Last Year’s Virginia Earthquake May Have Been Felt by More People Than Any Other Earthquake in U.S. History!

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

Most people living in the eastern U.S. probably think of places like California and Japan when they hear the word “earthquake.” But last year’s magnitude 5.8 earthquake in Virginia was a stark reminder that large and damaging earthquakes do occur in the eastern United States. And when eastern U.S. earthquakes occur, their effects can be quite dramatic compared to earthquakes of the same magnitude in the western United States. Here’s why:

Because of the difference in the structure of the Earth’s crust beneath the eastern U.S. versus the western U.S., seismic waves propagate more efficiently in the East than in the West. This difference is due to the fact that the crust beneath the East is older, warmer, and more solid than the crust underlying the West. The result of this difference is that when an earthquake of a given magnitude occurs in the eastern United States it is felt (and causes damage) over a much wider area than if that same size earthquake were to occur in the western United States.

This can be dramatically seen in the figure below, which shows areas where the Virginia earthquake was reported as being felt, as compared to areas where a magnitude 6 earthquake in California was reported as felt. The Virginia earthquake, although similar in size to the California quake (actually a bit smaller), was felt over an area about six times as large as that of the California earthquake.

This difference in the efficiency of seismic wave propagation, coupled with the fact that the region surrounding the Virginia quake is so densely populated, results in a surprising conclusion:

It is quite likely that the Virginia earthquake was felt by more people than any other earthquake in the history of the United States!

So even though earthquakes are obviously more frequent in California than they are in the East (due to California being on the plate boundary between the North American and Pacific plates), earthquake shaking and damage are nonetheless very real phenomena in the eastern United States.

If you felt this earthquake, please tell us your story by submitting a comment here.

References and Additional Reading:

For additional information about the Virginia earthquake, and more about the differences between eastern versus western U.S. earthquakes, see:

Learning from the 2011 Virginia Earthquake, Congressional briefing testimony of J.W. Horton, Congressional Hazards Caucus, March 29, 2012, www.hazardscaucus.org/briefings/horton-eqeast0312.pdf.

Mineral Virginia Earthquake Illustrates Seismicity of a Passive-Aggressive Margin, E. Wolin, S. Stein, F. Pazzaglia, A. Meltzer, A.L. Kafka, and C. Berti (2012), Geophysical Research Letters, 39(L02305), doi:10.1029/2011GL050310, www2.bc.edu/alan-kafka/ Virginia_082311/Virginia_GRL.pdf.

The Enigma of Why a Magnitude 5.8 Earthquake Occurred in Virginia, A.L. Kafka, akafka.wordpress.com/2011/12/27/the-enigma-of-why-a-magnitude-5-8-earthquake-occurred-in-virginia.

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An Amazing 40-Year-Long Seismogram: A Whole New Way of Seeing Our Planet Quake

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

This seismogram, produced by the Albuquerque Seismological Laboratory (one of the all-time best seismic stations on Earth), shows a 40-year record of seismic recording!

(Higher-quality versions of this seismogram can be found here and here.)

This is a view of the Earth quaking that has never been seen before. It’s that mind-boggling!

Also shown below for comparison is a more typical 24-hour seismogram, with each line representing an hour. In the 40-year seismogram, instead of each line representing an hour, each line represents a month. The vertical axis shows the year, the horizontal axis shows the day of the month, and the labels on the right side of the vertical axis identify significant, globally recorded earthquakes.

Three relatively small earthquakes can be seen in the 24-hour seismogram (recorded in Massachusetts at Weston Observatory on August 23, 2011):

Virginia, Magnitude 5.8
Colorado, Magnitude 5.3
Colorado, Magnitude 4.6

Three mega-quakes, the largest earthquakes recorded since 1972, can be seen on the 40-year seismogram:

Sumatra 2004, Magnitude 9.1
Japan 2011, Magnitude 9.0
Chile 2010, Magnitude 8.8

Also seen on the 40-year seismogram are the many smaller earthquakes that happen every month. The smallest earthquakes that can be seen on this seismogram are about magnitude 6 (i.e., about as big as the Virginia earthquake shown on the 24-hour seismogram).

