Locating The Epicenters’ Of Earthquakes
2014-15
Background Information: To locate the epicenter, a minimum of three circles are drawn on a map. (This process is called
triangulation.) You can determine the distance from a seismograph station using the graph you created in "Difference in Arrival
Times for P- and S- Waves”. This distance is the radius of the circle that you draw around that seismograph station. Once you have
done the same for at least two more seismograph stations the circles should meet at the epicenter (distortion and estimation error
may cause the circles not to meet exactly). With the advent of video cameras being used in so many places of business, the method
of locating the epicenter of an earthquake has evolved. When the primary wave (longitudinal) arrives it moves the earth back and
forth like a slinky. Seismologists note the direction that stuff falls when the P-wave hits. Using Newton’s law of inertia, they know
that if stuff falls toward the east the earthquake came from the east. Knowing the exact location and compass direction of the falling
stuff the seismologists triangulate to locate the epicenter. The magnitude, as measured by a seismogram, is scaled by the Richter
scale (open ended). The amount of energy released by an earthquake increases by 31 times for each magnitude number. For
example, an earthquake with a magnitude of 6 is 31 times stronger than an earthquake with a magnitude of 5 and 1,000 times
stronger than an earthquake with a magnitude of 4. (An earthquake magnitude-4 which occurs over a larger fault plane is going to
release more energy than an earthquake magnitude-4 over a smaller fault plane. Recently this has been accounted for by a scale
called magnitude moment.)
Purpose: You will be determining the epicenter of an earthquake by the process of triangulation.
Materials: the graph created in "Difference in Arrival Times for P- and S- Waves”, pencil, United States map, compass, textbook,
articles “EASTQUAKE: Countdown to Catastrophe” and “Why Must Earthquakes Be This Devastating?”
Procedure:
• Read page 219 and 222-223. Additionally, read the articles entitled “EASTQUAKE: Countdown to Catastrophe” and “Why Must
Earthquakes Be This Devastating?”
• Determine the distance to each seismograph station for the earthquake from the data table below. Rockville, MD has a difference
in arrival time of 2 minutes and 00 seconds. By looking on the "Difference in Arrival Times for P- and S- Waves” graph you will see
that the distance to the focus is 1,100 kilometers. Record this in the data table.
• Do the same for all 5 cities.
• Using the scale on the United States map, separate the compass so that the point and the pencil are 1,100 kilometer apart.
• Place the point of the compass on Rockville, MD.
• Draw a circle. This circle shows the distance of 1,100 kilometers from Rockville, MD. Which is how far from Rockville, MD the focus
of the earthquake is away.
• Following the same procedure for drawing the circles, do the other cities for this earthquake.
• The epicenter is located where all the circles come closest to meeting. Place an X at that spot to show where you believe the
epicenter was.
Analysis: Answer the following questions in completely.
1. What is the location of the epicenter of each of these earthquakes (use a map of the USA on pages 708-9 of the text)?
2. What is the minimum number of seismographs needed to accurately locate the epicenter of an earthquake ?
3. How are you able to locate the epicenter of an earthquake if you know where the focus is ?
4. How is distance of a seismograph from the earthquake related to the arrival time of the body waves (P- and S-) ?
5. What causes the ground to become liquefied ? Give an example where it occurred and why is can be so devastating.
6. What is a fire storm ?
7. What is an aftershock ?
8. What is a tsumani ?
9. The New Madrid earthquake was felt over the northeastern United States. An equally severe earthquake in California, however,
would not be felt much beyond California. Explain why ?
10. List and explain the reasons why an earthquake of similar magnitude on the east coast of the United States would cause more
severe damage than on the west coast of the United States.
