The San Andreas fault at Parkfield, California, apparently late in an interval between repeating magnitude 6 earthquakes, is yielding to tectonic loading partly by seismic slip concentrated in a relatively sparse distribution of small clusters (<20-meter radius) of microearthquakes. Within these clusters, which account for 63% of the earthquakes in a 1987-92 study interval, virtually identical small earthquakes occurred with a regularity that can be described by the statistical model used previously in forecasting large characteristic earthquakes. Sympathetic occurrence of microearthquakes in nearby clusters was observed within a range of about 200 meters at communication speeds of 10 to 100 centimeters per second. The rate of earthquake occurrence, particularly at depth, increased significantly during the study period, but the fraction of earthquakes that were cluster members decreased.

6This paper gives an overview of the advances made in the field of risk assessment and risk management 7 of geologic CO 2 storage (GCS) since the publication of the IPCC Special Report on Carbon Capture and 8Storage in 2005. Development and operation of a wide range of demonstration projects coupled with 9 development of new regulations for safe injection and storage of CO2 has led to development and 10 deployment of a range of risk assessment approaches. New methods and tools have been developed for 11 quantitative and qualitative risk assessment. These methods have been integrated effectively with 12 monitoring and mitigation techniques and deployed in the field for small-scale field tests as well as 13 large-scale commercial projects. An important development has been improved definition of risks, 14 which can be broadly classed as site performance risks, long-term containment risks, public perception 15 risks and market risks. Considerable experience has now been gained on understanding and managing 16 site performance risks. Targeted research on containment risks and induced seismicity risks has led to 17 improved understanding of parameters and processes influencing these risks as well as identifying key 18 uncertainties that need to be targeted. Finally, significant progress has been made to effectively 19 integrate communication strategies with risk management approaches to increase stakeholder 20 confidence in effectiveness of deployed risk management approaches to manage risks. 21

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Almost 4 million metric tons of CO 2 were injected at the In Salah CO 2 storage site between 2004 and 2011. Storage integrity at the site is provided by a 950-m-thick caprock that sits above the injection interval. This caprock consists of a number of low-permeability units that work together to limit vertical fluid migration. These are grouped into main caprock units, providing the primary seal, and lower caprock units, providing an additional buffer and some secondary storage capacity. Monitoring observations at the site indirectly suggest that pressure, and probably CO 2 , have migrated upward into the lower portion of the caprock. Although there are no indications that the overall storage integrity has been compromised, these observations raise interesting questions about the geomechanical behavior of the system. Several hypotheses have been put forward to explain the measured pressure, seismic, and surface deformation behavior. These include fault leakage, flow through preexisting fractures, and the possibility that injection pressures induced hydraulic fractures. This work evaluates these hypotheses in light of the available data. We suggest that the simplest and most likely explanation for the observations is that a portion of the lower caprock was hydrofractured, although interaction with preexisting fractures may have played a significant role. There are no indications, however, that the overall storage complex has been compromised, and several independent data sets demonstrate that CO 2 is contained in the confinement zone.carbon sequestration | geomechanics

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Explosions near the Earth's surface excite both seismic ground motions and atmospheric overpressure. The energy transferred to the ground and atmosphere from a near-surface explosion depends on yield (W) as well as the height-of-burst/ depth-of-burial (HOB/DOB) for above/belowground emplacements. We report analyses of seismic and overpressure motions from the Humble Redwood series of low-yield, near-surface chemical explosions with the aim of developing quantitative models of energy partitioning and a methodology to estimate W and HOB/DOB. The effects of yield, HOB, and range on amplitudes can be cast into separable functions of range and HOB scaled by yield. We find that displacement of the initial P wave and the integral of the positive overpressure (impulse) are diagnostic of W and HOB with minimal scatter. An empirical model describing the dependence of seismic and air-blast measurements on W, HOB/DOB, and range is determined and model parameters are found by regression. We find seismic amplitudes for explosions of a given yield emplaced at or above the surface are reduced by a factor of 3 relative to fully contained explosions below ground. Air-blast overpressure is reduced more dramatically, with impulse reduced by a factor of 100 for deeply buried explosions relative to surface blasts. Our signal models are used to invert seismic and overpressure measurements for W and HOB and we find good agreement (W errors < 30%, HOB within meters) with groundtruth values for four noncircular validation tests. Although there is a trade-off between W and HOB for a single seismic or overpressure measurement, the use of both measurement types allows us to largely break this trade-off and better constrain W and HOB. However, both models lack resolution of HOB for aboveground explosions.

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S U M M A R YWe present a physically based methodology to predict the range of ground-motion hazard for earthquakes along specific faults or within specific source volumes, and we demonstrate how to incorporate this methodology into probabilistic seismic hazard analyses (PSHA). By 'physically based,' we refer to ground-motion syntheses derived from physics and an understanding of the earthquake process. This approach replaces the aleatory uncertainty that current PSHA studies estimate by regression of empirical parameters with epistemic uncertainty that is expressed by the variability in the physical parameters of the earthquake rupture. Epistemic uncertainty can be reduced by further research. We modelled wave propagation with empirical Green's functions. We applied our methodology to the 1999 September 7 M w = 6.0 Athens earthquake for frequencies between 1 and 20 Hz. We developed constraints on rupture parameters based on prior knowledge of the earthquake rupture process and on sources within the region, and computed a sufficient number of scenario earthquakes to span the full variability of ground motion possible for a magnitude M w = 6.0 earthquake with our approach. We found that: (1) our distribution of synthesized ground motions spans what actually occurred and that the distribution is realistically narrow; (2) one of our source models generates records that match observed time histories well; (3) certain combinations of rupture parameters produced 'extreme,' but not unrealistic ground motions at some stations; (4) the best-fitting rupture models occur in the vicinity of 38.05 • N, 23.60 • W with a centre of rupture near a 12-km depth and have nearly unilateral rupture toward the areas of high damage, which is consistent with independent investigations. We synthesized ground motion in the areas of high damage where strong motion records were not recorded from this earthquake. We also developed a demonstration PSHA for a single magnitude earthquake and for a single source region near Athens. We assumed an average return period of 1000 yr for this magnitude earthquake and synthesized 500 earthquakes distributed throughout the source zone, thereby having simulated a sample catalogue of ground motion for a period of 500 000 yr. We then used the synthesized ground motions rather than traditional attenuation relations for the PSHA.In this paper, we present a physically based methodology to predict a range of ground motions at a particular site that may occur from a particular magnitude earthquake along a specific fault or within a specific source volume, and demonstrate a means to incorporate this into traditional probabilistic seismic hazard analyses (PSHA). The prediction methodology is based upon the work first presented by Hutchings (1991Hutchings ( , 1994 and further developed by . The physical model proposed by the previous studies has been further developed in this study and the methodology expanded to include PSHA. We apply the methodology to the M w = 6.0, 1999 Athens earthquake. The full methodology i...