A 5-km gap in ground rupture and a decrease in aftershock epicenter density associated with the 1992 M 7.3 Landers earthquake occurs at the intersection of the Johnson Valley and Pinto Mountain faults, near the town of Yucca Valley, California. Field mapping reveals a family of curvilinear faults occupies this region, undisturbed by the Landers seismic event. Wholly contained within Township Section 25, this family of faults strike N20°W at the northern border of the Section and rotates eastward, striking N70oW where last seen at the eastern boundary, with dips ranging between 60° to 80° NE. Where relative motion on the Johnson Valley fault is predominately dextral strike-slip, field evidence supports the curvilinear faulting to contain a component of high angle dip-slip. This study concludes the curvilinear nature of the faulting is the product of warping within the regional rock stresses due to the normal intersection of the Pinto Mountain and Johnson Valley faults. Each fault influences the other, creating a distortion in their respective stress fields subsequently followed by the curvilinear faults.
Field findings suggest the lack of ground rupture and seismic activity within this sector of the Lander earthquake resulted, at least in part, from the presences of the curvilinear and the Pinto Mountain faults. Seismic inactivity to a depth of 3 km beneath the surface trace of the curvilinear faults may be attributed to the oblique orientation and opposing dips of the curvilinear fault planes relative to the north-south aligned stress field of the Landers earthquake. As a result, the curvilinear faults resisted slip that occurred elsewhere during the Landers event. At depths greater than 3 km, seismic activity truncates in the subsurface against a plane plunging 77° N that originates at the surface trace of the Pinto Mountain fault. Field mapping also discovered high-angle reverse thrust faulting (named the Sawtooth fault) along the central axis of the Sawtooth Range to the west of the Section 25. These data are consistent with the Pinto Mountain fault under-thrusting the Mojave Block and uplifting this eastern extension of the San Bernardino Mountains. Field relationships also suggest this occur coincident with the activation of the San Andreas fault and rotation of the East Transverse Range province, thus suggesting a late Neogene to Holocene age of faulting.
Due to the under-thrusting of the Sawtooth range by the Pinto Mountain fault, dip-slip took local precedent over strike-slip separation that was otherwise associated with the Lander event. Consequently, the ground rupture gap represents a localized tectonically locked region. Because of this, the area between the Sawtooth Range and the Johnson Valley fault may be experiencing extension and pull apart. This is supported by a northeast-southwest strike in the fault trace orientation exhibited by the southern-most surface expressions in the Johnson Valley fault.
Presentation Date: Wednesday, October 17, 2018
Start Time: 8:30:00 AM
Location: 211A (Anaheim Convention Center)
Presentation Type: Oral