Summary

Ground rupture that occurred during the 28 June 1992 Landers-Big Bear, California, earthquake sequence has provided an unusual opportunity to map structures that form near the ground surface during strike-slip faulting. The largest structure, which is illustrated in an index-scale map, is the Landers-Big Bear rotating block. This block is defined on the east by an arcuate rupture 80-90 km long consisting of en echelon, right-stepping right-lateral fault zones that change orientation from north to south. The entire 1992 fault rupture defines an arc of about 60° with a radius of 70-80 km centered near the San Andreas fault zone to the west. In 1992 the Landers-Big Bear block rotated counterclockwise relative to surrounding ground, and deformed internally Deformation within the rotating block may have caused the left-lateral earthquake sequence near Big Bear Lake.

• Full Text
• Plate 1 - Traces of fractures along the Johnson Valley and Homestead Valley fault zones
• Plate 2 - Fractures along the Emerson fault zone northwest of Bessemer Mine Road
• Plate 3 - Fractures and belt of shear zones along Emerson fault at Single-Tower Transmission Line Road
• Plate 4 - Fractures in Two Ranches area of Homestead Valley fault zone
• Plate 5 - Fractures and relative displacements in Emerson fault zone at Tortoise Hill ridge
• Plate 6 - Fractures in the Charles en echelon zone
• Plate 7 - Fractures in left-lateral fault at Pipes Wash step in Homestead Valley fault zone