nan Kamal

Ground motion estimation in Delhi from postulated regional and local earthquakes

Ground motions are estimated at 55 sites in Delhi, the capital of India from four postulated earthquakes (three regional Mw = 7.5, 8.0, and 8.5 and one local). The procedure consists of (1) synthesis of ground motion at a hard reference site (NDI) and (2) estimation of ground motion at other sites in the city via known transfer functions and application of the random vibration theory. This work provides a more extensive coverage than earlier studies (e.g., Singh et al., Bull Seism Soc Am 92:555–569, 2002; Bansal et al., J Seismol 13:89–105, 2009). The Indian code response spectra corresponding to Delhi (zone IV) are found to be conservative at hard soil sites for all postulated earthquakes but found to be deficient for Mw = 8.0 and 8.5 earthquakes at soft soil sites. Spectral acceleration maps at four different natural periods are strongly influenced by the shallow geological and soil conditions. Three pockets of high acceleration values are seen. These pockets seem to coincide with the contacts of (a) Aravalli quartzite and recent Yamuna alluvium (towards the East), (b) Aravalli quartzite and older quaternary alluvium (towards the South), and (c) older quaternary alluvium and recent Yamuna alluvium (towards the North).

A fractal model of earthquake occurrence: theory, simulations and comparisons with the aftershock data

Our understanding of earthquakes is based on the theory of plate tectonics. Earthquake dynamics is the study of the interactions of plates (solid disjoint parts of the lithosphere) which produce seismic activity. Over the last about fifty years many models have come up which try to simulate seismic activity by mimicking plate plate interactions. The validity of a given model is subject to the compliance of the synthetic seismic activity it produces to the well known empirical laws which describe the statistical features of observed seismic activity. Here we present a review of one such, purely geometric, model of earthquake dynamics, namely The Two Fractal Overlap Model. The model tries to emulate the stick-slip dynamics of lithospheric plates with fractal surfaces by evaluating the time-evolution of overlap lengths of two identical Cantor sets sliding over each other. As we show later in the text, some statistical aspects of natural seismicity are naturally captured by this simple model. More importantly, however, this model also reveals a new statistical feature of aftershock sequences which we have verified to be present in nature as well. We show that, both in the model as well as in nature, the cumulative integral of aftershock magnitudes over time is a remarkable straight line with a characteristic slope. This slope is closely related to the fractal geometry of the fault surface that produces most of thee aftershocks. We also go on to discuss the implications that this feature may have in possible predictions of aftershock magnitudes or times of occurrence.

A test of the superconducting gravimeter as a long-period seismometer

Abstract Normal modes excited by the Minhasa peninsula earthquake of 18 April 1990 (Ms = 7.5) are used to test the response of the Canadian superconducting gravimeter (GWR 12) in the long-period seismic band. The periods and associated Q values compare very well with those determined elsewhere. Splitting of a number of the higher order spheroidal modes (0S13 to0S26) has been observed because of the low ambient noise at the site. The combined instrument and ground noise is nearly flat and close to 1 × 10−19 cm2 s−4 (0.1 nGal2) in the 1–5 mHz band.

Macroseismic field observations of January 26th, 2001 Kachchh earthquake and its seismotectonics

Abstract On the morning of January 26, 2001, a devastating earthquake of magnitude (Mw) 7.7, origin time 8 h 46 min 42.9 s, struck the Kachchh peninsula situated in the western India and was widely felt over the Indian subcontinent. The epicenter was located northeast of Bachau at 23.40° N, 70.28° E, with a focal depth of 25 km. It was the largest earthquake in India during the last five decades. Maximum intensity X according to the European Macroseismic Scale-98 (EMS-98) was observed in the meizoseismal zone. An isoseismal map shows an E–W trend with an elongation to the southeast direction. This earthquake caused widespread damage to poorly built buildings and framed structures up to 300 km from the epicenter zone. The damage distribution shows an asymmetric and heterogeneous pattern. The intensity attenuation is rapid in the northwest direction and slow towards the east and southeast direction. This earthquake produced widespread liquefaction in the Banni, Rann and along the coastal regions. The majority of 185 dam sites in the Kachchh region suffered minor to major damage. From the damage distribution and seismogenic effects, it can be inferred that the main shock was initiated along an E–W trending fault, north–northeast of Bachau. The pear shape isoseismal pattern might be caused by different seismic moment-rate release along this propagating rupture.

Modeling of strong motion generation area of the Uttarkashi earthquake using modified semiempirical approach

The semiempirical approach based on envelope summation method given by Midorikawa (Tectonophysics 218:287–295, 1993) has been modified in this paper for modeling of strong motion generation areas (SMGAs). Horizontal components of strong ground motion have been simulated using modifications in the semiempirical approach given by Joshi et al. (Nat Hazard 71:587–609, 2014). Various modifications in the technique account for finite rupture source, layering of earth, componentwise division of energy and frequency-dependent radiation pattern. In this paper, SMGAs of the Uttarkashi earthquake have been modeled. Two different isolated wave packets in the recorded accelerogram have been identified from recorded ground motion, which accounts for two different SMGAs in the entire rupture plane. The approximate locations of SMGAs within the rupture plane were estimated using spatio-temporal variation of 77 aftershocks. Source parameters of each SMGA were calculated from theoretical and observed source displacement spectra computed from two different wave packets in the record. The final model of rupture plane responsible for the Uttarkashi earthquake consists of two SMGAs, and the same has been used to simulate horizontal components of acceleration records at different station using modified semiempirical technique. Comparison of the observed and simulated acceleration records in terms of root mean square error confirms the suitability of the final source model for the Uttarkashi earthquake.

