Seismic Risk Analysis AP 5000 Note

Seismic Risk Analysis AP 5000 Note Some data used in this study belongs to Geopower Basel AG. Data and concepts published in this study may not be further used without written approval of Geopower Basel AG. SERIANEX AP 5000 Seismic Risk Analysis AP 5000 AP 5000 Report Report title: AP5000 Report - Seismic Hazard and Risk assessments during three reference time periods (normal, stimulation and circulation) Author(s): Ramon Secanell David Carbon François Dunand Christophe Martin Checked by: Christophe Martin Approved by: Bertrand Grellet Company: GEOTER S.A.S Pôle Géoenvironnement 3, rue Jean Monnet 34830 Clapiers France Version: Final Report – Rev. 1 Ref.: Report GTR/CBAL/1009-698 Report date: 15th October 2009 SERIANEX AP 5000 - 1/124 Seismic Risk Analysis AP 5000 EXECUTIVE SUMMARY The region of Basel is an area with high population density that has suffered damages caused by tectonic earthquakes during the past centuries. The objective of the work package AP5000 was to develop a probabilistic seismic risk assessment, intended to quantify the additional risk generated by the operation of the geothermal field. The seismic risk is first quantified in a normal period, during which only the seismicity of tectonic origin is taken into account. Next, the risk is estimated by considering two different phases of the geothermal field exploitation: a) the stimulation phase, which covers a short time period characterized by a high rate of hydraulic fracturation and induced seismicity, and b) the circulation phase, which corresponds to the lifespan of the field, during which induced and triggered seismicity are considered. For each time period, a probabilistic seismic hazard assessment (PSHA) was conducted with the objective of defining the hazard curves, in a region covering a radius of 12 km around the site. The predicted parameter of the seismic motion is the macroseismic intensity, because the vulnerability curves are defined as a function of the intensity. The PSHA method is consistent with the state-of-the-art: the uncertainties are propagated in the model by means of logical trees and Monte Carlo samplings. The seismic hazard, in a normal period, is coherent with previous studies published in the literature. A macroseismic intensity of VII-VIII has a return period of 475 years (90% probability of non-exceedance in 50 years). To consider the seismic activity during the stimulation period, two models stemming from the AP3000 workpackage are taken into account. The first one relies on empirical considerations to determine the maximum magnitude and the activity rate. It can be qualified as the more optimistic model, with the lowest activity rate. The second model is obtained from the AP3000 numerical simulations. It is considered as the more pessimistic model, with activity rate 10 times higher than the activity rate of the empirical model. Two synthetic models are also included in a logic tree to model the induced seismic activity during the circulation period. They differ by the annual rate of activity. Using the outputs from AP4000 workpackage. the impact of the potential triggered seismicity on near regional faults is also considered. The hazard comparison highlights several outcomes. • At macroseismic intensities greater than VI-VII at the site itself, and at intensities greater than V-VI at a distance of 15 km to the site, the seismic activity caused by the geothermal field exploitation has no impact in terms of seismic hazard. Above those intensity thresholds, the seismic hazard is fully controlled by the natural seismic activity (of tectonic origin). Below, the seismic hazard is mainly controlled by the induced activity. SERIANEX AP 5000 - 2/124 Seismic Risk Analysis AP 5000 • The hazard increment is significantly higher during the stimulation period than during the circulation period. On the site, the probability of exceedance of an intensity V is increased by a factor 50 during the stimulation period. During the circulation period, it is only multiplied by a factor 3. The hazard increment decreases with increasing distance to the site, but remains significant even 15 km away. The multiplication factor becomes 7 during the stimulation and is close to 1,3 during the circulation. • The seismic activity rate appears to be the main factor controlling the hazard. Because of very different activity rates associated to the empirical and synthetic models, the variability of the hazard curves is high during the stimulation period. • Compared to the induced seismicity, the triggered seismicity has a negligible impact on the hazard. In order to develop a probabilistic risk assessment at the scale of the urban area, the hazard curves are used in conjunction with vulnerability functions associated with the different building typologies,. The