Simanchal Padhy

Characteristics of Body-Wave Attenuations in the Bhuj Crust

Abstract The frequency-dependent attenuations of P and S waves in the crust beneath Bhuj are estimated using the extended coda-normalization method for varying the exponent, γ , in the geometrical spreading function, r γ ( r is the hypocentral distance). The analysis is based on the measurement of 378 aftershocks of the 2001 Bhuj earthquake ( M w xa07.6). The events were recorded by six stations of the network in the frequency range of 1–24xa0Hz. The values of and , corresponding to spectral amplitude decays, show strong frequency dependence and are expressed as (0.052±0.019) f -(1.1±0.06) and (0.02±0.01) f -(1.0±0.04) , respectively, when γ is fixed at unity. The ratio is found to be larger than unity for the whole frequency range. A model of random heterogeneity characterized by an exponential autocorrelation function with a root mean square fractional fluctuation e of 6%–8%, a correlation length a of 0.4–1.2xa0km, and e 2 / a ≈10 -2 –10 -3 km for the frequency range from 1 to 24xa0Hz can simulate the observed attenuation properties. This suggests that a scattering model well reproduce the frequency-dependent characteristics of and obtained in Bhuj.

Frequency-Dependent Attenuation of Body and Coda Waves in the Andaman Sea Basin

We estimated frequency-dependent attenuation of coda waves (![Graphic][1] ) and body waves (![Graphic][2] and ![Graphic][3] ) in 1.5–24xa0Hz by applying the single isotropic scattering theory and the extended coda-normalization method, respectively, in the crust beneath the Andaman Sea. We used 43 aftershocks of the 13 September 2002 earthquake ( M wxa06.5) in the Andaman Sea recorded by three stations installed in the Andaman Islands. The coda Q factors calculated from the amplitude decay rate of the S -wave coda show a dependence on frequency and lapse time. We found that with the increase in lapse time window from 10 to 40xa0s, Q ( Q C at 1xa0Hz) increases from 55 to 153, while the frequency-dependent coefficient n decreases from 1.1 to 0.94. The average frequency-dependent relations of ![Graphic][4] vary from 0.02 f -1.1 to 0.01 f -0.94 with an increase in lapse time window from 10xa0s to 40xa0s, respectively. The values of ![Graphic][5] and ![Graphic][6] corresponding to spectral amplitude decays show strong frequency dependence and are expressed as 0.02 f -1.01 and 0.01 f -1.0, respectively. Our results are consistent with those of other seismically active regions. The ratio ![Graphic][7] is found to be larger than unity for the whole frequency range. We separated intrinsic absorption (![Graphic][8] ) and scattering attenuation (![Graphic][9] ) using the independent estimates of ![Graphic][10] and ![Graphic][11] . The results show that ![Graphic][12] is close to ![Graphic][13] and both of them are larger than ![Graphic][14] , suggesting that coda decay is predominantly caused by intrinsic attenuation.nn [1]: /embed/inline-graphic-1.gifn [2]: /embed/inline-graphic-2.gifn [3]: /embed/inline-graphic-3.gifn [4]: /embed/inline-graphic-4.gifn [5]: /embed/inline-graphic-5.gifn [6]: /embed/inline-graphic-6.gifn [7]: /embed/inline-graphic-7.gifn [8]: /embed/inline-graphic-8.gifn [9]: /embed/inline-graphic-9.gifn [10]: /embed/inline-graphic-10.gifn [11]: /embed/inline-graphic-11.gifn [12]: /embed/inline-graphic-12.gifn [13]: /embed/inline-graphic-13.gifn [14]: /embed/inline-graphic-14.gif

Spatial and Temporal Variations in Coda Attenuation Associated with the 2011 Off the Pacific Coast of Tohoku, Japan (Mw 9) Earthquake

