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Originally posted by amongus
Did we have any sort of magnitude scale in the early 1800's? I'm confused, since I sure as hell know they didn't use decimals back then for such things...or maybe they did. How did they know ANY magnitude back then?
This is NOT a slam on the OP. I just would like to be educated.
As far as what were the actual magnitudes of the quakes - well, I have done the energy calculations myself, going with the data I have been able to personally gather. I did the estimate for the 2 main quakes on December 16, 1811, and for the last large quake of the day of February 7, 1812.
I can look in the region and see the rock strata data that shows hundreds of quakes of that magnitude in that time.
Constraints on the faulting that took place during the 1811-1812 earthquakes comes primarily from three sources: (a) historical accounts, including far-field intensity data and eyewitness reports from the epicentral zone (Figures 1 and 2), (b) seismological effects remaining from the earthquakes, such as preserved liquefaction features and present-day seismicity in the rupture zone (Figure 3), and (c) the physical structure of the crust of the 1811-1812 fault zone (Figure 4). Two additional and potentially powerful constraints are: (d) forward modeling of coseismic static strain fields to match with known topographic changes in the meisoseismal area and (e) geodetic/GPS measurement of the postseismic viscoelastic crustal relaxation still occurring today from the 1811-1812 faulting. Exploratory studies in both areas have been done (Gomberg 1992, Gomberg & Ellis 1994, Rydelek & Pollitz 1994), but the topographic and strain-rate databases must be considerably improved before they contribute significant constraints that rank with (a)- (c).
A historical perspective is important for assessing both the near- and farfield effects of the New Madrid earthquakes. Figure 1 shows that the epicentral NEW MADRID EARTHQUAKES region was on the forward edge of European settlement. Kentucky and Tennessee became states in 1792 and 1796, respectively; they were the only two states of the Union with territory in the meisoseismal zone, which lay primarily in what would become the states of Missouri (in 1821) and Arkansas (in 1836). The restricted population distribution (contoured in Figure 1) is a major problem in estimating the earthquakes' sizes from far-field isoseismal data (see Section 5).
On the basis of the extensive macroseismic effects including massive liquefaction, fissuring, subsidence, or uplift of landforms; violent disturbance of the river; and destruction of extensive tracts of forests (Table 2), both Davison (1936) and Richter (1958) considered the principal New Madrid events to be "great" earthquakes. In Richter's case, this presumably means a Richter magnitude MR ³ 8, although he never explicitly assigned a magnitude value. In Davison's case, this put New Madrid in the company of the famous 1755 Lisbon, 1897 Assam, 1906 San Francisco, and 1891 Nobi earthquakes, among others. Both authors' estimates were based on the work of Fuller (1912) and consisted more of informed judgment than of quantitative analysis.
Globally it dominates all other documented earthquakes of stable continental regions (SCR) (Johnston et al 1994), a category of plate interiors that incorporates roughly 25% of all crust and fully two thirds of all continental crust. Why the New Madrid earthquakes are unique remains an enigma. Perhaps, given sufficient time, other stable continental plate interiors will experience earthquakes of the magnitude and numbers of the New Madrid sequence, although the worldwide historical record does not reveal a comparable sequence.