Scenario Earthquakes for Urban Areas Along the Atlantic Seaboard of the United States
Abstract
Based on the historic (300-year) and instrumentally monitored (50-year) seismic records, many portions of the Atlantic seaboard of the U.S. and adjacent Canada are prone to earthquakes at low to moderate rates of seismicity. The observed moderate rates do allow for large (magnitude M = 6 and 7) earthquakes to occur, albeit at long recurrence intervals. Some of the larger historic events on record along the Atlantic margin of North America include the 1886 Charleston, SC, quake (Mw = 7.6.3) and the 1929 Grand Banks earthquake (Mw = 7.4.2). The largest historic event in the New York City area is the 1884 M = 5+ earthquake that occurred offshore; and the largest in the Boston area is the 1755 M = 6+ event near Cape Ann. Most of the larger earthquakes in the eastern U.S. have occurred prior to major population increases or, more recently, in less densely settled areas. These fortuitous circumstances have lulled the eastern U.S. population into the misleading perception that earthquakes do not pose a major threat.
The magnitudes to be considered for economic consequences range from relatively frequent quakes of magnitudes M = 4.5 typically causing only minor losses of at most a few million dollars, to rare magnitude M = 7 to 7.5 earthquakes potentially inflicting catastrophic losses in the tens to hundreds of billions of dollars, depending on distance from a major city, the soil conditions, and the value and vulnerability of the city’s built assets. From the relationship between observed frequency of occurrence, and magnitude based on historic seismicity, magnitude-distance pairs can be computed as a function of recurrence period for each city. For instance, a conservative estimate for New York City is that M = 5, 6 and 7 earthquakes can be expected to recur about once every 500 years at median distances of about 20, 50 and 130 km, respectively; and during a 2,500 year period at the much shorter distances of 10, 25 and 60 km, respectively. These numbers are approximately equivalent to a 10% chance for such events to occur during exposure times of 50 and 250 years, respectively.
Two region-specific factors contribute to a potentially high seismic hazard in the eastern U.S., once a magnitude of an earthquake is given. First, seismic shaking, especially at high frequencies (5 Hz) reaches in the eastern U.S. to larger distances than in the western U.S.; and second, ground motions can vary more strongly between sites on very hard rock and very soft soils. Another contributing hazard is soil liquefaction, a condition whereby a sandy soil becomes a liquid and cannot support structures.
We discuss examples of past earthquake effects in the eastern U.S., adjacent Canada, and from geologically similar situations elsewhere. From these examples, we develop scenario events centered on magnitudes near M = 6 since they fall somewhere between the "manageable" losses from M = 5 events, and the virtually "unmanageable" catastrophic losses from M = 7 events, at least if they are centered on any major eastern city such as Boston, New York, Philadelphia, Baltimore, Washington D.C., or Atlanta.
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