Bradford H. Hager

A geological and geophysical context for the Wenchuan earthquake of 12 May 2008, Sichuan, People's Republic of China

On 12 May 2008, a magnitude 7.9 earthquake ruptured the Longmen Shan margin of the eastern Tibetan plateau. This event occurred within the context of long-term uplift and eastward enlargement of the plateau. The area has numerous geological features not typical of active convergent mountain belts, including the presence of a steep mountain front (>4 km relief) but an absence of large-magnitude low-angle thrust faults; young high topography (post ca. 15 Ma) and thickened crust but low global positioning system (GPS) shortening rates (<3 mm/yr); and no coeval foreland subsidence. In our interpretation, crustal thickening beneath the eastern Tibetan plateau occurred without large-scale shortening of the upper crust but instead is caused by ductile thickening of the deep crust in a weak (lowviscosity) layer. Late Cenozoic shortening across the Longmen Shan could be as little as 10–20 km, with folding and faulting mainly accommodating differential surface uplift between the plateau and the Sichuan Basin. The earthquake of 12 May probably reflects long-term uplift, with slow convergence and right-slip, of the eastern plateau relative to the Sichuan Basin. GPS-determined rates in the vicinity of the 12 May event suggest an average recurrence interval of ~2,000–10,000 yr.

Space geodetic measurement of crustal deformation in central and southern California, 1984-1992

A laboratory type of analyzer for quantitatively determining the percent third element content of a hydrocarbon sample. A unique rhodium/americium radioactive source is disclosed.

Estimates of Seismic Potential in the Marmara Sea Region from Block Models of Secular Deformation Constrained by Global Positioning System Measurements

We model the geodetically observed secular velocity field in north- western Turkey with a block model that accounts for recoverable elastic-strain ac- cumulation. The block model allows us to estimate internally consistent fault slip rates and locking depths. The northern strand of the North Anatolian fault zone (NAFZ) carries approximately four times as much right-lateral motion (24 mm/yr) as does the southern strand. In the Marmara Sea region, the data show strain accu- mulation to be highly localized. We find that a straight fault geometry with a shallow locking depth of 6-7 km fits the observed Global Positioning System velocities better than does a stepped fault geometry that follows the northern and eastern edges of the sea. This shallow locking depth suggests that the moment release associated with an earthquake on these faults should be smaller, by a factor of 2.3, than previously inferred assuming a locking depth of 15 km. Online material: an updated version of velocity-field data.

Conman: vectorizing a finite element code for incompressible two-dimensional convection in the Earth's mantle

Abstract We discuss some simple concepts for vectorizing scientific codes, then apply these concepts to ConMan, a finite element code for simulations of mantle convection. We demonstrate that large speed-ups, close to the theoretical limit of the machine, are possible for entire codes, not just specially constructed routines. Although our specific code uses the finite element method, the vectorizing concepts discussed are widely applicable.

Subduction zone dip angles and flow driven by plate motion

Abstract Kinematic models of the large scale flow in the mantle accompanying the observed plate motions are calculated by neglecting thermal buoyancy forces. The large scale flow is therefore determined by the mass flux imposed by the moving plates. The energy and momentum equations decouple, and with the assumption of a radially symmetric Newtonian viscosity, the flow accompanying the plate motions can be obtained using harmonic analysis and propagator matrices. The resulting flow models predict remarkably well the observed dips of subducted slabs if the flow extends into the lower mantle. The plates drag along a thick boundary layer which should be included in models of the heating of subducted slabs.

Present day kinematics of the Eastern California Shear Zone from a geodetically constrained block model

We use Global Positioning System (GPS) data from 1993–2000 to determine horizontal velocities of 65 stations in eastern California and western Nevada between 35° and 37° N. We relate the geodetic velocities to fault slip rates using a block model that enforces path integral constraints over geologic and geodetic time scales and that includes the effects of elastic strain accumulation on faults locked to a depth of 15 km. The velocity of the Sierra Nevada block with respect to Nevada is 11.1±0.3 mm/yr, with slip partitioned across the Death Valley, (2.8±0.5 mm/yr), Panamint Valley (2.5±0.8 mm/yr), and Airport Lake/Owens Valley (5.3±0.7/4.6±0.5 mm/yr) faults. The western Mojave block rotates at 2.1±0.8°/My clockwise, with 3.7±0.7 mm/yr of left lateral motion across the western Garlock Fault. We infer 11±2 mm/yr of right lateral motion across the Mojave region of the Eastern California Shear Zone.

