Sunday 28 July 2019

Magnitude 6.5 Earthquake off the south coast of Honshū Island, Japan.

The Japan Meteorological Agency recorded a Magnitude 6.5 Earthquake at a depth of about 420 km, off the south coast of the Honshū Island, slightly after 3.30 am Japan Standard Time on Sunday 28 July 2019 (slightly after 6.30 pm on Saturday 27 July GMT). The event was felt across much of Honshū Island, though there are no reports of any damage or casualties. This is roughly what would be expected from an Earthquake of this size at this depth; the quake is big enough to be felt over a wide area, but most of its energy has dissipated before the shock-waves reach the surface.

The approximate location of the 28 July 2019 Honshū Earthquake. USGS.

Japan has a complex tectonic situation, with parts of the country on four different tectonic plates. Eastern Honshū area lies on the boundary between the Pacific, Eurasian and Philippine Plates, where the Pacific Plate is passing beneath the Eurasian and Philippine Plates as it is subducted into the Earth. This is not a smooth process; the rocks of the two plates constantly stick together, only to break apart again as the pressure builds up, causing Earthquakes in the process. 
 The movement of the Pacific and Philippine Plates beneath eastern Honshū. Laurent Jolivet/Institut des Sciences de la Terre d'Orléans/Sciences de la Terre et de l'Environnement.
Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.
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