Reassessment of Focal Depths of Shallow Moderate-Sized Earthquakes in Mariana Subduction Zone with Short-Period Rayleigh-Wave Amplitude Spectra

Abstract Focal depths provide vital information for seismogenic mechanisms and tectonic structures. Because of sparse seismic stations in the Mariana subduction zone, accurately determining the focal depth of shallow earthquakes is challenging. The amplitude spectra of short-period Rayleigh waves are highly depth sensitive and are applicable at local and regional distances with sparse seismic station coverage. In this study, we collect seismic data at local and regional distances in the Mariana region, and then use the optimal period of short-period Rayleigh-wave amplitude spectra to determine the depths of shallow moderate-sized earthquakes. The difference between our resolved depths and the Incorporated Research Institutions for Seismology (IRIS) results ranges from −12.5 to 17.5 km, whereas the difference between our resolved depths and the Global Centroid Moment Tensor (Global CMT) depths has a range of −7.5 to 12.5 km. To further verify the estimated source depth, we used depth phase modeling to constrain the source depth of the largest Mw 5.9 earthquake. The result of depth phase modeling (27.9 ± 0.3 km) is slightly larger than the depth of 24.4 ± 1.4 km using the optimal period method. In comparison, the estimated depth is quite different from the fixed values of 10 and 12 km in the IRIS and Global CMT catalogs, respectively. The study demonstrates the efficacy of this approach on shallow offshore earthquakes and the need to reassess the depth of earthquakes in the Mariana subduction zone.