ALBERT

Description: Quantifying transitions in seismic activity related to wastewater injection is an important step for accurate seismic-hazard assessment. It is also a challenging task due to the uncertainty in the relationship between injections and transitions; however, a consistent statistical analysis of instrumental seismic records allows for detecting and quantifying induced seismicity. In this study, we develop a statistical method for modeling a seismic sequence involving non-stationary-induced seismicity. It is composed of two steps: first, we select a model for the integrated seismicity (i.e., natural and induced) within the framework of the epidemic-type aftershock sequence via the Bayesian Model Comparison section. Second, we perform Bayesian inference within that model to assess the seismic activity and associated parameters. The method is applied to the analysis of the events from Oklahoma, demonstrating that it is able to provide a consistent representation of the occurrence of the dataset. Results show that the overall seismic rate (including mainshock and aftershock events) for events with local magnitudes ( M L ) above 2.5 has been escalated by a factor of more than 100, from 0.05 to more than five events per day, between January 1975 and August 2014. For this overall increase, the contribution of the mainshock events is estimated to be ~56%. Assuming the b -value of the Gutenberg–Richter law is 1.0, the probability of exceeding M L  5.0 in a 2-month period is predicted to have increased from about 0.05 to more than 0.5 during the study period. A sensitivity analysis is presented to show how the probabilistic inference is affected by the assumed b -value and the assumed maximum event magnitude. The proposed method can provide a statistical basis for quantitatively assessing the process of induced seismicity. In addition, it can be employed as a decision-support tool to identify areas with increasing levels of hazard and to guide strategies for risk mitigation.

Description: It is well documented that wastewater disposal by underground injection can increase seismic activity. However, it is challenging to predict whether and when this effect occurs, as it can manifest itself even many years after the beginning of disposal operations. Continuous instrumental monitoring of the seismic activity is a first step enabling early detection of symptoms of change, updating of site and regional risk assessment, and prompt response. To avoid misdetection, however, seismic records have to be rationally analyzed, properly modeling the statistical features of the earthquake occurrence process. In this study, we develop a statistical approach to detect increments in seismic rate, accounting for model uncertainty (which is particularly acute when the monitoring period is short) and interdependence among events. The approach is composed of two steps: (1) stochastic earthquake declustering identifies mainshocks, and (2) the hypothesis of a constant rate of mainshocks is statistically tested. The method is applied to the analysis of the Oklahoma injection region, demonstrating that it is able to detect an increment in seismic rate before the change is large enough to produce severe consequences. We also investigate the statistical power of the method using synthetic data simulated for a wide range of scenarios. We believe that the method can be employed as a preliminary decision-support tool to identify areas with trends in seismicity that require further investigation and to guide adaptive management and operation of disposal wells.

Description: Historical archives of earthquake occurrence provide a millennial view of fault system behavior with precision beyond the capability of radiometric dating techniques. Understanding the long historical record of earthquake activity in China presents a great opportunity to develop such an archive. However, the complex distributed nature of faulting here leads to uncertainty and requires matching historic events to the paleoseismic record from fault excavations. We present paleoseismic evidence for the most recent rupture of the north Danghe Nanshan thrust (NDNT) and correlate the age of this event to nearby historical archives. We use high-resolution topography generated from airborne drone photography to measure an average coseismic fault throw of 0.8±0.2 m. Three trenches excavated across these small scarps show shortening of ~1.3 m, consistent with event magnitude of M w 7.0±0.5 that probably ruptured the entire ~45-km-long northern strand of the eastern NDNT system. Geochronology data and the historical record from Dunhuang and Anxi together indicate that this event likely occurred at A.D. 1289, during the Yuan Dynasty. Electronic Supplement: Description of the test procedure of optically stimulated luminescence (OSL) samples, and figure showing natural OSL decay curve, growth curves, and equivalent dose (De) distributions.

Description: The mantle transition zone (MTZ) structure beneath eastern China is studied by using P -wave receiver functions. The 410 km discontinuity is relatively flat whereas the depth of the 660 km discontinuity varies significantly under the study area. This indicates that the 660 is more influenced by laterally varying temperature. The 520 km discontinuity is continuously observed at ~520 km depth under a limited area. Within the South China Block, the MTZ thickness under the Yangtze Craton is larger than normal whereas that under the Cathaysia Block is nearly normal. This indicates colder temperature affecting the MTZ beneath the Yangtze Craton. Combined with seismic tomography, our result suggests that the stagnant ancient slab beneath eastern China is mainly trapped around the base of the MTZ beneath the Yangtze Craton in the study area.