Determination of the fault plane and rupture size of the 2013 Santa Cruz earthquake, Bolivia, 5.2Mw, by relative location of the aftershocks

Abstract

The Central Andes of southern Bolivia is a highly seismic region with many active faults, that could generate earthquakes up to 8.9 Mw. In 2013, an earthquake of 5.2 Mw occurred in Santa Cruz de la Sierra, in the sub-Andean belt, close to the Mandeyapecua fault, one of the most important reverse faults in Bolivia. Five larger aftershocks were reported by the International Seismological Centre (ISC) and 33 smaller aftershocks were recorded by the San Calixto Observatory (OSC) in the two months after the mainshock.Distances between epicenters of the aftershocks and the mainshock were up to 36 km, which is larger than expected for an earthquake of this magnitude. Using data from South American regional stations and the relative location technique with Rayleigh waves, the epicenters of the five larger aftershocks of the Santa Cruz series were determined in relation to the mainshock. This method enabled to achieve epicentral locations with uncertainties smaller than one km. Additionally, using data of three Bolivian stations (MOC, SIV and LPAZ) eight smaller aftershocks, recorded by the OSC, were relocated through correlation of P and S waves. The results show a NNW-SSE trend of epicenters and suggest an E dipping plane. The maximum distance between the aftershocks is 14 km, that is not consistent with the expected subsurface rupture length, in accordance with the magnitude of the mainshock. The events are located away from the Mandeyapecua fault and show an opposite dip, demonstrating that these events were generated by another fault in the area, that had not been well studied yet.