Anthony R. Lowry

Transient fault slip in Guerrero, southern Mexico

The Guerrero region of southern Mexico has ac- cumulated more than 5 m of relative plate motion since the last major earthquake. In early 1998, a continuous GPS site in Guerrero recorded a transient displacement. Modeling indicates that anomalous fault slip propagated from east to west along-strike of the subduction megathrust. Campaign GPS and leveling data corroborate the model. The moment release was equivalent to an Mw≥6.5 earthquake. No M> 5 earthquakes accompanied the event, indicating the frictional regime is velocity-strengthening at the location of slip.

Dynamic elevation of the Cordillera, western United States

We introduce a methodology that synthesizes topography, gravity, crustal-scale seismic refraction velocity, and surface heat flow data sets to estimate dynamic elevation, i.e., the topography deriving from buoyancy variations beneath the lithosphere. The geophysical data independently constrain the topographic effects of surface processes, crustal buoyancy, and thermal boundary layer thickness. Each of these are subtracted from raw elevation of the western U.S. Cordillera to reveal dynamic elevation that can exceed 2 km and is significant at >95% confidence. The largest (∼1000 km diameter) of the dynamic elevation anomalies resembles a numerical model of a hypothetical Yellowstone hotspot swell, but the swell model does not account for all of the significant features seen in the dynamic elevation map. Other dynamic elevation anomalies are spatially correlative with Quaternary volcanism, but partial melt can contribute no more than a few hundred meters of elevation. Hence much of the dynamic elevation likely derives from other thermodynamic anomalies. Possible alternative mechanisms include both superadiabatic upwelling and adiabatic phase boundary deflections maintained by latent heat effects. Comparison of seismicity and volcanism to effective viscosity gradients, estimated from lithospheric flexural rigidity to facilitate the numerical swell model, suggests that tectonism focuses where lithosphere with negligible mantle viscosity abuts lithosphere with significant uppermost mantle viscosity.

Strength and rheology of the western U.S. Cordillera

Effective elastic thickness Te depends primarily on temperature, composition, and state of stress of the lithosphere. In this paper, we examine high-resolution spectral estimates of Te and their relationships to regional heat flow, age of the lithosphere, seismic properties, stress orientations, and earthquake focal depths of the western U.S. Cordillera. The relationship of Te to heat flow indicates that ductile flow accommodates long-term (∼106 to 108 years) isostatic response at different levels of the crust and upper mantle, depending principally on age (and, by implication, bulk composition) of the lithosphere. Isostatic response is primarily controlled by the upper mantle in Archean lithosphere of the middle Rocky Mountains, whereas Te depends on lower crustal flow in Early Proterozoic lithosphere of the Colorado Plateau. The Yellowstone-Snake River Plain volcanic field and significantly extended regions in the Basin-Range and northern Rocky Mountains are associated with latest Proterozoic aged lithosphere and indicate middle to upper crustal control of long-term Te. We also show that azimuthal variations of Te reflect deviatoric stress in the lithosphere. Te is found empirically to approximate the 95th percentile focal depth of background seismicity. The latter relationship is inconsistent with brittle-ductile control of focal depth, indicating that another rheological transition (e.g., from stick-slip to stable sliding frictional behavior) is responsible. Tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere. Moreover, we find a spatial correlation of the Intermountain Seismic Belt to a marked transition in Te, implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current-day plate boundary interactions.

