GNGTS 2018 - 37° Convegno Nazionale

72 GNGTS 2018 S essione 1.1 Intensity regression with distance and comparison with previous events was tested through a GIS-based approach. In order to calibrate a distance-based attenuation equation, we analyzed a calibration dataset of published macroseismic fields for large instrumental subduction earthquakes between Ecuador and Chile (Mw = 7.7 - 8.3). For each earthquake, first we digitized in a GIS environment the instrumental epicentral location and the macroseismic field and then, following the approach proposed by Bakun and Wentworth (1997), we investigated the epicentral distance of each isoseismal area. Since the distribution and the amount of data are not comparable among all the events (i.e., number and density of IDPs for each subset), we adopted a modified version of the procedure originally proposed by Bakun andWentworth (1997), using the published isoseismals as input data instead of the originally listed IDPs. Then, we calculated distance statistics of the sampled points belonging to each isoseismal (i.e., median, average and standard deviation) and, as suggested by Bakun and Wentworth (1997), we used the median of distance to calculate an attenuation equation. Median values have the considerable advantage to automatically smooth the effect of outliers (i.e., sites showing site amplification phenomena). We used a linear regression for attenuation relationship, as in Bakun and Wentworth (1997). Results. We found systematic difference in the assessed earthquake intensity between ESI- 07 and traditional scales and in the intensity attenuation with distance. A comparison of our dataset with macroseismic data of six, instrumental, large subduction earthquakes occurred between Ecuador and Chile, suggests that for the Pedernales earthquake the ESI-07 works better in the near field whereas damage-based scales are more reliable in the far field. Fig. 2 - Macroseismic fields for the 2016 Pedernales earthquake (Mw 7.8): a) Regional map of the EMS-98 IDPs; b) Isoseismal map for EMS-98 scale; c) Isoseismal map for ESI-07 scale. Legend: ESM Esmeraldas, MUI Muisne, MOM Mompiche, PED Pedernales, SIS San Isidro, CAN Canoa, BDB Boca da Briceño, TOS Tosagua, POR Portoviejo, JIP Jipijapa.