The March 25, 1990 (M_w = 7.0, M_L = 6.8), earthquake at the entrance of the Nicoya Gulf, Costa Rica: Its prior activity, foreshocks, aftershocks, and triggered seismicity

Abstract

On March 25, 1990 a large earthquake (Mw = 7.0, ML = 6.8) occurred at the entrance of the Nicoya Gulf, Costa Rica, at 1322:55.6 UTC, producing considerable damage in central Costa Rica and generating much interest about whether or not the Nicoya seismic gap (Nishenko, 1989) had broken. The local country‐wide seismographic network recorded 6 years of activity prior to this large earthquake, 16 hours of foreshocks, the mainshock, and its aftershocks. This network is operated jointly by the Costa Rica Volcanological and Seismological Observatory at the National University (OVSICORI‐UNA), and the Charles F. Richter Seismological Laboratory at the University of California, Santa Cruz (CFRSL‐UCSC). We obtained high resolution locations from this network and located the mainshock at 9°38.5′N, 84°55.6′W (depth is 20.0 km) and the largest foreshock (Mw = 6.0, March 25, 1990, at 1316:05.8 UTC) at 9°36.4′N, 84°57.1′W (depth is 22.4 km). We find that the aftershock zone abuts the southeast boundary of the Nicoya seismic gap, suggesting that the seismic gap did not rupture. Since the installation of the local network in April 1984 to March 24, 1990, nearly 1900 earthquakes with magnitudes from 1.7 to 4.8 (318 with magnitude 3.0 or larger) have been located at the entrance of the Nicoya Gulf, one of the most active regions in Costa Rica. The March 25 earthquake occurred at the northwest edge of this region, where a sequence of foreshocks began 16 hours prior to the mainshock. The spatial‐temporal distribution of aftershocks and directivity analysis of the mainshock rupture process using teleseismic records both indicate a southeast propagating rupture. The mainshock ruptured an asperity of approximately 600 km2 of area, with this area expanding to 4000 km2 after 7 days. We present evidence that suggests that the ruptured asperity is produced by the subduction of a seamount. Inversion of teleseismic broadband and long‐period P and SH waves yields a thrust faulting mechanism with the shallow plane striking 292°, dipping 26°, and with a rake of 88°, in agreement with the subduction of the Cocos plate under the Caribbean plate. Local first motions for the largest foreshock and the mainshock agree with this solution. We also present evidence suggesting that the March 25, 1990, earthquake triggered and reactivated several seismic swarms in central Costa Rica and temporally decreased the activity in the epicentral area of the July 3, 1983 (Ms = 6.2), Pérez Zeledón earthquake.

Slow slip events in Costa Rica detected by continuous GPS observations, 2002–2011

“…The last large earthquake in this region occurred in 1990 (Mw = 7.0) [Protti et al, 1995]. Norabuena et al [2004] studied the post-seismic relaxation process in this area and their results showed negligible impacts in GPS time series caused by post-seismic deformation.…”

“…The focal plane solutions were obtained from the Harvard CMT catalog. The approximate location of the 1990 earthquake is from Protti et al [1995]. Station velocities are relative to a fixed Caribbean plate.…”

“…The subduction rate in the trench-normal direction is 74-84 mmyr À1 [Outerbridge et al, 2010], while fore-arc motion is in the range of 8-14 mmyr À1 [DeMets, 2001;Norabuena et al, 2004;LaFemina et al, 2009]. The subducting Cocos plate is formed at both the fast spreading East Pacific Rise (EPR), with relatively smooth seafloor topography, and the slow-spreading Cocos-Nazca Ridge (CNR), with relatively rough seafloor topography [Protti et al, 1995;Barckhausen et al, 2001]. The location of the Nicoya peninsula, close to the subduction plate boundary and above the seismogenic portion of the plate interface, provides an excellent location for geodetic and seismic studies of shallow subduction zone processes [Lundgren et al, 1999; Outerbridge et al, 2010;Feng et al, 2010;Walter et al, 2011].…”

A strong-motion database from the Central American subduction zone

“…The more recent database compiled herein (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) American Seismic Centre (CASC) were also used in this study when available. In addition to the above sources of information, locations from detailed regional seismicity studies such as that of Ambraseys & Adams (2001) and from special studies with accurate relocations (e.g., Protti et al, 1995;Warren et al, 2008;Vallée et al, 2003) were included. The subduction events were classified by type (i.e., interface or intraslab), on the basis of focal mechanism, epicentral location, depth, and position with respect to trench axis.…”

“…More recently, a relatively large magnitude event occurred on 25 March 1990 (M W 7.3) event at the entrance of the Nicoya Peninsula. Although this latter event was initially associated with a rupture along the Nicoya seismic gap, detailed studies (Protti et al, 1995;Husen et al, 2002) indicated that the event was located to the southeast of the gap and instead it was due to the rupture of a seamount which acted as an asperity.…”