We propose a model for earthquake afterslip based on rate and state variable friction laws. In the model, afterslip is attributed to the interaction of a velocity‐weakening region at depth (within which earthquakes nucleate) with an upper region of velocity‐strengthening frictional behavior. The existence of this upper region is supported by independent seismologic observations and the results of laboratory friction experiments. In our model, afterslip is the result of relaxation of a stress perturbation within the velocity‐strengthening region, which arises when an earthquake propagates into that region from below. We derive the stress perturbation and its decay from the friction constitutive law using a simple, 1 degree‐of‐freedom approximation for the elastic interaction between the fault and its surroundings. This approximation is based on thickness‐averaged displacements and slip velocities within the velocity‐strengthening region, which is assumed to slip as a rigid block. Coseismic and postseismic slip are coupled through the thickness‐averaged stiffness k of the velocity‐strengthening region. We assume k to be inversely proportional to the thickness of this region, which means that thicker velocity strengthening regions have a greater tendency to arrest coseismic slip. We model the afterslip‐time histories of the 1966 Parkfield and 1987 Superstition Hills earthquakes and relate the model parameters to physical parameters which may govern the rheologic behavior of the faults. In accord with field observations, our model predicts (1) that afterslip on some faults scales with the thickness of the (unconsolidated) sedimentary cover and (2) that proportionally more afterslip occurs for earthquakes in which coseismic surface slip is small compared with coseismic slip at depth. Velocity‐strengthening frictional behavior is to be expected for faults within poorly consolidated sediments and for those that contain significant gouge zones (about >500 m) within their shallow regions (<3–5 km). Combining our results with those of recent laboratory friction studies indicates that relatively young faults with little accumulated fault gouge should exhibit little afterslip.

Abstract. We use geodetic techniques to study the India-Eurasia collision zone. Six years of GPS data constrain maximum surface contraction rates across the Nepal Himalaya to 18 4-2 mm/yr at 12øN 4-13 ø (1or). These surface rates across the 150-km-wide deforming zone are well fitted with a dislocation model of a buried north dipping detachment fault striking 105 ø, which aseismically slips at a rate of 20 :k i mm/yr, our preferred estimate for the India-to-southern-Tibet convergence rate. This is in good agreement with various geologic predictions of 18 4-7 mm/yr for the Himalaya. A better fit can be achieved with a two-fault model, where the western and eastern faults strike 112 ø and 101ø, respectively, in approximate parallelism with the Himalayan arc and a seismicity lineament. We find eastward directed extension of 11 4-3 mm/yr between northwestern Nepal Lhasa, also in good agreement with geologic and seismic studies across the southern Tibetan plateau. Continuous GPS sites are used to further constrain the style and rates of deformation throughout the collision zone. Sites in India, Uzbekistan, and Russia agree within error with plate model prediction.

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.

Global Positioning System (GPS) data from eight sites on the Caribbean plate and five sites on the South American plate were inverted to derive an angular velocity vector describing present-day relative plate motion. Both the Caribbean and South American velocity data fit rigid-plate models to within ؎1-2 mm/yr, the GPS velocity uncertainty. The Caribbean plate moves approximately due east relative to South America at a rate of ϳ20 mm/yr along most of the plate boundary, significantly faster than the NUVEL-1A model prediction, but with similar azimuth. Pure wrenching is concentrated along the approximately east-striking, seismic, El Pilar fault in Venezuela. In contrast, transpression occurs along the 068؇-trending Central Range (Warm Springs) fault in Trinidad, which is aseismic, possibly locked, and oblique to local plate motion.

Spirit leveling data from the Nepal Himalaya between 1977 and 1990 indicate localized uplift at 2–3 mm/yr in the Lesser Himalaya with spatial wavelengths of 25–35 km and at 4–6 mm/yr in the Greater Himalaya with a wavelength of ≈40 km. Leveling data with significantly sparser spatial sampling in southern Tibet between 1959 and 1981 suggest that the Himalayan divide may be rising at a rate of 7.5±5.6 mm/yr relative to central Tibet. We use two‐dimensional dislocation modeling methods to examine a number of structural models that yield vertical velocity fields similar to those observed. Although these models are structurally nonunique, dislocation models that satisfy the data require aseismic slip rates of 2–7 mm/yr on shallow dipping faults beneath the Lesser Himalaya and rates of 9–18 mm/yr on deep thrust faults dipping at ≈25°N near the Greater Himalaya. Unfortunately, the leveling data cannot constrain long‐wavelength uplift (>100 km) across the Himalaya, and unequivocal estimates of aseismic slip in central Nepal are therefore not possible. In turn, the poor spatial density of leveling data in southern Tibet may inadequately sample the processes responsible for the uplift of the Greater Himalaya. Despite these shortcomings in the leveling data, the pattern of uplift is consistent with a crustal scale ramp near the Greater Himalaya linking shallow northward dipping thrust planes (3–6°) beneath the Lesser Himalaya and southern Tibet. Aseismic slip on the potential rupture surface of future great earthquakes beneath the Nepal Himalaya south of this ramp appears not to exceed 30% of the total convergence rate between India and southern Tibet resulting in an accumulating slip deficit of 13±8 mm/yr.