Paleoseismic analysis of the San Vicente segment of the El Salvador Fault Zone, El Salvador, Central America

Abstract

The El Salvador earthquake of February 13^th 2001 (Mw 6.6) was associated with the tectonic rupture of the El Salvador Fault Zone. Paleoseismic studies of the El Salvador Fault Zone undertaken after this earthquake provide a basis for examining the longer history of surface rupturing earthquakes on the fault. Trenching at five sites along the San Vicente segment, a 21km-long and up to 2km-wide central section of the El Salvador Fault Zone, shows that surface fault rupture has occurred at least seven times during the past 8ka. Single-event displacements identified at each trench vary from several decimetres to at least 3.7m. Fault trace mapping, geomorphic analysis, and paleoseismic studies indicate a maximum magnitude for the El Salvador Fault Zone is c. M_w 7.6, with a recurrence interval of around 800yr. Earthquakes of M_w 6.6 or smaller, such as the February 2001 event are unlikely to be identified in the paleoseismic trenches, so our observations represent the minimum number of moderate to large earthquakes that have occurred on this part of the El Salvador Fault Zone. We observe significant variability in single-event displacement in the trenches, which we interpret as possible cascade rupture of several segments of the El Salvador Fault Zone. Combining displacements of river courses and the timing of events revealed in the trenches, we calculate a slip rate of c. 4mm/yr for El Salvador Fault Zone, identifying the fault zone as a major tectonic feature of the region, and a major source of seismic hazard and risk in El Salvador.