Aftershock pattern of the July 9, 1997 Mw=6.9 Cariaco earthquake in Northeastern Venezuela

Present-day deformation along the El Pilar Fault in eastern Venezuela: Evidence of creep along a major transform boundary

“…The upper and seismogenic part of the fault, represented by the upper dislocations (Table 3), are nearly vertical for the two segments; our favoured solution is compatible with the published focal mechanisms indicating right-lateral strike-slip on nearly vertical planes for earthquakes located in the upper crust (Baumbach et al, 2004;Deng and Sykes, 1995;Doser and Vandusen, 1996;Mendoza, 2000;Russo et al, 1993;Speed et al, 1991). As an example, for the 1997 earthquake, the hypocenter depths were respectively estimated by the Bulletin ISC (BISC, 1998), by Pérez (1988), and by Romero et al (1999), at 5 ± 2.3, 11 ± 1.5, and 9.4 km, whereas the aftershocks are located between the surface and 12 km depth (Baumbach et al, 2004).…”

“…The weighted differences between observed and simulated velocities using the Okada formulations of Green's functions have been minimized. We have minimized the number of free parameters using (1) geometrical constrains: the geometrical continuity between upper and deeper dislocations has been assumed, (2) geophysical constraints: the locking depth and the dip of the upper part of the fault have been determined with the distribution versus depth of microseismicity (Baumbach et al, 2004), (3) geological data: the location and orientation of the fault at surface have been determined by field data or high resolution seismic profiles (Van Daele et al, in press).…”

“…5). The transition between the upper and lower dislocations has been fixed at 12 km according to the depth distribution of aftershocks (Baumbach et al, 2004). The location of the El Pilar has been determined with the precise location of the outcrops of the Fault.…”

“…The recent activity of this major fault was shown by field survey on Pleistocene deposits along the fault (Vierbuchen, 1978;Macsotay, 1976Macsotay, , 1977Campos, 1981;Beltrán et al, 1996Beltrán et al, , 1999, high resolution seismic profiles in the Cariaco Gulf (Van Daele et al, in press) and, overall, by seismicity (several historical major earthquakes damaging the city of Cumaná (Paige, 1930;Audemard, 1999Audemard, , 2007. Detailed studies were conducted after the last major event which struck the city of Cariaco, located at the eastern end of the Gulf of Cariaco (Baumbach et al, 2004;Audemard, 2006). A recently performed high resolution seismic survey of the Gulf Bellizzia et al (1976), Stéphan et al (1980), Campos (1981, Beck (1986), Chevalier (1987); locations in Fig.…”

“…It must be underlined that there are no records of very large earthquakes during the last five centuries, which suggests that the segmentation of the fault by transpressional or transtensional relays reduces the size of the rupture area (Pérez and Aggarwal, 1981;Audemard, 2007). Focal mechanisms along the El Pilar Fault indicated right-lateral strike-slip on nearly vertical planes for earthquakes located in the upper crust (Baumbach et al, 2004;Audemard, 2006;Doser and Vandusen, 1996;Mendoza, 2000;Russo et al, 1993;Speed et al, 1991). Altamimi et al (2007).…”