Tsunamis Working Group Report
Leaders – Eric Geist (USGS - CMG), Aurelio Mercado (U. Puerto Rico)
Members:
Jim Lander (U. Colorado / NOAA), Victor Huerfano (U. Puerto Rico), Juan Luis Trias (USGS-WRD), Guy Gelfenbaum (USGS-CMG)
INTRODUCTION
The tsunami hazards for Puerto Rico and the Virgin Islands are unique in that practically all of the known causes of tsunamis are present in the Caribbean. These include earthquakes, primarily, but also landslides (north of Puerto Rico), submarine volcanic eruptions, catastrophic edifice collapse, and teletsunamis. Recent modeling studies of the 1918 earthquake and tsunami in Puerto Rico that killed 116 people (Mercado and McCann, 1998) and the compilation of a tsunami catalog over the last 500 years (Lander et al., in press) has highlighted the significant hazard in the Caribbean. We stress here that tsunami hazards are a real threat in the Puerto Rico region and adequate mitigation measures must be taken to reduce the exposure of infrastructure (lifelines, communication, emergency response), commercial interests, and the general populace to this hazard.
In the U.S. tsunami community, there has been an emphasis on tsunami hazards along the Pacific margin, based on geological evidence, and in Hawaii based on historical events. Destructive tsunamis in Hawaii in 1946 and 1960 led to the establishment of a tsunami warning system. The Great Alaska earthquake of 1964 highlighted the tsunami hazard both locally in Alaska and long distances in California. Recently, the local tsunami hazard in the Pacific Northwest has been illuminated, in part, by analysis of prehistoric tsunami deposits. We should not wait until a catastrophic disaster occurs in order to implement a tsunami hazard mitigation program in Puerto Rico and the Virgin Islands. The combination of widespread tectonic faulting with steep slopes due to the Puerto Rico Trench raises much concern in light of the recent experience in Papua-New Guinea. Quoting from an article in Science News (Vol. 154, October 3, 1988), "The main lesson is that small, local faults have a much greater potential for tsunami generation than we had thought earlier". The educational message from the Papua-New Guinea tsunami adopted by the western seaboard states of the USA is that "it can happen here and we need to deal with this problem now". Ditto for Puerto Rico and the US Virgin Islands.
In this summary report we will present issues that can be addressed by the Geologic Division of the USGS, and also issues that will need the intervention of other agencies.
WHAT NEEDS TO BE DONE?
Tsunami Sources
To systematically assess the tsunami hazards in Puerto Rico, the entire spectrum of sources must be considered, placing special emphasis on those sources most likely to occur and those sources that cause the largest hazard (severity and extent). Taking into account both of the components of hazard, severity and extent of tsunami damage, it is important to first identify the range of local earthquakes that might produce a tsunami. Using identical earthquake scenarios that are used to produce probabilistic seismic hazard maps must be done with great caution because of the difference between how earthquakes effect ground shaking and how they effect tsunami runup. In particular, a proposed interplate earthquake located in very deep water (1946 and 1787?) will dominate the tsunami hazard over all other scenarios. Special emphasis must be placed on determining the likelihood and probability of such an event using quantitative mechanisms derived from seismograms (not just qualitative inferences derived from geology).
Analysis of tsunami deposits and eyewitness observations has traditionally yielded critical information on earthquakes themselves. Although tide gauge records are not available for the major historic events near Puerto Rico (1918, 1946), there is the potential to extract tsunami observations of earlier events from local and Spanish archives for these important events. Analysis of prehistoric tsunami deposits can establish not only possible recurrence intervals for major earthquakes in the region, but possibly the magnitude of such events. For the Caribbean region especially, it is important to define criteria for identifying tsunami deposits and discriminating them from storm and hurricane deposits.
Numerical Tsunami Inundation Mapping and Near-shore Topography and Bathymetry
Tsunami inundation mapping provides the basis of deriving essential products for tsunami hazard mitigation. Land use planning decisions, evacuation maps, communication and transportation plans all rely on accurate tsunami inundation maps. In turn, the primary information needed to produce these maps (aside from a calibrated propagation and runup model) is near shore bathymetry and topography. Tsunami waves dramatically decrease in wavelength: as they approach the shore, as a function of bathymetry near the source tsunami waves maybe 100 km's long whereas in the nearshore region, wavelengths can be reduced by two orders of magnitude. The numerical stability criteria for propagation and runup models are directly related to having a sufficient number of grid points for a given wavelength. Whereas in the deep ocean ETOPO2 data is often sufficient to develop a grid to run tsunami propagation models, in the near-shore region where bathymetry is <100 m, greater horizontal resolution is necessary to produce accurate inundation maps (International Workshop on Bathymetry and Coastal-Topography Data Management, University of Washington, Seattle, WA, March 20-21, 1998; H. Yeh, "Tsunami Researchers Outline Steps for Better Data", EOS, Vol. 79, No. 40, 1998, pp. 480,484).
