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 4.21. 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 4.22 shows descriptions for levels of earthquake intensity on the MMI scale compared to the Richter scale. Seismic shaking is typically the greatest cause of losses to structures during earthquakes. Table 4.21 – Richter Scale
Magnitude Less than 3.5
Effects
Generally not felt, but recorded. Often felt, but rarely causes damage.
3.5 – 5.4
At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions.
5.4 – 6.0
6.1 – 6.9 7.0 – 7.9
Can be destructive in areas up to 100 kilometers across where people live. Major earthquake. Can cause serious damage over larger areas.
8.0 or greater
Great earthquake. Can cause serious damage in areas several hundred kilometers across.
Source: FEMA
Table 4.22 – Comparison of Richter Scale and Modified Mercalli Intensity (MMI) Scale
MMI
Richter Scale
Felt Intensity
I
0 – 1.9
Not felt. Marginal and long period effects of large earthquakes. Felt by persons at rest, on upper floors, or favorably placed.
II
2.0 – 2.9 3.0 – 3.9
III
Felt indoors. Hanging objects swing. Vibration like passing of light trucks. Duration estimated. May not be recognized as an earthquake. Hanging objects swing. Vibration like passing of heavy trucks. Standing motor cars rock. Windows, dishes, doors rattle. Glasses clink the upper range of IV, wooden walls and frame creak. Felt outdoors; direction estimated. Sleepers wakened. Liquids disturbed, some spilled. Small unstable objects displaced or upset. Doors swing, close, open. Pendulum clocks stop, start. Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Books, etc., fall off shelves. Pictures fall off walls. Furniture moved. Weak plaster and masonry D cracked. Small bells ring. Trees, bushes shaken. Difficult to stand. Noticed by drivers of motor cars. Hanging objects quiver. Furniture broken. Damage to masonry D, including cracks. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices. Some cracks in masonry C. Waves on ponds. Small slides and caving in along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged. Steering of motor cars is affected. Damage to masonry C; partial collapse. Some damage to masonry B. Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory
IV
4.0 – 4.3
V
4.4 – 4.8
VI
4.9 – 5.4
VII
5.5 – 6.1
VII
6.2 – 6.5
Wake County Multi-Jurisdictional Hazard Mitigation Plan 2019
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