Source inversion of regional intensity patterns of five earthquakes from south-western Norway

An intensity-based source-inversion technique, that earlier was applied only to largerearthquakes, has been tested on the basis of regional macroseismic intensity patternsfrom four recent earthquakes in south-western Norway for which high-qualityinstrumental data have provided independent assessments of magnitudes, locationsand fault-plane solutions. In spite of the modest magnitudes of the earthquakes (ML4.0-5.2) and the asymmetry of the sampled area due to the proximity to the coast, thetest was satisfactory enough in that the inversion results for three out of the four eventsstudied came close to the independent solutions from instrumental data. In addition,we inverted the intensity data from a 1954, ML4.5, earthquake from the same regionfor which independent instrumental data are lacking, and found results that areconsistent with our present understanding of the seismotectonics in this region. Themethod is based on the inversion of a kinematic function that represents the groundmotion, at a given point on the surface, from a line source. Since the problem is non-linear and possibly also bimodal, we used a sharing Niching Genetic Algorithm toperform the inversion. The inversion includes a number of source parametres, the mostsensitive of which were the hypocentral coordinates and the fault-plane parametres.The parametres obtained for the south-western Norway earthquakes studied here weregenerally stable except for the epicentral longitude, due to the partial lack of datatowards the offshore (North Sea) region to the west. The present study has shown thatit is possible to extend this type of inversion of earthquake intensity data to lowermagnitudes, sometimes even in cases when the sampling from the felt area isasymmetric. As such, the results achieved encourage the use of this method also forhistorical, pre-instrumental earthquakes also from low-seismicity regions. This opensthe way for significant advances based on historical data that earlier could be usedonly for a broader assessment of earthquake locations and magnitudes.