GNGTS 2018 - 37° Convegno Nazionale

38 GNGTS 2018 S essione 1.1 First comparison between the first polarities focal mechanism and moment tensor solutions. First motion focal solutions are based on the polarity of P-wave first motion: errors in first-motion observations may occur because of station polarity reversal or incorrect direct P-arrival picks due to low signal-to-noise ratio.Methods based onwaveform inversion (fromhigh quality data) provide stable and reliable focal mechanism: in addition, it is clear the speed and efficiency of these methods in respect to the extremely time-consuming method of determining the mechanism by reading polarities of first motions. However, these two different methods are not directly compatible. The first motion data describe only the early part of the source which may or may not be representative of the whole source process for an earthquake. In contrast, the moment tensor solution is derived from filtering of the whole wave train and so represents an average source process, therefore a comparison of the two methods should be made (Anderson, 1988). As first observations, agreement of the first motion and the moment tensor solution is good as a whole, very good in some regions as in Montecilfone (CB, southern Italy), where the main earthquakes of the 2018 were caused by strike- slip mechanisms (Fig. 1). At the same time there are many solutions that differ from the moment tensor often for the presence of minor strike-slip component. It is possible that difference does not originate from uncertainty of the solutions but represents real difference in source process. References Anderson H.; 1988: Comparison of centroid-moment tensor and first motion solutions for western Mediterranean earthquakes Physics of the Earth and Planetary Interiors, 52, 1—7 Elsevier Science Publishers. Lahr, J.C.; 1999, revised 2012: HYPOELLIPSE: a computer program for determining local earthquake hypocentral parameters, magnitude, and first-motion pattern U.S. Geological Survey Open-File Report 99–23, version 1.1, 119 p. and software, available at https://pubs.usgs.gov/of/1999/ofr-99-0023/. Pondrelli S., Salimbeni S., Morelli A., Ekström G., Postpischl L., Vannucci G. and Boschi E.; 2011: European- Mediterranean Regional Centroid Moment Tensor Catalog: solutions for 2005-2008 , Phys. Earth Planet. Int., 185(3), 74-81. Reasenberg, P.A. and Oppenheimer, D.; 1985: FPFIT, FPPLOT and FPPAGE: FORTRAN Computer Programs for Calculating and Displaying Earthquake Fault-Plane Solutions. US Geological Survey Open-File Report 85- 739, 109 p. Scognamiglio, L., Tinti, E., Michelini A .; 2009: Real-Time Determination of Seismic Moment Tensor for the Italian Region , Bulletin of the Seismological Society of America, 99(4):2223-2242, DOI: 10.1785/0120080104. Fig. 1 - An example: solutions available for the 2018 Montecilfone (South Italy) seismic sequence (black solutions: FPFIT; blue solutions: TDMT; red solutions: RCMT.