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Aftershock Risk in Japan Following Tohoku Earthquake Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 10NCEE Anchorage, Alaska AFTERSHOCK RISK IN JAPAN FOLLOWING TOHOKU EARTHQUAKE Nilesh Shome1 and Chesley Williams2 ABSTRACT The recent Tohoku Japan earthquake clearly demonstrated that aftershocks following large magnitude earthquakes can be a source of significant hazard. In this study, we have followed the Reasenberg and Jones (1989) approach to describe the aftershock activity based on: 1) Gutenberg-Richter model (GRM) specifying the relationship between the magnitude and total number of earthquakes in any region and time period, and 2) modified Omori model (MOM) defining the decay of the aftershock activity with time. The aftershock occurrence is represented by a non-stationary Poisson process whose rate varies with time after the main shock. Since the Tohoku earthquake occurred in a seismically active region, it is essentially impossible to differentiate aftershocks from background earthquakes in the catalog. Hence this study fitted the MOM and GRM to all the events in the catalog in order to estimate the post-Tohoku seismicity rates. We have used the JMA catalog with magnitude larger than or equal to M3.5 in order to estimate the seismicity that may cause damage to the built environment. The Method of Maximum Likelihood is used to estimate the parameters in MOM and GRM. The pre-Tohoku seismicity is calculated based on the observed seismicity since 1983 and that for post-Tohoku is based on the seismicity for one year following the Tohoku event. In this study, we consider only the change in the seismicity in the background sources. The change in seismicity rates in the shallow crustal faults, deep background sources, or the subduction sources are not considered in this study. In order to find out the change in the seismic risk in Japan after the Tohoku earthquake, we have estimated the average annual loss (AAL) for a well distributed building portfolio in Japan. We have calculated the AAL for each year from 2013 to 2017 in order to illustrate the evolution of risk with time following the Tohoku event. There is significant uncertainty in the estimation of the GRM and MOM parameters resulting in uncertainty in the estimation of risk and this is also addressed in this study. In addition, we investigate the suitability of MOM in forward prediction of post-event risks in a regional scale. The observed seismicity in the second year following the Tohoku earthquake is compared with those estimated based only on the first year post-event data. 1Senior Director, Model Development, Risk Management Solutions, Newark, CA 94560 2Senior Project Director, Model Development, Risk Management Solutions, Newark, CA 94560 Shome, N and Williams, C. Aftershock risk in Japan following Tohoku earthquake. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014. DOI: 10.4231/D3GX44V5K Aftershock Risk in Japan Following Tohoku Earthquake Nilesh Shome1 and Chesley Williams2 ABSTRACT The recent Tohoku Japan earthquake clearly demonstrated that aftershocks following large magnitude earthquakes can be a source of significant hazard. In this study, we have followed the Reasenberg and Jones (1989) approach to describe the aftershock activity based on: 1) Gutenberg- Richter model (GRM) specifying the relationship between the magnitude and total number of earthquakes in any region and time period, and 2) modified Omori model (MOM) defining the decay of the aftershock activity with time. The aftershock occurrence is represented by a non- stationary Poisson process whose rate varies with time after the main shock. Since the Tohoku earthquake occurred in a seismically active region, it is essentially impossible to differentiate aftershocks from background earthquakes in the catalog. Hence this study fitted the MOM and GRM to all the events in the catalog in order to estimate the post-Tohoku seismicity rates. We have used the JMA catalog with magnitude larger than or equal to M3.5 in order to estimate the seismicity that may cause damage to the built environment. The Method of Maximum Likelihood is used to estimate the parameters in MOM and GRM. The pre-Tohoku seismicity is calculated based on the observed seismicity since 1983 and that for post-Tohoku is based on the seismicity for one year following the Tohoku event. In this study, we consider only the change in the seismicity in the background sources. The change in seismicity rates in the shallow crustal faults, deep background sources, or the subduction sources are not considered in this study. In order to find out the change in the seismic risk in Japan after the Tohoku earthquake, we have estimated the average