We have produced a high-resolution microseismic image of a hydraulic fracture stimulation in the Carthage Cotton Valley gas field of east Texas. Gas is produced from multiple, low-permeability sands within an interbedded sand-shale sequence. We improved the precision of microseismic event locations 4-fold over initial locations by manually repicking the waveforms in a spatial sequence, allowing us to visually correlate waveforms of adjacent sources. The new locations show vertical containment within individual, targeted sands, suggesting little or no hydraulic communication between the discrete perforation intervals simultaneously treated within an 80-m section. Treatment lengths inferred from event locations are about 200 m greater at the shallow perforation intervals than at the deeper intervals. The highest quality locations indicate fracture zone widths as narrow as 6 m. Similarity of adjacent-source waveforms, along with systematic changes of phase amplitude ratios and polarities, indicate fairly uniform focal mechanisms (fracture plane orientation and sense of slip) over the treatment length. Composite focal mechanisms indicate both left-and right-lateral strike-slip faulting along near-vertical fractures that strike subparallel to maximum horizontal stress (S Hmax). The focal mechanisms and event locations are consistent with activation of the reservoir's prevalent natural fractures, fractures that are isolated within individual sands and trend subparallel to expected hydraulic fracture orientation (S Hmax direction). Shear activation of these fractures indicates a stronger correlation of induced seismicity with low-impedance flow paths than is normally found or assumed during injection stimulation.

We analyzed precisely located microearthquake data detected during five hydraulic fracture treatments in the Carthage gas field of east Texas. The treatments were conducted in two adjacent boreholes within interbedded sands and shales of the Upper Cotton Valley formation. The microearthquakes were induced within narrow horizontal bands that correspond to the targeted sandstone layers. Events throughout all the treatments show strike-slip faulting occurring uniformly along vertical fractures trending close to maximum horizontal stress direction. These events are consistent with the reservoir's prevalent natural fractures, known to be isolated within the sands and trending subparallel to the expected hydraulic fracture orientation. When this uniform fracture system was activated exclusively, the detected shear deformation, measured as the moment release per unit volume of fluid injected, was constant, independent of various fluid viscosities and flow rates used. Within the base of the Upper Cotton Valley formation, anomalous event counts and moment release occurred within dense clusters that delineate bends or jogs in the fracture zones. The mechanisms are also strike-slip, but the fault planes are more favorably oriented for failure. The dense clusters show location patterns diverging in time, suggesting the expulsion of fluids from compressive fault jogs. Fluid flow forced by the adjacent slip-induced loading appears to initially extend the treatments, but the loading also tends to lock up and concentrate stress at the jogs, as evident by fewer but larger events populating the structures as treatments progress. As a result, effective drainage lengths from the boreholes may be shorter than would be inferred from the seismicity extending past the jogs. These high-moment asperities are similar to dense patches of seismicity observed along creeping sections of the San Andreas fault, where they have been attributed to localized zones of strength or stress concentration, surrounded by larger regions undergoing stable, aseismic slip. This similarity, plus large moment deficits in terms of volume injected, suggests a large component of aseismic slip is induced by the Cotton Valley treatments.

S U M M A R YA high-rate injection of 20 000 m 3 of water into granite between 2.8 and 3.4 km depth at the Soultz hot dry rock (HDR) test site in France in 1993 September led to a 200-fold increase in borehole transmissivity and produced a subvertical cloud of microseismicity of dimensions 0.5 km wide, 1.2 km long, 1.5 km high and oriented 25 • NW. The resulting data set is unusually complete and well suited to studying permeability creation/enhancement processes in crystalline rock and the utility of microseismic data for revealing them. Although the microseismic cloud defined using joint hypocentre determination (JHD) locations was diffuse and showed little structure, application of the collapsing method showed it to be composed largely of discrete tubes and planes that propagated coherently. One prominent structure that extended 350 m downwards from the vicinity of a flow inlet early in the injection and that appears to contain a major flow path was subjected to detailed investigation to establish its hydrogeologic nature and the mechanisms underpinning its inferred permeability enhancement. High-resolution microseismic mapping techniques (i.e. multiplets and clustering) showed it to be a subvertical, NNW-SSE striking, fracture zone of width 10-20 m. The strike and scale of the structure identifies it as a member of a family of hydrothermally altered, cataclastic shear structures that constitute the primary permeable paths for fluid migration within the rock mass, both under ambient and forced fluid flow conditions. The microseismicity occurred on subvertical, smallscale fractures within the cataclastic shear zone whose azimuths scatter within 22 • of parallel to the parent structure. Although the structure is likely to have been naturally permeable to some degree, its permeability appears to have been significantly enhanced as a consequence of the injection. The most likely mechanism of permeability enhancement, which is in accord with the strong preference for the microseismicity to grow downwards, involves strike-slip shearing, which produced the opening of vertical tubes at along-strike jogs in the fault (the so-called Hill mesh). Seismic moment release averaged over the structure suggests shear displacements of at least 0.3 mm occurred, which are sufficient to generate aperture changes that are hydraulically significant. The preponderance of discrete structures within the microseismic cloud after collapsing suggests that significant flow and permeability enhancement (i.e. stimulation) within the rock mass is largely confined to the interiors of shear zones that appear to have a spacing of approximately 100 m.

We develop a Regional Seismic Travel Time (RSTT) model and methods to account for the first-order effect of the three-dimensional crust and upper mantle on travel times. The model parameterization is a global tessellation of nodes with a velocity profile at each node. Interpolation of the velocity profiles generates a 3-dimensional crust and laterally variable upper mantle velocity. The upper mantle velocity profile at each node is represented as a linear velocity gradient, which enables travel time computation in approximately 1 millisecond. This computational speed allows the model to be used in routine analyses in operational monitoring systems. We refine the model using a tomographic formulation that adjusts the average crustal velocity, mantle velocity at the Moho, and the mantle velocity gradient at each node. While the RSTT model is inherently global and our ultimate goal is to produce a model that provides accurate travel time predictions over the globe, our first RSTT tomography effort covers Eurasia and North Africa, where we have compiled a data set of approximately 600,000 Pn arrivals that provide path coverage over this vast area. Ten percent of the tomography data are randomly selected and set aside for testing purposes. Travel time residual variance for the validation data is reduced by 32%. Based on a geographically distributed set of validation events with epicenter accuracy of 5 km or better, epicenter error using 16 Pn arrivals is reduced by 46% from 17.3 km (ak135 model) to 9.3 km after tomography. Relative to the ak135 model, the median uncertainty ellipse area is reduced by 68% from 3070 km 2 to 994 km 2 , and the number of ellipses with area less than 1000 km 2 , which is the area allowed for onsite inspection under the Comprehensive Nuclear Test Ban Treaty, is increased from 0% to 51%.

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