig. 1C,
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Top: map of the Jérémie earthquake cluster located in zone 3. Colored dots are the epicenters selected for the spatio-temporal migration analysis (see text), color-coded as a function of their date. Gray dots show the entire catalog. The dashed line shows the segment along which seismicity propagation is observed, from the southeast to the northwest. Bottom: time-distance plot of the selected seismicity showing the progression of a seismicity front along the segment identified by a dashed line on the top panel. Gray squares show the propagation front determined every 10 days. The dashed red line is a linear fit to the propagation front at an average velocity of 35 m/day, the black dashed line is a fit to a diffusion law in $\sqrt{\text{time}}$. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

propagating upward into an overlying elastoplastic crust. With this kinematics the model predicts (1) reverse faulting in the footwall ahead of the fault tip, where its surface projection tends to extend as a reverse fault, consistent with reverse faulting on the Jérémie–Malpasse fault system, (2) strike-slip faulting in the hanging wall near the surface and just above the fault tip at depth, consistent with observations along the EPGF or Ravine du Sud faults, and (3) oblique–reverse slip in the rest of the hanging wall, consistent with the initial rupture phase of the 2010 and 2021 earthquakes within that crustal volume.

We illustrate a possible geometry associated with this model for the study area on Fig. 12. We draw a south-dipping oblique-slip fault at depth, through the mantle, necessary to accommodate the oblique convergence between the Gonâve and Caribbean plates determined geodetically (Calais et al., 2023). The lower part of this fault may be the locus of the tremors identified by Aiken et al. (2016) south of the surface trace of the EPGF at a depth 20–30 km following the 27 February 2010 Mw 8.8 Maule earthquake in Chili. In this interpretation, the deep fault and its surface projection onto the Jérémie–Malpasse fault system
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Orientation of the principal axes of the stress tensor derived from the inversion of the 73 focal mechanisms of Fig. 6, shown on lower hemisphere equal-area projection. Azimuth and plunge are indicated in degrees for each principal stress. Bottom panel shows the shape factor $R = (σ_3 - σ_1)/(σ_2 - σ_1)$. The best-fit solution, indicated by a red dot, has a score of 0.95 and a shape factor $R < 0$ indicative of a compressional stress regime. Gray dots show solutions with scores between 0.95 and 0.92, white dots show lower scores. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

allow for the north-verging thrusting of oceanic plateau material from the Caribbean Large Igneous Province (CLIP, (Duncan and Hargraves, 1984)), which constitutes the core of the Southern Peninsula (e.g., Maurasse et al., 1979; Wessels et al., 2019), onto island arc terranes of the Cretaceous–Paleocene orogenic belt commonly named “Great Arc of the Caribbean” (Burke, 1988).

In our interpretation, the deep fault branches upward, through the crust, in the form of a so-called “flower structure”, typical of strike-slip regimes (e.g., Harding, 1985; Sylvester, 1988). Following the model of Bowman et al. (2003), and in accordance with observations, localized oblique slip at depth partitions, through the crust, into strike-slip and reverse slip on distinct structures. The Jérémie–Malpasse fault accommodates shortening in a ~N40°E direction, as exemplified by the largest aftershocks of the 2010 and 2021 earthquakes. These were actually triggered events on two segments of the Jérémie–Malpasse fault system, the Trois Baie Fault in 2010, the Anse-à-Veau–Miragoâne segment in 2021. The EPGF accommodates the shear component of the Gonâve–Caribbean oblique motion and abuts at depth against the main north-dipping fault. The early parts of the 2021 rupture on the north-dipping Nippes fault, or in 2010 on the Léogâne fault, with oblique but largely reverse slip, locate within the triangular crustal sliver between these two main faults where oblique strain is predicted.

Other processes may also influence the stress/strain field in and around the Southern Peninsula, for instance the stresses induced by the horizontal gradients in gravitational potential energy between the topography, which culminates at 2300 m in the Macaya massif, and the −4000 to −3500 m-deep offshore bathymetry on either sides of the

peninsula (Fig. 1). The active role of the sublithospheric mantle, the effect of fluid pore pressure on effective fault friction, or local block rotations may also be invoked. Further data and analysis is required to directly determine the fault geometry at depth and present-day stress/strain field in the study area.

