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Finding faults

Most recent discussion of earthquake risk in the Delta have been based on the Delta Risk Management Strategy (DRMS).  DRMS was prepared by consultants (URS Corporation) for the Department of Water Resources.

Experts disagree about many of the findings in DRMS, which assumes five Delta fault systems.

For example, new modeling of the “Montezuma Hills Zone” fault by Shell Oil (which is doing a pilot test for CO2 injection wells) has confirmed what PG&E apparently knew years ago: that one of the faults identified in DRMS doesn’t exist, but is instead an “erosional feature” resulting from a meander of the Sacramento River.

Deirdre Des Jardins notes that the U.S. Geologic Survey did not include any faults in the Delta in their catalog of recently active faults – recently being within the last 11,000 years.

Yet all five of these “possible faults” were included in the URS seismology analysis for the DRMS and were used in the estimates for a high probability of earthquakes in the Delta.

The CALFED Science Program Independent Review Panel looked at DRMS found that

“the estimated frequency for flooding from seismic events is much larger than what is supported based on available information. The return period for an earthquake causing at least one levee failure is estimated to be about ten years, while a single event of this type has not occurred in over 100 years of history. Even considering only the past 20 years of history in which the configuration of the levees has been more similar to that at present, the analysis predicts that there would have been two failures on average and only a 16-percent chance of observing what has actually been observed: no failures. This unbalanced treatment of risks provides a potentially biased result, especially when comparing between seismic and flooding effects in evaluating mitigation measures.”

The Review Panel also said, “Although there has potentially been seismically induced damage to Delta levees, there have been no known failures of islands due to earthquakes in at least the last 100 years.”

Notes engineer Dr. Robert Pyke, DRMS exaggerated estimates of levee fragility in an earthquake.  This doesn’t mean that there is NO seismic risk to the Delta.  Says Dr. Pyke, “There remain a lot of uncertainties about the tectonics of the Delta region.  The solution is to build fat levees which address multiple issues including earthquake ground motions from any source.”

Meanwhile, it appears that the Ortigalita Fault that lies under San Luis Dam is much more active than faults identified as being under the Delta. There are real risks associated with faults under and around San Luis Dam and along the west side of the San Joaquin Valley where canals and aqueducts carry water south.

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