Chi-Chia Tang

Seismic and Geologic Evidence of Water‐Induced Earthquakes in the Three Gorges Reservoir Region of China

Earthquakes induced by reservoir impoundment pose great risk to property and lives worldwide, but studies demonstrating relationships between reservoir water levels, specific faults, and the local geology are rare. Here we show that the 2013 M 5.1 Badong Earthquake, the largest earthquake so far in the Three Gorges Reservoir (TGR) region of China, occurred at a shallow depth on a right-lateral south-dipping strike-slip fault. The fault is at least 15 km long and intercepts the TGR at the NS-running Shennongxi River. We find that the earthquake and its foreshocks and aftershocks are confined to a fractured Triassic carbonate formation that crops out in the reservoir. The precise locations of earthquakes coupled with the local geology suggest that the sequence was induced by high pore pressure due to reservoir water infiltration in a specific rock type. The newly identified fault has the potential to generate earthquakes of magnitude approaching the designed seismic intensity limit of the Three Gorges Dam. Plain Language Summary Earthquakes induced by reservoir impoundment pose great risk to property and lives worldwide, but studies demonstrating relationships between reservoir water levels, specific faults, and the local geology are rare. In this study, we determined focal mechanism and depth of the 2013 M 5.1 Badong Earthquake, the largest earthquake so far in the Three Gorges Reservoir (TGR) region of China. We also obtained high-precision locations of 205 events in this earthquake sequence. The results show that these earthquakes occurred at shallow depths between 1 and 5 km on a steep south-dipping fault. The fault is at least 15 km long and intercepts TGR at the NS-running Shennongxi River. A field survey indicates that the earthquakes are confined to a fractured rock formation that crops out in the reservoir. The precise locations of earthquakes coupled with the local geology suggest that they were induced by high pore pressure due to reservoir water infiltration. The newly identified fault is a seismic risk to the Three Gorges Dam. The findings are important because (1) they highlight the importance of understanding the detailed local hydrologic system in risk assessment, and (2) the fact that the seismogenic fault was missed before demonstrates the limitations of preconstruction risk-assessment surveys.

Empirical Mw–ML, mb, and Ms Conversions in Western ChinaEmpirical Mw–ML, mb, and Ms Conversions in Western China

Local magnitude ( M L), body‐wave magnitude ( m b), and surface‐wave magnitude ( M s), which are saturated at certain values and may lead to an incorrect energy estimation of a large earthquake, are largely used in quantifying the size of an earthquake in western China. Based on the catalog from the China Earthquake Data Center (CEDC) and moment magnitude ( M w) provided by the Global Centroid Moment Tensor (Global CMT) Project, we test the general orthogonal regression (GOR) and the ordinary least‐squares (OLS) methods in M w– M L, M s, and m b conversions for earthquakes in three different tectonic structures of western China. For M L and m b, linear trends vary according to tectonic structures, which implies different body‐wave attenuations in the three structures. For M s, the results are similar, whether using the GOR or the OLS method, but slopes of regression lines are not close to 1, which indicates routine misestimates of M s in western China. The GOR slopes are uniformly larger than the OLS slopes in all magnitude conversions. Standard deviations are between 0.06 and 0.13 for the GOR method but are between 0.12 and 0.25 for the OLS. Thus, the GOR is found to be superior to the OLS method and its use is recommended. Conversions of different magnitudes to M w not only imply different patterns of seismic‐wave attenuations but will also benefit immediate assessment of seismic damage after occurrence of a destructive earthquake in the future. Online Material: Earthquake catalog and tables showing differences between magnitude estimates for M w, M L, m b, and M s.

EmpiricalMw–ML,mb, andMsConversions in Western China

Local magnitude ( M L), body‐wave magnitude ( m b), and surface‐wave magnitude ( M s), which are saturated at certain values and may lead to an incorrect energy estimation of a large earthquake, are largely used in quantifying the size of an earthquake in western China. Based on the catalog from the China Earthquake Data Center (CEDC) and moment magnitude ( M w) provided by the Global Centroid Moment Tensor (Global CMT) Project, we test the general orthogonal regression (GOR) and the ordinary least‐squares (OLS) methods in M w– M L, M s, and m b conversions for earthquakes in three different tectonic structures of western China. For M L and m b, linear trends vary according to tectonic structures, which implies different body‐wave attenuations in the three structures. For M s, the results are similar, whether using the GOR or the OLS method, but slopes of regression lines are not close to 1, which indicates routine misestimates of M s in western China. The GOR slopes are uniformly larger than the OLS slopes in all magnitude conversions. Standard deviations are between 0.06 and 0.13 for the GOR method but are between 0.12 and 0.25 for the OLS. Thus, the GOR is found to be superior to the OLS method and its use is recommended. Conversions of different magnitudes to M w not only imply different patterns of seismic‐wave attenuations but will also benefit immediate assessment of seismic damage after occurrence of a destructive earthquake in the future. Online Material: Earthquake catalog and tables showing differences between magnitude estimates for M w, M L, m b, and M s.