Study on simulating strong ground motion by fractal stochastic method

Abstract

The time history of strong ground motion can be synthesized by empirical Green's function (EGF) method. Firstly a large seismic event is discretized into a series of subevents; secondly recordings of earthquakes with proper size and spatial distribution are chosen as time history (EGF) of those subevents; finally the EGFs are summated to get the time history of ground motion caused by the large event. It is certain for the regions with abundant strong earthquake records to calculate broadband time histories of strong ground motion, which is useful for engineering, by using EGF method (Hartzell, 1978). But usually there may be no recordings used as EGF. Under this condition, stochastic band-limited white-noise synthetics (Boore, 1983; Schneider et al, 1993; Beresnev and Atkinson, 1997) may be a good choice. That is, we can distribute the causative fault into several sub-faults in a certain rule and regard them as point sources for random waves, then calculate ground motions caused by each sub-sources in the receiving point, and finally get the target ground motion by summation. A large Ms=7.0 earthquake occurred in Lijiang county, Yunnan Province, on February 3, 1996. The focal depth is I0 km, and the area of its meizoseismal region is 225 km 2, and intensity is IX. After the earthquake, 9 aftershocks with Ms>4.0 have been recorded by four macroseismic observation stations arranged around the seismic zone, among them is the greatest aftershock (Ms=6.0) of this earthquake whose time history records are recorded by two stations. The rupture and propagating model of the great Lijiang Ms=6.0 aftershock is set up as a sample test in the present paper. After a fractal subfault distribution based on the finite fault model, sub-source accelerations are simulated by stochastic method and summated to get ground motion of the target event. Because geometrical relations between sub-sources and the receiving point are also taken into consideration, time delay by rupture and propagation of each sub-sources are added into the summation procedure. By doing so, we simulate ground motion of the two stations which recorded the greatest aftershock, they are the Earthquake Administration of Lijiang County (site 1) and the Financial Bureau of Jinshan Village (site 2). Their response spectra are also calculated. Through comparison of simulations and observations of the two sites, combining with field investigations of earthquake damage, differences in earthquake damage of the two sites are thorough studied so as to test the feasibility of the method.