Evidence for Vertical Partitioning of Strike-Slip and Compressional Tectonics from Seismicity, Focal Mechanisms, and Stress Drops in the East Los Angeles Basin Area, California

Abstract

We have synthesized the characteristics of the seismogenic zone in the east Los Angeles basin by analyzing earthquake data recorded during the past 30 years . The seismicity is distributed along the Whittier fault, with the majority of earthquakes located adjacent to the south side, in the depth range from 0 to 9 km, with b value of 1:1 0:05 and mostly normal and strike-slip faulting. Within the depth range of 9-12 km, the seismicity is scattered uniformly across the region, the b value is 1:0 0:05, and all three faulting styles are present. At the deepest depths (12-18 km), seismicity is sparse and primarily limited to a few clusters striking north; these deeper earthquakes primarily have reverse fault motion, and the b value is 0:78 0:04. Inversion of high-quality focal-mechanism data for the orientation of the regional stress field showed that the direction of maximum compressional stress rotates from N12°W at shallow depth to due north at the bottom of the seismogenic zone. Similarly, a depth dependence is observed in stress drops calculated from P-wave source spectra, which indicate stress drop generally increases from ∼7 MPa at shallow depth (3 km) to ∼53 MPa at the base of the seismogenic zone (17 km). Overall, our results provide new evidence for the vertical partitioning of styles of deformation and state of stress within this complex fault system in the east Los Angeles basin.Online Material: Stress drop fits to source spectra.

Computing a Large Refined Catalog of Focal Mechanisms for Southern California (1981-2010): Temporal Stability of the Style of Faulting

“…The bvalue decreases with the styles of faulting following the order from normal faulting, to strike-slip faulting, to reverse faulting, which is consistent with the findings of previous studies (e.g., Hauksson 1990;Schorlemmer et al, 2005;Yang and Hauksson, 2011). Earthquakes with reverse faulting occurred at relatively deeper depth compared to earthquakes of the other two faulting styles (Fig.…”

“…For depths shallower than ∼8 km, the probability densities of strike-slip and normal faulting are higher than for reverse faulting, while the opposite holds for depths beneath 8 km. Such a pattern of mechanism styles with depth is different from what was found for the east Los Angeles area by Yang and Hauksson (2011). This is in part because the east Los Angeles area has clear vertical differential movement controlled by tectonic compression and escape tectonics between the Transverse Range and the Peninsular Range, while the southern California region as a whole is dominated by the horizontal right-lateral strikeslip plate boundary motion.…”

Mechanics, slip behavior, and seismic potential of corrugated dip‐slip faults

“…The slip direction on faults is often used to determine the orientation of the regional principal stresses [e.g., Angelier et al , 1985; Becker et al , 2005; Kaven et al , 2011; Yang and Hauksson , 2011]. However, analysis of slip vectors on modeled corrugated fault surfaces suggests that that the slip direction on a corrugated fault does not uniquely define a loading direction (Figure 15).…”

Ambient tectonic stress as fragile geological feature

“…It is relevant to apply to appraise this assumption, as geophysicists have not measured tectonic stress in these rocks. As in the Parkfield area, strike‐slip faults and folds above blind‐thrust faults accommodate deformation [ Yeats , ; Meigs et al ., ; Marshall et al ., ; Yang and Hauksson , ]. The compressional strain rate is approximately 10 −7 yr −1 [ Marshall et al ., ] (∼1 mm yr −1 over 10 km across strike).…”