Analysis of Seismicity in the New Madrid Seismic Zone
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Abstract
Seismicity in the New Madrid seismic zone has been analyzed. The annual frequency in the number of earthquakes spanning the periods between 1963 and 2013 confirms that the zone is still very much active. The number of earthquakes increased from about 220 in 2009 to 910 in 2010, and to 960 in 2011. A decline was observed in following years. While earthquakes having magnitudes 1-1.9 formed the highest proportion of events in 2010, magnitudes 2-2.9 formed the majority in 2011. The highest recorded event from 1963 to 2014 had a magnitude of about of 5.6. Earthquakes in the New Madrid seismic zone have shallow focal depths, with over 60% occurring less than 15km from the surface. Hourly variation of earthquakes from 2010 to 2014 suggests a drop in sensitivity of the majoring instruments between 7am and 5pm due to heightened cultural noise, especially during midday. During this period, magnitude 1-1.9 are at their lowest peaks but magnitudes ≥ 2 show significant high values. The observed frequency of occurrence of earthquakes from 1963 to 2014 suggests that the New Madrid seismic zone remains active.
Introduction
It is a well-known fact that the New Madrid seismic zone has remained active in seismicity following the 1811-12 New Madrid sequence, which was responsible for four of the most devastating earthquakes in the history of the United States; one of which was later considered an aftershock following the first main shock (Morgan and Susan, 2014). The earthquakes occurred in December 1811, January 1812 and February 1812 (Lanbo et al., 1992 ). The New Madrid seismic zone is an area of intraplate earthquakes located at the edge between Missouri, Arkansas, Tennessee and Kentucky (Barz and Zoback, 2001).
Intraplate seismicity is a consequence of internal plate deformation rather than by motion along plate boundary and unlike the San Andreas Fault in California where the fault system is relatively exposed at surface, the fault system in the New Madrid seismic zone remains hidden in the subsurface, beneath 100ft-200ft of soft soil where traces and fault scarps quickly erode (Morgan and Susan, 2014). According to McKeown, 1982, an ancient active rift in Precambrian/or early Paleozoic gave rise to the seismic zone. The active fault system is observed from seismogram recordings of micro-seismic earthquakes which indicates a fault length of approximately 125 miles from Marked Tree Arkansas, through Missouri, Tennessee, Kentucky to Cairo in Illinois (Geology Survey program; Facts about the New Madrid seismic zone).
Intraplate earthquakes can have tremendous consequences because the magnitudes are comparable with those of plate boundary events (Johnson, 1996). Large occurrence of earthquakes in this region have the potential of affecting parts of Illinois, Missouri, Kentucky, Indiana, Tennessee, Arkansas, Missouri (Stelzer, 1999). This zone has been characterized as the ‘most active seismic zone in the midcontinent region east of the Rocky Mountains in the United States (Missouri Department of Natural Resources).
Although the magnitudes of the 1811-12 earthquakes was gleaned from historical records and may have been inaccurate, sand blows in areas of about 10,400km2 believed to be associated with the New Madrid sequence suggests the earthquakes were very large in amplitude (Tuttle et al, 2002). The estimated moment magnitude of the earthquakes falls between 8.1 and 8.3 (Johnson and Kanter, 1990).
Archaeological as well as dendrochronological studies have been pivotal in unravelling past seismic events prior to 1811 as well as confirming the occurrence of the 1811-12 earthquake events. Van Arsdale et al., 1998, used tree ring records from Cypress trees in Reelfoot Lake to establish a record for the 1811-12 events, noted in the form of fractures in the tree ring followed by speedy growth after the inundation. Dating of sand blows, soil horizons and artifacts indicated that events such as the 1811-12 occurred around 2350Bc, AD 300, AD 900 and AD 1450 (Tuttle et al., 2002; Tuttle et al., 2006).
In an attempt to explain the ongoing seismicity in the New Madrid seismic zone, Nuttli, 1973 and Hough, 2009, proposed that it was due to long lived aftershocks following the 1811-12 events. Morgan and Hough, 2014 in a similar study in the ongoing seismicity in the New Madrid seismic zone discarded the aftershock hypothesis citing strain accrual as a more probable cause.
A number of studies have focused on possible triggers of seismicity in the New Madrid seismic zone. Barz and Zoback, 2001 proposed that deglaciation of the Laurentide ice sheet which covered a large part of the Northern US until ca. 20ka changed the stress field in the New Madrid seismic zone and caused seismic strain rates to increase. Grana and Richardson, 1996 suggested a stress field change due a rift pillow. Liu and Zoback, 1997 proposed that the high seismicity was as a result of increased ductile strain of the crust and upper mantle following locally elevated heat flow.
In this paper, the seismicity of the zone is analyzed by paying reference to the frequency of the number of earthquakes observed yearly as well as its hourly distribution.
Data
The earthquake catalog used in the study was obtained from the Advanced National Seismic Systems (ANSS). It contained ten thousand and thirty nine (10039) earthquakes with magnitudes ≥ 1. The catalog contained events from 27 June 1962 to 8 December 2014 in a 10o by 10o area centered on New Madrid, Missouri (Fig 1). The study area covered parts of Iowa, Missouri, Illinois, Indiana, Kentucky, Arizona, Tennessee, Mississippi, Alabama and Loiusiana.
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