Strong ground motion characteristics of 2016 Central Italy earthquakes and implications for ground motion modeling

The 2016 Central Italy earthquake sequence produced three mainshocks: (1) M6.1 24 August, (2) M5.9 26 October, and (3) M6.5 30 October. Each mainshock was followed by many aftershocks, some of which with M > 5.0. All earthquake events occurred on southeast-northwest trending normal faults. As part of reconnaissance activities of these events performed by the Geotechnical Extreme Events Reconnaissance Association (GEER), ground motion data was processed and analyzed. After processing all data using procedures developed during the latest Next Generation Attenuation (NGA-West2) project, we analyze strong motion characteristics of all three mainshocks, two selected large aftershocks (M5.3 24 August and M4.8 26 August) and a foreshock (M5.4 26 October). Our analysis shows that stations near the hanging wall, exhibit fling-step in some cases but no obvious rupture directivity effects. We compare ground motion intensity measures (including peak ground acceleration and velocity, PGA and PGV, respectively) to Italy-specific and global ground motion models. Overall, the data exhibit fast attenuation at large distance (>100 km), which is captured by Italy-adjusted global models, but not by Italy-specific models. We also found that global models tend to over-predict ground motions at short periods. Both features were also observed from the 2009 L’Aquila earthquake data and may represent regional features. We estimate the spatial distribution of PGA for the three mainshocks by means of a Kriging analysis performed on within-event residuals using a global semi-variogram model. We found that the ground motion is most intense south-west of the Mt.Vettore-Mt.Bove normal fault. Given the importance of Italian normal fault earthquakes in worldwide ground motion databases, this data set is of global significance for studies of normal fault ground motions.