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.

We examine a century of seismicity and earthquake-related deformation on the western margin of the Indian plate in the Pakistan region of northern Baluchistan. Several catalogs of earthquakes for this region currently exist, but early catalogs in particular suffer from errors, incompleteness, inhomogeneity, and location bias. We form a new catalog of more than 1000 earthquakes using original sources to confirm macroseismic locations and assign M S magnitudes to earthquakes since 1892. In doing so we reveal a systematic east-northeast bias in locations caused largely by the uneven global distribution of seismic stations used in their determination. An appendix provides narrative accounts and historical references to 34 significant earthquakes in the region.The pattern of seismicity in the past century shows activity over a 700-km-long, 200-km-wide segment of the plate boundary with predominantly strike-slip faulting to the west and thrust faulting to the east. At its narrowest near 29Њ N, transpression of the plate boundary is partitioned into reverse and strike-slip components separated by approximately 100 km. The M 7.3 1931 Mach earthquake (slip 1.1 m on a 40Њ east-southeast-dipping reverse fault) released fault-normal stresses that may have "unclamped" the subsequent M 7.7 left-lateral Quetta earthquake 4 years later. The northern Chaman fault system in the past century has been largely inactive, suggesting that this time period is not representative of long-term activity in the region and that up to 4 m of potential slip is currently available to drive one or more future M Ͼ 7 earthquakes. Despite triangulation installed in the late nineteenth and early twentieth centuries, no recent geodetic data are available to permit plate boundary velocities to be measured directly.

Landsat satellite image (LE70390372003084EDC00) showing location of surface slip triggered along faults in the greater Salton Trough area. Red bars show the generalized location of 2010 surface slip along faults in the central Salton Trough and many additional faults in the southwestern section of the Salton Trough. Surface slip in the central Salton Trough shown only where verified in the field; slip in the southwestern section of the Salton Trough shown where verified in the field or inferred from UAVSAR images.

The magnitude 7.8 Gorkha earthquake in April 2015 ruptured a 150-km-long section of the Himalayan décollement terminating close to Kathmandu 1-4 . The earthquake failed to rupture the surface Himalayan frontal thrusts and raised concern that a future M w ≤ 7.3 earthquake could break the unruptured region to the south and west of Kathmandu. Here we use GPS records of surface motions to show that no aseismic slip occurred on the ruptured fault plane in the six months immediately following the earthquake. We find that although 70 mm of afterslip occurred locally north of the rupture, fewer than 25 mm of afterslip occurred in a narrow zone to the south. Rapid initial afterslip north of the rupture was largely complete in six months, releasing aseismic-moment equivalent to a M w 7.1 earthquake. Historical earthquakes in 1803, 1833, 1905 and 1947 also failed to rupture the Himalayan frontal faults, and were not followed by large earthquakes to their south. This implies that significant relict heterogeneous strain prevails throughout the Main Himalayan Thrust. The considerable slip during great Himalayan earthquakes may be due in part to great earthquakes tapping reservoirs of residual strain inherited from former partial ruptures of the Main Himalayan Thrust.The spatial distribution of surface deformation in the April M w 7.8 Nepal earthquake and its M w 7.3 aftershock ten days later was captured by several InSAR scenes and by continuously operating GPS receivers 1-8 . These data indicate that an average slip of 3.5 m occurred on a 60-km-wide × 150-km-long rupture of the northdipping décollement, the Main Himalayan Thrust (MHT), with maximum slip locally reaching 7 m approximately 15 km north of Kathmandu. Rupture propagated eastwards along the region of maximum interseismic strain near the Greater Himalaya, but terminated to the south near the Kathmandu valley (Fig. 1).The interseismic convergence rate in Nepal observed geodetically 9,10 is 18-20 mm yr −1 , similar to the rate of advance of the Himalaya over the Indian plate inferred from geological evidence 11 . The Indian plate descends beneath Tibet aseismically north of a seismically active transition zone in northern Nepal, south of which three decades of geodetic data indicate that the MHT is effectively locked 12 . The occasional rupture of this locked décollement accommodates the incremental northward passage of the Indian plate beneath the Himalaya. A several-thousand year record of this slip is emerging from the exhumation of surface ruptures of Himalayan frontal faults 13-15 (Main Frontal Thrust, MFT); however, the 2015 earthquake is one of several M ≤ 7.8Himalayan earthquakes whose southward rupture failed to rupture the MFT, and whose contribution to facilitating Himalayan moment release thereby eludes palaeoseismic investigation.InSAR data reveal that the 2015 earthquake did, however, trigger ∼5 cm of near-surface slip on a subsidiary branch of the frontal fault, the Main Dun Thrust (MDT), with no significant coseismic slip on the décollement between the...