On the hunt for unusual subduction zone earthquakes - Clues from secondary phases in seismograms of local earthquakes

Abstract:

Intermediate-depth earthquakes in subduction zones are generally believed to occur in the slab mantle and in the slab crust. Recent observations point out that some intermediate-depth earthquakes may occur in other, unusual places, such as on the deep portion (>50 km) of the subduction interface, and in the overlying mantle wedge. These regions are separated by strong seismic discontinuities that are typically imaged with techniques using teleseismic waves, while local earthquakes are generally located based on the timing of first arriving P- and S-waves. While this combination of imaging and earthquake locations provides a good initial overview of where the earthquakes are located in the system, the uncertainties associated with the two approaches are too large (i.e., several kilometers) to robustly identify on which side of a discontinuity (with thickness >100 m) the earthquakes are located. Here we present results from a study that explores how the waveforms of small, local earthquakes can be exploited to determine the source region of subduction zone earthquakes more robustly. Our investigation involves a three-step approach and includes an application to data from the Western Hellenic subduction zone, where we have previously observed clusters of earthquakes in the mantle wedge and in the slab. First, to identify characteristic secondary phases, we computed and analyzed synthetic seismograms from a generic 2-D subduction zone. Second, to enhance the visibility of secondary phases in field data, we implemented a workflow to process three-component seismograms from eleven densely spaced stations. Third, to identify individual secondary phases in the data, we matched their timing to theoretical arrivals computed in a 3-D model of seismic velocities and discontinuities. We identified on average two to three secondary arrivals per station. These include P- and S-reflections from the plate interface which indicate hypocenters in the mantle wedge, and P-reflections from the slab Moho which indicate hypocenters on the plate interface and in the subducting crust. Our results highlight the strong potential of full-waveform analysis of local earthquakes to locate hypocenters precisely relative to subsurface structure and, in doing so, hunt for earthquakes that may occur in unusual environments.