Modeling of strong motion generation areas of the Niigata, Japan, earthquake of 2007 using modified semi-empirical technique

Abstract The Niigata prefecture in Japan was devastated by a large shallow earthquake (Mw 6.6, MJMA 6.8) on July 16, 2007. This earthquake has been recorded at 305 stations of Kiban Kyoshin network (KiK-net). Source model of this earthquake has been computed from accelerograms recorded by KiK-net at near-field stations surrounding source of earthquake. Several isolated wave packets were seen in recorded accelerograms at near-field stations surrounding source of this earthquake. Each wave packet in recorded accelerogram represents an isolated patch of envelope of accelerogram released from a rupture plane and is considered to be an independent source of strong motion generation area. Three different isolated wave packets have been identified within the rupture plane of the Niigata earthquake from recorded accelerograms. These isolated wave packets were considered as strong motion generation areas (SMGAs) in the rupture plane. Source parameters of each SMGA were calculated from the source displacement spectra. The approximate locations of SMGAs over the source fault were estimated using spatio-temporal variation of 48 aftershocks recorded by KiK-net and K-NET. Modified semi-empirical method has been used to simulate strong ground motion at various stations. Comparison of the observed and simulated acceleration waveforms is made in terms of root-mean-square error. Comparison of NS and EW component of observed and simulated records at eight stations confirms the suitability of final source model consisting of three SMGAs and efficacy of the modified semi-empirical technique to simulate strong ground motion.

3D basin-shape ratio effects on frequency content and spectral amplitudes of basin-generated surface waves and associated spatial ground motion amplification and differential ground motion

This paper presents the effects of basin-shape ratio (BSR) on the frequency content and spectral amplitudes of the basin-generated surface (BGS) waves and the associated spatial variation of ground motion amplification and differential ground motion (DGM) in a 3D semi-spherical (SS) basin. Seismic responses were computed using a recently developed 3D fourth-order spatial accurate time-domain finite-difference (FD) algorithm based on the parsimonious staggered-grid approximation of the 3D viscoelastic wave equations. The simulated results revealed the decrease of both the frequency content and the spectral amplitudes of the BGS waves and the duration of ground motion in the SS basin with the decrease of BSR. An increase of the average spectral amplification (ASA), DGM and the average aggravation factor (AAF) towards the centre of the SS basin was obtained due to the focusing of the surface waves. A decrease of ASA, DGM and AAF with the decrease of BSR was also obtained.

Using fast Hartley transform to study the free oscillations of the earth

In recent years, Hartley Transform (HT) as a substitute of much widely used Fourier Transform (FT) has been practised in science and industries. The advantage of faster computation of HT is enormous when one is dealing with very long data sets. One such application arises in computation of parameters of Free Oscillations of the Earth (FOE), where one needs to study very long period vibrations of the earth, excited after a large earthquake. We demonstrate here an application of HT to determine the parameters of these normal modes of the earth after the Minahasa Peninsula earthquake of 18 April 1990 (Ms = 7.5).

Study of Effects of Sediment-Damping, Impedance Contrast, and Size of Semi-Spherical Basin on the Focusing and Trapping of the Basin-Generated Surface Waves

This article presents the effects of sediment-damping, impedance-contrast (IC), and size of semi-spherical (SS) basin on the focusing and trapping of the basin-generated surface (BGS) waves and the spatial-variation of average-spectral-amplification (ASA), differential ground motion (DGM), and average-aggravation-factor (AAF). A frequency-dependent focusing of the BGS-wave is inferred. Increase of ASA, DGM, and AAF with increase of size of the SS-basin with a fixed-shape-ratio revealed that the BGS-wave focusing has counter-balanced the sediment-damping effects. It is concluded that the BGS-wave focusing and trapping in the SS-basin is more sensitive to change of IC as compared to the similar change of sediment-damping.

A scenario of ground shaking hazard in intracratonic circular basins developed by basin-generated surface waves: an earthquake engineering perspective

AbstractThis paper presents the comparative scenario of ground motion amplifications in the 2D semi-cylindrical (SC) basin and 3D semi-spherical (SS) intracratonic basin caused by the basin-generated surface (BGS) waves and the associated spatial variations of average spectral amplification, differential ground motion (DGM) and average aggravation factor (AAF). The time-domain seismic responses of both the basins were computed using a recently developed 3D fourth-order accurate viscoelastic finite-difference algorithm based on the well-known GMB-EK rheological model. The analysis of simulated results revealed that AAF and DGM are comparable near the basin-edge in both the basins but the differences are increasing towards the centre of basins due to the focusing of the BGS-waves in the SS-basin. The obtained many-fold increase of 1D/2D-AAFs and DGM at the centre of the SS-basin as compared to the SC-basin reflects the inadequacy of 1D or 2D response of an intracratonic circular basin for the seismic hazard assessment. It may be inferred that the level of damage near the edges of the 2D and 3D circular basins may be comparable but unexpected damage may occur in the central part of a circular basin due to the focusing of the BGS-waves.