Global patterns of radiated seismic energy and apparent stress
Abstract
Radiated energies from shallow earthquakes with magnitudes ≥5.8 that occurred between 1986 and 1991 are used to examine global patterns of energy release and apparent stress. In contrast to traditional methods which have relied upon empirical formulas, these energies are computed through direct spectral analysis of broadband seismic waveforms. Velocitysquared spectra of body waves are integrated after they have been corrected for effects arising from depth phases, frequencydependent attenuation, and focal mechanism. The least squares regression fit of energy E_{s} to surface wave magnitude M_{s} for a global set of 397 earthquakes yields log E_{s} = 4.4 + 1.5M_{s}, which implies that the GutenbergRichter relationship overestimates the energies of earthquakes. The least squares fit between E_{s} and seismic moment M_{0} is given by the relationship E_{s} = 1.6 × 10^{5} M_{0}, which yields 0.47 MPa as the average global value of apparent stress. However, the regression lines of both E_{s}M_{s} and E_{s}M_{0} yield poor empirical predictors for the actual energy radiated by any given earthquake; the scatter of data is more than an order of magnitude about each of the regression lines. On the other hand, global variations between E_{s} and M_{o}, while large, are not random. When subsets of E_{s}M_{0} are plotted as a function of seismic region and faulting type, the scatter in data is substantially reduced. The E_{s}M_{0} fits for many seismic regions and tectonic environments are very distinctive, and a characteristic apparent stress τ_{c} can be derived. The lowest apparent stresses (<1.5 MPa) are associated with thrust earthquakes at subduction zones. The highest apparent stresses (>3.0 MPa) are associated with strikeslip earthquakes that occur at oceanic ridgeridge transforms and in intraplate environments seaward of island arcs. Intermediate values of apparent stress (1.5 < τ_{a} < 3.0 MPa) are associated with strikeslip earthquakes at incipient or transitional plate boundaries. In general, the dominant mode of failure for a tectonic environment is associated with the faulting type that has the lowest apparent stress. An energy magnitude M_{E} can complement moment magnitude M_{w} in describing the size of an earthquake. M_{E}, being derived from velocity power spectra, is a measure of seismic potential for damage. M_{w}, being derived from the lowfrequency asymptote of displacement spectra, is more physically related to the final static displacement of an earthquake. When earthquake size is ranked by moment, a list of the largest events is dominated by earthquakes with thrust mechanisms. When earthquake size is ranked by energy, the list of the largest events is dominated by strikeslip earthquakes.
 Publication:

Journal of Geophysical Research
 Pub Date:
 September 1995
 DOI:
 10.1029/95JB01969
 Bibcode:
 1995JGR...10018205C
 Keywords:

 Seismology: Earthquake parameters;
 Seismology: Earthquake dynamics and mechanics;
 Seismology: Seismicity and seismotectonics;
 Tectonophysics: Stressescrust and lithosphere