Gillian R. Foulger

Upper-mantle origin of the Yellowstone hotspot

Fundamental features of the geology and tectonic setting of the northeast-propagating Yellowstone hotspot are not explained by a simple deep-mantle plume hypothesis and, within that framework, must be attributed to coincidence or be explained by auxiliary hypotheses. These features include the persistence of basaltic magmatism along the hotspot track, the origin of the hotspot during a regional middle Miocene tectonic reorganization, a similar and coeval zone of northwestward magmatic propagation, the occurrence of both zones of magmatic propagation along a first-order tectonic boundary, and control of the hotspot track by preexisting structures. Seismic imaging provides no evidence for, and several contraindications of, a vertically extensive plume-like structure beneath Yellowstone or a broad trailing plume head beneath the eastern Snake River Plain. The high helium isotope ratios observed at Yellowstone and other hotspots are commonly assumed to arise from the lower mantle, but upper-mantle processes can explain the observations. The available evidence thus renders an upper-mantle origin for the Yellowstone system the preferred model; there is no evidence that the system extends deeper than ∼200 km, and some evidence that it does not. A model whereby the Yellowstone system reflects feedback between upper-mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep-mantle plume hypothesis.

Non‐double‐couple earthquakes 2. Observations

Most studies assume that earthquakes have double-couple (DC) source mechanisms, corresponding to shear motion on planar faults. However, many well-recorded earthquakes have radiation patterns that depart radically from this model, indicating fundamentally different source processes. Seismic waves excited by advective processes, such as landslides and volcanic eruptions, are consistent with net forces rather than DCs. Some volcanic earthquakes also have single-force mechanisms, probably because of advection of magmatic fluids. Other volcanic earthquakes have mechanisms close to compensated linear vector dipoles and may be caused by magmatic intrusions. Shallow earthquakes in volcanic or geothermal areas and mines often have mechanisms with isotropic components, indicating volume changes of either explosive or implosive polarity. Such mechanisms are consistent with failure involving both shear and tensile faulting, which may be facilitated by high-pressure, high-temperature fluids. In mines, tunnels are cavities that may close. Deep-focus earthquakes occur within zones of polymorphic phase transformations in the upper mantle at depths where stick-slip instability cannot occur. Their mechanisms tend to be deviatoric (volume conserving), but non-DC, and their source processes are poorly understood. Automatic global moment tensor services routinely report statistically significant non-DC components for large earthquakes, but detailed reexamination of individual events is required to confirm such results.

Non-double-couple microearthquakes at Long Valley caldera, California, provide evidence for hydraulic fracturing

Abstract Most of 26 small (0.4≲ M ≲3.1) microearthquakes at Long Valley caldera in mid-1997, analyzed using data from a dense temporary network of 69 digital three-component seismometers, have significantly non-double-couple focal mechanisms, inconsistent with simple shear faulting. We determined their mechanisms by inverting P - and S -wave polarities and amplitude ratios using linear-programming methods, and tracing rays through a three-dimensional Earth model derived using tomography. More than 80% of the mechanisms have positive (volume increase) isotropic components and most have compensated linear-vector dipole components with outward-directed major dipoles. The simplest interpretation of these mechanisms is combined shear and extensional faulting with a volume-compensating process, such as rapid flow of water, steam, or CO 2 into opening tensile cracks. Source orientations of earthquakes in the south moat suggest extensional faulting on ESE-striking subvertical planes, an orientation consistent with planes defined by earthquake hypocenters. The focal mechanisms show that clearly defined hypocentral planes in different locations result from different source processes. One such plane in the eastern south moat is consistent with extensional faulting, while one near Casa Diablo Hot Springs reflects en echelon right-lateral shear faulting. Source orientations at Mammoth Mountain vary systematically with location, indicating that the volcano influences the local stress field. Events in a ‘spasmodic burst’ at Mammoth Mountain have practically identical mechanisms that indicate nearly pure compensated tensile failure and high fluid mobility. Five earthquakes had mechanisms involving small volume decreases, but these may not be significant. No mechanisms have volumetric moment fractions larger than that of a force dipole, but the reason for this fact is unknown.

The 1994–1995 seismicity and deformation at the Hengill triple junction, Iceland: Triggering of earthquakes by minor magma injection in a zone of horizontal shear stress

Since July 1994 an unusually persistent swarm of earthquakes (M<4.0) has been in progress at the Hengill triple junction, SW Iceland. Activity is clustered around the center of the Hromundartindur volcanic system. Geodetic measurements indicate a few centimeters uplift and expansion of the area, consistent with a pressure source at 6.5±3 km depth beneath the center of the volcanic system. The system is within the stress field of the south Iceland transform zone, and the majority of the recorded earthquakes represent strike-slip faulting on subvertical planes. We show that the secondary effects of a pressure source, modeled as a point source in an elastic half-space, include horizontal shear that perturbs the regional stress. Near the surface, shear stress is enhanced in quadrants around the direction of maximum regional horizontal stress and diminished in quadrants around the direction of minimum regional stress. The recorded earthquakes show spatial correlation with areas of enhanced shear. The maximum amount of shear near the surface caused by the expanding pressure source exceeds 1 μstrain, sufficient to trigger earthquakes if the crust in the area was previously close to failure.

