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Shattered Rock and Precarious Rock Evidence for Strong Asymmetry in Ground Motions during Thrust Faulting

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Recent thrust faulting models, both physical and numerical, have indicated very high ground motions on the hanging wall and relatively low motions on the footwall of thrust faults. In this article, recent field evidence supporting this strong asymmetry is described. Earlier field observations of objects thrown in the air also suggest that ground motions must have exceeded 1g and 100 cm/sec. New shattered rock evidence on the hanging wall of thrust faults in Southern California supports the occurrence of intense ground motions. On the other hand, the lack of shattered rock and the presence of precariously balanced rocks on the footwall of two thrust faults in Southern California indicate relatively low ground motions, thus indicating strong asymmetry in ground motions near the fault trace for thrust faults. Some current ground-motion attenuation curves and hazard maps may give too low values on the hanging wall and too high values on the footwall. This could be of critical importance to estimating earthquake hazard from potentially large thrust fault earthquakes in the Los Angeles Basin and elsewhere in the world. The new evidence also supports the idea that dynamic inertial detachment of the hanging wall from the footwall occurs in nature in the manner suggested by some recent physical and numerical models. If such detachment occurs, it could also have important implications for understanding thrust fault rupture mechanics, for example, the paradox of large overthrusts and the heat flow paradox for thrusts.

Bulletin of the Seismological Society of America Vol. 91, June 2001, pp. 443–445

The photographs in figures 2, 3, and 4, printed incorrectly in black and white, are shown here in color.

View larger version (574K): [in this window] [in a new window]   Figure 2. Examples of thrust fault hanging-wall shattered rock within a few to several km of the fault outcrop: (a) Sierra Madre fault, San Gabriel Mountains, Big Tajunga Canyon; (b) Sierra Madre fault, San Gabriel Mountains, State Highway 39; (c) White Wolf fault system, Grapevine area, U.S. Interstate 5, (d) Banning fault system, San Gorgonio Mountains, State Highway 38; (e) Close-up of shattered rock, showing typical appearance on a small-scale. Note separation of cracks and lack of evidence of shearing.

View larger version (248K): [in this window] [in a new window]   Figure 3. Examples of precarious or semiprecarious rocks on the foot wall of the White Wolf fault, Southern California: (a) Balanced rock showing range front scarp of the White Wolf fault at the foot of Bear Mountain (in clouds). Distance from the fault trace is about 5 km. Height of rock is about 1 m. (b) Tall, thin balanced rock in the same area. Height of rock is about 1.5 m. (c) Large, delicately balanced rock in same area. Height of rock is about 4 m.

View larger version (306K): [in this window] [in a new window]   Figure 4. Examples of precarious or semiprecarious rocks on the foot wall of the Banning Thrust fault, south of San Gorgonio pass, along State Highway 243: (a) Balanced rock about 5 km south of the fault. Height of rock is about 1 m. Town of Banning is visible in the distance. View is to the north toward the San Gorgonio mountain range, hidden in smoke. (b) Balanced rock about 4 km south of the Banning fault. (c) Balanced rock about 15 km south of Banning fault, near the large balanced rock shown in an earlier publication, Figure 2m in Brune (1996).

This Article Abstract Figures Only Full Text (PDF) Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Brune, J. N. Search for Related Content