Abstract
Spatial variations in underthrusting earthquake seismic magnitude differentials (mb − Mw) are examined for plate boundary megathrusts in the vicinity of the 26 December 2004 Sumatra-Andaman (Mw 9.2), 2010 Maule, Chile (Mw 8.8), and 11 March 2011 Tohoku, Japan (Mw 9.1) great earthquakes. The magnitude differentials, corrected for ω-squared source spectrum dependence on seismic moment, provide a first-order probe of spatial variations of frequency-dependent seismic radiation. This is motivated by observations that the three great earthquakes all have coherent short-period radiation from the down-dip portions of their ruptures as imaged through back-projections, but little coherent short-period energy from shallower regions where large coseismic slip occurred. While there is substantial scatter in the magnitude measures, all three regions display some increase in relative strength of short-period seismic waves with depth, with the pattern being strongest for Sumatra and Japan where the deeper portion of the seismogenic zone is below the overriding crust. Other regions such as the Kuril Islands, Aleutians, Peru, and Southern Sumatra/Sumba show little, if any, depth pattern in the magnitude differentials. Variation in material and frictional properties over particularly wide seismogenic megathrusts likely produce the depth-dependence observed in both mb−Mw residuals and great earthquake seismic radiation.
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