Estimation of Temporal Probability in Offshore Geohazards Assessment

Abstract

Abstract Risk assessment for sea floor installations exposed to offshore geohazards requires an estimation of the occurrence probability of a hazardous event during a reference time period, for example the annual probability of the installation being impacted by a submarine mass gravity flow. The paper describes the procedures developed in recent offshore geohazards studies for the estimation of the temporal probability in two situations:When there is a clear trigger for initiating the slide that would develop into a mass gravity flow andWhen there is evidence of slide activity, but no obvious trigger for slide initiation. In the first situation, the assessment of temporal probability requires a probabilistic description of the frequency and intensity of the trigger(s) releasing the submarine slide, a probabilistic model for calculating the response of the slope to the trigger, and a probabilistic model for evaluating the runout of the released mass gravity flow. Using these models, the probability of a mass gravity flow impacting the seafloor installation(s) should then be computed for all relevant scenarios and return periods in order to derive the annual or lifetime probability. However, analyzing all possible scenarios and return periods could be time-consuming and impractical. The paper presents a simplified procedure and demonstrates its application through a case study for earthquake-triggered slides. In the second situation, where it is difficult to identify the trigger(s) initiating the slide, one must rely on the identification and dating of recent (in the geological sense) slide events in the area. The dating results and other relevant geological evidence are then integrated into a Bayesian framework to establish the annual probability of slope instability. The paper also presents the theoretical framework and example calculations for this situation.