Foundation, Concrete and Earthquake Engineering

Tectonic Summary of Magnitude 7.1 - Near the East Coast of Honshu, Japan 07/04/2011

Last month's quake (Mw= 8.9 Earthquake- Honshu, Japan) struck at 32km deep near the east coast of Honshu, Japan. More than 12,700 people are known to have died in the disaster and nearly 15,000 people remain unaccounted for. Hundreds of thousands have been made homeless.

The April 7, 2011 earthquake near the east coast of Honshu, Japan occurred as a result of thrust/reverse faulting on or near the subduction zone plate boundary between the Pacific and North America plates. At the latitude of this earthquake, the Pacific plate moves approximately westwards with respect to the North America plate at a rate of 83 mm/yr, and begins its westward descent beneath Japan at the Japan Trench. Note that some authors divide this region into several microplates that together define the relative motions between the larger Pacific, North America and Eurasia plates; these include the Okhotsk and Amur microplates that are respectively part of North America and Eurasia.

The epicenter and focal-depth of the April 7 earthquake are consistent with the event having occurred very close to the main interface thrust-fault of the subduction zone plate boundary. Preliminary focal-mechanisms, however, imply slip on a fault with steeper dip than that of the main interface thrust-fault, which may imply an intraplate source is more likely.

Legend for Earthquake Location FIG: Major Tectonic Boundaries: Subduction Zones -purple, Ridges -red and Transform Faults -green 
Earthquake Location:  38.25 N 141.64E Depth: 25.6 Km

Earthquake Location:  38.25 N 141.64E Depth: 25.6 Km

This earthquake can be considered an aftershock of the March 11, 2011 Mw 9.0 Tohoku earthquake. The aftershock sequence of that event has been ongoing since March 11, and has included 58 earthquakes of M 6 or greater up until April 7 2011, two of which were greater than M 7 (M7.7 and M7.9, both on March 11). Over the two days preceding the March 11 earthquake, a series of large foreshocks had occurred, beginning on March 9th with a M 7.2 event approximately 40 km from the epicenter of the March 11 earthquake, and continuing with another three earthquakes greater than M 6 on the same day. Prior to March 9 2011, the Japan Trench subduction zone had hosted nine events of magnitude 7 or greater since 1973. See the tectonic summary of the March 11 event for more details of the historic seismicity in this region.

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