Accurate, up-to-date, near-shore topography is currently not available for much of the Puerto Rico coastline. The available shelf bathymetry, while dense and recent for the south and east coasts, it is almost a century old for the west and north coasts and, at the same time, is completely missing for the critical near-shore area for all of the north coast (with the exception of the waters just off the San Juan metropolitan area). Recent advances in technology have made it possible to collect this data with airborne systems. In summary, an essential product that the USGS Geologic Division can contribute to tsunami hazard assessment is the acquisition of coastal bathymetric and topographic data. This will be of importance not only for tsunami modeling but also for storm surge modeling, and ocean surface gravity waves transformation studies.
Tsunami Deposits Mapping
By looking at the Quaternary sedimentary record, geologists have begun to infer the occurrence of past tsunamis in the northwest coast of Puerto Rico and thus interpret the risk of future tsunami events (Moya, 1999). Except for the limited available written or verbal history, obtaining a record of past events may be nearly impossible. Since scientists cannot yet predict when a tsunami will occur, obtaining a geologic record of past events may be one of the best means to assess future risk.
Recent work in Puerto Rico and elsewhere has shown that tsunamis leave a sedimentary record. Furthermore, if a sedimentary deposit is left, inference might be made regarding the size or extent of the tsunami. The continuation of the work begun by Moya (1999) is highly recommended for the Atlantic and Caribbean coasts of Puerto Rico.
Education, Outreach, and a Puerto Rico and Virgin Islands Tsunami Warning System
Based on the fact that in the last 150 years tsunami-related fatalities in the Caribbean were nearly 5 times greater than in Hawaii, Alaska, and the U.S. West Coast combined, it is important that, as soon as possible, a tsunami warning program be implemented in the U.S.A. Caribbean possessions. During the 1997 Caribbean Tsunami Workshop, sponsored by the Sea Grant College Program of the University of Puerto Rico, and held in Mayaguez, P.R., the following resolution was drafted:
Resolution
The community of concerned citizens, emergency managers, educators, and scientists, gathered by the University of Puerto Rico 11-13 June 1997 in Mayaguez, having:
Heightened awareness that per event, loss of life in the Caribbean due to tsunamis is at least equal to losses due to the major regional hazard, hurricanes;
Recognized that Caribbean tsunamis have been and will continue to be caused by local earthquakes, volcanic eruptions, landslides, and by distant seismic events;
Appreciated that our region’s population has grown ten’ fold since the great Virgin Islands tsunami of 1867 with most of that growth concentrated near the coast;
Debated and considered the overwhelming potential for catastrophic loss and the current state of technology’s ability to mitigate such a significant toll on life and property,
Do hereby collectively petition our governments to extend natural hazard protection to include tsunami risk reduction with integrated system components of education, warning, management, and research.
End of Resolution
Through the proposed warning and tsunami mitigation program we are proposing to: 1) identify the areas exposed to this hazard, 2) raise the public’s consciousness about this "forgotten hazard", and 3) set the groundwork for what would be the first tsunami warning system in the Caribbean region.
The implementation of a tsunami hazard mitigation program and warning system in Puerto Rico and the US Virgin Islands will have application beyond the boundaries of the US territories in the Caribbean basin. They will be of value to other US interests in the region, where there is a growing awareness of a potential tsunami threat. It will help reduce loss of life and infrastructure in USA’s own backyard, which at the end will result in reduced human suffering and aid cost in the event of a tsunami strike. Here we list a series of short-term goals:
Preparation and supply of tsunami flooding and evacuation maps. This is an essential step to the development of effective hazard planning. This will also include estimates of the travel time interval expected between earthquake/tsunami generation and its arrival at different locations along the south, north, east, and west coasts of Puerto Rico, also including the US Virgin Islands. Through inundation mapping we can assess the vulnerability of schools, hurricane shelters, and critical infrastructure. Critical, essential facilities should be safe guarded against tsunami attack, or moved to safer ground. This has already started at the University of Puerto Rico under the sponsorship of the Puerto Rico State Civil Defense, FEMA, and the University of Puerto Rico Sea Grant College Program (UPRSG). The inundation mapping results should be adapted to a widely used GIS package so that it can be easily distributed to other government agencies, specially those responsible for land-use planning, emergency response, and critical facilities. It should be emphasized that the results of the coastal inundation mapping, whether for tsunamis or storm surges, are as reliable and accurate as the input topographic and bathymetric data.
