Dillsburg, PA, re-visited

Here's a post I started about a month ago, but got deterred by North Korea:

What's happening under Carroll Township?

By TERESA ANN BOECKEL
Daily Record/Sunday News

Updated: 05/30/2009 11:04:38 PM EDT Figure caption: (Daily Record/Sunday News -- Carrie Hamilton and Teresa Ann Boeckel)

Charles Scharnberger, retired earth sciences professor with Millersville University [and stalwart member of the LDCSN]:

Plate tectonics, or movements in the Earth's crust, plays a role in the big picture, he said. The North American Plate, which stretches from the middle of the Atlantic Ocean to California, is moving westward. The immediate picture is that rock is fracturing under the ground, and it's happening at a fairly shallow depth. The timing of the quakes and why they're happening in Dillsburg is what geologists don't understand. Many other places, besides Dillsburg, have strong contrasting bodies of rock -- igneous rock intruding into shale -- where stress is concentrated. It could be an ancient fault being reactivated, or it could be a new fault forming, he said.

Wayne Pennington, chairman of the Geological and Mining Engineering and Sciences Department at Michigan Technological University:
Earthquakes in general are caused by stress concentrations within the earth. The rocks break and slide. Faults are not big openings in the earth; they're like cracks in a sidewalk, he said. Rocks on one side won't line up with those on the other. Parts of the earth's crust are shifting, and stress can build up. It can be released in weaker spots. If a fault system is identified, the size of the fault will give experts a good idea of its maximum potential. It takes bigger faults to produce large earthquakes. The San Andreas fault, for example, is hundreds of miles long. If one exists in the Dillsburg area, it's probably not more than a few kilometers long, he said. A kilometer equals 0.62 miles. "It's, in a sense, the earth resettling," he said. Induced seismic activity, such as mines collapsing, is another possibility. It would result from the readjusting of rock near the stress void. The depths of the earthquakes would determine whether the mines are involved. If the mines are collapsing, the seismic activity that the area has been experiencing probably will not get any bigger.

George H. Myer, a professor of earth and environmental science at Temple University:
Earthquakes differ in California and the East Coast. In California, it's a side-to-side motion, known as a strike-slip fault. In Pennsylvania, they're known as normal or vertical faults. Normal faults push part of the rock downward, and vertical faults push the rock upward, he said. Geological maps show countless faults in the Dillsburg area, and it's possible that a reverse fault is at play. That's because of the booms residents are hearing. The primary waves refract off the surface and into the air, creating a vibrating noise that people can hear.

San Andreas fault - amazing image

UAVSAR (Uninhabited Aerial Vehicle Synthetic Aperture Radar) image of the San Andreas fault in the San Francisco Bay area just west of San Mateo and Foster City. The fault runs diagonally from upper left to lower right. The body of water along the fault line is Crystal Springs Reservoir. Image credit: NASA/JPL

Nuclear test statistics

I've done a fair number of blogs on small earthquakes in areas - Dillsburg, PA; Albany, NY - where the seismic threat is, after all, not very high.

Here are some historical statistics from 1945-2006 on nuclear testing (note the log scale on the y-axis):

Source: CTBTO (Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization)

This is a graph of tests, not warheads. Maybe we'll look at that in another post.

Comprehensive Nuclear Test Ban Treaty

North Korea detonated a nuclear device on October 9, 2006. (a) A map of the region shows the location of the test (red star), nearby earthquakes (blue dots), and seismic monitoring stations (white triangles) at Mudanjiang in northeast China and Taejon, South Korea. (b) Seismograms recorded during the explosion (red wave) and a recent earthquake (blue wave) near that experiment show distinct P and S seismic waves for the earthquake, but not for the nuclear test (S&TR).

One more entry from Science & Technology Review, a publication of Lawrence Livermore Labs, which has an article on seismic verification of nuclear testing in its latest edition, although not late enough to mention the latest nuclear test of North Korea.

Monitoring the Comprehensive Nuclear Test Ban Treaty

"The U.S. ceased nuclear testing in 1992 in anticipation of the acceptance of the Comprehensive Nuclear Test Ban Treaty (CTBT). In 1996, President Bill Clinton and many other heads of state signed this multilateral treaty to prohibit all nuclear testing. Although most signatory countries ratified the treaty, the U.S. did not, and several countries required for the treaty to enter into force did not sign it. Expectations are high that the administration of President Barack Obama will reevaluate the CTBT’s role in nonproliferation policy.

