“Cellular Seismology”: Does Past Seismicity Delineate Zones Where Future Large Earthquakes are Likely to Occur?

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

Although earthquake prediction is not (currently?) possible, people of course still want to plan as well as they can for the impact of future large earthquakes. An obvious component of such planning, if it were possible, would be knowing where future large earthquakes are likely to occur.

Plate tectonics provides a very successful model for describing where the vast majority of earthquakes occur, i.e., at or near plate boundaries. The development of the theory of plate tectonics in the 1950s and 60s depended to a large extent on the fact that most earthquakes repeatedly occur within the same zones, zones that we now know are plate boundaries. But one of the fundamental questions that still remains to be answered is: What causes some (occasionally quite large) earthquakes to occur outside of those zones, in locations that are not near plate boundaries?

Figure 1: Global seismicity for two 19-year time periods, 1973-1991 and 1992-2010 (magnitude 5 and greater). For any time period long enough to represent a good statistical sample, the vast majority of earthquakes map out plate boundaries, demonstrating that on a global scale most earthquakes keep occurring in the same places.

These non-plate boundary (NPB) earthquakes are either strictly “intraplate” earthquakes (i.e., those that occur deep in the interiors of plates) or earthquakes that occur in diffuse zones near, but not on, plate boundaries. Although we know that, on a global scale, most earthquakes repeatedly occur in well-defined zones, we don’t know if that is the case for NPB earthquakes. Do NPB earthquakes repeatedly occur in specific zones, or do the zones they occur in “migrate” over time from one place to another such that they might eventually occur essentially anywhere in plate interiors? If NPB earthquakes occur in specific zones that remain stationary over time, then given a long enough record of past seismicity, we should be able to discern where large NPB earthquakes will occur in the future. But if NPB seismicity migrates over time, then past seismicity will not be a useful indicator of where future large NPB earthquakes will occur.

If we are to ever understand the cause of NPB earthquakes, we will have to resolve this question of whether or not the locations where these NPB earthquakes are occurring are persistent. And we will have to determine the characteristics of Earth processes in those zones that are seismically active, as compared to zones that aren’t. My attempt to answer these types of questions has led me to invent the method known as “Cellular Seismology” (CS). Details of my CS research have been published in scientific/technical papers (e.g., Kafka, 2002; Kafka, 2007; and Kafka and Ebel, 2011). What follows here is my explanation of the essence of CS in (what I hope to be) simple, non-technical language.

CS is an intentionally simple method of systematically investigating the relationship between locations of past and future earthquakes in a given region. The name “Celluar Seismology” was chosen because it is analogous to a cellular phone system, with past earthquakes acting analogously to a cell phone tower. The cell tower is associated with a circular zone, extending some radius away from the tower, within which cell phones can receive a signal from the tower. Analogously, we envision that some circular zone surrounding the epicenter of a past earthquake is a zone that presumably has the necessary geophysical characteristics to generate future earthquakes.

CS involves analyzing what seismologists refer to as “earthquake catalogs”, i.e., databases of times, locations and magnitudes of earthquakes in a given region. To implement CS we construct circles of a given radius around each epicenter in an earthquake catalog (which we call the “Pre-CAT”), and investigate the percentage of later-occurring earthquakes (in what we call the “Post-CAT) that were located within that radius of at least one previous earthquake. These Post-CAT earthquakes that occurred near a Pre-CAT earthquake are referred to as “hits.”

We then systematically analyze the observed percentages of hits in an attempt to discern the extent to which patterns emerge in the relationship between locations of past and future earthquakes. We have found what seems to be a stable pattern of at least 2/3 to 3/4 of future earthquakes occurring near past earthquakes in most regions, and we are now investigating how the patterns compare and contrast for different regions.

