Geologic investigations of a “slip gap” in the surficial ruptures of the 1992 Landers earthquake, southern California

TitleGeologic investigations of a “slip gap” in the surficial ruptures of the 1992 Landers earthquake, southern California
Publication TypeJournal Article
Year of Publication1995
AuthorsSpotila JA, Sieh K
JournalJournal of Geophysical Research: Solid Earth
Volume100
Pagination543-559
ISBN Number2156-2202
Keywords8110 Tectonophysics: Continental tectonics—general, 8150 Tectonophysics: Plate boundary—general
Abstract

A 3-km-long gap in the dextral surficial rupture of the 1992 Mw = 7.3 Landers earthquake occurs at the north end of a major fault stepover between the Johnson Valley and Homestead Valley faults. This gap is situated along a segment of the Landers rupture that has been modeled geophysically as having a deficit in average slip at depth. To better evaluate the nature of the slip gap, we document in detail the character and distribution of surficial rupture within it. Along the gap, is a northwest trending thrust fault rupture with an average of less than 1 m of northeast directed reverse-slip and nearly no oblique right slip. We interpret this rupture to be limited to the shallow crust of the northern end of the stepover and to have been the secondary result of dextral shear, rather than a mechanism of rigid-block slip-transfer from the Landers-Kickapoo fault. A zone of en echelon extensional ruptures also occurs along the slip gap, which we interpret as the secondary result of diffuse dextral shear that accommodated less than 0.5 m of west-northwest extension. These secondary ruptures represent a discontinuity in the surficial dextral rupture of the Landers earthquake, which we propose resulted from the lack of a mature fault connection between the Johnson Valley and Homestead Valley faults. The rupture pattern of the slip gap implies a significant deficit in net surficial slip, which compares favorably with some geophysical models. Aspects of this rupture pattern also suggest a temporal sequence of rupture that compares favorably with geophysical interpretations of the dynamic rupture propagation.

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