Chapter 5 Paleoseismology of Compressional Tectonic Environments

Publisher Summary Large compressional earthquakes in the upper crust, and even larger plate boundary earthquakes in subduction zones, produce surface deformation recorded by displacements on reverse or thrust faults by growth of surface folds and by changes in the elevation of the land surface. Study of such stratigraphic and geomorphic features yields information about the size and recurrence of large earthquakes that is not available from historic sources in many regions. This chapter presents an overview of compressive structures on regional scales and focuses on paleoseismic evidence for coseismic compressive deformation on local and site scales. Most examples are drawn from our experience in western North America. The most common seismogenic structure in compressional tectonic environments is the thrust fault (a low-angle reverse fault). Movement on thrusts results in thickening of the vertical section of crust cut by the fault and shortening of the horizontal distance across the fault. In many geologic environments, thrust faults bifurcate as they project toward the surface, producing complex fault systems at the surface. Individual fault splays at all scales separate thrust-bounded slices of rock, and these slices are often stacked by repeated fault offset. At depth the splays merge and the dips of the faults decrease until they merge into near horizontal detachments below which shortening does not occur through brittle deformation.