Diverse Volumetric Faulting Patterns in the San Jacinto Fault Zone

Abstract

We examine locations, magnitudes, and faulting types of post‐2000 earthquakes in the trifurcation area of San Jacinto fault zone to clarify basic aspects of failure processes in the area. Most M ≥ 3.5 events have strike‐slip mechanisms, occur within 1 km of the main faults (Clark, Buck Ridge, and Coyote Creek), and have hypocenter depths of 10–13 km. In contrast, many smaller events have normal source mechanisms and hypocenters in intrafault areas deeper than 13 km. Additional small events with hypocenter depth <13 km occur in off‐fault regions and have complex geometries including lineations normal to the main faults. Five moderate earthquakes with M 4.7–5.4 have high aftershock rates (~150 M ≥ 1.5 events within 1 day from the mainshock). To obtain more details on aftershock sequences of these earthquakes, we detect and locate additional events with the matched filter method. There are almost no aftershocks within 1 km from the mainshocks, consistent with large mainshock stress drops and low residual stress. The five aftershock sequences have almost no spatial overlap. While the mainshocks are on the main faults, most aftershocks are located in intrafault and off‐fault regions. Their locations and spatial distribution reflect the mainshock rupture directions, and many also follow structures normal to the main faults. The significant diversity of observed features highlights the essential volumetric character of failure patterns in the area. The increasing rate of moderate events, productive aftershock sequences, and large inferred stress drops may reflect processes near the end of a large earthquake cycle.

Off‐Fault Focal Mechanisms Not Representative of Interseismic Fault Loading Suggest Deep Creep on the Northern San Jacinto Fault

“…The correlation between the slip sense of focal mechanisms in the San Bernardino basin and patterns of off‐fault stressing rate from interseismic models with ~10‐km locking depth on the SJf suggests that the interseismic microseismicity of the basin records a component of permanent distributed off‐fault deformation in the basin. This result is consistent with a recent study of normal slip focal mechanisms along the Anza section of the SJf (Cheng et al, ). If the focal mechanisms of the basin were inverted to estimate interseismic stresses on the SJf and SAf, they would predict normal loading contrary to the long‐term slip record of these faults.…”

“…Yang et al (2012) report temporary changes in focal mechanism slip sense after large magnitude earthquakes in Southern California. Cheng et al (2018) report off-fault aftershocks that have different slip sense from the earthquakes that occur along the Anza segment of the SJf, to the south of the study area of this paper. Some of the normal slip earthquakes within the San Bernardino basin have been associated with secondary normal faults revealed by geophysical imaging of the top of the basement (Anderson et al, 2004).…”

“…If the ongoing SJf slip is contributing to the off‐fault normal slip microseismicity, then the fault below this depth may be creeping. Along the Anza section of the SJf, south of this study area, normal slip microseismicity also occurs near the SJf at depths of 10–13 km (Cheng et al, ). The discrepancy between locking depth of the Anza section of the SJf inferred from geodesy (11 ± 3 km; Fialko, ) and the base of seismicity in this region (17 ± 3 km) led to the inference of local creep below 10 km (Wdowinski, ), which is consistent with the depths of off‐fault normal microseismicity along this section of the SJf (Cheng et al, ).…”

Volumetric and shear processes in crystalline rock approaching faulting

“…events and between left-/right-lateral shear events may occur in the form of conjugate microseismicity and faulting or distributed deformation near complex fault geometries, as well as in the surrounding volume (26,39). High-resolution earthquake catalogs in well-instrumented areas show that ongoing seismicity can occur in zones with width of the order of 10 km around major faults (1,3). While the largest events occur on the main faults, off-fault seismicity and aseismic processes of the type documented in this study can play key roles in the localization or relocalization of faulting in geomaterials.…”

“…Highresolution earthquake catalogs show abundant microseismicity in the volumes surrounding major faults (1,2). The interaction between seismicity along faults and in the surrounding rock volume is critical for understanding the spatiotemporal evolution of earthquakes in active areas (3,4). Laboratory experiments demonstrate that microfracture generation produces macroscopic dilation of rocks before macroscopic shear failure occurs (5)(6)(7) and controls the transition from distributed damage to shear localization (8)(9)(10)(11)(12).…”