Figure 2: Hypothetical region, showing how Cellular Seismology works. When a red (Post-CAT) earthquake occurs within a green zone, i.e., a region surrounding a Pre-CAT earthquake, that Post-CAT earthquake is referred to as a “hit.”
Figure 3: Cellular Seismology results for Northeastern United States. This analysis is shown for the case of earthquakes with magnitude of 3 and greater. The circle radius around the Pre-CAT earthquakes is chosen so as to fill 33% of the map area within the blue polygon. For 33% map area, there are 84% hits for this case.
Figure 4: Cellular Seismology results for California. This analysis is shown for the case of earthquakes with magnitude of 4 and greater. The circle radius around the Pre-CAT earthquakes is chosen so as to fill 33% of the map area within the blue polygon. For 33% map area, there are 93% hits for this case.

This CS research has been occurring within the context of a resurgence of interest among some seismologists in earthquake forecasting. It is well accepted among most seismologists that “earthquake prediction” (which refers to predicting the specific time, place and magnitude of an earthquake) is not currently possible, but some level of “earthquake forecasting” (which refers to more long-term estimates of the probability of earthquakes occurring within some region) is considered to be a reasonably attainable goal.

We applied CS to earthquakes in California where there have been recently published earthquake forecasts based on past seismicity. The results are somewhat counter-intuitive. We are finding (so far?) that there does not appear to be anything in the record of past seismicity that is any more predictive of where future earthquakes are likely to occur other than the simple notion that future earthquakes tend to occur near past earthquakes.

The underlying philosophy behind CS is to prefer the simplest (most parsimonious) approach to analyzing any phenomenon of interest. We tried using more complicated approaches to analyzing the relationship between past seismicity and later-occurring earthquakes, and found that more complicated methods showed insufficient gain in predictability to warrant any more complicated approach to this problem than the simple CS method. Thus, we argue that before invoking a complicated solution to predicting locations of future earthquakes, that complicated approach should be checked to see if it performs any better than CS, which we think is a reasonable, least astonishing, hypothesis.

References:

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Magnitude 5.6 Earthquake in Oklahoma on September 3, 2016

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

On September 3, 2016, a magnitude 5.6 earthquake that occurred in Pawnee, Oklahoma, was recorded by Weston Observatory (see seismogram below). This earthquake is tied with another magnitude 5.6 earthquake in Prague, OK (November 6, 2011) as the two largest known earthquakes in Oklahoma.

OK_090316

For more information about the effects of this earthquake, see:

Oklahoma Quake Prompts Shutdown of Gas-Linked Wells (USA Today) 

Check back here for updates on this earthquake.

Additional information about this earthquake can also be found on the U.S. Geological Survey earthquake monitoring web site.

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Magnitude 7.0 Earthquake in Japan on April 15, 2016

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

On a April 15, 2016, a magnitude 7.0 earthquake that occurred on the Kyushu Island of Japan, was recorded by Weston Observatory (see seismogram below). The strong signal at the bottom few hours of the seismogram is the Japan quake. The other strong signals are other earthquakes that also occurred on the same day. The signal at the top is a magnitude 6.4 earthquake in Vanuatu, and the smaller signal about an hour before the Japan quake is a magnitude 6.1 earthquake that occurred in Guatemala.

Japan_041516_NESN_WES

Today’s magnitude 7.0 Japan quake occurred very near a magnitude 6.2 quake that occurred two days earlier. There is, of course, the likelihood of strong aftershocks of today’s main shock, but there is no way of knowing whether or not the occurrence of these two events would lead to any more large earthquakes in this area.

A tsunami warning was initially issued for this earthquake, but the warning was later lifted.

Check back here for updates on this earthquake.

Additional information about this earthquake can be found on the U.S. Geological Survey earthquake monitoring web site.

 

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Students Use BC Library Seismograph to Monitor Earthquakes and Storms, and Test Prototype of Seismographs in Public Places

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

Weston Observatory and the Boston College Lynch School of Education recently installed a seismograph in the BC O’Neill Library (first floor study area). This seismograph display is a prototype of our new version of seismographs operating in public places.

BC_Library_Blog_Fig1
Figure 1: Seismograph display in the Boston College O’Neill Library.

Our first recorded earthquake at this site occurred beneath the Kamchatka Peninsula of Russia . Because the epicentral area is sparsely populated and the earthquake was 100 miles deep, this earthquake is not likely to have caused serious damage or casualties.

 BC_Library_Blog_Fig2
Figure 2: Earthquake recorded by the BC O’Neill Library seismograph.

