Rupture Characteristics of Large (Mw ≥ 7.0) Megathrust Earthquakes from 1990-2014

Abstract:

Seismic wave radiation from megathrust earthquakes provides an important probe of fault zone properties and rupture attributes. Depth-varying rupture characteristics of short-period seismic radiation for earthquakes along megathrusts have been inferred from several recent giant earthquakes and large tsunami earthquakes. To quantify any depth-dependent variations more extensively, we performed systematic analysis of 112 M_w ≥ 7.0 thrust-faulting earthquakes on circum-Pacific megathrust faults using teleseismic body wave finite-fault inversions and source spectrum determinations. Large tsunami earthquakes and some other shallow events at depths less than about 18 km have unusually long source durations, low static stress drops and strongly depleted short-period radiation. Deeper events have no clear trend with source depth for moment-normalized centroid duration, static stress drop, moment-scaled total radiated energy, apparent stress, or radiation efficiency. Scaling with seismic moment supports self-similarity for several source characteristics for the intermediate-size earthquake population, albeit with substantial scatter. However, the source spectra have high-frequency spectral decay slopes averaging ~-1.5, rather than -2, the expected value for a standard ω^-2 model. This may result from compound nature of megathrust ruptures with varying scale lengths of heterogeneity, but is also influenced by attenuation assumptions. There is a systematic increase in short-period spectral levels manifested in reduced high-frequency spectral decay slope with increasing depth. The ratio of high-frequency (0.3-1Hz) radiated energy to total energy increases correspondingly. Correlation of higher estimated average megathrust temperature at 30 km depth with higher spectral decay rate indicates that the depth-varying pattern may result from frictional properties being influenced by temperature variations or from a systematic reduction of average attenuation with increasing depth along the megathrust.