Hobbies And Interests
Vibration Detectors
A seismograph̵7;s detectors produce electronic signals in response to tiny vibrations in the ground. The detectors are all mounted on a heavy concrete slab in direct contact with the ground; this prevents small motions from being lost in flooring and other vibration-damping materials. The detector may consist of a mass on a spring connected to an electronic pickup; earthquake vibrations, transmitted through the ground, set the mass in motion, producing an electronic signal. A seismograph typically has three detectors to pick up east-west and north-south ̶0;S̶1; earthquake waves and up-down compressive ̶0;P̶1; waves.
Electronic Amplifier
A cable connected to the detectors carries the signals to an amplifier system; the signals, being weak and noisy, require amplification and processing. In addition to boosting the signal, a set of filters removes frequencies other than those that indicate earthquake activity -- this helps the seismometer screen out vibrations from people walking in the hall and trucks passing in the street.
Paper Drum
A seismometer̵7;s drum recorder produces a graphical tracing of the signals picked up in the detector and processed by the electronics. A slowly-turning metal drum has a piece of chart paper attached; as the drum moves, a pen records earthquake movements on the paper. When the drum has made a complete turn, a technician changes the paper and files the finished chart.
Computer System
Computerized seismometers use a computer system in place of the paper chart recorder. The computer converts the amplified signals into digital data and stores it onto a hard drive. Software on the computer displays a variety of charts from the stored information; a scientist can see charts from any date recorded on the hard drive.
Combining Seismographs
Although a single seismograph provides useful information, scientists combine readings from multiple seismographs in different geographic regions to determine the exact location and timing of earthquakes. The P and S waves travel through the earth at known speeds, so a few seismometers will pick up the waves early, then others will detect signals later, making it possible to backtrack and find the earthquake̵7;s exact position.
Seismic Signatures
A scientist experienced in reading seismographs can distinguish between different types of earthquakes as well as nearby rockslides, the movement of people, animals and vehicles, volcanic activity and the blast from nuclear weapons. The seismograph of these events has a characteristic ̶0;signature̶1; composed of the shapes and timing of recorded waves. An earthquake, for example, is a complex event, similar to wood fracturing in slow motion; the seismograph records several signals from the earthquake as the underground faults move. By contrast, an atomic bomb test consists largely of a single blast, its vibrations moving uniformly outward in all directions.