Earthquakes induced by water injection at ～3 km depth within the Rongchang gas field, Chongqing, China

摘要

Unwanted water, amounting more than 1 million m3, has been injected intermittently at a pumping pressure of 2.1–2.9 MPa (over hydrostatic) at 2.6–2.9 km depth within the Rongchang gas field, western Chongqing, China, since July 1988. The injections have induced more than 32,000 surface-recorded earthquakes, including 2 of M L ≥ 5, 14 of M L ≥ 4, and more than 100 of M L ≥ 3 up until the end of 2006. We examined the epidemic-type aftershock sequence (ETAS) model and the temporal evolution of several statistics of the earthquake sequence. The random component in the ETAS model can be considered as an indicator of fluid-driven activity, while the rate of Omori-law-type aftershocks indicates stress triggering of the preceding earthquakes. We observed three distinct seismic activity phases that were coincident with different injection periods. Phase I corresponds to initial injections with low injection rate (long time interval) and shows a vast majority of Omori-type events and only 8% externally forced activity. Phase II corresponds to a period of high injection rates and shows intense earthquake activity characterized by a high percentage (45%) of fluid-driven earthquakes. Phase III demonstrates decreasing seismicity as a result of a lowering in the injection rate; more than 70% of earthquakes occurred randomly over time, indicating the dominance of external triggering. The physical mechanism that induced the Rongchang earthquake sequence was a change in Coulomb's failure stress caused by pore pressure diffusion and transformed from seismic slip of preceding earthquakes. A pressure increase of 2.1–2.9 MPa at the injection well is more than sufficient to induce failure within the local formations.