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A High-Resolution 4D Source Image of the 2015 M7.9 Bonin Deep-Focus Earthquake

A primary way of investigating the enigmatic nature of deep-focus earthquakes is by studying the source properties of these events. At these great depths, the orientations of rupture planes are difficult to determine given the limited knowledge of preexisting faults and the lack of aftershocks generally associated with large deep-focus events. This lack of rupture plane information hinders many source modeling methods and limits insights that may constrain the mechanisms that promote these enigmatic events. In this study, we utilize P waves recorded at the Hi-net array in Japan with a back-projection method to study the source properties of the 2015 Mw 7.9 Bonin deep-focus earthquake. To limit artifacts caused by seismic phases other than the direct P waves, a genetic algorithm method is applied to the selection of data that maximizes the point source nature of back-projection results from small events in the study region. This approach combined with the excellent aperture of the local Hi-net array allows for high-resolution 4D imaging of the earthquake source. This analysis identifies three rupture segments of this event based upon the locations of imaged energy. Segments 1 and 3 propagate with a horizontal orientation to the northwest and are separated by 15 km. Segment 2 exhibits both westward and upward propagation and terminates at the initiation point of segment 3. Combined, the orientations of these three segments cannot be fit with a single plane, suggesting either failure of multiple rupture planes during this event or failure on a curved surface. Interestingly, the orientations of segments 1 and 3 are consistent of the near vertical nodal plane of the focal mechanism of this event, while segment 2 is only consistent with the near horizontal nodal plane. One interpretation of these results is that the earthquake involved failure on conjugate rupture planes. This failure may have been induced by extensional stresses in the heel of the subducting slab as it is deflected to a horizontal orientation upon encountering the 660 km discontinuity.

Presenting Author: Eric Kiser

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