How do seismologists measure earthquakes?
In the United States, large-scale seismological networks are generally run by federal agencies (such as the USGS, the Bureau of Reclamation, and the Department of Energy), by state agencies, and by public and private universities. These networks are designed to monitor earthquake activity and to provide data for research into earth science problems. The Council of the National Seismic System is an organization of institutions involved in seismic monitoring with the goal of coordinating efforts to record and analyse seismic data.
For example, UC Berkeley operates a seismic network in northern and central California for the purposes of monitoring seismic activity and furthering earthquake research. The seismographic instrumentation used in the Berkeley Digital Seismic Network (BDSN) includes broadband seismometers to sense weak ground motions and accelerometers to sense strong ground motions. Both types of sensors utilize force-feedback circuitry to determine the overall response, linearity, stability, and dynamic range of the sensors. UC Berkeley operates several different broadband seismometers and accelerometers in order to cover the widest range of frequencies and ground motions:
The primary limitations at the low-frequency end of the spectrum are the seismic background noise level, the instrumental noise level, and the thermal stability of the sensor and the data logger. At the best BDSN sites, the semi-diurnal gravitational tide signal is readily apparent in the raw data while at the noisier sites it is not resolvable.
The primary limitations at the high-frequency end of the spectrum are the digital sampling rate, the instrumental noise level, and the attenuation and scattering of the surface weathered layer (the upper ~100 meters of the crust). The best BDSN stations, with sensors installed in boreholes, typically see 200+ Hz signals generated by local earthquakes. The best BDSN stations, with sensors installed on the surface, typically do not register significant energy at frequencies above ~30 Hz.
(periods of 4+ hours) | |
(frequency band which usually causes structural damage during strong ground shaking) |
The capabilities of the modern generation of seismic instrumentation was driven by the needs of pure research and made possible by the advent of large scale integrated circuit technology. As our knowledge of earth structure and our ability to model the earth at higher frequencies improves, accurate recordings at yet higher frequencies will become useful. At lower frequencies, primarily associated with secular deformation of the earth's crust, data are provided by continuously operating Global Positioning System (GPS) receivers. The UC Berkeley Seismographic Station is collaborating with a number of agencies in northern California to form the Bay Area Regional Deformation (BARD) Network to monitor crustal deformation as well as seismic activity. Many of the BARD GPS receivers are co-located with BDSN instrumentation.
Information on the installation of the seismic instrumentation is available from the BDSN Installation Guide. You can also find information on making your own seismometer. Finally, if you are interested in seeing what an earthquake looks like, here are some examples of earthquakes recorded by the BDSN.