"Although the CTBT is not in force, signatory countries and the U.S. are active participants in the International Monitoring System, which is overseen by the International Data Centre in Vienna, Austria, an organization established specifically to verify the CTBT. Every country supporting the system has a national data center. Livermore provides research and development support to the U.S. National Data Center at Patrick Air Force Base in Florida, which is responsible for U.S. nuclear test monitoring and international treaty verification.

"The International Monitoring System comprises a worldwide network of 337 sensitive monitoring stations and laboratories to detect nuclear explosions. Seismic stations anchored to bedrock record underground elastic waves, infrasound stations collect acoustograms from low-frequency sound waves aboveground, hydroacoustic stations in the oceans record underwater sound waves, and radionuclide stations measure airborne radioactive gases or particles. More than 230 of the recording systems now send data to the International Data Centre on a provisional basis. This unique network is designed to detect nuclear explosions anywhere on the planet—in the oceans, underground, or in the atmosphere.

"After the treaty enters into force, the signatory countries will have the role of identifying an event as a violation. The treaty also specifies several ways to resolve concerns about suspicious events, from consultation and clarification through a protocol that could lead to on-site inspections."

Comment on explosions and fault plane solutions

Image from Wikipedia

I haven't yet figured out how to make the Comments in this blog visible without having to first click on the Comments blog. If you know how to do so, please clue me in.

Anyway, good question from my last entry, if you didn't see it: "Is there any way you'd consider sharing a higher-resolution version of that figure (say, big enough for a PowerPoint) with me? I'd like to use if for educational purposes (specifically in explaining the foundations of beach ball diagrams)."

I usually provide links to the text and figures I shamelessly borrow from other sources. So the best resolution you're gonna get is to go back to the source I used. In many cases, you can get a better resolution of the figures I post by clicking on the figure itself, and a better copy will open in a new window. Of course, I'd encourage you to retain the reference to the source if you re-use the figures in your own presentations.

One of the classic ways to distinguish seismic waves emanating from earthquake vs. seismic waves original from blasts, including underground nuclear tests, is that the energy from explosions is all directed radially outwards, while the initial pulse of energy from earthquakes is compressional in some quadrants, but dilatational in others. The global distribution of these compressional and dilatational first motions of seismic waves can be used to infer the orientation of the fault plane on which the earthquake occurred, and also whether the sense of motion is normal, reverse, or strike slip. This kind of analysis is known as a first motion study, fault plane solution, or focal mechanism solution. So earthquakes and explosions will yield quite distinct fault plane solutions.

I found a couple of decent resources if you want to learn more about earthquakes and fault plane solutions. There is a very nice online tutorial from Arild Andresen, University of Oslo. The Wikipedia page on focal mechanism look ok, and has two links at the end to a pdf file with more technical material for the geologist, and a cool page from the University of Bristol which lets you construct your own fault plane diagram.

But for your classroom demonstration, you have to see if you can find a beach ball that has four quadrants, not six. Faulty analogy, perhaps?

More on seismic verification of nuclear tests

Science & Technology Review, a publication of Lawrence Livermore Labs, coincidentally has an article on seismic verification of nuclear testing in its latest edition, although not late enough to mention the latest nuclear test of North Korea. So the North Korea tests referred to are from 2006, not last month.

Part II of my cribbed comments focus on the Walter diagram for discriminating earthquakes from nuclear tests:

Various kinds of seismic events can be grouped on a source-type, or Hudson, plot based on their ground motion. A perfectly symmetric underground explosion would appear at the apex of the plot. By analyzing the seismic waves produced by the disturbance that rocked the Crandall Canyon Mine in Utah in August 2007, Laboratory seismologists determined that the event was an implosive tunnel collapse, not the sideways slippage of an earthquake.

"Distinguishing an earthquake from a nuclear event requires a close examination of the seismic waves. Such waves fall into two major categories: surface waves, which move along Earth’s surface, and body waves, which move through Earth and bounce off structures inside. Body waves may be primary (P) or secondary (S). Seismic P waves are compressional waves, similar to sound waves in the air. S waves are shear, or transverse, waves, similar to those that propagate along a rope when one end is shaken. Underground explosions radiate P waves in a relatively symmetric spherical shape. Earthquakes, which result from plates sliding along a buried fault, strongly excite the transverse motions of S waves, producing a distinct radiation pattern. Explosions thus show strong P waves and weak S waves. Earthquakes, in contrast, typically show weak P waves and strong S waves.

