Researchers say parts of the San Andreas and San Jacinto fault systems are carrying more tectonic strain than at any point in the past millennium, though the findings do not predict when a major earthquake will occur.
By yourNEWS Media Newsroom
Key sections of the San Andreas and San Jacinto fault systems have accumulated more tectonic stress than at any point in the past 1,000 years, according to a new study renewing concerns about Southern California’s long-term earthquake risk.
The research, published last month in the Journal of Geophysical Research: Solid Earth, does not forecast when a major earthquake will strike.
Scientists said the prolonged absence of a large rupture has nevertheless allowed strain to continue building beneath one of the most densely populated regions in the United States.
“We keep accumulating that earthquake energy, and it has to be released,” U.S. Geological Survey geologist and study co-author Kate Scharer told the Los Angeles Times. “And the only way it gets released is through large earthquakes. The small ones don’t really do it.”
Researchers examined portions of the San Andreas and San Jacinto faults near Cajon Pass in San Bernardino County.
The team developed a computer model combining a 1,000-year record of earthquakes with satellite observations of tectonic-plate movement and measurements of deformation in Earth’s crust.
Lead author Liliane Burkhard said the model showed that present-day stress has surpassed all previously estimated levels in the record.
The model “found that tectonic stress has now reached higher levels than at any point in that entire record,” Burkhard told the Times.
Researchers also compared current conditions with those believed to have existed before major historical earthquakes.
“From the model, we see that the conditions that historically preceded large joint ruptures crossing both fault systems are now approaching,” Burkhard said.
According to the study, sections of both fault systems now contain more accumulated stress than scientists estimate was present before several major earthquakes during the past millennium.
The findings are particularly significant because of the interaction between the San Andreas and San Jacinto systems near Cajon Pass.
A rupture involving both faults could affect a broad portion of Southern California and expose millions of residents to severe shaking.
Scientists noted that Southern California has experienced an unusually long interval without a major rupture on some fault sections.
Large earthquakes along portions of the San Andreas fault typically occur at intervals of approximately 100 to 150 years, although the timing can vary and substantially longer gaps are possible.
The southern San Andreas fault near the Salton Sea is believed to have last produced a major earthquake about 300 years ago.
That extended period without a large release of energy does not establish that an earthquake is imminent. It does mean that tectonic movement has continued adding strain to the fault system.
The study reinforces longstanding warnings from the USGS that Southern California faces a substantial probability of a damaging earthquake.
The agency estimates a 60% chance that a magnitude 6.7 or greater earthquake will strike the Los Angeles region by 2045.
A major rupture on the southern San Andreas could affect Los Angeles, Orange, Riverside, San Bernardino, Ventura and Imperial counties.
The USGS examined such an event in its 2008 ShakeOut scenario, which modeled the effects of a hypothetical magnitude 7.8 earthquake on the southern San Andreas fault.
That scenario projected violent shaking across much of the region and estimated approximately 1,800 deaths.
The simulation also illustrated the potential for widespread damage to buildings, roads, utilities, water systems and other critical infrastructure.
Researchers emphasized that the new study should not be interpreted as a prediction of a specific earthquake date, location or magnitude.
Earthquakes cannot currently be predicted with the precision needed to identify exactly when a fault will rupture.
Computer models also depend on assumptions about historical earthquakes, fault behavior, crustal movement and the transfer of stress between different sections of a fault system.
The authors acknowledged those limitations while maintaining that the unusually high modeled stress levels should be taken seriously.
The fault systems are “more loaded than at any point in the 1,000-year record,” Burkhard said.
The findings offer a long-term assessment of earthquake potential rather than an immediate warning. Current stress conditions could persist for years, or a major rupture could occur sooner.
Scientists cannot determine which possibility is more likely based on the model alone.
The study’s practical significance lies in preparedness.
Southern California’s population density and dependence on complex transportation, communications, energy and water networks mean even a moderate earthquake can create widespread disruption.
A major event could cause structural collapses, fires, landslides, transportation failures and extended utility outages.
Residents can reduce risk by securing heavy furniture and appliances, preparing emergency water and food, maintaining medical supplies and identifying safe locations inside homes and workplaces.
Officials and property owners can also strengthen older buildings, improve infrastructure resilience and review emergency response plans.
Burkhard said the central lesson of the research is not that residents should expect an earthquake on a particular day, but that the region should remain prepared for a large event.
“In the end, the most important message is a simple one: Let’s make sure we are prepared,” she said.
Original article
Comments