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Earthquakes: Expert Q&A

  • Posted 01.06.11
  • NOVA

On January 20, 2011, Frank Vernon answered questions about earthquakes, including whether one quake can trigger others, what effect a large tremblor can have on the Earth’s rotation, and where around the world future destructive earthquakes are mostly likely to strike.

Frank Vernon

Frank Vernon

Research Seismologist
University of California at San Diego
Full Bio

Photo credit: Courtesy Frank Vernon

Frank Vernon

Frank Vernon is a Research Seismologist at the Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California at San Diego. His current research interests focus on developing distributed, networked, real-time sensor networks on land and underwater. Vernon is the current Director for the USArray Array Network Facility for the National Science Foundation’s EarthScope program. EarthScope deploys thousands of seismic, GPS, and other geophysical instruments to study the structure and evolution of the North American continent and the processes that trigger earthquakes and volcanic eruptions. Vernon is also Principal Investigator on the ANZA broadband and strong motion seismic network. Since 1982, this network has provided real-time seismic monitoring capability for southernmost California, one of the most active seismic zones in the U.S. As part of this project, he is studying the physics of the earthquake source from local quakes. Vernon earned his B.S. from UC San Diego and his Ph.D. from Scripps.

Q: Dont u think it is possible that by taking these samples & the collection process (therefore disturbing already high risk earthquake land) from deep down in the earth has any baring on the strength effect earthquakes have and/or how soon they occur? Michele Arndorfer, Portland, OR

Actually most significant earthquakes initiate rupture starting around 5 kilometers (3.1 mi) depth or deeper. Almost all of our measuring instrumentation is located on the surface; in a few places we can place instruments about 100 meters (330 ft) underground, which is very near the surface. It is extremely difficult to install instruments in a fault zone due to the geology of the fault itself, so essentially all instruments are placed at some distance from the fault and will not affect fault behavior.

Q: Do earthquakes exceeding a certain very high magnitude commonly speed up the rotation of the earth? And could this effect the rate of global climate change by giving the Earth's surface less time to cool down from Solar radiation? Clay C., Blacksburg, SC

Very high magnitude earthquakes can slightly change the rotation rate of Earth. For example, the 2010 magnitude-8.8 earthquake in Chile sped up the rotation rate of the Earth by 1.26 millionths of a second. But this is a very small number and will not significantly affect the rate of global climate change.

Q: Having lived most of my life in Portland, Oregon, I'm very aware of the Cascadia Subduction Zone. What kinds of studies (if any) are actively being conducted in order to attain greater information about this major fault? Thanks. Larry Zollner Larry Zollner, Tampa, Florida

Many different types of studies are being made on the Cascadia Subduction Zone. Since 2000 there has been the discovery of episodic tremor on the Cascadia Subduction Zone by scientists at the Canadian Pacific Geoscience Centre based on GPS and seismic data ( http://gsc.nrcan.gc.ca/geodyn/ets_e.php ). Following this discovery there has been a lot of additional research by U.S. scientists, led by the University of Washington and the University of Miami.

Currently there are regional (Pacific Northwest Seismic Network, http://www.pnsn.org/ ) and national ( Canadian National Seismograph Network, http://earthquakescanada.nrcan.gc.ca/stndon/CNSN-RNSC/index-eng.php ; Advanced National Seismic System http://earthquake.usgs.gov/monitoring/anss/ ) seismic and GPS networks that monitor the region. In addition, a large number of geological field experiments are being conducted to understand the region’s geology and the earthquake history.

There are two seafloor observatories that have sensors to study Cascadia as well as other aspects of the geological, biological, oceanographic, and atmospheric properties of the region. The first system is the Canadian NEPTUNE system, which started deployment in 2009 ( http://www.neptunecanada.ca/ ). The second system, which is under construction, is the U.S. National Science Foundation MRE Ocean Observatories Initiative Regional Scale Nodes ( http://rsn.apl.washington.edu/ , http://www.oceanleadership.org/programs-and-partnerships/ocean-observing/ooi ).

Finally, under U.S. American Recovery and Reinvestment Act funding, there is an emphasis by the NSF to improve the monitoring capability in the Cascadia region by building and deploying ocean bottom seismometers as well as redeploying EarthScope Transportable Array stations ( http://www.oceanleadership.org/2010/nsf-cascadia-initiative-workshop/ ).

Q: Dear Dr. Vernon, How do scientists determine how much stress has built up on a fault? And to what degree can they use that calculation to predict the next time it will release that stress? Thanks. --Amy W. Amy Wilson, New York, New York

This is a very difficult problem. We cannot make a direct measurement of stress on the fault. What we can do is measure strain along faults using high-precision GPS measurements and/or shallow borehole strainmeters. Using the history of earthquakes on a fault, both from observed earthquakes as well as results from paleoseismology, we can develop models of where future earthquakes may be likely to occur. The timing of the events is still unpredictable.

