The Puente Hills Blind Thrust:
L.A.'s latest earthquake hazard find
In the March 5th issue of Science Magazine, John Shaw of Harvard University and Peter Shearer of the University of California at San Diego report discovering an active new blind-thrust fault beneath metropolitan Los Angeles. They identified this fault, which they named the Puente Hills Thrust, from seismic reflection profiles, petroleum wells, and relocated earthquakes. They believe that a segment of this fault likely caused the 1987 Whittier Narrows (Mw 6.0) earthquake. The seismic reflection profiles define a 40-kilometer-long east-west fault that is inclined at 27° to the north below the land surface. It includes three distinct parts, which the authors call Los Angeles, Santa Fe Springs, and Coyote Hills segments. Assuming that the entire fault system extends to the depth of the Whittier Narrows earthquake (about 15 kilometers), the authors estimate that the individual fault segments could generate moment magnitude (Mw) 6.6 to 6.7 earthquakes, or if all three segments ruptured at once, a Mw 7.0 shock.
ABS Consulting estimates that a Mw » 7 earthquake on the Puente Hills Thrust would cause in excess of $100 billion in damage. This is over double the damage in the 1994 Northridge (Mw 6.7) earthquake. Such an event, if credible, should not be ignored, especially if it can occur every 250 to 2000 years as postulated by Shaw and Shearer. ABS Consulting is dedicated to providing state-of-the-art earthquake hazard and risk information to its clients. As such, we have developed an earthquake risk model for, not only the Los Angeles metropolitan area, but the entire United States that captures all of the seismic hazard that is known and quantifiable at the present time. However, as responsible professionals, ABS Consulting's scientists and engineers use only that information that is formed from "good" scientific fact or consensus opinion, leaving the highly speculative, unsubstantiated assumptions that are often reported through the media or short turn-around scientific journals to be sufficiently evaluated by the scientific community before being adopted.
Given the evidence presented by Shaw and Shearer, the Puente Hills Thrust clearly exists. However, the authors have to make many assumptions to postulate its downward extent and its potential "seismogenic" (earthquake-producing) capability. The fault is only defined to a depth of about 5 to 7 kilometers in the seismic profiles. At this point, the fault is confined to upper Tertiary-age sediments that are not believed to be seismogenic based on past earthquakes. They must extrapolate this fault to the depth of the Whittier Narrows earthquake to project it into harder rock which is believed to be capable of generating a significant earthquake. Furthermore, only part of the Sante Fe Springs segment is defined to this depth, making the projection of the other segments to seismogenic depth even more speculative. Given such a limited seismogenic rupture surface, the potential magnitude that this fault system could generate, even if extrapolated to the depth of the Whittier Narrows earthquake, would be smaller than the authors indicate.
The quantification of earthquake risk requires that the frequency as well as the size of a potential earthquake be known. If a fault generates an earthquake once every 100,000 years, then it does not pose much of a risk. The recurrence intervals suggested by Shaw and Shearer for the Puente Hills Thrust of 250 to 2000 years are short enough to make the fault a significant risk. However, these estimates are based on fault slip rates that are inferred from crustal deformations observed on the surface and could be quite inaccurate. Given the still speculative extent and seismogenic potential of the Puente Hills Thrust, ABS Consulting scientists believe that it would be irresponsible to include it in its earthquake risk model at this time. However, we will carefully monitor the developing information on this fault and will include it in our model as soon as the scientific community has reached a consensus on its geographic extent, seismogenic potential, and recurrence frequency.
That is not to say that we are ignoring the potential for blind-thrust earthquakes in the Los Angeles metropolitan area. Based on the latest scientific consensus information, we currently include in our earthquake risk model the Elysian Park Thrust (Mw 6.7) that lies beneath Los Angeles, the Compton Thrust (Mw 6.8) that lies beneath Long Beach, the Northridge Thrust (Mw 6.9) that lies beneath the San Fernando Valley, and several blind-thrust faults beneath the Santa Barbara Channel (Mw 6.6-7.4). The Elysian Park Thrust is particularly important, since it covers virtually the same area as the inferred seismogenic part of the Puente Hills Thrust, if the latter is extrapolated from its known depth of no more than 5 to 7 kilometers to the depth of the Whittier Narrows earthquake. Although Shaw and Shearer claim that the Puente Hills Thrust is distinct from the Elysian Park Thrust, this distinction may not be so clear at the seismogenic depths important to quantifying a fault's potential for generating strong ground shaking. Furthermore, with both faults being so close to one another, the surface deformations used to estimate the earthquake frequencies will be the same. The recurrence interval of a postulated Mw 6.7 earthquake on the Elysian Park Thrust is 550 years, at the lower range of recurrence estimates for the Puente Hills Thrust.
ABS Consulting is also closely following the development of a body of scientific information being developed by scientists at the Southern California Earthquake Center that indicates that there may be an active blind thrust fault beneath the San Joaquin Hills in southern Orange County. This evidence is every bit as valid as that used by Shaw and Shearer to infer the Puente Hills Thrust at depth, but the scientists involved have been careful not to over-speculate regarding its seismic potential until they have more evidence with which to form solid scientific consensus.
In conclusion, ABS Consulting will continue to follow the development of scientific information regarding blind thrust faults and other potential sources of earthquake risk in the Los Angeles metropolitan area. We will include these faults in our earthquake risk model when there is sufficient scientific consensus on its earthquake generating potential.
Additional Notes
Blind Thrust Faults: Blind thrust faults and ramps constitute a new, previously unrecognized class of faulting. They are termed "blind" faults because they do not extend to the earth's surface. Blind thrust faults are inclined downward into the earth at a relatively steep angle (typically steeper than 45° as measured from the horizontal); blind thrust ramps, in contrast, are inclined at a low angle (typically less than 45°). Both can generate earthquakes that have a significant vertical shaking component. The 1994 Northridge earthquake (magnitude 6.7) occurred on a previously unknown blind thrust ramp.
Recent advances in seismology have identified broad areas that are underlain by blind thrust ramps. Three of the blind thrust ramps in the Southern California area, for example, are the Elysian Park, Compton, and Santa Monica Mountains ramps. Relatively little is known about such ramps, such as their precise recurrence intervals, locations, and size of the fault planes. However, approximations based on current knowledge of the thrust ramps' sizes and locations were used herein to account for this important phenomenon.
Contact Information: For more information, contact Dr. Kenneth Campbell at kcampbell@absconsulting.com.
|
