SEISMOGRAPHS

 Earthquake engineering Project

Seismographs

 

Definition

•        Seismograph is an instrument that makes a record of seismic waves caused by an earthquake, explosion, or other Earth-shaking phenomena.

Seismographs are equipped with electromagnetic sensors that translate ground motions into electrical changes.

A record produced by a seismograph on a display screen or paper printout is called a seismogram.

 

•        The physics behind the sensor is Newton's Law of Inertia:

"A body in motion tends to stay in motion unless acted upon by a force, and a body at rest tends to remain at rest unless acted upon by another force."

 

•        Although originally designed to locate natural earthquakes, seismographs have many other uses, such as:

o   Petroleum exploration

o   Investigation of Earth’s crust and lower layers

o   Monitoring of volcanic activity

 

•        Basic principles of the modern seismograph

o   Pendulum is free to swing in one direction.

o   If the ground moves back and forth (oscillates) and if the period of ground motion (the time necessary for one complete oscillation) is sufficiently shorter than the period of free oscillation of the pendulum, the pendulum will lag, and the movement of the ground relative to the pendulum can be recorded. The magnitude of that movement is commonly amplified electrically.

o   When the period of the pendulum is comparable to that of the ground motion, the seismograph will not exactly record Earth’s movement. The correction, however, can readily be computed mathematically.

 

•        The ground can move in any of three directions, two horizontal and one vertical. Because each kind of movement must be separately recorded, three pendulums, one for each direction, are needed for a complete seismograph.

 

•        Technological developments in electronics have given rise to higher-precision pendulum seismometers and sensors of ground motion.

In these instruments, the electric voltages produced by motions of the pendulum or the equivalent are passed through electronic circuitry to amplify and digitize the ground motion for more exact readings.

 

•        Seismometers are usually designed to record signals over a specified range of frequencies (or periods)

So, discuss instruments based on the range of vibration frequencies that they can detect.

Thus, one way to characterize seismometers is to describe the range of vibration frequencies that they can detect.

 

•        The broadband instrument senses most frequencies equally well;

•        The long-period and short-period instruments are called "narrow" bands, because they preferentially sense frequencies near 1/(15 s) and 1 hertz respectively.

•        The yellow region is the low end of the frequency range audible to most humans (we can hear waves around 20 hertz to 20,000 hertz).

•        The left panel is a comparison of a modern broadband seismometer response and the classic World-Wide Standard Seismic Network (WWSSN) long- and short-period instruments.

•        The same broad-band response is shown in the right panel, to compare the response with a special short-period instrument, the Wood-Anderson, and an accelerometer.

The Wood-Anderson short-period instrument was the one that Charles Richter used to develop his magnitude scale for southern California.

The accelerometer is an instrument designed to record the large amplitude and high-frequency shaking near large earthquakes.

Those are the vibrations that are important in building, highway, etc. design.

 

•        Seismographs are divided into three types:

o   Short-period

o   Long- (or intermediate-) period

o   Ultralong-period, or broadband, instruments.

 

•        Short-period instruments are used to record P and S body waves with high magnification of the ground motion.

For this purpose, the seismograph response is shaped to peak at a period of about one second or less.

The intermediate-period instruments of the type used by the World-Wide Standardized Seismographic Network had a response maximum of about 20 seconds.

 

•        The results of different recording instruments on the measurements of ground motion (displacement) for an earthquake that occurred in Texas, in 1995.

The observations were recorded on a broad-band instrument and the signals that would have been recorded on the WWSSN instrument types were simulated using a little mathematics since all the vibrations that would be detected by the long- and short-period seismometers are also recorded by the broadband seismometer.

 

•        Ground displacement observed near Tucson, Arizona, caused by an earthquake in southwestern Texas.

The top panel shows the vibrations measured using a broadband seismometer

The middle panel shows the vibrations as they would be detected by the long-period sensor

The bottom panel the vibrations that would be sensed by a short-period sensor (scaled by a factor of 10 so we can see them better)

The displacements are shown in microns, which are 1x10^-6 meters.

