The August 14, 2021 Nippes, Haiti Earthquake Mainshock and Aftershock Sequence Recorded by The Ayiti-seismes Citizen Network

Description

Since July 2019, The Ayiti-seismes citizen network (ASCN) has been detecting and locating, in real time, earthquakes occurring in Haiti of magnitude ≥ 2.5. ASCN is based on both Seiscomp3 modules and Géoazur in-house software, and uses seismic signals from (1) low-cost, low-maintenance Raspberry Shake (RS) instruments installed in Haiti and (2) permanent seismic stations in neighboring countries. RS stations are part of the seismological contribution to a "seismic citizen" project and are hosted by citizens who provide internet connection and power supply. Most of these instruments, however, are installed in noisy sites which may reduce their sensitivity to low magnitude seismic events. The ASCN recorded the Nippes earthquake of August 14, 2021, Mw 7.2 in the Southern Peninsula of Haiti and its aftershock sequence. At 21 km from the mainshock epicenter, an RS has been monitoring the seismic signal on 4 channels (1 vertical velocity component, 3 accelerometric components). Two hours after the mainshock, two other RS in the epicentral area were reconnected by their hosts, making their data available, followed five days later by two new RS stations installed in the aftershock zone. From August 14 to September 20, more than 1200 aftershocks of magnitude Mlv 1.5 to 5.8 were detected and located by the ASCN. To improve the location of the hypocenters, we used the Nonlinloc software and (1) determined an appropriate velocity model of the area, (2) used manual and automated P and S arrival picks, (3) corrected to source-specific station terms, and (4) relocated the events using a waveform similarity method. RS data in the aftershock area are essential for constraining focal depths. The mainshock and its aftershocks have been relocated north of the Enriquillo Plantain Garden Fault (EPGF). The mainshock is ~ 4 km from the EPGF while the aftershocks extend over a length of ~ 80 km west of the mainshock, and split into two main clusters: an eastern NW-trending cluster that surrounds the mainshock and dips to the NNE, and a NE-trending western cluster composed mainly of shallow events that joins westward with sparse events alined on ~30 km along the EPGF, confirming a westward rupture propagation of the mainshock and probably the activation of shallow faulting.

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