Crust and Lithospheric Structure – Natural Source Portable Array Studies of the Continental Lithosphere

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The advent of plate tectonics brought with it a recognition that seismology lacked the tools to image the deep continental lithosphere and the sublithospheric mantle at a resolution sufficient to piece together how plate tectonics formed and shaped the continents over geologic time. This recognition led directly to the formation of the Program for the Array Seismic Studies of the Continental Lithosphere (PASSCAL) and the Incorporated Research Institutions for Seismology (IRIS) consortium within the United States and more recently to other major national efforts in portable array seismology, particularly in Europe and China. The strong community consensus that was behind IRIS/PASSCAL (and other seismological initiatives around the globe) led to greatly increased funding for seismology that underwrote an advanced program for the development and acquisition of a large complement of standardized data loggers and sensors to study the continental lithosphere. Concurrently, independent breakthroughs in low-power microprocessors and compact mass storage devices for the consumer market, advances in force-feedback sensor microelectronics, and the launching of the Global Positioning System with its precise positioning and absolute timing capabilities fortuitously provided the essential ingredients for the manufacture of a powerful new generation of low-power portable seismic instrumentation. This low-power instrumentation for the first time enabled the continuous unattended broadband recording of seismic signals anywhere in the world for months or years at a time. Armed with these powerful and highly versatile seismic systems, seismologists fanned out across the globe to install portable arrays in many of the most scientifically challenging regions of the continents. The birth of this new brand of seismology, termed ‘passive array seismology,’ made high-resolution imaging of 3-D geologic structures deep in the lithosphere a reality. Data from these portable broadband arrays in turn spurred a new generation of advanced methodologies and animated fresh areas of seismic research, notably in regional body wave and surface wave tomography, direct waveform imaging of deep discontinuity structures, and shear-wave splitting and surface wave measurements of mantle anisotropy.