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How Seismic Events are Located



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Earthquakes can be terribly destructive, but as with all seismic events, they also reveal things to us that otherwise would be deeply hidden, like natural resources or even the secret testing of a nuclear bomb.  This is why many people, not just geologists, need to know how seismic events are located.

What is a seismic event

Somewhere far below our feet, two blocks of solid rock suddenly slip past each other.  An earthquake has occurred, releasing energy that immediately starts radiating outward as seismic waves.

An earthquake like this is called a tectonic event because it occurs within the Earth's crust.  Nontectonic events happen, too.  Any movement that briefly releases energy into the Earth, producing seismic waves, counts.  This includes, among other things, traffic and other routine human activity, landslides, explosions, and avalanches.

Today's geoscientists are on the lookout for seismic events all the time, using networks of very sensitive instruments that monitor the structure and movements of the Earth more closely than ever could have been hoped for in the distant past.

Dragons, soot, and electronics - detecting seismic events

Earthquakes were long considered random acts of God.  As human societies became more complex and organized, decision makers needed to know about major events as soon as possible.  Almost 2 millennia ago, during the Han dynasty's reign in China, Zhang Heng invented a device that indicated when a large earthquake occurred, even far from the imperial capital.  This seismoscope looked like a large bronze vessel with eight dragon heads around the top, each with a little bronze ball in its mouth.  The very first earthquake was recorded when the ball fell from the dragon that faced east—although no ground motion was felt at the court, the next day, a rider arrived from the east with news of an earthquake.

It proved difficult to get more information about earthquakes.  In the West, a number of isoseismal maps were developed, using lines to connect points where there was evidence, based on verbal reports from survivors and descriptions of damage, of similar intensity of shaking (“isoseismal” comes from Greek words for “same” and “earthquake”).  These maps showed roughly where an earthquake had been centered and what the intensity of shaking had been at various distances from the central zone, and different intensity scales were developed from them.

One of these, the Modified Mercalli scale, is still in use today as it provides valuable information from places where there is little monitoring equipment.  However, seismographs, or seismometers—instruments that continuously record both time and ground motion—arrived in the late 19th century and they now provide the most detailed information about a seismic event.

At first, seismometers consisted of a container with a very heavy mass inside that was mounted on a spring.  When the ground moved, the container moved but not the mass, because of its inertia and the spring mount.  Early seismometers had a stylus that would scratch a record of the motion onto soot-covered glass.  The information obtained this way is still valuable.  In the 1980s, reevaluation of such records gave scientists better details of exactly how the San Andreas Fault moved during the 1906 San Francisco earthquake.

Today, seismometers are electronic and measure ground motion based on how much force it takes to hold a small mass steady as the container around it moves.  These come with varying degrees of sensitivity and are deployed throughout the world in networks that allow geoscientists to locate seismic events with good precision.

Triangulate this - locating seismic events

To understand exactly how scientists locate a seismic event, let's return to the earthquake example.  Two blocks of rock far below us slip, releasing kinetic energy that radiates out as seismic waves from the hypocenter (the point inside the Earth where the first movement happens).  Some of this energy reaches the surface; if factors are right, these surface waves can cause a lot of destruction at and around the epicenter (where we are standing on the surface, directly above the hypocenter).  Other seismic waves travel through Earth and so are called body waves.

Body waves are the most useful in finding the hypocenter because they travel right through the planet:  sensitive instruments anywhere in the world can often detect them.  Some of these waves move by compressing tiny particles ahead of them; these are the first ones to reach the seismometer where they are recorded as primary (P) waves.  Another sort of body wave travels more slowly because it is shearing its way through the Earth, displacing particles side to side.  This is the secondary (S) wave that reaches the seismometer.

A relationship exists between the distance from the seismometer to the earthquake and the interval between the arrival of the quake’s P and S waves.  If the waves arrive 1.5 minutes apart, the earthquake was 900 km away from the station, 3 minutes apart, 1800 km; and 5 minutes apart, 3300 km.

This distance is actually the radius of an imaginary circle that has the seismometer at its center.  While scientists still don’t know the direction of the quake, as it could have occurred at any point on the circumference of this circle, all that is needed now is a similar circle from two more seismographs at different locations.  The earthquake happened at the point where the circles from all three seismographs intersect.

While other location methods can be used, depending on the number of instruments available or the historical record, triangulation is the basic approach used everywhere to locate seismic events.

Real-world examples

The exact shape of the waves on their instruments tells geoscientists quite a bit about what sort of seismic event happened.  This is why seismometers are used to verify compliance with the Comprehensive Test Ban Treaty and to locate rogue nuclear tests, such as the ones recently conducted by North Korea.

Scientists also generate seismic waves at the surface and then study how the waves are reflected by different layers and objects underground.  This seismic profiling technique was shown in the movie "Jurassic Park" and is also used to map the structure of Earth as well as to search for oil and gas deposits.  For instance, many landowners allowed seismic profiling of their property during the investigation into possible oil and gas resources in the Eastern Overthrust Belt of New York and New England in the early 1980s, which showed that resources are there but currently are too expensive to tap.

Hawaii

Perhaps the best example where all this comes together to benefit both science and the public is Hawaii.  Scientists have been intensively studying volcanoes there for almost a century now, even as the island population has dramatically increased.  As of 2009, an estimated 178,000 people shared the Big Island with five volcanoes, three of them active.  That is over 50% more people than lived there in 1983, when Kilauea began its current eruption (now in its 24th year and still going strong).

Because of the large at-risk population, volcanologists and other geoscientists in Hawaii keep a very close watch on seismic events and nowadays can accurately locate all but the smallest seismic events.  Most are related to volcanism, but a few are tectonic, generally either a direct or indirect result of the size of the Big Island volcanoes, especially Mauna Loa, the largest and most massive volcano in the world.

Summary

Quite often, dangers can be best handled by learning as much about them as possible.  Earthquakes and other seismic events are some of the biggest calamities we will ever face, but mankind has developed the necessary tools to deal with them.  This process is ongoing, but we understand seismic events much better now and so are able to minimize their risks, as well as sometimes even benefit from the hidden secrets they have revealed to us.

Sources:

USGS Earthquake Hazards Program.  Multiple pages.

“Measuring Earthquakes” in Earthquakes - General Interest Publication, USGS.

What are seismic events?  Wisegeek.

History of early isoseismal maps.  Varga, P. Acta Geod. Geoph. Hung., Vol. 43(2–3), pp. 285–307 (2008)

Is It Time To Revisit the Eastern Overthrust Belt of New York and New England?  Martin, John P.

Forensic Seismology and the Comprehensive Test Ban Treaty.  Bowers, D. and Selby, N. D. Annual Review of Earth and Planetary Sciences Vol. 37: 209-236 (abstract only)

USGS Hawaiian Volcano Observatory.  Multiple pages.

Seismometer.  Wikipedia.

How are Earthquakes Located?  The IRIS Consortium.

Hawaii County Quick Facts.  US Census Bureau.

County of Hawaii Data Book Section 1:  Population.   County of Hawaii.

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