Date of Award

6-2014

Document Type

Open Access

Degree Name

Bachelor of Science

Department

Geology

First Advisor

Matthew Manon

Language

English

Keywords

travel, event, seismic, earthquake

Abstract

As part of a Keck Geology Consortium project on the Nicoya Peninsula, Costa Rica, a dense broadband seismic array was installed directly above the rupture zone of the long-anticipated Nicoya September 5th, 2012, Mw= 7.6 earthquake. 5 Nanometrics Trillium compact seismometers and Taurus digitizers were installed, defining a ~200km triangular area with one triangular side oriented parallel to the trench and the apex located ~15km inland from the coast. This seismic network was operating from July 2nd to July 18th, and with this data combined with the permanent stations of the Nicoya broadband network, 254 events were initially located. Comparison of velocity models and Vp/Vs ratios was performed to reduce the amount error between the calculated travel times versus the expected travel times of each event. On June 23rd, a Mw= 5.7 aftershock was recorded in the area of rupture of the September 5th event, the largest event to hit the Peninsula during data collection. The aftershocks of this Mw= 5.7 event on June 23 and June 24 were located at depths of 20.9 ± 5.7 km, along the plate interface beneath the Nicoya Peninsula. This event ruptured an area of the plate interface that did not fully rupture during the September 5th event. Analysis of focal mechanisms for individual interplate events indicates dominant thrust faulting motion, consistent with underthrusting along the seismogenic zone, and combined focal mechanisms provide accurate evidence of normal fault motion. The normal faulting earthquakes most likely formed from extension at the outer rise, which were then subducted and reactivated by dehydration reactions beneath the Nicoya Peninsula. This study, combined with previous and future seismic studies, has allowed for a better understanding of where seismogenic zone earthquakes occur beneath the Nicoya Peninsula following a large magnitude event, exemplifying the benefit of dense local seismic networks to monitor for smaller magnitude seismicity that otherwise would have gone unnoticed.

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