Nav: Home

Slow Earthquakes Seen As Complex As Regular Earthquakes

September 05, 1996

In December 1992, two borehole strainmeter devices located close to the San Andreas Fault picked up a series of deformation signals that the instruments, deployed in 1984, had never before detected. The signals, which lasted for about a week, were from a "slow" earthquake. Slow earthquakes release their strain energy much more slowly than do regular earthquakes and are usually more difficult to detect. Accompanying the slow quake was a series of small regular earthquakes.

The analysis of the San Andreas event, reported in this week's issue of Nature, has yielded a surprisingly complex picture of the slow process; in the few cases previously reported, the slow rupture appeared to be rather simple. Carnegie's Alan Linde and colleagues* report that the slow 1992 quake had a total displacement across its rupture surface of only a few centimeters but was slower by two orders of magnitudes than any previously detected slow event. Its complexity, they write, was comparable to that of a regular earthquake.

The borehole strainmeters which picked up the slow earthquake signals are located near the transition between locked and stably sliding segments of the San Andreas Fault in central California. (In stably sliding faults, slip usually occurs through creep and small earthquakes. Locked faults often produce large ruptures; the 1906 San Francisco earthquake occurred in the locked partion of the San Andreas to the north of the study area.) The slow strain changes extended over 30 square km (about 11 square miles) of fault surface area with a total strain rupture corresponding to a magnitude 4.8 earthquake. They included a series of slow events with several episodes of varying slip time. Depth ranged from 0.1 km to at least 4 km, and perhaps as much as 8 km. The small regular earthquakes accompanying the slow earthquake sequence were each no larger than magnitude 3.7.

Together with previous slow earthquake analyses, this new work suggests that a relation exists between the amount of slow redistribution of stress and the size of associated earthquakes. For example, a slow earthquake preceding the great 9.5-magnitude Chilean earthquake of 1960 was very large in extent and had a slip of several meters. A 1978 slow event in Japan had a slip of about one meter and was followed by earthquakes as large as magnitude 5.8. The 1992 California event, displacing only a few centimeters, was accompanied by very small earthquakes.

What this means for understanding the earthquake process is not clear. Several scientists, among them Linde and his colleagues at Carnegie's Department of Terrestrial Magnetism in Washington, D.C., have previously suggested that slow earthquakes are an integral part of the seismic faulting process. Such events, they say, may be important in earthquake nucleation, i.e., they may act as triggers in initiating the process leading to normal earthquakes. Or, they may account for the excess of plate convergence over seismic slip in subduction zones. (The rate at which two plates converge is often greater than the detected rate of seismic slip; therefore, some other mechanism, such as a slow earthquake, must account for the difference.) This particular slow earthquake, located where locked and stably sliding segments of the San Andreas join, may have some bearing on the nature of such fault transitions.

However, it is not easy to detect slow earthquakes. Because rupture takes place slowly, seismic waves are not generated. Thus, the usual techniques for detecting earthquakes (with seismometers) are not available. Additionally, the total surface displacements of slow quakes are often too small to be seen by conventional distance measuring techniques, such as the satellite- based Global Positioning System. As well, the signal amplitudes of slow earthquakes decrease rapidly with increasing distance from the source. With only a few arrays of instruments like borehole strainmeters sensitive enough to detect them, progress in determining their role in the seismogenic process is likely to be slow.

Research in seismology has been a part of the Carnegie Institution's Department of Terrestrial Magnetism for decades. DTM scientists, for example, helped develop the Sacks-Evertson borehole strainmeter, in which a hydraulic amplification system permits detection of extremely small changes in rock strain. A Sacks-Evertson strainmeter was one of the two instruments used in this study. (The other was a Gladwin tensor strainmeter.) Both instruments are cemented under compression into subsurface rock, typically 200 km deep.

DTM, led by Sean Solomon, is one of five research arms of the Carnegie Institution of Washington, a nonprofit scientific and educational organization founded by Andrew Carnegie in 1902. The president of the Institution is the biologist Maxine Singer.

