Macroseismic estimation of earthquake parameters

The derivation of earthquake parameters from macroseismic (intensity) data is an inveterate problem. Yet for earthquakes in the pre-instrumental period (roughly, before 1900) intensity data points (IDPs) are the only form of numerical data available to the seismologist. In order to produce a numerate, consistent catalogue of historical earthquakes that can be combined in a compatible way with modern instrumental data requires some system for estimating what instrumental parameters would have been obtained had seismometers been in operation. Successive catalogue authors have had to deal with this problem as they saw fit; but as most earthquake catalogues have been compiled as national initiatives, one finds that one type of method has been used in one country, something else in another, and so on. This leads to obvious problems of inconsistency when it comes to studies that need to transcend national borders. A major aim of the NA-4 module of the European Framework project NERIES is to produce a catalogue of European earthquakes before 1900 in which there is the greatest possible level of internal consistency in the determination of earthquake parameters. This means the use of uniform procedures for determining earthquake parameters over the whole of Europe. Finding suitable procedures that can be used for this is a difficult task, and is the subject of this report. The parameters to be determined are essentially the location and the size of each earthquake. Precisely how one defines location in this context is arguable – one speaks of the “macroseismic epicentre”, but this is not necessarily exactly the same as an epicentre in the sense of the surface projection of the point where an earthquake rupture initiates. Where an earthquake rupture is large, while the distribution of high intensities may delineate its extent, there is no possibility to determine the initiating point – and probably not a lot of interest in doing so either. The co-ordinates that will be used will be those of the centre point of the rupture; something approximating to the focus from which the seismic energy radiated. For this reason, the term barycentre is sometimes preferred (Cecić et al 1996). From the point of view of seismic hazard, arguably such a point is of greater interest, in terms of reconstructing the seismic field. Since earthquakes occur in three dimensions, as well as the latitude and longitude co-ordinates, one needs also some sort of depth of focus. This is evidently more meaningful for smaller earthquakes of limited rupture dimensions. “Size” has to be considered here to mean “magnitude”; whereas many earlier historical earthquake catalogues were content to use epicentral intensity, Io, as a size measure, for modern applications, and for consistency with modern data sets, this is not enough, and magnitude, preferably moment magnitude, Mw, has to be estimated.