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7.7 mg Oaxaca Mexico

easyrider

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if it is 7.4 on the Mercalli scale, it is the equivalent of about 6 on the Richter scale, not very strong

 

easyrider

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if it is 7.4 on the Mercalli scale, it is the equivalent of about 6 on the Richter scale, not very strong


The m 7.4 would be a major earthquake according to the USGS. Kind of strong, imo.

Bill
 

bbodb1

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@easyrider - what a cool resource - thanks for posting this!

Try zooming OUT and see how many earthquakes there were today! 474 (and counting) Wow!
Alaska is busy!
 

T_R_Oglodyte

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if it is 7.4 on the Mercalli scale, it is the equivalent of about 6 on the Richter scale, not very strong

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.

1592944567052.png


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.

1592945788462.png
 
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T_R_Oglodyte

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If the 7.4 magnitude is Richter, that would be a quake capable of causing significant local damage, especially in with unconsolidated soils and shallow groundwater. Areas with dense soil and rock will be much less affected.
 

T_R_Oglodyte

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if it is 7.4 on the Mercalli scale, it is the equivalent of about 6 on the Richter scale, not very strong

That's a generalization but is often inaccurate in the specifics. Even a small quake can generate an off the chart MMI depending on the location of the quake.

This is a big issue in the Los Angeles basin. Just because of where faults lie, most strike slip quakes (e.g., most of the San Andreas and related fault systems), occur in mountainous or hilly areas, where the epicenter is rocky. Rock have much less movement during a quake, so by the time the seismic waves reach vulnerable areas the energy is already dissipating. The LA Basin, we are learning, is crossed by numerous deeper thrust faults that are in the sedimentary areas, that are prone to high shaking. So a smaller Richter magnitude event occurring on one of those faults can have a much larger effect than a larger San Andreas strike-slip event occurring north of the city.
 
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