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来对世界俯冲带进行分类，或者更具体地说，是“地震耦合”。马里亚纳岛板块通常被认为是从岛弧“拉回”，从而引发了活跃的弧后扩张。马里亚纳斯型俯冲带不太可能发生大的逆冲(巨型逆冲)地震。在全球定位系统测量确定汤加岛弧迄今具有最大的弧后扩张速度以及板块会聚之后。大多数地球物理学家的结论是，汤加实际上是世界上地震耦合最少的俯冲带，而不是马里亚纳群岛。2006年5月3日，汤加南部的哈阿派群岛附近发生了一次(Mw=7.9)浅层大地震。其质心矩张量(CMT)的震源机制与板内沿浅层板间逆冲带(即大型逆冲)会聚的预期机制几乎完全一致。如果这种解释是正确的，这将是过去20年来最令人惊讶的地震之一，它将需要对俯冲和地震成因的机制进行重大反思。然而，所有的震源机制都是模棱两可的，另一种解释是，这次事件有一个高角度断层面，在这种情况下，它将构成一次巨大的板块撕裂事件。浅层板块撕裂事件很少见，特别是在经历向下倾斜挤压的板块中，而且还没有使用GPS和宽带地震台阵对此类事件进行仔细研究。在震后形变保持强劲的同时，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使人们能够快速调查破裂带及其周围环境，部署宽带地震台阵研究余震，调查全球定位系统测量以确定以前建立的台站的同震运动感，并结合测量和连续的全球定位系统测量以监测所有大地震产生的震后形变场。还包括对该地区海岸线的快速地貌调查，以帮助限制同震和地震后的垂直运动。没有人使用现代地震和GPS仪器观测到大而浅的板块撕裂地震的震后瞬变和余震。该项目是三个美国机构以及汤加和环太平洋国家的合作伙伴之间的合作。这项工作还可能对汤加沿线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.