Clustering of Large Earthquakes Explained by Random Variability?

Sumatra, 2004. Chile, 2010. Japan, 2011. Odds are these names and dates ring a bell. The scars are still too fresh and widespread for them to have slipped from memory. They identify, of course, the three largest recorded earthquakes that have occurred in the last 45 years — with magnitudes of 9.1, 8.8 and 9.0, respectively. Add in the 2005 magnitude 8.6 quake that also hit Sumatra, likely triggered by the 2004 event, and you’re talking about the four largest earthquakes in roughly the last half century all occurring within a few years of each other. It seems strange that they have been so bunched up, doesn’t it? 
 

An aerial view of Minato, Japan taken about a week after the
March 11, 2011 magnitude 9.0 earthquake and resulting tsunami that
devastated large swaths of the Japanese coast. (Credit: Lance Cpl.
Ethan Johnson, U.S. Marine Corps, Creative Commons Attribution
2.0 Generic)

In fact, there was a similar sequence of major earthquakes in the middle of the last century as well: Kamchatka, magnitude 9.0, 1952; Chile, magnitude 9.5, 1960 (the largest known earthquake); and Alaska, magnitude 9.2, 1964. No other 9.0+ events were recorded in the 20th century. Throw in a few more 8.5+ quakes in the same time frame, and you’ve got quite a cluster of destructive temblors occurring over just a decade and a half.

Contrast these turbulent stretches with the decades-long periods before 1950 and from about 1965 until 2004, when the planet was relatively calm, seismically speaking, and it certainly appears that these enormous earthquakes timed so close together are connected by more than coincidence, right? And if so, shouldn’t we be expecting more major quakes in the near future as one popular author suggested following the earthquake off the coast of Japan last spring?

Crowdsourcing Science

In this age of the ever-expanding scope and complexity of cutting-edge science, researchers are increasingly using any and all resources at their disposal to expand their capacity for data collection and analysis.  This may mean borrowing time on massive, multi-user super-computers to run complex simulations (climate models, for example), or it may mean larger and larger interdisciplinary collaborations among scientists who each have their own equipment and expertise.

In this age of ever-expanding wired interconnectivity, there are also a growing number of opportunities for members of the everyday public to voluntarily offer up their services in the name of scientific awareness and progress.  These citizen scientists have been helping in everything from surveys of wildlife (see, for instance, my last post about the updated winter wolf count in Wisconsin, which relies in part on observations from knowledgeable amateurs) to surveys of interstellar gravitational waves.

Liquefaction and the Dancing Red Giraffes

This post follows on others (here and here) I have recently shared regarding the study of and hazards associated with earthquakes in the Puget Sound region in Washington state.

Among the fascinating stories I had the privilege of hearing while visiting PNSN and the Seattle field office of the USGS was a firsthand account of liquefaction in action during the 2001 Nisqually earthquake from USGS geophysicist Bob Norris.

Liquefaction is the process whereby dry sandy or silt-rich soil becomes a water-saturated slurry and loses its mechanical strength.  It is a frequent side-effect of earthquake-induced ground shaking, and is a particular hazard throughout the Puget Sound region where structures are built on unlithified till and deltaic deposits.

"Big Steel Giraffes": Commercial shipping cranes at the Port of Seattle.  Harbor Island (center) is tucked between SoDo (foreground) and West Seattle (background).  (Image by Vmenkov via Wikimedia Commons)

On the morning of February 28, 2001, as Norris arrived to collect data from a seismometer located on Harbor Island in Seattle, his truck began rocking “from side to side.”  It took him several seconds to realize what was happening, and although there was no noise from the shaking, it was strong enough that he “thought I was going to get whiplash,” he said.

Shake, rattle, and roll...to scale

Last week was spring break here in Madison, WI.  It’s been a while since I took part in the annual mass exodus, but I decided to take advantage of the time off, pack up, and get out of Dodge for a few days.  My destination was chilly, drizzly Seattle for an in-person crash course about the study of earthquakes and their associated dangers in the Puget Sound area.  I also went out to meet some of the people—both professional scientists and volunteer citizens—who are helping this effort along.

Spring Break 2011!  Downtown Seattle as seen from West Seattle across Elliott Bay.

The timing of my trip during the week after Japan’s subduction-related magnitude 9.0 Tohuku earthquake and the resulting tsunami, and not long after a shallow crustal temblor hit Christchurch, NZ, was purely coincidental.  It did, however, provide an engaging, albeit tragic, backdrop for discussing earthquake hazards in the area and individual motivations for contributing to the study of these hazards.  The susceptibility of the Puget Sound to both of these types of earthquakes, as well as to large deep earthquakes such as the 2001 Nisqually quake, and the parallels to these recent events is not lost on many in the region.

Nisqually...10 Years On

Headline from the Seattle Post-Intelligencer, March 1, 2001.

Earthquake occurrences in the past seven days.
(Screen grab from www.usgs.gov)

A quick glance at the USGS' running tally of recent earthquakes in the U.S. (right) reminds us of the country's most tectonically active areas .  It's pretty clear: California and Alaska, followed by several other geographic pockets of notable activity.  For residents of one of those pockets--Seattle and the greater Puget Sound area--the February 28 dateline on the map at right carries special significance.

Today is the 10th anniversary of the magnitude 6.8 Nisqually earthquake, which struck the area at 10:54 a.m. local time on Wednesday, February 28, 2001.  Although it ranks as only the 82nd largest earthquake by magnitude (by my count) in the U.S., it is the third largest on record and the most recent significant quake in Washington state.

The quake originated at a depth of 52 km in the subsurface Juan de Fuca plate, which is subducting under the North American plate in the Cascadia subduction zone.  The epicenter (47.15N 122.72W) was located toward the southern end of Puget Sound, 17.6 km northeast of Olympia, WA and 57.5 km south southwest of Seattle.

An Abbreviated Numerical History of the Great New Madrid Earthquakes

200: Years since a series of massive earthquakes, originating in the subsurface New Madrid fault system of southeastern Missouri and northeastern Arkansas, began in 1811.  The quakes are some of the largest in U.S. history and are the largest ever (recorded) to occur east of the Rocky Mountains.

(Image courtesy of USGS)

4: Number of principal quakes that occurred during the series.  The first major quake occurred at 2:15 am local time on December 16, 1811, followed by the second five hours later.  The third occurred on January 23, 1812 and the fourth on February 7, 1812.  The second quake is sometimes regarded as an aftershock rather than a principal quake, because it was smaller and occurred so soon after the first.  About 200 aftershocks of magnitude 4.0 or greater were also recorded, along with numerous smaller quakes.

7.0: Minimum estimated magnitude (on the Richter scale) of each of the principal quakes according to the United States Geological Service.  Seismographs were not in use at the time in North America, so the magnitudes have been estimated by later researchers based on accounts of the earthquakes.  The USGS has estimated the magnitudes, in chronological order, as 7.7, 7.0, 7.5, and 7.7., although other estimates suggest that several of them were magnitude 8.0 or higher.  The largest earthquake ever recorded in the U.S. was magnitude 9.2, which occurred in Alaska on March 3, 1964.