In the absence of large instrumentally recorded events in southern Haiti before 2010, the notion has long been that it should be the locus of large, purely strike-slip earthquakes on the EPGF. Historical earthquakes are indeed typically interpreted in that framework (e.g., Ali et al., 2008; Hough et al., 2023). The growing evidence for strain partitioning in this oblique convergence area questions this assumption and raises the issue of the activation of earthquake ruptures within such a transpressional and partitioned tectonic system. Wang et al. (2018) proposed that earthquakes alternate over centuries between strike-slip rupture of long segments of the EPGF and oblique slip on shorter, transpression-related fault segments. The 2010 and 2021 events appear to argue for a different scenario, in which a single earthquake combines the two kinematics in two sub-events. Both earthquakes indeed initiated with reverse slip – here on a segment of the Jérémie–Malpasse fault system – then jumped onto a nearby segment of the EPGF with pure strike-slip motion. The NE-SW compressional stress regime of southern Haiti actually favors the rupture of well-oriented reverse faults oblique to the EPGF, which can, in turn, activate a nearby segment of the EPGF. Douilly et al. (2015) and Yin et al. (2024) explored the conditions under which such a dynamic propagation may occur and show that this was a physically plausible scenario for both the 2010 and 2021 earthquakes. A similar scenario occurred in Alaska on 3
<figure><img src="image_15.png" /></figure>

**Fig. 12.** Interpretative synthetic cross-section of the study area. Earthquake hypocenters, plotted with white dots, are actual hypocenters from sections 6 and 14 on Fig. 5. Main faults inferred are shown with solid black lines. The fault segments involved in the 2021 Nippes earthquake sequence described in this paper are highlighted with a dashed red line. The yellow star indicates the nucleation location of the $M_w$ 7.2 Nippes earthquake from Calais et al. (2022). The colored image in the background, from Bowman et al. (2003), shows optimal failure planes for an elastic plastic model with slip partitioning resulting from oblique slip on a 75° dipping fault. Blue and red segments represent optimal failure planes using a lower hemisphere projection of the failure surfaces (see Bowman et al. (2003)). The model is consistent with reverse faulting on the Jérémie–Malpasse fault (JMF) and on the initial patch of the 2021 Nippes rupture, strike-slip on the second rupture patch – possibly the EPGF or Ravine du Sud faults. Moho depth is interpreted from Douilly et al. (2015) and Lee et al. (2025). CLIP = Caribbean Large Igneous Province; GAC = Great Arc of the Caribbean. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

November 2002, with a $M_w$ 7.0 reverse slip earthquake on the Susitna Glacier fault which dynamically triggered a 220 km–long rupture of the Denali strike-slip fault, resulting in a $M_w$ 7.9 event (Eberhart-Phillips et al., 2003; Hreinsdottir et al., 2003; Aagaard et al., 2004).

## 7. Conclusions

Using data from a local temporary broadband seismic network, a national network of low-cost seismometers, and regional seismic stations, we obtained a catalog of 5341 precisely relocated events following the 14 August 2021, $M_w$ 7.2, Nippes earthquake in southern Haiti. We used the NLL-SSST-Coherence method, which allows us to reduce hypocenter uncertainties to under 3 km for 88% to 94% of the events, highlighting the effectiveness of the method in complex fault zones.

The seismicity, which concentrates north of the EPGF, can be split into aftershocks directly related to the Nippes earthquake rupture process, and two off-rupture clusters. A first one concerns the Anseà-Veau – Miragoâne area and corresponds to the aftershock sequence of two $M_w$ 5.3 and 4.9 earthquakes that occurred on 24 January 2022. This cluster defines a planar structure between 4 and 12 km depth, dipping ~34° to the southwest, that is likely a segment of the offshore Jérémie–Malpasse reverse fault system. A second cluster, offshore Jérémie, started immediately after the Nippes mainshock and continued during the entire time interval of the present study. Its epicenters cluster close to the offshore trace of the Jérémie–Malpasse reverse fault. Its swarm-like temporal distribution as well as evidence for directional propagation of the epicenters indicate that this sequence was likely driven by fluid migration. The main aftershock sequence,

which highlights the Nippes earthquake rupture as determined independently from geodetic and teleseismic studies, shows two groups of events defining north-dipping structures. An eastern one is oriented ~100°–110°E, oblique to the EPGF trend, with events reaching down to 30 km depth. A western group, parallel to the EPGF and confined to 0–15 km depth, projects at the surface along the trace of the Ravine du Sud strike-slip fault.

