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We present a case of remotely triggered seismicity in Southwest China by the 2015/04/25 M7.8 Gorkha, Nepal earthquake. A local magnitude M L 3.8 event occurred near the Qijiang district south of Chongqing city approximately 12 min after the Gorkha mainshock. Within 30 km of this M L 3.8 event there are 62 earthquakes since 2009 and only 7 M L > 3 events, which corresponds to a likelihood of 0.3% for a M L > 3 on any given day by a random chance. This observation motivates us to investigate the relationship between the M L 3.8 event and the Gorkha mainshock. The M L 3.8 event was listed in the China Earthquake National Center (CENC) catalog and occurred at shallow depth (∼3 km). By examining high-frequency waveforms, we identify a smaller local event ( ∼ M L 2.5 ) ∼ 15 s before the M L 3.8 event. Both events occurred during the first two cycles of the Rayleigh waves from the Gorkha mainshock. We perform seismic event detection based on envelope function and waveform matching by using the two events as templates. Both analyses found a statistically significant rate change during the mainshock, suggesting that they were indeed dynamically triggered by the Rayleigh waves. Both events occurred during the peak normal and dilatational stress changes (∼10–30 kPa), consistent with observations of dynamic triggering in other geothermal/volcanic regions. Although other recent events (i.e., the 2011 M9.1 Tohoku-Oki earthquake) produced similar peak ground velocities, the 2015 Gorkha mainshock was the only event that produced clear dynamic triggering in this region. The triggering site is close to hydraulic fracturing wells that began production in 2013–2014. Hence we suspect that fluid injections may increase the region's susceptibility to remote dynamic triggering.

The October 17, 1989 M w 6.9 Loma Prieta earthquake provides the first opportunity of probing the crustal and upper mantle rheology in the San Francisco Bay Area since the 1906 M w 7.9 San Francisco earthquake. Here we use geodetic observations including GPS and InSAR to characterize the Loma Prieta earthquake postseismic displacements from 1989 to 2013. Pre-earthquake deformation rates are constrained by nearly 20 yr of USGS trilateration measurements and removed from the postseismic measurements prior to the analysis. We observe GPS horizontal displacements at mean rates of 1–4 mm/yr toward Loma Prieta Mountain until 2000, and ∼2 mm/yr surface subsidence of the northern Santa Cruz Mountains between 1992 and 2002 shown by InSAR, which is not associated with the seasonal and longer-term hydrological deformation in the adjoining Santa Clara Valley. Previous work indicates afterslip dominated in the early (1989–1994) postseismic period, so we focus on modeling the postseismic viscoelastic relaxation constrained by the geodetic observations after 1994. The best fitting model shows an elastic 19-km-thick upper crust above an 11-km-thick viscoelastic lower crust with viscosity of ∼ 6 × 10 18 Pa s , underlain by a viscous upper mantle with viscosity between 3 × 10 18 and 2 × 10 19 Pa s . The millimeter-scale postseismic deformation does not resolve the viscosity in the different layers very well, and the lower-crustal relaxation may be localized in a narrow shear zone. However, the inferred lithospheric rheology is consistent with previous estimates based on post-1906 San Francisco earthquake measurements along the San Andreas fault system. The viscoelastic relaxation may also contribute to the enduring increase of aseismic slip and repeating earthquake activity on the San Andreas fault near San Juan Bautista, which continued for at least a decade after the Loma Prieta event.

