Earthquake Preparedness: Not Just for Left Coasters
It was a warm winter night 200 years ago when the ground and buildings started shaking from a strong earthquake in the upper Mississippi River valley near New Madrid, Mo. The first in a series of quakes struck on Dec. 16 around 2:15 a.m., and the few thousand pioneer settlers in the region did not understand what was happening, much less what to do about it. People panicked and ran outside during several long minutes of strong shaking. Unfortunately, this frightened response would likely be repeated in the central U.S. today, as it was when a magnitude 5.8 earthquake struck central Virginia earthquake on Aug. 23, and again when a magnitude 5.6 quake struck east-central Oklahoma on Nov. 6.The havoc 200 years ago was caused by a powerful earthquake near the southern portion of the Missouri Bootheel, then the edge of the western frontier. Little did the settlers also know that this would be the start of a remarkable series of powerful earthquakes, each 500 to 1000 times stronger than the recent Virginia and Oklahoma events, and each with their own aftershock sequence. Some of the aftershocks were large enough to cause additional damage. At least seven of the earthquakes were felt on the east coast of the United States. One of these East Coast residents was President James Madison in Washington D.C., who probably felt at least three of these earthquakes, and remarked in a letter to Thomas Jefferson on Feb. 7, 1812 (the day of the 3rd and final magnitude 7+ earthquake in the sequence):
"The re-iteration of earthquakes continues the uproar from certain quarters. They have slightly reached the state of N. Y. and have been severely felt W. & S. Westwardly. There was one here this morning at 5 or 6 minutes after 4 o'C. It was rather stronger than any preceeding one, & lasted several minutes, with sensible tho very slight repetitions throughout the succeeding hour."
Because there were no seismographs in 1811, considerable evidence about the size of the New Madrid earthquake sequence comes from hundreds of written accounts like this. Many eyewitnesses in the New Madrid region described a sense of almost continual shaking for weeks, and during the largest earthquakes, "geysers of sand and water several feet high erupting on to the ground surface." Boatmen on the Mississippi River reported feeling the shaking in their small barges and noted the collapse of river banks and the disappearance of islands in the river channel. Scientists know these ground-failure effects today as tell-tale signs of liquefaction, and these accounts are corroborated by the evidence of widespread liquefaction (sand blows) still visible on the ground surface today. In effect, the size and distribution of the sand blows act as historic seismographs, helping today's scientists gauge the amount of ground shaking occurring over centuries.The 1811-1812 New Madrid sand blows are among the largest yet found on Earth, and although it does not mean that these quakes were the largest to occur (far from it), they do tell us much about the strong shaking in the region between Memphis and St. Louis and the potentially devastating consequences of a repeat of similar earthquakes in this region: millions of acres of valuable farmland would be flooded and clogged with sand, railroad and highway infrastructure would be damaged, the billion-dollar Mississippi River transportation system would likely be shut down for an extended period, and unreinforced brick buildings and chimneys could be damaged to distances of 600 miles from New Madrid, Mo.
We know the 1811-1812 earthquake sequence was not just a freakish one-time event. Digging in to the sand blows geologists have also found widespread evidence of pre-historic sand blows telling of similar sequences of earthquakes around 1450, 900, and 300 A.D. The USGS earthquake hazard map, which is a compilation of scientific studies over the past 40 years, demonstrates the earthquake hazard created by this earthquake history and that much of the nation is at risk from damaging earthquakes. For example, historical accounts document two other powerful earthquakes, one that struck Cape Ann, Ma. in 1755, and the other Charleston, S.C. in 1886. Both were prior to instrumental recording, but their magnitudes are estimated to have been about magnitude 6.3 and 7.3 respectively. Of course, there was not as much to shake in those days, but should there be a repeat of earthquakes like this, they would strike large areas, and possibly a few populous cities (St. Louis, Nashville, Memphis, and Little Rock) without a history of earthquake preparedness. Strong, damaging earthquakes are clearly not just confined to the tectonic plate boundaries along the west coast of North America.
Thankfully, recent magnitude 5.3 to 5.8 earthquakes in Illinois, Colorado, Virginia, and Oklahoma did not cause loss of life, and there were no major failures of critical infrastructure. Some brick and stone buildings suffered damage and will need repairs. The fact that these quakes did not strike closer to urban areas was a stroke of luck and was largely responsible for the safety of the citizenry, not the citizenry's earthquake preparedness and response. Nevertheless, it is natural to fear earthquakes because unlike hurricanes, fires, tsunamis, or tornadoes, we cannot see them coming. But knowing what to do when an earthquake strikes, that we are prepared for the worst, and that we live in an earthquake-safe environment can bring much peace of mind. So what SHOULD people do if they find themselves being shaken by an earthquake and how can they be better prepared? Here are some quick tips:
- If you are inside, stay inside. Get under a sturdy table or desk and hold on. Stay away from windows, bookcases, and other falling objects and glass - Drop, Cover, and Hold
- If you are outside, stay outside, unless you are right next to a building that could have falling glass or bricks. In that case, duck into a doorway for shelter.
- If driving in a car, stay in the car, but safely pull over to a stop, ideally not on or under a bridge or overpass.
- AFTER the event it is natural that you will want to check on loved ones, but send out a text message rather than call; it has far less impact on communication systems.
- BEFORE the next event, think about how you can make your surroundings safer. The FEMA website has abundant information on what to do in case of an earthquake, and how to make your home, business, or community more earthquake resilient.
Marcia McNutt
Director, US Geological Survey
No comments:
Post a Comment