Raising awareness of potentially affected populations. For the majority of historical Caribbean tsunamis, which tend to be local, there is universal agreement within the tsunami and emergency response community that technology alone cannot protect coastal inhabitants in the immediate area of a near-source tsunami. Local population at risk must be able to recognize the signs of an impending tsunami hazard such as strong, prolonged ground shaking, and seek higher ground immediately. Communities need to know what areas are likely to be flooded through inundation maps that define the evacuation area and designate evacuation routes and safe regions in which to assemble evacuees. Planners, emergency responders, and residents need to understand the multi-hazard ramifications of a very large local earthquake that will disrupt much of the community infrastructure. Local decision-makers need to understand the nature of the risk and be provided with mitigation tools in order to make reasoned long-term planning decisions. A sustained public outreach program is needed to gain the long-term grass-root support of the coastal populations and to institutionalize tsunami mitigation in an all-hazard approach to risk reduction. This will include community meetings at exposed locations throughout the island for the purpose of raising consciousness about this "forgotten" hazard. The same will be done at the school level through outreach programs, including videos and talks. Reflective tsunami hazard signs for placement in coastal regions can be reproduced for placement at the exposed locations based on the results of numerical modeling. These signs can give information on evacuation routes, or give brief evacuation instructions in case of an earthquake. Radio and television will be used, as is already done for the earthquake hazard. Training and evacuation drills will be carried out for exposed schools. For all of these activities we can count on the support the of the Puerto Rico Civil Defense and the Marine Advisory Services of the UPRSG. These have already taken a strong leadership in the aspect of tsunami hazard mitigation in the Caribbean through its sponsorship of the Caribbean Tsunami Workshop (1997) and three tsunami-related research projects. These are: 1) "Determination of the tsunami hazard for western Puerto Rico from local sources", Principal Investigators: A. Mercado and W. McCann; 2) "Estimate of the tsunami hazard in the Greater Antilles from local, earthquake-related tsunami sources", Principal Investigators: A. Mercado and W. McCann; 3) "Investigation of the Potential Tsunami Hazard on the North Coast of Puerto Rico due to Submarine Slides Along the Puerto Rico Trench", Principal Investigators: A. Mercado, N. Grindlay - Univ.of North Carolina, and P. Liu - Cornell - the main sponsor of this latter project is the PR Civil Defense and FEMA). Sea Grant has offered support in the preparation of workshops for planners, community organizations, and educators. They have also offered the use of their printing facilities for information and publication materials, including press kits.
Develop warning messages and protocol in the event that a potentially tsunamigenic event is detected. Information on a felt earthquake, magnitude and location, that could reach affected communities in a few minutes would go a long way towards relieving confusion and concerns and possibly averting an unnecessary evacuation. Warning protocols will be developed at the municipal, state, and federal levels, and also within the rest of the Caribbean community. The key agencies in the Pacific states for warning messages and emergency response coordination are NOAA (Pacific Tsunami Warning Center and the National Weather Service, NWS), the USGS regional seismic networks, and state and local emergency response agencies. A similar setup will be established in Puerto Rico and the USVI, and will include the Puerto Rico Seismic Network (PRSN) of the University of Puerto Rico, NWS, and the Puerto Rico Civil Defense. Encourage the participation of Puerto Rico’s Warning Coordination Meteorologist - WCM - in the regular Tsunami Warning Coordination Meteorologists Training Workshops held on the West coast; the Pacific states WCM’s have now included tsunamis as part of their coastal hazard community preparedness activities. The WCM’s have been designated the point of contact between the States and NOAA.
With the goal of establishing a Caribbean Tsunami Warning System for Puerto Rico and the Virgin Islands, evaluate local, regional, national, and global seismic networks and equipment, and their role in a tsunami warning system. At-risk regions need real-time determination of earthquake source information to assess the nature of the hazard in order to optimize emergency response. In 1998 FEMA and the University of Puerto Rico funded a project to upgrade the instrumentation of the Puerto Rico Seismic Network (PRSN) of the Department of Geology of the University of Puerto Rico at Mayaguez. With the new 9 telemetered broad-band stations large magnitude events will be recorded without the possibility of the recorded signal going off scale which will permit the determination of the seismic parameters. The goal is for it to be capable of notification of a major earthquake within 2 minutes of the initial rupture. This initial notification will be followed within 3 minutes by detailed seismic parameters that provide an understanding of the likelihood of a tsunami. Once the seismic parameters are calculated the expected tsunami inundation areas can be determined through interpolation of the results from previous simulations. In this way the Puerto Rico earthquake and tsunami warning systems will be integrated under the same facility. This will require hardware and software upgrades and developments in the PRSN. NOAA is also considering the establishment of a Caribbean Tsunami Warning Center at Mayaguez, according to a letter from John J. Kelly, Assistant Administrator for Weather Services, NOAA.
