Task 6 - Gubbio - Deliverable D20: Bedrock shaking scenarios

Abstract

The main goal of Task 6 is the computation of the bedrock and site seismic scenarios in the Gubbio town and in the Gubbio basin (Central Italy). This area represents one of the prediction case studies, planned in the framework of Project S3 which aims at simulating ground shaking scenarios for moderate magnitude earthquakes. Deterministic shaking scenarios, described in this report, are computed for the Maximum Credible Earthquake. This is associated with two sources capable of generating an earthquake with M equal to 6 and located at about 10km from the sites. Starting from these sources bedrock shaking scenarios at different level of complexity are computed by ground motion prediction equations (scenarios of level 0), high frequency (f>0.5Hz) simulations (scenarios of level 1) and broad band (0-20 Hz) simulations (scenarios of level 2). However since many sites have to be simulated for accurately sampling the basin, the bedrock ground motions for evaluating the seismic response of the valley are generated at level 1. For four test sites a comparison between time series computed at different levels of complexity have been performed in order to verify the approximations introduced in level 1. Before computing the predicted shaking scenarios, we evaluate the modelling capability of the adopted simulation techniques reproducing the observed ground motions occurred during the 1984 Gubbio earthquake (M 5.7). Finally we apply the probabilistic-deterministic approach for a characteristic earthquake scenario proposed by Convertito et al. (2006) to perform hazard analysis considering the two seismogenic faults at three sites of interest in the Gubbio area (paragraph 3.3). In this way, respect to the deterministic approach, we produce ‘dynamic shaking scenarios’ introducing the time variable. The shaking scenarios generated in the Gubbio area allow us to show how the prediction of the ground motion can be tackled using different methodologies and which strategy should be followed to select mean shaking scenario. As the available information increases, we could gradually adopt approaches more and more sophisticated and provide shaking scenarios that account for specific effects related to the source and propagation (e.g., directivity, radiation pattern distribution,…). Furthermore, the adoption of different approaches allows a cross check of the results guaranteeing the reliability of the ground motion estimates.