Q: Can one earthquake trigger another far away? I don’t mean aftershocks of the first earthquake but a whole separate giant earthquake on the same or a different fault? Did we see this in 2010, which seemed to have a lot of major quakes? Patricia McCurdy, Wayne, PA

I will answer these questions in reverse order. There is no known mechanism that can cause one large earthquake to remotely trigger another large earthquake in a different part of the world. If we look at global earthquake statistics, the number and magnitudes of earthquakes in 2010 were not significantly different from other years. The largest magnitude earthquake was the magnitude-8.8 that occurred in Chile. Looking at statistics, we see earthquakes of this size occurring on average about once every 10 years. We will see about one to two magnitude-8.0 earthquakes per year on average.

What was unusual in 2010 was the amount of news coverage about earthquakes. Two factors are at work. The first is that the global communications system has been rapidly improving over the last few decades, making it easier to get information in the form of news and video to the public. The second factor was that while the number of earthquakes around the world was not unusual, the human loss and the devastation caused by the Haitian earthquake was quite unusual. This led to a large amount of coverage by the news media worldwide, which can lead to the perception that there were more earthquakes.

However, there is possible triggering along the same fault or along faults that are in the same region of a major earthquake. There is evidence, in some cases, that once a large earthquake occurs on one section of a fault, within a few years a large earthquake does occur on the adjacent section of the fault. Some examples are in Indonesia, which since the beginning of 2000 has had a series of 22 magnitude-7.0–7.9 events, two magnitude-8.0–8.9 events, and one magnitude-9.5 in 2004. Another example is found in Turkey ( http://en.wikipedia.org/wiki/Triggered_earthquake ). And a third example occurred in southern California with the 1992 magnitude-6.1 Joshua Tree earthquake (April 23), followed by both the magnitude-7.3 Landers earthquake (June 28) and, 3 hours later, the magnitude-6.5 Big Bear Earthquake, and, finally, in 1999 by the magnitude-7.1 Hector Mine earthquake. While these correlations are very intriguing, the physical mechanism is not well understood at this time.

Q: I live in Vancouver Washington I would like to see What the likely hood and how bad the area is for earthquakes? John Sellberg, Vancouver Washington

Some good places to look are:

http://www.ess.washington.edu/SEIS/PNSN/

http://earthquake.usgs.gov/earthquakes/states/washington/hazards.php

http://earthquake.usgs.gov/earthquakes/states/?region=Washington

Q: What are the places in the entire world that are probably going to experience a dangerous earthquake in the next 250 years? christianne, Virginia

The most likely places in the world that may experience a dangerous earthquake are those located along the boundaries of the tectonic plates. A good map of the global hazard can be found at http://www.seismo.ethz.ch/static/GSHAP/ where the red areas are the most likely places to experience a large earthquake and the white areas are the least likely.

Q: Hello Mr Vernon, Is it true that quakes generally produce mostly lateral (horizontal) movement of the ground? If so, is there much variation in vertical movements in different types of quakes or different material densities / locations? Thank you, Ryan Flaherty, Colorado

Many earthquakes are strike-slip earthquakes, which can produce significant horizontal movement of the ground where they rupture, especially in the form of shear waves. But there are other types of earthquakes, either normal or thrust events, which generate significant vertical motion at the earthquake fault. In general, any earthquake will generate both horizontal and vertical movement of the ground. On the surface of the Earth, the vertical and horizontal shaking are influenced by the location and size of the earthquake, the orientation of the fault, the topography of the ground, and the geology of the area.

Q: I learned at school that there is a certain distance that an earthquake can travel. But i dont get it. When there is a spot where it stops, is it like where you are standing in the shaky part and move and all of a sudden there is not shaking? Naomi Chia, Irvine, CA

Unfortunately, Earth does not work that way. The part of the fault that actually breaks or ruptures can be relatively small in area compared to the area where earthquake shaking is felt or measured. Generally the farther you are away from the earthquake, the less shaking you will feel. For example, during the April 4, 2010 Cucapah magnitude-7.2 earthquake near the border of California and Mexico, the earthquake rupture was about 60 kilometers (37.2 mi) long and those of us in San Diego over 120 kilometers (74.4 mi) away felt shaking, although nowhere near as strong as if we were much closer to the earthquake.

Q: is there an earthquake fault close to Salt Lake City or the continental divide? Judy Stroble, Breckenridge, Co.

The Wasatch Fault Zone runs through the Salt Lake City area. This is a significant fault zone, which can potentially generate up to a magnitude-7.0 earthquake. Current understanding of seismic hazard in the Intermountain West does not show a high seismic hazard along the continental divide.

Websites you may be interested in include:

Utah: http://geology.utah.gov/utahgeo/hazards/eqfault/index.htm

Utah: http://earthquake.usgs.gov/earthquakes/states/index.php?regionID=44

Colorado: http://earthquake.usgs.gov/earthquakes/states/index.php?regionID=6

Intermountain West: http://earthquake.usgs.gov/regional/imw/

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