Faculty and Collaborators: Ramon Arrowsmith (Arizona State), John Baldwin (Lettis and Assoc.), James Dolan (USC), Eldon Gath (ECI), Lisa Grant (UCI), Martha House (SLU), Ed Keller (UCSB), Keith Kelson (Lettis and Assoc.), Scott Lindvall (Lettis and Assoc.), Sally McGill (Cal State-San Bernardino), Karl Mueller (Colorado), Lewis Owen (UCR), Dan Ponti (USGS), Tom Rockwell (SDSU), Charlie Rubin (Central Washington), John Shaw (Harvard), Kerry Sieh (Caltech), Glenn Tinsley (USGS), Jerry Treiman (CDMG), Ray Weldon (Oregon), Bob Yeats (OSU)

Postdocs: Chris Goldfinger (Oregon State), Gary Huftile (Brisbane), Jan Vermilye (Columbia), Doug Yule (Caltech)

Students: Lesley Ballenger (Chapman), Tim Dawson (SDSU), Safaa Dergham (Cal State-Long Beach), Maggi Glasscoe (UC-Davis), Dawn Grant (Cal State-San Bernardino), Larry Gurrola (UCSB), Ross Hartleb (USC), Robert Langridge (Oregon), Nate Onderdonk (UCSB), Mike Oskin (Caltech), Meredith Robertson (USC), Jim Spotila (Caltech), Elizabeth Stone (Arizona State), Molly Trecker (UCSB), Allan Tucker (USC), Chris Walls (San Diego State), Kris Weaver (USC)

SCEC geologists worked on several neotectonic and paleoseismic problems throughout 1998. As in years past, our efforts focused on determining the parameters of active faults that are relevant to understanding the character and frequency of future earthquakes.

Ernie Duebendorfer and Jan Vermilye have published their initial investigation of the possibility that aseismic deformation might account for a large fraction of the north-south shortening across the LA urban region that is seen in geodetic measurements and geologic. Although they have not yet quantified the amount of pressure solution, they have found that it is common in the rocks of the Ventura Basin. They suggest that it might well absorb 10 to 20% of the north-south contraction across the Los Angeles region. This amount of aseismic deformation in the depth range of 10 to 20 km would lessen the need for future large earthquakes within the region, but would still leave most of the seismic "deficit" that has been proposed by Dolan and others in 1995.

Earthquake Sources in the Metropolitan Los Angeles Region

Several SCEC investigators have continued their work on source characterization in the greater Los Angeles region in 1998. Rubin, Lindvall and Rockwell published their paleoseismic work at the Loma Alta site, across a strand of the Sierra Madre fault in Altadena. They interpret radiometric- and soils-age estimates of two colluvial wedges to indicate that this fault at the base of the San Gabriel mountains has sustained two slippages totaling about 10.5 meters in the past 15,000 years. Such large slip events would be consistent with earthquake magnitudes above 7. This result supports the hypothesis that large earthquakes on the large, composite fault systems in the region relieve most of the north-south strain. But the very long recurrence times are several times longer than those proposed by Dolan et al. (1995). Oskin and Sieh have just submitted their manuscript on the active folds and fold scarps of the central Los Angeles area. Their geomorphic, geochronologic and stratigraphic evidence has led them to conclude that these structures accommodate about 1 mm/yr of north-south contraction across the Elysian Park anticline. They contend that this is evidence of an active reverse fault, which extends 20 km from Hollywood to Whittier, beneath the central LA region. They calculate that a nominal characteristic earthquake on this structure would be about Mw 6.5 to 6.8 and would recur about every 1,000 to 4,000 years (about as frequently as the 1994 Northridge event). Other work published in the past year includes Dolan and others' study of the Hollywood fault, which was made possible by SCEC's collaboration with the Metropolitan Transit Authority.

San Joaquin Hills

A few years ago, SCEC geologists Lisa Grant, Eldon Gath and Karl Mueller began working on the raised marine terraces in the hills behind Newport Beach and Laguna Beach in coastal Orange County These terraces suggested the possibility of large earthquakes generated by a blind thrust fault beneath coastal Orange County. Their work has led to determination of an uplift rate of about 0.25 mm/yr. The shape of the anticlinal uplift and U-Th ages have yielded an uplift rate of about 0.25 mm/yr and determination that the causative fault is a blind thrust that dips seaward from beneath the coastal communities of central Orange County. Recent discovery of an emerged late (?) Holocene terrace in upper Newport Bay may lead to establishment of the timing of the most recent large seismic event produced by this structure.

The San Andreas fault

Jim Spotila completed his geomorphologic and geochronologic work on the active structures adjacent to the San Andreas fault in the San Bernardino Mountains His definition of the geometry of the ancient uplifted surface that forms most of the mountain range was a critical for interpreting the geometry, cause, and rate of slip of the large thrust fault that underlies the range. The 70-km-long thrust fault that forms the prominent scarp on the northern flank of the San Bernardino mountains has slipped about 3.1 km in the past 2 million years. This yields an average slip rate of about 1.6 mm/yr. The fault has formed in response to the 15-km-wide contractional bend in the San Andreas fault on the southern margin of the range, near San Gorgonio Pass. The three-dimensional structure of this thrust fault is now well-enough known and its activity well enough established to warrant efforts to estimate strong ground motions that it could produce. Spotila's Uranium-Helium thermochronometry revealed that the Yucaipa block, a narrow sliver of crust between the Mill Creek and South Branch strands of the San Andreas fault, has risen several km in the past 1.5 million years. This rapid uplift has very important implications for the current geometry and seismic potential of that portion of the fault zone between San Bernardino and Palm Springs. By the end of the year, Yule and Sieh will have completed a manuscript that uses these and other geologic and seismographic data to interpret the 3-D geometry of the San Andreas fault in this region. The complexity and lack of contiguity of the fault zone suggests that future large ruptures will produce unusually complex earthquakes. Yule and Sieh have also begun to excavate the Burro Flat paleoseismic site, which is on the western edge of the 15-km-wide contractional jog in the San Andreas. They expect to have additional evidence for the timing of large ruptures of the San Andreas fault by the end of 1998. Sally McGill has continued her excavations of a paleoseismic site along the San Andreas near Plunge Creek, in the San Bernardino valley. That site consists of carbon-rich alluvial outwash at the front of the San Bernardino mountains. Results from earlier work are almost ready for publication and include evidence for two recent events -- one between AD 1440 and AD1640 and the other between AD 1240 and AD 1440. Her new excavations, several hundred feet away, on a younger alluvial fan are expected to expose a younger section. Tom Rockwell, Tim Dawson and Scott Lindvall expect to begin new excavations at their Frazier Park site, near the middle of the great 1857 rupture, before the end of 1998. Problems with ground-water have delayed their new excavations. Their preliminary work has yielded evidence for the 1857 earthquake and a previous large event in the 16th or 17th century. Still farther north, between Parkfield and the Carrizo Plain, Grant and Arrowsmith have found records of section lines surveyed across the San Andreas fault both before and after the great 1857 earthquake. They hope that comparison of these records will enable geodetic determinations of the amount of offset associated with the 1857 event.

[Thrust 1] [Thrust 3] [Thrust 4] [Thrust 5]

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