SECTION 4: RISK ASSESSMENT
All of North Carolina is subject to earthquakes, with the western and southern region most vulnerable to a damaging earthquake. The state is affected by both the Charleston Fault in South Carolina and New Madrid Fault in Tennessee. Both of these faults have generated earthquakes measuring greater than 8.0 on the Richter Scale during the last 200 years. In addition, there are several smaller fault lines in eastern Tennessee and throughout North Carolina that could produce less severe shaking. EXTENT Earthquakes are measured in terms of their magnitude and intensity. Magnitude is measured using the Richter Scale, an open-ended logarithmic scale that describes the energy release of an earthquake through a measure of shock wave amplitude. A detailed description of the Richter Scale is given in Table 4.25. Although the Richter scale is usually used by the news media when reporting the intensity of earthquakes and is the scale most familiar to the public, the scale currently used by the scientific community in the United States is called the Modified Mercalli Intensity (MMI) scale. The MMI scale is an arbitrary ranking based on observed effects. Table 4.26 shows descriptions for levels of earthquake intensity on the MMI scale. Seismic shaking is typically the greatest cause of losses to structures during earthquakes. Table 4.25 – Richter Scale Magnitude Effects Less than 3.5 Generally not felt, but recorded. 3.5 – 5.4 Often felt, but rarely causes damage.
At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions. Can be destructive in areas up to 100 kilometers across where people live.
5.4 – 6.0
6.1 – 6.9 7.0 – 7.9
Major earthquake. Can cause serious damage over larger areas.
Great earthquake. Can cause serious damage in areas several hundred kilometers across.
8.0 or greater
Source: FEMA
Table 4.26 – Modified Mercalli Intensity (MMI) Scale MMI Shaking Description/Damage I Not felt
Not felt except by a very few under especially favorable conditions. Felt by a few persons at rest, especially on upper floors of buildings.
II
Weak
III
Weak
Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
IV
Light
V
Moderate
VI
Strong
VII
Very strong
Wake County, NC Multi-Jurisdictional Hazard Mitigation Plan
WSP June 2024 Page 107
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