Landslides and Earthquakes
A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments. Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability. Typically, pre-conditional factors build up specific sub-surface conditions that make the area/slope prone to failure, whereas the actual landslide often requires a trigger before being released. U.S. Geological Survey scientists found that last year's magnitude 5.8 earthquake in Virginia triggered landslides at distances four times farther—and over an area 20 times larger—than previous research has shown. "We used landslides as an example and direct physical evidence to see how far-reaching shaking from east coast earthquakes could be," said Randall Jibson, USGS scientist and lead author of this study. "Not every earthquake will trigger landslides, but we can use landslide distributions to estimate characteristics of earthquake energy and how far regional ground shaking could occur."
"Scientists are confirming with empirical data what more than 50 million people in the eastern U.S. experienced firsthand: this was one powerful earthquake," said USGS Director Marcia McNutt. "Calibrating the distance over which landslides occur may also help us reach back into the geologic record to look for evidence of past history of major earthquakes from the Virginia seismic zone."
This study will help inform earthquake hazard and risk assessments as well as emergency preparedness, whether for landslides or other earthquake effects.
This study also supports existing research showing that although earthquakes are less frequent in the East, their damaging effects can extend over a much larger area as compared to the western United States.
The research is being presented at the Geological Society of America conference, and will be published in the December 2012 issue of the Bulletin of the Seismological Society of America.
The USGS found that the farthest landslide from the 2011 Virginia earthquake was 150 miles from the epicenter. This is by far the greatest landslide distance recorded from any other earthquake of similar magnitude. Previous studies of worldwide earthquakes indicated that landslides occurred no farther than 36 miles from the epicenter of a magnitude 5.8 earthquake.
"What makes this new study so unique is that it provides direct observational evidence from the largest earthquake to occur in more than 100 years in the eastern U.S," said Jibson. "Now that we know more about the power of East Coast earthquakes, equations that predict ground shaking might need to be revised."
It is estimated that approximately one-third of the U.S. population could have felt last year's earthquake in Virginia, more than any earthquake in U.S. history. Shaking reports came from southeastern Canada to Florida and as far west as Texas.
Scientists plotted the landslide locations that were farthest out and then calculated the area enclosed by those landslides. The observed landslides from last year's Virginia earthquake enclose an area of about 33,400 km2, while previous studies indicated an expected area of about 1,500 km2 from an earthquake of similar magnitude.
The difference between seismic shaking in the East versus the West is due in part to the geologic structure and rock properties that allow seismic waves to travel farther without weakening.
For further information see Shaking.
Shake Map image via USGS.