← Back to Sessions

Observations of the Aftershock Sequences of Intermediate-Depth Earthquakes Beneath Japan

Description: 

The increasing pressure and temperature with increasing depth in the Earth should prevent earthquakes from occurring >50 km depth, yet they are observed occurring down to ~700 km. At intermediate depths, the two most commonly proposed mechanisms are dehydration embrittlement and thermal shear instability. These two mechanisms have different implications for aftershock production — dehydration embrittlement would lead to aftershock patterns similar to those observed for shallow earthquakes whereas thermal shear instability would result in few aftershocks — so observing and quantifying the temporal decay of intermediate-depth aftershock sequences can help distinguish the causative source mechanism. Japan's high earthquake activity, earthquake catalog with a low magnitude of completeness, and contrasting properties of the subducting Pacific and Philippine Sea Plates make it an excellent location to study intermediate-depth aftershock sequences and how different factors contribute to aftershock productivity. We analyze the 199 earthquakes in the JMA Unified Earthquake Catalog from 1985-2021 with MJ≥5.7 at 70-350 km depths and are able to fit the modified Omori's Law to quantify the temporal decay for 21 of the aftershock sequences. The aftershocks tend to be clustered around the mainshock, so an aftershock zone can be drawn that separates the aftershocks from the surrounding background seismicity. Earthquakes within this subjective aftershock zone are used to characterize the aftershock sequences. In comparison with shallow aftershock sequences, the characterized aftershock sequences have a similar temporal decay exponent p≅1 but much lower productivity. Preliminary results suggest that productivity is higher in the Pacific Plate than in the Philippine Sea Plate, in the upper plane of the double seismic zone than in the lower plane, and for shallower earthquakes. The characterized sequences last from ~40 days to >30 years. The characterized aftershock sequences are closer to our expectations for dehydration embrittlement than thermal runaway. Variations in productivity may relate to the amount of fluid available.

Session: New Insights into the Development, Testing and Communication of Seismicity Forecasts [Poster Session]

Type: Poster

Date: 5/2/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Linda

Student Presenter: No

Invited Presentation: 

Authors