SECTION 4: RISK ASSESSMENT
MMI
Richter Scale
Felt Intensity stacks, monuments, towers, elevated tanks. Frame houses moved on foundations. Decayed piling broken off. Branches broken from trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on steep slopes. General panic. Masonry D destroyed; masonry C heavily damaged, sometimes with complete collapse; masonry B seriously damaged. (General damage to foundations.) Serious damage to reservoirs. Underground pipes broken. Conspicuous cracks in ground. In alluvial areas sand and mud ejected, earthquake fountains, sand craters. Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes, embankments. Large landslides. Water thrown on banks of canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches and flat land. Rails bent slightly.
IX
6.6 – 6.9
X
7.0 – 7.3
XI
7.4 – 8.1
Rails bent greatly. Underground pipelines completely out of service. Damage nearly total. Large rock masses displaced. Lines of sight and level distorted. Objects thrown in the air.
XII
> 8.1
Masonry A: Good workmanship, mortar, and design; reinforced, especially laterally, and bound together by using steel, concrete, etc.; designed to resist lateral forces. Masonry B: Good workmanship and mortar; reinforced, but not designed in detail to resist lateral forces. Masonry C: Ordinary workmanship and mortar; no extreme weaknesses like failing to tie in at corners, but neither reinforced nor designed against horizontal forces. Masonry D: Weak materials, such as adobe; poor mortar; low standards of workmanship; weak horizontally. Source: Oklahoma State Hazard Mitigation Plan. The most severe earthquake to impact the Wake County area was the Charleston earthquake of 1886. It is estimated to have been felt as a 7 or 8 on the MMI Scale. Since then, six earthquakes have been felt in Wake County, and all were at an MMI Scale of 4 or lower. Impact: 1 – Minor Spatial Extent: 4 – Large Historical Occurrences The USGS Earthquake Hazards Program maintains a database of all historical earthquakes of a magnitude 2.5 and greater. These events are illustrated in the following pages. Figure 4.6 shows historical earthquakes by magnitude in relation to North Carolina and the Quaternary Faults identified by USGS. This includes events from 1973 to 2019. Figure 4.7 provides a more detailed view of earthquakes that have occurred within 50 and 100 miles of Wake County.
Wake County Multi-Jurisdictional Hazard Mitigation Plan 2019
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