Can Mantle Transition Zone beneath the Caroline Plate, Equatorial Western Pacific Be Resolved Seismologically Using Available Dataset?

Abstract:

The Caroline Plate, located in the equatorial Western Pacific, appears to be underlain by a stagnant slab. Even though the plate is surrounded by seismic zones of deep- and intermediate-depth earthquakes (originating from the Philippines, Banda Sea, Mariana and New Britain Island), available seismic dataset of source-receiver pair is limited due to the scarcity of land-based seismic stations. In particular, seismic stations are limited in the western and eastern sides of the Caroline Plate. In this study, we perform investigations on the seismic resolution using waveform inversion approach. Earthquake data sets in the last decade obtained from publicly available seismic stations were used to examine the presumed mantle transition zone (MTZ), whose depth ranges from 410 to 660 km in three dimensions. Our analysis shows that, although the current coverage may be marginally sufficient to assess the presence or absence of stagnant slab, the number of ray paths, especially towards the equator, is too limited to obtain high-resolution three-dimensional structure. Hence, it is important to install additional seismic stations (most likely, in the form of the broadband ocean bottom seismometers) around the Caroline Plate. A number of optimal sites are suggested based on our resolution tests. It is hoped that by placing additional ocean bottom seismometers several important issues regarding the nature of MTZ beneath Caroline Plate can be explored that could not be addressed by global tomography models alone such as its origin.