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Subduction Zone Events Around Japan and Wavefield Anomalies - Structure Beyond Tomography

Description: 

The dense networks of seismic stations across the Japanese archipelago allow detailed investigation of the patterns of ground motion and the nature of the seismic wavefield, in an area with a complex configuration of subduction involving both the Pacific and Philippine Plates. Tomographic studies can delineate the general morphology of the subducted material because it has higher seismic wavespeeds than its surroundings. However, the wavespeed contrasts alone are not sufficient to duct high-frequency energy from deep events to the surface. Internal heterogeneity with longer correlation lengths (10-20 km) along slab than across (0.5-1.0 km) provides a good description of the behaviour, but the high frequencies require a boost from the low wavespeeds associated with a modified olivine wedge for the deepest events.  The presence of only a small area with high frequency waves from the 2015 Mw 7.9 Ogasawara event at 680 km depth indicates that this event lies outside the main slab.

During moderate to deep (35–260 km) earthquakes within the Philippine slab along the Ryukyu arc, distinctive later phases after S are observed across the Japanese archipelago for epicentral distances from 1500 km to 2200 km, producing anomalous amplification of ground motion in central and northern Japan, associated with reflections from the upper mantle discontinuities and the top of the Pacific slab. The character of these reflections indicates that the weak tomographic signal for the PAC slab below 200 km beneath western Japan is more likely associated with a thinned than a torn slab.

Session: Structure and Properties of Subducting Slabs and Deep Earthquakes

Type: Oral

Date: 4/19/2023

Presentation Time: 02:45 PM (local time)

Presenting Author: Brian L. N. Kennett

Student Presenter: No

Invited Presentation: 

Authors