We analyzed the coseismic and early postseismic deformation of the 2015, Mw 8.3 Illapel earthquake by inverting 13 continuous GPS time series. The seismic rupture concentrated in a shallow (<20 km depth) and 100 km long asperity, which slipped up to 8 m, releasing a seismic moment of 3.6 × 1021 Nm (Mw = 8.3). After 43 days, postseismic afterslip encompassed the coseismic rupture. Afterslip concentrated in two main patches of 0.50 m between 20 and 40 km depth along the northern and southern ends of the rupture, partially overlapping the coseismic slip. Afterslip and aftershocks confined to region of positive Coulomb stress change, promoted by the coseismic slip. The early postseismic afterslip was accommodated ~53% aseismically and ~47% seismically by aftershocks. The Illapel earthquake rupture is confined by two low interseismic coupling zones, which coincide with two major features of the subducting Nazca Plate, the Challenger Fault Zone and Juan Fernandez Ridge.

The coseismic‐induced ionospheric total electron content (TEC) perturbations were analyzed following the Mw 7.8 Nepal earthquake (28.147°N, 84.708°E; depth ~15 km) that occurred on 25 April 2015 at 06:11:26 UTC. The ionospheric response is due to both the modes, i.e., shock acoustic waves (slow mode) and Rayleigh wave induced (fast mode). The continuous Global Positioning System (GPS) data at about 60 sites from various GPS networks have been used in the present study. All the sites within epicentral distance of ~2400 km and 70°–170° azimuth recorded the Rayleigh wave‐induced TEC response, while the sites within ~400–2200 km in the same azimuth recorded the response from both the modes. The maximum coseismic‐induced peak‐to‐peak TEC amplitude is ~1.2 total electron content unit, 1 TECU = 1016 el m−2. From Hodochron plot, the apparent Rayleigh wave velocity has been determined as ~2400 m/s on the average and the acoustic wave velocity as 1180 m/s, both these waves being discernible beyond ~1200 km of epicentral distance as also evident from Hodochron plot and wavelet spectrographs. We reckoned the Rayleigh wave group velocities using ionospheric response at selected radial pairs of stations and validated. The ionospheric response distribution seen mainly depending on the epicentral distance, satellite geometry, directivity of radiation pattern, and the upper crustal heterogeneity. This study highlights the characteristics of ionospheric response consequent to the 2015 Nepal earthquake.

The 11 April 2012 East Indian Ocean earthquake is unique because of its largest ever recorded aftershock. The main earthquake occurred with a magnitude of 8.6 Mw and was followed by a strong aftershock (8.2 Mw). Our analysis of the main shock indicates that the rupture was a mixture of strike-slip and thrust faults, and significant vertical surface displacements were observed during the event. The prime interest here is to study the post-seismic ionospheric disturbances, along with their characteristics. As both earthquakes had large magnitudes, they provided an opportunity to minimize the ambiguity in identifying the corresponding seismic-induced ionospheric disturbances. Approximately 10 min after both seismic events, the nearby ionosphere started to manifest electron density perturbations that were investigated using GPS-TEC measurements. The epicenters of both events were located south of the magnetic equator, and it is believed that the varying magnetic field inclination might be responsible for the observed north-south asymmetry in the post-seismic total electron content (TEC) disturbances. These disturbances are observed to propagate up to approximately 1,500 km towards the north side of the epicenter and up to only a few hundred kilometers on the south side. The frequency analysis of the post-seismic TEC disturbances after both earthquakes exhibits the dominant presence of acoustic frequencies varying between approximately 4.0 to 6.0 mHz. The estimated propagation velocities of the post-seismic TEC disturbances during the main shock (0.89 km/s) and aftershock (0.77 km/s) confirm the presence of an acoustic frequency as the generative mode for the observed TEC fluctuations.

No abstract

No abstract

SUMMARY Underthrusting of the Indian Plate beneath the Himalaya has been the cause of many hazardous thrust‐faulting earthquakes along the arc. Moderate earthquakes with magnitudes of ≤5 occur frequently in this region, releasing the elastic strain accumulated over many years around the plate boundary. These events can be attributed to slip deficits where the Indian and Eurasian Plates are locked during interseismic periods. Geodetic measurements can help discriminate the distribution of the interlocking areas and the steadily slipping areas beneath the Himalaya. To understand the deformation across the Central and Western Himalaya and the associated slip on thrust faults, campaign‐mode GPS data were collected in the Garhwal–Kumaun region of the Western Himalaya. GPS sites velocities show that the deformation is currently concentrated between the Lesser and Higher Himalaya. Horizontal velocities are used to estimate the slip distribution. For the estimation, a model of the interseismic surface deformation caused by buried non‐uniform creep dislocation (NUC) on a curved fault surface is used. The slip distribution shows that there might be structural discontinuity on the fault between the Kumaun and Garhwal regions of the Himalaya. The estimated slip rate at the depth around 20–40 km in the Central Himalaya and at the depth of ∼15 km in the Western Himalaya is 10 mm yr–1. The NUC model indicates that the shallow part (<20 km) of the thrust fault system along the plate boundary is almost locked. The locking depth appears to be deep in the Central Himalaya and shallower in the Western Himalaya. Further, most of the historical large seismic events are observed to have occurred in an area with a slip velocity less than 10 mm yr–1 (i.e. in a locked zone) at the plate interface.

The India-Eurasia collision, driven by tectonic forcing, is modulated by nontectonic forcing allied to seasonal variations in the neighboring regions. To decipher the ground deformation in response to hydrological mass variations of the Himalaya and North India, we analyzed continuous Global Positioning System (cGPS) observations from 50 sites together with Gravity Recovery and Climate Experiment (GRACE) data for the period 2004-2015. Vertical components of surface deformation derived from GPS and GRACE show moderate to high-level amplitude correlation with a slope value of 0.76 and a level of phase delay from ±25°to ±30°. The average weighted root-mean-square reduction (WRMS) of 17.72% suggests the prominence of hydrological mass variations particularly over the sub-Himalaya and Indo-Gangetic Plain (IGP). GPS-derived vertical deformation after correcting the hydrological effects utilizing GRACE observations suggests that the sub-Himalaya and IGP are undergoing subsidence and the surrounding areas show uplift. In addition to the tectonic and nontectonic forcings, an unsustainable consumption of groundwater associated to irrigation and other anthropogenic uses influence the subsidence rate in the IGP and sub-Himalaya. Further, 2-D elastic dislocation modeling suggests that GRACE correction to the GPS vertical velocity causes a reduction in the subsurface slip rate estimation over the Main Himalayan Thrust (MHT) system by 12.06% and improves the chi-square misfit by 20.32%.

No abstract