The Mineral, Virginia (USA), earthquake occurred at 17:51:3.9 UTC (Coordinated Universal Time) on 23 August 2011; the hypocenter was at 37.905°N, 77.975°W and depth was 8 km. The widely reported moment magnitude (Mw) was 5.7 ± 0.1. The mb (teleseismic short-period) magnitude estimated here using 26 global network stations is 5.77 ± 0.23, in agreement with mb values reported by national and international data centers, and not significantly different from Mw. However, the mbLg magnitude of the earthquake determined here using 386 stations in eastern North America is 6.28 ± 0.26. The mbLg magnitude is a short-period magnitude correlated with mb that is based on the amplitude of the Lg phase at regional distances. Lg is a crust-guided phase that represents the largest amplitudes observed on short-period seismograms at regional distances in eastern North America. The mbLg magnitude was the primary magnitude appearing in catalogs of eastern United States earthquakes until superseded recently by Mw. The catalog of previous earthquakes in central Virginia is keyed to mbLg, rather than Mw. The Mineral shock reveals large regional variations in the Lg phase attenuation in the eastern United States. The expected value for the return period of mbLg 6.3 and larger earthquakes in the Central Virginia seismic zone is 752 yr, with a 95% confidence interval of 385–1471 yr. The Mineral earthquake caused Modified Mercalli Intensity (MMI) VIII damage in the epicentral area, with several instances of partial and total collapse of masonry chimneys and walls. A finite-fault, full wavefield simulation of the motions within 30 km of the epicenter fits the velocity recordings and Fourier spectral amplitudes in the 1–10 Hz frequency band, at the only strong-motion station in that distance range. The strongest motions are predicted to have occurred in two areas offset to the northwest and southeast of the epicenter, within which peak ground accelerations may have approached 2 g, and peak velocities were probably well in excess of 20 cm/s. The only factor mitigating damage in this earthquake was the brief (<3 s) duration of strong shaking.

The 23 August 2011 M 5.7 intraplate earthquake occurred in the Central Virginia seismic zone near Mineral, Virginia (USA), far from the nearest plate boundaries. I suggest here that this earthquake, as well as others occurring in this region since 1774, was triggered by pore-fluid pressure diffusion associated with groundwater recharge. Using finite element modeling (FEM) estimates are made of the magnitude and timing of pressure diffusion with respect to the time of the earthquake. Two scenarios are considered: (1) the diffusion took place along vertical to near-vertical diffusion paths, or (2) the diffusion was restricted to a hydraulically transmissive fracture zone that was later illuminated by thousands of aftershocks. Both scenarios have merit. The fracture zone may not have been entirely generated by the main shock. For either model a fractured crust is assumed to be recharged at the surface of the Earth by groundwater recharge with subsequent pore-fluid pressure diffusion propagating to the hypocenter. The transient behavior of pore-fluid pressure is examined at the focal depth of 8 km. These results are compared with the duration and timing of base flow (groundwater recharge) as estimated from a hydrograph separation. The delays of the peaks in pore-fluid pressure diffusion as computed by the FEM simulations are found to be consistent with the start and duration of groundwater recharge with respect to the timing of the Virginia earthquake.

