SGER: Collaborative Research: A Rapid Geophysical Response to the Great 2006 Tonga EQ

20多年来，地球物理学家和地质学家一直倾向于根据它们的耦合程度对世界上的俯冲带进行分类，或者更具体地说是“地震耦合”。马里亚纳板块通常被视为从岛弧“拉回”，从而引起活跃的弧后扩张。马里亚纳型俯冲带不太可能产生巨大的逆冲下（大逆冲）地震。经过GPS测量，汤加岛弧的弧后扩张速度和板块辐合速度都是最快的。大多数地球物理学家得出结论，汤加，而不是马里亚纳群岛，实际上是世界上地震耦合最小的俯冲带。2006年5月3日，汤加南部Ha'apai岛群附近发生了一次大地震（里氏7.9级）。其质心矩张量（CMT）的震源机制几乎完全符合沿浅板间逆冲带（即大逆冲带）的板内辐合。如果这种解释是正确的，这将是近20年来最令人惊讶的地震之一，这将需要对俯冲机制和地震成因进行重大反思。然而，所有的震源机制都是模糊的，另一种解释是，这次事件有一个高角度的断层面，在这种情况下，它将构成一个巨大的板块撕裂事件。浅层板块撕裂事件很少发生，特别是在经历下倾压缩的板块中，并且没有使用GPS和宽带地震阵列仔细研究过这种事件。在震后形变仍然剧烈的情况下，快速进行近场测量是至关重要的。这笔用于探索性研究的小额赠款将允许对断裂带及其周围环境进行快速调查，部署宽带地震阵列来研究余震，调查GPS测量以在先前建立的站点建立同震运动感，并将调查和连续GPS测量相结合，以监测所有大地震产生的震后变形场。还包括对该地区海岸线的快速地貌调查，以帮助限制同震和震后垂直运动。从来没有人用现代地震和GPS仪器观察过一次大而浅的板块撕裂地震的震后瞬态和余震。该项目是美国三个机构以及汤加和环太平洋国家的合作伙伴之间的合作项目。这项工作还可能对汤加沿岸的地震和海啸灾害产生重大影响。

For more than 20 years geophysicists and geologists have tended to classify the world's subduction zones in terms of their degree of coupling, or more specifically 'seismic coupling'. The Marianas slab is often seen as 'pulling back' from the island arc, thereby giving rise to active backarc spreading. Marianas-type subduction zones are very unlikely to generate great underthrusting (megathrust) earthquakes. After GPS measurements established that the Tonga island arc has by far the greatest rates of backarc spreading as well as plate convergence. Most geophysicists have concluded that Tonga, rather than the Marianas, is actually the least seismically-coupled subduction zone in the world. On 3 May 2006 a great (Mw = 7.9) shallow earthquake occurred near the Ha'apai island group in southern Tonga. The focal mechanism for its centroid moment tensor (CMT) almost exactly matches that expected for intraplate convergence along the shallow interplate thrust zone, i.e., a megathrust. If this interpretation is correct, this would be one of the most surprising earthquakes of the last twenty years, and it would require major rethinking on the mechanics of subduction as well as seismogenesis. However, all focal mechanisms are ambiguous, and the alternative explanation is that this event had a high angle fault plane, in which case it would constitute a great plate-tearing event. Shallow plate tearing events are rare, especially in slabs experiencing downdip compression, and no such event has been subject to careful study using GPS and broadband seismic array. It is vital to make near-field measurements quickly, while postseismic deformation remains vigorous. This small grant for exploratory research will allow a rapid survey of the rupture zone and its immediate surroundings, deployment of a broadband seismic array to study the aftershocks, survey GPS measurements to establish the sense of coseismic movements at previously established stations, and a combination of survey and continuous GPS measurements to monitor the postseismic deformation field produced by all great earthquakes. Also included is a rapid geomorphological survey of the region's coastlines to help constrain coseismic and postseismic vertical movements. No one has ever observed postseismic transients and aftershocks of a great and shallow plate tearing earthquake using modern seismic and GPS instruments. This project is a collaboration between three US institutions as well as partners in Tonga and the Pacific Rim countries. The work also could have significant implications for earthquake and tsunami hazards along Tonga.