Early impressions of the Sendai earthquake

A great earthquake, moment magnitude 8.9, has occurred off the coast of Sendai, Japan. This earthquake was located on the Japan Trench, the plate boundary between the Okhotsk and Pacific plates. It appears to have ruptured southward from about latitude 39° N to 35°45′ N. If this is the case, one would expect significant shaking in Tokyo, as the earthquake rupture propagated in that direction. Indeed, early reports indicate this to be true.

This earthquake was preceded by a magnitude 7.2 earthquake two days ago near the epicenter. In light of today’s quake, that earthquake is considered a foreshock. It is possible that that earthquake triggered this larger event by increasing the stress on the fault at today’s epicenter. This is only conjecture at this point, and more detailed analysis will be done by researchers around the world in the coming months to investigate this possibility.

This earthquake did not occur in one of the closely-watched seismic gaps bordering Japan. The most hazardous of these is considered to be the Nankai Trough, about 800 km southwest of this earthquake. The region of this earthquake last ruptured in 1938 in a magnitude 7.8 earthquake somewhat south of the epicenter, while the epicentral region and the fault extending northward last ruptured in a series of earthquakes between 1897 and 1901 (the exact location and size of these events is unknown, as there were no seismometers at the time).

This earthquake was very shallow, only about 24 km from the surface at the hypocenter (the point at depth below the epicenter). It is quite likely that this earthquake ruptured all the way to the surface of the seabed, which is why it has generated such a significant tsunami.

At this time the full extent of the damage is unknown. The USGS PAGER system is estimating between 10 and 1000 deaths, and between $1 billion and $100 billion in damage, but these estimates are based on the shaking from the earthquake and do not include tsunami damage. The tsunami is forecast to reach Hawaii around 5am PST, and a tsunami warning is in effect there. A tsunami warning is additionally in effect for the Aleutian island chain in Alaska.

Had Seismic Warning Systems had regional earthquake warning system in place in Japan, it might have provided an estimated 10-20 seconds of warning in Sendai and the epicentral region, and as much as 45-60 seconds in Tokyo.

We will update this blog as information develops.

Gilead Wurman
Chief Seismologist

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