*The authors of the Nature paper are Alan T. Linde of DTM, Michael T. Gladwin and Ross L. Gwyther, both of the Commonwealth Scientific and Industrial Research Organisation, Queensland Center for Advanced Technologies, Australia; Malcolm J. S. Johnston, USGS, Menlo Park, CA; and Roger G. Bilham, University of Colorado, Boulder.

Carnegie Institution for Science

Related Earthquake Articles:

From where will the next big earthquake hit the city of Istanbul?
Scientists reckon with an earthquake with a magnitude of 7 or greater in this region in the coming years.
Dissection of the 2015 Bonin deep earthquake
Researchers at Tohoku University's Department of Geophysics, have been studying the deep earthquake which occurred on May 30, 2015, to the west of Japan's Bonin Islands.
The search for the earthquake nucleus
Where a tectonic plate dives under another, in the so-called subduction zones at ocean margins, many strong earthquakes occur.
Better understanding post-earthquake fault movement
Preparation and good timing enabled Gareth Funning and a team of researchers to collect a unique data set following the 2014 South Napa earthquake that showed different parts of the fault, sometimes only a few kilometers apart, moved at different speeds and at different times.
The maximum earthquake magnitude for North Turkey
The Istanbul metropolitan region faces a high probability for a large earthquake in the near future.
Double dose of bad earthquake news
A team of researchers, including one from the University of California, Riverside, has discovered that earthquake ruptures can jump much further than previously thought, a finding that could have severe implications on the Los Angeles area and other regions in the world.
Discovery of hidden earthquake presents challenge to earthquake early-warning systems
Seismologists at the University of Liverpool studying the 2011 Chile earthquake have discovered a previously undetected earthquake which took place seconds after the initial rupture.
Babe Ruth and earthquake hazard maps
Northwestern University researchers have turned to an unusual source -- Major League Baseball -- to help learn why maps used to predict shaking in future earthquakes often do poorly.
Earthquake rupture halted by seamounts
Experts expected for some time that one of the next mega earthquakes occurs off northern Chile.
Catastrophic landslides post-earthquake
In the last few months, it has once more become clear that large earthquakes can solicit catastrophic landsliding.

Related Earthquake Reading:

by Seymour Simon (Author)

Seymour Simon knows how to explain science to kids and make it fun. He was a teacher for over twenty years, has written more than 250 books, and has won multiple awards. In Earthquakes, Simon introduces elementary-school readers to earthquakes through engaging descriptions and stunning full-color photographs. He teaches readers why and how earthquakes happen and the damage they can cause through pictures, diagrams, and maps. He also gives real life examples of earthquakes that have occurred all over the world. This book includes a glossary and index.

Supports the Common Core... View Details

Earthquakes (Let's-Read-and-Find-Out Science 2)
by Dr. Franklyn M. Branley (Author), Megan Lloyd (Illustrator)

Read and find out about one of nature’s most mysterious forces—the earthquake. Some earthquakes are so small that you don’t even feel them, while others can make even big buildings shake! Learn why earthquakes happen, where they are most likely to occur, and what to do if one happens near you.

Now rebranded with a new cover look and with updated text and art, this classic picture book describes the causes and effects of earthquakes (including a tsunami). This book features rich vocabulary and fascinating cross-sections of mountains, volcanoes, and faults in the earth’s moving... View Details

Earthquakes (A True Book)
by Ker Than (Author)

A study of earthquakes and the science behind them. View Details

The Great Quake: How the Biggest Earthquake in North America Changed Our Understanding of the Planet
by Henry Fountain (Author)

New York Times Book Review Editors' Choice

In the bestselling tradition of Erik Larson’s Isaac’s Storm, The Great Quake is a riveting narrative about the biggest earthquake in North American recorded history -- the 1964 Alaska earthquake that demolished the city of Valdez and swept away the island village of Chenega -- and the geologist who hunted for clues to explain how and why it took place.

At 5:36 p.m. on March 27, 1964, a magnitude 9.2. earthquake – the second most powerful in world history – struck the young state of Alaska. The... View Details

Earthquake in the Early Morning (Magic Tree House #24) (Magic Tree House (R))
by Mary Pope Osborne (Author), Sal Murdocca (Illustrator)

The #1 bestselling chapter book series of all time celebrates 25 years with new covers and a new, easy-to-use numbering system!