annual loss (AAL) for a well distributed building portfolio in Japan. We have calculated the AAL for each year from 2013 to 2017 in order to illustrate the evolution of risk with time following the Tohoku event. There is significant uncertainty in the estimation of the GRM and MOM parameters resulting in uncertainty in the estimation of risk and this is also addressed in this study. In addition, we investigate the suitability of MOM in forward prediction of post-event risks in a regional scale. The observed seismicity in the second year following the Tohoku earthquake is compared with those estimated based only on the first year post-event data. Introduction The Tohoku-oki (henceforth “Tohoku”) earthquake (Mw 9.0) of March 11, 2011, was the largest event in the recent history of Japan. Like any other large magnitude events, large number of aftershocks followed the Tohoku earthquake around the rupture zone. The number of aftershocks, however, has become fewer over the time. This spatial and temporal clustering of earthquakes is an important aspect of aftershocks and is a source of significant increase in hazard in short term (e.g., InsuranceInsight [4] reported up to 50% increase in reported earthquake insurance following the Tohoku event). Temporal clustering, as commonly observed during 1Senior Director, Model Development, Risk Management Solutions, Newark, CA 94560 2Senior Project Director, Model Development, Risk Management Solutions, Newark, CA 94560 Shome, N and Williams, C. Aftershock risk in Japan following Tohoku earthquake. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014. aftershock sequences, constitutes strong evidence for time-dependent behavior of the seismic process. This is a departure from a simple spatially-variable, time-independent Poisson process, which is commonly assumed to model seismicity in earthquake risk calculations. In this paper, we represent the aftershock hazard by looking into increased recurrence rate over the long-term rate. Fusakichi Omori [6], a pioneer Japanese seismologist, based on the observed frequency of felt earthquakes per day, following the 1891 Nobi earthquake (M8) in central Japan, theorized that aftershocks decrease regularly with time and this is known as Omori’s law. Utsu [7] later on updated the law, known as modified Omori Model (MOM) based on observation of additional aftershocks. Lately, Reasenberg and Jones [8] proposed a simple comprehensive model (referred here as RJM) where the productivity is modeled by Gutenberg and Richter model (GRM) for magnitude-frequency relationship of earthquakes and the aftershock rate decay with time by MOM. In this study, we use this model to predict the Tohoku aftershock seismicity rate. A similar model is used by USGS in the Short-Term Earthquake Probability (STEP) model [9] to calculate time-dependent earthquake hazard for clustering of earthquakes. This model is no longer in use because of difficulty in prediction of spatial decay of aftershock seismicity rates, particularly for main shocks on faults. This is, however, not an issue in this study since we are estimating the spatial decay based on the observation of earthquakes during the first year after the Tohoku earthquake. There are other approaches for predicting aftershock seismicity rates, which are the stochastic Epidemic Type Aftershock Sequence model (ETAS) [10] and a physical approach proposed by Dieterich[11] based on the rate- and state-model of fault friction. Researchers also explored static stress change to provide explanation for seismicity rate increase in the aftershock zone. It is found [12] that about 60% of earthquakes following large magnitude earthquakes (M>7) within the zone of positive stress change follow MOM and the rest are associated with stress decrease decaying much faster than those predicted by MOM. All these approaches are expected to predict the seismicity rate, which is different from the approach presented here. Recently Risk Management Solutions (RMS) researched to find out the change in earthquake occurrence rates in the Tohoku region due to static stress changes [2]. In this study, RMS chose 13 well accepted finite fault slip models and the rate change is calculated based on the static stress changes on the receiver sources for each of those models. It is found that the model-to- model variability in stress change is high (coefficient of variation > 1) and the mean recurrence rate change of earthquakes is expected to be even higher. The approach presented in this paper is followed for updating the Japan Earthquake Model of Risk Management Solutions (RMS) to take into account the increased seismicity from the Tohoku earthquake [13]. Seismologists
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