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Andrew Alden

8.0 Earthquake in Samoa

By September 29, 2009

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Early Tuesday local time, a major earthquake struck the western Pacific near Samoa. A tsunami watch was put into effect. Updates as the day progresses. Follow developments on the USGS's dedicated page for this event. The location of the quake is at the sharp bend at the north end of the Kermadec subduction zone, south of the Samoan Islands. Typical events are shallow thrust quakes, but this was east of the subduction zone and had a different mechanism.

UPDATES: First analyses point to an "energy magnitude" Me of 7.8 (later raised to 8.1) and hypocenter depth of about 35 km. CNN reported a 3-meter tsunami in Pago Pago, Samoa, which caused a good deal of damage. The Pacific Tsunami Warning Center reported a 1.6-meter tsunami there. Hawaii and the rest of the Pacific got a negligible tsunami.

The magnitude was raised from 7.9 to 8.0 by the USGS. Focal mechanisms are in at the USGS page; it was a normal event, meaning that the fault's hangingwall moved down upon the footwall. And the depth has been better determined at about 10 km.

The European Geofon seismic monitoring network put the magnitude of the quake at 8.2. An AP story cited 8.3 from the tsunami center's bulletin. Twitterers enthusiastically used the highest available number. But other than that, I must say that Twitterers passed around sound news and corrected themselves quickly. @Hypocentre pointed out that the various agencies are estimating surface-wave magnitude Ms (8.0), moment magnitude Mw (8.3) and energy magnitude Me (8.1).

The tsunami watch and warning was lifted six hours later. Hawaii has recorded tsunamis no higher than 50 centimeters. More than a hundred people have died in Samoa. As the aftershocks fade and Samoa resumes ordinary life, scientists will remain interested in this earthquake because it is very large for a normal-faulting event.

Friday, Chris Rowan posted a nice explanation of this earthquake and the Indonesian quake the next day.

What are earthquakes?
The three types of fault
Earthquake focal mechanisms
About subduction zones
About tsunamis


October 10, 2009 at 4:05 pm
(1) tree says:

Truly too bad for such beautiful places to be destroyed, but … we are definitely living in a time of “climatic changes & challenges.” Good thing the Bible tells us about it … and the better days coming before too long.

October 10, 2009 at 8:32 pm
(2) Geology Guide says:

How convenient that earthquakes, which occur at all times, can be relied on to justify placid attitudes among good Christians.

October 26, 2012 at 7:26 am
(3) AA says:

Turns out this was in fact a set of 3 earthquakes:
the normal event and another 2 thrust events of magnitude 7.8 further south, triggered by the normal event. These 2 events might be the reason for the bulk of the tsunami, as they were very shallow for thrust events and their wave patterns might produce impressive wave heights at places, due to diffraction.

And I didn’t read it in the bible, you spammers.

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