Thickness Of Seismogenic Layer Research Articles

Block versus continuum deformation in the Western United States

The relative role of block versus continuum deformation of continental lithosphere is a current subject of debate. Continuous deformation is suggested by distributed seismicity at continental plate margins and by cumulative seismic moment sums which yield slip estimates that are less than estimates from plate motion studies. In contrast, block models are favored by geologic studies of displacement in places like Asia. A problem in this debate is a lack of data from which unequivocal conclusions may be reached. In this paper we apply the techniques of study used in regions such as the Alpine-Himalayan belt to an area with a wealth of instrumental data—the Western United States. By comparing plate rates to seismic moment release rates and assuming a typical seismogenic layer thickness of 15 km it appears that since 1850 about 60% of the Pacific-North America motion across the plate boundary in California and Nevada has occurred seismically and 40% aseismically. The San Francisco Bay area shows similar partitioning between seismic and aseismic deformation, and it can be shown that within the seismogenic depth range aseismic deformation is concentrated near the surface and at depth. In some cases this deformation can be located on creeping surface faults, but elsewhere it is spread over a several kilometer wide zone adjacent to the fault. These superficial creeping deformation zones may be responsible for the palaeomagnetic rotations that have been ascribed elsewhere to the surface expression of continuum deformation in the lithosphere. Our results support the dominant role of non-continuum deformation processes with the implication that deformation localization by strain softening must occur in the lower crust and probably the upper mantle. Our conclusions apply only to the regions where the data are good, and even within the Western United States (i.e., the Basin and Range) deformation styles remain poorly resolved. Nonetheless, we maintain that block motion is the deformation style of choice for those continental regions where the data are best.

Normal faulting in the upper continental crust: observations from regions of active extension

Observations of present-day normal faulting in regions of active continental extension may be helpful when interpreting the geological record in older extensional basins. The most obvious manifestation of active extension is normal faulting in earthquakes. Earthquake foci are mostly confined to the upper (seismogenic) continental crust, whose thickness imposes a scale on the observed deformation. Large earthquakes move faults with lengths similar or large compared with the thickness of the seismogenic layer. Large seismogenic normal faults on the continents appear to be restricted to a dip range of around 30–60°: dips significantly gentler than 20° have not been observed in fault plane solutions of large earthquakes. Large seismographic normal faults are approximately planar in cross-section and cut through the base of the upper seismogenic layer, rotating about a horizontal axis as they move. A reasonable estimate of the regional extension can often be made from the dip of the large normal faults and the tilt of the blocks they bound (the ‘domino model’). Such faults are rarely continuous for more than 15–20 km, but commonly change strike or step in an en échelon fashion. This segmentation may occur on a scale controlled by the thickness of the seismogenic layer, and is an important influence on sedimentation and drainage. These observations are common to large seismogenic normal faults from a variety of tectonic settings, and suggest that the kinematics of normal faulting may not be strongly influenced by the forces responsible for the extension, which can vary widely in nature and magnitude. Small earthquakes, which move faults that are small compared with the seismogenic layer thickness, show no simple pattern and often can be interpreted as internal deformation of blocks bounded by large faults. In some places the seismogenic basement faults do not reach the surface but are decoupled from the sedimentary cover by layers of weak lithology. Faults in the sedimentary cover may be strongly curved in cross-section, requiring their hangingwalls to deform internally. Estimating extension from such faults is not straightforward and requires a knowledge of the hangingwall deformation. Estimates made in this way are often very non-unique and prone to large errors. We postulate that the thickness of the seismogenic upper crust controls both the maximum length of large normal fault segments along strike and the maximum size of blocks that can rotate coherently about a horizontal axis.