Localized water reverberation phases and its impact on back-projection images

Abstract

Coherent radiators imaged by backprojections (BP) are commonly interpreted as part of the rupture process. Nevertheless, artifacts introduced by structure related phases are rarely discriminated from the rupture process. In this study, we use a calibration event to discriminate between rupture and structure effects. We reexamine the waveforms and BP images of the 2012 M_w 7.2 Indian Ocean earthquake and a calibration event (M_w 6.2). The P wave codas of both events present similar shape with characteristic period of approximately 10 s, which are backprojected as coherent radiators near the trench. S wave BP does not image energy radiation near the trench. We interpret those coda waves as localized water reverberation phases excited near the trench. We perform a 2-D waveform modeling using realistic bathymetry model and find that the steep near-trench bathymetry traps the acoustic water waves forming localized reverberation phases. These waves can be imaged as coherent near-trench radiators with similar features as that in the observations. We present a set of methodologies to discriminate between the rupture and propagation effects in BP images, which can serve as a criterion of subevent identification.