An overview of the global variability in radiated energy and apparent stress

A global study of radiated seismic energies E R and apparent stresses T a reveals systematic patterns. Earthquakes with the highest apparent stress occur in regions of intense deformation and rupture strong lithosphere. In oceanic settings, these are strike-slip earthquakes (T a up to 27 MPa) occurring intraplate or at evolving ends of transform faults. At subduction zones and intracontinental settings, these are strike-slip earthquakes with T a up to 7 MPa. Normal-fault earthquakes exhibit a more complex pattern. Higher T a 's (up to 5 MPa) are found for intraslab events at depths from 35 to 70 km that occur near zones of intense deformation such as a sharp slab bend or the juncture of colliding slabs. Lower T a 's (< 1 MPa) are found for normal-fault earthquakes at the outer rise and outer trench wall or deep in flat warm slabs. The lowest average τ a (0.3 MPa) is found for thrust-fault earthquakes at subduction zones. The variation of average apparent stress with tectonics suggests a relationship with lithospheric strength and fault maturity. Mature faults, such as plate boundaries that have experienced large cumulative slip, appear to have low strength and tend to yield earthquakes with low apparent stresses. Immature faults, in contrast, are stronger and yield high apparent stresses because either they are the result of fresh-rock fracture or at least their cumulative fault slip is quite small. These results have implications of use to the seismic engineering community because E R and its magnitude counterpart M e are reliable indicators of the potential for damaging ground motion.