BROAD-BAND MODELING ALONG A REGIONAL SHIELD PATH, HARVARD RECORDING OF THE SAGUENAY EARTHQUAKE

The Saguenay earthquake, 1988 November 25, is one of the first large shield type events recorded by a broadband-high dynamic range instrument, the Streckeisen system, installed at Harvard station (HRV). The event is sufficiently large to be well recorded teleseismically and thus the source characteristics can be determined by independent means and considered known. This allows a detailed study of the propagational effects along this path, at an epicentral distance of 625 km, where the strengths of the surface waves can be compared with the crustal body phases. Broadband modelling using standard analytical techniques and flat layered models works amazingly well over the period range of 0.5-20s. A detailed strategy for modelling broadband regional phases is given in terms of a decomposition of the vertical and radial seismograms into three segments: P(nl) (containing P(n), pP(n), sP(n), P(m)P, P coupled PL-waves); S(nl) (containing S(n), sS(n), S(m)S, etc.); and the fundamental Rayleigh waves. Information about the upper crust is obtained from the fundamental Rayleigh waves while crustal thickness and velocity gradients in the mantle are obtained from P(nl) and S(nl). This particular crustal model has a thickness of 35 km with a sharp Moho and a substantial gradient in the top 20 km of the mantle, 0.01 km s-1 per km for both P and S velocities. The mantle velocities, alpha = 8.2 and beta = 4.55 km s-1 are slower than expected for a shield environment. Attenuation is not required for waveform modelling or for absolute amplitude estimation.