Local and Duration Magnitudes in Northwestern Italy, and Seismic Moment Versus Magnitude Relationships

Abstract

In the present work, we develop some local magnitude scales for northwestern Italy based on vertical short-period records. This study is motivated by the possibility of applying the computed scales to an instrumental catalog of more than 25,000 local earthquakes, as this region has been continuously monitored by 12 short-period vertical-component (1c) stations since the mid-1980s. Furthermore, a digital network of three-component (3c) broadband or 5 second sensors has monitored northwestern Italy since 1996. Today, a significant number of earthquakes have been simultaneously recorded by both networks, allowing the calibration of the 1c local scale by using magnitudes computed according to a scale derived for the 3c digital network. Moreover, because station Sant’ Anna di Valdieri houses both a 3c (code stv2) and 1c (code stv) sensors, the magnitude scales for the two networks can be developed using the same reference station. The magnitude scale M L = log A + log( R /100) + 0.0054( R − 100) + 3 − S is derived for the 3c digital network with the requirement that the correction S of station stv2 is zero. This scale is based on 10,057 maximum amplitudes (2822 earthquakes) computed from horizontal synthesized Wood-Anderson seismograms, in the hypocentral distance 10 to 310 km and in the range 0 ≤ M L ≤ 5. With respect to an carlier magnitude scale derived for the 3c network constraining the sum of all the station corrections to zero, the magnitudes predicted by the previous equations show an average bias of (−0.2 ± 0.1), which can be ascribed to the different constraint applied to the station corrections. The magnitudes predicted by the scale for the 3c network are used to calibrate magnitude scales based on either total duration or maximum amplitude from synthesized Wood-Anderson seismograms computed for each short-period vertical recording. The magnitude scale obtained considering maximum amplitudes from vertical short-period recordings is M L = log A + log( R /100) + 0.0041 ( R − 100) + 3 − S ′. The reliability of the obtained magnitude scales is assessed using 827 earthquakes different from those we considered in the regression analysis. Finally, the following seismic moment versus local magnitude relations are valid in the western Alps in the range 0 where M L3C is the local magnitude computed starting from the horizontal component of broadband (flat frequency response, from 0.033 to 50 Hz) or semibroadband (flat frequency response, from 0.2 to 40 Hz) sensors and M L1C is the magnitude computed starting from the vertical short-period recordings. [1]: /embed/graphic-1.gif