| In 1997, the U.S. Geological Survey's (USGS) Working Group on California Earthquake Probabilities was expanded to include more than 100 geologists, seismologists, geophysicists, and mathematicians. This group, known as WG99, calculated new quake odds for the San Francisco Bay region based on insights gained since the 1989 Loma Prieta earthquake. They concluded that there is a 70% probability (±10%) of at least one magnitude 6.7 or greater quake, capable of causing widespread damage, striking the region before 2030. The process used to determine these odds is described below. |
Balancing Plate Motions and Earthquakes
Earthquake probabilities for the San Francisco Bay region are derived by balancing two processes: (1) the motions of tectonic plates, and (2) the slippage on faults, which occurs primarily during earthquakes. The continual northwestward motion of the Pacific Plate past the North American Plate loads strain onto the network of active faults that cut through the region. Earthquakes sporadically release and redistribute this strain.
One side of the balance is the rate at which plate motions load strain onto faults. Development of the Global Positioning System (GPS) has allowed geophysicists to make accurate measurements of how the current plate motions (totaling 1.5 inches per year across the entire region) distributes strain onto individual faults. Geologic studies also contribute to this understanding by documenting long-term fault motions, which must match the strain-loading rate.
The other side of the balance is the slip on faults, which over time must account for the strain built up by the plate motions. Slip on faults (movement of one side of a fault relative to the other) can occur either during earthquakes or during slow, aseismic (without earthquakes) creep. For example, creep on the Hayward Fault is slowly offsetting street curbs, even in the absence of large quakes. Where aseismic creep occurs, it affects the balance between plate motions and earthquakes by relieving strain, which can either change the likelihood or lower the magnitude of future quakes. Nevertheless, most slip on faults occurs during earthquakes: the larger the quake, the greater the slip.
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Quake probabilities are derived by balancing two processes: (1) the continual motions of the plates that make up the Earth's outer shell (represented by pouring sand onto the left tray) and (2) the slip on faults, which occurs primarily during earthquakes (equivalent to adding balls to the right tray). The slip on faults over time must balance the strain built up by the plate motions. The total amount of slip during an earthquake, shown here by proportional volumes of the spheres, depends on its magnitude (M). The larger the quake, the more strain released. |