Spatial variability of the b-value and seismic potential in Costa Rica

Abstract

Costa Rica is a seismically active region located in a subduction zone. Combining earthquake data from the National Seismological Network and several Central American catalogs from 1522 to 2020, we present a new estimation for the Gutenberg-Richter relationship. The seismic catalog compiled with ~122,000 earthquakes was studied by calculating the magnitude of completeness (Mc) and by applying a space-time window declustering method. The residual catalog and a previously proposed seismic zonation were used to determine the b-value, the maximum magnitude (Mmax), and the mean recurrence interval (MRI). Our results show a temporal and geographic variation in the Mc, decreasing from 7.0 since 1793 to 3.0 since 1995, and with the lowest estimate (2.5) in Central Costa Rica. The b-values of 0.85 for the entire catalog and ~0.83 for the interplate zones, are similar to other regions worldwide with young subducting slabs. There is a general trend of higher and more variable b-values among the upper-plate (average 0.90) and intraslab (1.14) zones as compared to the interplate regions (0.85). The upper-plate variability is explained in terms of the diversity in geological units and faulting style, whereas in the interplate and the subducting slab is connected to the stress level imposed by different seafloor morphologies and hydration states along the margin. Our data suggest a seismic potential of moment magnitude (Mw) 7.9–8.0 for the Nicoya and Southern Nicaragua interplate zones and for the Limon region. The Gutenberg-Richter distribution shows that a Mw 7.0 has a longer MRI for the intraslab zones (~72 years) than for the subduction interplate region (~15 years) and plate and microplate boundaries (~40–45 years). Interplate earthquakes have occurred recently (1991 Mw 7.7 and 2012 Mw 7.6) but no major intraslab since 1948 (Mw 7.0).