Collaborative Research: Making the San Andreas Fault at the Mendocino Triple Junction

一组密集的宽带地震仪正在被用来调查门多西诺三重连接的结构，这是加州著名的圣安德烈亚斯断层系统的最北端。研究区域从加利福尼亚州-俄勒冈州海岸延伸到西经120 °，北纬37 °至43 °。门多西诺三联点是北美板块、太平洋板块和戈尔达板块的交汇点，相对于固定的北美参照系，它随时间向北迁移。 一组相互作用的过程与迁移的门多西诺三重结，从表面延伸到软流层水平，作用于修改北美岩石圈从一个俯冲复合体在卡斯卡迪亚的圣安德烈亚斯转换板块边界，离开沿海加州在其尾流。地幔通过控制板块强度和运动学以及从软流圈到北美板块的热量和质量通量，对MTJ附近发生的过程进行了主导控制。1993- 1994年广泛的主动震源地震研究的结果提供了对MTJ区域极好的地壳控制。 地震调查提供了MTJ区域地幔和下地壳的结构、组构以及地震和地震变形的改进成像。戈尔达地壳和上覆沉积物俯冲，并成为从卡斯卡迪亚俯冲带的增生碎屑合并为一体，因为这一质量流入新兴的转换边缘，并成为结构化的圣安德烈亚斯走滑系统。与此同时，戈尔达板块南部的板块缺口导致软流圈上升和减压熔融，这使圣安德烈亚斯断层附近的北美板块发生了岩浆作用。再加工和膨胀的岩石圈变厚，形成一个小的门多西诺角造山高原，而侵蚀通量的大部分上地壳回俯冲带沿着构造控制的北向河流。附近的戈尔达-胡安·德富卡俯冲导致喀斯喀特火山，这也有助于大陆地壳的增长，分离和循环。 将作为本项目的一部分开发的地壳和上地幔三维地震速度模型将被纳入北方加州和俄勒冈州南部的事件和强地面运动特征描述中，这两个地区可能发生毁灭性大地震。

A dense array of broadband seismographs are being used to investigate the structure of the Mendocino Triple Junction, the northernmost terminus of California's famed San Andreas fault system. The study area extends from the California-Oregon coast to 120o W longitude, and from 37 to 43o North latitude. The Mendocino Triple Junction, the site where the North America, Paclific and the Gorda plates meet, migrates northward with time relative to a fixed North American reference frame. A set of interacting processes associated with the migrating Mendocino Triple Junction, extending from the surface to asthenospheric levels, act to modify North America lithosphere from a subduction complex at Cascadia to the San Andreas transform plate boundary, leaving coastal California in its wake. The mantle imparts a dominant control on the processes occurring near the MTJ through its control on plate strength and kinematics and the flux of heat and mass from the asthenosphere to the North American plate. Results of an extensive active source seismic study in 1993-94 provide excellent crustal control in the MTJ region. The seismic investigations provide improved imaging of the structure, fabric, and seismic and aseismic deformation of the mantle and lower crust in the MTJ region.. Gorda crust and overlying sediment subduct and become incorporated with accretionary terranes from the Cascadia subduction zone as this mass flows into the emerging transform margin and becomes structured into the San Andreas strike-slip system. Simultaneously, slab gap opening south of the Gorda slab causes asthenospheric ascent and decompression melting, which magmatically underplates North America near the San Andreas fault. The reprocessed and inflated lithosphere thickens to create a small Cape Mendocino orogenic plateau, while erosion fluxes large fractions of the upper crust back to the subduction zone along tectonically controlled north-trending rivers. Nearby, Gorda-Juan de Fuca subduction results in the Cascade volcanoes, which also contribute to continental crust growth, segregation and recycling. The 3-D seismic velocity models of the crust and upper mantle, which will be developed as part of this project, will be included in event and strong ground motion characterization for northern California and southern Oregon, a site of potentially devastating great earthquakes.