Edge wave and non-trapped modes of the 25 april 1992 Cape Mendocino tsunami

The 25 April 1992 Cape Mendocino earthquake generated a tsunami characterized by both coastal trapped edge wave and non-trapped tsunami modes that propagated north and south along the U.S. West Coast. Both observed and synthetic time series at Crescent City and North Spit are consistent with the zero-order edge wave mode solution for a semi-infinite sloping beach depth profile. Wave amplitudes at Crescent City were about twice that observed at North Spit, in spite of the fact that the source region was three times farther from Crescent City than North Spit. The largest observed amplitude was due to an edge wave which arrived almost three hours after the initial onset of the tsunami; since such waves are highly localized nearshore, this suggests that the enhanced responsiveness at Crescent City is at least partly due to local dynamic processes. Furthermore, the substantially delayed arrival of this wave, which was generated at the southern end of the Cascadia Subduction Zone, has significant implications for hazard mitigation efforts along the entire U.S. West Coast. Specifically, this study demonstrates that slow-moving but very energetic edge wave modes could be generated by future large tsunamigenic earthquakes in the CSZ, and that these might arrive unexpectedly at coastal communities several hours after the initial tsunami waves have subsided.     

Progress in NTHMP Hazard Assessment

The Hazard Assessment component of the U.S. National Tsunami Hazard Mitigation Program has completed 22 modeling efforts covering 113 coastal communities with an estimated population of 1.2 million residents that are at risk. Twenty-three evacuation maps have also been completed. Important improvements in organizational structure have been made with the addition of two State geotechnical agency representatives to Steering Group membership, and progress has been made on other improvements suggested by program reviewers.     

Real-Time Tsunami Forecasting: Challenges and Solutions

A new method for real-time tsunami forecasting will provide NOAAs Tsunami Warning Centers with forecast guidance tools during an actual tsunami event. PMEL has developed the methodology of combining real-time data from tsunameters with numerical model estimates to provide site- and event-specific forecasts for tsunamis in real time. An overview of the technique and testing of this methodology is presented.     

Extraction of tsunami source coefficients via inversion of DART \circledR^{\circledR} buoy data

The ability to accurately forecast potential hazards posed to coastal communities by tsunamis generated seismically in both the near and far field requires knowledge of so-called source coefficients, from which the strength of a tsunami can be deduced. Seismic information alone can be used to set the source coefficients, but the values so derived reflect the dynamics of movement at or below the seabed and hence might not accurately describe how this motion is manifested in the overlaying water column. We describe here a method for refining source coefficient estimates based on seismic information by making use of data from Deep-ocean Assessment and Reporting of Tsunamis (DART ^\circledR^{\circledR}) buoys (tsunameters). The method involves using these data to adjust precomputed models via an inversion algorithm so that residuals between the adjusted models and the DART ^\circledR^{\circledR} data are as small as possible in a least squares sense. The inversion algorithm is statistically based and hence has the ability to assess uncertainty in the estimated source coefficients. We describe this inversion algorithm in detail and apply it to the November 2006 Kuril Islands event as a case study.     

The NTHMP Tsunameter Network

A tsunameter (soo-NAHM-etter) network has been established in the Pacific by the National Oceanic and Atmospheric Administration. Named by analogy with seismometers, the NOAA tsunameters provide early detection and real-time measurements of deep-ocean tsunamis as they propagate toward coastal communities, enabling the rapid assessment of their destructive potential. Development and maintenance of this network supports a State-driven, high-priority goal of the U.S. National Tsunami Hazard Mitigation Program to improve the speed and reliability of tsunami warnings. The network is now operational, with excellent reliability and data quality, and has proven its worth to warning center decision-makers during potentially tsunamigenic earthquake events; the data have helped avoid issuance of a tsunami warning or have led to cancellation of a tsunami warning, thus averting potentially costly and hazardous evacuations. Optimizing the operational value of the network requires implementation of real-time tsunami forecasting capabilities that integrate tsunameter data with numerical modeling technology. Expansion to a global tsunameter network is needed to accelerate advances in tsunami research and hazard mitigation, and will require a cooperative and coordinated international effort.     

A method for estimating casualties due to the tsunami inundation flow

This study develops a method for estimating the number of casualties that may occur while people evacuate from an inundation zone when a tsunami has inundated an area. The method is based on a simple model of hydrodynamic forces as they affect the human body. The method uses a Tsunami casualty index (TCI) computed at each grid point of a numerical tsunami model to determine locations and times within the tsunami inundation zone where evacuation during the tsunami inundation is not possible and therefore where casualties are likely to occur. The locations and times can be combined with information about population density to compute the potential number of casualties. This information is useful in developing tsunami evacuation routes that avoid such locations. To illustrate the method, it is applied to the Seattle waterfront in Washington State, USA, that is under the threat of possible tsunami disasters due to Seattle Fault earthquakes. Preliminary results suggest that the tsunami casualties may occur within the Seattle waterfront for 15 min, during the time interval from 3 to 18 min after a large Seattle Fault tsunami is generated when the background tide level is mean high water.