Geoscience Reference
In-Depth Information
In this regard, the major challenge for tsunami warning is that tsunamis are controlled by
the lowest frequency part of a seismic source, with periods of 500 to 2,000 seconds, whereas
routinely recorded seismic waves have energy in the treble domain, with periods ranging from
0.1 to 200 seconds, exceptionally 500 seconds. In addition, seismic waves fall into several cat-
egories. Body waves travel through the interior of the earth at average velocities of 10 km/sec,
take seconds to minutes to reach recording stations, and their high-frequency components
are a good source of information. By contrast, surface waves travel around the surface at con-
siderably slower speeds (3-4 km/sec) and take as much as 90 minutes to reach the most distant
stations. The surface waves carry low-frequency signals; that is, the part of the spectrum most
relevant to tsunami warning, although high-frequency body wave methods can also resolve
event duration and rupture length (e.g., Ishii et al., 2005; Ni et al., 2005). For this latter case, the
high-frequency body waves have not yet been exploited by the USGS's National Earthquake
Information Center (NEIC) or the TWCs. In short, the evaluation of earthquake size for tsunami
warning faces a double challenge: extrapolating the trebles in the earthquake source to infer
the bass, and doing this as quickly as possible to give the warning enough lead time to be
useful.
Magnitudes can be obtained from various parts of the seismic spectrum, and expectedly
such different scales have been “locked” to each other to quantify an earthquake with a single
number. This is achieved through the use of “scaling laws,” which assert that the spectrum of a
seismic source (the partitioning of its energy between bass and treble) is understood theoreti-
cally and can be estimated as a function of earthquake size. However, this universal character of
scaling laws is far from proven, especially in its application to mega-earthquakes, which trigger
the far-ield tsunamis of major concern. In addition, scientists have identiied a special class of
generally smaller events, dubbed “tsunami earthquakes” by Kanamori (1972), whose source
spectra systematically violate scaling laws (see Appendix G). Therefore, characterizing an earth-
quake source with a single number representing magnitude cannot describe all its properties,
especially in the context of tsunami warning.
A detailed technical review of these topics is given in Appendix G, and the special case of
tsunami earthquakes is reviewed in Appendix H. A summary of the conclusions of Appendix G
are:
•
Classical magnitudes routinely determined by conventional seismological methods
are inadequate for tsunami warning of great and mega-earthquakes.
•
The authoritative measurement of earthquake size, the moment tensor solution, is
based on normal modes and long-period surface waves arriving too late to be used
for tsunami warning.
•
The TWCs currently use an algorithm named
M
wp
which integrates the long-period
components of the irst arriving P-waves to infer the low-frequency behavior of the
seismic source.
•
PTWC has recently implemented the use of the “W-phase” algorithm as well as the
M
wp
algorithm.
