Seismic evidence for conjugate slip and block rotation within the San Andreas Fault System, southern California

Abstract

The pattern of seismicity in southern California indicates that much of the activity is presently occurring on secondary structures, several of which are oriented nearly orthogonal to the strikes of the major through-going faults. Slip along these secondary transverse features is predominantly left-lateral and is consistent with the reactivation of conjugate faults by the current regional stress field. Near the intersection of the San Jacinto and San Andreas faults, however, these active left-lateral faults appear to define a set of small crustal blocks, which in conjunction with both normal and reverse faulting earthquakes, suggests contemporary clockwise rotation as a result of regional right-lateral shear. Other left-lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data. Many of these structures predate the modern San Andreas system and may control the pattern of strain accumulation in southern California. Geodetic and paleomagnetic evidence confirm that block rotation by strike-slip faulting is nearly ubiquitous, particularly in areas where shear is distributed, and that it accommodates both short-term elastic and long-term nonelastic strain. A rotating block model accounts for a number of structural styles characteristic of strike-slip deformation in California, including: variable slip rates and alternating transtensional and transpressional features observed along 1Also atPaper number 6T0103.0278-7407/86/006T-0103510.00 strike of major wrench faults; domains of evenly-spaced antithetic faults that terminate against major fault boundaries; continued development of bends in faults with large lateral displacements; anomalous focal mechanisms; and differential uplift in areas otherwise expected to experience extension and subsidence. Since block rotation requires a detachment surface at depth to permit rotational movement, low-angle structures like detachments, of either local or regional extent, may be involved in the contemporary strike-slip deformation of southern California. A block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge-bounding faults, but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction. This behavior, coupled with the presence of possible regional detachments, accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges. Although fault displacements along secondary structures associated with block rotations remain small, they may still influence the nucleation and the characteristic rupture length of large earthquakes. A more complete description of what these structures are, and how they interact, may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard. N.J., 1981.

Aftershocks and Triggered Events of the Great 1906 California Earthquake

“…This agrees well with the findings of Mendoza and Hartzell (1988), who made similar observations looking at aftershock patterns and mainshock faulting associated with a number of earthquakes in California andIdaho between 1966 and1986. This characteristic is also consistent with the conclusions of Liu et al (1999Liu et al ( , 2003, that most of the aftershocks of the 1992 Landers earthquake are not candidates for rerupture of the mainshock faults, and of Rubin and Gillard (2000), who showed that aftershocks of microearthquakes on the central SAF tend not to occur within a distance approximately equal to the radius of the first rupture.…”

“…(In the 1979 earthquake, which involved rupture on the Brawley fault and the Imperial fault south to the international border, the intensity was uniformly MMI VII from Brawley to Calexico; see Reagor et al [1982] and Nason [1982].) Alternatively, slip along one of the northeast-trending cross faults southeast of the Salton Sea could be responsible; candidates include the fault involved in the strongest aftershock of the 1979 event (Johnson and Hutton, 1982) and the fault involved in the 1981 Westmorland earthquake (Nicholson et al, 1986 (Gurrola and Rockwell, 1996). We also rule out a location on the SAF near or northwest of Bombay Beach because Bombay Beach is approximately halfway between Brawley and Coachella, but the intensities are much higher in Brawley and to the south than they are in Coachella and to the north.…”

Aftershocks of the 2010 Mw 7.2 El Mayor‐Cucapah earthquake reveal complex faulting in the Yuha Desert, California

“…A variety of fault geometries, including flower structures (e.g., where complex surface faults coalesce onto simpler structures at depth) and conjugate faulting (e.g., where complex surface faults remain individual planes at depth) have been previously identified along major strike-slip faults. Weldon II et al [2002] and Harding [1985,1988] observe active flower structures along the San Andreas fault and other prominent strike-slip faults in Southern California, while Sharp [1967], Nicholson et al [1986], and Hauksson et al [2002] suggest that conjugate faulting is common in immature fault systems. Here multiple planar faults that extend the full depth of the seismogenic zone are apparent in three-dimensional visualizations of the relocated seismicity.…”

Recent and Long-Term Behavior of the Brawley Fault Zone, Imperial Valley, California: An Escalation in Slip Rate?

“…perial fault. The BSZ is a diffuse zone of seismicity that extends north-northwest from the BFZ toward the San Andreas fault (SAF) at Bombay Beach; focal mechanisms and seismicity lineaments within the BSZ indicate that most of the earthquakes occur on left-lateral northeast-trending and right-lateral northwest-trending cross-faults (e.g., Fuis et al, 1982;Nicholson et al, 1986;P. Shearer, unpublished data).…”