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

密集的宽带地震仪阵列正被用来调查门多西诺三重枢纽的结构，门多西诺三重枢纽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-俄勒冈海岸延伸到西经1200度，从北纬37度到430度。门多西诺三重交汇处是北美、太平洋和戈尔达板块交汇的地方，相对于一个固定的北美参照系，它随着时间向北移动。一系列与门多西诺三重连接点迁移有关的相互作用过程，从地表延伸到软流层，改变了北美岩石圈，从卡斯卡迪亚的俯冲复核到圣安德烈亚斯转换板块边界，将加利福尼亚沿海地区置于其后。地幔通过控制板块强度和运动学以及软流层向北美板块的热量和质量通量，对发生在MTJ附近的过程起着主要的控制作用。1993- 1994年广泛的活源地震研究结果为MTJ地区提供了良好的地壳控制作用。地震调查改善了MTJ地区地幔和下地壳的结构、结构、地震和地震形变成像。戈达地壳和上覆沉积物俯冲，并与卡斯卡迪亚俯冲带的增生地体合并，这些物质流入新兴的转换边缘，并构造成圣安德烈亚斯走滑体系。同时，戈尔达板块以南的板块裂口打开，引起软流圈上升和减压融化，使北美洲在圣安德烈亚斯断层附近的岩浆板块底部。经过再加工和膨胀的岩石圈增厚，形成了一个小的门多西诺角造山带，而侵蚀作用将大部分上地壳沿着构造控制的北向河流流回俯冲带。在附近，戈达-胡安-德富卡的俯冲形成了喀斯喀特火山，这也促进了大陆地壳的生长、分离和再循环。作为这个项目的一部分，地壳和上地幔的三维地震速度模型将被包括在加利福尼亚北部和俄勒冈南部的事件和强地面运动特征中，这是一个潜在的毁灭性大地震的地点。

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.