Story courtesy of California Watch.
By Susanne Rust
Scientists know the San Andreas Fault is going to rupture.
But why it hasn't triggered a major temblor in more than 100 years has baffled geologists and seismologists.
And it suggests that a mega-quake may be around the corner.
Scientists from the Scripps Institution of Oceanography in La Jolla and the U.S. Geological Survey say ruptures along the San Andreas Fault may be tied to fluctuations of the Salton Sea and Colorado River. They say the diversions and dams engineered along the Colorado River may have upset a natural cycle of flooding that perhaps triggered several of the fault's quakes.
And without the periodic release of energy and pressure those flood-related quakes triggered, the pressure currently stored in the fault could generate a monstrous magnitude 7.5 or larger earthquake.
"We've been baffled as to why the southern San Andreas hasn't gone. It's been compared to a woman who is 15 months pregnant," said Scripps seismologist Debi Kilb, a co-author of the report. "Now, this paper offers one explanation why."
Studying sediment deposits along the floor of the ancient Lake Cahuilla - once a large lake in the Imperial Valley fed by the Colorado River - the researchers found a correlation between flooding and the rupture of step-over faults.
Step-over faults are smaller faults that run at an angle to larger faults, in this case the San Andreas Fault. At times, the faults may have triggered larger ruptures along the main fault.
"These step-over zones really need to be considered when assessing earthquake hazards and need to be examined as potential triggers for destructive earthquakes on larger faults," said Danny Brothers, an author of the report and researcher at the Geological Survey.
The researchers say that during the past 1,000 years, five major earthquakes occurred along the southern San Andreas. These earthquakes are estimated to have been larger than magnitude 7 and occurred roughly every 180 years.
The last large quake on this section of the fault took place more than 300 years ago.
The authors say that while floodwaters in the Salton trough no longer affect these step-over faults, seismic and tectonic forces still play a role, and they are capable of generating a major temblor along the San Andreas.
In a statement, Neal Driscoll, another Scripps co-author, said:
"Earthquake simulations reveal that shaking of large metropolitan areas such as Riverside and Los Angeles will be larger if the earthquake propagates from south to north - our research suggests that the Salton Sea step-over zone may provide a trigger for such a propagation direction."
But not every one buys the flood/quake connection.
"When you take a hard look at the data," Ray Weldon, a geologist at the University of Oregon, told Science News, "you have to ask yourself: 'Is this just a coincidence?' "