Coulomb Stress Change Sensitivity due to Variability in Mainshock Source Models and Receiving Fault Parameters: A Case Study of the 2010–2011 Christchurch, New Zealand, Earthquakes

Strong aftershocks following major earthquakes present significant challenges for infrastructure recovery as well as for emergency rescue efforts. A tragic instance of this is the 22 February 2011 Mw 6.3 Christchurch aftershock in New Zealand, which caused more than 100 deaths while the 2010 Mw 7.1 Canterbury mainshock did not cause a single fatality (Figure 1). Therefore, substantial efforts have been directed toward understanding the generation mechanisms of aftershocks as well as mitigating hazards due to aftershocks. Among these efforts are the prediction of strong aftershocks, earthquake early warning, and aftershock probability assessment. Zhang et al. (1999) reported a successful case of strong aftershock prediction with precursory data such as changes in seismicity pattern, variation of b-value, and geomagnetic anomalies. However, official reports of such successful predictions in geophysical journals are extremely rare, implying that deterministic prediction of potentially damaging aftershocks is not necessarily more scientifically feasible than prediction of mainshocks

Phase Behavior of Hydrocarbon Fluids in Shale Systems from Molecular Simulation

Production from shale reservoirs is getting more attention from the oil industry. However, the shale is not understood as well as conventional reservoirs. One complexity is the unclear fluid phase behavior in shale nanopores. Since the knowledge of the hydrocarbon phase behavior is fundamental for the petroleum reservoir simulation, the phase behavior in shale reservoirs has received significant attention in recent years. Since hydrocarbon fluids are stored inside nanopores of shale matrices, a great interaction exists among the pore boundary and fluid molecules. Thus, fluid phase behavior in a shale reservoir is substantially different from conventional behavior. Due to this interaction, the fluid molecules are distributed heterogeneously inside the nanopores and the phase diagrams are shifted under confinement. Molecular simulation techniques have been developed in previous studies for an advanced performance in the phase behavior description under confinement. In this work, a new molecular simulation method, gauge-GCMC, is presented to study the phase behavior of multiple component fluids considering the confinement effect. This method is verified by matching the phase diagrams of simple and complex hydrocarbons with theoretical results and the simulation data from other techniques. The simulation results show that the density differences between vapor and liquid phases are reduced while critical densities increase under confinement. Also, the confined phase behavior has a great change in the fluid compositions, because heavier components have a stronger adsorption effect than that of lighter components. Shale rocks usually have a wide pore size distribution (PSD) and the traditional single pore-size models are not accurate enough to represent a real shale system. To understand the PSD effect on the phase behavior, the gauge-GCMC is used to generate phase diagrams based on two types of cylindrical models (single pore and multiple pores, including one based on Eagle Ford shale rock). The simulation results show that with an increasing pore size, the phase equilibrium properties approach the bulk values. In addition, the small pore causes a stronger shift in the phase diagram, compared with large pores. The small pores are filled before the large ones, which means that liquid condensation will first happen in the small pores. In the Eagle Ford test, it is possible to use a single pore model with a 10 nm diameter to represent the phase diagrams of this complex pore system. To investigate the contribution from boundary material on the fluid phase behavior, two types of pore models (slit and cylinder), which are built from three materials (two inorganic minerals and one kerogen), are used to generate the phase diagrams of pure fluids (Cv1 and Cv3) and one ternary fluid (Cv1/Cv3/nCv5). Under confinement, liquid densities are reduced while vapor densities are increased in both pore models. Critical points are shifted to lower densities. For the ternary case, a large shift of the nCv5 composition is shown in the vapor phase ternary diagrams, while only small changes have been observed in the liquid composition. When the temperature increases to one typical shale condition, phase separation of the ternary fluid is available in slit pore tests, while only one phase is formed in tests of cylinder pores. Based on the comparison of all results, the cylinder pore, which has more adsorption surface area, can provide a stronger adsorption effect than the slit pore. The calcite models have a greater confinement effect on fluid properties, and the other two materials cause the similar shift effect on phase diagrams

Stress changes on major faults caused by 2013 Lushan earthquake and its relationship with 2008 Wenchuan earthquake

On April 20, 2013, an M s7.0 earthquake occurred in Ya'an-Lushan region, Sichuan Province, China, killing and injuring more than one thousand people. Therefore, it is critical to outline the areas with potential aftershocks before reconstruction and re-settlement as to avoid future disasters. Based on the elastic dislocation theory and multi-layered lithospheric model, we calculate the co- and post-seismic stress changes caused by the Wenchuan and Lushan earthquakes to discuss the relationship between M w7.9 Wenchuan earthquake and M s7.0 Lushan earthquake, the influences on the distribution of aftershock caused by the Lushan earthquake, and the stress changes on major faults in this region. It is shown that the Coulomb failure stress increment on the hypocenter of Lushan earthquake caused by the Wenchuan earthquake is about 0.0037-0.0113 MPa. And the possible maximum value (0.0113 MPa) is larger than the threshold of stress triggering. Therefore, the occurrence of Lushan earthquake is probably effectively promoted by the Wenchuan earthquake. The aftershock distribution is well explained by the co-seismic stress changes of Lushan earthquake. By the two ends of the rupture of Lushan earthquake with increased Coulomb failure stress, a lack of aftershock recordings indicates the high seismic hazard. The stress accumulation and corresponding seismic hazard on the Kangding-Dafu segment of the Xinshuihe fault, the Beichuan-Yingxiu fault, the Pengxian-Guanxian fault, and the Ya'an fault are further increased by the Lushan earthquake and post-seismic process of Wenchuan earthquake.8 page(s

The co-seismic Coulomb stress change and expected seismicity rate caused by 14 April 2010 Ms=7.1 Yushu, China, earthquake

A moderate and widely felt earthquake of Ms7.1 (Mw6.9) occurred on April 14, 2010 in Yushu district of Qinghai province (China) which destroyed a large number of buildings and killed more than two thousand people. The distribution of the aftershock sequence is sparse, but with relatively large magnitudes (Ms6.3 and Ms5.7 aftershocks were recorded), strong aftershocks are still highly potential to occur in the near future. Therefore, it is critical to delineate areas where potential aftershocks could occur. Based on static stress triggering theory, we calculated the coseismic stress changes induced by the mainshock in Yushu County and its surrounding regions by adopting elastic dislocation theory and a multilayered crustal model. According to the rate- and state-variable friction law, we calculated the expected seismicity rate and probability of occurrences of M ≥ 5.0 earthquake in the next ten years. It is observed that the Coulomb stress changes increase apparently on the segment of the Wudaoliang-Qumalai Fault, the Batang-Luoxu and Dangjiang-Longbao segments of the Ganzi-Yushu Fault and the whole Wulanwula Lake-Yushu Fault. Based on the theory of Coulomb stress triggering, the probability of earthquake occurrence on these faults would increase in the near future. Considering the historical seismicity, the strain accumulation and the Coulomb stress changes, the probability of M ≥ 5 earthquake and M ≥ 6 earthquake occurrence in the entire study area in the next 10 years are 79–87% and 26.7–33.6%, respectively. Actually, a moderate earthquake of Ms5.3 occurred to the northwest of the Nangqian County on June 26, 2011