Geoscience Reference
In-Depth Information
Tsunami Earthquakes
In 1972, H. Kanamori deined a special class of earthquakes, which he called “tsunami earth-
quakes,” whose tsunamis are signiicantly larger than expected from their seismic magnitudes,
especially conventional ones. Such events generally feature an exceptionally slow progression
of the seismic rupture along the earthquake fault and can be very treacherous because they
lack the high frequencies felt by humans in the near-ield, which serve as a natural warning
for local populations, while hiding in their enhanced low-frequency spectrum the capability
to generate disastrous tsunamis. Examples include the catastrophic events in Sanriku (Japan,
1896) and Unimak (Aleutian Islands, 1946). The real-time identiication of tsunami earthquakes
remains a challenge in modern tsunami warning, especially because these events are relatively
rare; only a dozen have been documented in the past 113 years with only ive since the advent
of modern digital seismometers.
A case study . On September 2, 1992, an earthquake occurred off-shore Nicaragua with
magnitudes m b = 5.3 and M s = 7.2. Note the disparity between the body- and surface-wave
magnitudes. The former meant that the earthquake was deprived of the high frequencies typi-
cal of ground shaking and felt by humans in the near-ield. Indeed, in some coastal communities,
the earthquake was not even felt by the population, who thus had no natural warning of the
impending disaster. Its higher surface-wave magnitude indicates a “red” source, enriched in low-
frequency energy, as was later conirmed by a Global Centroid-Moment-Tensor (CMT) solution
equivalent to M w = 7.6, measured at periods of 135 s. The earthquake generated a tsunami that
ran up to more than 10 m and killed 170 people on the shores of Nicaragua. 1 Similar scenarios
took place in Sanriku, Japan (1896; 27,000 dead), Java (1994, 2006), and Peru (1996); other
tsunami earthquakes have been described in the Kuril Arc (1963, 1975), the Aleutians (1946), and
Tonga (1982). 2
A major challenge regarding tsunami earthquakes is to identify them in real time from
their seismic records. Once an estimate of the seismic moment is obtained, the earthquake
is analyzed for possible extended source duration by computing an estimate of the high-
frequency energy carried in its P-waves. The result allows a comparison between the behavior
of the source in the bass and treble parts of its spectrum, and if an anomaly is detected,
identiies the earthquake as a violator of scaling laws, that is, as a tsunami earthquake, whose
tsunami potential is greater than would be expected by its initial seismic waves. This algorithm,
which uses the concept of the slowness parameter θ, 3 has been implemented at the Paciic
Tsunami Warning Center (PTWC). 4 It was used to successfully identify in real time the slowness
of the Java earthquake of July 17, 2006.
Another, more general challenge is to understand the origin of the anomalous rupture
in tsunami earthquakes and in particular in what geological environments they can occur. At
least two different (and somewhat contradictory) scenarios have been proposed, involving the
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