For regional tsunamis this goal can be implemented in coordination with the existing Caribbean wide World Meteorological Organization weather network and the Middle-America Seismic Data Center at the PRSN. Since 1998 the PRSN operates the data center for the Middle-America Seismograph Consortium (MIDAS). A protocol has been established for the participating seismic networks of the Caribbean, Central America, South America, and North America, to submit in near real time data on significant events in the area. This issue has already been widely discussed in the two Caribbean tsunami workshops already held (Intergovernmental Oceanographic Commission - Caribbean Region - UNESCO - Consultation of Experts on Tsunami Hazards Meeting, St. John, USVI, 1996, and the Caribbean Tsunami Workshop held at Mayaguez, P.R. on June 12-13, 1997) and in the Peru, 1997, meeting of the International Co-ordination Group for the Tsunami Warning System in the Pacific (ITSU-XIV) XVI. At several MIDAS meetings the desirability of having seismic data exchanged in real time has been discussed for tsunami warning applications.
The need to reduce false alarms will be emphasized. False alarms are not only costly (a May 1986 Hawaii false alarm estimated cost due to loss of business productivity is between $30,000,000 and $40,000,000), but they also erode the credibility of the local emergency managers. But the cost of failing to evacuate for a real event or incorrectly estimating the duration risk can be much greater. The human loss due to a unannounced repetition of the 1867 or 1918 tsunami events would be huge.
Develop a bilingual WEB site to post general tsunami information, results of tsunami-related investigation, inundation flood maps, history of Caribbean tsunamis, information on potential tsunamigenic events, and links to other tsunami and Emergency Response-related WEB sites.
Produce a tsunami video from the Caribbean perspective. This video will include the history, hazards, and protective measures concerning tsunamis.
Seek the participation of Puerto Rico, as an observer or full-fledged member, in the National Tsunami Hazard Mitigation Program, which presently include the states of Washington, Alaska, Oregon, California, and Hawaii. This program is sponsored by FEMA, NOAA, USGS, and state co-sharing. Contact with the relevant agencies has been already established and, for example, the University of Puerto Rico has been included in a proposal to the National Science Foundation for the establishment of a Facility for the Analysis and Comparison of Tsunami Simulations (FACTS). The other agencies or institutions participating are the Pacific Marine Environmental Laboratory (PMEL) of NOAA, the USGS, the Pacific Tsunami Warning Center (NOAA), University of Southern California, University of Alaska, the Oregon Graduate Institute, Cornell University, Novosibirsk Tsunami Laboratory, and the Maui High Performance Computing Center.
Develop a Caribbean Historical Tsunami Data Base. This would help in deciding which tsunamigenic events to include in future modeling activities, and in the determination of fault parameters for the modeling. Modeling can be done for each historical event, and for similar, non-historical (i.e., "synthetic"), but plausible, events. A database of the simulated coastal inundation maps due to each of these events (historical and made up) will be stored and used in real-time during an actual event for evacuation purposes. This is similar to what has been done for hurricane evacuation purposes here in the Caribbean, where inundation maps have been prepared by running a storm surge model many times using as input different intensity (on the Saffir/Simpson scale) "synthetic" hurricanes. The same should be done using the Tsunami Travel Time software, allowing for a data base of elapsed travel times to different locations along Puerto Rico’s coastline due to tsunami sources located at different potentially tsunamigenic earthquake or landslide Caribbean locations. All of this would obviate the need to do the modeling in real-time which, for a local tsunami is almost impossible.
Promote and seek support and partnerships at state and federal level for long-term tsunami hazard mitigation. This partnership should also include the private sector, for example, insurance industry.
References
Mercado, A., and W. McCann, 1998. Numerical simulation of the 1918 Puerto Rico tsunami. J. Natural Hazards, Vol. 18:57-76.
Moya, J. C., 1999. Stratigraphic and morphologic evidence of tsunami in northwestern Puerto Rico. Executive Summary. Submitted to Sea Grant College Program, University of Puerto Rico, Mayaguez Campus.
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