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Seismic Education

Learn more about recent highlighted earthquakes at the Earthquake Details page

Use a Travel-Time Curve Chart to Approximate the Distance to an Earthquake

One of the best places for learning about seismology and earthquakes is at the
United States Geological Survey (USGS) earthquake websites at http://earthquake.usgs.gov/learn.
Frequently asked questions about earthquakes

Can you predict earthquakes?

No. Neither the USGS nor Caltech nor any other scientists have ever predicted a major earthquake.
 They do not know how, and they do not expect to know how any time in the foreseeable future.
However based on scientific data, probabilities can be calculated for potential future earthquakes.
For example, scientists estimate that over the next 30 years the probability of a major EQ occurring
 in the San Francisco Bay area is 67% and 60% in Southern California.

The USGS focuses their efforts on the long-term mitigation of earthquake hazards by helping to
improve the safety of structures, rather than by trying to accomplish short-term predictions.

Why are we having so many earthquakes?
Has earthquake activity been increasing? Does this mean a big one is going to hit?
We haven't had any earthquakes in a long time; does this mean that the pressure is building up?

Although it may seem that we are having more earthquakes, earthquakes of magnitude 7.0 or greater have
 remained fairly constant throughout this century and, according to our records,
have actually seemed to decrease in recent years.

There are several reasons for the perception that the number of earthquakes, in general,
and particularly destructive earthquakes is increasing.

1) A partial explanation may lie in the fact that in the last twenty years, we have definitely had an increase
 in the number of earthquakes we have been able to locate each year. This is because of the tremendous
 increase in the number of seismograph stations in the world and the many improvements in global communications.

In 1931, there were about 350 stations operating in the world; today, there are more that 4,000 stations
 and the data now comes in rapidly from these stations by telex, computer and satellite.
This increase in the number of stations and the more timely receipt of data has allowed us and other
seismological centers to locate many small earthquakes which were undetected in earlier years,
and we are able to locate earthquakes more rapidly.

The NEIC now locates about 12,000 to 14,000 earthquakes each year or approximately 50 per day.
Also, because of the improvements in communications and the increased interest in natural disasters,
the public now learns about more earthquakes. According to long-term records (since about 1900),
we expect about 18 major earthquakes (7.0 - 7.9) and one great earthquake (8.0 or above) in any given year.
However, let's take a look at what has happened in the past 32 years, from 1969 through 2001, so far.
Our records show that 1992, and 1995-1997 were the only years that we have reached or exceeded the
 long-term average number of major earthquakes since 1971. In 1970 and in 1971 we had 20 and 19 major
earthquakes, respectively, but in other years the total was in many cases well below the 18 per year which
we may expect based on the long-term average.

2) The population at risk is increasing. While the number of large earthquakes is fairly constant, population
 density in earthquake-prone areas is constantly increasing. In some countries, the new construction that
comes  with population growth has better earthquake resistance; but in many it does not. So we are now
seeing increasing casualties from the same sized earthquakes.

3) Better global communication. Just a few decades ago, if several hundred people were killed
by an earthquake in Indonesia or eastern China, for example, the media in the rest of the world would
not know about it until several days, to weeks, later, long after such an event would be deemed newsworthy.
So by the time this information was available, it would probably be relegated to the back pages of the
newspaper, if at all. And the public Internet didn't even exist. We are now getting this information almost immediately.

4) Earthquake clustering and human psychology. While the average number of large earthquakes per year
 is fairly constant, earthquakes occur in clusters. This is predicted by various statistical models,
and does not imply that earthquakes that are distant in location, but close in time, are causally related.
But when such clusters occur, especially when they are widely reported in the media, they are noticed.
However, during the equally anomalous periods during which no destructive earthquakes occur,
no one deems this as remarkable.

A temporal increase in earthquake activity does not mean that a large earthquake is about to happen.
Similarly, quiescence, or the lack of seismicity, does not mean a large earthquake is going to happen.
A temporary increase or decrease in the seismicity rate is usually just part of the natural variation in the
seismicity. There is no way for us to know whether or not this time it will lead to a larger earthquake.
Swarms of small events, especially in geothermal areas, are common, and moderate-large magnitude
earthquakes will typically have an aftershock sequence that follows.
All that is normal and expected earthquake activity.

