Collaborative Research: Cascadia Tilt

墨尔本合作研究：卡斯卡迪亚倾斜太平洋西北部每隔几百年就会发生一次大的俯冲地震，每隔几年就会发生一次破坏性较小的地震。 一个令人惊讶的发现（来自GPS大地测量学）是，每13-14个月，胡安·德富卡板块和地表下约20公里的北美之间就会发生大约10厘米的增量移动。 这种缓慢的转变持续1-3周，产生低的亚音速隆隆声（2s-5 Hz），最重要的是，对未来的破裂带施加压力，使其逐渐接近下一次大地震。 也就是说，未来的缓慢滑动事件很可能是压垮骆驼的最后一根稻草--可能发生的里氏9级地震破坏了西雅图和温哥华，并产生海啸破坏了远至夏威夷和日本。为了更好地了解这些缓慢滑动事件，在华盛顿州安装了10根1公里长的水管（倾斜仪），每根水管的末端都有传感器，这些传感器可以检测到1微米的相对水高度变化（倾斜相当于旧金山弗朗西斯科相对于纽约升高了一张纸的厚度）。 倾斜仪将监测接下来的四个缓慢事件，以精确地确定滑动是如何发生的以及在哪里发生的，以及这些缓慢滑动事件与随后发生的中度破坏性浅源地震之间的关系。这些数据将以可上网查阅的格式以接近真实的时间提供，以提高公众的认识。 中央华盛顿大学和位于博尔德的科罗拉多大学的研究生将参与对数据的不小处理。该项目包括公众宣传的几个方面，既要让公众了解科学方法的细节，又要确保该地区的公众、工程师和城市规划者了解太平洋西北部的地震危险。 每一个慢事件都将是一个机会，提供新闻突破性的数据，同时向公众提供有关测量及其对地震灾害的重要性的定制信息。 警报之间的时间间隔为14个月，几乎完美地提醒公众地震安全是一个持续的问题，而不会使他们过度暴露于假警报。 最近几次缓慢事件中有两次的结束是以浅震和有感地震为标志的，其中一次（2001年2月28日的尼斯夸利地震）造成了2.5亿美元的损失。

EAR-0310293MelbourneCOLLABORATIVE RESEARCH: CASCADIA TILT Great subduction earthquakes occur beneath the Pacific Northwest every few hundred years, and damaging but smaller earthquakes every few years. One surprising discovery (from GPS geodesy) is that every 13-14 months, an incremental shift of roughly 10cm occurs between the Juan de Fuca plate and North America some 20 km beneath the surface. This slow shift has a duration of 1-3 weeks, generates a low sub-sonic rumble (2s-5Hz), and most importantly, stresses the future rupture zone incrementally towards the next great earthquake. That is, a future slow slip episode may well be the final straw that breaks the camel's back - a possible Richter M=9 earthquake damaging both Seattle and Vancouver, and generating tsunami damage as far as Hawaii and Japan. In an attempt to learn more about these slow slip events, ten 1-km-long water pipes (tiltmeters) are being installed in Washington state each terminated by sensors that can detect 1 micron of relative water height change (a tilt equivalent of San Francisco rising the thickness of a sheet of paper relative to New York). The tiltmeters will monitor the next four slow episodes to ascertain precisely how and where slip occurs, and the relationship between these slow slip events and moderately damaging shallow earthquakes that appear to follow them. The data will be available in near real time in a web accessible format to engender public awareness. Graduate students at Central Washington University and the University of Colorado at Boulder will be involved in the not inconsiderable processing of the data. The project includes several aspects of public outreach, both to make the public aware of the details of the scientific method, and to ensure that the public, engineers, and urban planners in the region know the seismic hazards of the Pacific Northwest. Each slow event will be an opportunity to deliver news-breaking data, accompanied by a tailored message to the general public about the measurements and their significance to seismic hazards. At 14 months, the time interval between alerts is almost perfectly designed to remind the public that seismic safety is an ongoing issue, without overexposing them to false alarms. The end of two of the last several slow events has been marked by shallow, felt earthquakes, one of them (the Nisqually earthquake 28 Feb 2001), resulting in $250M of damage.