Seismographs are the principal instruments of the researchers who study earthquakes and those who map the Earth's inside. The Earth's motion during an earthquake is measured in terms of its movement relative to some object that remains independent of the ground motion. In a seismograph, this object is a seismometer which consists of a mass suspended on springs within a case. During an earthquake the mass remains still while the case around it moves with the ground.
As it moves, the instrument records diplacements of case relative to the mass as they change with time. The record is known as a seismogram.
Each seismogram (LHT, LHR and LRZ) shows something different.
L means "long period" which typically refers to a sampling rate of one measurement of Earth displacement per second.H means "high gain" which means the instrument is not intended to be placed in the neighborhood of active faults.
|***transverse (or perpendicular) Earth displacement||***radial or along the direction of the propagation of the wave||***vertical to the direction of the wave|
Below is a map of IRIS/IDA (Incorporated Research Institutions for Seismolgy/ International Deployment of Acceleromters) seismograph stations across the globe. IDA/IRIS is just one example of a global network of stations sharing data.
*The symbols in GREEN represent IDA stations equipped with La Coste gravimeters.
*The symbols in RED show the location of IRIS GSN Broadband Stations.
*The BLUE symbol marks the location of an independent station contributing data via the IDA Data Collection Center.
*The ORANGE symbols represent stations which are currently under construction and will soon be part of Project IDA.
The graph below shows seismographs from three stations located at different distances from an earthquake. It is obvious that seismic waves take longer to arrive at stations that are farther away. By dividing the change in distance by the change in time, which is equal to the slope of the line connecting arrivals, one can determine the average velocity of a wave. Variations in the type of rock through which the waves travel are illustrated by variations in slope, thus variations in velocity.