Use of spectral acceleration data for determination of three-dimensional attenuation structure in the Pithoragarh region of Kumaon Himalaya

Three-dimensional attenuation structures are related to the subsurface heterogeneities present in the earth crust. An algorithm for estimation of three-dimensional attenuation structure in the part of Garhwal Himalaya, India has been presented by Joshi (Curr Sci 90:581–585, 2006b; Nat Hazards 43:129–146, 2007). In continuation of our earlier approach, we have presented a method in which strong motion data have been used to estimate frequency-dependent three-dimensional attenuation structure of the region. The border district of Pithoragarh in the Higher Himalaya, India, lies in the central seismic gap region of Himalaya. This region falls in the seismic zones IV and V of the seismic zoning map of India. A dense network consisting of eight accelerographs has been installed in this region. This network has recorded several local events. An algorithm based on inversion of strong motion digital data is developed in this paper to estimate attenuation structure at different frequencies using the data recorded by this network. Twenty strong motion records observed at five stations have been used to estimate the site amplification factors using inversion algorithm defined in this paper. Site effects obtained from inversion has been compared with that obtained using Nakamura (1988) and Lermo et al. (Bull Seis Soc Am 83:1574–1594, 1993) approach. The obtained site amplification term has been used for correcting spectral acceleration data at different stations. The corrected spectral acceleration data have been used as an input to the developed algorithm to avoid effect of near-site soil amplification term. The attenuation structure is estimated by dividing the entire area in several three-dimensional block of different frequency-dependent shear wave quality factor Q _β (f). The input to this algorithm is the spectral acceleration of S phase of the corrected accelerogram. The outcome of the algorithm is given in terms of attenuation coefficient and source acceleration spectra. In the present study, this region has been divided into 25 rectangular blocks with thickness of 10 km and surface dimension of 12.5 × 12.1 km, respectively. Present study gives three-dimensional attenuation model of the region which can be used for both hazard estimation and simulation of strong ground motion.     

Depth determination from a non-stationary magnetic profile for scaling geology

The conventional spectral analysis method for interpretation of magnetic data assumes stationary spatial series and a white‐noise source distribution. However, long magnetic profiles may not be stationary in nature and source distributions are not white. Long non‐stationary magnetic profiles can be divided into stationary subprofiles following Wiener filter theory. A least‐squares inverse method is used to calculate the scaling exponents and depth values of magnetic interfaces from the power spectrum. The applicability of this approach is demonstrated on non‐stationary synthetic and field magnetic data collected along the Nagaur–Jhalawar transect, western India. The stationarity of the whole profile and the subprofiles of the synthetic and field data is tested. The variation of the mean and standard deviations of the subprofiles is significantly reduced compared with the whole profile. The depth values found from the synthetic model are in close agreement with the assumed depth values, whereas for the field data these are in close agreement with estimates from seismic, magnetotelluric and gravity data.     

The chemical composition of soil from the Apollo 16 and Luna 20 sites

The concentrations of the rare earth elements (REE), K, Rb, Sr, Ba, U, Zr and Cr for the Luna 20 soil and four different Apollo 16 soils are reported. These trace element abundances imply: (1) that the lunar highlands consist of a mixture of rocks rich in large ion lithophile (LIL) elements and LIL-element impoverished anorthosites; or (2) that the bulk of the aluminum-rich crust did not originate by upward segregation of plagioclase in a primitive liquid shell. The Luna 20 soil is distinguished from the Apollo 16 soil by lower aluminum and LIL element abundances.     

