Recalibration of the distance correction term for local magnitude (ML) computations in Italy

• Autores: Barbara Lolli, Paolo Gasperini, Francesco Mariano Mele, Gianfranco Vannucci
• Localización: Seismological Research Letters, ISSN 0895-0695, ISSN-e 1938-2057, Vol. 86, Nº. 5, 2015, págs. 1383-1392
• Idioma: inglés
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• Resumen

  • Even though the most reliable and physically consistent definition of the size of an earthquake is the moment magnitude Mw, small earthquakes located in Italy and in other regions of the world are traditionally calculated using local magnitude ML. Because such magnitude was calibrated using a set of southern California earthquakes, a specific recalibration is required in regions with different attenuation properties. We determine the amplitude attenuation function for Italy using various datasets and different functional forms. We also estimate separate attenuation equations for a subdivision of the Italian area in two regions with different crustal properties.

    ML was defined by Richter (1935) as the decimal logarithm of the ratio between the maximum amplitude A in millimeters on a standard short‐period torsion seismometer (Anderson and Wood, 1925) and the maximum amplitude A0 that the same seismometer would record for a standard earthquake (ML 0): Embedded Image(1) Although Wood–Anderson (WA) seismometers were dismissed by most observatories a couple of decades ago, ML estimates consistent with the original definition can still be computed using synthetic WA (SWA) waveforms obtained from modern seismometers after digital convolution of the recorded waveforms with the WA response function (e.g., Uhrhammer and Collins, 1990).

    The above definition (1) of ML is accompanied by a table, reporting the distance correction term (−logA0) for epicentral distances D ranging from 25 to 600 km (later improved and extended to distances <25  km by Gutenberg and Richter, 1942), which was determined by Richter (1935), by constraining A0=10−3  mm at D=100  km.

    Jennings and Kanamori (1983), using strong‐motion records, found a significant …