Apparent stress variations near the Osa Peninsula, Costa Rica, influenced by subducted bathymetric features

Abstract

[1] We compute apparent stress for 114 aftershocks (0.9 ≤ M L ≤ 3.7) of the 1999 M w = 6.9 Quepos, Costa Rica, thrustfaulting earthquake to examine the influence of subducting plate topographic complexity near the Osa Peninsula on earthquake rupture. Using seismic coda techniques, we find a heterogeneous distribution in apparent stress of 0.1-2.5 MPa (mean 0.6 MPa) for these aftershocks. Mean aftershock apparent stress is more than twice the global mean for thrust-faulting earthquakes at oceanic subduction zones and 1.5 times the mean for events just northward along the margin near the Nicoya Peninsula where the subducting plate has lower relief. We also find constant source scaling for the Osa aftershocks. The variation in apparent stress found near the Osa Peninsula, and high mean as compared to global and regional values, suggest areas of stress concentration in the region of bathymetric complexity in the subduction zone.

Low stress drop earthquakes in the rupture zone of the 1992 Nicaragua tsunami earthquake

“…Events in a pair consist of any events in the cluster with at least one common station and narrowband frequency envelope. We use the spectral ratio method described by Mayeda et al [2007], which has been further developed in more recent studies [e.g., Phillips et al, 2008;Moyer et al, 2011;Fisk and Phillips, 2013]. Coda waves are scattered off heterogeneities along indirect paths between the source and receiver [e.g., Aki, 1969;Aki and Chouet, 1975], and the coda amplitudes are stable with respect to epicentral distance, azimuth, source mechanism, and laterally varying path effects [e.g., Aki, 1969;Aki and Chouet, 1975;Rautian and Khalturin, 1978;Phillips and Aki, 1986;Mayeda et al, 2007;Mayeda and Malagnini, 2010].…”

“…Source parameter results based on data from coda are similar to those based on direct wave data, with reduced scatter in the spectral ratios [e.g., Mayeda et al , ; Mayeda and Malagnini , ; Abercrombie , ; Fisk and Phillips , ; Yoo and Mayeda , ; Somei et al , ]. We use the spectral ratio method described by Mayeda et al [], which has been further developed in more recent studies [e.g., Phillips et al ., ; Moyer et al ., ; Fisk and Phillips , ].…”