Planet Earth/6d. You Can’t Fake an Earthquake: How to Read a Seismograph
An Earthquake?
[edit | edit source]Horizontal coal layers erode out of the cliffs in central Utah west of the town of Huntington. The Crandall Canyon Mine provided coal for the nearby Huntington Power station, which uses coal to generate electricity. In the early morning of August in 2007, the roof of the mine collapsed trapping six miners 1,800 feet beneath the surface inside the coal mine. Emergency responders raced to the scene to find that the mine was completely buried in the collapse. As new media arrived to the scene and attempts were made to rescue the six miners, the owner of the mine, Bob Murray, emphatically argued that the mine collapse was due to a naturally occurring Earthquake, rather than an over-weakened mine shaft that had collapsed. As rescue attempts proved fruitless, and another 3 people died in attempts to locate the trapped Utah miners, the question of whether the mine collapse was due to poor supports holding the roof, or whether the mine collapse was a result of an Earthquake became a central question. Unknown to the mine owner, one of the largest geological experiments ever conceived was being conducted that summer in Utah, as hundreds of high-quality, portable seismographs were being deployed across the United States to record the motion of the North American continent. This network of seismographs is known as the US Array. High resolution GPS stations deployed by the National Science Foundation’s Earthscope initiative recorded the actual motion of the continental plate, with partnerships with many university seismologists who added to the research through the IRIS program (Incorporated Research Institutions for Seismology). This massive government effort to measure the motion of the Earth resulted in amazing insight into the dynamic nature of the North American lithospheric plate. Measurements demonstrated the quick motion of Southern California northwest, while in Oregon and Washington the motion is toward the northeast. The Pacific lithospheric plate under Southern California slides across the San Andreas fault, at a measured rate of over 50 millimeters a year. The San Andreas fault is a transverse plate boundary that separates the Pacific plate (including the Pacific Coast of California) from the North American plate, rather than the plates diverging or subducting they are actually moving laterally against each other making the region highly prone to Earthquakes. Utah on the other hand, in the interior of the continent, does not move very quickly, with motion restricted to only a few millimeters a year. Much of this motion is in the Great Basin in Western Utah near the Nevada border. The North American plate is colliding with the Pacific Plate, and spinning slowly clockwise as it spins, but only few millimeters a year. This motion is slowly pulling apart western Utah, forming an ever-widening basin region, called the Great Basin. [See: https://www.unavco.org/software/visualization/GPS-Velocity-Viewer/GPS-Velocity-Viewer.html] However, in eastern Utah the motion is nearly 0, with no motion detect at the many GPS stations deployed to the area. The region around the Crandall Canyon Mine is not near any plate boundaries, nor active or extinct volcanoes, and is in a tectonically quiet region of North America. It seemed unlikely that an Earthquake caused the mine collapse.
Locating Earthquakes
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Seismographs not only record the motion of the Earth, but they can be used to detect exactly where an Earthquake originates from. The ability to pin-point an Earthquake’s location is a vital tool to geologists. To locate an Earthquake requires a minimum of three seismographs positioned near the Earthquake. The Earthquake will produce three propagating seismic waves. P-Waves, S-Waves and Surface Waves. P-Waves are the fastest traveling waves, followed by S-Waves with Surface Waves the slowest traveling seismic wave. P-Waves will always arrive at each seismograph first, while S-Waves will be the second arriving seismic wave, followed by the Surface Waves; each recorded as squiggly lines. Both P-Waves and S-Waves are called Body Waves, since they also travel through the subsurface (the body of the rock layers). The difference between the arrival times of the P-Waves and S-Waves is known as the S-P Interval, and is proportional to the distance from the Earthquake. Think of the P and S seismic waves as runners in a race on a track. They both start at the same line. The faster runner will win the race (P-Wave), while the slower runner will come in second (S-Wave). If the faster runner outpaces the slower runner at a constant speed, the distance between the two runners will grow farther apart, the farther they are from the starting line, if they run 10 meters they will be closer together than if they run 1000 meters. The greater the distance between the arrival times of P-Waves and the S-Waves, the farther the Earthquake is located from the seismograph. Each S-P Interval will give a unique distance from the station, and using three seismograph stations circles of each of these measured distances can pinpoint the epicenter of an Earthquake. An epicenter is the location on the surface of the Earth directly above the location of the Earthquake, and is found by triangulation of three (or more) seismographs. A focus is the actual location of the Earthquake in the subsurface, underground. The epicenter and focus of the 3.9 magnitude Earthquake that morning was directly over the Crandall Canyon Mine, but that did not necessarily distinguish a mine collapse from a natural occurring Earthquake.
