| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025
Abstract
The Ms 7.1 Loma Prieta earthquake of 18 October 1989 produced abundant ground ruptures in an 8 by 4 km area along Summit Road and Skyland Ridge in the Santa Cruz Mountains. Predominantly extensional fissures formed a left-stepping, crudely en echelon pattern along ridges of the hanging-wall block southwest of the San Andreas fault, about 12 km northwest of the epicenter. The fissures are subparallel to the San Andreas fault and appear to be controlled by bedding planes, faults, joints, and other weak zones in the underlying Tertiary sedimentary strata of the hanging-wall block. The pattern of extensional fissures is generally consistent with tectonic extension across the crest of the uplifted hanging-wall block. Also, many displacements in Laurel Creek canyon and along the San Andreas and Sargent faults are consistent with right-lateral reverse faulting inferred for the mainshock. Additional small tensile failures along the axis of the Laurel anticline may reflect growth of the fold during deep-seated compression. However, the larger ridge-top fissures commonly have displacements that are parallel to the north-northeast regional slope directions and appear inconsistent with east-northeast extension expected from this earthquake. Measured cumulative displacements across the ridge crests are at least 35 times larger than that predicted by the geodetically determined surface deformation. These fissures also occur in association with ubiquitous landslide complexes that were reactivated by the earthquake to produce the largest concentration of co-seismic slope failures in the epicentral region. The anomalously large displacements and the apparent slope control of the geometry and displacement of many co-seismic surface ruptures lead us to conclude that gravity is an important driving force in the formation of the ridge-top fissures. Shaking-induced gravitational spreading of ridges and downslope movement may account for 90¿ or more of the observed displacements on the linear fissures. Similar fissures occurred in the same area and elsewhere near the San Andreas fault during the predominantly right-lateral 1906 San Francisco earthquake and suggest that the Loma Prieta ground ruptures may, in large part, be independent of fault kinematics.
This article has been cited by other articles:
|
|
 |
|
  W. H. Bakun Seismic activity of the San Francisco Bay region Bulletin of the Seismological Society of America, June 1, 1999; 89(3): 764 - 784. [Abstract] [PDF]
|
|
|
|
|
|
M. Bouchon and J. S. Barker Seismic response of a hill: The example of Tarzana, California Bulletin of the Seismological Society of America, February 1, 1996; 86(1A): 66 - 72. [Abstract] [PDF]
|
|
|
|
|
|
A. Aydin and Y. Du Surface rupture at a fault bend: the 28 June 1992 Landers, California, earthquake Bulletin of the Seismological Society of America, February 1, 1995; 85(1): 111 - 128. [Abstract] [PDF]
|
|
|
|
|
|
S. H. Hartzell, D. L. Carver, and K. W. King Initial investigation of site and topographic effects at Robinwood Ridge, California Bulletin of the Seismological Society of America, October 1, 1994; 84(5): 1336 - 1349. [Abstract] [PDF]
|
|
|
|
|
|
C. S. PRENTICE and D. P. SCHWARTZ Re-evaluation of 1906 surface faulting, geomorphic expression, and seismic hazard along the San Andreas fault in the southern Santa Cruz Mountains Bulletin of the Seismological Society of America, October 1, 1991; 81(5): 1424 - 1479. [Abstract] [PDF]
|
|
|
|
|
|
T. C. HANKS and H. KRAWINKLER The 1989 Loma Prieta earthquake and its effects: Introduction to the special issue Bulletin of the Seismological Society of America, October 1, 1991; 81(5): 1415 - 1423. [PDF]
|
|
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
