Rupture characteristics of the 2005 Tarapaca, northern Chile, intermediate-depth earthquake: Evidence for heterogeneous fluid distribution across the subducting oceanic plate?

Abstract

We examined the rupture of the 2005 Tarapaca, northern Chile, earthquake at about 110 km depth with respect to both kinematic and dynamic characteristics by using regional and teleseismic waveforms. The earthquake has a downdip tensional focal mechanism. The subhorizontal rupture is characterized by two patches of large slip and high stress drop which are aligned nearly in the east-west direction, being perpendicular to the direction of the Chile Trench. Rupture initiated in the eastern patch and then propagated to the western patch. Between the two patches, there exists a region of nonpositive stress drop and high strength excess, which can cause subshear rupture to propagate from the eastern to the western patches but radiates little seismic waves. Seismic radiation energy from this earthquake tends to be low, which is consistent with the nonpositive stress drop and high strength excess between the two patches. While the physical mechanism of intermediate-depth earthquakes is still controversial, current leading hypotheses are associated with dehydration within subducting plates. The rupture characteristics of the Tarapaca earthquake can be related to heterogeneous fluid distribution due to the dehydration. The spatial separation and dominant stress of the two large-slip patches agree with the characteristics of the previously reported double seismic zone beneath Chile. The two patches may be the manifestation of the double seismic zone where dehydration reactions can release fluid. Using a numerical simulation of 3-D dynamic rupture, we have shown that weakening due to fluid can account for the rupture characteristics of the Tarapaca earthquake.