Earthquake Magnitude
[edit | edit source]In 1930 Beno Gutenberg left Germany, and his failing father’s soap factory for a new life in the United States, it was a good choice to leave the country when he did. In the United States Gutenberg established the California Institute of Technology Seismological Laboratory to study Earthquake activity under the active Pacific Plate in California. Although struggling with English and the new culture of Southern California, he continued his research on seismic activity. In 1936 he hired an awkward physicist named Charles Richter. Richter was a native of Los Angeles, and involved in Earthquake research. The two scientists were opposite to each other, the reserved older German Gutenberg and the young American Richter who spent his free time at nudist colonies with his long-time wife (a romance writer), and rubbing shoulders with Hollywood stars of the 1930s. The two worked closely to develop a magnitude scale. The scale was a logarithmic scale to classify the strength of Earthquakes. Looking at seismographs, the vertical squiggles (amplitude) were taller the closer the Earthquake was, but also taller the bigger the Earthquake was. This was the same issue facing astronomers who classified a star’s brightness; as it is difficult to tell if a star is bright because it is close, or because it is big. The same was true of Earthquakes, so they developed an independent way to classify an Earthquake’s magnitude independent of its distance from the recording seismograph, this became known as the Richter Scale. To determine the Richter scale, a geologist will look at all the records of seismographs recorded of an event and select the largest ground motion recorded (highest amplitude). The logarithm of the vertical height of the maximum amplitude is taken (measured in micrometers/microns). A correction is applied based on the distance between the first arrival time and the log of the maximum amplitude. This simple procedure allowed earthquakes to be quickly assessed as to their Richter magnitude. On the Richter scale, Earthquakes between 0 to 3 are rarely felt, but are recorded on seismographs. Earthquakes above 3 to 5 are felt but do not cause much damage. Earthquakes above 5 are destructive, depending on where they occur. The 1906 San Francisco Earthquake was 7.9 on the scale, while the largest ever recorded Richter scale Earthquake was the 1960 Great Chilean earthquake, which was recorded as 9.5. The 3.9 magnitude Earthquake recorded directly over the Crandall Canyon Mine was not enough to cause much damage and would have been slightly felt in the motion of Earth to a nearby observer.
Push and Pull of Earthquakes
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Earthquakes are produced by the motion across a fault. A fault is a plane or surface between two moving blocks of solid rock. Faults can be oriented as either mostly up/down vertically or more horizontally depending on the stresses subjected to the solid rock. Normal faults are faults that are oriented more vertically and are caused due to extension, when the rocks are pulled apart. In a normal fault one block of rocks will drop by sliding down the fault surface. Reverse faults are also oriented vertically, but are caused due to compression, when the rocks are pushed together. One rock block will ride over the other. Reverse faults are very rare, hence most vertically oriented faults are normal faults. Horizontally oriented faults are called thrust faults, and are a more commonly a result of compressional forces applied to rock blocks. In thrust faults one block will slide over another block, often in a more horizontally oriented plane. Normal faults are common in regions that are experiencing stretching or extension, while thrust faults are common in regions that are experiencing collision or compression. A strike-slip fault is a fault that where two blocks slide next to each other without much if any vertical motion. Faulting only occurs in rock layers that are within the brittle zone of the crust, rocks below this zone will plastically deform into folds. Structural geologists study the dynamic nature of faulting and folding observed in rock layers on the Earth.
Earthquakes resulting from the motion of a fault will produce either a push or pull depending on their orientation and motion. If the first motion of the P-wave is upward on a seismograph, the motion was a push away from the epicenter, while if the first motion of the P-wave is downward on a seismograph, the motion was a pull away from the epicenter. If an Earthquake moves across a typical strike-slip fault, seismographs from each quadrate will record either push or pull motions. If an Earthquake moves across a thrust fault or normal fault, half the seismographs will record a pull and half a push. Study of the push-pull record in a group of seismographs can help geologists orient the direction of motion across a fault.
In 2008 Douglas S. Dreger, Sean R. Ford and William R. Walter examined the record of seismographs recorded during the Crandall Canyon Mine disaster in Utah as part of the US Array experiment. They noticed that all the seismographs recorded a pull motion, or dilation in the first arrival of the P-waves. Such motion suggested that the mine collapsed, resulting in a pull motion on the surrounding rocks inward into the mine. This was recorded by seismographs. The mine disaster thus was concluded as being caused by the roof of the mine collapsing on the workers, and not an Earthquake.
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e. The Rock Cycle and Rock Types (Igneous, Metamorphic and Sedimentary). |