The dynamics of the erosive central Andean forearc vary significantly along strike. In northern Chile at 20°S–27°S, and particularly at 22°S–25°S, the forearc in the Coastal Cordillera has been undergoing extension since at least the Pliocene, reactivating steep E-dipping faults of the Mesozoic Atacama fault system. This has been explained by forearc uplift driven by underplating, shallow slab dip, subduction of bathymetric features, and elastic rebound during the earthquake cycle. These processes, however, are active over a much wider area of the central Andean forearc than Coastal Cordillera extension and therefore cannot explain why extension is localized to the northern Chilean onshore outer forearc. We compiled crustal seismicity and the depth of lithospheric boundaries from existing studies to investigate other possible explanations for onshore forearc extension. At 22°S–25°S, seismicity increases above the background subduction-related level present to the north and south. Extensional focal mechanisms, consistent with steep E-dipping faults active at depths up to ~40 km, are also present onshore within this latitude range, but absent to the north and south; this is consistent with the distribution of mapped active fault scarps. The Salar de Atacama crust is seismically active at depths up to ~40 km. Thick lithosphere is present beneath the forearc, and the longitudinal axis of thickest lithosphere is deflected to the east at the latitude of the Salar de Atacama. To the east, the Puna Plateau lithosphere has been thinned by lithospheric removal events. The most robust correlation with onshore forearc extension is the thick, cold, strong crust and mantle lithosphere beneath the anomalous Salar de Atacama in the inner forearc of northern Chile. The combination of underplating-driven outer forearc uplift, the presence of the preexisting structure of the Atacama fault system favorable for reactivation, and the negative buoyancy beneath the Salar de Atacama is inferred to drive Coastal Cordillera normal faulting at this latitude. Recent (<10 Ma) lithospheric removal beneath the Puna Plateau to the east may have enhanced the effect of the negatively buoyant Salar de Atacama lithosphere on the forearc. This implies that both preexisting lithospheric structure and lithospheric processes in the hinterland may influence forearc dynamics.

An analysis of seismicity in the northern Mojave block, southern Walker Lane belt, and southern Sierra Nevada provides a detailed snapshot of the kinematics of active deformation within a young and probably evolving intracontinental plate boundary in eastern California. Earthquakes in this region were relocated using joint hypocentral inversion, double-difference, and waveform cross-correlation techniques. Groups of focal mechanisms were inverted for the components of a reduced deformation rate tensor, and the inversion results were synthesized in maps of the regional seismogenic deformation field. In general, seismogenic deformation in the Eastern California shear zone and Walker Lane belt is characterized by distributed simple shear and primarily accommodates northwest translation of the Sierra Nevada–Central Valley (Sierran) microplate relative to stable North America. Crustal thinning in the southern Walker Lane belt is subordinate to northwest-directed dextral shear and typically is associated with releasing geometries among dextral faults. There is no significant variation in strain geometry in the transition from the Eastern California shear zone to the Walker Lane belt across the Garlock fault; we interpret this to indicate that this structure is being sheared and rotated clockwise by distributed northwest dextral shear. In contrast, seismogenic deformation in the southeastern Sierra Nevada near latitude 36°N is characterized by horizontal elongation and locally by flattening, i.e., extension in two perpendicular horizontal directions. The transition from wrench tectonics in the southern Walker Lane to flattening in the High Sierra occurs across a 25- to 35-km-wide transtensional zone west of the Sierran range front. The elongation and flattening occur directly east of the “Isabella anomaly,” a high-velocity anomaly in the upper mantle interpreted to be lower Sierran lithosphere that is foundering or convectively descending into the asthenosphere. The seismogenic thinning of the southern Sierra may be related to thickening above the descending lithosphere to the west and/or may be driven by tensile buoyancy forces. The seismotectonics of this region may provide insights into the process by which the Walker Lane belt has progressively expanded westward into the Sierran microplate during late Cenozoic time.

The Ilan Basin of northern Taiwan forms the western limit of the Okinawa Trough, where the trough meets the compressional ranges of central Taiwan. Apatite fission-track ages of 1.2 ± 0.5 Ma and 3.5 ± 0.5 Ma, measured north and south of the basin, respectively, indicate faster exhumation rates in the Hsüehshan Range to the north (>1.6 mm/yr) than in the Backbone Range to the south (0.7 mm/yr). Reconstructed subsidence rates along the northern basin margin are also faster than in the south (6–7 compared with 3–5 mm/yr). Global positioning system (GPS) and active seismological data indicate motion of the southern basin margin to the east and southeast. We propose that the Ilan Basin is being formed as a result of extension of northern Taiwan, largely controlled by a major southeast-dipping fault, modeled at ∼30° dip, and mapped as a continuation of the Lishan Fault, a major thrust structure in the Central Ranges. Flexural rigidity of the lithosphere under the basin is low, with elastic thickness ∼3 km. A southwest-migrating collision between the Luzon Arc and southern China, accompanied by subduction polarity reversal in the Ryukyu Trench, has allowed crustal blocks that were previously held in compression between the Eurasian and Philippine Sea plates to move trenchward as they reach the northern end of the collision zone. Subduction polarity reversal permits rapid extension and formation of the Ilan Basin and presumably, at least, the western Okinawa Trough, as a direct consequence of arc-continent collision, not because of independent trench rollback forces. This conceptual model suggests that migrating arc-continent collision causes the rapid formation of deep marginal basins that are then filled by detritus from the adjacent orogen, and that these should be common features in the geologic record.