 

•        The Press-Ewing seismograph, developed in the United States for recording long-period waves, was widely used throughout the world.

 

•        SMAs, or strong motion accelerographs, are usually all that is required for monitoring the response of a structure during an earthquake, whether this a building, bridge, dam, power station, or any other critical infrastructure that could be affected by a large earthquake.

 

•        Recently, in order to provide as much flexibility as possible for research work, the trend has been toward the operation of very broadband seismographs with a digital representation of the signals. This is usually accomplished with very long-period pendulums and electronic amplifiers that pass signals in the band between 0.005 and 50 hertz.

 

•        Because many strong-motion instruments need to be placed at unattended sites in ordinary buildings for periods of months or years before a strong earthquake occurs, they usually record only when a trigger mechanism is actuated with the onset of ground motion.

 

•        A real-time seismic recording system with digital storage and satellite communications.

Ground vibrations are detected by the sensor, digitally recorded, and then transmitted via satellite.

 

•        Another important class of seismometers was developed for recording large amplitude vibrations that are common within a few tens of kilometers of large earthquakes - these are called strong-motion seismometers.

Strong-motion instruments were designed to record the high accelerations that are particularly important for designing buildings and other structures.

 An example set of accelerations from a large earthquake that occurred in near the coast of Mexico in September of 1985 are shown in the diagram below.

 

•        The ground displacement was recorded at a strong-motion seismometer that was located directly above the part of a fault that ruptured during the 1985 Mw = 8.1, Michaocan, Mexico earthquake.

 

•        The left panel is a plot of the three components of acceleration (one vertical and two horizontal).

From the curve, we can see that strong, high-frequency shaking lasted almost a minute in the region.

The peak acceleration was about 150 cm/s^2. Often, we will report such numbers as a fraction of the gravitational acceleration at Earth's surface, which we call "g" and which is about 980 cm/s^2.

Thus, the peak acceleration in this region was about 150/980 g, or about 0.15g, or equivalently 15% of g.

 

•        One place with which we are all familiar with acceleration changes is an elevator.

The acceleration that you experience in an elevator is about 2 m/s^2 or about 0.2g.

However, in an elevator, the transition from 0g (not moving) to 0.2g is smooth and comfortable.

During the earthquake, you can see that the ground accelerations were varied between -0.1g to +0.1g several times each second, for at least 10-15 seconds.

That is not very gentle shaking.

•        The middle panel shows the velocity of ground movement, which we can calculate using calculus

The velocity is the integral of the acceleration.

The peak velocity for this site during that earthquake was about 20-25 cm/sec.

And if we integrate the velocity, we can compute the displacement, which is shown in the right-most panel.

•        From the displacement plot, we can see that the permanent offsets near the seismometer were up, west, and south, for a total distance of about 125 centimeters.

 

•        Today's global seismic network is a cooperative, international effort that consists of more than 100 seismic recording stations.

From seismograms, we can quickly estimate an earthquake origin time (when the rupture began), location (its depth, latitude, and longitude), and faulting parameters such as strike, dip, and slip directions.

 

•        Locations of seismic recording stations that are part of the Global Seismic Network - a cooperative federation of international seismology organizations that share data.

 

•        After an earthquake we often descend on the epicentral region with portable seismic instruments to carefully and closely monitor aftershock sequences that follow most large earthquakes.

Portable seismic recording systems have been designed for this purpose and they are similar to the permanent stations but often run-on battery and solar power.

The data are retrieved by a scientist who visits the site and downloads the digital signals from a computer hard disk that's part of the portable seismic recording system.

 

•        In addition to their value in monitoring and studying earthquakes and aftershocks, seismograms can be used to probe the internal structure of Earth.

•        To facilitate such studies, we often deploy temporary seismometer networks for several months or years in geographic regions that we want to study more closely.

•        Some recent study areas are Tibet, East Africa, Fiji, Antarctica, and the western United States.

 

References:

o   http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/seismometers.html

o   https://www.britannica.com/science/Press-Ewing-seismograph