Evaluation of Tsunami Sources with the Potential to Impact the U.S. Atlantic and Gulf Coasts An Updated Report to the Nuclear Regulatory Commission By Atlantic and Gulf of Mexico Tsunami Hazard Assessment Group Evaluation of Tsunami Sources with the Potential to Impact the U.S. Atlantic and Gulf Coasts An Updated Report to the Nuclear Regulatory Commission By Atlantic and Gulf of Mexico Tsunami Hazard Assessment Group: Uri ten Brink1, David Twichell1, Eric Geist2, Jason Chaytor1,3, Jacques Locat4, Homa Lee2, Brian Buczkowski1, Roy Barkan5, Andrew Solow3, Brian Andrews1, Tom Parsons2, Patrick Lynett6, Jian Lin3, and Mylène Sansoucy4 1 U.S. Geological Survey, Woods Hole Science Center, Woods Hole, Massachusetts, USA 2 U.S. Geological Survey, Menlo Park, California, USA 3 Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA 4 Department of Geology and Geological Engineering, Université Laval, Sainte-Foy, Québec, Canada 5 Department of Geophysics and Planetary Sciences, Tel Aviv University, Ramat Aviv, Israel 6 Department of Civil Engineering, Texas A&M University, College Station, Texas, USA This report supersedes The Current State of Knowledge Regarding Potential Tsunami Sources Affecting U.S. Atlantic and Gulf Coasts, submitted to the Nuclear Regulatory Commission on September 30, 2007 April 30, 2007 Revised September 30, 2007 Revised August 22, 2008 NOTICE from USGS This publication was prepared by an agency of the United States Government. 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Although all data published on this report have been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and related materials and (or) the functioning of the software. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of these data, software, or related materials. This report should be cited as: Atlantic and Gulf of Mexico Tsunami Hazard Assessment Group, 2008, Evaluation of Tsunami Sources with the Potential to Impact the U.S. Atlantic and Gulf Coasts - A Report to the Nuclear Regulatory Commission: U.S. Geological Survey Administrative Report. This report was compiled for the Nuclear Regulatory Commission under NRC Job No: N6480 Acknowledgements: We would like to thank Thierry Mulder, David Piper, Bill Normark, Brian McAdoo, Roger Urgeles, Brian Collins, David Mosher, Rob Kayen, Dave Tappin, Valerie Paskevich, VeeAnn Cross and many others who have contributed their thoughts, insights, technical assistance and reviews on the various papers included in this volume. Cover: Maximum wave amplitude projected on a sphere from our preferred location of the 1755 Lisbon Earthquake. Warm colors indicate high amplitudes and cold colors low amplitudes. Black lines are Great Circle paths between the source and various locations along the U.S. East Coast and the Caribbean. Note the predicted low amplitude along most of the U.S. East Coast (with the exception of Florida) due to the scattering of energy by submarine topography in the Eastern Atlantic Basin. Executive Summary The 2004 Sumatra tsunami, which took place in an area with no historical record of a similar event, has brought awareness to the possibility of tsunamis along the U.S. Atlantic and Gulf of Mexico coasts. While these rare events may not have an impact on tsunami probability calculations for flood insurance rates, they need to be considered in long-range planning, such as for the placement of nuclear power plants. The U.S. Geological Survey was tasked by the Nuclear Regulatory Commission to prepare an evaluation of tsunami sources and their probability to impact the U.S. Atlantic and Gulf of Mexico coasts. This report is an updated evaluation based on additional data analysis and modeling. It provides a general review of potential tsunami sources, and provides a geotechnical analysis and hydrodynamic model for one landslide offshore North Carolina. The evaluation also identifies geographical areas with limited information and topic for further study. Finally, the updated report present new theoretical developments that may aid in quantitative evaluation of tsunami probability. The work included in this report represents both review of published work and original work. Eight of the 14 topical chapters in this report represent original work, and the remaining 6 chapters are based on literature reviews. The original work is in the process of being published as eight papers in a special issue of Marine Geology, an international peer-reviewed scientific journal. The main findings of the study so far include: 1. Landslides along the U.S. Atlantic margin have the potential to cause tsunami locally. These landslides are concentrated along the New England and Long Island sections of the margin, outward of major ancient rivers in the mid-Atlantic margin and in the salt dome province offshore North Carolina. The landslides generally removed a relatively thin (a few 10s of meters) layer of mostly unconsolidated sediments. The mapped landslides follow a log-normal size distribution centered at a volume of about 1 km3. However, some parts of the upper continental slope, particularly off Long Island New England, have not yet been mapped in detail. Relatively few large landslides from the entire mapped inventory (9 landslides over 500 km2 and 16 landslides over 10 km3) could have caused a damaging tsunami. The criteria for devastating tsunami is presently based solely on modeling of the Currituck slide offshore North Carolina. Additional simulations off New England are needed to refine the threshold for damaging tsunamis. A review of known ages of submarine landslides around the Atlantic Ocean and worldwide i shows a factor of 1.7-3.5 lower frequency of occurrence during the past 5000 years relative to the last glacial period and the first 5000 years after the end of glaciation, suggesting that the majority of the observed landslides along the U.S. Atlantic margin are older than 5000 years. 2. Earthquake sources that can generate trans-oceanic tsunamis, are located west of Gibraltar and in the Puerto Rico trench. Tsunami simulations from the 1755 Lisbon earthquake show that seafloor topography between the source area and the Azores Islands plays a major role in scattering the wave energy traveling toward the U.S. East Coast. This conclusion matches the lack of tsunami reports from parts of the U.S. East coast that were populated at the time (Boston, New York, Chesapeake Bay, Savannah, Charleston). However, simulations show that should a large tsunamigenic earthquake take place in the Puerto Rico trench, the resultant tusnami may be destructive to many parts of the U.S. East Coast. To date, no evidence was found for large historical or pre-historical earthquakes in the Puerto Rico trench, and the ability of this plate boundary to generate large earthquakes is being investigated. 3. Far-field landslides, such as in the Canary Islands, are not expected to cause a devastating tsunami along the U.S. Atlantic coast. 4. Large landslides in the Gulf of Mexico are found in the submarine canyon and fan provinces extending from present Mississippi and other former larger rivers that emptied into the Gulf. These large landslides were probably active before 7500 years ago. In other areas, landslides continue to be active, probably because of salt movement, but are small and may not pose tsunami hazard. Very little is known about the threat of landslide-generated tsunamis from the Mexican coast, particularly the Campeche escarpment. Tsunamis generated by earthquakes do not appear to impact the Gulf of Mexico coast. 5. Several approaches to quantitatively calculate the probability of tsunamis impacting the U.S. East and Gulf Coasts have been developed, but their accuracy depends in large part on the available of observations of size distribution, recurrence interval, and geotechnical parameters of the sea floor. ii Contents Chapter 1: Introduction and Background........................................................................1 Section 1: Landslides Chapter 2: Geologic Controls on the Distribution of Submarine Landslides along the U.S. Atlantic Continental Margin.....................................................................................3 Introduction ....................................................................................................................3 Geologic Setting ..............................................................................................................4 Methods ..........................................................................................................................5 Bathymetry .....................................................................................................................5