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4 Tsunami Detection and Forecasting
Pages 109-162

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From page 109...
... tsunami warning centers ( TWCs) paying specific attention to the infrastructure of the earth and ocean observation networks and to the data processing and tsunami modeling that occur at the TWCs.
From page 110...
... coastal communities subject to uncertain tsunami warnings. Although both sea level gauge networks have already proven their value for tsunami detection, forecasting, and model development, fundamental issues remain concerning gaps in coverage, the value of individual components of the network, and the risk to the warning capability due to coverage gaps, individual component failures, or failures of groups of components.
From page 111...
... Looking to the future, the committee concludes that the numbers, locations, and prioritizations of the DART stations and coastal sea level gauges should not be considered static, in light of constantly changing fiscal realities, survivability experience, maintenance cost experience, model improvements, new technology developments, and increasing or decreasing international contributions. The committee finds of great value NOAA's continual encouragement and facilitation of researchers, other federal and state agencies, and nongovernmental organizations (NGOs)
From page 112...
... Technological warning of a tsunami that has been generated without a detectable earthquake will likely require detection of the tsunami waves themselves by waterlevel gauges. Seismic Networks Used by the Tsunami Warning Centers Both TWCs access the same extensive seismic networks that provide near-real-time information on earthquakes from around the world.
From page 113...
... The GSN is sufficiently robust to support warnings for events far from the recording devices and provides good global coverage (U.S. Indian Ocean Tsunami Warning System Program, 2007)
From page 114...
... Algorithms for determining the geographical location and depth of an earthquake source from seismic arrival times are based upon the concept of triangulation (U.S. Indian Ocean Tsunami Warning System Program, 2007)
From page 115...
... Reliable and well-accepted determinations of earthquake size (the "moment tensor solution" -- or the product of fault area with the amount of slip) are possible, but these estimates are necessarily based on long-period surface waves arriving too late to be useful for tsunami warning, which strives for initial estimates within five minutes of the first measurements having been received.
From page 116...
... • 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 Mwp which integrates the long-period components of the first arriving P-waves to infer the low-frequency behavior of the seismic source.
From page 117...
... With the availability of the new tsunami forecasting methods and sea level observations (as described below) , the TWCs rely more on sea level data and numerical models than on details of earthquake parameters after the issuance of the initial warning product.
From page 118...
... Conclusion: The complex seismic processing algorithms used by the TWCs, given the available seismic data, quickly produce adequate estimates of earthquake location, depth, and magnitude for the purpose of tsunami warning. The methodologies are inexact, partly because of the physically variable nature of tsunami-generating earthquakes (one model does not fit all)
From page 119...
... A T WC must, therefore, not only provide timely warning of a destructive tsunami, but also must avoid causing unnecessary evacuations with their attendant negative impacts. The detection and forecasting process requires real-time observations of tsunamis from both coastal sea level gauges and open-ocean sensors (such as provided by the DART stations)
From page 120...
... system into the TWCs (e.g., Weinstein, 2008; see Section Forecasting of a Tsunami Under Way) places additional emphasis on the importance of the proper operation of the sea level stations, especially the open-ocean DART stations whose sea level observations of the tsunami waves are not distorted by bathymetric irregularities and local harbor resonances that affect the coastal sea level observations.
From page 121...
... Therefore, a tsunami warning system should not only provide timely warning of a destructive tsunami, but also should avoid issuing "false alarms." Although the DART stations have their greatest value in discerning tsunami propagation characteristics in the open ocean, the inundation problem requires, ideally, sea level sensors along tsunami-prone coastlines because of the spatial variations in tsunami height that are produced by local bathymetry, coastal geometry, and the resultant system responses (e.g., coastal and harbor resonances)
From page 122...
... Indian Ocean Tsunami Warning System Program, 2007) for sea level stations that are intended to provide data for tsunami warning.
From page 123...
... NOS Sea Level Stations for Tsunami Detection In the several decades leading up to 2004, NOAA's NOS Center for Operational Oceanographic Products and Services (CO-OPS; http://tidesandcurrents.noaa.gov/) operated long 
From page 124...
... term tide stations, and the National Weather Service (NWS) utilized the data to support the national tsunami warning system.
From page 125...
... SOURCE: Weinstein, 2008; Pacific Tsunami Warning Center, NOAA. At the current time, CO-OPS operates tide stations on all U.S.
From page 126...
... As well, NOAA describes in its Tsunami Warning Center Reference Guide (U.S. Indian Ocean Tsunami Warning System Program, 2007)
From page 127...
... stations. To the committee's knowledge, the level of adherence of international stations used by the TWCs to either NWLON or Tsunami Warning Center Reference Guide (U.S.
From page 128...
