Monday, June 4, 2012

The Largest Possible Bay Area Earthquake

Some questioners have serious worries about the risks in where they live – polluted groundwater from industrial plants or fracking, exposure to volcanic eruptions, and risk from earthquakes, hurricanes, and tornadoes. Sometimes people are just trying to get some reference they can use for legal action or engineering decisions they are required to make… or justify. In this example it’s not clear what the objective is, but I hope the answer is educational.

Please provide me with Web links or literature citations to the current estimates of maximum credible earthquakes in the San Francisco Bay Region.

Thank you. – Robert Z.

I'll lay out selected references first, then explain the principles underlying them.

The largest historic earthquake to strike northern California remains the M = 7.9 event of 1906. These aren't that unusual; I remember vividly being thrown out of my bed as a small child by a magnitude 7.7 earthquake in the southern San Joaquin Valley.

... we find that "Determining whether the intraslab events occur within the crust or mantle portions of the slab is not only important for understanding the rupture process of these events, but also for estimating the maximum possible magnitude. Normal-faulting earthquakes confined to the 7 km thick subducted oceanic crust are not likely to exceed the magnitudes of the three large (M6.5-7.1) Cascadia intraslab earthquakes, while allowing a thicker seismogenic zone suggests that much larger earthquakes could occur."

...we read "The Calaveras Fault plays a major role in accommodating plate-motion slip in the San Francisco Bay region.  Geodetic modeling, historical creep data and paleoseismic trenching suggest a fault slip rate of about 15 mm/yr on the Central Calaveras Fault, which extends from San Felipe Lake on the southeast to Calaveras Reservoir on the northwest.  Within the uncertainty of limited geologic data, the long-term slip rate on the Central Calaveras Fault is consistent with the short term rate estimated from aseismic creep and geodetic modeling.  However, a critical question is whether or not the Central Calaveras Fault produces large-magnitude earthquakes, or whether the fault relieves strain only by aseismic creep and small to moderate earthquakes.  Existing seismic source characterization models generally assume or strongly weight scenarios in which the fault may rupture in earthquakes up to magnitudes of about M6.2.  Understanding the maximum size of earthquakes possible along the Central Calaveras Fault is critical to estimating probabilities of future earthquakes in the San Francisco Bay region. ".

In general, the maximum possible moment magnitude correlates closely with the amount of fault surface that actually breaks - and by how much (surface area * throw).

A subduction earthquake such as Cascadia in January 1700 (or Tohoku, 11 March 2011) presents a much larger potential slip-surface because it dips relatively shallowly, and can reach down-dip much farther (~200 km for the Tohoku event) before it reaches the plastic zone (pressures and temperatures high enough) of the Mantle. The San Andreas is a roughly vertical-dipping, right-lateral transform fault, and also has multiple bifurcations and bends, all of which would tend to limit the surface area where slip can actually take place. This suggests that M ~ 8 is the maximum that could be expected for the San Francisco Bay area.


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