Locating The Epicenters’ Of Earthquakes
2014-15
Earthquake #1
Seismograph Station
Difference in arrival times of P- & S- waves
(minutes and seconds)
Rockville
2:00
Newport
4:00
Rapid city
2:30
Tucson
3:20
McMinnville
0:30
Seismograph Station
Difference in arrival times of P- & S- waves
(minutes and seconds)
Rockville
1:50
Newport
4:00
Rapid city
2:50
Tucson
4:15
McMinnville
2:15
Seismograph Station
Difference in arrival times of P- & S- waves
(minutes and seconds)
Rockville
4:20
Newport
1:50
Rapid city
1:25
Tucson
1:45
McMinnville
3:30
Distance to epicenter
(kilometers)
Earthquake #2
Distance to epicenter
(kilometers)
Earthquake #3
Distance to epicenter
(kilometers)
Locating The Epicenters’ Of Earthquakes
EASTQUAKE: Countdown to Catastrophe
By Robert L. Ketter
2014-15
The Washington Post
December 04, 1988
California earthquakes and the San Andreas Fault used to be sure-fire joke material on the East Coast. But even New Yorkers aren’t laughing any
more. Not since the Quebec quake struck at 6:46 p.m. on November 25, 1988. Measuring 6.0 on the Richter scale, it centered on a sparsely
populated area 90 miles north of Quebec City. Within minutes it had sent rattles from Maine to Michigan and far enough south to rock the White
House. Suddenly easterners were shaken with the realization that it could happen here. And that the results could be horrible: When a quake of
about the same magnitude as Quebec’s hit Whittier, California, in 1987, it did $358 million worth of damage in 15 seconds. In fact, it will happen
here: Scientists have determined that the eastern United States is virtually certain to experience a destructive earthquake within two decades. Yet
to date there has been no serious federal effort to plan for coping with such a catastrophe.
Cracking the Crust
It is generally accepted that the outermost shell of Earth is composed of a dozen or so very large plates. These plates range from 45 to 95 miles
thick, and they move slowly and continuously with respect to each other at the rate of an inch or so per year. One of these, the North American
plate, extends from California and Mexico’s Baja Peninsula in the west to the middle of the Atlantic Ocean where it borders the African and
Eurasian plates. Along the mid-Atlantic ridge, the plates are moving apart and a new oceanic crust is continually being formed as molten lava rises
within the Earth’s mantle. At the boundaries of the other plates, different situations exist. In some areas one plate is forced downward under
another. In others- for example, along the San Andreas fault in California plates slide horizontally relative to each other in a :stick-slip” type of
motion. Corresponding sudden releases of energy are responsible for the earthquakes which occur in the California region. About 95 percent of
the world’s earthquakes occur along or near the boundaries of these crustal plates. However, many ever destructive ones occur within the plates
themselves, for reasons that are not fully understood. The currently accepted explanation is that several hundred million years ago the central part
of the United States was pushed upward by molten rock from the underlying mantle. This material was heavier than the surrounding rocks, and
when the upwelling ceased, the entire region subsided. Seas covered the area, laying down thick layers of sediment which eventually hardened
into limestone, sandstone and shale. Later, during the various cycles of plate movement, rifts formed- breaks in the hard lithosphere circling the
earth. Still later, molten rock from under the lithosphere moved upward along these faults, cooling before it reached the surface. Once again, the
ocean advanced over the area, depositing sands, clays and gravels. At present, due to the continual spreading that is occurring in the mid-Atlantic,
these ancient faults are subjected to east-west compressive forces. The forces acting on these faults, it is believed, are responsible for most
intraplate earthquakes. Unfortunately, we do not know the magnitude or distribution of the forces or even the locations of all the faults. We,
therefore, cannot predict with any certainty when or where the next crustal fracture- and therefore the next earthquake- will occur.
Shaking Out the Past
Scientists have identified the locations of many earthquakes that have occurred in the eastern United States since 1534. The number of
occurrences, however, is not the only- nor necessarily the best- indicator of the earthquake threat that exists for this region today. The largest
earthquakes known to have occurred in North America were intraplate quakes in the Mississippi valley near New Madrid, Mo., in 18811 and 1812.
It has been estimated that three had surface wave magnitudes of 8.6, 8.4 and 8.8 on the Richter scale. (Devised in 1935, the logarithmic scale is
used to compare magnitudes of quakes. Tremors are felt at 2; values over 6 cause damage to buildings. The hardest quake ever recorded registered
8.9.) Church bells 1,000 miles away in Boston rang; the earth was observed to “roll” in waves a few feet in height; large areas of land were uplifted
while others sank; existing lakes drained and new ones were created. Sand blows- a mixture of water and soil- erupted to the surface, spreading
debris over large areas, where it is still visible today. Another intraplate quake (Richter 7.7) occurred in 1886 in Charleston, S.C. The earthquake
menace in the central and eastern part of the United States is compounded by the fact that rocks in these regions transmit seismic energy much
more efficiently than do those that exist on the West Coast. For one thing, eastern rocks are older and denser than the comparatively recent
formations in the West. For another, the many fault-lines existing in the West serve to impede the travel of shock waves; the East has many fewer
faults to dampen the intensity. Thus the damage area for the 1811 New Madrid earthquake, which had a magnitude approximately the same as the
San Francisco earthquake of 1906, resulted in an area of similar damage that was 15 times larger. The size of the area affected by Charleston
earthquake also was very large. Moreover, large numbers of people live and work in the central and eastern states, where few structures have
been designed or built to resist earthquake damage. The consequences of a major seismic event, therefore, would be felt by many.
