# 5948
Since 2004 the earth has seen an increase in the number of extremely large earthquakes (over the numbers recorded in the previous hundred years) which has led some people to wonder if they might be connected, or part of a pattern.
The chart below, from the USGS, shows twelve 8.0 magnitude or greater quakes since 2004, or about 50% more than would normally be expected.
This pattern has led to a great deal of speculation – particularly on the internet - that we have entered an `age of earthquakes’.
The USGS posted a short analysis – based on a recently published paper in the GRL (Geophysical Research Letters)- that states that this rise in seismic activity in within the range that one might expect from pure chance.
First the USGS statement, then a link to the article.
Spate of Large Earthquakes Not Related
Released: 11/7/2011 4:00:00 PM
MENLO PARK, Calif. – The magnitude-7.2 earthquake on October 23rd in Turkey and the magnitude-9.0 quake that impacted Japan in March are leading many to wonder if these events are part of a larger global trend toward giant earthquakes. After combing through 110 years’ worth of global seismic records, USGS seismologist Dr. Andrew Michael concluded that the recent increase in the number of large earthquakes may just reflect random occurrence.
Using three distinct statistical tests, Dr. Michael studied whether variations in the number of large, global earthquakes could be explained as a random fluctuation, once local aftershocks of the large earthquakes are taken into account. In a recently published paper, he explains how he tested whether the intervals between earthquakes have followed a clustering pattern that would be suggestive of quakes related to each other. He then developed a specific earthquake-triggering statistical model to determine if global seismicity increased after the largest earthquakes, examining the effect of the largest earthquakes on smaller ones. Finally, he tested for clustering in the energy released by earthquakes.
In each test, he found that the apparent clustering among large earthquakes can be described as a random fluctuation and cannot be used to predict future events.
"Large earthquakes, in statistical terms, do not happen very often, even though they are noteworthy for their enormous consequences. In events that do not happen very often, random processes are highly variable and may appear clustered even if they are not," said Dr. Michael. "While the probability of future large mainshocks has not increased, neither has it decreased."
Dr. Michael concludes that global forecasts of earthquakes and the damage they inflict should use the longest possible historical record, rather than just the record from the recent past.
"This research is a good example of how science can empower our decisions with knowledge rather than fear," said USGS Director Marcia McNutt. "Those who live in earthquake country should work to make their surroundings as safe as possible and know what to do if an earthquake happens regardless of how quiescent or active the rest of the planet may be."
The paper "Random variability explains apparent global clustering of large earthquakes" appears in Geophysical Research Letters, a publication of the American Geophysical Union, and is available upon request.
The abstract to the article in the GRL may be accessed at the link below:
GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L21301, 5 PP., 2011
doi:10.1029/2011GL049443Random variability explains apparent global clustering of large earthquakes
key Points
- Large global earthquakes are temporally random plus aftershocks
- Apparent global clustering proposed by others is due to random variability
- Current risk of future earthquakes is not elevated except for local aftershocks
Andrew J. Michael
U.S. Geological Survey, Menlo Park, California, USA
The bottom line, according to the author:
Except for regional aftershocks, which may occur for months or years after a major quake, recent earthquake activity is not a good predictor of the future.
Areas that have been seismically quiet for decades can suddenly, and without warning, experience a strong quake.
Which is why FEMA, READY.GOV, and other agencies continue to urge everyone to be prepared for all types of emergencies, including earthquakes.
Everyone needs an appropriate disaster plan, just as everyone should have a good first aid kit, a `bug-out bag’, and sufficient emergency supplies to last a bare minimum of 72 hours.
For more on disaster preparedness, I would invite you to visit Ready.gov or revisit these blogs:
The L. A. County Emergency Survival Guide
An Appropriate Level Of Preparedness
And reminder that early next year, two regions will be holding major earthquake exercises.
The Great Central U.S. Shakeout On February 7, 2012
The Great Utah ShakeOut, April 17, 2012
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