Aftershock pattern of the July 9, 1997 Mw=6.9 Cariaco earthquake in Northeastern Venezuela
Michael Baumbach a, Helmut Grosser a,*,1, Gloria Romero Torres b,2, Jorge L. Rojas Gonzales c,3, Monika Sobiesiak a, Wolfgang Welle a,1
a GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany ´ ´ ´ Fundacion Venezolana deInvestigaciones Sismologicas, Apartado Postal 76880, Caracas 1070-A, Venezuela c ´ ´ Centro de Sismologıa, Universidad de Oriente, Cerro el Medio, Av. Universidad, Cumana. Edo. Sucre, Venezuela b
Abstract The Mw = 6.9 Cariaco earthquake of July 9, 1997 was the strongest earthquake that occurred on the El Pilar fault between ´ Cumana and the Gulf of Paria in the 20th century. The El Pilar fault isthe most prominent strike-slip fault in the transition zone between the Caribbean plate and the South American plate in this region. We installed 18 seismic and 10 strong motion stations for recording aftershocks. We calculated the hypocenters of 1306 aftershocks with magnitudes between À 0.5 and 4.1. Most aftershocks concentrated near the documented trace of the El Pilar fault. The spatialdistribution of the aftershocks allowed us to separate the aftershock region on the El Pilar fault into six segments with different dip angles. We computed and classified faultplane solutions of 222 aftershocks. The majority of fault-plane solutions are of strike-slip type. Some segments show differences in the dominating type of focal mechanisms. From the aftershock distribution and spatial variationof b-values we inferred a rupture length of the Cariaco earthquake of 60 km, and thus an average displacement of f 1.3 m during the main shock is estimated. Two out of four stepovers, which were found on this length, were seismically activated during the monitoring time interval. The b-value distribution on the fault plane appears to define two areas of major stress relaxation: beneath Cariaco andhalf way between Casanay and El Pilar close to the surface. D 2003 Elsevier B.V. All rights reserved.
Keywords: El Pilar fault; Cariaco earthquake; Surface rupture; Aftershocks; Focal mechanism; b-value; Fault segmentation; Stepover
1. Introduction The 1997 Cariaco earthquake in Northern Venezuela (Mb = 6.0, Mw = 6.9 after the Bulletin of the ISC, 1997) was the largest earthquake that occurredin the
* Corresponding author. Fax: +49-331-2881295. E-mail address: firstname.lastname@example.org (H. Grosser). 1 Fax: +49-331-2881295. 2 Fax: +58-2-2579860. 3 Fax: +58-93-521466. 0040-1951/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2003.10.018
20th century on a segment of the east –west trending El ´ Pilar fault between the city of Cumana in the West and theGulf of Paria in the East. The El Pilar fault is part of the fault system separating the South American (SA) plate from the Caribbean (Ca) plate. The main shock caused 73 fatalities and heavily damaged the town of Cariaco (EERI, 1997). More isolated damage occurred in the west along the southern coast of the Gulf of Cariaco as far as ´ Cumana and in the east to Carupano at the coast of the CaribbeanSea northeast of Casanay. Details of
M. Baumbach et al. / Tectonophysics 379 (2004) 1–23 Table 1 Observed surface ruptures and liquefaction phenomena after the 1997 Cariaco earthquake Longitude (jW) 63.906 63.786 63.669 63.617 63.611 63.554 63.549 63.520 63.492 63.487 63.459 63.431 63.413 Latitude (jN) 10.456 10.442 10.473 10.482 10.483 10.488 10.488 10.492 10.495 10.495 10.500 10.50010.500 13 34 31 15 30 25 29 landslide 78 25 12 Offset (cm) Remarks sand volcanoes slipping of coastal road
´ damage analysis can be found in Perez (1998) and Lang et al. (2003). ´ Invited by the Fundacion Venezolana de Investi´ nes Sismologicas (FUNVISIS), Caracas, the ´ gacio German Task Force for Earthquakes came to Venezuela in order to join the geophysical and engineering studies related to...