Seismic evidence for a tilted mantle plume and north-south mantle flow beneath Iceland

Shear waves converted from compressional waves at mantle discontinuities near 410- and 660-km depth recorded by two broadband seismic experiments in Iceland reveal that the center of an area of anomalously thin mantle transition zone lies at least 100 km south of the upper-mantle low-velocity anomaly imaged tomographically beneath the hotspot. This offset is evidence for a tilted plume conduit in the upper mantle, the result of either northward flow of the Icelandic asthenosphere or southward flow of the upper part of the lower mantle in a no-net-rotation reference frame. fl 2002 Elsevier Science B.V. All rights reserved.

Plate dynamics near divergent boundaries: Geophysical implications of postrifting crustal deformation in NE Iceland

The bulk of a tectonic plate is thought to move continously at a rate consistent with the geologic average. On the other hand, movements are highly episodic at plate boundaries. We study the plate dynamics that relate to these two different modes by modelling the displacements observed using the global positioning system in Northeast Iceland 1987–1990. These observations were made about 10 years after an episodic divergent movement between the North American and Eurasian plates 1975–1981. The horizontal displacement field fits well a two-dimensional model of postrupture stress relaxation assuming a thin elastic layer overlying a layer of Newtonian viscosity. This analysis indicates values of about 10 m2/s for the stress diffusivity and 0.3–2 × 1018 Pa s for the Newtonian viscosity of the lower layer. However, no significant correlation exists between the observed and modeled vertical displacements probably because of the relative inaccuracy of the vertical component observations. Assuming that contemporary plate motion is the sum of many displacements that have diffused from boundaries where episodic displacements occur periodically, we simulate the spatial transition from episodic to continuous plate movements. The plate “boundary zone” where movements are episodic or quasi-episodic is of the order of 100 km wide, depending on the stress diffusivity (which may be fairly uniform throughout the world) and the frequency of episodic movements.

Postrifting anelastic deformation around the spreading plate boundary, north Iceland: 1. Modeling of the 1987–1992 deformation field using a viscoelastic Earth structure

A third Global Positioning System (GPS) survey of a regional network surrounding the Krafla volcanic system, north Iceland, was conducted in 1992 following a major crustal spreading episode which began in this system in 1975. Differencing the 1992 results with those from 1987 and 1990 reveals a regional deformation field with a maximum, rift-normal expansion rate of 4.5 cm/year near the rift, decreasing to 3 cm/year at large distances. The time-averaged spreading rate in north Iceland, 1.8 cm/year, cannot account for this deformation. The vertical deformation field reveals regional uplift throughout the network area at its maximum closest to the rift and decreasing with distance. Three different models are applied to study the postdike injection ground deformation : (1) stress redistribution in an elastic layer over a viscoelastic half-space, (2) stress redistribution in an elastic-viscous layered medium, and (3) continued opening at depth on the dike plane in an elastic half-space. Using model 1, the effects of historical episodes in the region are subtracted from the observed displacement fields, and the remaining motion is modeled as relaxation following the recent Krafla rifting episode. The best fit model involves a half-space viscosity of 1.1 x 10 18 Pa s, a relaxation time of 1.7 years, and an elastic layer thickness for northeast Iceland of 10 km. The vertical field indicates that the Krafla dike complex rifted the entire elastic layer. Using model 2, the motion 1987-1990 and 1990-1992 can be simulated adequately given the survey errors, but the 1987-1992 deformation is poorly fitted, suggesting that a more realistic geophysical model is required. Using model 3, a range of dikes will fit the deformation field.

Three-dimensional seismic image of a geothermal reservoir: The Geysers, California

Three-dimensional seismic travel-time tomography of The Geysers geothermal area, in the coast ranges of northern California, shows a strong (−9%) anomaly in VP/VS, the ratio of the compressional and shear wave speeds, that is not evident in VP alone and corresponds closely to the most intensively exploited part of the geothermal reservoir. This anomaly probably indicates low pore pressure and relatively dry conditions, caused partly by boiling of pore water as steam is extracted. Steam pressure decreases over the last decade have probably caused seismologically measurable changes in wave speeds. Tomographic measurement of VP/VS is a promising technique both for identifying geothermal resources and for monitoring them during exploitation.

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.

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.