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. 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 figure. 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.

An earthquake generates a series of waves that penetrate the entire Earth and travel at and through its surface. Each wave has a characteristic time: each has its own move 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.

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.

Figure 2. Travel-time curves with idealized seismograms (earthquake records superimposed).

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 jumps 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.

The simplest method of locating an earthquake on a globe is to find the time interval between the P- 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. Great-circle arcs are drawn on the globe using the distance of the earthquake to the station as a radius. All the arcs should intersect at a common point - the epicenter.

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 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.

Figure 3. Propagation paths of combinations of P, S, and L waves from an earthquake focus.

Abridged from Earthquake Information Bulletin. Vol. 2, No. 5, September - October, 1970.

Analog Seismometers

Old seismometers were all analog. Analog instruments are called "analog" because the analog signal is converted into digital information at the site of data processing. This means that the analog signal must be sent, in this case over phone lines, from each station to the central site. Each station's signal is then converted from analog to digital by hardware and processed by computers.

Signals from analog stations go off-scale quickly because the electronics and analog phone lines have limited dynamic range. However, each analog station is somewhat simpler, the time stamping of the data is done simultaneously, and the data conversion hardware is at the central site, so the analog stations are somewhat easier to maintain.

Ranger

Photo of the Ranger seismometer

The Ranger seismometer was developed in the late 1950/s for a hard (in excess of about 3kg or 6.6lbs) landing on the Moon. The mass weighs about 1.5 kg (about 3.3 lbs) and is actually a suspended ring magnet.


Wood-Anderson

Photo of the Wood-Anderson seismometer

Harry Wood and John Anderson developed the Wood-Anderson seismometer in the 1920's to record local earthquakes in southern California. It photographically records the horizontal motion.


Viking 75

Photo of the Viking 75 seismometer

This is a model of the seismometer that was first developed to be placed on the surface of mars in 1976 during the Viking Mars Mission. It records shaking in all three dimensions.


Digital Seismometers

Digital stations, on the other hand, have high and low gain sensors and do their data conversion at the sensing site itself with 24 bit digitizers, thus allowing both small and large signals to stay on scale. The digital information is then sent via digital data link to the central site where it is able to be used immediately by the computers processing and storing the data.

Using digital stations instead of analog stations provides several important benefits:

The K2 Strong Motion Accelerograph

Image of a K2 accelerometer and recorder
Click on image to view a larger version.

The K2 data logger with force balanced accelerometers, the seismometer, and communications equipment that are typically used.


Image of a K2 installation
Click on image to view a larger version.

The K2 as it would be set up at an actual digital site. The metal frame is used to keep things from bumping up against the seismometer while it is in operation. The K2 is crooked in its frame for a specific reason. It must be aligned in a northerly direction so that the North-South and East-West ground motions are indeed what are sensed by the device.


Image of the internal componets of the K2 accelerometer
Click on image to view a larger version.

A closer view of the internals of the K2 accelerograph. The metal bar in the lower left hand corner of the case is the battery holder. The three ground motion sensors (for vertical, N-S, and E-W motions) are in the top left hand corner of the device, covered by a protective grille. The data conversion/storage/communications hardware is in the right half of the device. The orange rectangular object to the left is the rechargeable battery which powers the K2 during power shortages.

FBA-23

Photo of the FBA-23 Force Balance Accelerometer

The Force Balance Accelerometer measure the acceleration of the ground as it is shaking during an earthquake. It uses a feedback system in which the output signal from the transducer is amplified and fed back to a device that moves the mass to the original unperturbed position.


STS-1

Photo of the STS-1 seismometer

This electronic seismometer is a state-of-the art instrument. It can accurately record both small local earthquakes and large distant ones. It can also record earth tides and the sonic boom produced by the Space Shuttle as it flies into Edwards Air Force Base.


Episensor

Photo of the Episensor

This instrument is a newer version of the FBA-23. It records ground acceleration much more accurately and with greater sensitivity.


The Central Site Data Acquisition System

image of the Central data acquisition system
Click on image to view a larger version.

This is a picture of the central site data acquisition system. The tower contains the actual data receiving hardware, as well as a computer and data storage system. The large terminal on the table allows system administrators to modify and maintain the system.


A Comparison of Analog and Digital Recordings

Here is an image of an analog recording of an earthquake. The relatively flat lines are periods of quiescence and the large and squiggly line is an earthquake.

Image of a classic analog seismogram

Below is a digital seismogram. The data is stored electronically, easy to access and manipulate, and much more accurate and detailed than the analog recordings.

Image of a Digital seismograph

Earthquake research has assisted engineers in determining better construction and design of retrofitting of homes and buildings that can withstand the shaking that earthquakes generate. Earthquake information, such as location, magnitude, and shaking distribution, is immediately available within minutes after an earthquake to everyone via broadcast media or the internet.