Geoscience Reference
In-Depth Information
have been proposed for detecting tsunamis in the wake of the Indian Ocean event of 2004.
Although potentially promising, there has not been any demonstration of a viable operational
alternative to the current systems, perhaps due to lack of funding. In general, most alternatives
are not adequately sensitive to serve as a replacement for present technologies, with which
small waves (<1 cm) can be observed and used for wave model inputs, ine-tuning of forecasts
and warnings (including cancellation of warnings), and tsunami research. Nevertheless, contin-
ued research and development may prove fruitful. The descriptions below of some interesting
technologies and methodologies are provided simply to indicate possibilities and should not
be interpreted as endorsements of their utility by this committee.
Duration of High-Frequency P-Waves for Earthquake
Moment Magnitude Estimation
Because of the dificulty of obtaining reliable estimates of seismic moments at the long
periods relevant to tsunami generation, research is needed to explore the possibility of using
other methods, possibly drawing on different technologies, in order to improve the accuracy of
moment estimates, and the ability to detect unusual events, such as tsunami earthquakes.
One approach to the near-real-time investigation of large seismic sources consists of target-
ing their duration in addition to their amplitude. The comparison between the amplitude and
duration reveal violations of scaling laws (e.g., slow events such as tsunami earthquakes). Follow-
ing the 2004 Sumatra earthquake, Ni et al. (2005) noted that source duration can be extracted
by high-pass iltering of the P-wave train at distant stations, typically between 2 and 4 Hz. Only
P-waves escape substantial inelastic attenuation, so that this procedure eliminates spurious
contributions by later seismic phases and delivers a “clean” record of the history of the source.
This approach has been pursued recently by Lomax et al. (2007) and Okal (2007a). In par-
ticular, the latter study has applied techniques initially developed in the ield of seismic source
discrimination (of manmade explosions as opposed to earthquakes) to characterize the dura-
tion of the source through the time
τ
1/3
over which the envelope of the high-frequency P-wave
is sustained above one third of its maximum value. It is shown, for example, that this approach
would have clearly recognized the 2004 Sumatra earthquake as a great earthquake, or the
2006 Java tsunami earthquake rupture as exceptionally slow. In addition, alternative methods
for the rapid identiication of source duration of major earthquakes are presently the topic of
signiicant research endeavors, e.g., by Lomax et al. (2007) and Newman and Convers (2008).
The high-frequency band of the Sumatra earthquake was recorded in Japan using the
Hi-Net seismic array comprising 700 borehole instruments at an approximate 20 km spacing.
Ishii et al. (2005) used the data from the array to produce back-projected images of the earth-
quake rupture over approximately eight minutes across a 1,300 km long aftershock region
including both the slip history and overall extent of the seismic zone. A comparison of the sub-
sequent fault image, when compared to previous great earthquakes, supported the hypothesis
that the moment magnitude of the earthquake was 9.3—the largest earthquake ever recorded
with modern seismic instruments. The authors believe that such images of the aftershock
