Analysis of seismic magnitude differentials (m b−M w) across megathrust faults in the vicinity of recent great earthquakes

Abstract

Spatial variations in underthrusting earthquake seismic magnitude differentials (m b −M w ) are examined for plate boundary megathrusts in the vicinity of the 26 December 2004 Sumatra-Andaman (M w 9.2), 2010 Maule, Chile (M w 8.8), and 11 March 2011 Tohoku, Japan (M w 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 m b −M w residuals and great earthquake seismic radiation.

Depth‐varying rupture properties of subduction zone megathrust faults

“…There are relatively few moderate size events with very shallow depths near the toe of the megathrusts, which is probably a manifestation of the conditionally stable, or completely locked conditions there. Rushing and Lay [2011] consider similar magnitude differential patterns for other regions near large earthquakes without finding great consistency in trends, but few other regions have the exceptionally wide seismogenic zones of the events in Figure 8.…”

“…In order to avoid biases in the m b values for large events with longer than several second rupture durations, we limit the analysis to events with m b 5.0 to 6.5. Even across that magnitude range there is some expected scaling of the 1 Hz energy level relative to seismic moment, and we apply a correction to the magnitude differentials for moment scaling of an ω ‐squared source spectrum with a 3 MPa constant stress drop [ Rushing and Lay , 2011].…”

“…The approximate depth ranges of the domains from Figure 6 are shown on the middle panel. Only events in the range 5 ≤ M w ≤ 6.5 are included to avoid saturation in the m b measurements due to fixed time window [modified from Rushing and Lay , 2011].…”

Subduction zone megathrust earthquakes

“…Tsunami earthquakes, given their shallow slip with limited high-frequency radiation, can be identified by their relatively low m b relative to M w measurement (e.g., Kanamori, 1972;Kanamori and Kikuchi, 1993;Lay et al, 2011a). Rushing and Lay (2012) examined the differences between m b and M w for a large catalog of likely megathrust events with M w ≥ 5 in several subduction zones (Chile, Japan, Sumatra, Kuril, Aleutians, Sumba, Peru). They found positive m b -M w perturbations for deeper megathrust events, reflecting enhanced deep short-period radiation in Japan, northern Sumatra, and central Chile.…”

Frictional properties of sediments entering the Costa Rica subduction zone offshore the Osa Peninsula: implications for fault slip in shallow subduction zones

“…This variation likely reflects spatial variations in frictional properties along the seismogenic portion of plate-boundary megathrusts (e.g., Bilek and Lay 1998;Scholz 1998;Bilek 2007;Wang and He 2008;Rushing and Lay 2012).…”