On January 23-25, we recorded a snowstorm in the Boston area, as well as a magnitude 7.1 earthquake that occurred in Alaska.

 BC_Library_Blog_Fig3
Figure 3: Snowstorm and Alaska earthquake recorded by the BC O’Neill Library seismograph.

 

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Seismic Monitoring of North Korea Nuclear Tests

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

People have been asking why Weston Observatory didn’t record the recent “seismic event” in North Korea.

It was much too small (magnitude 5.1) for us to see it this far away at our New England Seismic Network (or BC-ESP) stations. For us to see a seismic event at that distance, it would probably have to be about a magnitude 6.0 or higher. However, Weston Observatory seismologists also track recordings at seismic stations operated by collaborating observatories that are closer to North Korea.

Here are the seismograms at Weston, MA where it wasn’t recorded and at the IRIS/USGS station at Mudanjiang, China (MDJ) where it was recorded very well. This figure shows the MDJ seismogram and also the “spectrogram” (multi-colored plot, calculated by Dr. Jay Pulli, Visiting Scholar at Weston Observatory).

NKorea_010616_Fig1(Click to enlarge.)
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In this next figure (from IRIS and USGS) you can see the 1/6/2016 event seismogram, superposed on three other seismograms of previous North Korea nuclear tests. The seismograms are so similar that it is hard to distinguish the 2016 event (shown in red). This was one of the first clues that the event was probably a North Korea nuclear test. The biggest difference is just the relative sizes of the nuclear tests.
NKorea_010616_Fig2(Click to enlarge.)
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​Seismologists will be studying these, and other, seismograms of North Korea nuclear tests for forensic analysis of details of the nature of the 1/6/2016 event.
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Spectrograms: Visualizing How the Frequency of Seismic Waves Varies With Time

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences, Boston College

These (very cool, and beautiful, I think) graphics are “spectrograms” of some of our recordings of the magnitude 4.7 earthquake that recently occurred in Oklahoma.

The seismograph in Texas is operated by Kristi Rasmusson Fink, and the seismographs in MA are operated by Weston Observatory. The data processing was done by Jay Pulli.

A spectrogram is a particular way of visualizing the vibrations present in a seismogram. It shows how the frequency of the motion varies over time, and how different frequencies of vibration appear at different times in the record. Yellow colors represent stronger signals, and blue colors represent weaker signals.

spectograms
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More (Fracking Related?) Oklahoma Quaking…

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences, Boston College

Just this past week alone, there have been 32 earthquakes detected in Oklahoma (magnitude 2.0 to 4.7). Our seismograms and “spectrograms” of today’s magnitude 4.7 quake are shown below.

(The seismograph in Texas is operated by Kristi Fink and the seismographs in MA are operated by Weston Observatory. The data processing was done by Jay Pulli.)

113015_Spectrograms

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Students Make Their Own Earthquake!

Nock_Jump_Graphic

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Do I Think That Fracking Causes Earthquakes?

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

Note: My analysis presented here is based on what was known about this issue at the time that I wrote this (June 2014). I don’t think the essence of this story has changed (yet?), but as more earthquakes occur near fracking sites the details of the issue are evolving.

These days, I can hardly go anywhere without being asked: “So, tell me, does fracking cause earthquakes?” Not wanting to get into a complex scientific and political argument on the spot (especially in social situations), I generally try to change the subject.

But, I guess there’s no avoiding it. Being a seismologist, I do have a responsibility to answer. So, here’s my take on it.

Short and simple answer: Yes.
Longer answer: But, it’s complicated…

“Hydraulic fracturing,” also known as “fracking”, is a method of injecting fluid into the ground to fracture rock for extracting natural gas and oil. Since an earthquake is the release of energy due to fracture of rock inside the Earth, we would have to say that the process of fracking definitely creates many small earthquakes. These small earthquakes are generally too small to be felt and are not (so far?) the earthquakes that have been considered the big problem in terms of causing structural damage or injury. However, part of a fracking operation involves the use of high volumes of water to release natural gas from dense rock, and disposing of the associated wastewater involves injecting it into deep rock formations. That wastewater injection can “induce” or “trigger” earthquakes in faults that have been dormant (and might have otherwise remained dormant) for a very long time. These wastewater-injection induced earthquakes are not necessarily so small, and can be damaging. And that is (so far?) where the fracking/earthquake problems and controversies lie.