Fault slip may involve slow aseismic creep and fast seismic rupture, radiating seismic waves manifested as earthquakes. These two complementary behaviors accommodate the long-term plate convergence of major subduction zones and are attributed to fault frictional properties. It is conventionally assumed that zones capable of seismic rupture on the subduction megathrust are confined to between about 10 to 50 km depth; however, the actual spatiotemporal distribution of fault mechanical parameters remains elusive for most subduction zones. The 2011 Tohoku Mw 9.0 earthquake ruptured with > 50 m slip up to the trench, thus challenging this conventional assumption, and provides a unique opportunity to probe the mechanical properties of the Japan subduction zone. Drawing on the inferred distribution of coseismic and postseismic slip, it has recently been suggested that portions of the megathrust are capable of switching between seismic and aseismic behavior. Kinematic models of the coseismic rupture and 15-month postseismic afterslip of this event suggest that the coseismic rupture triggered widespread frictional afterslip with equivalent moment magnitude of 8.17–8.53, in addition to viscoelastic relaxation in the underlying mantle. The identified linear relation between modeled afterslip, slip inferred from repeating earthquakes on the plate interface, and the cumulative number of aftershocks within 15 km distance of the subduction thrust suggests that most aftershocks are a direct result of afterslip. We constrain heterogeneous rate-state friction parameters of the subduction thrust from the computed coseismic stress changes and afterslip response. Our results indicate a variable pattern along dip and strike, characterizing areas down-dip and south of the main rupture zone as having velocity-strengthening properties. In agreement with seismic tomographic models of plate boundary elastic properties and geologic evidence for previous M > 8.5 megathrust earthquakes on this section of the plate boundary, we suggest that the obtained pattern of the frictional properties is characteristic of subducted material and thus persistent in time and space.

Article history: Accepted 23 October 2013 Available online xxxx Editor: P. Shearer USArray has facilitated significant advancement in tomographic models and methodologies. While there persists a fundamental tradeoff between horizontal and vertical resolution due to the components of the wave train analyzed, advances in joint inversions are continuing to refine the tomographic images generated with USArray. The DNA13 model incorporates teleseismic P observations, independent SH and SV observations, and surface-wave phase velocities from both teleseismic earthquakes and ambient noise to constrain the relative wave-speed from the crust down into the lower mantle. We address the validity of our models through forward prediction of observables and compare the predictive power of the DNA13 models to other models. In the shallow portion of DNA13, we image the Archean age Wyoming Province, which exhibits evidence of ocean closure at its northern and southern ends. The Llano Province in Central Texas is of Grenville age and still contains lithospheric evidence of subduction as the province accreted to North America. Comparison of these two provinces highlights the role of fossil slabs as part of the cratonic architecture. Analysis of the deep portion of the models highlights variations within the Farallon plate, including two distinct high wave-speed anomalies in the eastern U.S. and a shallow feature in the center of these two anomalies. We propose this is evidence of an oceanic plateau, which provides the necessary positive buoyancy to promote flat-slab subduction of the Farallon plate. Published by Elsevier B.V.

Using seismic surface waves recorded with Earthscope's Transportable Array, we apply surface wave imaging to determine 3D seismic velocity in the crust and uppermost mantle. Our images span several Proterozoic and early Cambrian rift zones (Mid-Continent Rift, Rough Creek Graben—Rome trough, Birmingham trough, Southern Oklahoma Aulacogen, and Reelfoot Rift). While ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences, the Reelfoot Rift is unique in its association with low mantle seismic velocity. Its mantle low-velocity zone (LVZ) is exceptionally pronounced and extends down to at least 200 km depth. This LVZ is of variable width, being relatively narrow ( ∼ 50 km wide) within the northern Reelfoot Rift, which hosts the New Madrid Seismic Zone (NMSZ). We hypothesize that this mantle volume is weaker than its surroundings and that the Reelfoot Rift consequently has relatively low elastic plate thickness, which would tend to concentrate tectonic stress within this zone. No other intraplate ancient rift zone is known to be associated with such a deep mantle low-velocity anomaly, which suggests that the NMSZ is more susceptible to external stress perturbations than other ancient rift zones.

