SR 06:209 - Laserfiche WebLink

Earthquake magnitude is a measure of overall earthquake size at the epicenter, and is recorded by the Richter Scale, a logarithmic scale related to seismograph readings. In addition, seismologists use "moment magnitude" to measure the amount of energy released by an earthquake. See Table V-I, below. The moment magnitude is proportional to the area of the fault plane that has slipped and thus is directly related to the fault length. An earthquake may have one moment magnitude but a range of intensities. "Intensity" is a measure of the effect of an earthquake at a specific location. The most conunonly used measurement of earthquake intensity and ground shaking is the Modified Mercalli Intensity Scale. Table V-I, above, summarizes the Modified Mercalli Intensity Scale (MMI) in relation to the Richter Scale, as well as perceived shaking hazard, potential damage, and earthquake effects as related to groundshaking severity. Almost all of the major faults in the Bay Area are strike-slip faults where the rupture along the fault plane extends almost vertically into the ground, and the ground on one side moves past the ground on the other side. In some earthquakes, the surface of the ground can rupture along the fault, or a landslide can be triggered, or a number of other incidents may occur. But in all earthquakes the ground shakes, and the shaking of the ground itself causes most earthquake damage. Earthquakes can cause a number of specific hazards. 1 GTOllnrl Sh~klng Portions of the Planning Area that are underlain by loosely compacted soils may experience the greatest amount of ground-shaking and damage, even though they may not be closest to the fault rupture. As shown on Figure V-3 and further described in Table V-I, above, the ground-shaking hazard in the Pleasanton Planning Area is mostly within the "severe" to "violent" range. Damage resulting from severe groundshaking would be moderate to heavy while damage due to violent groundshaking would be heavy. The intensity of earthquake ground shaking in anyone area varies for a number of reasons: the magnitude of the earthquake; the distance from the site of the fault source; the direction of propagation of the rupture; soil saturation or groundwater; and the type of geologic materials underlying the site, with stronger shaking occurring on the softer soils. ? P~rthcpl~ln" Rpl~tpc1 T ~nc1d-irlpo;;. Landslides could occur generally in areas shown in Figure V-I, above, as a result of groundshaking and other physical conditions as discussed above. The causes of slope failure, which normally produce landslides and differential settlement, are augmented during earthquakes by strong ground motion. An earthquake occurring in conjunction with a season of heavy rainfall when soils are saturated with water would create the most severe danger of landslides. Public Safet)' 082906, dean V-s City Council 8/29/2006