The Earth's Interior

Five billion years ago the Earth was formed by a massive conglomeration of space materials. 
The heat energy released by this event melted the entire planet, and it is still cooling off today.
Denser materials like iron (Fe) sank into the core of the Earth, while lighter silicates (Si), other 
oxygen (O) compounds, and water rose near the surface. The earth is divided into four main layers: 
the inner core, outer core, mantle, and crust. The core is composed mostly of iron (Fe) and is so hot
that the outer core is molten, with about 10% sulfur (S). 
The inner core is under such extreme pressure that it remains solid.

Earth's Interior

Most of the Earth's mass is in the mantle, which is composed of iron (Fe), magnesium (Mg), aluminum (Al), 
silicon (Si), and oxygen (O) silicate compounds. At over 1000 degrees C, the mantle is solid but can deform
slowly in a plastic manner. The crust is much thinner than any of the other layers, and is composed of the 
least dense calcium (Ca) and sodium (Na) aluminum-silicate minerals. 

Being relatively cold, the crust is rocky and brittle, so it can fracture in earthquakes. 

This is a brief summary of our knowledge of the earth's interior. For further information, read a more detailed
description at the University of Nevada's Seismological Laboratory pages.

P and S Waves

When an earthquake occurs, it releases energy in the form of seismic waves that radiate from the
earthquake source in all directions. The different types of energy waves shake the ground in different ways
and also travel through the earth at different velocities. The fastest wave, and therefore the first to arrive 
at a given location, is called the P wave. The P wave, or compressional wave, alternately compresses 
and expands material in the same direction it is traveling. The S wave is slower than the P wave and arrives next,
shaking the ground up and down and back and forth perpendicular to the direction it is traveling. 
Surface waves follow the P and S waves.

See also Figure 2 "Time-travel Curves" below for more discussion of the propagation
of P and S waves through the earth.

Hypocenter (or Focus) vs Epicenter

The earthquake hypocenter (also commonly referred to as the earthquake focus) is the
point within the earth where an earthquake rupture starts.
The epicenter is the point directly above the hypocenter (focus) at the surface of the Earth.

Epicenter vs Hypocenter

Seismographs - Keeping Track of Earthquakes
Ref: http://earthquake.usgs.gov/learn/topics/seismology/keeping_track.php

Throw a rock into a pond or lake and watch the waves rippling out in all directions from the point of impact.
Just as this impact sets waves in motion on a quiet pond, so an earthquake generates seismic waves that
radiate out 
through the Earth.

Seismic waves lose much of their energy in traveling over great distances.
But sensitive detectors (seismometers) can record theses waves emitted by even the smallest earthquakes.
When these detectors are connected to a system that produces a permanent recording, they are called seismographs.

There are many different types seismometers, but they all are based on the fundamental principle - that the differential
motion between a
free mass (which tends to remain at rest) and a supporting structure anchored in the ground
(which moves with the vibrating Earth) can be used to record seismic waves.


Figure 1.
 Simple Seismographs

Seismographs are designed so that slight earth vibrations move the instruments; the suspended mass (M),
however, tends to remain at rest, and its recording stylus records this difference in motion.
The horizontal seismograph shown here moves only in the horizontal plane.

Vertical seismographs, like the simple one shown here, use a "soft" link between the earth-anchored
instrument and the suspended mass. In this design, the mass hangs from a spring, which absorbs some
of the motion and causes the mass to lag behind actual motion.  This principle is illustrated in the Figure 1 above.
Vertical support AB holds mass M in position by wire AM and by strut BM at point B.  
The system becomes a seismometer when the vertical support is embedded in a concrete pier attached to
the Earth.  If there is no friction at the point B and mass M is reasonably large, the movement of the pier
and the attached upright support in response to an earthquake wave will  set up a differential motion between
the mass and  the pier (the inertia of the mass will make it remain at rest).

This motion - the signal of an earthquake wave - can then be recorded on a revolving drum.
When the pier is steady, the pen attached to the mass writes a straight line. But when the pier shakes,
the mass and strut wiggle, recording waves from the earthquake that started the boom in motion.

