Deep earthquakes

Tsuchiyama, A., & Nakajima, J. (2021). Diversity of deep earthquakes with waveform similarity. Physics of the Earth and Planetary Interiors, 314, 106695. (https://doi.org/10.1016/j.pepi.2021.106695)

Deep earthquakes occur at depths of > 70 km along the Wadati-Benioff zone in a subducting slab. The depth range of the seismicity generally depends on environmental factors of the slab, such as the plate age, temperature, and convergence rate.

I am interested in the mechanism of deep earthquakes and their diverse characteristics in different subduction zones. In my studies, I focused on two specific regions; central Japan (working at Tokyo Tech) and Bucaramanga in Columbia (working at MIT).

central Japan

Having utilized the dense network of seismic stations on the land, we measured source parameters of similar earthquakes through a spectral-ratio method. Consequently, our result suggests that the dominant rupture mechanism of deep earthquakes could be diverse even for similar events on the same fault plane [Tsuchiyama & Nakajima, 2021].

bucaramanga Nest in colombia

This is my first project for my PhD at MIT. The Bucaramanga Nest has several distinctive features such as intense seismicity, variations of focal mechanisms including non-DC components, and no clear evidence for the aftershocks (although the other intermediate-depth earthquake nests indicate aftershock sequences...!). Currently, I am working on a systematic detection of intermediate-depth earthquakes in the Bucaramanga Nest to portray spatio-temporal seismicity more precisely.

What is a scientific mystery about deep earthquakes?

The generation mechanism is puzzling. Deep earthquakes are unique because they occur in a deeper portion of the subduction zone under extremely high temperature and pressure conditions. Because of the extreme environment, some seismologists assume that the mechanism of deep earthquakes is different from that of shallow earthquakes. Other seismologists expect that there is a standard mechanism to trigger earthquakes in shallow and deep regions.

Thanks to advanced technology, we are able to observe earthquakes even at a depth of > 600 km. The histogram of earthquakes in subduction zones shows a bimodal trend of the event number along the depths. How could we explain this unique feature of deep earthquakes? It is still a mystery...

Which part of the world has many deep earthquakes?

The most active region is the Tonga-Fiji subduction zone, where the largest-deep earthquake (Mw 8.2 Tonga earthquake) occurred in 2018. This subduction zone generates the majority of deep earthquakes around the world.

The Bucaramanga Nest in Colombia has the most highly-productive activity of intermediate-depth earthquakes. There are two other nests where intermediate-depth earthquakes intensively occur around the world: Valencia and Hindu-Kush nest.

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