A seismometer is an instrument for measuring earthquakes and other ground motions.

Basic principles

Seismometers are composed of several basic elements:

1. A frame securely affixed to the earth. This may be embedded quite deep in the earth to reduce noise induced by passing surface vehicles or located in quiet regions. A mounting upon solid bedrock is preferable.
2. An inertial mass suspended in the frame by some method, using springs or gravity to establish a steady-state reference position.
3. A damper system to prevent long term oscillations in response to an event.
4. A means of recording the motion of the mass relative to the frame. Early seismometers used motion-amplifying mechanical linkages, while modern instruments use electronic amplification of signals generated by position or motion sensors.

Passing seismic waves will move the frame, while the mass will tend to stay in a fixed position due to its inertia. It is this relative motion that is sensed and recorded.

An early example

The principle can be shown by an early special purpose seismometer. This consisted of a large stationary pendulum, with a stylus on the bottom. As the earth starts to move, the heavy mass of the pendulum has the inertia to stay still in the non-earth frame of reference. The result is that the stylus scratches a pattern corresponding with the earth's movement. This type of strong motion seismometer recorded upon a smoked glass (glass with carbon soot). While not sensitive enough for the detection of distant earthquakes these were useful indicators of the direction of the initial pressure waves and thus an aid in determining the epicenter of a relatively local earthquake — such instruments were useful in the analysis of the 1906 San Francisco earthquake. Further re-analysis was performed in the 1980's using these early recordings.

Modern instruments

Modern instruments use electronic sensors, amplifiers, and recording instruments. They come in two forms: analog and digital. Analog instruments come in three main varieties: short period, long period and broad-band. The short and long period measure velocity and are very sensitive however they 'clip' or go off-scale for ground motion that is strong enough to be felt by people.

The modern broad-band seismometer (so called because of the capacity to record a very broad range of frequencies) consists of a small 'proof mass', confined by electrical forces, driven by sophisticated electronics. As the earth moves, the electronics attempt to hold the mass steady through a feedback circuit. The amount of force necessary to achieve this is then recorded.

Accelerometers output this directly as acceleration (remembering Newton's F=ma), but broad-band seismometers use an integrating circuit to output velocity.

Another type of seismometer is a digital strong-motion seismometer. A strong-motion seismometer measures acceleration and outputs acceleration; this can be mathematically integrated later to give velocity. One drawback of strong-motion seismometers is they are not as sensitive to ground motions as an analog instrument but they do stay on scale during the strongest of seismic shaking.

Locating an event

Pressure and shear waves

An earthquake induces two types of waves in the ground - a pressure wave (called the P wave) and a number of shear waves (called S waves). The P wave can be likened to a sound wave, alternately compressing and rarifying (by a small amount) the density of rock, while the S wave is more akin to the ripples on a still pond when a stone is dropped into it. These two waves travel at different velocities (the P wave is much faster). The P wave can also travel directly, deep in the earth, while the S wave is a surface wave. With appropriate adjustments the difference in time of arrival of these two waves at a seismic observatory can be used to determine the distance to the event. Observations from at least three stations may be used to determine the event location.

Before the advent of precise electronic clocks and modern high-speed electronic communications this was the principal method of determining the location of distant events.

Interconnected seismometers

Seismometers spaced in an array can also be used to precisely locate, in three dimensions, the source of the earthquake, using the time it takes for seismic waves to propagate away from the epicenter, the point of fault rupture. Interconnected seismometers are also used to detect underground nuclear test explosions.

See also

• Earthquake
• Seismology
• Plate tectonics

External Links

• The history of early seismometers
• A Java code applet demonstrating the operation of a damped-mass seismometer