Calibration of Crustal Historical Earthquakes from Intra-Carpathian Region of Romania

Abstract

The main task of the presented study is to elaborate a set of relations of mutual conversion macroseismic intensity - magnitude, necessary for the calibration of the historical crustal earthquakes produced in the Intra - Carpathian region of Romania, as a prerequisite for homogenization of the parametric catalogue of Romanian earthquakes. To achieve the goal, we selected a set of earthquakes for which we have quality macroseismic data and the Mw moment magnitude obtained instrumentally. These seismic events were used to determine the relations between the Mw and the peak/epicentral intensity, the isoseist surface area for I=3, I=4 and I=5: Mw = f (Imax / Io), Mw = f (Imax / Io, h), Mw = f (A3, A4; A5). We investigated several variants of such relationships and combinations, taking into account that the macroseismic data necessary for the re-evaluation of historical earthquakes in the investigated region are available in several forms. Thus, a number of investigations provided various information resulted after revising initial historical data: 1) Intensity data point (IDP) assimilated or not with the epicentre intensity after analysis of the correlation level with recent seismicity data and / or active tectonics / seismotectonics, 2) Sets of intensities obtained in several localities (IDPs) with variable values having maxims that can be considered equal to epicentral intensity (Io), 3) Sets of intensities obtained in several localities (IDPs) but without obvious maximum values, assimilable with the epicentral intensity, 4) maps with isoseismals, 5) Information on the areas in which the investigated earthquake was felt or the area of perceptiveness (e.g. I = 3 EMS during the day and I = 4 EMS at night) or the surfaces corresponding to a certain degree of well-defined intensity. The obtained relationships were validated using a set of earthquakes with instrumental source parameters (localization, depth, Mw). These relationships lead to redundant results meaningful in the process of estimating the quality and credibility of the primary data used (e.g. IDPs, isoseismals) and in the correct determination of Mw.