SYNTHETIC ISOSEISMALS OF 3 EARTHQUAKES IN CALIFORNIA AND NEVADA

Recent tests on a series of earthquakes in California and Nevada sugge st that in some regions the approximate shapes of the territories with equal earthquake-induced damage (expressed in terms of macroseismic i ntensity) could be synthetically traced out with a simple formula. Thi s formula takes into account some gross features of the source: depth and length, unilateral or bilateral rupture, radiation patterns, ruptu re velocity, and directivity. Having been formulated on an empirical b asis, the formula is however compatible with the so-called asymptotic approach, in which the far-field component of the Green's function is used. This paper presents the synthetic isoseismals of the earthquakes at Cedar Mountain, Nevada, 1932; Fairview Peak-Dixie Valley, Nevada, 1954; and Coalinga, California, 1983. An overall consistency, from acc eptable to remarkable, between the observed intensity patterns and the synthetically back-predicted intensity has been obtained for them. Wh ere the detailed modelling techniques available today are inapplicable , due to insufficient information on the features of the seismic sourc es, or to save time and money, the new formula may be utilizable for i mproving seismic hazard calculations. The formula was also used invers ely for back-predicting geometric-kinematic parameters of the Coalinga 1983 earthquake from macroseismic maps. This gave characteristics for its source which are in good agreement with the majority of data infe rred from modelling and from analyzing modern instrumental recordings. This striking result opens new perspectives in retrieving information on the source of ancient earthquakes for which only macroseismic info rmation is available. (C) 1997 Elsevier Science Limited.