Monday, October 26, 2015

Living on the Fault Line In America

Updated November 2016

In its most recent iteration of its United States National Seismic Hazard Maps, the United States Geological Survey (USGS) has updated the predictions of geoscientists regarding the areas of the U.S. that are most prone to earthquakes, how frequent the earthquakes will be and the magnitude of the earthquakes.

According to the USGS, of the 50 states, 42 have a reasonable chance of experiencing a damaging earthquake over a fifty year period and 16 of those states have a relatively high chance of experiencing earthquake-related damage as you can see on this map:


Not surprisingly, the risk is particularly high along the west coast of the United States and in the western intermountain area; this would include the states of Alaska, California, Hawaii, Nevada, Montana, Washington, Oregon and Wyoming.  

While most of us tend to think of the aforementioned states when we think of earthquakes in the U.S., in fact, when you look at the map, you will notice the dark coloured area in the south central part of the United States.  This area, encompassing parts of Missouri, Arkansas, Tennessee, Kentucky and Illinois as well as peripheral states including Illinois and Mississippi, are adjacent to the New Madrid Seismic Zone or NMSZ.   Here is a map from the USGS showing the faults of the New Madrid Seismic Zone:


Here is a map showing the history of earthquake epicentres along the New Madrid Seismic Zone with the circles being scaled according to the magnitude of the earthquake and coloured according to time (red indicating earthquakes that occurred from 1974 to 2002 with magnitudes greater than 2.5 and green circles indicating earthquakes that occurred prior to 1974):


Here is an even more detailed map showing the epicentres of the major earthquakes as well as the smaller earthquakes along the offset fault lines from a 2008 study by the Missouri Department of Insurance:


Unlike the faults in the Pacific region of the United States that are often seen at or very close to the surface, the faults of the NMSZ are poorly understood because they are buried by between 100 and 250 feet of soil deposited by rivers and an additional 2500 feet of older sedimentary rocks.  Scientific knowledge about the NMSZ faults are gained from studying the seismographic records obtained through the use of more than 30 seismographs throughout the area.  

Here is a block diagram showing the geology of the fault:


The fault itself is about 125 miles long and has four main offset segments. The present day faults consist of reactivated faults that began as a continental rift zone roughly 750 million years ago.  If you don't understand the concept of a rift zone, think of a tear in the earth's crust similar to what we see today in the Red Sea and through the Rift Valley of eastern Africa.  As the earth's crust spreads, igneous rocks from the mantle rise into the crust; in the New Madrid Seismic Zone this occurred about 200 million years ago.  Today, the motion along the fault is both thrusting due to lateral compression and strike-slip where one part of the earth slides laterally past another, similar to the San Andreas fault.

Geoscientists have observed that there are roughly 200 microseismic (magnitude 1.0 to 2.0) earthquakes every year along the NMSZ.  Between 1974 and 2008, there were an estimated 4000 earthquakes   Every so often, there are very significant earthquakes along the NMSZ; one of the largest earthquakes to strike the continental United States occurred during the winter of 1811 - 1812 along the New Madrid Seismic Zone.   While there were no seismographs that recorded the events of December 16, 1811, January 23, 1812 and February 7, 1812, looking at the historical records of the damage done by the trio of earthquakes (note, that the second and third earthquakes are not believed to be aftershocks) has led geoscientists to believe that the earthquakes each had a magnitude of between 6.8 and 8.0 depending on the study as shown on this chart:


On top of that, studies have estimated that an additional 200 moderate to large earthquakes shook the region between December 16, 1811 and March 15, 1812.  For your illumination, San Francisco's 1906 earthquake had a magnitude of 7.8.

One of the problems facing residents of the area is the type of sediment that is deposited at the surface, overlying the fault zones.  These river sediments, also known as alluvium, can liquify when shaken, resulting in structural instability which negatively impacts the foundations of buildings and other infrastructure.  This occurs when an earthquake has a magnitude of 6.0 or greater.

The USGS predicts that the likelihood of a 7.7 magnitude earthquake hitting the region over the next fifty years at less than 7 to 10 percent (compared to an estimate of 20 to 40 percent by the Central United States Earthquake Consortium), however, loss calculations from an earthquake with a magnitude of this size for Missouri alone would look like this:


Given that the New Madrid earthquake of 1811 shook the ground in Boston, Massachusetts, 1000 miles away, causing church bells to ring, the damage zone from a high magnitude earthquake along the NMSZ could cover a large geographic area.  With that in mind, the Mid-American Earthquake Centre projects that total economic losses for the states along the NMSZ would look like this:


Since the areas impacted include the cities of St. Louis, Missouri and Memphis, Tennessee with a combined population of nearly 1 million people and that most of the buildings in the region were not built to the same earthquake-proof standards as those on much of the Pacific coast, the damage to both the economy and to the human population caused by a significant earthquake along the New Madrid Seismic Zone could prove to be catastrophic.

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