Effect of depth-dependent shear modulus on tsunami generation along subduction zones

Estimates of the initial size of tsunamis generated by subduction zone eart hquakes are significantly affected by the choice of shear modulus at shallo w depths. Analysis of over 360 circum-Pacific subduction zone earthquakes i ndicates that for a given seismic moment, source duration increases signifi cantly with decreasing depth (Bilek and Lay, 1998; 1999). Under the assumpt ion that stress drop is constant, the increase of source duration is explai ned by a 5-fold reduction of shear modulus from depths of 20 km to 5 km. Th is much lower value of shear modulus at shallow depths in comparison to sta ndard earth models has the effect of increasing the amount of slip estimate d from seismic moment determinations, thereby increasing tsunami amplitude. The effect of using depth dependent shear modulus values is tested by mode ling the tsunami from the 1992 Nicaraguan tsunami earthquake using a previo usly determined moment distribution (Ihmle, 1996a). We find that the tide g auge record of this tsunami is well matched by synthetics created using the depth dependent shear modulus and moment distribution. Because excitation of seismic waves also depends on elastic heterogeneity, it is important, pa rticularly for the inversion of short period waves, that a consistent seism ic/tsunami shear modulus model be used for calculating slip distributions.