The role of crustal quartz in controlling Cordilleran deformation

Large-scale deformation of continents remains poorly understood more than 40 years after the plate tectonic revolution. Rock flow strength and mass density variations both contribute to stress, so both are certain to be important, but these depend (somewhat nebulously) on rock type, temperature and whether or not unbound water is present. Hence, it is unclear precisely how Earth material properties translate to continental deformation zones ranging from tens to thousands of kilometres in width, why deforming zones are sometimes interspersed with non-deforming blocks and why large earthquakes occasionally rupture in otherwise stable continental interiors. An important clue comes from observations that mountain belts and rift zones cyclically form at the same locations despite separation across vast gulfs of time (dubbed the Wilson tectonic cycle), accompanied by inversion of extensional basins and reactivation of faults and other structures formed in previous deformation events. Here we show that the abundance of crustal quartz, the weakest mineral in continental rocks, may strongly condition continental temperature and deformation. We use EarthScope seismic receiver functions, gravity and surface heat flow measurements to estimate thickness and seismic velocity ratio, vP/vS, of continental crust in the western United States. The ratio vP/vS is relatively insensitive to temperature but very sensitive to quartz abundance. Our results demonstrate a surprising correlation of low crustal vP/vS with both higher lithospheric temperature and deformation of the Cordillera, the mountainous region of the western United States. The most plausible explanation for the relationship to temperature is a robust dynamical feedback, in which ductile strain first localizes in relatively weak, quartz-rich crust, and then initiates processes that promote advective warming, hydration and further weakening. The feedback mechanism proposed here would not only explain stationarity and spatial distributions of deformation, but also lend insight into the timing and distribution of thermal uplift and observations of deep-derived fluids in springs.

Flexural rigidity of the Basin and Range-Colorado Plateau-Rocky Mountain transition from coherence analysis of gravity and topography

Stochastic inversion for flexural loads and flexural rigidity of the continental elastic layer can be accomplished most effectively by using the coherence of gravity and topography. However, the spatial resolution of coherence analysis has been limited by use of two-dimensional periodogram spectra from very large (> 105 km 2) windows that generally include multiple tectonic features. Using a two-dimensional spectral estimator based on the maximum entropy method, the spatial resolution of flexural properties can be enhanced by a factor of 4 or more, enabling more detailed analysis at the scale of individual tectonic features. This new approach is used to map the spatial variation of flexural rigidity along the Basin and Range transition to the Colorado Plateau and Middle Rocky Mountains physiographic provinces. Large variations in flexural isostatic re- sponse are found, with rigidities ranging from as low as 8.7x102o N m (elastic thickness T, = 4.6 km) in the Basin and Range to as high as 4.1x1024 N m (T, = 77 km) in the Middle Rocky Mountains. These results compare favorably with independent determinations of flexural rigidity in the region. Areas of low flexural rigidity correlate strongly with areas of high surface heat flow, as is expected from the contingence of flexural rigidity on a temperature-dependent flow law. Also, late Cenozoic normal faults with large displacements are found primarily in areas of low flexural rigidity, while deformation fronts of Mesozoic/Tertiary overthrusts occur 0 to 100 km east of the low-rigidity region. The highest flexural rigidity is found within the Archean Wyoming craton, where evidence suggests that deeply rooted cratonic lithosphere may play a role in deter- mining the distribution of tectonism at the surface.

On the recovery of effective elastic thickness using spectral methods: Examples from synthetic data and from the Fennoscandian Shield

There is considerable controversy regarding the long-term strength of continents (Te). While some authors obtain both low and high Te estimates from the Bouguer coherence and suggest that both crust and mantle contribute to lithospheric strength, others obtain estimates of only <25 km using the free-air admittance and suggest that the mantle is weak. At the root of this controversy is how accurately Te can be recovered from coherence and admittance. We investigate this question by using synthetic topography and gravity anomaly data for which Te is known. We show that the discrepancies stem from comparison of theoretical curves to multitaper power spectral estimates of free-air admittance. We reformulate the admittance method and show that it can recover synthetic Te estimates similar to those recovered using coherence. In light of these results, we estimate Te in Fennoscandia and obtain similar results using both techniques. Te is 20-40 km in the Caledonides, 40-60 km in the Swedish Svecofennides, 40-60 km in the Kola peninsula, and 70-100 km in southern Karelia and Svecofennian central Finland. Independent rheological modeling, using a xenolith-controlled geotherm, predicts similar high Te in central Finland. Because Te exceeds crustal thickness in this area, the mantle must contribute significantly to the total strength. Te in Fennoscandia increases with tectonic age, seismic lithosphere thickness, and decreasing heat flow, and low Te correlates with frequent seismicity. However, in Proterozoic and Archean lithosphere the relationship of Te to age is ambiguous, suggesting that compositional variations may influence the strength of continents. Copyright 2004 by the American Geophysical Union.

Resonant slow fault slip in subduction zones forced by climatic load stress.

