2010 Metro Mitigation Plan
2010 Metro Mitigation Plan
2010 Metro Mitigation Plan
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The Mercalli scale is the most effective means of determining the approximate<br />
magnitude of a quake that occurred in historic time prior to the advent of uniform<br />
seismic detection devices and the Richter Scale.<br />
Table 4-19. Comparison of Richter Magnitude<br />
and Modified Mercalli Intensity Scales<br />
Richter Mercalli<br />
Magnitude Scale<br />
Effects<br />
2 I – II Usually detected only by instruments<br />
3 III Felt Indoors<br />
4 IV – V Felt by most people; slight damage<br />
5 VI – VII Felt by all; damage moderate<br />
6 VII – VIII Damage moderate to major<br />
7 IX – X Major damage<br />
8+ X - XII Total and major damage<br />
Ground Motion Amplification<br />
Ground motion is the movement of the earth‟s surface due to earthquakes or explosions. It is<br />
produced by waves generated by a sudden slip on a fault or sudden pressure at the explosive<br />
source and travels through the earth and along its surface. Ground motion is amplified when<br />
surface waves of unconsolidated materials bounce off of or are refracted by adjacent solid<br />
bedrock. The seismic hazard in the <strong>Metro</strong> area is shown in Figure 4-8, which uses contour<br />
values to indicate the earthquake ground motions that have a common probability of being<br />
exceeded in 50 years.<br />
In developing Figure 4-8, the ground motions being considered at a given location are those<br />
from all future possible earthquake magnitudes at all possible distances from that location.<br />
The ground motion coming from a particular magnitude and distance is assigned an annual<br />
probability equal to the annual probability of occurrence of the causative magnitude and<br />
distance.<br />
The method assumes a reasonable future catalog of earthquakes, based upon historical<br />
earthquake locations and geological information on the recurrence rate of fault ruptures.<br />
When all the possible earthquakes and magnitudes have been considered, a ground motion<br />
value is determined such that the annual rate of its being exceeded has a certain value.<br />
Therefore, as presented on Figure 4-8, for the given probability of exceedance, two percent,<br />
the locations shaken more frequently will have larger ground motions.<br />
<strong>Metro</strong>politan Nashville - Davidson County<br />
Hazard Identification<br />
Multi-Hazard <strong>Mitigation</strong> <strong>Plan</strong> Page 4-45<br />
April <strong>2010</strong>
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