City of Irvine

2020 Local Hazard Mitigation Plan

Figure 3-10: Liquefaction Zones

damage. The MMI scale has replaced the Richter scale, which is no longer used since it loses effectiveness when measuring larger earthquakes. Since the degree of shaking, and consequently damage, generally decreases as the seismic energy travels further away from the fault rupture’s point of origin, differen t sections of a city or region can report different MMI measurements in different locations. Given Irvine’s size, it is likely that different sections of the City would report different MMI measurements. The MMI scale depicted in Table 3-12 uses Roman numerals on a 12-point scale to measure each degree of shaking intensity. Another scale for measuring seismic shaking is the moment magnitude scale (MMS, denotedMw or simply M). The MMS measures the energy released by the fault rupture beginning at 1.0 and increases as the energy of the earthquake grows. The MMS is a logarithmic scale, meaning that the difference between numbers on the scale multiplies as they increase. As an example, a 5.0M earthquake is approximately 1.4 times greater than a 4.9M event, 32 times greater than a 4.0M event, and 1,000 times greater than a 3.0M event. Seismic shaking can also be measured in relationship to force of Earth’s gravity (g), or percent g. This method is useful for geographically displaying areas of seismic shaking potential. Percent g is computed

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