Note: The title of this blog has been updated from “BC-ESP Discussion Forum” to “Musings in the Quake Zoneto reflect what it has evolved into.

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What is Earthquake Magnitude?

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

Earthquake magnitude is a concept that can be quite confusing. Although it is well-recognized as a measure of the size of an earthquake, the principles underlying earthquake magnitude are not as simple as one might expect. Here is a short, and hopefully not too technical, explanation of the essential characteristics of earthquake magnitude:

Magnitude is a measure of the size of an earthquake at the location where it occurred. This is not the same thing as the amplitudes of the seismic waves where they are recorded, because (as can be seen in the figures below) the amplitudes of those waves decay with distance as they travel from the epicenter to the station. To estimate the magnitude of the earthquake from seismograms recorded at stations around the world, it is necessary to adjust the amplitude you observed at your station for the amount that the waves decayed as they traveled from the earthquake to your station.

A magnitude scale is a scheme for making these amplitude adjustments to determine a number that represents the size of the earthquake. Since exactly how to make these adjustments is a science in itself, there are many formulas that have been developed by seismologists to make the necessary adjustments. The original magnitude formula was developed by Charles Richter (in 1935), which is why the magnitude of an earthquake is often loosely referred to as the size of the earthquake on the “Richter Scale.”

Since the development of the Richter Scale, there have been many other magnitude formulas developed by other seismologists. This can lead to quite a bit of confusion, but all of these formulas should give roughly the same result. To minimize this confusion, the U.S. Geological Survey (USGS) publishes an “official” magnitude for each earthquake they report. This official magnitude is the USGS’s estimate of the magnitude that is most appropriate to release to the public given all of the complications discussed above. These official magnitudes reported by the USGS are often the values that seismologists are referring to when they discuss a given earthquake publicly. But sometimes a seismologist’s reported magnitude of an earthquake might be that of a seismic observatory or research institution other than the USGS. This sometimes happens when an earthquake occurs in an area where seismologists operate seismographs close to the epicenter, and thus they feel they have a more accurate estimate of the magnitude of that particular earthquake than the magnitude reported by the USGS.

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The Enigma of Why a Magnitude 5.8 Earthquake Occurred in Virginia

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

Holiday parties are a time for me to connect with old friends, meet new friends, and get asked a lot of questions about why the Earth quakes along the East Coast.

This year, fellow party-goers here in New England have been telling me that they felt the earthquake that occurred in Virginia on August 23 – and they want to know why a magnitude 5.8 earthquake occurred there. My inevitably disappointing answer: “Nobody really knows.”

They finally have the attention of a seismologist and get to ask the big question, and all they get is “Nobody really knows”? You call that an answer? Have they been cheated out of a great scientific story? I don’t think so. In fact, I would argue that it’s the mysteries of science that are the great and engaging stories. The stories about what we know are much less exciting to me than the explorations of what we don’t know. This is the realm of wonder, awe, suspense, excitement, and fun that makes science a great endeavor.

Now, I should clarify: It’s not that we don’t know anything about the cause of Eastern US earthquakes.

California, and the West Coast in general, is the place in the US that many people think of as “earthquake country.” Most California earthquakes occur near a major boundary separating two of the Earth’s tectonic “plates”, the North American plate and the Pacific plate. Nonetheless, earthquakes are well known to occur in plate interiors as well, and those earthquakes are called intraplate earthquakes. Intraplate earthquakes are common occurrences, globally as well as specifically in the Eastern United States.

A commonly espoused explanation for the cause of intraplate earthquakes is that “ancient zones of weakness” are being reactivated by forces currently active in plate interiors. In 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. While this concept of reactivation of old zones of weakness is commonly assumed to be valid, in reality the identification of individual active geologic features has proven to be quite difficult. Unlike the situation for many plate boundary earthquakes, it is not at all clear whether faults mapped at the Earth’s surface in the Eastern United States are the same faults along which the earthquakes are occurring.

