Ultralong Period Seismic Study of the December 2004 Indian
Ocean
Earthquake and Implications for Tsunami Hazards
Seth Stein
Department of Geological Sciences
Northwestern University, Evanston IL
The December 2004 Sumatra earthquake was the third largest earthquake since the
seismometer was invented about hundred years ago. Hence it is
being studied extensively to understand how and where such rare giant
earthquakes occur and how to improve warning systems for the devastating
tsunamis that can result. Analysis of the earth's longest period normal modes
of oscillation, with periods of 20 minutes or longer, shows that the earthquake
was much
larger and involved slip on a much longer fault than initially inferred. The
normal modes indicate that the 1200 km long aftershock
zone slipped by about 10 m, consistent with the large tsunami amplitudes in
Thailand, Sri Lanka, and India because tsunamis have maximum
amplitude at right angles to the fault. Because the entire aftershock zone
slipped, strain accumulated from subduction of India beneath Burma
on this part of the plate boundary has been released. This has important
implications for reconstruction because it leaves no immediate
danger of a similar oceanwide tsunami from slip on this segment of the plate
boundary for hundreds of years. However, stress transferred by the
earthquake increased the danger of a large tsunami resulting from a great
earthquake on segments of the Sumatra trench to the south, as in
fact occurred in March 2005. The December earthquake was much larger than
expected from a previously proposed relation, based on ideas about
the mechanics of the interface between the overriding and subducting plates, in
which such earthquakes and hence oceanwide tsunamis occur
only when young lithosphere subducts rapidly. Moreover, a global reanalysis
finds little support for this correlation. Hence it appears
that much of the apparent differences between subduction zones, such as some
trench segments but not others being prone to M > 8.5 events and
hence oceanwide tsunamis, may be sampling artifacts from the short earthquake
history available to us. This possibility, which is
supported by the variability in rupture mode at individual trench segments,
means that many more trench segments may be capable of
generating oceanwide tsunamis.
