The Seismic Strong Motion Array Project (SSMAP) and September 5, 2012 (Mw 7.6) Nicoya, Costa Rica Earthquake Investigation During 2006-18

Date: 4/25/2019

Time: 06:00 PM

Room: Grand Ballroom

Seismic gaps along the subduction zones are locations where large earthquakes have not occurred in a long time. These areas are considered locked and are accumulating large amounts of strain energy that will ultimately be released in major earthquake. The Nicoya Peninsula in northwestern Costa Rica was considered a zone with this type of seismic gap. The previous major earthquakes in Nicoya occurred on 1853, 1900 and 1950, which indicates about a 50-year recurrence interval for the characteristic earthquake cycle. With the goals to: 1) record and locate strong subduction zone mainshocks [and foreshocks, “early aftershocks”, and preshocks] in Nicoya Peninsula, at the entrance of the Nicoya Gulf, and in the Papagayo Gulf regions of Costa Rica, and 2) record and locate any moderate to strong upper plate earthquakes triggered by a large subduction zone earthquake in the above regions, a seismic strong motion array (SSMAP project) was installed in the Nicoya Peninsula, array composed of 10 sites with Geotech A900 accelerographs. Also, the OVSICORI-UNA network was upgraded with ES-T episensors that could record the large 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 the 50 years recurrence interval, but was instead 62 years later. The main shock focal mechanism was thrust faulting of the Cocos plate in the Middle America trench with strike N54W and dip 20 degrees NE. The mainshock location was 9.671 N and 85.878 W. The maximum accelerations from two A900 stations perpendicular to the trench, Fortuna (distance 112 km) and Pedernal (distance 128 km) were: 13.8% and 8.9 % g; although the main acceleration was recorded at Dulce Nombre de Nicoya 122% g. The October 10 (M_W 5.3) and 24 (M_w 6.6) aftershocks recorded at Tamarindo were accelerations of 2.4% and 8.2% g. We also relocated 60 events from 2006 to 2018 for moderate magnitudes (4 < M_w< 6.5), mainly located in Nicoya Peninsula region which are analyzed for spatio-temperal relations.

Presenting Author: Gerald Simila

Authors