Tectonic geodesy revealing geodynamic complexity of the Indo-Burmese arc region, North East India

The plate boundary between India and Sunda plates across the Indo-Burmese arc (IBA) region is probably the most neglected domain as far as the plate motion, crustal deformation and earthquake occurrence processes are concerned. Because of the limited or no geodetic measurements across the IBA region, debate continues on the most appropriate plate boundary model for the region. Subduction along this boundary occurred in geological past, but whether it is still active is a debatable issue. It is believed that the predominantly northward India–Sunda relative plate motion of about 36 mm/year is partitioned between the Indo-Burmese wedge (IBW) and the Sagaing Fault (SF). However, it is not clear how relative plate motion between India and Sunda plates is accommodated across the IBA region – whether localized, partitioned or distributed, and in particular what is the slip rate and mode of slip accommodation across faults in the region? In such cases, Global Positioning System (GPS) measurements of crustal deformation have proved to be the best and probably the only tool. Our detailed seismo-tectonic study, crustal deformation study using high precision GPS measurements of eight years, strain rate estimates, field studies, analytical and finite element modelling of GPS data from the IBW region in North East India provide evidence for present-day active deformation front (or the plate boundary fault) between the India and Burma plates. On the basis of our extensive studies, it is now suggested that the Churachandpur–Mao Fault (CMF), a geologically older thrust fault, accommodates motion of about 16 mm/year through dextral strike–slip manner. The motion across the CMF constitutes about 43% of the relative plate motion of 36 mm/year between the India and Sunda plates. The remaining motion is accommodated at SF. On the basis of modelling, which suggests low friction along the CMF, absence of low-magnitude seismicity along the CMF, lack of historic and great and major earthquakes on the CMF and regions around it, and field studies, it is proposed that the motion across the CMF occurs predominantly in an aseismic manner. Such behaviour of the CMF significantly lowers the seismic hazard in the region

Gravity, GPS and geomagnetic data in India

Gravity, Global Positioning System (GPS) and Geomagnetic data sets in India are acquired by different research, academic and government institutions, under various projects. These data sets have extensively been utilized for natural resources and lithopsheric explorations, earthquake studies, atmospheric and ionospheric studies, control surveys, aircraft navigation, etc. The data are archived at individual institutions and have different modes of procurement considering some of the data, e.g., gravity data are classified in nature. Some of these data sets are contributed to the international observational network for example IGS and INTERMAGNET and are available as open source for the scientific communities. Present article provides information about different types of available Gravity, GPS and Geomagnetic data, their archival and mode of availability to the user community

Preparations for future great earthquakes seen in levelling observations along two lines across the outer Himalaya

An important set of levelling observations across Central Nepal have been reported in the literature recently. We infer from these observations that recoverable elastic strains are accumulating in the upper crust of the region and will lead to a great earthquake in course of time. Limited levelling data from the Dehradun region in northwestern Himalaya show remarkable similarities to the Nepalese data and require a similar interpretation regarding preparation for the next great earthquake in the region

Coulomb stress changes and aftershocks of recent Indian earthquakes

During the past decade remarkable progress has been made in studies related to fault interactions and how the occurrence of an earthquake perturbs the stress field in its neighbourhood, which may trigger aftershocks and subsequent earthquakes. These studies have significant implications on the seismic hazard assessment of a region, as the change in stress can cause either a delay or an advance in the occurrence of future earthquakes. Further, since the assessment of seismic hazard is dependent on the rupture parameters of past earthquakes, it is important to reliably estimate such parameters, viz. rupture location, geometry, and extent of past earthquakes. Here, we report the constraints on some of the rupture parameters of Indian earthquakes during the past 14 years, namely the 2001 Bhuj, 1999 Chamoli, 1997 Jabalpur, 1993 Killari and 1991 Uttarkashi earthquakes, which are derived using the available aftershock data and assuming that these aftershocks occurred in the zones of increased static stress. Analyses of teleseismic waveform of these earthquakes poorly constrained such parameters

Application of pattern recognition algorithm in the seismic belts of Indian convergent plate margin - CN algorithm