Pre-4.2 AE mare-basalt volcanism in the lunar highlands

The concept that the plutonism of the lunar highlands and the mare-type volcanism are two separate problems in both time (> 4.4 AE versus < 3.95 AE) and space is seriously questioned by the discovery of a 4.23-AE low-Ti mare basalt from Fra Mauro Formation.Apollo 14 breccia 14305 contains a clast (,122) which is an olivine gabbronorite that is texturally and mineralogically similar to several Apollo 12 basalts (e.g., 12005, 12035, 12040). It consists of cumulus olivine (40 modal %; Fo 62–70) and Ti-chromite (2.5 modal %); post-cumulus phases include low-Ca pyroxene (29 modal %; Wo 7–13 En 68–75), augite (10 modal %; Wo 31–40 En 47–50), plagioclase (15 modal %, An 82–93), and ilmenite (4 modal %, 5–7 MgO). The TiO₂ content of this rock = 4.3%; CaO/Al₂O₃ ? 1.0, CaO = 5.1%; MgO/FeO ? 1.0, MgO = 21.9%. The REE pattern, normalized to chondritic abundances, is approximately 30 × Ch and “hump-shaped” with a pronounced Eu depletion and a non-KREEPy signature. A four-point Rb-Sr isochron reveals an age of 4.23 ± 0.05 AE. The sample has a low initial ⁸⁷Sr/⁸⁶Sr= 0.69911 ± 3.The data presented here show that non-KREEPy, mare-type volcanism commenced at least as early as 4.2 AE in the Fra Mauro region and probably across much of the lunar surface. Massive bombardment during the “terminal cataclysm” and the subsequent veneer of younger mare basalts has obliturated most of the evidence for these ancient volcanic events. These old, mare-type volcanics may be related to basin-forming events such as made Procellarum (i.e., impact-triggered igneous activity).     

An assessment of seismicity parameters in northwest Himalaya and adjoining regions

In this study, an assessment of seismicity parameters in the northwest Himalaya and adjoining regions using an earthquake catalog from India Meteorological Department covering a period from June 1, 1998 to June 30, 2011 has been carried out. The spatial distributions of seismicity parameters, namely magnitude of completeness, M _C, a value, b value, and correlation fractal dimension, D _C, are estimated for the studied region. The M _C, a, and b values are found to be 2.5, 4.601, and 0.83, respectively. Despite significant gaps, the spatial distributions of a and b values are seen to follow similar trend and are found scattering in between Main Boundary Thrust (MBT) and South Tibet Detachment, adjoining areas of Mahendragarh-Dehradun Fault (MDF), Delhi-Haridwar Ridge (DHR) and Moradabad Fault (MF), and the southern flank of Karakoram Fault and Indus-Tsangpo Suture Zone. The estimated spatial distribution of b and a values is within 90 % of confidence level, thereby indicating non-uniform stress accumulation or higher rock fracturing density in the studied region caused by strong tectonization following several earthquakes. Negative correlation between low b value and high D _C is observed predominantly in the region between the MBT and Munsiari Thrust or Main Central Thrust-I of Garhwal and Kumaon Himalaya, adjoining zones of MDF, DHR, and MF of Indo-Gangetic plain, and the eastern flank of the studied region, suggesting the presence of asperities in the zone. At the same time, active creeping process can be inferred in between the MBT and Main Central Thrust of Garhwal Himalaya and the surrounding areas of Shimla region of the Himalayan arc to the northwestern part of the studied region from the positive correlation between b value and D _C. The results indicate that the structural heterogeneity caused by different stress accumulation and rock fracturing densities exists due to continuous tectonic adjustments between different geomorphic features of the studied region. An attempt has also been made to classify the studied region into smaller seismic zones by observing the spatial patterns of b value and D _C that are fractal properties of the observed seismicity, along with the prevalent fault networks.     

Source parameters of 1<Superscript>st</Superscript> April 2015 Chamoli earthquake (Mw 4.8) vis-à-vis seismotectonics of the region

The paper presents a detailed analysis of 1^st April 2015 earthquake, whose epicenter (30.16° N, 79.28° E) was located near Simtoli village of Chamoli district, Uttarakhand. The focal depth is refined to 7 km by the grid search technique using moment tensor inversion. The source parameters of the earthquake as estimated by spectral analysis method suggested the source radius of ~1.0 km, seismic moment as 1.99E+23 dyne-cm with moment magnitude (Mw) of 4.8 and stress drop of 69 bar. The fault plane solution inferred using full waveform inversion indicated two nodal planes, the northeast dipping plane having strike 334° and dip 5° and the southwest dipping plane with dip 86° and strike 118°. The parallelism of the nodal plane striking 334° with dip 5° as indicated in depth cross sections of the tectonic elements suggested the north dipping Main Boundary Thrust (MBT) to be the causative fault for this earthquake. Spatio-temporal distribution of earthquakes during the period 1960-2015 showed seismic quiescence during 2006-2010 and migration of seismicity towards south.