The magnitude 5.8 Virginia earthquake occurred in the “Central Virginia Seismic Zone”, a zone of seismicity where earthquakes have occurred before. But the specific cause of earthquakes in this seismic zone, including that of the recent magnitude 5.8, remains an enigma.

In 1875, an earthquake in this zone shook bricks from chimneys, broke plaster and windows, and overturned furniture. Based on felt effects and damage from the 1875 earthquake, it has been estimated to have had a magnitude of about 4.8. On December 9, 2003, a magnitude 4.5 earthquake occurred in this region causing minor damage, and over the years other small earthquakes have also occurred in the Central Virginia Seismic Zone.

So it is not surprising to seismologists that an earthquake would occur in Virginia, but why it occurred at that particular location, at that particular time, and why it was a magnitude 5.8 remains a mystery. And this is the kind of mystery that drew me to a life obsessed with the enigma of why earthquakes occur in the Eastern United States, deep in the interior of the North American plate.

>>>>

Click here for a paper summarizing the Virginia earthquake: Mineral, Virginia, Earthquake Illustrates Seismicity of a Passive-Aggressive Margin, by Emily Wolin, Seth Stein, Frank Pazzaglia, Anne Meltzer, and Alan Kafka, Geophysical Research Letters, 39(L02305), doi: 10.1029/2011GL050310.

Click here to read more about earthquakes in New England: Why Does the Earth Quake in New England?, by Alan Kafka.

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Podcast About the Japan Mega-Quake

Click here to listen to Boston Museum of Science podcast of Alan Kafka’s description of the March 2011 Japan mega-quake and earthquakes in general. Click on image above to enlarge.

(On the Boston Museum of Science website, click on “Open audio”, then click on the forward arrow.)

Correction: The ocean floor moved upward by about 10 ft, not “a couple of feet” (as it says in the interview).

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Financial Costs of Earthquakes and Wars

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

Today I read that the cost of the 2011 Japan megaquake might be as much as $305 billion dollars. I don’t usually think in terms of billions of dollars (my salary is somewhat lower than that), and I wanted to get some sense of what that number means in the context of things that cost many billions of dollars.

I compared the potential $305B cost of the Japan earthquake to some other earthquakes, Hurricane Katrina, the Gulf Oil Spill, the wars in Iraq and Afghanistan, and some potential scenario earthquakes that might occur in the future. The comparison with wars is not intended to be a political statement, but I think it does give a good visual picture of the context of what earthquakes and other disasters cost (see figure). (Click on figure for an enlarged version.)

The numbers plotted in the figure show that the 2011 Japan megaquake would (if these estimates are right) be the most costly earthquake by quite a large margin. Big earthquakes that might occur in the future near US cities are shown by the scenario earthquakes, plotted in green. The costs of those potential earthquakes could be comparable to that of the Japan megaquake. The cost of the Gulf Oil Spill is about comparable to that of the Chile earthquake, and the cost of Hurricane Katrina is comparable to that of the Kobe, Japan earthquake of 1995.

Note: These numbers are just order of magnitude estimates, and were not very carefully sourced for this blog. There are probably better numbers available in detailed studies. The idea here was just to create a quick and simple visual picture of the order of magnitude differences in the costs of these events. The websites and references that I used for the numbers shown here are given in this Excel spreadsheet.

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Why Didn’t the Foreshocks Predict the Great Japan Earthquake?

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

In the aftermath of the devastating magnitude 9.0 earthquake in Japan, many people are asking whether we “should have known” that the smaller earthquakes that preceded it were warnings of worse to come. But while it is clear in retrospect that there were foreshocks during the few days preceding the main shock, including a magnitude 7.3 foreshock and two other magnitude 6.0 foreshocks, before the fact there was no way to know that they were indeed foreshocks.