The earthquake catalogue from 1964 to August 1991 is used to identify the times of increased probabilities (TIPs) of the earthquake mainshocks of magnitudes greater than or equal to 6.4 and are associated with the Indian convergent plate margins, in retrospect. In Pakistan and Indo-Burma regions, the analysis was repeated for magnitude threshold 6.2 and 7.0 respectively. All the earthquakes (except one in the Hindukush region and one in Indo-Burmese region) in Pakistan, Hindukush-Pamir, Himalaya and Indo-Burmese regions were preceded by the special activation and hence were predicted. Approximately 23 ± 10% of the total time (1970 to August 1991) is occupied by the TIPs in all the regions. The reasons for failure to predict the two earthquakes in these regions are discussed. Our analysis gives a better picture of the regionalization and the size of the space-time volume for the preparation of an earthquake. The high success ratio of the algorithm proves that it can be applied in this territory for further prediction in the real time, without any significant changes in its parameters

Stress triggering of normal aftershocks due to strike slip earthquakes in compressive regime

A few cases of occurrence of normal aftershocks after strike slip earthquakes in compressive regime have been reported in the literature. Occurrence of such aftershocks is intriguing as they occurred despite the apparent stabilizing influence of compressive plate tectonic stresses on the normal faults. To investigate the occurrence processes of such earthquakes, we calculate change in static stress on optimally oriented normal and reverse faults in the dilational and compressional step over zones, respectively, due to slip on a vertical strike slip fault under compressive regime. We find that change in static stress is much more pronounced on normal faults as compared to that on reverse faults, for all values of fault friction. Change in static stress on reverse fault is marginally positive only when the fault friction is low, whereas for normal faults it is positive for all values of fault friction, and is maximum for high fault friction. We suggest that strike slip faulting in compressive regime creates a localized tensile environment in the dilational step over zone, which causes normal faulting in that region. The aftershocks on such normal faults are considered to have occurred as an almost instantaneous response of stress transfer due to strike slip motion

Effect of the Zipingpu reservoir impoundment on the occurrence of the 2008 Wenchuan earthquake and local seismicity

The occurrence of the 2008 May 12 Wenchuan earthquake (M 7.9) near the eastern edge of the Tibetan Plateau triggered a debate whether it was influenced by the newly impounded Zipingpu reservoir, located only about 21 km east of the earthquake epicentre. We calculated change in stresses due to the 3-D reservoir water load using Boussinesq theory and pore pressure diffusion due to the actual variation of water level in the reservoir. Using the stresses and pore pressure due to the reservoir impoundment, we calculated the Coulomb stress on the NW dipping fault plane of the 2008 Wenchuan earthquake. We find that the reservoir operation did not cause any increase in the Coulomb stress at the earthquake hypocentre and the region around it, rather it decreased the stress by a nominal amount of about 1 KPa. Thus, we suggest that the reservoir probably did not play a role in the occurrence of this earthquake. However, our analysis suggests that the slightly enhanced low-magnitude shallow seismicity after the reservoir impoundment, but prior to the 2008 Wenchuan earthquake, was positively influenced by the reservoir impoundment

Temporal variation of seismicity in a Himalayan tectonic block and the 1991 Uttarkashi earthquake, India

Spatial and temporal variation of the seismicity between 1970 and August 1991 in the northwestern Himalaya and adjoining areas of Tibet, immediately to the north, were investigated using a function of the CN algorithm. The aim of the exercise was to detect a pattern which, if known earlier, could have been used to predict the Uttarkashi earthquake of 19 October 1991 in the Garhwal Himalaya. It is observed that strong earthquakes occurring in a block, defined by longitudes 75° and 83°E and latitudes 30° and 38°N approximately, yield a distinctive pattern of temporal variation in the seismicity for the purpose. We conclude from the analysis that the seismicity in the northwestern Himalayan segment of the Alpide-Himalayan seismic belt may not be independent of seismicity in southwestern Tibet

Analysis of coseismic water-level changes in the wells in the Koyna–Warna region, Western India

We test the hypothesis that coseismic water-level changes in wells are proportional to coseismic volumetric strain by analyzing available data from the Koyna–Warna region of western India. A total of 18 cases of water-level changes have been reported at ten wells corresponding to six earthquakes of M≥4.3 that occurred in the region from 1997 to 2005. Out of these, clear unambiguous steplike coseismic water-level changes have been observed in ten cases at five confined wells. We used basic poroelastic theory to simulate volumetric strain and corresponding water-level changes and find that all cases show consistency in sign between reported coseismic water-level changes and simulated volumetric strain. All confined wells with high strain sensitivity that are located near earthquake epicenters show good agreement in magnitude between simulated and reported volumetric strain, thereby supporting this hypothesis