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B.R. Julian

Publications and source records attributed to B.R. Julian.

24 records · Page 2Linked to original sources
USGS1997

Industrially induced changes in Earth structure at the geysers geothermal area, California

Industrial exploitation is causing clearly-measurable changes in Earth structure at The Geysers geothermal area, California. Production at The Geysers peaked in the late 1980s at ∼3.5 × 10³ kg s −1 of steam and 1800 MW of electricity. It subsequently decreased by about 10% per year [ Barker et al. , 1992] because of declining reservoir pressure. The steam reservoir coincides with a strong negative anomaly (∼0.16, ∼9%) in the compressional-to-shear seismic wave speed ratio V P / V S , consistent with the expected effects of low-pressure vapor-phase pore fluid [ Julian et al. , 1996]. Between 1991 and 1994 this anomaly increased in amplitude by up to about 0.07 (∼4%). This is consistent with the expected effects of continued pressure reduction and conversion of pore water to steam as a result of exploitation. These unique results show that V P / V S tomography can easily detect saturation changes caused by exploitation of reservoirs, and is a potentially valuable technique for monitoring environmental change. They also provide geophysical observational evidence that geothermal energy is not a renewable energy source.

Geophysical Research Letters↗
USGS1997

Non-double-couple earthquake mechanisms at the Hengill-Grensdalur volcanic complex, southwest Iceland

The Hengill-Grensdalur area in Iceland generates frequent small non-double-couple earthquakes with explosive volumetric components. We collected high quality three-component digital recordings of 4,000 earthquakes on a purpose-designed, 32-station network in 1991, and determined focal mechanisms for 100 of the best-recorded earthquakes by inverting amplitude ratios. Many of the mechanisms are consistent, within the errors, with simultaneous shear and tensile faulting, with tensile faults parallel to the local spreading ridge, and shear faulting similar to that in the South Iceland transform-fault zone. Some events cannot be explained by this model, however, and require other processes, such as crack closing and partial compensation of tensile cracks by fluid flow.

Geophysical Research Letters↗
USGS1996

Earthquake mechanisms from linear-programming inversion of seismic-wave amplitude ratios

The amplitudes of radiated seismic waves contain far more information about earthquake source mechanisms than do first-motion polarities, but amplitudes are severely distorted by the effects of heterogeneity in the Earth. This distortion can be reduced greatly by using the ratios of amplitudes of appropriately chosen seismic phases, rather than simple amplitudes, but existing methods for inverting amplitude ratios are severely nonlinear and require computationally intensive searching methods to ensure that solutions are globally optimal. Searching methods are particularly costly if general (moment tensor) mechanisms are allowed. Efficient linear-programming methods, which do not suffer from these problems, have previously been applied to inverting polarities and wave amplitudes. We extend these methods to amplitude ratios, in which formulation on inequality constraint for an amplitude ratio takes the same mathematical form as a polarity observation. Three-component digital data for an earthquake at the Hengill-Grensdalur geothermal area in southwestern Iceland illustrate the power of the method. Polarities of P , SH , and SV waves, unusually well distributed on the focal sphere, cannot distinguish between diverse mechanisms, including a double couple. Amplitude ratios, on the other hand, clearly rule out the double-couple solution and require a large explosive isotropic component.

Bulletin of the Seismological Society of America↗
USGS1996

Non-double-couple earthquake mechanisms at the Geysers geothermal area, California

Inverting P- and S-wave polarities and P:SH amplitude ratios using linear programming methods suggests that about 20% of earthquakes at The Geysers geothermal area have significantly non-double-couple focal mechanisms, with explosive volumetric components as large as 33% of the seismic moment. This conclusion contrasts with those of earlier studies, which interpreted data in terms of double couples. The non-double-couple mechanisms are consistent with combined shear and tensile faulting, possibly caused by industrial water injection. Implosive mechanisms, which might be expected because of rapid steam withdrawal, have not been found. Significant compensated-linear-vector-dipole (CLVD) components in some mechanisms may indicate rapid fluid flow accompanying crack opening. Copyright 1996 by the American Geophysical Union.

California↗
USGS1992

Post-rifting stress relaxation at the divergent plate boundary in northeast Iceland

Interaction of the elastic lithosphere with the underlying anelastic asthenosphere causes strain to propagate along the Earth's surface in a diffusion-like manner following tectonism at plate boundaries. This process transfers stress between adjacent tectonic segments and influences the temporal tectonic pattern along a plate boundary. Observations of such strain transients have been rare, and have hitherto been confined to strike-slip and underthrusting plate boundaries 1 . Here we report the observation of a strain transient at the divergent (spreading) plate boundary in Iceland. A Global Positioning System survey undertaken a decade after an episode of dyke intrusion accompanying several metres of crustal spreading reveals a spatially varying strain field with the expected diffusion-pulse shape and an amplitude three times greater than the 5.7 cm that would be expected from the average spreading rate 2 . A simple one-dimensional model with a thin elastic layer overlying a viscous layer fits the data well and yields a stress diffusivity of 1.1 á±0.3 m 2 s −1 . Combined with struc-tural information from magnetotelluric measurements, this implies a viscosity of 0.3−2 × 10 19 Pa s—a value comparable to that derived for Iceland from post-glacial rebound 23 , but low compared with estimates for mantle viscosity obtained elsewhere 3 .

Nature↗