if it is 7.4 on the Mercalli scale, it is the equivalent of about 6 on the Richter scale, not very strong
What's the difference between Mercalli Scale and Richter Scale? While the Mercalli scale describes the intensity of an earthquake based on its observed effects, the Richter scale describes the earthquake's magnitude by measuring the seismic waves that cause the earthquake. The two scales have...
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You can't directly equate MMI (Modified Mercalli Intensity) to Richter magnitude. Richter is a measure of energy released. Mercalli relates to ground shaking, and is related to both Richter and geologic conditions. For given earthquake, there is only one Richter measurement. But there will be multiple Mercalli intensities, depending on locations. MMI is often depicted using a color-coded shakemap.
Below, for example, is the shakemap for the Richter 6.7 Northridge earthquake, which generated peak Mercalli intensity X+, the top of the scale. That was in a localized area of coarse unconsolidated soils with shallow groundwater, in the Sylmar area. Other high intensity zones are in the other unconsolidated valley floors in the Simi and San Fernando Valleys.
Or here is the 1868 Hawyard quake shakemap (Richter 6.8 ). That had max MMI = X. Again, almost all of the high intensity movement occurs on unconsolidated sediments on the valley floors. The hill areas have much less shaking, even in areas a mile or so from epicenter.
