Seismotectonics of the San Andreas Fault System between Point Arena and Cape Mendocino in northern California: Implications for the development and evolution of a young transform

Abstract

The northernmost and relatively youthful segment of the San Andreas fault system is situated within a 100+ km wide zone of northwest trending strike-slip faults that includes, from west to east, the San Andreas, Maacama, and Bartlett Springs faults. Although the San Andreas fault is the principal strike-slip fault in this system, it has been virtually aseismic since the 1906 earthquake. Moderate levels of seismicity locate to the east along the Maacama fault and, to a lesser extent, the Bartlett Springs fault at focal depths typical of other strike-slip faults within the San Andreas fault system in central California. North of the San Andreas fault system, within the Cape Mendocino area, earthquakes occur at depths of up to 40 km and primarily reflect internal deformation of the subducting Garda slab, and slip along the Mendocino Fracture Zone. Seismicity along the Maacama and Bartlett Springs faults is dominated by right-lateral to oblique-reverse slip along fault planes that dip 500-75 ø to the northeast. The northern extent of seismicity along these faults terminates near the surface projection of the southern edge of the Garda slab. The onset of seismicity along these faults may be related to the abrupt change in the elastic thickness of the North American plate as it enters the asthenaspheric window. The Maacama and Bartlett Springs faults are strike-parallel with active reverse faults within the forearc region of the Cascadia subductian zone. This preexisting structural fabric of northwest trending reverse faults in the forearc area appears to have strongly influenced the initial slip and complexity of these faults. Continuation of the moderately dipping Maacama fault to the southeast along the steeply dipping Healdsburg and Rodgers Creek fault zones and the near-vertical Hayward and Calaveras fault zones in the San Francisco Bay area suggests that these faults evolve toward a more vertical dip to minimize the shear stresses that tend to resist plate motion.

Evidence for Moho‐lower crustal transition depth diking and rifting of the Sierra Nevada microplate

“…Imaging of the structural complexities associated with the slab window and the Gorda slab in northern California, south of the latitude of the MTJ, illustrates the impact of subduction processes extending south and southeast of the triple junction into north central California [Castillo and Ellsworth, 1993;Furlong and Schwartz, 2004;Liu et al, 2012] and reflecting a stress field increasingly modified by the Cascadia domain northward. Clearly, plate boundary driven shear and extensional faulting are concurrent in the modern Walker Lane and eastern Sierra [Faulds and Henry, 2008;Wesnousky, 2005;Wesnousky et al, 2012]; the combined contribution of plate boundary shear, northward evolution of the MTJ, and Cascadia domain subduction processes to the deformation and evolution of the northern Walker Lane and the northern Sierra is unclear.…”

Toward a better understanding of the Late Neogene strike-slip restraining bend in Jamaica: geodetic, geological, and seismic constraints

“…Nearly all relocated earthquakes are found between depths of 12 and 27 km, remarkably deep in comparison with continental settings such as northern and central California, where almost all seismicity is confined above crustal depths of 11 -12 km (e.g. Castillo & Ellsworth 1993). Focal mechanisms reveal an island dominated by east -west-directed, left-lateral shear with a lesser north -south convergent component (Wiggins-Grandison 2003;Wiggins-Grandison & Atakan 2005).…”

Structural analysis of hypocentral distribution of an earthquake sequence using anisotropic wavelets: Method and application

“…Our test concerns the 2288 M ≥ 2 seismic events that have occurred between 1 January 1980 and 31 December 1997 in the San Francisco Bay area (121.53°W to 122.87°W, 37.25°N to 38.32°N) (Figure 3a). There several well‐known vertical or subvertical faults or fault segments are marked by densely aligned epicenters (Hayward, Greenville, Concord faults, segment of San Andreas fault through the San Francisco peninsula, and central segment of the Calaveras fault [ Castillo and Ellsworth , 1993]; see Figure 3b). The lengths of these seismic features, within the study area, are ∼40 km (except the Hayward fault, which exceeds 80 km).…”

“…(a) Map of the 2228 epicenters (data from NCEDC, 1 January 1980 to 31 December 1997). (b) Mapped faults from Castillo and Ellsworth [1993]. (c) NOAWC map obtained for a resolution equal to the mean estimated error on location ( a = 8 pixels ∼ 4 km).…”