Global Positioning System (GPS) measurements at subduction plate boundaries often record fault movements similar to earthquakes but much slower, occurring over timescales of ∼1 week to ∼1 year. These ‘slow slip events’ have been observed in Japan, Cascadia, Mexico, Alaska and New Zealand. The phenomenon is poorly understood, but several observations hint at the processes underlying slow slip. Although slip itself is silent, seismic instruments often record coincident low-amplitude tremor in a narrow (1–5 cycles per second) frequency range. Also, modelling of GPS data and estimates of tremor location indicate that slip focuses near the transition from unstable (‘stick-slip’) to stable friction at the deep limit of the earthquake-producing seismogenic zone. Perhaps most intriguingly, slow slip is periodic at several locations, with recurrence varying from 6 to 18 months depending on which subduction zone (or even segment) is examined. Here I show that such periodic slow fault slip may be a resonant response to climate-driven stress perturbations. Fault slip resonance helps to explain why slip events are periodic, why periods differ from place to place, and why slip focuses near the base of the seismogenic zone. Resonant slip should initiate within the rupture zone of future great earthquakes, suggesting that slow slip may illuminate fault properties that control earthquake slip.

Distributed Deformation across the Rio Grande Rift, Great Plains, and Colorado Plateau

We use continuous measurements of GPS sites from across the Rio Grande Rift, Great Plains, and Colorado Plateau to estimate present-day surface velocities and strain rates. Velocity gradients from five east-west profiles suggest an average of ∼1.2 nanostrains/yr east-west extensional strain rate across these three physiographic provinces. The extensional deformation is not concentrated in a narrow zone centered on the Rio Grande Rift but rather is distributed broadly from the western edge of the Colorado Plateau well into the western Great Plains. This unexpected pattern of broadly distributed deformation at the surface has important implications for our understanding of how low strain-rate deformation within continental interiors is accommodated.

GPS monitoring of crustal deformation at Taal Volcano, Philippines

Data from a dual-frequency GPS network operated continuously from May 1998‐October 1999 on Taal volcano, Philippines, were processed on a daily basis to monitor processes of crustal deformation associated with volcanic activity. During the 16-month period of observations, displacements totaled nearly 30 mm in the horizontal and 50 mm in the vertical over 2.7 and 5.8 km baselines. Relative site velocities, estimated from daily site coordinates using 60-day tapered windows, vary significantly and can exceed 150 mm yr 21 in the horizontal. Velocity estimates were used to invert for parameters of a point-source model of elastic strain. During periods in which velocities are significant, the motions have a localized source at very high confidence, and the source magnitude term fluctuates between inflationary and deflationary behavior on time scales of weeks to months. The largest site velocities (and corresponding deformation model parameters) are time-correlative with anomalous bursts of hydrothermal activity and high-frequency local seismicity. In each instance the onset of deformation precedes both seismicity and hydrothermal activity, and the hydrothermal event coincides with a rapid shift in the velocity behavior. The relative timing of these phenomena suggests that deformation and seismicity both are responding to punctuated migration of hydrothermal fluids. q 2001 Published by Elsevier Science B.V.

Use of GPS for estimation of bending angles of radio waves at low elevations

The paper compares three methods of calculating the bending angles of radio waves propagated from space to a ground-based receiver: (1) from refractivity climatology corrected for refractivity at the receiving antenna, (2) from radiosonde refractivity profiles, and (3) from the refractivity at the antenna and the measured Doppler frequency shift of the GPS signals. The methods are tested with the use of radiosonde and GPS observations collocated in space and in time. We analyzed seven cases during October-November 1999 where GPS satellites were observed to below 0.5° elevation from Point Loma, California, and which coincided closely in time with radiosonde launches from the nearby Miramar station. In all cases the bending angles calculated from Doppler and from radiosondes agree fairly well at all elevations, but in a number of cases both differ significantly at low elevations from the bending angles calculated from climatology corrected for the refractivity at the antenna. Thus GPS has the potential of being used for the correction of radar observations at low elevations instead of (or complementary to) radiosondes. The differences between the bending angles calculated from climatology corrected for the refractivity at the antenna and those calculated from the Doppler frequency shift indicate anomalies in the refractivity profile in the lower troposphere and can thus be used as an indicator of ducting conditions.