Localization of the gravity field and the signature of glacial rebound

The negative free-air gravity anomaly centred on Hudson Bay, Canada, shows a remarkable correlation with the location of the Laurentide ice sheet, suggesting that this gravity anomaly is the result of incomplete post-glacial rebound. This region, however, is also underlain by higher-than-average mantle seismic velocities, suggesting that the gravity low might result instead from dynamic topography associated with convective downwellings. Here we analyse the global gravity field as a simultaneous function of geographic location and spectral content. We find that the Hudson Bay gravity low is unique, with anomalously high amplitude in the spectral band where the power from the Laurentide ice load is greatest and the relaxation times predicted for viable models of viscous relaxation are longest. We estimate that about half of the Hudson Bay gravity anomaly is the result of incomplete post-glacial rebound, and derive a mantle viscosity model that explains both this gravity signature and the characteristic uplift rates for the central Laurentide and Fennoscandian regions. This model has a jump in viscosity at 670 km depth, comparable to that in dynamic models of the geoid highs over subducted slabs,, but lacks a low-viscosity asthenosphere, consistent with a higher viscosity in the upper mantle beneath shields than in oceanic regions.

Melt intrusion as a trigger for lithospheric foundering and the eruption of the Siberian flood basalts

Any viable model of the Siberian flood basalt (SFB) eruption must provide for a massive pulse of magma, initially erupted below sea level. We propose as a triggering mechanism a limited precursory melt that intrudes and heats the mantle lithosphere, lowering its viscosity, and increasing its density as the melt freezes into eclogite. This warm, dense mantle lithosphere is then removed via a Rayleigh-Taylor instability that creates surface subsidence. Removal of the mantle lithosphere lengthens the melting column, and the melt volume of the SFB can be produced in less than a million years. The model is permissive of the existing geologic data for the SFB, which rule out a traditional hot, deep mantle plume. Numerical models demonstrate that the ability of a mantle upwelling to remove lithosphere is dependent primarily upon the lithospheric rheology, not on the temperature or size of the upwelling.

Re-examination of the lunar magma ocean cumulate overturn hypothesis: melting or mixing is required

Abstract There is a long-standing hypothesis that the last fraction of the lunar magma ocean crystallized into a layer of dense, Ti-rich cumulate minerals at shallow depths (∼100 km) early in the moon’s history. Many questions remain about the stability of these high-Ti cumulates. It has been suggested that the cumulates subsequently sank deep into the moon because of gravitational instability, but high-Ti material is required at shallower depths by 3.5 Ga to create the high-Ti mare basalts and picritic glasses. The high-Ti material may have re-erupted from depth, or some or all of it may have remained at shallow depths throughout lunar history. Data on phase stabilities, bulk compositions, densities, and temperatures of melting and crystallizing in addition to results from numerical modeling suggest that the high-Ti cumulates would sink only under highly specific conditions. Five scenarios for sinking high-Ti cumulate materials are examined, and only two are found plausible. In particular, it is found that simple sinking of solidified high-Ti cumulates is unlikely because the temperature at which the cumulates solidify is low, and viscosity under these conditions is very high. It is, however, possible that high-Ti cumulates mixed with a substantial fraction of olivine would have viscosity low enough to allow them to sink as solids. Further, because clinopyroxene and ilmenite melt in a ratio of 2:1, remelted high-Ti cumulates would be negatively buoyant and sink as liquids, percolating downward through the underlying mantle and beginning to recrystallize ilmenite at 200 km depth, making a hybrid, heterogeneous mantle.

Toroidal-Poloidal Partitioning of Lithospheric Plate Motions

A spherical harmonic expansion of tectonic plate motions on the Earth requires both poloidal and toroidal harmonics. Each spectrum decays fairly uniformly as l-2 (l is the spherical harmonic degree), and the toroidal-poloidal ratio of the degree power is between 0.5 and 1.0 for all degrees up to 128. Convection in a laterally homogeneous medium will excite only poloidal motions; hence the question of why the toroidal component of plate motion is so large. Numerical models of 3-D convection with surface plates show that the rheological heterogeneity represented by plate boundaries can account for the excitation of toroidal surface motions from an underlying poloidal convective flow. Plates also account for the l-2 decay of the spectra, which is a simple geometric consequence of the plate-like velocity field. Lateral viscosity variations can also account for the net rotation of the lithosphere in the hot spot reference frame; this requires order of magnitude lateral viscosity variations. The spectra of plate motions depends on the geometries of the plates as well as their relative motions. A Monte Carlo simulation of plate motions shows that the observed toroidal-poloidal ratio for all degrees is less than would be expected for most plate motions, given the existing geometry. Relatively simple numerical models of 3-D convection with surface plates evolve to a final steady state (when it exists) that minimizes the toroidal-poloidal ratio of plate motion. This suggests that the much more complex system of plates on the Earth may be similarly governed.