The structural style of the southeastern region of the Dezful Embayment (Zagros fold-and-thrust belt) was defined through the interpretation of satellite images, field surveys, well stratigraphy, and magnetotelluric and seismic profiles. The compressional setting of the area derives from the last phases of Cenozoic continental collision between the Arabian Plate and Eurasia. The shortening affected an ∼13 km thick sedimentary succession comprising Proterozoic units (Hormuz Formation?), Cambrian to Early Miocene platform sediments (mainly carbonates), the Gachsaran evaporites, and Mio-Pliocene foredeep clastic deposits. Although in the Dezful Embayment there is no evidence for outcropping Hormuz salt, the structural features of the area (the style of folds, their lack of consistent vergences, the low-angle tectonic taper) suggest the existence of a lower ductile detachment level of possible great thickness and low frictional resistance. The rounded folds in the carbonates are forced folds detached over a deep ductile level, subsequently faulted with progressive deformation by characteristic steepening up reverse faults. Second-order asymmetric folds affecting the limbs of the main structures indicate a flexural slip mechanism of folding in a multilayered and mechanically heterogeneous (carbonates and shales) succession. The Gachsaran Formation is deformed by shorter, strongly disharmonic folds and thrusts, and acted as a decoupling level for the overlying folds, whose synclines often directly overlie the carbonate anticlines. A regional restored cross section indicates a shortening of 11.5% in the area. The involvement of the crystalline basement in thrusting and the deep structural style of the area are discussed in terms of earthquake mechanisms, focal depths, structural analysis, and stratigraphic considerations.

High-resolution seismicity data from Costa Rica and southern Nicaragua have been used to image the geometry of the Wadati-Benioff zone under southern Central America. The data include 9,514 events with computed horizontal and vertical errors smaller than 4 and 5 km respectively. We found that under the Nicaragua-Costa Rica border the Wadati-Benioff zone contorts (from steep to shallow dip angles, northwest to southeast) but does not show evidence of a brittle tear, as postulated by others. Further to the southeast—northeast from Puerto Caldera—the Wadati-Benioff zone does, however, show a tear (the Quesada Sharp Contortion) at intermediate depths (h > 70 km). Overall the dip angle of the Wadati-Benioff zone decreases from 84° under Nicaragua to 60° under central Costa Rica. The maximum depth of intraplate Wadati-Benioff zone earthquakes also decreases from northwest (-200 km under Nicaragua) to southeast (-125 km under central Costa Rica). In southern Costa Rica, east of 83°55′W, we Find no evidence of the Wadati-Benioff zone below 50 km. This geometry of the Wadati-Benioff zone and other tectonic features related to the subduction of the Cocos plate beneath the Caribbean plate have been integrated into a model that correlates them with along-trench variations in age of the subducted Cocos plate. These tectonic features are (1) the shallowing of the Middle America Trench bathymetry from northwest to southeast, (2) differences in coupling between Cocos and Caribbean plates, (3) the termination of the Central America Volcanic Chain in central Costa Rica, and (4) distinct stress field variations on the overriding Caribbean plate. The subduction of the Cocos Ridge under southern Costa Rica is partially responsible for some of these features (shallowing of the Middle America Trench in southern Costa Rica) and for the high uplift rates of the outer arc. However, since the presence of the Panama Fracture Zone limits the subducted extension of the Cocos Ridge to less than 100 km from the trench, we propose that the overall geometry of the Wadati-Benioff zone is controlled by abrupt along-trench changes in the age of the subducted Cocos plate.

