Evidence for Large Earthquakes in Metropolitan Los Angeles
Department of Geological Sciences Research
Charles Rubin (Department of Geology, Central Washington University, Ellensburg, WA), Scott Lindvall (William Lettis & Associates, Valencia, CA), and Tom Rockwell (Department of Geological Sciences, San Diego State University, San Diego, CA)
The Sierra Madre fault, along the southern flank of the San Gabriel Mountains in the Los Angeles region, has failed in magnitude 7.2 to 7.6 events at least twice in the past 15,000 years. Restoration of slip on the fault indicates a minimum of ~4.0 m of slip from the most recent earthquake and suggests a total cumulative slip of ~10.5 m for the past two prehistoric earthquakes. Large surface displacements and strong ground-motions resulting from M > 7 earthquakes within the Los Angeles region are not yet considered in most seismic hazard and risk assessments.
Loma Alta site -- selected photographs
Click on each snapshot for a high resolution photograph.
Images from the Science Paper -- Selected PDF images
Location map:Map of southern California showing major faults. CF, Cucamonga fault; C-S, Clamshell-Sawspit fault; EPBT, Elysian Park blind thrust; JPL, Jet Propulsion Laboratory; LA, Los Angeles; HF, Hollywood fault; P, Pasadena; PV, Palos Verdes; RF, Raymond fault; SM, Santa Monica; V-ER, Verdugo-Eagle Rock fault. The Sierra Madre fault shown in heavy red lines; 1971 San Fernando earthquake surface rupture shown in blue heavy lines. Northridge aftershock zone shown in blue hatched lines. Yellow dashes surround the dense metropolitan population of the Los Angeles, San Gabriel, and San Fernando basins. Click here for a JPG image.
Trench log: Cross-section through sediments in trench wall showing fault traces, stratigraphic units, and radiocarbon dates. A, angular detrital charcoal fragment ; R, rounded detrital charcoal fragment. Faults are shown in heavy lines. Scale shown in meters; no vertical exaggeration. Radiocarbon ages are quoted in 14C years before present, except calendric ages quoted as ka (yrs. B.P.). Click here for a GIF image.
Model: Schematic development of colluvial wedges from two successive earthquakes. (A) Earthquake 1 ruptures the ground surface; scarp is known schematically as an unstable overhang. (B) Scarp collapses, degrades, and sheds debris forming colluvial wedge 1. (C) Earthquake 2 ruptures ground surface and offsets colluvial wedge 1. (D) Scarp collapses, degrades, and sheds debris to form colluvial wedge 2. Note that surfical soil and colluvial wedge 1 are no longer preserved in the upper plate. Click here for a GIF image.
Restoration: . (A) Restoration from the most recent earthquake yields a minimum of slip of 3.8-4.0 m. In this restoration, the tip of the upper plate is restored to below the lower colluvial wedge; the restoration does not account for any slip on secondary faults within the hanging wall. (B) Restoration of slip from the last two earthquakes showing a minimum of ~ 10 m. Here, the upper plate is restored to below the Bt horizon and the buried paleosurface surface is matched to the topographic profile of the fault scarp. (C) Detailed topographic profile of fault scarp surveyed using an electronic EDM/theodolite. Click here for a GIF image.
NEW STUDY BY CWU RESEARCHER REVEALS UNEXPECTED EARTHQUAKE THREAT TO L.A.
July 10, 1998
Prehistoric earthquake evidence recently unearthed near Pasadena, Calif., is likely to shake up emergency preparedness and building code officials throughout southern California, according to Dr. Charles Rubin, Central Washington University geologist.
His article, titled "Evidence for Large Earthquakes in Metropolitan Los Angeles," which appears in the July 17 issue of Science magazine -- a weekly publication of the American Association for the Advancement of Science -- is sure to capture public attention, as well. Rubin co-authored the paper with Scott Lindvall, of William Lettis & Assoc., a Valencia, Calif., earth sciences consulting firm; and Tom Rockwell, geology professor at San Diego State University.
According to Rubin and his colleagues, the Sierra Madre fault, which runs along the southern flank of the San Gabriel Mountains -- and passes less than a dozen miles from downtown Los Angeles -- has produced earthquakes ranging in magnitude from 7.2 to 7.6 on the Richter scale at least twice in the past 15,000 years. In comparison, a 7.2 magnitude quake would release six times as much energy as the 1994, 6.7 magnitude Northridge, Calif., earthquake.
By excavating a trench across the Sierra Madre fault to look at the buried evidence (strata of soil, gravel and sand) at a site near Pasadena, and by radiocarbon dating the charcoal they found in the various layers, they estimate that at least two prehistoric earthquakes have shifted one edge of the fault upward more than 30 feet.
"Yet, most seismic hazard and risk assessments of the L.A. region don't even consider such large vertical ground displacements and strong ground-motions, which result from earthquakes of 7.0 or larger," Rubin says. According to the Science article, the potential for damage from earthquakes along reverse faults (those that move the ground vertically) in the L.A. region has been dramatically illustrated by four sizeable quakes in the past 30 years. Most earthquake hazard assessments in southern California -- where literally millions of people could be affected -- have traditionally focused on the San Andreas fault and its neighboring strike-slip faults. (Strike-slip faults move horizontally on the same plane, like two cars passing each other -- going in opposite directions -- on a freeway).
"Although there is debate on the maximum size of earthquakes in the greater Los Angeles region, our data suggest that prehistoric earthquakes at the site we excavated are substantially larger than other historically observed earthquakes along reverse faults in the Los Angeles region," Rubin says.
Rubin warns that damage from large magnitude earthquakes along the Sierra Madre fault would be substantially different from that produced in the 1994, 6.7 Northridge earthquake. "Unlike the Northridge earthquake, that ruptured northward away from the metropolitan region, a magnitude 7+ earthquake on the Sierra Madre fault would rupture southward, directing energy into the densely populated basin," he says. "Large surface displacements, as well as strong ground-motions, have the potential for disrupting lifeline systems (roads, rail, phone, power and water) and producing significant damage to modern buildings," Rubin reports. "Until our recent excavation work, earthquake researchers didn't have any direct evidence about how big the reverse faults in this area could be -- we had no clue until now that an earthquake of a magnitude 7.5 had ever occurred in the region," he says.
"This work really changes that. This is
direct evidence that earthquakes in the region have been substantially
larger than ever before documented -- and suggests that they could
happen again," Rubin warns. Rubin has done extensive earthquake
research in California for more than seven years. His most recent
study was partially supported by grants from the Southern California
Earthquake Center and the National Science Foundation.