Background and Scope - Tohoku, Japan Earthquake and Tsunami of 2011 - page 4

references the parameters against a pre-computed database of over 100,000
tsunamigenic scenarios to issue a tsunami forecast (JMA 2011).
In recent events, up to and including the Great East Japan earthquake, the tsunami
forecast has included a specific estimate of expected wave heights. JMA is currently
evaluating this practice — for large earthquakes, initial magnitude estimates (usually
low) tend to fall below actual tsunami-generated wave heights. In the 2011 event,
JMA estimated an initial earthquake magnitude of 7.9 with expected waves of up to 3
meters; however, based on later seismic data, JMA revised the magnitude upward,
eventually forecasting wave heights in excess of 10 meters. (Cyranoski 2011)
With its warning system activated, JMA uses additional data ground motion and
wave data to update the tsunami forecasts and warnings. An important data source for
the warning system is a series of coastal GPS wave buoys operated by Japan's Port
and Airport Research Institute. These buoys report wave motion in real time. Other
wave data sources include a national system of tide gauges and sea bed pressure
sensors.
In contrast to Japan's system, the U.S. maintains separate but coordinated earthquake
and tsunami warning centers. The National Earthquake Information Center (NEIC),
part of the U.S. Geological Survey (USGS), Department of the Interior, serves as an
earthquake warning and information center. The NEIC relies on a combination of
national and worldwide seismic data, automated tools, and human review to issue
moment magnitude and location information as soon as possible after an event. The
NEIC is currently developing an earthquake early warning system.
The National Weather Service, part of the National Oceanographic and Atmospheric
Administration (NOAA) under the Department of Commerce, operates two tsunami
warning centers. The Pacific Tsunami Warning Center (PTWC) takes responsibility
for most of the Pacific Basin and interim responsibility for portions of the Caribbean,
Indian Ocean, and South China Sea. The West Coast Alaska Tsunami Warning
Center (WC/ATWC) responsible for the coastline of the continental U.S., parts of
Canada, and Alaska. Both of the U.S. tsunami warning centers use seismic data either
from NEIC or from any global member of the Tsunami Warning System (TWS) to
determine the earthquake hypocenter (location, depth) and magnitude. For an
underwater earthquake at a depth less than 100 km and above a certain magnitude
threshold, the center issues an appropriate tsunami message. As more seismic and
wave data become available, the center employs additional methods to measure the
earthquake's tsunamigenic potential. Additional sources of wave data include tide
gauges and NOAA's Deep-ocean Assessment and Reporting of Tsunami (DART)
buoys, a network of 39 stations throughout the Pacific and western Atlantic basins.
The warning centers use NOAA's Pacific Marine Environmental Laboratory (PMEL)
propagation models to estimate tsunami arrival times. NOAA is currently developing
a real-time tsunami detection and forecasting system.