The Paradox Valley Unit (PVU) is operated by the U.S. Bureau of Reclamation (USBR) and is built to control the water quality of the Dolores River - a feeder of the Colorado River. Brine is extracted along the river from several shallow wells. Before it is injected into a 4.8km deep well for long-term storage, it is filtered at a surface-treatment facility. The target zone of the injection is a subhorizontal formation of a Mississippian-age limestone. The first injection test started in 1991. Continuous injection started in 1996 and is still ongoing. Micro-seismicity induced by the injection is monitored with the 15-station Paradox Valley Seismic Network operated by USBR, and more then 5700 events have been located during the 20 years since the start of injection. The locations of the seismic events provide crucial insights to the migration pathways of the injected fluid. In this study we analyze the seismicity up to the end of 2011, which does not include the magnitude 3.9 event that caused a temporary shut down of the PVU in January 2013. The largest event included in our study period is an event with M4.3 that occurred in May 2000. The majority (75%) of the events have magnitudes of 1 or smaller; only 74 events have magnitudes larger or equal to 2.5, of which only 4 are larger or equal to 3.5. About 80% of the seismicity occurred within 4km of the injection well. However, one zone more then 10 km away from the well first became active in late 2010, and more than 500 microearthquakes occurred within several weeks. The goal of this study is to understand the processes occurring during a long-term subsurface fluid injection when there is no circulation. While other wastewater injection projects exist, none has been monitored as well as the Paradox Valley seismicity or has been going on for such a long time. The first step is to characterize in detail the frequency magnitude distributions (FMD) of the ongoing seismicity at Paradox Valley. We then infer the locations of several small faults from the seismicity and relate the b-values of the Gutenberg Richter relationship to those faults. We find an overall correlation between the orientation of the faults and the b-values. In addition, we investigate the influence of the different injection regimes on the behavior of the FMD and thus the b-value.

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