An adventure that will shake you up! That's what Jack and Annie get when the Magic Tree House whisks them back to California in 1906. As soon as they arrive, the famous San Francisco earthquake hits the city. Can Jack and Annie save the day? Or will San Francisco be destroyed first?

Did you know that there’s a Magic Tree House book for every kid?

Magic Tree House: Adventures with Jack and Annie, perfect for readers who are just beginning... View Details

Quakeland: On the Road to America's Next Devastating Earthquake
by Kathryn Miles (Author)

A journey around the United States in search of the truth about the threat of earthquakes leads to spine-tingling discoveries, unnerving experts, and ultimately the kind of preparations that will actually help guide us through disasters. It’s a road trip full of surprises.
Earthquakes. You need to worry about them only if you’re in San Francisco, right? Wrong. We have been making enormous changes to subterranean America, and Mother Earth, as always, has been making some of her own. . . . The consequences for our real estate, our civil engineering, and our... View Details

Earthquake Terror (Puffin Novel)
by Peg Kehret (Author)

When Jonathan and his family go camping on Magpie Island, they look forward to a fun, relaxing weekend. But their fun quickly vanishes when Jonathan, his sister, Abby, and their dog, Moose, find themselves in the middle of a natural disaster. A devastating earthquake has hit, destroying their camper, knocking out the only bridge to the mainland, and leaving Jonathan, Abby, and their dog with no food, water, or shelter. Alone in the woods, can Jonathan manage to keep calm and save Abby and Moose—and stay alive himself? View Details

Earthquake: San Francisco, 1906 (Survivors)
by Kathleen Duey (Author), Karen A. Bale (Author)

Can two kids survive the biggest disaster of 1906? Find out in this gripping tale of historical fiction, part of the Survivors series.

Brendan O’Connor is delivering pastries to the bustling businesses and elegant hotels of San Francisco, dreaming that someday he will be part of that life.

Li Dai Yue is running from the isolated security of Chinatown, distraught over the marriage her uncle has arranged for her.

Chance throws them together on the day of the San Francisco earthquake. Can two strangers from such different worlds work together to survive the terror of... View Details

Easter Earthquake: How Resurrection Shakes Our World
by James A. Harnish (Author)

Have you ever been in an earthquake? Earthquakes shake our most basic assumptions: that the ground will remain steady beneath our feet, that the world's current existence is the way it will always be. But when tectonic plates shift under the earth's surface and the ground shakes beneath our feet, it rattles the bedrock assumptions on which we build our lives.

The Gospel of Matthew reports that on the first Easter morning, an earthquake rocked the earth, ripped open the tomb, and scared the Roman guards at the tomb, who "shook with fear and became like dead men" (Matthew 28:4).

This is... View Details

Earthquakes! (TIME FOR KIDS® Nonfiction Readers)
by Teacher Created Materials;Cy Armour (Author)

Early elementary readers find out what causes earthquakes and what to do to stay safe if one occurs in this helpful nonfiction reader. Featuring informational text, colorful maps, diagrams, and vibrant photos, this book keeps children engaged and fascinated at the same time!

About Shell Education
Rachelle Cracchiolo started the company with a friend and fellow teacher. Both were eager to share their ideas and passion for education with other classroom leaders. What began as a hobby, selling lesson plans to local stores, became a part-time job after a full day of teaching,... View Details

Best Science Podcasts 2018

We have hand picked the best science podcasts for 2018. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Confronting Stigma
Why do we harshly judge certain behaviors or conditions, making it harder to talk honestly about them? This hour, TED speakers confront stigmas around addiction, depression, HIV and sex work. Guests include journalist Johann Hari, TV/film producer and mental health advocate Nikki Webber Allen, HIV awareness educator Arik Hartmann, and sex worker and activist Juno Mac.
Now Playing: Science for the People

#461 Adhesives
This week we're discussing glue from two very different times. We speak with Dr. Jianyu Li about his research into a new type of medical adhesive. And Dr. Geeske Langejans explains her work making and investigating Stone Age and Paleolithic glues.