Geodetic measurements from a network of permanent GPS stations along the Pacific coast of Mexico reveal a large “silent earthquake” along the segment of the Cocos‐North American plate interface identified as the Guerrero seismic gap. The event began in October of 2001 and lasted for 6–7 months. Average slip of ∼10 cm produced measurable displacements over an area of ∼550 × 250 km2. The equivalent moment magnitude of the event was Mw ∼ 7.5. Recognition of this and previous slow event here indicate that the seismogenic portion of the plate interface is not loading steadily, as hitherto believed, but is rather partitioning the stress buildup with episodic, as opposed to steady‐state or periodic, slip downdip of the seismogenic zone. This process increases the stress at the base of the seismogenic zone, bringing it closer to failure. These results call for a reassessment of the seismic potential of Guerrero and other seismic gaps in Mexico.

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, v(P)/v(S), of continental crust in the western United States. The ratio v(P)/v(S) is relatively insensitive to temperature but very sensitive to quartz abundance. Our results demonstrate a surprising correlation of low crustal v(P)/v(S) 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.

Abstract. The Guerrero region of southern Mexico has accumulated 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 M•o_>6.5 earthquake. No M>5 earthquakes accompanied the event, indicating the frictional regime is velocity-strengthening at the location of slip.

[1] There is considerable controversy regarding the long-term strength of continents (T e ). While some authors obtain both low and high T e 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 T e can be recovered from coherence and admittance. We investigate this question by using synthetic topography and gravity anomaly data for which T e 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 T e estimates similar to those recovered using coherence. In light of these results, we estimate T e in Fennoscandia and obtain similar results using both techniques. T e 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 T e in central Finland. Because T e exceeds crustal thickness in this area, the mantle must contribute significantly to the total strength. T e in Fennoscandia increases with tectonic age, seismic lithosphere thickness, and decreasing heat flow, and low T e correlates with frequent seismicity. However, in Proterozoic and Archean lithosphere the relationship of T e to age is ambiguous, suggesting that compositional variations may influence the strength of continents.

Abstract. 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.

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.

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 km2) 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 response are found, with rigidities ranging from as low as 8.7×1020 N m (elastic thickness Te = 4.6 km) in the Basin and Range to as high as 4.1×1024 N m (Te = 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 determining the distribution of tectonism at the surface.

[1] The flexural rigidity or effective elastic thickness of the lithosphere, T e , primarily depends on its thermal gradient and composition. Consequently, maps of the lateral variability of T e in continents reflect their lithospheric structure. We present here a new T e map of South America generated using a compilation of satellite-derived (GRACE and CHAMP missions) and terrestrial gravity data (including EGM96 and SAGP), and a multitaper Bouguer coherence technique. Our T e maps correlate remarkably well with other proxies for lithospheric structure: areas with high T e have, in general, high lithospheric mantle shear wave velocity and low heat flow and vice versa. In this paper we focus on the T e of the stable platform. We find that old cratonic nuclei (mainly Archean and Early/Middle Proterozoic) have, in general, high T e (>70 km), while the younger Patagonian Phanerozoic terrane has much lower T e (20-30 km), suggesting that T e is related to terrane age as has already been noted in Europe. Within cratonic South America, T e variations are observed at regional scale: relatively lower T e occurs at sites that have been repeatedly reactivated throughout geological history as major sutures, rift zones, and sites of hot spot magmatism. Today, these low T e areas are surrounded by large cratonic nuclei. They concentrate most of the intracontinental seismicity and exhibit relatively high surface heat flow and low seismic velocity at 100 km depth. This implies that intracontinental deformation focuses within relatively thin, hot, and hence weak lithosphere, that cratonic interiors are strong enough to inhibit tectonism, and that the differences in lithospheric rigidity, structure, and composition between stable cratons and sites of intracontinental deformation are not transient, and may have been maintained, in some cases, for at least 500 m.y.