Locating The Epicenters’ Of Earthquakes
2014-15
The New Madrid Scenario
In general, all of the major earthquakes that have occurred in the eastern and central parts of the United States have exhibited precursor activities.
With that in mind, consider that more than 2,000 earthquakes have been recorded in and around the New Madrid area between 1974 and 1983and they are still occurring. It would be difficult for anyone to say that this region is not a prime candidate for a major seismic event. One of the
most frequently asked, and least satisfactorily answered, questions concerning earthquakes is: “When will the next major one happen ?”
Unfortunately, seismology cannot now answer that question in a deterministic fashion- that is, accurately predict when an earthquake will occurnor will be able to in the near future. A probabilistic assessment, however, is possible, and this has been done for New Madrid as well as a number
of other potentially active sites. Based on those calculations, the probability of a destructive earthquake occurring at any single specific geographic
site in the eastern part of the country within the next 15 to 25 years is relatively low. However, the probability of one occurring somewhere in the
eastern United States before the year 2000 can be considered better than 75 to 95 percent. Before the year 2010, nearly 100 percent. In 1985, two
researchers estimated the probability of occurrence of two large New Madrid earthquakes for two separate time periods- by the end of the
century, and by the year 2035. The earthquakes they considered were presumed to have surface-wave magnitudes of 6.3 and 8.3 on the Richter
scale, the latter corresponding to one of the 1811 quakes. It was found that the probability of occurrence between now and the year 2000 for
smaller earthquakes would be 40 to 60 percent; for larger, only 0.3 percent to 1 percent. By the 2035, however, there would be an 86 to 97 percent
probability of occurrence for the smaller one, and a 2.7 percent to 4 percent probability of occurrence for the larger. Assuming the larger of the
two postulated New Madrid earthquakes, we can hypothesize a scenario for the surrounding regions using the Modified Mercalli Intensity values.
(A 12-degree scale that ranks seismic events according to, the perceived amount of shaking that occurs.) For example, for areas experiencing an
intensity of VI- that is, for Chicago, Pittsburgh, Knoxville and so forth- the shaking would be felt by everyone. Many people would be frightened;
dishes, glassware and some windows would be broken; church and school bells would ring; and trees and bushes would shake visibly. Closer, in
regions experiencing an intensity of VII- St. Louis, Carbondale and Nashville there would be fright and signs of panic; some masonry walls would fall;
frame houses, if not securely bolted down, would move on their foundations; branches would break on trees; and cracks would appear in wet soil
and on steep slopes. In the general vicinity of the epicenter (around Memphis) where the intensity has been projected to be X, most masonry and
frame structures would be destroyed with their foundations; there would be serious damage to dams, dikes and embankments, and there would
be large landslides. Railroad tracks would be bent. Should the postulated event occur at night, when most people are in their homes which are
primarily wood construction- it is estimated that in the six cities nearest to the event- Memphis, Paducah, Carbondale, Evansville, Poplar Bluff and
Little Rock- there would result 676 deaths due to structural failure. If the event occurred during the daylight hours, that number would increase to
4,907 deaths. Damage to transportation systems would seriously hamper rescue and relief efforts. Railway networks would be similarity affected.
River ports would be extensively disrupted, there would be partial or limited availability of major airport facilities. These six cities would experience
serious impairment or loss of their utility (electric, water, gas and sewer) systems. Were the earthquake to occur at a time when high-water
conditions existed, flooding of low-lying areas would take place. Widespread individual or small-group structural fires would occur, but it is unlikely
that giant fires or conflagrations would result. For this six-city region of major impact, 460,000 persons would require shelter because of damaged
residences. Finally, it is estimated that restoration and replacement costs would exceed $51 billion. All of the consequences thus far described are
“local” consequences. Equally of national importance would be the effects felt by areas remote to the site, if the epicenter were located in the
vicinity of one of the major pipeline routes, and if that route passed through an area where major soil liquefaction took place. In all likelihood, such
an earthquake would disrupt the flow and distribution of transport crude oil and natural gas. Not only would there be the potential for serious
environmental contaminations, but it would be reasonable to expect significant energy distribution outages in heavily populated areas located
several hundred miles away. Although the probability of an earthquake occurring at a singular particular site in the immediate future is not large,
the probability of one occurring somewhere within the eastern part of the country is quite large. Moreover, the overall consequences of such an
event could be enormous.