The seismotectonic pattern determined along a 65-km-wide corridor across the central California Coast Ranges (Dehlinger and Bolt, 1987), from the San Andreas fault to seaward of the Hosgri fault, is used to identify associated structures in the upper crust. The seismogenic zone is approximately 12 km thick in the corridor, and forms a 90-km-wide border zone of the upper Pacific lithospheric plate. This border includes three provinces (from northeast to southwest): the San Andreas fault zone, an adjacent 40- to 50-km-wide seismically quiescent province, and a 40- to 50-km-wide compressive province along the coastline. These provinces are characterized by distinct focal parameters and distinct rock types, and transition boundaries between these provinces are relatively narrow. The upper crust in the quiescent province consists of high-strength granites of the Salinian block; in both the San Andreas and the contractional provinces, this part of the crust consists of low-strength Franciscan rocks. We conclude that differential strengths of the upper crustal rocks in the corridor have modified the broader, more regional stress fields acting across the North American-Pacific plates to produce the observed seismicity. The extent to which the earthquake data in the corridor corroborate the existence of a proposed deep detachment surface is examined. The detachment model has been suggested to account for the crustal shortening observed across the Coast Ranges (Crouch et al., 1984; Eaton, 1985), where lower crustal materials are being recycled into the mantle. The set of earthquake data analyzed here does not imply the presence of a detachment within the seismogenic zone; neither, although less directly, does it imply such a detachment at greater depth. If a deep detachment due to horizontal shortening does exist, it would be restricted to the southwest half of the corridor, as strike-slip, not horizontal shortening, predominates in the northeast half of the corridor. An alternate model, in which deformation beneath the seismogenic zone occurs by creep and flow over an extensive depth range, can be made to conform with upper crustal shortening in the southwest part of the corridor and with horizontal slip in the quiescent and the San Andreas provinces. Such types of deeper deformation are more consistent with the earthquake focal parameters in the corridor than is a detachment at or below the base of the seismogenic zone.

The combination of moderate seismic activity, sparsity of seismograph stations and relatively low density of population makes it difficult to assign quantitative seismicity values to most of the central United States. In the New Madrid seismic zone, where the level of seismic activity is higher and the number of seismograph stations is more adequate, one can delineate the active fault zone and determine a magnitude-recurrence relation. These capabilities will be extended to other seismic zones as arrays of seismographs are installed for recording microearthquakes, which will give information on fault delineation, focal mechanism, and magnitude frequency. Only after such information is available will we be able to make positive statements relating seismic activity to specific geologic features.The seismicity data suggest that earthquakes that occur outside the recognized seismic zones or major structural features will have a maximum body-wave magnitude mb of 5.5 and that this maximum value will occur only infrequently. Experience shows that if these relatively minor earthquakes are only a few kilometres deep they may have an epicentral intensity at least as large as VII (observed for an earthquake of mb = 3.8), but their magnitude and area of perceptibility will be small. With the exception of the New Madrid seismic zone and possibly the Wabash Valley seismic zone, a conservatively reasonable value for the maximum body-wave magnitude to be expected in the major seismic and structural zones of the central United States is 6.5. For the New Madrid seismic zone an earthquake of body-wave magnitude equivalent to that of a great earthquake (mb = 7.5) can be expected, on the basis of what has already been experienced in 1811-1812.Because of low anelastic attenuation, the earthquakes in the central United States are felt and cause damage over much wider areas than earthquakes of comparable magnitude in the western United States. Further consequences are that the ground shaking has a longer duration and that the ground-motion spectrum shifts at the larger distances to lower frequencies, which results in relatively low ground acceleration for relatively large ground displacements and a greater effect on high-rise than low-rise structures.