Earthquake archaeology

By Gregory Vogel


            Fire, flood, famine, war, plague... all major disasters, including earthquakes, leave distinctive signatures in the archeological record.  An odd specialty within archaeology concerns evidence from ancient catastrophes: how often did they occur?  What caused them?  How have civilizations prepared, or failed to prepare, for disasters?  Aside from simply filling the gaps in our knowledge of the past, these studies have the potential to teach us practical lessons about what has and hasn't worked in the face of calamity.


            The recent earthquakes in Illinois and Missouri bring to mind stories of the 1811 and 1812 quakes in the New Madrid Seismic Zone immediately to the south.  At 5.2 and below on the Richter Scale, the recent quakes were not so strong as to cause major damage.  The scale of the historic New Madrid events is difficult to imagine, with two quakes measuring an estimated 8 on the Richter Scale, six or more an estimated 7, and over 1,000 smaller quakes occurring on a nearly daily basis for several months and even years later.  


            There were no seismic instruments to record the quakes at the time, but personal accounts give enough information to estimate the strength of the events and to convey the stark terror they caused.  Survivors recall visible waves rolling across the ground surface.  People were thrown out of their beds and unable to walk or even stand upright during the most violent shaking.  The Mississippi River was re-routed and even flowed backwards for a time.  New lakes were created as large tracts of land were lowered 20 feet and more.  Church bells as far away as Boston, 1,000 miles from the epicenter, rang because of the shaking.


            Seismologists predict earthquake hazards based on a region's seismic history, but because large earthquakes in the central U.S. are uncommon, there isn't much history to go on.  Large earthquakes are recorded in sediments where lateral movement shifts blocks of earth and where deeply buried sand is forced upward from the pressure and shaking.  Such "sand blows" resemble small volcanoes, as tall spouts of sand shoot out of the ground and form low cones 100 or more feet across.  Where these features are preserved in sediments they can be used to estimate the extent and power of past earthquakes, but determining how long ago they formed is difficult. 


            This is where archeology can help.  Large river valleys of the central U.S. are rich in prehistoric Native American sites, which can be dated by numerous methods honed over the last 100 years of archeological research.  Where an archaeological site is found below earthquake features in the sedimentary record, we know the earthquake is younger than the archeology; where a site is found above the same features, the opposite is true.  Creating a solid archaeological sequence thus helps us understand earthquake history.  


            Paleoseismologists (geologists who study ancient earthquakes) work closely with archaeologists on sites in the Mississippi valley to pin down the history of past seismic events.  While the record is so far quite sketchy, several ancient earthquakes have been documented.  One of the earliest occurred southwest of the New Madrid Seismic Zone 5,000 to 7,000 years ago, and one of the latest occurred sometime between A.D. 1300 and 1600. 


            And just how did large earthquakes affect ancient populations?  Determining this is even more difficult than dating the events, but there is one intriguing possibility.  In late prehistoric times, between about A.D. 1000 to 1300, large portions of the central Mississippi Valley and adjacent areas held large Native American populations in both small village sites and large centers like Cahokia.  The villages and cities appear to have been almost completely abandoned shortly after then, leading archaeologists to call this the "Vacant Quarter".  There are several hypotheses to account for the abandonment: environmental degradation, epidemic disease, war or perhaps some other social upheaval. 


            Several archaeologists, however, have pointed out that the vacant quarter roughly corresponds to the region of greatest impact within the New Madrid and Wabash Valley Seismic Zones.  Recall that there is evidence for at least one major seismic event around the time of the Vacant Quarter.  The hypothesis is clear, even if it is difficult to test: a series of very violent earthquakes simply frightened whole populations into leaving.  The quakes of 1811/1812 certainly had that effect.


            Are there lessons for today?  The ancient record doesn't provide any direct guidelines.  The earth and wood architecture of Native Americans, whether or not it survived large quakes, didn't leave a great deal of rubble to tell us how it fared.  Perhaps the most important lesson is simply that the next "big one", sooner or later, is all but certain.  The U.S. Geological Survey estimates "the probability of a magnitude 6 to 7 earthquake occurring in this seismic zone within the next 50 years is higher than 90%.", and the probability of an event 8 or greater is between 7 to 10 percent."  Vague guidelines at best.


            Because of our modern infrastructure and population density it would be nearly impossible for us to completely abandon such a large area anymore – no chance of a new Vacant Quarter.  The best we can do is study hard to understand earthquake histories to better predict future events.  And prepare.  Archaeologically, today's society is always tomorrow's ancient civilization.  Prepared or not, the rubble left from much of our modern architecture will certainly be a boon to future paleoseismologists.  Small comfort, even to an archaeologist.