Abstract
We invert for the state of stress in the southern California crust using a catalogue of high quality earthquake focal mechanisms (1981–2010). The stress field is best resolved where seismicity rates are high and sufficient data are available to constrain the stress field across most of the region. From the stress field, we determine the maximum horizontal compressive stress (S_Hmax) orientations and the style of faulting across southern California. The trend of S_Hmax exhibits significant regional and local spatial heterogeneities. The regional trend of S_Hmax varies from north along the San Andreas system to NNE to the east in the Eastern California Shear Zone as well as to the west, within the Continental Borderland and the Western Transverse Ranges. The transition zones from one state of stress to the other occur over a distance of only a few kilometres, following a trend from Yucca Valley to Imperial Valley to the east, and the western edge of the Peninsular Ranges to the west. The local scale heterogeneities in the S_Hmax trend include NNW trends along the San Andreas Fault near Cajon Pass, Tejon Pass and the Cucapah Range, as well as NNE trends near the northern San Jacinto Fault and the Wheeler Ridge area. The style of faulting exhibits similar complexity, ranging from predominantly normal faulting in the high Sierra Nevada, to strike-slip faulting along the San Andreas system, to three consecutive bands of thrust faulting in the Wheeler Ridge area and the Western Transverse Ranges. The local variations in the style of faulting include normal faulting at the north end of the San Jacinto Fault and scattered areas of thrust faulting. The regional variations in the SHmax trends are very similar to the pattern of the GPS-measured maximum shortening axes of the surface strain rate tensor field although the strain field tends to be smoother and appears to capture some of the upper-mantle deformation field. The mean trend of S_Hmax departs about approximately 14° to the east from the trend of the maximum shortening directions derived from anisotropy in the upper mantle.
Highlights
The geometrically complex tectonic boundary between the Pacific and the North America plates cuts across southern California causing a broad distribution of seismicity (Fig. 1)
The Western Transverse Ranges and Continental Borderland (WTRCB) zone extends to the west of the grey and solid dashed curves (‘Western Peninsular Ranges Line (WPR-L)’ in Fig. 8), which is located to the west of the San Andreas Fault (SAF) inside the Pacific Plate
We find that the SHmax orientation in the seismogenic layer is rotated about 14◦ clockwise relative to the maximum shortening direction indicated by anisotropy in the upper mantle
Summary
The geometrically complex tectonic boundary between the Pacific and the North America plates cuts across southern California causing a broad distribution of seismicity (Fig. 1). The inset panel on the top right-hand corner shows the relative location of the mapped area in the whole state of California, with the San Andreas Fault in bold, and relative plate motions with arrows. Using focal mechanisms from ∼50 000 earthquakes recorded by the Southern California Seismic Network (SCSN) from 1981 to 1999, Hardebeck & Hauksson (2001b) inverted for the state of stress in southern California. Their results showed that the SAF in southern California is a weak fault in a low-strength crust. Our results provide new insights into the regional and local tectonics along the plate boundary
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