The spatial and temporal variations of coda attenuation (![Graphic][1] ) were studied in the source region of the 2011 Off the Pacific Coast of Tohoku, Japan ( M wxa09) earthquake. The ![Graphic][2] values were determined from the amplitude decay rate of the S ‐wave coda in narrower overlapping frequency bands in the range f =1.0–24u2009u2009Hz, based on a single isotropic scattering model for more than 400 earthquakes ( M JMAu20093∼6.5) in the region recorded in a period from January 2005 to August 2011, including pre‐ and postseismic period. Our estimates of the spatially averaged ![Graphic][3] value, ![Graphic][4] in f =1.0–24u2009u2009Hz, is almost stable with small variations ( increases by about 10%–16% after the 2011 Tohoku event in f =1.25–3.5u2009u2009Hz in some stations of northern Japan, which is confirmed by a statistical t ‐test at 99.9% confidence level. The change in ![Graphic][6] is spatially limited to the rupture zone, while other paths remain nearly unaffected, suggesting local changes of scattering properties in the vicinity of the Tohoku‐Oki source volume. This change may be attributed to an increase in the density of open microcracks in the mainshock source volume (such as due to increase in stress induced by the 2011 event) and probably the fluid content in fractures in the rocks. A model of heterogeneity due to coseismically opened cracks (dominant scale length of a =0.6–1.8u2009u2009km in f =1.25–3.5u2009u2009Hz) enhanced by increased stress change possibly controls the increased ![Graphic][7] after the 2011 event in the source rupture region.nn [1]: /embed/inline-graphic-1.gifn [2]: /embed/inline-graphic-2.gifn [3]: /embed/inline-graphic-3.gifn [4]: /embed/inline-graphic-4.gifn [5]: /embed/inline-graphic-5.gifn [6]: /embed/inline-graphic-6.gifn [7]: /embed/inline-graphic-7.gif

Inversion of Seismogram Envelopes Using a Multiple Isotropic Scattering Model in Garhwal Himalaya

We estimated intrinsic attenuation ![Graphic][1] , scattering attenuation ![Graphic][2] , site amplification Z , and source energy S from inversion of three-component coda envelopes of the 1999 Chamoli earthquake of India for central frequencies 1.5, 3, 6, 12, and 24xa0Hz. The multiple isotropic scattering of S waves was numerically simulated by using a Monte Carlo method based on the radiative transfer theory. Isotropic sources and acoustic scattering in a full space were assumed. Adapting a grid search for scattering coefficient g and least-squares inversion for intrinsic attenuation parameter b , and source energy S , we inverted the observed envelopes of ten aftershocks ( M L≥3.5) in 1–24xa0Hz.nnOur results reveal that both ![Graphic][3] and ![Graphic][4] are weakly frequency dependent with the power-law forms of (0.006±0.004) f -(0.89±0.33) and (0.003±0.0005) f -(0.84±0.08), respectively. High scattering loss can be interpreted to be due to the presence of large lateral velocity heterogeneities in the crust. The total attenuation Q -1 decreases with frequency, taking the power-law form of (0.009±0.003) f -(0.87±0.19). The mean free path ranges from 30 to 300xa0km, with an average value of 100xa0km, and the intrinsic attenuation parameter b ranges from 0.01 to 0.05 sec-1, with an average value of 0.03 sec-1. Our estimates of source energy are in good agreement with the values obtained assuming an ω 2-source model. Site effects estimated using the fixed values of S k , b , and g exhibit less scatter, ranging from 0.73 to 2.54 with no significant frequency dependence consistent with the rock sites.nn [1]: /embed/inline-graphic-1.gifn [2]: /embed/inline-graphic-2.gifn [3]: /embed/inline-graphic-3.gifn [4]: /embed/inline-graphic-4.gif

Site-specific ground motion simulation and seismic response analysis for microzonation of Nanded City, India

