Tectonic tremor on Vancouver Island, Cascadia, modulated by the body and surface waves of the Mw 8.6 and 8.2, 2012 East Indian Ocean earthquakes

The 2012 East Indian Ocean earthquake (Mw 8.6), so far the largest intraoceanic plate strike-slip event ever recorded, modulated tectonic tremors in the Cascadia subduction zone. The rate of tremor activity near Vancouver Island increased by about 1.5 times from its background level during the passage of seismic waves of this earthquake. In most cases of dynamic modulation, large-amplitude and long-period surface waves stimulate tremors. However, in this case even the small stress change caused by body waves generated by the 2012 earthquake modulated tremor activity. The tremor modulation continued during the passage of the surface waves, subsequent to which the tremor activity returned to background rates. Similar tremor modulation is observed during the passage of the teleseismic waves from the Mw 8.2 event, which occurs about 2 h later near the Mw 8.6 event. We show that dynamic stresses from back-to-back large teleseismic events can strongly influence tremor sources

Locked and loading megathrust linked to active subduction beneath the Indo-Burman Ranges

The Indo-Burman mountain rangesmarkthe boundary between the Indian and Eurasian plates, north of the Sumatra–Andaman subduction zone. Whether subduction still occurs along this subaerial section of the plate boundary, with 46mm/yr of highly oblique motion, is contentious. About 21mm/yr of shear motion is taken up along the Sagaing Fault, on the eastern margin of the deformation zone. It has been suggested that the remainder of the relative motion is taken up largely or entirely by horizontal strike-slip faulting and that subduction has stopped. Here we present GPS measurements of plate motions in Bangladesh, combined with measurements from Myanmar and northeast India, taking advantage of a more than 300 km subaerial accretionary prism spanning the Indo-Burman Ranges to the Ganges–Brahmaputra Delta. They reveal 13–17mm/yr of plate convergence on an active, shallowly dipping and locked megathrust fault. Most of the strike-slip motion occurs on a few steep faults, consistent with patterns of strain partitioning in subduction zones. Our results strongly suggest that subduction in this region is active, despite the highly oblique plate motion and thick sediments. We suggest that the presence of a locked megathrust plate boundary represents an underappreciated hazard in one of the most densely populated regions of the world

InSAR and GPS measurements of crustal deformation due to seasonal loading of Tehri reservoir in Garhwal Himalaya, India

We report unique observations of crustal deformation caused by the seasonal water level changes of Tehri reservoir in the Garhwal region ofNWHimalaya from GPS measurements and Interferometric Synthetic Aperture Radar (InSAR) analysis. All GPS sites along the Himalaya are strongly influenced by seasonal hydrological and atmospheric loading. However, the GPS siteKUNRlocated near the reservoir additionally exhibits anomalous variations due to seasonal water loading and unloading by the reservoir. Our InSAR analysis confirms that the seasonal filling of the reservoir causes measurable subsidence in its neighbourhood. In addition to the elastic deformation caused by the seasonal reservoir loading and the negligible poroelastic deformation caused by associated fluid pressure changes, there is an unaccounted biannual deformation in the east component of the GPS time-series which we suspect to be caused by altered hydrological conditions due to the reservoir operations. Understanding crustal deformation processes due to such anthropogenic sources helps in separating deformation caused by tectonic, hydrological and atmospheric effects from that caused by these activities

Coseismic Offsets due to Intermediate Depth 16 April 2013 Southeast Iran Earthquake (M-w 7.8)

The 16 April 2013 southeast Iran earthquake (M-w 7.8) is the second major earthquake in the region, including the Makran subduction zone, since 1945. This intraslab earthquake, within the subducting Arabian plate, occurred as a result of normal faulting at an intermediate depth. We report coseismic offsets from the two nearby continuously operating Global Positioning System sites that are located within 300 km of the epicenter. The coseismic offsets are consistent with the slip on the northward steeply dipping fault plane. This earthquake probably occurred in response to the slab- detachment process, and its occurrence has implications for the plate interface coupling of the Makran subduction zone

Anomalous transients in GPS measurements due to induced changes in local site conditions

Transients in GPS time series can occur due to post-seismic deformation, seasonal hydrological loads, sea-level changes, flood and drought conditions, excessive groundwater withdrawal and recharge, etc. We report two new cases where the application of external loading, namely, earthquake loading and surface loading due to impoundment of hydroelectric reservoir, probably altered the local hydrological conditions to cause anomalous transients in the surface displacement. In the first case, moderate shaking due to the 2015 Gorkha earthquake at Patna (Bihar, India) caused transients in ground deformation in the following 50–60 days of the earthquake which are recorded by a continuous GPS site at Patna. In the second case, impoundment of the Tehri reservoir and its seasonal variations in the Garhwal Himalaya probably altered the local hydrological conditions which is causing anomalous biannual cyclic deformation at a site KUNR, near the reservoir