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January 2012 Issue

Start with Science to Predict Tsunami Risks, Seismic Hazards

By Dr. Marcia McNutt
U.S. Geological Survey

Over the last year, I have been reminded repeatedly that the past is the key to the present. If we are to have any hope of defining worst-case scenarios of extreme natural hazards, our definition of the past must go beyond human history and incorporate clues left in the rock record.

On March 11, a magnitude-9.0 earthquake off the coast of Japan generated a tsunami that inundated a large area on the northeast coast of Honshu, the country's main island, resulting in widespread devastation. The earthquake was one of the five most powerful recorded in the world since instrumental detection and record-keeping began around 1900. The resulting tsunami flooded dozens of coastal cities, numerous ports and the broad coastal plain around Sendai—the nearest major city to the earthquake, at a distance of 130 kilometers from the epicenter. The specter of devastation and loss of life recalled memories of the Sumatra-Andaman earthquake and tsunami that struck the Indian Ocean coastlines on December 26, 2004. These two events point compellingly to the need for a multidisciplinary approach to evaluate the potential magnitude and probability of incidents that are both humanitarian disasters and also affect jobs and commerce on the sea and at ports and harbors.

Geologic, Seismic Hazards Investigations
Global seismic networks, which provide crucial information on earthquake location and magnitude for the tsunami warning system, are critical for estimating tsunami-generating potential. Dense GPS-based networks record the accumulation of elastic strain along submarine faults, provided these faults are within about 100 kilometers from shore. Geologic investigations of drowned forests, turbidity flows in the deep ocean, and tsunami deposits and liquefaction evidence along the coast extend the historical record over a few millennia. An understanding of tectonic and sediment transport processes, which are responsible for submarine earthquakes and landslides, can help assess the potential for extreme events in areas where no historical or prehistoric evidence was found.

Tsunami Concerns for the Eastern United States
The U.S. Geological Survey (USGS) has conducted extensive studies in the northeastern Caribbean from 2005 to 2011 and in the Atlantic continental margin since 2007 as part of assessing distal threats to coastal communities in the eastern United States. There have been several earthquakes of magnitude 7 to 8 and three devastating tsunamis in the northeastern Caribbean in the past 150 years. The number of tsunami casualties there is nearly six times more than that of the U.S. West Coast, Hawaii and Alaska combined since 1842.

Multibeam bathymetry mapping of the northeastern Caribbean, conducted from 2002 to 2009, has laid the foundation for earthquake and tsunami hazard assessments of this region. This bathymetry has been augmented by high-resolution seismic reflection profiles, coring, installation of GPS stations on outer islands, re-evaluation of historical seismicity, a search for paleotsunami deposits and recording earthquake activity offshore with ocean-bottom seismometers. These assessments are being incorporated into USGS seismic hazard maps and National Tsunami Hazard Mitigation Program evaluations of select coastal locations in the Caribbean.

The potential hazard along the Puerto Rico Trench, a subduction zone that is similar in geometry and relative plate motion to the Sumatra-Andaman subduction zone, is also of prime interest to the assessment of tsunami hazards along the Atlantic continental margin of the U.S. and Canada.

Assistance to the Nuclear Regulatory Commission
The presence of nuclear power plants along the U.S. East Coast, in addition to the prospect of building new plants, has prompted the U.S. Nuclear Regulatory Commission in 2006 to ask the USGS for help assessing tsunami hazards in this region. The historical record includes 91 reported tsunamis in the Caribbean Basin since the 16th century, including the 1929 Grand Banks landslide tsunami, which impacted the Atlantic coast, and evidence of one possible tsunami affecting Long Island.

For the evaluation of extreme tsunamis that might affect critical infrastructure such as nuclear power plants, the probabilistic method becomes less defined, owing to a lack of data on very rare but high-impact tsunami sources such as enormous submarine landslides. Relating the maximum size of submarine landslides to earthquake magnitude may present an opportunity to evaluate a conservative landslide probability using the earthquake catalog.

The challenge to quantitative tsunami hazard assessment here is twofold. First, submarine landslides along the continental slope and outer rise are thought to constitute the primary potential tsunami source for the region, and the amplitude of a landslide-generated tsunami depends not only on landslide volume but also on speed of movement. However, geotechnical characterization and age determination of submarine landslides are inherently difficult, expensive and time-consuming. Second, the assessment has to consider a maximum possible run-up during a period of 10,000 years or longer, a time during which sedimentary processes on the margin have undergone profound changes. The USGS has mapped and assembled a multibeam bathymetry database for the margin that is under development, and continues to augment it with new high-resolution seismic reflection profiles, coring and deployment of ocean-bottom seismometers. The purpose of the studies is to determine the volume and absolute or relative ages of the landslides and to understand the geological conditions that promote slope failure.

Learning from Past US Tsunamis
After the 2004 Sumatra earthquake, the USGS Tsunami Source Working Group was created to define the geometry of the shallow interplate thrust faults in the western Pacific and to determine if there were geologic and geophysical factors that favor the generation of magnitude-8.5 or greater earthquakes. These objectives are part of an overall effort to build a database of tsunami propagation scenarios for simulations. The USGS has also had a more local focus on nearshore faults along the California coast, including the offshore San Andreas, Hosgri and the Newport-Inglewood fault zones, where multibeam bathymetry, high-resolution seismic reflection and near-bottom ROVs have been used to help assess their recurrence interval.

Natural hazards should ideally be evaluated probabilistically. This procedure has been applied to the evaluation of earthquake hazards in the U.S. and elsewhere, and forms the basis for the widely used USGS seismic hazard maps. The occurrence of very rare and very large earthquakes at subduction zones or very rare earthquakes in intraplate settings pose a challenge. Even more challenging is creating a probabilistic hazard assessment for tsunamis, which are still more rare than earthquakes.

In both these cases of extremely rare events, the underlying distribution and statistics necessary for hazards assessment is poorly constrained and the subject of active research. Tsunami hazard methods based on earthquake hazard analyses were introduced in 2008. These methods are probabilistic in nature and rely on geological information such as fault slip rates from long-term tectonic motion, rather than solely on the occurrence of historical events.

In 2005, the first-ever probabilistic tsunami inundation maps were created in a joint NOAA-USGS project for Seaside, Oregon, based on the Federal Emergency Management Agency's Flood Insurance Rate Map design probabilities.

Challenges in Making Probabilistic Hazards Assessments
Probabilistic hazard assessments for extreme events are only as good as the data used to construct them. The challenge for the coming years for the USGS and its research partners is to continue to collect and analyze quality geological and geophysical data in the U.S. continental margin that will contribute to earthquake, tsunami and landslide hazard assessments.

This challenge can only be met by sharing resources, such as ships with NOAA, the academic fleet and equipment with National Science Foundation-funded equipment pools, and by addressing the science needs of other government agencies such as NOAA, the U.S. Nuclear Regulatory Commis'sion, the Department of State and the Bureau of Ocean Energy Management.

For a list of references, visit http://bitly.com/vWuEBQ.

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