FORENSICS 152: WHAT’S SHAKING

This essay might be of special interest to writers of detective and mystery stories who would like to enrich their stories by providing their readers with a gift of some extra details. It might also be of general interest to many other readers.

Many harbor the thought that seismology has to do only with measuring and recording energy releases of earthquakes and has nothing to do with forensics. A definition of the word “forensics” states that it generally refers to scientific methods used to investigate crimes. This piece describes how a geoscientist, Terry Wallace, who had no apparent connection to criminology, used seismology and smarts to discover a number of criminals and to enable him to actually witness their criminal acts.

Terry was a member of a group scientifically studying ground movement in the area of the Andes Mountains along the Bolivian-Chilean border. He was puzzled by recorded measurements of occasional micro-earthquakes emanating from an out-of-the-way plain located more than 13,000 feet above sea level.. This itself was not too unusual, but the quaking was mysteriously happening only late at night.

Micro-earthquakes are those having a magnitude of less than 2.0 on the Richter magnitude scale and are too feeble to be felt. Many readers will recall having heard of the Richter scale. It is used to quantify energy released by earthquakes and is often mentioned by newscasters when reporting them. A few readers might remember that the Richter magnitude scale is base-10 logarithmic. For example, an earthquake having a magnitude measurement of 5.0 on the Richter scale would indicate that the earthquake had a shaking amplitude of ten times that of an earthquake having a magnitude measurement of 4.0. Accordingly, the amount of energy it released would be equal to the square root of 1,000, or 31.6, times that released by the 4.0 earthquake. Those who stayed home from school with the flu on the day that the Richter magnitude scale was being taught need not be concerned. The Richter magnitude referred to in this piece is less than 1.0.

Knowing that earthquakes were not the only phenomenon that shook seismographs, Terry developed a theory about what might be causing the quaking. Close examination of seismographic tracings led him to believe that the source might be trucks driving across the plain. The approach from one direction would have been across flat land. From the opposite direction, it would have been uphill. He believed he could determine the direction of the trucks by the different traces made when the trucks had been shifted into low gear while driving uphill. Eventually, Terry even began estimating the weight of passing trucks.

Given the route along which they were traveling and the time of night they were doing it, Terry figured that smugglers might be transporting contraband. Confirming that trucks were the source of the seismic disturbances might have cost him his life, but he was sufficiently adventurous to meet such a challenge. He hid one night and observed, as he had suspected he would, a passing parade of blacked-out trucks.

In addition to detecting the passage of trucks in the night, seismologists use thousands of sensitive instruments permanently located around the world to pinpoint and monitor, as they were originally designed to do during the Cold War, nuclear weapons tests. The instruments also provide information bearing on such events as submarine explosions, aircraft crashes, tectonic plate movements, landslides and mine tragedies. Other gathered information aids in predicting tsunamis, earthquakes and volcanic actions.

According to Terry, water conducts sound so well that a submerged seismometer (hydrophone) can detect an underwater explosion of a single stick of dynamite anywhere in the world.

ADDITIONAL INFORMATION:

A magnitude 4.0 on the Richter scale would rattle loose items within buildings but would not likely cause serious damage to the buildings. A magnitude 5.0 might cause major damage to poorly constructed buildings, but would not likely cause more than slight damage to those that were well constructed.

The magnitude 9.0 earthquake that caused the devastation in Japan in March of 2011 had a seismic energy yield equivalent to the energy yield of 480 million tons of TNT.

To put energy yields in some perspective, the asteroid that struck the Yucatan Peninsula in Mexico some 65 million years ago had an energy yield equivalent to that of 96 million, million tons (a teraton) of TNT. The asteroid was estimated to have been 6.2 miles in diameter, and it left a crater (the Chicxulub Crater) that was 110 miles in diameter. The strike would have created a tsunami rising thousands of feet.

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