A New Approach for Aftershock Hazard Assessment That Takes Into Account Observed Mainshock Demand

Abstract eng:
Aftershock hazard assessment is a critical issue in the post-earthquake safety evaluation of damaged buildings. Unfortunately, misevaluation of aftershock risk claimed many lives in the recent seismic sequences. Aftershock hazard is often assessed by means of probabilistic seismic hazard assessment (PSHA). But unlike the conventional PSHA, the rate of earthquakes (i.e. aftershocks) is considered to decay as a function of time. In this proceeding, a new approach for aftershock hazard assessment is proposed. The novelty of the proposed approach is the consideration of indicators related to the mainshock event in the assessment of aftershock hazard. Specifically, indicators such as instrumental recordings of the mainshock earthquake at the site of interest or the macroseismic observations from the site, are considered in the proposed framework. The fundamental idea behind the Conditional Aftershock Hazard Assessment (CAHA) approach proposed in this proceeding is the estimation of the variability of aftershock ground motion intensity conditioned on observed mainshock intensity at the site of interest. For this purpose, the correlation between the mainshock and the aftershock ground motion intensities exhibited at a set of sites, are investigated. Specifically, the correlation between the epsilon values identified for the mainshock and the aftershock events are considered. The epsilon parameter is defined as the difference between measured and estimated spectral acceleration value divided by the standard deviation representing the total variability. The correlation structure of the epsilons is evaluated by analyzing pairs of recorded mainshock and aftershock ground motion records. Using the identified correlations, the aftershock ground motion variability is updated conditioned on the mainshock indicators based on the principles of probability theory. For the site where aftershock hazard is to be evaluated, the instrumental recording of the mainshock ground motion are very often not available. This is the case, for example, when the site of interest is located away from the strong motion stations. In such cases, the macroseismic indicators are the only observable evidences that can be utilized to infer the intensity of shaking exhibited at the site. The CAHA framework proposed here, enables utilizing such macroseismic observations in order to obtain improved estimates of aftershock hazard. Specifically, the Modified Mercalli Intensity, MMI is used as the macroseismic indicator. This approach is expected to be very useful for regions with inadequate density of strong motion stations. An example application of the proposed approach is presented. The aftershock hazard following the 2011 Van (Turkey) M7.2 earthquake, is considered in this application. The hazard at the site of a strong motion station located in the city of Van, is evaluated using the proposed CAHA method. The hazard curves estimates obtained using the CAHA method are compared against and the curves obtained using conventional methods. Subsequently, the likelihoods predicted for the actual strong motion intensities registered during the aftershocks were evaluated. The results indicate that the improved estimates of hazard could be obtained using the proposed CAHA method, compared to conventional approaches.

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