probabilistic risk assessment relies on an evaluation of the probabilities of losses during each time period and on risk scenarios, intended to provide F/N curves. The risk scenarios are obtained for intensities corresponding to various probabilities of exceedance. A deterministic scenario is also performed to appreciate the consequences due to an occurrence of the estimated maximum magnitude. In order to obtain consistent comparison between prediction and observation, the predictive risk model was first calibrated by reproducing the impact of the earthquake of December 8th, 2006. Several conclusions can be obtained from the risk assessment results. • The seismic activity due to the geothermal field exploitation, is not responsible for a human risk increment, but only generates additional financial losses. This is because only damages D1 of the EMS 98 scale are expected. Heavy damages or partial collapse are not expected. • The risk increment during the stimulation is significant. We consider that the most likely loss could be 45 M CHF within the few weeks of the stimulation period. It represents approximately 6 times the loss observed during the earthquake of December 8th, 2006. This amount is approximately 10 times higher than the predicted loss only associated to the seismic risk of natural origin. Considering the stimulation hazard curves associated to the percentiles 15 % and 85 %, that explain a large uncertainty, the financial losses could vary between 35 Millions CHF to 300 Millions CHF. The loss due to the stimulation could represent between 0,017% and 0,02% of the total insured value. • Other uncertainties, such as the uncertainties associated with the vulnerability functions or to the cost function, contribute to enlarge the abovementioned variability between 10 Millions CHF and 640 Millions CHF. • Compared to the stimulation, the risk increment between the circulation and the normal periods is low (multiplication factor of 1.06). However, because the lifespan of the field is 30 years, the absolute losses would be higher during the circulation period SERIANEX AP 5000 - 3/124 Seismic Risk Analysis AP 5000 than during the stimulation period. The corresponding loss increment represents 170 Millions of CHF over 30 years of operation. • In terms of insurance, the additional risk associated to the stimulation period could be covered by an insurance prime of 430 CHF/building. The annual insurance prime to cover the tectonic risk is estimated to be close to 840 CHF/building/year. 50 CHF/building/year would be necessary to cover the risk increment during the circulation period. • Considering the different models of induced seismicity developed within the AP3000 workpackage, the estimated maximum magnitude (3.7±0.4) has a great chance to be bserved during the reservoir operation. In that case, the losses would be superior to the losses observed in 2006. We consider that they could vary between 50 Millions CHF (Mw=3.7) to 160 Millions CHF (Mw=4.1). These values must be balanced using multiplication factor of 0.5 and 2 to account for uncertainties. • The F/N curves resulting from the risk scenarios represent the annual frequency of exceedance of financial losses. They indicate that the highest risk increment would be observed during the first year of operation. During the first year of operation, the potential losses due to the stimulation and the circulation are cumulative. In this case, for an annual rate of exceedance of 1, the risk is approximately 17 times higher than the tectonic risk. The risk increment decreases with the annual rate of exceedance. For annual rates lower than 0,01, the risk increment becomes negligible, because the hazard remains controlled by the natural seismicity. SERIANEX AP 5000 - 4/124 Seismic Risk Analysis AP 5000 CONTENTS 1 Introduction – Scope of the study .................................13 1.1 OBJECTIVES AND SCOPE OF WORKS ................................................................. 13 1.2 REPORT ORGANIZATION .................................................................................. 17 2 Methodology ....................................................................18 2.1 KEY CONSIDERATIONS TO DEVELOP THE METHODOLOGY .................................... 18 2.1.1 Ground motion parameter ................................................................................................ 18 2.1.2 Minimum magnitude ......................................................................................................... 19 2.1.3 Introduction of a near-regional fault model....................................................................... 20 2.2 SPECIFICITIES OF THE HAZARD CALCULATION MODELS FOR EACH REFERENCE PERIOD ........................................................................................................

Load more