... No analysis has been undertaken to evaluate critical coverage gaps with regards to the tsunami warning decision process. Furthermore, no analysis has been undertaken to determine the relative importance of each existing coastal sea level gauge to the tsunami warning decision and evacuation decision processes.
From page 129...
... Such an analysis could also determine the relative importance of each existing coastal sea level gauge to the tsunami warning decision and evacuation decision processes. Although there is some degree of redundancy in coverage in the current sea level gauge network, there has been no evaluation of the associated risk and the vulnerability of the system to failures of single or multiple stations.
From page 130...
... Coastal Sea Level Data Processing In January 2008, NTHMP issued a report (National Tsunami Hazard Mitigation Program, 2008) intended to identify vulnerabilities in the U.S.
From page 131...
... This committee did not undertake an assessment of the processing, distribution, archiving, and long-term access to tsunami-relevant sea level data originating from international sea level stations. As previously stated, the near-real-time, tsunami-relevant sea level data available to the TWCs via the GTS (and archived at the IOC's SLSMF; http://www.vliz.be/gauges/)
From page 132...
... If no further events are detected, the system returns to standard mode after 4 hours. There have been two types of operational DART stations: the first generation DART stations (DART I)
From page 133...
... noaa.gov/dart/dart.shtml; National Data Buoy Center, NOAA.
From page 134...
... agencies, as indicated in the stations 4.6.eps legend. SOURCE: http://www.ndbc.noaa.gov/dart.shtml; National Data Buoy Center, NOAA.
From page 135...
... The technical memorandum provides a starting point for continued refinement of the siting decisions and extension of the DART array, if necessary, while also providing information to aid efforts by the international community to extend the network coverage. The net result of the deliberations on the siting of the DART stations is the current array displayed in Figure 4.6.
From page 136...
... Either the bottom unit or the surface buoy of a DART station may fail and, in remote locations, repair/replacement may not be an immediate option because of seasonal vulnerabilities of non-U.S. territories in the TWCs' AORs, could be filled by DART stations if the resources of international partners are insufficient to fill the gaps with coastal sea level stations.
From page 137...
... cludes that the numbers, locations, and prioritizations of the DART stations should not be considered static. These parameters of the DART network clearly deserve frequent re-consideration in light of constantly changing fiscal realities, survivability experience, maintenance cost experience, model improvements, new technology developments (even new DART designs)
From page 138...
... Conclusion: NOAA is to be commended for having developed a prioritization scheme for the distribution of the DART stations and for having rapidly deployed the DART array. There are no serious gaps in the geographic coverage of the DART network as designed, with regard to providing timely and accurate tsunami warnings and forecasts for at-risk U.S.
From page 139...
... Figure 4.7 indicates how network availability steadily declined to a low of 69 percent in February 2009. The number of DART stations deployed grew from 10 in July 2006 (7 new DART II systems, along with 3 older DART I systems)
From page 140...
... The issue of low network performance is exacerbated by the fact that clusters of nearby DART stations tend to be nonoperational for many months, leaving large gaps in DART coverage. For example, five stations cover the Aleutian Islands west of the Dateline, past the Kuril 70 0 65 3 600 56 3 52 9 500 Median Age of 49 6 Deployed Moorings 43 8 40 0 410 Median Age of Failed Moorings 30 0 273 274 200 10 0 0 2006 2007 2008 2009 20 06 20 07 20 08 20 09 Number of Deployed Stations : (15)
From page 141...
... As a consequence of the pervasive outages of the DART stations, the TWCs cannot depend on the DART network for tsunami forecasting. According to NDBC personnel, the budget only allows for annual routine maintenance and no funds are available for "discrepancy response" (that is, nonroutine maintenance for inoperative gauges)
From page 142...
... In order to maintain the current DART network configuration, adequate resources are needed for maintenance, including funding for unscheduled ship time to effect repair and replacement of inoperable DART stations. The alternative approach would be to invest the majority of resources into improving the DART station reliability to get closer to the 
From page 143...
... The second choice implies that DART stations are maintained sparingly, with only minimal attention to the integrity of the network's tsunami detection capability, until the reliability of the DART stations is improved. In this case, it must be understood and acknowledged that the DART network might be fully deployed but will not be fully functional until such time as the reliability of the DART stations gets much closer to the design goal of a four year lifetime than the present median time-to-failure of just over one year.
From page 144...
... A first step could be for NOAA to establish a strategic plan that determines whether (1) it is most important to maintain the DART II network at the highest level of performance right now (meaning that the first priority for resources is maintenance, including funding of costly ship time to repair and replace inoperative DART stations as soon as possible)
From page 145...
... and Global Observation Systems The coastal sea level data and metadata are available through the IOS Sea Level Monitoring Facility (http://www.vliz.be/gauges/index.php)
From page 146...