Let me explain.

Some large earthquakes have foreshocks, and some don’t. How to (in advance) distinguish a foreshock from any other earthquake remains an enigma. There are, for example, more than 100 magnitude 6 to 7 earthquakes occurring each year, the vast majority of which are not foreshocks of huge earthquakes like the Japan quake. It would, therefore, not be practical to issue an earthquake prediction and call for evacuation every few days when these inevitable magnitude 6 to 7 earthquakes occur, on the slight chance that they might be foreshocks of a huge quake.

So unfortunately, in spite of the now clearly identified foreshocks of the Japan earthquake, we still had no way of predicting the magnitude 9.0 earthquake. It is, however, very clear that a huge earthquake was inevitable along the coast of Japan, even though nobody knew when that earthquake was going to occur. It might not have occurred for 10 years, 100 years, 1,000 years, or even 10,000 years. It is, of course, wise to prepare for the inevitability of major earthquakes in seismically active regions like Japan, but that doesn’t mean that we are even close to being able to predict when those earthquakes will occur. The sober reality is that there is no reliable, scientifically proven method of predicting where and when the next big earthquake will occur.

As Yogi Berra said, “It’s tough to make predictions, especially about the future.”

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Is a Large Earthquake Likely to Occur at Indian Point Nuclear Power Plant?

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

In the aftermath of the March 2011 earthquake and tsunami in Japan, there is heightened interest (and fear) regarding the seismic safety of nuclear power plants in the United States. Of particular interest is the Indian Point nuclear power plant in Westchester County just north of New York City, and the question of whether a large earthquake is likely to occur there. Seismologists are expected to provide definitive answers to such questions, but unfortunately life — and Earth processes — are not so simple.

A recent report from msnbc.com described how, based on their analysis of the Nuclear Regulatory Commission’s assessment of nuclear power plants in the US, one of Indian Point’s reactors is No. 1 for risk of damage from an earthquake. Furthermore, a number of other eastern US plants are also higher on the list than nuclear power plants in California. The reason for this surprising result is that California power plants were designed to withstand larger earthquakes than these eastern US plants.

It is important not to minimize the real risk of earthquakes hitting nuclear power plants in the eastern US. It is certainly true that there is a real earthquake hazard in the eastern US, albeit significantly less than the hazard in California. The challenge is that unlike the California situation — where relatively well-understood plate tectonic processes dominate the hazard and thus large earthquakes are more likely to occur in places we expect — it is not clear where to expect the next large eastern earthquakes. In spite of advances in our research, there still remains great uncertainty about the fundamental question of where future large earthquakes are (and are not) likely to occur in the eastern US. Any attempt to discern whether or not any particular location is more prone to future large earthquakes than any other location is still fraught with uncertainty. We have a long way to go before we will be able to say (if ever) that a given location on a map of the eastern US is really more prone to future large earthquakes than any other location.

A 2008 study by seismologists at Columbia University’s Lamont-Doherty Earth Observatory argued that Indian Point is located at the intersection of two active seismic zones, and is thus one of the least favorable sites in the New York City area from an earthquake hazard and risk perspective. My own assessment of the situation is that the distribution of epicenters lends itself to many possible conjectures of hypothetical fault zones, all of which are based on circumstantial evidence. None of these hypotheses can be considered as concrete evidence that the Indian Point site is necessarily any more prone to future large earthquakes than many other sites in the New York City area.

It would of course be comforting if seismology could tell us which of these two assessments is right. In my many years of experience of being asked to respond to these types of questions, I am often struck by how uncomfortable people generally are with uncertainty. Many people are so desperate for certainty in an uncertain world that they seem to be more comforted being told “Yes, a tragic earthquake is certain to occur in your town next week” than to be told “We just don’t know.” But, the truth is that we just don’t (yet?) have definitive answers to the question of what the level of risk is to nuclear power plants in the eastern US.

