Source Parameters of Earthquakes in the Xiluodu Reservoir Area, Lower Jinsha River, Southwest China

Abstract. Following impoundment of the Xiluodu Reservoir, lower Jinsha River, southwest China, in May 2013, seismic activity increased significantly. Two magnitude 5 earthquakes (April 5 and August 17, 2014) generated multiple earthquake clusters with unique seismic characteristics. We used the spectral ratio method to calculate the source parameters of 369 earthquakes with local magnitudes of 2.0–4.3 in the Xiluodu Reservoir area. Stress drops generally ranged from 0.01 MPa to 60 MPa and were weakly positively correlated with magnitude. However, the data showed substantial scatter. The corner frequencies of earthquakes ranged from 2 Hz to 20 Hz. The source dimensions calculated from corner frequencies ranged from 50 m to 600 m and increased with increasing magnitude. Different earthquake clusters exhibited different stress-drop characteristics. For earthquakes of magnitude 2.0–2.9 with focal depths < 5 km, the average stress drop near the dam was significantly lower than that along the two M_S5 seismic belts. For earthquakes of magnitude 3.0–3.9 along the two M_S5 seismic belts, stress drops were significantly lower at focal depths < 5 km than at focal depths > 5 km. Most aftershocks occurring shortly after the M_S5.2 reservoir-triggered earthquake had relatively large stress drops. Our seismic activity analysis, combined with the results of previous studies, revealed complex earthquake generation mechanisms within the Xiluodu Reservoir area. Abundant small earthquakes and microearthquakes with low stress drops near the dam were likely induced by reservoir water infiltration, which increased the surrounding pore pressure and activated multiple unstable fractures and structural instabilities. Conversely, concealed faults already existed in the vicinity of the two seismic belts and background tectonic stress was relatively high. Thus, increased pore pressure promoted earthquake generation and facilitated greater stress release, resulting in medium to high stress drops.