Validation of the Intensity-Based Source Inversions of Three Destructive California Earthquakes

The objective of our work is to invert the intensities of preinstrumentaldestructive earthquakes to retrieve their principal source characteristics. This is rele-vant in countries (as Italy and Greece) where a large and high-quality data bank ofearthquake intensities during the past few centuries exists. We have previously val-idated our technique presented in this journal for theML5.9 1987 Whittier Narrowsearthquake, and here we demonstrate that our algorithm works on three other earth-quakes with known sources. Three more validations of the algorithm are presentedhere by using the site intensities observed by the U.S. Geological Survey for theM6.4 1983 Coalinga, theMS7.1 1989 Loma Prieta, and theMw6.7 1994 Northridgeearthquakes. Our simplifiedKFformula simulates the body-wave radiation from alinear source, and 11 source parameters are retrieved: the three nucleation coordi-nates, the fault-plane solution, the seismic moment, the rupture velocities, the along-strike and antistrike rupture lengths, and the shear-wave velocity in the half-space.To find the minima on the hypersurface of the residuals in the multiparameter modelspace, we use a genetic process with niching (Niching Genetic Algorithm) becausewe have already shown that the problem is bimodal for pure dip-slip mechanisms.The objective function of the nonlinear inversion is the sum of the squared residuals(calculated-minus-observed intensity at all sites). The three tests presented here weresuccessful because the results agreed with the sources already known from instru-mental measurements. A very accurate solution was found for Loma Prieta, notwith-standing the complexity of its source. In the two other cases, sources close to thereference ones were retrieved. The quality of our source determinations are obviouslylower than those of instrumentally based models, but would be highly noteworthyfor earthquakes without seismological recordings. The potential of the “KFinver-sions” seems particularly promising for European countries where seismicity ratesare not very high, and where a lot of information on earthquake damage is availablefor preinstrumental earthquakes. The present series of validations raises our hopethat further information on historic seismic events can be obtained from intensities,thus increasing the knowledge of seismotectonics and seismic hazard.