Source Characterization of the 13 June 2005 Tarapaca, Chile, Intermediate-depth Earthquake (Ms7.8)

We show source characteristics of the 2005 Tarapaca earthquake that occurred at a depth around 115 km (NEIC) in the east-dipping Wadati-Benioff zone beneath northern Chile. Accelerograms of the mainshock were successfully recorded by five strong-motion stations of Universidad de Chile within 300 km of the epicentral distance. We analyzed the data using the technique of Kuge (2003). The point-source moment tensor solution (Mw~7.7) we obtained from the displacement waveforms has horizontal and vertical nodal planes, both striking to the N-S direction. Our solution is similar to the Harvard CMT solution, agreeing with the observed polarities of the teleseismic P waves. Aftershocks were located in a horizontal region by Universidad de Chile, which suggests that the horizontal nodal plane was the fault plane. Two S-wave pulses, which can be the manifestation of the mainshock source process, are observed at a station above the source region. At stations to the northwest and southwest, however, the pulses are insignificant in the SH components, and the SH duration is comparable but shorter at the northwestern station. These observations cannot be explained by a horizontal line-source along the strike of the Wadati-Benioff zone. The slip distribution we obtained on the fault plane by modeling the waveforms is characterized by patches spreading in the NW-SE direction. The regions of high slip are located within 20 km of the relocated hypocenter. Most of the moment was released in 15 s. Our results also imply that part of initial slip could propagate to the east and that slip propagation in the eastern side could be more significant than that in the western side. By carefully examining teleseismic waveforms archived by IRIS, we found that the centroid time of the P displacement waveform is earlier in North America and that an initial, short pulse is observed for pP waveforms at stations to the east, although directivity in the teleseismic waves is generally weak. Additional analysis of modeling the teleseismic waveforms will further constrain the source process. Our values of the average stress drop and source duration for the Tarapaca earthquake are in the range of those previously reported for deep and intraplate earthquakes.