Geoscience Reference
In-Depth Information
(see below). In the present coniguration of worldwide networks, the large number of avail-
able stations provides robust location determination, although losing a signiicant number of
seismic stations could affect the accuracy of earthquake location and depth.
A great earthquake on a subduction thrust tends to nucleate beneath shallow water, or
even beneath land in the case of the giant 1960 Chile and 1964 Alaska earthquakes. The source
of such an earthquake, and of the ensuing tsunami, extends far beyond the earthquake's point
of nucleation (the hypocenter, on the fault plane; the epicenter, if projected to the earth's
surface). What matters for earthquake size, and for tsunami size as well, is the fault-rupture area,
which extends seaward into deep water as well as coastwise. The hypocenter is much like the
match that initiates a forest ire in which the damage depends on the total area burned. The
tendency to instead equate an earthquake with its hypocenter contributed to confusion dur-
ing the near-ield tsunami from the February 27, 2010, Chilean earthquake of magnitude 8.8.
Partly because this earthquake's hypocenter was located near the coast, the Chilean govern-
ment retracted a tsunami warning before the largest waves came ashore.
Depth determination is crucial to assessing an earthquake's tsunamigenic potential
because sources deeper than about 60 km generally pose no tsunami threat and are well
resolved by location algorithms. Finer resolution of depth for shallower earthquakes remains a
general seismological challenge, particularly in near-real time. This parameter can have some
inluence on the generation of tsunamis in the near-ield; however, for far-ield tsunamis gener-
ated by megathrust earthquakes, theoretical studies (Ward, 1980; Okal, 1988) have shown that
the probability of tsunami excitation is moderate for depths less than 60 km. This somewhat
paradoxical result relects the fact that a shallower source may create a locally larger deforma-
tion of the ocean loor, but over a smaller area. This acts to compensate for the effect on the
generation of the tsunami, which is controlled by the integral of the deformation over the
whole ocean loor. Given the techniques and data available, the committee found that the loca-
tion techniques used at the TWCs (Weinstein, 2008; Whitmore et al., 2008) were adequate in the
context of tsunami warning.
Determining an earthquake's magnitude is a more problematic aspect of the initial earth-
quake parameterization. The concept of magnitude is probably the most popular, yet most
confusing, parameter in seismology. In simple terms, it seeks to describe the size of an earth-
quake with a single number. Reliable and well-accepted determinations of earthquake size (the
“moment tensor solution”—or the product of fault area with the amount of slip) are possible,
but these estimates are necessarily based on long-period surface waves arriving too late to be
useful for tsunami warning, which strives for initial estimates within ive minutes of the irst
measurements having been received. Most seismologists agree that it is not currently possible
to predict how much of a fault will ultimately break based on the seismic waves propagating
away from the point of nucleation (the epicenter), and that only when the slip ends can the
true size or moment be inferred. For an event such as the Sumatra earthquake, the propaga-
tion of breakage along the fault surface alone takes nearly eight minutes (e.g., de Groot-Hedlin,
2005; Ishii et al., 2005; Lay et al., 2005; Tolstoy and Bohnenstiehl, 2005;
Shearer and B�rgmann,
2010). Magnitudes determined at shorter times will necessarily underestimate the true size of
the earthquake.
