Rupture Characteristics of Major and Great (MW ≥ 7.0) Megathrust Earthquakes from 1990-2015: Source Scaling and depth-dependence Relationships

Abstract:

Seismic wave radiation from megathrust earthquakes provides an important probe of fault zone properties and interplate rupture attributes. To quantify the source scaling relationships and any depth-dependence more extensively, we analyzed 114 M_w ≥ 7.0 thrust-faulting earthquakes on circum-Pacific megathrusts using the finite-fault inversions and source spectrum determinations. To address the limited resolution of source spatial extent from teleseismic observations, we performed finite-fault inversions with rupture expansion velocity V_r = 2.0, 2.5, and 3.0 km/s for 96 events lacking independent V_r constraints.

Source duration T_d and centroid time shifts T_c vary systematically with the cube root of seismic moment; and there is no strong dependence on M_w for moment-scaled radiated energy E_R/M₀, apparent stress σ_a, or radiation efficiency η_R. The slip-weighted stress drop Δσ_E from finite-fault slip models, with direct trade-off from V_r, has little dependence on M_w. However, the product V_r³Δσ_E is very stable for each event across the suite of models considered, and has little trend with M_W. Radiation efficiency tends to decrease with average slip for large events, and estimates of fracture energy increase steadily with slip.

Large tsunami earthquakes and some other shallow events (depth < ~18 km) have unusually long T_d, low Δσ_E and strongly depleted short-period radiation. Deeper events have no clear trend with source depth for moment-normalized T_c, Δσ_E, E_R/M₀, σ_a, or η_R. There is relative enrichment in short-period spectral levels with increasing depth manifested in reduced high-frequency spectral decay slope. The ratio of high-frequency (0.3-1Hz) radiated energy to total energy increases correspondingly. These observations suggest that overall dynamic rupture processes are relatively insensitive to depth, but varying scale lengths of megathrust heterogeneity may contribute to enrichment of short-period seismic radiation for deep events.