A simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the Basin and Range Province. However this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation (3–5 km) above core complex structures. Because topographic lows of this magnitude are not observed above core complexes today, it requires mountain ranges 3–5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat Moho as the core complexes developed. An alternate mode of isostatic compensation, whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust, provides an equally good fit to the observed detachment fault geometries, while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat Moho beneath these zones of upper crustal doming. If this interpretation is correct, then the effective elastic plate thickness below the Basin and Range Province probably cannot exceed a maximum of about 4 km, with a best guess estimate of about 0.5 to 1 km. Computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the Moho of about 100°C follows doming by several million years, perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks. Subsequent cooling of the mid to lower crust beneath the core complex could result in “freezing” of the initially ductile lower crust within the core of the dome, effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults.

The U.S. Bureau of Reclamation's Paradox Valley Unit (PVU) extracts aquifer brine from nine shallow wells along the Dolores River, Paradox Valley, in southwestern Colorado and, after treating, high pressure injects the brine 4.3-4.8 km below the surface. PVU injects at rates between ϳ800 and ϳ1300 L/min. Since 1991, PVU has emplaced over 4 ‫ן‬ 10 6 m 3 of fluid and induced more than 4000 surfacerecorded seismic events. The events are recorded on the local 15-station Paradox Valley Seismic Network. The induced seismicity at Paradox separates into two distinct source zones: a principle zone (Ͼ95% of the events) asymmetrically surrounding the injection well to a maximum radial distance of ϳ3 km, and a secondary, ellipsoidal zone, ϳ2.5 km long and centered ϳ8 km northwest of the injection well. The expansion of these zones has stabilized since mid-1999, about three years after the onset of continuous injection. Within the principal zone, hypocenters align in distinct linear patterns, showing at-depth stratigraphy and the local Wray Mesa fracture and fault system. The primary faults of the Wray Mesa system are aseismic, striking subparallel to the inferred maximum principal stress direction, with one or more faults, probably, acting as fluid conduits to the secondary seismic zone. Individual seismic events, in both zones, do not discernibly correlate with short-term injection parameters; however, a 0.5 km 2 region immediately northwest of the injection well responds to long-term, large-scale changes in injection rate and the surpassing of a threshold injection pressure. Focal mechanisms of the induced events are consistent with simple double-couple, strike-slip moments and subhorizontal extension to the northeast. In addition, the fault planes are consistent with principal stress directions determined from borehole breakouts. More than 99.9% of the PVU seismicity is below human detection (ϳM 2.5). However, approximately 15 events have been felt locally, with the largest being a magnitude M 4.3. Because of the M 4.3 and two earlier-felt M ϳ3.5 events and injection economics, PVU changed injection strategies three times since 1996. These changes reduced seismicity from ϳ1100 events/year to as low as ϳ60 events/year.

The Paradox Valley Unit (PVU), a salinity control project in southwest Colorado, disposes of brine in a single deep injection well. Since the initiation of injection at the PVU in 1991, earthquakes have been repeatedly induced. PVU closely monitors all seismicity in the Paradox Valley region with a dense surface seismic network. A key factor for understanding the seismic hazard from PVU injection is the maximum magnitude earthquake that can be induced. The estimate of maximum magnitude of induced earthquakes is difficult to constrain as, unlike naturally occurring earthquakes, the maximum magnitude of induced earthquakes changes over time and is affected by injection parameters. We investigate temporal variations in maximum magnitudes of induced earthquakes at the PVU using two methods. First, we consider the relationship between the total cumulative injected volume and the history of observed largest earthquakes at the PVU. Second, we explore the relationship between maximum magnitude and the geometry of individual seismicity clusters. Under the assumptions that: (i) elevated pore pressures must be distributed over an entire fault surface to initiate rupture and (ii) the location of induced events delineates volumes of sufficiently high pore-pressure to induce rupture, we calculate the largest allowable vertical penny-shaped faults, and investigate the potential earthquake magnitudes represented by their rupture. Results from both the injection volume and geometrical methods suggest that the PVU has the potential to induce events up to roughly M W 5 in the region directly surrounding the well; however, the largest observed earthquake to date has been about a magnitude unit smaller than this predicted maximum. In the seismicity cluster surrounding the injection well, the maximum potential earthquake size estimated by these methods and the observed maximum magnitudes have remained steady since the mid 2000s. These observations suggest that either these methods overpredict maximum magnitude for this area or that long time delays are required for sufficient pore-pressure diffusion to occur to cause rupture along an entire fault segment. We note that earthquake clusters can initiate and grow rapidly over the course of 1 or 2 yr, thus making it difficult to predict maximum earthquake magnitudes far into the future. The abrupt onset of seismicity with injection indicates that pore-pressure increases near the well have been sufficient to trigger earthquakes under pre-existing tectonic stresses. However, we do not observe remote triggering from large teleseismic earthquakes, which suggests that the stress perturbations generated from those events are too small to trigger rupture, even with the increased pore pressures.