Friday focal mechanisms

A post by Chris RowanSince I missed last Friday, this week features the significant earthquakes from the past two weeks. Hopefully the maps help in understanding what’s going on; I’ll be continuing to develop this format in the coming weeks.

Sunday 4th July: Magnitude 6.2, E Coast of Honshu, Japan, Depth 25 km

The focal mechanism indicates NW-SE compression. The rupture is located behind a trench where the Pacific plate is being subducted west beneath the Eurasian plate, and is at approximately the depth of the subduction interface.
USGS Page

Wednesday 7th July: Magnitude 5.5, S California, Depth 16 km

The focal mechanism indicates a strike-slip earthquake, and it’s probable association with the San Andreas Fault system suggests that is is the result of dextral strike slip along a NW-SE trending fault (identified in reports as the San Jacinto Fault). This earthquake is of interest due to a proposed correlation to April’s magnitude 7.2 earthquake further south in Baja California. This placed additional stress on faults in southern California, possibly making them more likely to rupture.
USGS Page

Saturday 10th July: Magnitude 6.3, S of Mariana Islands, Depth 10 km

This focal mechanism indicates strike-slip motion. The rupture is located in the Pacific plate, at a point before it is subducted beneath the Philippine plate at the Marianas trench. If you look at the bathymetry on Google Earth, a NW-SE trending linear feature can be traced through the rupture point; this is possibly an old fracture zone that has been reactivated by compression of the Pacific plate as it approaches the trench.
USGS Page

Monday 12th July: Magnitude 6.2, Antofagasta, Chile, Depth 114 km

The focal mechanism indicates E-W extension, which is somewhat surprising given that this rupture is located inland from the Chilean subduction zone. This could either represent bending of the subducting plate as it is thrust underneath South America, or gravitational collapse in the forearc. The focal depth of 114 km suggests the latter is probably more likely.
USGS Page

Wednesday 14th July: Magnitude 6.5, Bio Bio, Chile, Depth 29 km

The focal mechanism indicates E-W compression. Its location just inland from the Chilean trench, and the relatively shallow rupture depth, clearly associates this earthquake with subduction on the Chilean margin.
USGS Page

Categories: earthquakes, geophysics
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Comments (2)

  1. Hypocentre says:

    For the intermediate depth Antofagasta event, I’d go with slab pull as a mechanism rather than plate bending.

    The google maps to help to add context.