Modern earthquakes as a key to understanding those of the past: the intensity attenuation curve speaks about earthquake depth and magnitude

Abstract

Abstract. The Italian historical earthquake record is among the richest worldwide; as such it allows the development of advanced techniques for retrieving quantitative information by calibration with recent earthquakes. Building on a pilot elaboration of northern Italy earthquakes, we developed a procedure for determining the focal depth of all Italian earthquakes from intensity data alone. In a second step the procedure calculates their magnitude, taking into account the inferred depth. Focal depth exhibits a substantial variability countrywide, but so far received little attention: pre-instrumental earthquakes were routinely "flattened" at upper crustal level (~10 km), on the grounds that the calculation of focal depth is heavily dependent on the largely unknown local propagation properties. We gathered a learning set of 42 earthquakes documented by reliable instrumental data and by numerous intensity observations. Rather unexpectedly we observe (1) that within 50 km from the epicenter the ground motion attenuation rate is primarily controlled by focal depth and largely independent of magnitude; (2) that within this distance the fluctuations of crustal attenuation properties are negligible countrywide; and (3) that knowing both the depth and the expected epicentral intensity makes it possible to estimate a reliable magnitude.

Constraining earthquake fault sources through the use of intensity data and seismic scenarios: application to the Betic Cordillera (South Spain)

“…The most early scales were based on the effects of an earthquake on people, structures and nature alike, while recently developed scales focus more either on damage to buildings (usually considering several different structural types of building stock), such as the EMS-98; or on environmental and geological effects, as the ESI-07 (Guerrieri and Vittori, 2007). The 20th and 21st centuries have seen plethora of authors using intensity fields to estimate different earthquake parameters, such as magnitude (e.g., Nuttli, 1973;Toppozada, 1975;Nuttli et al, 1979;Ambraseys, 1985;Johnston, 1996;Johnston and Schweig, 1996;Bakun and Wentworth, 1997), epicenter location (e.g., Bakun and Wentworth, 1997;Gasperini et al, 1999;Gasperini et al, 2010), rupture size (e.g., Evernden, 1975;Gasperini et al, 1999;Gasperini et al, 2010;Canora et al, 2021) or hypocentral depth (e.g., Evernden, 1975;Sbarra et al, 2022;Sbarra et al, 2019).…”