Friday, March 11, 2011

The Big One clear across the Pacific

kw: observations, earthquakes, tsunamis, geology

At 11:46 AM local time (12:46 AM EST; 0546Z/UTC) the fifth-largest earthquake ever measured struck a long section of the subduction zone beneath the Japan Trench, measuring 8.8 or 8.9 on the Richter scale. There was a day of medium-size foreshocks (in the 6 range), and so far more than 100 aftershocks measuring 5 or stronger. This map from the USGS Earthquake Center shows a good summary compiled just before 1:00 PM EST (1800 Z/UTC) today. Click for a larger view.

The makings of an earthquake this size are evident on the map. The yellow blocks (almost hidden) denoting yesterday's foreshocks cover an area near the epicenter, as measured from a distance, of the giant quake. The actual area of fault that ruptured covers most of the area outlined by the blue blocks that denote today's aftershocks and the big one itself. It takes energy release along at least 200 miles (300+ km) of a fault, and throughout a crustal thickness of 5-10 miles (8-15 km), to produce such a quake. The magnitude 7.1 aftershock, the larger block more to the right on the map, required a rupture along a total fault length of about 10 miles (15 km).

When I studied earthquake geology some forty years ago, one of the first things I learned was that the intensity of local shaking caused by a "sixer", an earthquake in the range of 5.8-6.2, is as bad as it gets. Larger quakes spread similar levels of damage over larger and larger areas.

One question people are asking is why such a huge earthquake produced such a relatively feeble tsunami? The maximum wave measured in Japan, that I've read of so far, was 10 m (33 ft). The much larger tsunamis of a few years ago were produced by a much smaller earthquake.

This is primarily because of this quake's depth beneath the crustal surface. The hypocenter of the 8.9 temblor was 24.4 km (15.2 mi), and the aftershock hypocenters range from 16 to 45 km (10 to 28 mi) depth. This is nearly all beneath the "edge" of the oceanic crust. By the time the p- and s-waves reached the surface they had softened a bit, and there was much less vertical motion of the sea floor than a shallower event would cause. Secondly, there was no submarine landslide. The more frequent seismicity in and around Japan keeps most of the debris from accumulating on the slopes of the continental shelves. A huge slide was the major factor in the 2004 tsunamis.

A few hours ago, I was hearing of 32-33 dead in the tsunamis in Japan; a short while ago a CNN feed here at work reported "hundreds dead". Even a 10 m wave is nothing to despise. Though the quake's epicenter was about 100 km (60 mi) offshore, the waves took only 12 minutes to reach the coast, long before most people could get to higher ground, once they got up from being knocked over!

There is a lot of sorrow in Japan today. Let us offer a prayer of comfort for the survivors.

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