It seems the New Madrid Seismic Zone has gotten into the 4th of July spirit by providing an earthquake for the holiday. It was in Southeast Missouri with a magnitude of 2.8. For more information visit the CERI website and go to Missouri Earthquake. Read and enjoy the seismic information as you celebrate the holiday.
When one is looking for earthquake felt reports in the historic record the sometimes the easiest part is finding the report. The hard job then becomes trying to figure out the magnitude or force of the earthquake that the report is detailing. In the era prior to the Richter scale for instrumental earthquakes another scale was developed to help in quantifying felt reports. This scale was the Mercali scale. This scale was developed in the early 20th century from previous scaling measurements. The developer was an Italian vulcanologist Guiseppe Mercali hence the name of the scale. It was later refined by Richter to become the Modified Mercali Scale or MMI in scientific shorthand.
Let us look at the scale and how it is constructed and used. The scale is from one to eight and is written in roman numerals . It is based on the effects noted by eyewitnesses on buildings or the terrain caused by the earthquake.
Using this scale, gives eyewitness accounts a numerical value that can be used by seismologists to estimate the intensity of an earthquake where no instrumental data is present. This makes historic accounts usable for earthquake data that predates modern instrumentation. It also means that historic accounts are important not only for descriptions of damage but for helping assign a magnitude to an historic earthquake. In the next post I will give an example or two of how this is done with some historic accounts from the New Madrid era.
While all concerned were slumbering from a nice weekend, the New Madrid seismic zone nudged us to try and awaken the sleeping. At 1:24 am this morning a 3.1 earthquake occurred south of Ridgely, Tennessee. It was a light one and should have been felt in the vicinity surrounding the epicenter. For more information visit this site Local Earthquake for more information.
It seems that seismicity does not take the weekend off. During the weekend an earthquake happened outside of the New Madrid Seismic Zone but in the Southeastern United States in a location that usually does not experience earthquakes. The quake in question occurred on Saturday north of Aiken, South Carolina. Its depth was 4 kilometers(3 miles( and its magnitude was 4.1. What was interesting was the vast distance that the quake was felt. When you refer to the data page you will see is was noticed or felt in a multi state area. For more information visit South Carolina earthquake.
During the time period of the New Madrid earthquakes seismic activity was not common in that region. Here is what a Charleston South Carolina newspaper noted about seismic activity:
Source: "Communication," Charleston(S. C,) The Times, December 16, 1811, Page 3, Column 1, New Madrid Compendium Number 93.
"Columbia S. C, February 11", New Madrid Compendium Number 137
Two hundred and two years ago the largest of the New Madrid earthquakes occurred. It was noted by most observers as being the strongest of the quakes and was felt across the Eastern North American continent. In commemoration of that day here is a felt report from the quake . This accounts gives a vivid glimpse at the power the earthquake displayed even at a large distance from the epicenter. Read and enjoy.
Source "Columbia S. C. February 11," Raleigh Star, February 22, 1812, Page 31, Column 3.
This week the New Madrid seismic zone was a little busier . We had two small earthquakes one at the lower end of the zone then another at the end of the week near New Madrid. Both were small but could have been felt by people near the epicenter. The first one occurred on Tuesday at 5:55 pm near Cherry Valley Arkansas. The magnitude was 2.6 and the depth was 2 miles. For further information visit the data page at Arkansas Earthquake. The second earthquake occurred on Friday at 6:12 am and was located 7 miles south-southwest of New Madrid, Missouri. The magnitude was 2.5 and the depth was 5.9 miles. For further information visit the data page Missouri Earthquake.
Read and enjoy!
During the weekend while the area was thawing out briefly
from the current series of cold snaps, an earthquake occurred in Western
Kentucky.The magnitude was 2.6 and it occurred at 6:35
pm, the location was a few miles west southwest of Paducah, Kentucky.For more information visit the CERI data page