Geoscience Reference
In-Depth Information
earthquake, but also by material conditions at the source, such as source focal geometry, earth-
quake source depth, and water depth above the fault-rupture area.
Although estimating the size of a tsunami based on the magnitude of an earthquake has
severe limitations (see Appendix G), the initial warning from a seismically generated tsunami is
still based on the interpretation of the parent earthquake for several reasons:
•
most tsunamis are excited (or initiated) by earthquakes;
•
earthquake waves are easy to detect, and seismic instrumentation is available, plenti-
ful, and accessible in near-real time (latencies of seconds to a few minutes);
•
most importantly, seismic waves travel faster than tsunamis by a factor of 10 to 50,
thereby allowing an earthquake to provide an immediate natural warning for people
who feel it while leaving time for instrumental seismology to trigger oficial warnings
for coasts near and far from the tsunami source; and
•
earthquakes have been studied, and their sources are reasonably well understood.
Although most tsunamis result from earthquakes, some are triggered by landslides or
volcanic eruptions. Technological warning of a tsunami that has been generated without a
detectable earthquake will likely require detection of the tsunami waves themselves by water-
level gauges.
Seismic Networks Used by the Tsunami Warning Centers
Both TWCs access the same extensive seismic networks that provide near-real-time infor-
mation on earthquakes from around the world. Currently, about 350 independent channels
of seismic data are monitored and recorded by the TWCs (National Oceanic and Atmospheric
Administration, 2008a; Figure 4.1). Seismic networks that provide these data are operated and
funded by many different agencies and organizations, including the U.S. Geological Survey
(USGS), the National Science Foundation (NSF), the National Tsunami Hazard Mitigation Pro-
gram (NTHMP), the UN Comprehensive Nuclear Test-Ban Treaty Organization (CTBTO), various
universities in the United States, non-U.S. networks, and stations run by the Paciic Tsunami
Warning Center (PTWC) and the West Coast/Alaska Tsunami Warning Center (WC/ATWC) them-
selves. Many of the networks used by the TWCs are part of the USGS/NSF Global Seismographic
Network (GSN), which currently comprises more than 150 globally distributed, digital seismic
stations and provides near-real-time, open access data through the Data Management System
(DMS) of the Incorporated Research Institutions for Seismology (IRIS). The IRIS DMS also serves
as the primary archive for global seismic data. GSN is a partnership between the NSF/IRIS and
the USGS. The TWCs access seismic network data through dedicated circuits, private satellites,
and the Internet.
The GSN is widely recognized as a high-quality network, having achieved global coverage
adequate for most purposes, with near-real-time data access as well as data quality control
and archiving (National Science Foundation, 2003; Park et al., 2005). GSN stations have proven
