THE SEISMIC STRONG MOTION ARRAY PROJECT (SSMAP) AND THE SEPTEMBER 5, 2012 MW=7.6 NICOYA, COSTA RICA EARTHQUAKE

Since 2006, the seismic strong motion array project (SSMAP) for the Nicoya Peninsula in northwestern Costa Rica has been composed of 10 sites with Geotech A900/A800 accelerographs (three-component) and GPS timing. Our digital accelerograph array has been deployed as part of our ongoing research on large earthquakes in conjunction with the Earthquake and Volcano Observatory (OVSICORI) at the Universidad Nacional in Costa Rica. The country wide seismographic network has been operating continuously, and has been upgraded with broad-band seismometers and Episensors. The previous major earthquake in Nicoya was the 1950 M­_w 7.8 event. On September 5, 2012, a M_w=7.6 earthquake occurred in the seismic gap and appears to be the expected event based on recurrence interval, but Protti (2012) states that only 40% of the energy calculated to have accumulated since the 1950 quake was released by the September earthquake. The mainshock focal mechanism is thrust faulting of the Cocos plate in the Middle America trench with strike N54W and dip 20 degrees NE. The preliminary result for the mainshock and associated aftershocks is a rupture zone of 79km length and 42km width. The maximum accelerations from two A900 stations, Fortuna (distance 112km) and Cerro (distance 128 km) are: 19.5% and 17.8 % g, respectively. Additional aftershock acceleration data and previous events for 2010-12 will be presented.