Here’s my summary of the story:

  • There are well-documented cases of earthquakes clearly associated in time and space with fracking operations. But it’s not really the fracking itself that is the most likely source of damaging earthquakes. It’s the disposal of fluids via injection of highly pressurized wastewater into faults that more likely tends to induce the larger, potentially damaging earthquakes.
  • Most earthquakes associated with fracking operations have been smaller than magnitude 3. But a few larger and damaging earthquakes are suspected of having been induced by the injection of wastewater from natural gas (and also oil production) operations. On the seismogram shown below, you can see the Weston Observatory/BC-ESP recording of a magnitude 5.6 earthquake that occurred in Oklahoma on November 6, 2011 and is suspected of having been induced by injection of wastewater from oil extraction. Although that earthquake has not been directly linked to fracking, the injection of wastewater from oil extraction is essentially the same process as what occurs in fracking operations.
  • There is a well-known theoretical explanation of why injection of highly pressurized wastewater into faults could induce earthquakes. The increased pore pressure along the fault effectively lubricates the fault, causing it to release stress that might have been building up for many years, but might not have slipped without the excess pore pressure.
  • Out of many thousands of fracking operations, and the associated disposal of wastewater, so far only a small percentage of those operations have been clearly identified as being related to induced earthquakes of any significant size. The majority of operations have not, so the probability of a given fracking operation inducing damaging earthquakes is probably quite low.
  • Just how large and damaging a fracking/wastewater injection-related induced earthquake could be remains unknown. Although such earthquakes are generally smaller than magnitude 3, and the largest earthquakes claimed to be fracking-related are less than magnitude 6, even larger future earthquakes can not be ruled out. More research on this topic will be necessary to answer the question of just how big future wastewater injection-related induced earthquakes might ever be.
  • There are many other environmental issues related to fracking, such as heavy truck traffic and contamination of nearby well water used by local communities for drinking water. These are important issues to consider regardless of the question of whether or not fracking operations induce earthquakes.

Bottom line for me: These kinds of things are complicated. I think a more relevant question than “Does fracking cause earthquakes?” is: Given that wastewater injection procedures associated with fracking operations can induce earthquakes big enough to cause damage in some (small?) percentage of cases (and that there are other environmental hazards associated with fracking), what should we do about it? Fracking provides new sources of natural gas that could enhance our ability to generate electricity, heat homes, and provide fuel for transportation. Given the uncertainties, how do we find the right balance to make informed decisions about the extent to which the risks associated with fracking, or with any other process for finding new sources of energy, are worth the benefits?

This is the challenge for all of us as citizens of planet Earth.

oklahoma_fracking_quake

Weston Observatory/BC-ESP seismogram of magnitude 5.6 earthquake that occurred in Oklahoma on November 6, 2011. This earthquake is likely related to injection of highly pressurized wastewater (from oil extraction operations) into wells. It was big enough to cause injuries and damage more than a dozen homes. (Click on seismogram to enlarge.)

Further Reading:

For an excellent, more complete, analysis of the situation, see the USGS Science Feature article Man-Made Earthquakes Update, and follow the links in it.

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Clear and Simple Illustration of Mechanism to Explain Earthquakes in the Central and Eastern United States

This is a clear and simple illustration (adapted from the South Carolina Earthquake Education and Preparedness Program) of the commonly accepted “ancient zones of weakness” model for the cause of earthquakes in the Central and Eastern United States (and other “intraplate” regions far from plate boundaries). According to this model, preexisting faults and/or other geological features formed during ancient geological episodes persist in the intraplate crust, and, by way of analogy with plate boundary seismicity, earthquakes occur when the present-day stress is released along these zones of weakness.

IP_Quakes_Logs_AdaptedAlso see:

Why Does the Earth Quake in New England? and South Carolina Earthquake Education and Preparedness Program

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