California does have a higher earthquake hazard, but eastern US power plants have not been built to withstand earthquakes as well as those in California. Hopefully, over time seismologists will be able to reduce the uncertainty and more accurately assess the balance between these two effects. But for now, this will remain a matter of deciding the level of uncertainty that we are willing to accept in order to provide us with energy for our chosen lifestyle.  The tragic earthquake and tsunami in Japan provide a stark reminder that seismic risk assessment is both a science and an art.

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Calling the Japan Earthquake “Divine Retribution” is Bad for Science and Bad for Theology

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

After the magnitude 9.0 earthquake in Japan, Tokyo governor Shintaro Ishihara commented that the earthquake was “divine retribution” for Japanese egoism. If this kind of response to a natural disaster was just an isolated incident, there would be no reason for me to be writing about it. Unfortunately, however, I am quite often told some variation of this divine retribution story after a tragic earthquake has occurred. My “favorite” response came after I gave a lecture on the great Sumatra earthquake and tsunami of 2004. According to one woman in the audience, since the mainshock occurred the day after Christmas and the largest aftershock occurred the day after Easter, it’s obvious that the reason for this tragedy is that God is punishing us for our sins!

I find this kind of thinking to be very disturbing for two reasons: It diverts attention from what science actually can do to help mitigate the tragic effects of earthquakes, and it is also bad theology.

I would hardly claim to be an authority on matters of the theological realm (for discussion of those types of matters, see Rabbi Kafka’s blog), but this one seems theologically simple enough that even I can see the logical flaw: What would be the point of any deity (or natural/spiritual force of the universe) killing thousands of innocent people to punish “us” for our sins or to teach people a lesson about egoism? It is hard for me to imagine a worse view of life than to think that innocent people in Japan were killed to teach them (or “us”) the right and moral way to live. I sure hope that is not the way our world works!

There is plenty of thoughtful and fascinating philosophy and theology written on the question of why innocent people suffer from tragedies that have no apparent meaning. As seismologists involved in the study of events that are sometimes very tragic, I think it is valuable for us to ponder such imponderable questions, if only to sensitize us to the tragedy a world away from the fascinating and scientifically interesting seismograms we record. But, simplistic answers to such deep questions don’t help anybody.

I guess I should be pleased that Shintaro Ishihara publicly apologized for his outrageous comment, but unfortunately I don’t find his apology to be very comforting. I find that this disturbing kind of naïve understanding of the relationship between science and spirit is all too alive and well in our culture.

I think it is better for us to respond to natural disasters by devoting some energy to increasing our understanding of the causes of earthquakes, improving seismic hazard mapping, building more seismically resistant buildings, and developing better emergency management plans, than to blame the tragedy on some bizarre theological cause. Using science to investigate the causes of earthquakes and to help mitigate their effects is (regardless of any particular theology) positive action that leads to making people safer from the devastating effects of earthquakes.

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Reflections on the Powerful Earthquake in Japan

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

The tragic earthquake that occurred in Japan on March 11, 2011, once again reminds me of the importance of being sensitive in our roles as seismologists regarding the human tragedy caused by powerful earthquakes. It is always a challenge for seismologists to find the correct balance among conveying scientific information about “interesting” earthquakes, communicating our amazement at the incredible power of natural forces in the Earth, and remembering to be sensitive about the human tragedy caused by large earthquakes. But the scientific study of earthquakes is not just fascinating, it’s also practical. Through scientific research on earthquakes we can take positive action that leads to making people safer from the devastating effects of earthquakes. We can mitigate some of the consequences of earthquakes through increasing our understanding of the causes of earthquakes, improving seismic hazard mapping, building more seismically resistant buildings, and developing better emergency management plans to help the victims when tragic earthquakes do strike.

It is important in our roles as seismologists and educators not to forget that the phenomenon we study, and find to be so fascinating, has such tragic consequences for people. But that fascination with the incredible power of earthquakes also inspires people to study them and to unravel their mysteries so that we can take positive action to minimize their tragic consequences.

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