Popular Mechanics had an interview with Roger Musson, author of The Million Death Quake. Part of the interview:
Q: You warn of "rogue quakes." What are those?Here is a May 10, 2012, about the April 2012 rogue earthquake off Indonesia:
A: All earthquakes occur on fault lines, but faults are far more common than most people realize. The rocks we live on have been around for a long time. They’ve been broken up in response to episodes of mountain building that happened millions and millions of years ago. So faults are very common.
A "rogue earthquake" occurs on a fault that wasn’t identified as being particularly dangerous. That can happen in areas where earthquakes are very rare. You might get one every several thousands of years, so the last one was long before living memory.
The other sort of case is earthquakes that are a surprise but shouldn’t be a surprise, and that was very much the case with Haiti. The fault line that produced the Haiti earthquake was well-known . . . There was a historical record of a devastating earthquake back in the 18th century. But in that case, a lot of people hadn’t looked into the history of earthquakes in the area and didn’t realize that the fault was as dangerous as it is.
On April 11 of this year, one of the 10 biggest earthquakes ever recorded struck off the coast of Indonesia. It was felt from Bangladesh to Australia.
You may not have even heard of this magnitude 8.6 quake. It barely made the news in the U.S. because it did very little damage. Two people died, but there was no massive tsunami.
To understand why this quake was so big, yet not catastrophic, you need to know this: There are two kinds of major quakes in the world. One type happens when two plates of the Earth's crust slide past one another horizontally. That's what happens along California's San Andreas Fault.
The other kind of major quake occurs when two plates collide, and one slips beneath the other with a jolt.
"We expect that the largest earthquakes occur in that kind of a setting, called a megathrust," says Greg Beroza at Stanford University. In fact, he says, since seismologists started recording earthquakes about 120 years ago, every quake this huge has been a megathrust quake.
Or that was true, until last month.
The magnitude 8.6 quake off of Indonesia was actually the side-slipping kind, called a strike-slip. Fortunately, that kind of quake is much less likely to cause a tsunami because sideways motion doesn't generate big waves the way up-and-down motion does (unless it triggers an undersea landslide). That's why it caused relatively little damage.
. . . That makes it the biggest quake ever recorded that was not on a fault. It dwarfed the best-known example of quakes like that: the 1811 and 1812 quakes centered near New Madrid, Mo. Nobody measured that one at the time, but historical reconstructions estimate the magnitude at around 7.7.I had posted sometime before about some of the most destructive earthquakes in history, as well as the New Madrid Earthquake.
Beroza suspects the April quake in Indonesia — along with a companion quake the same day, magnitude 8.2 — are part of a gradual process that is making new faults and redefining the boundary between plates.
I've personally noticed news reports of various smallish earthquakes in locations generally thought to be geologically quiet.
ReplyDeleteThere have been a handful of quakes along the East Coast near Washington, D.C., and New York.
There have also been several small quakes in the North Texas (DFW area) over the past few years. Many people have attributed those quakes to "fracking" associated with natural gas drilling, which this commenter doubts. But, there is a large fault running through the DFW area, along with many smaller faults.