1. Section 1 1 Introduction 1 Section 2 2 Drought Hazard Profile 1
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- 3.3Probability of Occurrence
- 3.4Magnitude and Intensity
3.2Disaster HistoryA number of earthquakes have occurred over the past 200 years, 43 of which were recorded by the U.S. Geological Survey (USGS) (see Appendix A). Most of these earthquakes were very minor. Map 2 shows the location of all felt historical earthquakes during this period. The two most significant historic earthquakes in Louisiana were the New Madrid earthquakes (1811-1812) and the 1930 earthquake in Donaldsonville in southern Louisiana which measured 4.2 on the Richter scale. The New Madrid earthquake was one of the largest earthquake events ever to occur in the United States. Occurring near New Madrid, Missouri, from December 16, 1811, to February 7, 1812, a number of earthquakes originated in this region, with the three strongest earthquakes thought to have magnitudes between 8.4 and 8.7 on the Richter scale. The strongest tremors were felt from Map 2: Louisiana Earthquake Hazard Map New Orleans to Quebec and the course of the Mississippi River was permanently changed. The aftershocks went on for more than 5 years after the initial series of earthquakes. The largest recorded earthquake event in Louisiana occurred in Donaldsonville on October 19, 1930. The earthquake was felt over a 15,000-square-mile area of southeastern Louisiana. The towns that suffered the most damage were Donaldsonville, Gonzales, Napoleonville, and White Castle. There were reports of damaged brick chimneys, broken windows, and overturned small objects. Other towns affected were Morgan City, Franklin, Elemans, Berwick, and Plaquemine. These towns reported doors and windows rattling, houses creaking, and hanging objects swinging. 3.3Probability of OccurrenceF  igure 1 also displays the probability of exceeding a certain ground motion, expressed as peak ground acceleration (PGA). This particular map shows the 10% probability of exceeding normal ground motion in 50 years. This translates to a 1 in 475 chance of normal ground motion being exceeded by the amount shown on the map annually. The southern half of the state has the lowest PGA in the state of 1 percent gravity (1%g). Only the northeastern part of the state has a PGA of 3%g—quite low when compared to the New Madrid Seismic Zone, which has a PGA as high as 40%. It is important to note that this map expresses a 10% probability; there is a 90% percent chance that normal ground motions will not be exceeded.
3.4Magnitude and IntensityThere are several different ways to express the severity of an earthquake. The two most common are magnitude, which is the measure of the amplitude of the seismic wave and is expressed by the Richter scale, and intensity, which is a measure of how strong the shock was felt at a particular location, expressed by the Modified Mercalli Intensity (MMI) scale. The Richter scale represents a logarithmic measurement where an increase in the scale by one whole number represents a tenfold increase in measured amplitude of the earthquake. Table 3 shows the rough correlation between the Richter scale, Peak Ground Acceleration (PGA), and MMI. The relationship between PGA, magnitude, and intensity are, at best, approximate, and also depend upon such specifics as the distance from the epicenter and depth of the epicenter.
D  eveloped by the Central United States Earthquake Consortium (CUSEC), Figure 2 depicts a hypothetical earthquake in the New Madrid region with a magnitude of MMI VIII, similar to the magnitude of the 1811-1812 New Madrid earthquakes. An earthquake at this magnitude would be felt in the northern reaches of Louisiana at a magnitude of MMI VI. According to Table 3, earthquakes at this magnitude would be felt by all, move heavy furniture, and cause slight damage.
With a history of strong tremors originating from the New Madrid seismic zone reaching into northern portions of the State and an active fault system and past earthquakes in the southern half of the State, the evidence indicates that the most severe earthquakes in the state are most likely to occur to the very north (near the Arkansas - Mississippi border) and to the south (near the coast). Directory: recovery recovery -> Standard Specifications For Structural Supports for Highway Signs recovery -> Georgia Disaster History recovery -> Recovery of Damages recovery -> National Marine Fisheries Service Protected Resources Research and Management Services Download 1.88 Mb. Share with your friends:
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