Typical Event Magnitudes

This section discusses typical event spectra for various earthquakes at local, regional and teleseismic distances.

Local, Regional and Teleseismic Studies

Teleseismic events are those recorded at great distances, with the threshhold variously defined as 30° or 1000 km from the source. The anelasticity of the earth results in physical and temporal dispersion which means that surface waves tend to only be detectable at lower frequencies, from 1 mHz to 1 Hz. At even lower frequencies, free oscillations of the earth occur at 0.3 mHz to 5 mHz. In general lower frequencies allow deeper structures in the earth to be probed. Both of these spectral bands require the lowest possible noise floors at low frequencies.
In contrast, near-source or local seismic events are those recorded within several multiples of the length of the initial rupture, or typically within tens of kilometers of the source. Near-source events reveal detailed structure of the rupture and localized magnification effects. Amplitudes of ground motion can be much higher, and frequencies are typically from 0.1 to 100 Hz with most of the energy concentrated between 1 and 10 Hz. Regional events are those detected at distances between local and teleseismic from the source.
The spectra of typical events of various magnitude at local, regional and teleseismic distances were published by Clinton and Heaton (2002). The spectra presented there represent median peak amplitudes in octave bandwidths; for events of a given size and epicentral distance, 50% will exceed the amplitude given while 50% will fall short.
Some of these curves are shown in the following figure. The shift towards lower amplitudes and freqeuncies as the event is recorded at greater distances from the source is evident.


Site Seismicity

The location of the site with respect to tectonic plate boundaries is the main factor which determines the likelihood of seeing an event of a particular magnitude in a particular time period. There are many good sources for seismicity assessment, e.g. http://www.seismo.ethz.ch/GSHAP/. Since seismic hazard maps commonly plot contours of acceleration levels for which there is a 10% chance of exceedance in 50 years, they are appropriate for determining risk to humans but not for instruments, unless it is precisely the largest and rarest events which you intend to study.