Major Losses Projected For Earthquakes On Puente Hills Fault Beneath Los Angeles

SCEC is a research consortium funded by the National Science Foundation (NSF) and the USGS. SCEC involves more than 400 scientists at more than 50 research institutions, and is headquartered at the University of Southern California (USC).

"The Puente Hills results are important," said Kaye Shedlock, program director in NSF's division of earth sciences, which funds SCEC, "because they illustrate how much we've been able to learn about the complexities of earthquakes, the associated ground shaking, and the effects on society. The results also show us what we still need to learn to mitigate the loss of life and the economic impacts of earthquakes."

The new research results, published in the May 2005 issue of the Earthquake Engineering Research Institute's journal Earthquake Spectra, were based on shaking scenarios created using newly available software for seismic hazard analysis developed by SCEC and the USGS, coupled with loss-estimation software developed by FEMA.

The Puente Hills fault was discovered in 1999. In 2003, an SCEC study found the fault had ruptured at least four times in the past 11,000 years, with earthquake magnitudes ranging from 7.2 to 7.5. To determine probable losses from such earthquakes on the Puente Hills fault, scientists conducting the study created 18 scenarios depicting possible shaking levels throughout the region. They note the loss scenarios themselves are somewhat uncertain because of the many variables involved in predicting ground shaking, including whether the full fault ruptures or just part of it does. Also, the formal quantification of loss estimates is still a challenging and imprecise science, they say, even using the best available models and scientific data.

USGS researcher Ned Field, the lead scientist on the study, said that although the scenarios show that the vast majority of losses will occur in Los Angeles County, directly over the rupture surface, tangible losses are also predicted for San Bernardino and Orange Counties located east and south of the Puente Hills fault. The losses predicted for this event are greater than those observed for the 1994 Northridge earthquake, both because of the higher potential earthquake magnitudes and because the heavily shaken area during Northridge was made up mostly of wood-frame residential structures, whereas Puente Hills lies under older and more vulnerable commercial and industrial structures.

The authors also emphasize that a full Puente Hills fault rupture is a rare event, occurring once every 3,000 years or so. "In fact," said Field, "as an individual, your odds of dying of a heart attack or an auto accident are much greater than dying from this earthquake. That being said, there are other sources of earthquakes throughout the region, and it's not question of if, but when, so everyone should take necessary safety precautions. We're striving to prevent these natural hazards from becoming disasters." He also said that a Puente Hills earthquake would have widespread impact, so it's up to emergency and public policy officials to plan accordingly.

"Quantifying earthquake risk is difficult and fraught with many uncertainties," said Tom Jordan, director of SCEC, and a co-author of the paper reporting the study results. "One of the main goals of this study is to use our improved knowledge of seismic hazards in Southern California to evaluate--and hopefully reduce--the uncertainties in this type of risk analysis."

The researchers determined a probable range of estimated losses by averaging losses predicted under each scenario and model. The scenarios all assumed an earthquake occurring at 2 p.m. during a weekday, when many people are at work. The number of casualties would be significantly less in that area if an earthquake were to occur at night when most people are at home. Their results showed that:

* The estimated number of fatalities could range between 3,000 and 18,000, with an average of 7,600. The Northridge quake resulted in 33 direct fatalities, and the 1995 Kobe, Japan, earthquake resulted in 6,348 fatalities.

* The total number of injuries could range between 56,000 and 268,000, or an average of about 120,000.

* The number of displaced households could range from 142,000 to 735,000, with an average of 274,000.

* Relief agencies would have to provide short-term public shelter for 42,000 to 211,000 people, with an average of 80,000 people needing short-term public shelter.

* The amount of debris generated by such an earthquake would range between 30,000 and 99,000 tons, averaging 51,000 tons.

These projections assume that no efforts have occurred that would improve infrastructure to reduce earthquake losses before the quake happened, Jordan noted, adding: "If society chooses to invest in mitigation, many of these losses could be avoided."

SCEC gathers new information about earthquakes in Southern California, integrates this information into a comprehensive and predictive understanding of earthquake phenomena, and communicates this understanding to increase earthquake awareness, reduce economic losses, and save lives.