{"id":11611,"date":"2023-12-02T23:28:56","date_gmt":"2023-12-02T23:28:56","guid":{"rendered":"https:\/\/earthjay.com\/?p=11611"},"modified":"2023-12-30T21:22:50","modified_gmt":"2023-12-30T21:22:50","slug":"earthquake-report-m-7-6-offshore-of-the-philippines","status":"publish","type":"post","link":"https:\/\/earthjay.com\/?p=11611","title":{"rendered":"Earthquake Report: M 7.6 offshore of the Philippines"},"content":{"rendered":"

This morning (my time) I was awakened by the tsunami notification that I get on my phone. I lept into action.<\/p>\n

I get tsunami notifications so that I can respond to these potential tsunami events that may impact the coast of California as part of my job at the California Geological Survey<\/a>.<\/p>\n

There was a M 7.6 earthquake offshore of the island of Mindanao, Philippines. And, it was close to land (so a good chance that there may be casualties and damage). <\/p>\n

I immediately thought of the people who live or visit there and what they must be experiencing. It is frightening to experience such a large magnitude earthquake.<\/p>\n

https:\/\/earthquake.usgs.gov\/earthquakes\/eventpage\/us7000lff4\/executive<\/a><\/p>\n

This part of the world is tectonically complicated. The nation of Philippines is bordered by convergent plate boundaries called subduction zones. <\/p>\n

Subduction zones are where plates move towards each other. One of the plates dives down beneath the other plate. The interface of these plates is called the megathrust subduction zone earthquake fault.<\/p>\n

To the east of the Philippines is the Philippines trench, a an area of very deep water formed by the subduction zone there where the Philippine Sea plate subducts beneath the Sunda plate (part of Eurasia). To the west is the Manila trench, where the Sunda plate subducts eastwards beneath the Philippine Sea plate. <\/p>\n

Initially<\/strong>, it appeared that this earthquake may be a megathrust interface earthquake. However, the earthquake was too shallow to be on the subduction zone. The M 7.6 was actually somewhere in the upper plate.<\/p>\n

However, there have been quite productive series of aftershocks and triggered earthquakes. Most of these earthquakes are not really aftershocks, since they are on different faults. The largest magnitude earthquakes, other than the M 7.6, appear related to the megathrust (e.g., the M 6.4 and M 6.9).<\/p>\n

In fact, it appears that the M 7.6 has very few aftershocks. Most of the seismicity following the M 7.6 appear to be related to the megathrust triggered events.<\/p>\n

Given the depth of the earthquake, if there was to be a tsunami, it would probably be a smaller one.<\/p>\n

That is what came to be true. There is a tide gage near Davao City, in a location sheltered from a direct path from the M 7.4 epicenter (mapped location of the earthquake). <\/p>\n

The tsunami is evident in these tide gage data. I plot these data below:<\/p>\n


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UPDATE: 4 Dec 2023<\/p>\n

Above I write that the M 7.6 was likely on a thrust fault in the upper plate. This is based on the hypocentral (3-dimensional location of the earthquake) location relative to the USGS model of the subduction zone (“slab 2.0”). <\/p>\n

We may think about “splay faults,” faults that begin at depth along the megathrust, but split off as separate faults and, possibly, reach the surface of the seafloor.<\/p>\n

Dr. Judith Hubbard proposed an alternate hypothesis (see their tweet in the social media section below). The USGS slab 2.0 model may be too deep here. This is entirely possible, if not likely. <\/p>\n

If Hubbard’s hypothesis is correct, it would place the abundance of the sequence on faults within the subducting plate. <\/p>\n

This is also possible. However, I would have expected that the majority of these earthquakes would be along the megathrust, not within one of the plates.<\/p>\n

Below is my interpretive poster for this earthquake<\/font><\/H2><\/p>\n