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4. Detection of Tsunamigenic Earthquakes

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4. Detection of Tsunamigenic Earthquakes Study commissioned by Defra Flood Management The threat posed by tsunami to the UK June 2005 Statement of Use Dissemination Status Internal/External: Approved for release to Public Domain Keywords: tsunami, earthquakes, flood forecasting, flood warning, flood risk Research Contractor This report was produced by: British Geological Survey Murchison House West Mains Road Edinburgh EH9 3LA The work was carried out by a consortium with the following membership: British Geological Survey HR Wallingford Met Office Proudman Oceanographic Laboratory Project Manager Dr David Kerridge, [email protected] Defra Project Officer Mr David Richardson, [email protected] This document is also available on the Defra website www.defra.gov.uk/environ/fcd/studies/tsunami Department for Environment, Food and Rural Affairs, Flood Management Division Ergon House, Horseferry Road, London SW1P 2AL. Telephone 0207 238 3000 www.defra.gov.uk/environ/fcd © Queen’s Printer and Controller of HMSO 2005 Copyright in the typographical arrangement and design rests with the Crown. This publication (excluding the logo) may be reproduced free of charge in any format or medium provided that it is reproduced accurately and not used in a misleading context. The material must be acknowledged as Crown copyright with the title and source of the publication specified. The views expressed in this document are not necessarily those of Defra. Its officers, servants or agents accept no liability whatsoever for any loss or damage arising from the interpretation or use of the information, or reliance on views contained herein. Published by the Department for Environment, Food and Rural Affairs Departmental Foreword We welcome this thorough and detailed report and are grateful for the efforts of all the specialists concerned who completed the work at very short notice. This study has confirmed that Tsunamic events in the North East Atlantic and North Sea are possible but rare. In most plausible circumstances it is likely that such an event would be contained by current defences, designed to resist storm surges, for all major developed areas. We note, however, that any event of this type would be different in character from a storm surge and, in particular could occur at any time of the year without, for example, the build up of prior meteorological conditions associate with winter storms. We also note that there is some uncertainty about potential impacts of a wave that might be generated from a Lisbon-type earthquake in South West England, South Wales and Southern Ireland and that it may be possible to resolve some of this uncertainty through further more detailed modelling. We therefore propose to undertake further short term research that will: • Refine the potential impact envelope in the South West of England, South Wales (and Southern Ireland) from Lisbon-type events; • Give more consideration to the difference between tsunami-type events and storm surge waves in terms of coastal impact • Study typical impacts of near-coast events (e.g. on North Sea coast recreational beaches and other facilities seaward of defences) to identify the potential degree of hazard. In the light of these further investigations we will review the priority for further specific public awareness and/or emergency planning activities. However, because these types of event are so infrequent, we agree that warning systems are only sustainable if they are regularly used also for other purposes. We will therefore in the meantime: • Encourage discussions between the Environment Agency, Met Office and POL regarding a programme of selective improvements to the tide gauge network in terms of measurement frequency and data handling in light of the report recommendations and the further impacts work noted above. Also improvements in access to real-time data from gauges in other countries. All of these could have some benefits for normal storm forecasting. And with ODPM: • Encourage others to work on relevant enhancement and development of the European Broadband seismic network, data analysis and interpretation to support the aims identified in this report, especially the monitoring of potential near-field events as these will have other benefits for seismic research. • Encourage further dialogue between Met Office and British Geological Survey regarding routes for effective advice and warning dissemination relating to seismic events of all types with potentially hazardous consequences for the UK. We believe that this is a measured and appropriate response to the potential for the type of low probability events outlined in this report. Flood Management Division Defra June 2006 i ii Contents Executive Summary ......................................................................................... 1 1. Introduction .................................................................................................. 5 2. Possible sources of tsunamis affecting the UK ........................................ 7 2.1 Introduction .................................................................................................. 7 2.2 Possible tsunami sources............................................................................. 8 2.2.1 UK coastal waters .............................................................................. 9 2.2.2 NW European continental slope....................................................... 12 2.2.3 Plate boundary area west of Gibraltar.............................................. 20 2.2.4 Canary Islands ................................................................................. 23 2.2.5 Mid-Atlantic Ridge............................................................................ 24 2.2.6 Eastern North America continental slope ......................................... 25 2.2.7 Caribbean ........................................................................................ 26 2.3 Preliminary conclusions ............................................................................. 29 3. Models of tsunami propagation from potential source areas ................ 31 3.1 Introduction ................................................................................................ 31 3.2 Tsunami modelling ..................................................................................... 31 3.2.1 Generation of an initial tsunami profile............................................. 31 3.2.2 Propagation of tsunami to the coast................................................. 32 3.2.3 Estimate magnitude of tsunami waves at the coast ......................... 33 3.2.4 Estimation of tsunami inundation ..................................................... 35 3.3 Description of the numerical models .......................................................... 37 3.3.1 Results from Lisbon 1755 type tsunami ........................................... 40 3.3.2 Results from any North Sea tsunami with an origin that of the 1931 earthquake................................................................................................ 41 3.3.3 Results from tsunamis arising from a volcanic collapse on La Palma .................................................................................................................. 42 3.3.4 Celtic Sea......................................................................................... 42 3.4 Discussion of modelling results.................................................................. 43 3.5 Assessment of in-land flooding .................................................................. 58 3.5.1 A ‘first-order’ approach..................................................................... 58 3.5.2 Joint probability of different factors influencing flooding................... 58 4. Detection of tsunamigenic earthquakes .................................................. 60 4.1 Seismic characteristics of tsunamigenic earthquakes and slides............... 60 4.2 Tsunami earthquakes................................................................................. 61 4.3 Location, mechanism and depth ................................................................ 62 4.4 Submarine slides and landslides entering the sea ..................................... 63 4.5 Review of the current state and capability of seismograph networks ......... 65 4.5.1 Seismic monitoring equipment ......................................................... 65 4.5.2 Data processing ............................................................................... 66 4.5.3 Description of seismograph networks relevant to tsunami warning for northwest Europe...................................................................................... 67 4.5.4 Response to tsunamigenic events with the potential to have an impact on the UK ...................................................................................... 70 4.5.5 Strategy for tsunami monitoring in the UK and northwest Europe.... 71 4.5.6 Summary.......................................................................................... 73 5. Detection of tsunami using oceanographic and remote sensing instrumentation.............................................................................................. 75 5.1 Introduction ................................................................................................ 75 iii 5.2 Tsunami travel times................................................................................... 75 5.3 Technologies for tsunami detection ...........................................................
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