ALBERT

Description: Remarkably, the existing multi-hazard risk models that assess the physical vulnerability of exposed assets lack validation and do not offer the disaggregated effects per hazard scenario. We track that shortcoming while relying on the assumption that locally calibrated single-hazard vulnerability models available in the literature can be reused for multi-hazard risk assessment for building portfolios. This is done through a holistic method that probabilistically harmonises such single-hazard fragility models and allows us to assess the differential and cumulated damage that are expected from residential building stocks affected by cascading hazards. This is a modular approach that is composed of: (1) exposure models that classify the buildings into individual sets of classes for each hazard; (2) their spatial aggregation onto optimal geographical units whose variable resolution is compatible with the variability of the hazard intensities; (3) the probabilistic compatibilities between such sets of building classes and the damage states within their fragility models; (4) the use of state-dependent fragility functions for the second hazard. We test this methodology on the residential building stock of Lima (Peru), a coastal mega-city. Damage distributions and direct economic loss estimates are calculated for six independent mega-thrust-earthquake scenarios (main-shock) ranging from Mw 8.5 to 9.0. Thereby, the resultant distribution of damage states after the first earthquake is probabilistically converted to the reference frame of the tsunami and is later used to assess the cumulative damage using state-dependent fragility functions. This setting allows disaggregating the direct economic losses of each individual hazard to the entire cascading sequence.

Description: Multi-hazard risk assessments for building portfolios exposed to earthquake shaking followed by a tsunami are usually based on empirical vulnerability models calibrated on post-event surveys of damaged buildings. The applicability of these models cannot easily be extrapolated to other regions of larger/smaller events. Moreover, the quantitative evaluation of the damages related to each of the hazards type (disaggregation) is impossible. To investigate cumulative damage on extended building portfolios, this study proposes an alternative and modular method to probabilistically integrate sets of single-hazard vulnerability models that are being constantly developed and calibrated by experts from various research fields to be used within a multi-risk context. This method is based on the proposal of state-dependent fragility functions for the triggered hazard to account for the pre-existing damage, and the harmonisation of building classes and damage states through their taxonomic characterization, which is transversal to any hazard-dependent vulnerability. This modular assemblage also allows us to separate the economic losses expected for each scenario on building portfolios subjected to cascading hazards. We demonstrate its application by assessing the economic losses expected for the residential building stock of Lima, Peru, a megacity commonly exposed to consecutive earthquake and tsunami scenarios. We show the importance of accounting for damage accumulation on extended building portfolios while observing a dependency between the earthquake magnitude and the losses derived for each hazard scenario. For the commonly exposed residential building stock of Lima exposed to both perils, we find that classical tsunami empirical fragility functions lead to an underestimation of predicted losses for lower magnitudes (Mw) and large overestimations for larger Mw events in comparison to our state-dependent models and cumulative damage method.