"But this information alone is not foolproof because the structure of Earth imparts an imprint on the signal. One way to quantify the difference between these seismic disturbances is to determine the ratio of P-wave to S-wave energy measured from the seismograms. Explosions should have higher P:S ratios than earthquakes.

"Recent Livermore work led by Walter sought to clarify the characteristics of the P:S ratios that distinguish nuclear weapons tests from other tectonic activity. By examining regional amplitude ratios of ground motion in a variety of frequencies, his team empirically demonstrated that such ratios indeed separate explosions from earthquakes. The researchers used closely located pairs of earthquakes and nuclear explosions recorded at monitoring stations at or near the Nevada Test Site; Novaya Zemlya and Semipalatinsk, former Soviet Union test sites; Lop Nor, China; India; Pakistan; and the North Korea test.

" 'At high frequencies, above 6 hertz, the P:S ratio method appears to work everywhere we looked,” says Walter. 'Explosions have larger P:S amplitude values.' For example, a test in India on May 11, 1998, compares well with the October 9, 2006, North Korea test.

"However, west of Pakistan, in the tectonically complex Middle East, seismogram analysis becomes more complicated. S waves attenuate, or lose energy, more rapidly in regions that are geologically complicated and seismically active. Seismograms from these areas tend to have larger P-wave amplitude relative to their S-wave amplitude. As a result, earthquake signals may look like those from an explosion. Because of these wave propagation effects, Walter’s team applied a tomographic technique to measure the highly variable attenuation of S waves in the Middle East.

"Tomography is a mathematical operation that uses variations in the waves passing through a material to construct an image of the material’s structure. For example, a medical tomography scan uses variations in the waves of x radiation transmitted through the body to produce an image that a radiologist can analyze. Tomography of Earth uses seismic waves to generate comparable images of Earth’s inner structure.

"The tomographic technique developed by Walter’s team models structural deformation based on the attenuation occurring as S waves propagate through different geologic media. Even with the precise algorithms in this tomographic attenuation method, however, some known earthquakes have P:S ratios that look like those of explosions. 'We expect that analysis of ground motion at higher frequencies will help us better understand and use this method,' says Walter. 'But we have only recently had the computational power we need to model high frequencies in 2D and 3D Earth models.' "

North Korea nuclear test - seismic verification

Science & Technology Review, a publication of Lawrence Livermore Labs, coincidentally has an article on seismic verification of nuclear testing in its latest edition, although not late enough to mention the latest nuclear test of North Korea.

I'm too lazy to paraphrase an article already well written. It starts out as follows:

AN earthquake, a nuclear test, and a mine collapse all cause seismic disturbances that are recorded at monitoring stations around the world. However, these three types of events produce very different ground motions at their source. Earthquakes are caused by sideways slippage on a fault plane, while underground nuclear explosions push outward in all directions. A mine collapse is a massive vertical roof fall.

Lawrence Livermore is at the forefront of research to more accurately distinguish nuclear explosions from the rest of Earth’s never-ending seismic activity, including earthquakes large and small, volcanoes, and waves crashing on shore. The Laboratory’s work was unexpectedly put to the test following the August 2007 collapse of the Crandall Canyon coal mine in Utah, which killed six miners. Ten days later, another collapse killed three rescue workers. Both events were recorded on the local network of seismic stations operated by the U.S. Geological Survey (USGS) as well as on the USArray stations, which are part of EarthScope, a program funded by the National Science Foundation. There was considerable contention about whether the initial magnitude-3.9 event was caused by an earthquake or a collapse.

At the time, Livermore seismologists were working with colleagues from the University of California at Berkeley on a waveform-matching technique to distinguish among nuclear explosions, earthquakes, and collapse events. This technique compares seismograms produced by computer modeling with recorded data at local to regional distances (from 0 to 1,500 kilometers) for periods of 5 to 50 seconds. Livermore’s analysis of the August 2007 seismograms pointed to a collapse rather than an earthquake. The important result for the Laboratory team was being able to identify the Crandall Canyon event from its seismic signature despite its small magnitude.

Livermore’s seismological research is part of the Department of Energy’s support for the U.S. National Data Center in the area of nuclear treaty verification. (See the box below.) The team’s experience with the Crandall Canyon Mine has given the Livermore seismologists even greater confidence that they can identify a relatively small nuclear test using the same technique.

More to come on this. Or go to the link, and read the whole article now.