Destructive earthquakes have occurred in the past, and will undoubtedly occur in the future. The only real uncertainties are when and where. It is
important that we be prepared in order to mitigate the effects of such catastrophic events. A number of things can be done, and the role of
Washington will be a major importance both during the first several days and later on.
Locating The Epicenters’ Of Earthquakes
Why Must Earthquakes Be This Devastating?
By Mark Zoback
2014-15
The Washington Post
Sunday, January 4, 2004; Page B05
The toll from the Iranian earthquake -- at least 30,000 dead and an estimated 40,000 homeless in just a few seconds -- is difficult to comprehend.
Unfortunately, we can predict with reasonable certainty that sometime in the next few years, in a country with buildings unprepared to withstand
disaster, a catastrophic quake will happen again. It's one more way in which the gap between the developed and the developing world’s makes a
profound difference, and we seismologists have to decide how best to deal with that sad fact.
The evidence of this gap is startling. The 1989 Loma Prieta earthquake in California (registering a magnitude of 6.9) left 63 dead and more than
3,700 injured, while the 2001 quake in Bhuj, India, (magnitude 7.6) killed roughly 20,000 and injured more than 100,000. The 1999 Izmit
earthquake in Turkey (registering 7.5) caused more than 17,000 deaths and 50,000 injuries, and left as many as 600,000 homeless. As devastating
as these recent events have been, they almost pale compared to the largest earthquake losses on record. How does one conceptualize the 1976
earthquake in Tangshan, China, which killed somewhere between 250,000 and 500,000 people? Or the 1556 quake in China's Shensi province,
considered by many to be the greatest natural disaster in history, that left an unimaginable 800,000 dead? Yet here is perhaps the most sobering
fact: If those earthquakes were to recur today at the same magnitude, the losses might be nearly as severe. That's because in many parts of the
developing world, sun-baked mud bricks and clay tile roofs still serve as shelter for millions of people. In earthquake country, these reinforced
masonry structures are death traps waiting to happen. So are some larger buildings: Floors are added until the first signs of the lowest levels
yielding. Even a moderate amount of shaking is enough to cause a collapse. One image imprinted on my mind is that of relief workers standing atop
a once six-story building in Mexico City that was literally pancaked in 1978 by an earthquake 200 miles away. In stark contrast, the impact of large
earthquakes in the developed world is never as severe, evidence of the clear value of earthquake engineering. Reinforced structures almost never
collapse, and even if they sustain heavy damage, they usually protect the lives of occupants as they are designed to do. Take, for example, the 1995
earthquake in Kobe, Japan, which effectively destroyed more than 100,000 buildings and killed 5,500. Nearly all of those deaths could be traced to
older buildings whose construction predated the enactment of modern building codes. While tens of thousands of newer structures were damaged
beyond repair, they did not crumple. Despite the clear-cut evidence that earthquake engineering is effective, deploying it in the developing world
is a complicated matter. The extensive retrofitting that has strengthened dangerous buildings and infrastructure (power plants, dams, pipelines,
bridges, overpasses, etc.) in California and Japan is often seen as prohibitively expensive in the developing world, although international groups are
working to implement low-cost solutions to some of the most egregious hazards in earthquake-prone areas. Even the best engineering, however,
can't work without vigilance. Having essentially the same building codes as California doesn't protect anyone if lax enforcement or corruption
prevents their implementation (the recent earthquakes in Turkey being an obvious case in point).
The final reason it is often difficult to implement earthquake engineering in the developing world is geologic, not human. The devastating
earthquakes in China and India that I cited earlier were "intraplate" events -- that is, they occurred far from the boundaries of the great tectonic
plates where earthquakes are more common, such as the San Andreas Fault system in California. In many cases, intraplate earthquakes are not
associated with previously known faults and the geological reasons for their occurrence are not well understood (such as three severe earthquakes
that struck New Madrid, Mo., in the winter of 1811-1812 and the one that struck Charleston, S.C., in 1886).
