A Probabilistic Assessment of Tsunami Hazard in the Indian Ocean

The Indian Ocean Tsunami of December 26, 2004 made starkly evident the need for better information on tsunami hazard in the Indian Ocean. The tsunami threat faced by Indian Ocean countries consists of a complex mix of tsunami from local, regional and distant sources, whose effects at any particular location in the Indian Ocean are highly dependent on variations in sea floor shape between the source and affected coastlines, complicating tsunami disaster management and the design of tsunami warning systems for the Indian Ocean. In order to provide national governments in the Indian Ocean with the information they need to make informed decisions about tsunami mitigation measures, including development of a warning system, a comprehensive hazard and risk ass\n\nIn this presentation we discuss the results of this assessment. The study focused on tsunami caused by subduction zone earthquakes, because they are the most frequent source of large tsunami, and tsunami hazard is expressed as annual probability of a tsunami exceeding a given amplitude at a given offshore depth. Because so little is known about the recurrence rates of large megathrust earthquakes in the subduction zones bordering the Indian Ocean, it was decided to develop two hazard maps: a 'low-hazard' end member, based on only those earthquake sources of tsunami for which there is definite evidence, and a 'high-hazard' end member, based on all potential megathrust earthquake sources, including hypothetical ones for which there is no historical or geological evidence, that may affect Indian Ocean coastlines. The actual hazard lies somewhere between these two end members, and the difference between the low hazard and high hazard maps is a simple and effective way to express the uncertainty in the hazard assessment. This uncertainty reflects the lack of knowledge of tsunamigenic earthquake occurrence, and can only be reduced through a better understanding of earthquake and tsunami occurrence in the Indian Ocean.\n\nYou can also purchase hard copies of Geoscience Australia data and other products at http://www.ga.gov.au/products-services/how-to-order-products/sales-centre.html

2004年12月26日的印度洋海啸，有力地凸显了印度洋地区完善海啸危险性信息的迫切需求。印度洋沿岸国家面临的海啸威胁，由本地、区域及远洋等多类震源的海啸复杂组合而成，而印度洋任一特定区域的海啸影响，高度取决于震源与受影响海岸线之间的海底地形差异，这显著增加了海啸灾害管理与印度洋海啸预警系统设计的复杂性。为向印度洋沿岸各国政府提供所需信息，使其能够基于科学依据制定海啸减缓措施（包括预警系统建设），一项全面的海啸危险性与风险评估工作随即展开。

在本报告中，我们将阐述此次评估的研究成果。本次研究聚焦于俯冲带地震引发的海啸，因为这类地震是大型海啸最频发的诱因，且海啸危险性以“给定近海水深处，海啸波幅超过特定阈值的年发生概率”进行量化。由于目前对印度洋周边俯冲带内大型逆冲断层地震的复发周期所知甚少，因此决定编制两份危险性基准图：一份为“低危险性”端元情景，仅纳入有明确证据支持的海啸震源；另一份为“高危险性”端元情景，涵盖所有可能影响印度洋沿岸的潜在逆冲断层地震源，包括缺乏历史或地质证据的假想震源。实际的海啸危险性介于这两份端元情景之间，而高低危险性图之间的差值，可直观且有效地体现风险评估中的不确定性。该不确定性源于我们对海啸引发地震发生规律的认知不足，唯有通过加深对印度洋地区地震与海啸活动规律的理解，方能降低此类不确定性。

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