Great Valley Thrust Faults

We use the 1983 Coalinga earthquake (GV13) as the principal example for segmentation of the Great Valley thrust fault system (Figure 9). Stein and Ekstrom [1992] modeled the leveling data from before and after the 1983 event. Their model is in accord with the focal mechanism of the mainshock, the 3-dimensional pattern of the aftershocks, and the interpretation of crustal and geologic structures of the region by Wentworth and Zoback [1989]. We used a simplified version of their favored thrust fault mechanism as a template for a thrust fault system that extends as far north as the Rumsey Hills (39° N) shown as segments GV01 to GV14. Except for the northernmost two segments, a single rate of shortening 1.5 mm/yr is consistent with both the VLBI rate modeled for the Pacific plate-Sierran block boundary and local rates of uplift where such data were available [D.F. Argus, written commun. 1995; Wakabayashi and Smith [1994]). Unruh and Moores [1992] and Unruh and others [1995a,b] show that the structural setting of the Sacramento Valley is also compatible with the same variety of thrust mechanism.

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Figure 9. Great Valley thrust faults. Segments used in this study (GV01-GV14); heavy dashed lines indicate blind thrust tips buried at 7 km depth. Olive green rectangles show downdip extent of these hypothetical ruptures. Segments of Wakabayashi and Smith [1994](labeled WS-1 to WS-17) based mainly on geomorphic interpretation of range front. Quaternary faults of Jennings [1992] shown as orange and red lines for youngest faults and narrower green and purple lines for oldest faults. Other lines: antiforms (dashes and pluses), major piercements (MDA, Mount Diablo; NIA, New Idria) and surficial thrusts (continuous ticks, on hanging wall) [Jennings, 1977; Phipps, 1992]. Along Sacramento Valley margin, fine black lines indicate interpreted subsurface ramps (arrow lines point up-dip), flats (T's) and subsurface thrust tips (2-tick dashes) of Unruh and others [1995b]. Historical earthquakes shown as stars attributed by magnitude and year of occurrence [Ellsworth, 1990].

Segmentation follows the paradigm set by the Coalinga rupture. A stepover and a truncation of the principal antiformal structure adjoining the Great Valley synform axis distinctly limited the areal extent of the 1983 rupture [Stein and Ekstrom, 1992]. Stein and Ekstrom [1992] show by modeling years of postseismic deformation that aseismic slip probably continued downdip to 15 km, below the 7-10 km depth range attributed to the coseismic rupture. This postseismic creep is an important feature that tends to limit the amount of seismic moment available coseismically for such earthquakes, because we believe that slip between depths of 10-15 km occurs aseismically in this region. For segmentation, we mainly relied on major bends, stepovers and truncations of the principal antiformal structure immediately adjacent to the Great Valley synform. For comparison, in Figure 9 we show the generally similar segmentation results of Wakabayashi and Smith [1994] and of Unruh and others [1995b].