3D dynamic rupture simulation and local tomography studies following the 2010 Haiti earthquake
The 2010 M7.0 Haiti earthquake was the first major earthquake in southern Haiti in 250 years. As this event could represent the beginning of a new period of active seismicity in the region, and in consideration of how vulnerable the population is to earthquake damage, it is important to understand the nature of this event and how it has influenced seismic hazards in the region. Most significantly, the 2010 earthquake occurred on the secondary Léogâne thrust fault (two fault segments), not the Enriquillo Fault, the major strike-slip fault in the region, despite it being only a few kilometers away. We first use a finite element model to simulate rupture along the Léogâne fault. We varied friction and background stress to investigate the conditions that best explain observed surface deformations and why the rupture did not to jump to the nearby Enriquillo fault. Our model successfully replicated rupture propagation along the two segments of the Léogâne fault, and indicated that a significant stress increase occurred on the top and to the west of the Enriquillo fault. We also investigated the potential ground shaking level in this region if a rupture similar to the Mw 7.0 2010 Haiti earthquake were to occur on the Enriquillo fault. We used a finite element method and assumptions on regional stress to simulate low frequency dynamic rupture propagation for the segment of the Enriquillo fault closer to the capital. The high-frequency ground motion components were calculated using the specific barrier model, and the hybrid synthetics were obtained by combining the low-frequencies (< 1Hz) from the dynamic rupture simulation with the high-frequencies (> 1Hz) from the stochastic simulation using matched filtering at a crossover frequency of 1 Hz. The average horizontal peak ground acceleration, computed at several sites of interest through Port-au-Prince (the capital), has a value of 0.35g. Finally, we investigated the 3D local tomography of this region. We considered 897 high-quality records from the earthquake catalog as recorded by temporary station deployments. We only considered events that had at least 6 P and 6 S arrivals, and an azimuthal gap less then 180 degrees, to simultaneously invert for hypocenters and 3D velocity structure in southern Haiti. We used the program VELEST to define a minimum 1D velocity model, which was then used as a starting model in the computer algorithm SIMULPS14 to produce the 3D tomography. Our results show a pronounced low velocity zone across the Logne fault, which is consistent with the sedimentary basin location from the geologic map. We also observe a southeast low velocity zone, which is consistent with a predefined structure in the morphology. Low velocity structure usually correlates with broad zones of deformation, such as the presence of cracks or faults, or from the presence of fluid in the crust. This work provides information that can be used in future studies focusing on how changes in material properties can affect rupture propagation, which is useful to assess the seismic hazard that Haiti and other regions are facing.^
Andrew M. Freed, Purdue University.