Corner Frequency Variation in the Southeastern Region of the 1992 Nicaragua Tsunami Earthquake

Abstract:

At the Nicaragua portion of the Middle America Trench, where the Cocos Plate is subducting at ~85 mm/yr, a tsunami earthquake (m_b 5.3, M_s 7.2, M_w 7.6) occurred at 15 km depth on 2 September 1992, causing a tsunami up to 8 m high and >116 deaths. A tsunami earthquake is characterized by deficiency in high frequency radiated energy and large tsunami for its M_s. Tsunami earthquakes are relatively rare, but their occurrence presents a significant hazard to coastal populations, so the potential to identify tsunami earthquake regions has wide-reaching hazard implications. Here we examine the notion that the cause of the dominantly low frequency energy in the 1992 tsunami earthquake may also manifest in small earthquakes in the same area. We examine 241 events within and south of the 1992 rupture limits with hypocenters 8-30 km depth and 1.7 < M_w < 4.6, recorded by land station and ocean bottom seismometers in southern Nicaragua and northern Costa Rica from November 2005 to June 2006. Event source parameters moment, corner frequency, and stress drop are determined using the spectral ratios of S-wave coda. Mean stress drop inside the 1992 rupture area is 3.2 MPa, and immediately south of the 1992 rupture zone mean stress drop is 10.4 MPa. Inside and outside the 1992 rupture zone at similar depths, the increase in stress drop is by a factor of five and occurs abruptly over <50 km. The mean corner frequency of events in the 1992 rupture area is ~45% the mean corner frequency value for events south of the 1992 rupture zone. Therefore, our results demonstrate different source parameter characteristics for microseismicity in the region of a past tsunami earthquake. If this finding can be reproduced at other subduction zones, it may significantly improve subduction zone coastal hazard assessment.