Geodetic data indicate that the M7.1 Hector Mine, California, earthquake was followed by a brief period (a few weeks) of rapid deformation preceding a prolonged phase of slower deformation. We find that the signal contained in continuous and campaign global positioning system data for 2.5 years after the earthquake may be explained with a transient rheology. Quantitative modeling of these data with allowance for transient (linear biviscous) rheology in the lower crust and upper mantle demonstrates that transient rheology in the upper mantle is dominant, its material properties being typified by two characteristic relaxation times ∼0.07 and ∼2 years. The inferred mantle rheology is a Jeffreys solid in which the transient and steady-state shear moduli are equal. Consideration of a simpler viscoelastic model with a linear univiscous rheology (2 fewer parameters than a biviscous model) shows that it consistently underpredicts the amplitude of the first ∼3 months signal, and allowance for a biviscous rheology is significant at the 99.0% confidence level. Another alternative model – deep postseismic afterslip beneath the coseismic rupture – predicts a vertical velocity pattern opposite to the observed pattern at all time periods considered. Despite its plausibility, the advocated biviscous rheology model is non-unique and should be regarded as a viable alternative to the non-linear mantle rheology model for governing postseismic flow beneath the Mojave Desert.

The gravity anomaly resulting from mass redistribution from an earthquake source (point dislocation) is re-explored in a spherical Earth geometry. Using a previously developed method of determining the Green's functions for the static displacement field on a layered Earth, I obtain the corresponding Green's functions for the perturbed gravitational potential and gravity anomaly in a spherical harmonic expansion. The synthesized gravity anomaly includes the effects of changes in the intrinsic density distribution due to divergence of the displacement field and vertical displacement along level surfaces in density (stratification displacement). Numerical examples of the Bouguer gravity anomaly produced on a layered crustal model show that the contribution of intrinsic density changes dominates over the contribution of stratification displacement. Application to central Japan yields a new image of a buried thrust structure suspected of having ruptured in the 1891 Nobi earthquake.

The convergence of the Philippine Sea and Eurasia plates is accommodated primarily by plate subduction along the Nankai trough west of about 137.3°E, but additionally by widely distributed onshore and offshore deformation to the east. This zone, with a total dimension of 200 km (parallel to the Nankai trough) by 350 km, has developed as one highly coupled region since the collision of the Izu peninsula with mainland Japan about 1–2 m.y. ago. The onshore faulting in central Japan is consistent with a WNW-ESE direction of maximum horizontal stress, while the oceanward deformation is driven by diffuse NW-SE shortening of the Izu-Ogasawara volcanic ridge which produces a kinematic discontinuity with respect to the Philippine Sea oceanic crust to its west. The strain release associated with the 1891 Nobi earthquake, and the onshore deformation as a whole, suggests that the Philippine Sea-Eurasia convergence is partially accommodated by onshore shortening between the Tsuruga Bay-Ise Bay Tectonic Line and the Izu peninsula.

The absolute motion of Africa (relative to the hotspots) for the past 30 My is modeled with two Euler vectors, with a change occurring at 6 Ma. Because of the high sensitivity of African absolute motions to errors in the absolute motions of the North America and Pacific plates, both the pre-6 Ma and post-6 Ma African absolute motions are determined simultaneously with North America and Pacific absolute motions for various epochs. Geologic data from the northern Atlantic and hotspot tracks from the African plate are used to augment previous data sets for the North America and Pacific plates. The difference between the pre-6 Ma and post-6 Ma absolute plate motions may be represented as a counterclockwise rotation about a pole at 48 °S, 84 °E, with angular velocity 0.085 °/My. This change is supported by geologic evidence along a large portion of the African plate boundary, including the Red Sea and Gulf of Aden spreading systems, the Alpine deformation zone, and the central and southern mid-Atlantic Ridge. Although the change is modeled as one abrupt transition at 6 Ma, it was most likely a gradual change spanning the period 8-4 Ma. As a likely mechanism for the change, we favor strong asthenospheric return flow from the Afar hotspot towards the southwest; this could produce the uniform southwesterly shift in absolute motion which we have inferred as well as provide a mechanism for the opening of the East African Rift. Comparing the absolute motions of the North America and Pacific plates with earlier estimates, the pole positions are revised by up to 5° and the angular velocities are decreased by 10–20%.