We study strong ground motion characteristics and ground response analysis of different sites in Nanded City, India. Synthetic strong ground motion records are obtained for two scenario earthquakes of magnitude M 5.2 and M 6.8 in southern Peninsular India using the finite fault model. Shear-wave velocity averaged over top 30xa0m of the soil is obtained from multichannel analysis of surface wave survey at 60 sites at depths of 20–50xa0m below the ground surface. The sites in different parts of the city are characterized by peak ground acceleration (PGA) at the surface and bedrock level, 5xa0% damped response spectra at the ground surface and the amplification factor defined as the ratio of ground motion at the surface to that at the bedrock. The area east to the lineament is characterized by low VS30 values (<200xa0m/s), probably associated with Godavari River basin sediments with alluvial deposits, composed of brown clay with intercalated bands of sand and gravel. A relatively high PGA value at the surface of about 0.24xa0g for the M 5.2 target event and hence high ground motion amplification indicative of major damage are observed at sites E-NE to the lineament in the area that are associated with the top soil column consisting of unconsolidated river basin sediments with very low VS30 values (<200xa0m/s). The amplification factor for the city ranges from 3 to 8 for the highest target event of magnitude M 6.8. There is a strong lateral variation in PGA values at the surface for the part of the city lying to the east of the lineament and parallel to its trend, where a relatively high amplification factor of 6–8 is found at a dominant frequency of 1.5–2.5xa0Hz, which can be considered as a significant zone of amplification and hence areas with high seismic hazard. The site-specific response spectra show that the peak of spectral acceleration for different sites occurs around 3xa0Hz. There is a large spatial variation of spectral acceleration at any given frequency for the city. For example, site nos. 9 and 50 are characterized by a peak in spectral acceleration of 0.55 and 0.9xa0g, respectively, at 3xa0Hz frequency.

Decoupling of Pacific subduction zone guided waves beneath central Japan: Evidence for thin slab

The fine-scale seismic structure of the northeast Japan subduction zone is studied based on waveform analyses of moderate-sized (M4.5–6), deep-focus earthquakes (h >350u2009km) and the finite difference method (FDM) simulation of high-frequency (up to 8u2009Hz) wave propagation. Strong regional S wave attenuation anomalies for specific source-receiver paths connecting the cluster of events occurring in central part of the Sea of Japan recorded at fore arc stations in northern and central Japanese Islands (Honshu) are used to model the deeper structure of the subducting Pacific Plate, where recent teleseismic tomography has shown evidence for a possible slab tear westward beneath the Sea of Japan. The character of the observed anomalous S wave attenuation and the following high-frequency coda can be captured with the two-dimensional (2-D) FDM simulation of seismic waves in heterogeneous plate model, incorporating the thinning of the plate at depth, which is also compared with other possible causes of dramatic attenuation of high-frequency S wave due to low-Q or much weaker heterogeneities in the slab. The results of simulation clearly demonstrate that the dramatic loss of high-frequency S wavefield from the plate into the surrounding mantle occurred due to the variation in the plate geometry (i.e., thinning of the plate) at depth near the source rather than due to variation in physical properties, such as due to the lowered-Q and weaker heterogeneities in the plate. The presence of such a thin zone defocuses the high-frequency slab-guided S wave energy from the subducting plate into the surrounding mantle and acts as a geometric antiwaveguide. Based on the sequence of simulation results obtained, we propose thinning of Pacific Plate at depth subducting beneath northeastern Japan, localized to central part of Honshu, in agreement with the observations.

Effects of errors and biases on the scaling of earthquake spatial pattern: application to the 2004 Sumatra- Andaman sequence

This study discusses the scaling properties of the spatial distribution of the December 26, 2004, Sumatra aftershocks. We estimate the spatial correlation dimension D2 of the epicentral distribution of aftershocks recorded by a local network operated by Geological Survey of India. We estimate the value of D2 for five blocks in the source area by using generalized correlation integral approach. We assess its bias due to finite data points, scaling range, effects of location errors, and boundary effects theoretically and apply it to real data sets. The correlation dimension was computed both for real as well as synthetic data sets that include randomly generated point sets obtained using uniform distributions and mimicking the number of events and outlines of the effective areas filled with epicenters. On comparing the results from the real data and random point sets from simulations, we found the lower limit of bias in D2 estimates from limited data sets to be 0.26. Thus, the spatial variation in correlation dimensions among different blocks using local data sets cannot be directly compared unless the influence of bias in the real aftershock data set is taken into account. They cannot also be used to infer the geometry of the faults. We also discuss the results in order to add constraints on the use of synthetic data and of different approaches for uncertainty analysis on spatial variation of D2. A difference in D2 values, rather than their absolute values, among small blocks is of interest to local data sets, which are correlated with their seismic b values. Taking into account the possible errors and biases, the average D2 values vary from 1.05 to 1.57 in the Andaman–Nicobar region. The relative change in D2 values can be interpreted in terms of clustering and diffuse seismic activity associated with the low and high D2 values, respectively. Overall, a relatively high D2 and low b value is consistent with high-magnitude, diffuse activity in space in the source region of the 2004 Sumatra earthquake.