The inversion of focal mechanisms for 73 of the earthquakes relocated here shows a purely compressional regime, with $\sigma_1$ quasi-horizontal and oriented N44°E. This earthquake sequence therefore corresponds, overall, to the release of compressional interseismic stress. This begs the question of the role of the EPGF, typically viewed as the main plate boundary structure in southern Haiti. We argue that strain in the Southern Peninsula is partitioned between compression on the Jérémie–Malpasse fault system, shear on the EPGF, and hybrid deformation within the crustal sliver bounded by these two faults, where the 2010 and 2021 earthquakes initiated. This view is consistent with a mechanical model with oblique slip at depth on a north-dipping fault that accounts for oblique motion between the Gonâve and Caribbean plates.

The 2010 and 2021 earthquakes of southern Haiti both initiated with dip-slip motion on a north-dipping reverse fault that propagated westward on a purely strike-slip fault segment consistent with the EPGF strike and long-term faulting regime. This shows that the long-term strain partitioning observed in the geodetic and geological data is also observed at the quasi-instantaneous time scale of the rupture of those two earthquakes. Future seismic hazard assessment for the region should therefore account for faults other than the EPGF as potential sources, with mechanisms other than strike-slip.
CRediT authorship contribution statement

**Sylvert Paul:** Writing – original draft, Investigation, Formal analysis. **Tony Monfret:** Writing – review & editing, Validation, Supervision, Investigation, Conceptualization. **Eric Calais:** Writing – review & editing, Investigation, Funding acquisition. **Françoise Courboulex:** Writing – review & editing, Supervision, Investigation, Funding acquisition, Conceptualization. **Bertrand Delouis:** Writing – review & editing, Software, Investigation, Formal analysis. **Anthony Lomax:** Writing – review & editing, Validation, Software, Investigation, Formal analysis. **Bernard M. de Lépinay:** Investigation. **Steeve J. Symithe:** Investigation, Funding acquisition. **Anne Deschamps:** Writing – review & editing, Investigation. **David Ambrois:** Writing – review & editing, Data curation. **Sadrac St Fleur:** Supervision, Investigation. **Dominique Boisson:** Project administration, Funding acquisition.

Funding sources

We acknowledge funding from the FEDER European Community program through the Interreg Caraïbes "PREST" project, grant number 5236, and from the French National Research Agency, "OSMOSE" project, grant number ANR-21CE03-0010, and "CAST" project, grant number ANR-22-CE01-0019. We acknowledge the support of the Joint International Laboratory "Caribact". Sylvert Paul's PhD grant was funded by the Initiative of Excellence Université Côte d'Azur under reference number ANR-15-IDEX-01 and by the French Embassy in Haiti through the Anténor Firmin doctoral research program. Steeve J. Symithe acknowledges support from the Grant and Cooperative Agreement G20AC00100 between the U.S. Geological Survey (USGS) and the Faculté des Sciences (FDS) of the State University of Haiti (UEH). Funding from this grant was provided by the USAID Bureau of Humanitarian Assistance. Steeve J. Symithe also acknowledges funding from the "Fonds d'Appui à la Recherche" (FAR) of the Rectorat of the States University of Haiti (RUEH).

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We deeply thank the individuals and organizations hosting Raspberry Shake stations in Haiti. We thank colleagues from the University of Liège, Belgium, for providing data from two temporary broadband stations used in this study, as well as the agencies and staff operating the regional seismic networks in the Dominican Republic, Cuba, Jamaica, Canada, Puerto Rico, and the United States for making their data openly available. We thank Roby Douilly for providing access to the seismic data from the Nanometric stations used in this study. Some of the figures were made with the Generic Mapping Tools (GMT), (https://www Villiers et al. (2013)). We thank two anonymous reviewers for their comments, which helped improve the technical sections of the manuscript.

Appendix A. Supplementary data

Supplementary material related to this article can be found online at https://doi.org/10.1016/j.tecto.2025.230982.

Data availability

Broadband and strong motion seismic records from regional and global seismic stations were retrieved from the IRIS data center (https://ds.iris.edu/ds/nodes/dmc/). Data collected by the temporary broadband stations (network code Z2) deployed in Haiti will be made available through the IRIS Data Center in October 2026. Data from Raspberry Shake stations, which are part of the HY network (https://doi.org/10.7914/sn/hy) are openly available, with real-time streaming, and can be obtained from the FDSN Webservice of the Ayiti-séismes platform (https://ayiti.unice.fr/ayiti-seismes/fdsnws/). The final earthquake catalog from this work is openly available at https://zenodo.org/record/17423384.

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