Usually, the drum rotates on a screw-threaded axle so that the recording pen moves on a continuously advancing
record and does not simply repeat the same circle over and over. Because time - both the time of day
and the synchronization of events - is an important element in seismology,
clocks are always part of a seismograph system.

A single seismograph pendulum works in only one direction, and cannot give a complete picture of
wave motions from other directions. To overcome this problem, modern seismograph stations have
three separate instruments to record horizontal waves - (1) one to record the north-south waves,
(2) another to record east-west waves, and (3) a vertical one in which a weight resting on a spring
tends to stand still and record vertical ground motions. The spring-suspended mass lags behind the
motion caused by the earthquake, making the pen record the waves on the drum. This combination of
instruments tells a seismologist the general direction of the seismic wave source, the magnitude at
its source, and the character of the wave motion. Instruments at other stations must be used to get a
precise fix on the earthquake's epicenter.

As explained earlier, an earthquake generates a series of waves that penetrate the entire Earth, and travel at
and through its surface. Each wave has a characteristic velocity and mode of travel.
They are quite complex, but a few basic facts will explain how they travel through the Earth and how
an earthquake's epicenter  can be determined from seismograph records.

Locating Earthquakes

There are four basic types of seismic waves; two preliminary body waves that travel through the Earth,
and two that travel only at the surface (L waves). Combinations, reflections, and diffractions produce an
infinity of other types, but body waves are the main interest in this discussion.

Body waves are composed of two principal types; the P (primary) wave, comparable to sound waves,
which compresses and dilates the rock as it travels forward through the Earth; and the S (secondary) wave,
which shakes the rock sideways as it advances at barely more than half the P-wave speed. 

P and S Waves

The P wave is designated the primary preliminary wave because it is the first to arrive at a seismic station
after an earthquake. It travels at a speed usually less than 6 kilometers per second in the Earth's crust and
up to 13 kilometers per second through the core.

The S wave is the secondary preliminary wave to be recorded. It follows paths through the
Earth quite similar to those of the P-wave paths, except that no consistent evidence has yet been found
that the S wave penetrates the Earth's core.

The lines labeled P, S, and L in the curves shown on Figure 2 represent the travel time required
for each phase at distances of 0 to 1300 kilometers from the earthquake's epicenter.
They mark the points on the record at which these waves first arrive at the station.

Time-Travel Curves

Figure 2.

Travel-time curves with idealized seismograms (earthquake records superimposed).

Use a Travel-Time Curve Chart
to Approximate the Distance to an Earthquake

The simplest method of locating an earthquake on a globe is to find the time interval between the P-wave
and S-wave arrivals at several seismograph stations. The distance to the earthquake from each station
is then determined from standard travel-time tables and travel-time curves.

To find the distance to the earthquake from a particular station, look at the event's seismogram and find the time
difference between the P wave and S-wave arrivals.  A rough approximation of this distance can be rapidly made
using graphical methods and a travel-time curve chart, such as the one shown below: click on it to open a larger
image, then save and print it out, or work directly on the on-screen image.
1.  measure the distance on the curve's left (vertical) axis that equals the P-S wave arrival time difference
(hint: use a piece of blank paper placed next to the travel-time curve chart's time scale; make two
marks along one edge of the  paper, at "0", and at the value of the
seismogram's P-S wave arrival time difference).
2.  Slide the paper (keeping the edge vertical) upward and to the right along the P and S curves until the
separation between the two curves lines up with the two marks you just made.  At that point, drop
vertically down the chart and read the distance to the earthquake epicenter.

 Travel Time Curve


Another method of locating an earthquake is to use the P-wave arrival-time minus origin-time (P - O) interval
instead of distance. This method is more common because the time can be taken directly from surface
focus travel-time tables assuming an origin of 00:00 hours. This method, however, requires that travel-time tables
be available for various depths of focus. For locating a deep shock, one 700 kilometers deep, for example,
travel-time tables and travel-time curves for that depth have to be used to calculate the origin time and distances.

Other wave types can be generated inside the Earth by P and S waves, as shown in Figure 3. As many as five different
wave groups or phases can emerge when a P or S wave encounters a discontinuity or interface within the Earth.

Seismic Waves 1

Seismic Waves 2

Figure 3.

Propagation paths of combinations of P, S, and L waves from an earthquake focus


More to come ...

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