... The contributions of optimization algorithms to the network design process could be explored more fully as well. Station Prioritization Recommendation: NOAA should prioritize the existing DART stations and coastal sea level gauges (both U.S.
From page 147...
... the declining performance of the DART network, (2) the importance of both the DART and coastal sea level networks for tsunami detection and forecasting, and (3)
From page 148...
... As most far-field tsunamis generated in the North Pacific take less than 7 hours to strike Hawaii, the entire forecast, including data acquisition, data assimilation, and inundation projections, must take place within 4 hours or less. Although this sounds like a comfortable margin, in fact it is quite a short time period compared to many other natural disasters, especially since it can take anywhere from 30 minutes to 3 hours to acquire sufficient sea level data (Whitmore et al., 2008)
From page 149...
... This scaling process can achieve results as soon as the full wavelength of the leading wave is observed and is updated with observations of the full wave time series. When the wave arrives at the next buoy, the tsunami wave heights are corrected again, although the experience to 
From page 150...
... , are developed: Once the combinations of wave fields from the pre-computed scenarios are constrained by the DART sea level data using the least squares fit technique, the database is queried for wave height and fluid velocity time series at all sea-boundaries of the region targeted for the inundation forecast. At each boundary point, the time histories of heights and velocities are used to initialize the boundary conditions.
From page 151...
... The forecast models were run in near-real time before the tsunami reached the locations shown. The4.9.eps for Santa Barbara exhibited a 9-minute Figure model data early arrival (0.8-1 percent error accumulated during the propagation simulation)
From page 152...
... on the nearby coast, offshore open-ocean gauges that provide near-real-time, rapidly sampled sea level observations are needed. This need motivated the placement of five DART stations off the coasts of California, Oregon, Washington, and British Columbia (see Figure 4.6)
From page 153...
... Among the sensors are those useful for tsunami detection; for example, bottom pressure sensors, seismometers, current meters, hydrophones, gravimeters, and accelerometers. The cable can deliver relatively high amounts of electric power to support many sensors acquiring data at high sampling rates.
From page 154...
... From a pragmatic operational point of view, the utilization of NEPTUNE-Canada and the OOI sensors for tsunami detection could be expected to eliminate the need for the DART buoys off Washington and Oregon, thus freeing up those resources for other purposes. In Japan, cabled observatories already exist that are focused on collecting measurements of earthquakes and tsunamis.
From page 155...
... It is appropriate therefore to briefly review nascent technologies and methodologies that might be able to improve the ability of the U.S. TWCs and their international counterparts to provide quicker and more accurate tsunami warnings.
From page 156...
... The descriptions below of some interesting technologies and methodologies are provided simply to indicate possibilities and should not be interpreted as endorsements of their utility by this committee. Duration of High-Frequency P-Waves for Earthquake Moment Magnitude Estimation Because of the difficulty of obtaining reliable estimates of seismic moments at the long periods relevant to tsunami generation, research is needed to explore the possibility of using other methods, possibly drawing on different technologies, in order to improve the accuracy of moment estimates, and the ability to detect unusual events, such as tsunami earthquakes.
From page 157...
... . With the development of the UN International Monitoring System of the CTBTO, several state-of-the-art hydrophone stations have been deployed in the world ocean, offering an opportunity for complementary use in the context of tsunami warning.
From page 158...
... Once such techniques reach an operational status, they could contribute to tsunami warning. An additional aspect of SOFAR hydrophone sensors is that they can record pressure variations accompanying the passage of the tsunami, and in this sense could supplement the network of DART buoys, as their sensors (in both cases pressure detectors)
From page 159...
... In the case of Nias and Sumatra, both continuous GPS data as well as campaign GPS data were available. He tested the model against satellite altimetry measurements of the tsunami wave using Topex, Jason, and Envisat data (altimetry profiles included time epochs of 1:55-2:10, 1:48-2:03, and 3:10-3:22 [hr:min after the origin time]
From page 160...
... There are few new technologies that promise such revolutionary approaches for improving tsunami warning, especially in the near-field region. Conclusion: GPS geodesy, exploiting near-real-time data telemetry from permanent geodetic stations, holds great promise for extending the current seismic networks to include capabilities for measuring displacements in the coastal environment for great and mega-earthquakes.
From page 161...
... At the present time, however, the NEXT constellation is not being touted as a tool for operational tsunami warning. Tsunami-Induced Sea-Surface Roughness and "Tsunami Shadows" Godin (2004)
From page 162...
... Conclusion: Novel and potentially useful approaches to the estimation of earthquake magnitude and tsunami detection are emerging. Some of these approaches could become operational in the not-too-distant future with proper support for research and testing.


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