Landscape evolution in the eastern Mojave Desert is recorded by sys1ematic changes in Pliocene to latest Pleistocene volcanic: land• forms that show discrete periods of eolian deposition, surface stabilization, drainage-network expansion, and erosion on basaltic lava nows. These processes are documented by K•Ar dating in conjunction with morphometric, sedimentologic. pedologic, and geophysica1 stud

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.

No abstract

One of the persistent problems with numerical solutions to the elastic wave equation is the finite size of the numerical grid. As with a physical body, the grid boundaries will reflect incident energy. If not eliminated or reduced substantially, these reflections will invade the grid interior and interfere with the desired solution. One method for eliminating reflections is creating a large and/or expanding grid. This method may be impractical since it can be quite costly in both computer time and memory. Another method is making the grid boundary “transparent” to outgoing energy. This method is ideally done by designing absorbing or nonreflecting boundaries which are mathematically equivalent to a one‐way, or outgoing, elastic wave equation only. In practice, an outgoing elastic wave equation is an approximation since the wave equation is not generally separable into outgoing and incoming parts. Two absorbing boundary condition approximations commonly used are those from Reynolds (Reynolds, 1978) and Clayton and Engquist, (Clayton and Engquist, 1977).