Large earthquakes on intraplate faults occur infrequently, and the killer faults responsible for history's most damaging events may not have moved
for thousands of years. This week's earthquake that ravaged Iran's ancient city of Bam destroyed a 2,000-year-old citadel. From a geologic
perspective, this earthquake occurred in the Zagros collision belt, a broad region of faulting and crustal deformation resulting from the collision of
the Arabian and Central Asian plates. As efforts to implement earthquake engineering and preparedness move forward in the developing world,
what else might be done to limit the devastating consequences of such events? The answer may be the holy grail of seismology, earthquake
prediction. Earthquake prediction would not prevent much of the physical damage that has accompanied earthquakes in the developing world, but
it would be enormously effective in reducing the death tolls. The successful prediction of the 1975 Haicheng earthquake in China undoubtedly
saved thousands, if not tens of thousands, of lives. The Haicheng prediction may have been fortuitous, as the area had experienced numerous
foreshocks (relatively small quakes that sometimes precede a main shock). Unfortunately, since that event there have been no scientifically
acknowledged accurate earthquake predictions. After almost 30 years with no successful predictions (and no apparent progress toward prediction),
many seismologists have all but given up on earthquake prediction. At one level it is difficult to argue with this logic. From a scientific perspective,
Locating The Epicenters’ Of Earthquakes
2014-15
not only do we not know how to predict earthquakes, we do not even know if, in general, earthquakes are predictable. We do know prediction is
not easy. It has been difficult to "catch" an earthquake early with detailed monitoring systems. For example, despite the high probability of
earthquakes on various sections of the San Andreas Fault system, none of the significant earthquakes that have struck California in the past 15
years (Loma Prieta, Landers, Northridge, Hector Mine or the recent San Simeon event) actually occurred on the San Andreas. Given the paucity of
direct observations of earthquake processes at depth, it has also been impossible to test many of the theories about such processes that have been
proposed over the past several decades. Today, we still do not know if all, most, some or any damaging earthquakes will ever be predictable in a
way that would benefit people.
Still, there are many reasons for optimism. Like our colleagues in astronomy, earthquake scientists are living in an era of unparalleled progress due
to the data now coming in from exciting new observational systems. For example, permanently installed, high-resolution Global Positioning System
(GPS) instruments have recently discovered episodic, slow-slip events at depth on the great subduction faults threatening Japan, the Pacific
Northwest and Central America.
Time will tell if such slow movements play a role in initiating earthquakes on these major faults. It is heartening to know that the discoveries being
revealed by the roughly 1,000 permanent GPS instruments installed throughout Japan will soon be complemented by about 800 such instruments
installed along major fault systems and volcanic centers in western North America as part of the Plate Boundary Observatory of the National
Science Foundation's EarthScope initiative. Satellite radar observations (known as InSAR, for Interferometric Synthetic Aperture Radar) have shown
us "breathing volcanoes" (episodes of inflation and deflation without eruption) as well as large portions of the Earth's crust deforming slowly in
response to large earthquakes. Might they also show anomalous deformation prior to earthquakes?
Another component of the EarthScope initiative, SAFOD (the San Andreas Fault Observatory at Depth), will soon penetrate the San Andreas Fault to
address long-standing questions about the physics of earthquake processes. In addition, for the very first time, it will be possible to make direct
observations at the depth of the initiation, propagation and arrest of earthquake ruptures, and test for observable phenomena that might someday
lead to reliable earthquake predictions. To me, this week's tragedy in Iran emphasizes the fact that the stakes are simply too high to leave
earthquake prediction to quacks and charlatans. So what should we do to limit future catastrophes? In military parlance, we attack across a broad
front. We support efforts to implement earthquake engineering and preparedness programs, especially in the developing world, and we continue
to do everything we can to better understand the physics of earthquakes.
If we can do that, perhaps someday we will develop prediction capabilities we can trust, with widespread benefit to the hundreds of millions of
people who live in quake-threatened areas around the world. At worst, this research will lead to a better understanding of long-term earthquake
hazards and dramatic improvements in our knowledge of how the earth works.
Not bad for a fallback position.
Locating The Epicenters’ Of Earthquakes
2014-15
Locating The Epicenters’ Of Earthquakes
2014-15
Locating The Epicenters’ Of Earthquakes
2014-15
Locating The Epicenters’ Of Earthquakes
2014-15