Seismic intensity map of South India for estimated future earthquakes

In this study, an attempt has been made to prepare the seismic intensity map for south India considering the probable earthquakes in the region. Anbazhagan et al. (Nat Hazards 60:1325-1345, 2012) have identified eight probable future earthquake zones in south India based on rupture-based seismic hazard analysis. Anbazhagan et al. (Eng Geol 171:81-95, 2014) has estimated the maximum future earthquake magnitude at these eight zones using regional rupture character. In this study, the whole south India is divided into several grids of size 1(o) x 1(o) and the intensity at each grid point is calculated using the regional intensity model for the maximum earthquake magnitude at each of the eight zones. The intensity due to earthquakes at these zones is mapped and thus eight seismic intensity maps are prepared. The final seismic intensity map of south India is obtained by considering the maximum intensity at each grid point due to the estimated earthquakes. By looking at the seismic intensity map, one can expect slight to heavy damage due to the probable earthquake magnitudes. Heavy damage may happen close to the probable earthquake zones

Pseudo-Spectral Damping Reduction Factors for the Himalayan Region Considering Recorded Ground-Motion Data

Ground-motion prediction equations that are used to predict acceleration values are generally developed for a 5% viscous damping ratio. Special structures and structures that use damping devices may have damping ratios other than the conventionally used ratio of 5%. Hence, for such structures, the intensity measures predicted by conventional ground-motion prediction equations need to be converted to a particular level of damping using a damping reduction factor (DRF). DRF is the ratio of the spectral ordinate at 5% damping to the ordinate at a defined level of damping. In this study, the DRF has been defined using the spectral ordinate of pseudo-spectral acceleration and the effect of factors such as the duration of ground motion, magnitude, hypocenter distance, site classification, damping, and period are studied. In this study, an attempt has also been made to develop an empirical model for the DRF that is specifically applicable to the Himalayan region in terms of these predictor variables. A recorded earthquake with 410 horizontal motions was used, with data characterized by magnitudes ranging from 4 to 7.8 and hypocentral distances up to 520 km. The damping was varied from 0.5-30% and the period range considered was 0.02 to 10 s. The proposed model was compared and found to coincide well with models in the existing literature. The proposed model can be used to compute the DRF at any specific period, for any given value of predictor variables

Selection of shear modulus correlation for SPT N-values based on site response studies

Shear modulus plays a fundamental role in the estimation of the ground response parameters in seismic microzonation studies. A large number of site response studies are still being carried out using SPT data, considering existing correlations between SPT N-values and shear modulus without knowing the effectiveness of the shear modulus correlation for the type of soil column. To the best of our knowledge, there is no clear-cut guideline regarding the use of a suitable shear modulus correlation to estimate representative shear stiffness for a specific soil column in response studies. In this study, therefore, an attempt has been made to identify a suitable correlation for estimating shear modulus (G(max)) for different types of soils, such as sand, clay and gravel or a mixture of all three. Sites with earthquake data recorded at the surface (soil profiles along with SPT N-values and shear wave velocity), are selected from the K-NET (Japanese website) data set. The collected earthquake data consists of moment magnitudes (Mw) ranging from 5.0 to 9.0, which were recorded at different epicentral distances. Nonlinear site response studies have been carried out by considering earthquake data recorded at a rock site as an input ground motion to the soil profiles, as published in the K-NET data site. Surface ground motion and response spectrums were further obtained from different G(max) correlations and were compared with surface recorded time histories for the same event. This study shows that peak ground acceleration (PGA), response spectrums (RS) and amplification factors (AF) obtained from a very few G(max) correlations are comparable with the recorded PGA, response spectrum and amplification factor

Maximum magnitude estimation considering the regional rupture character

The main objective of the paper is to develop a new method to estimate the maximum magnitude (M (max)) considering the regional rupture character. The proposed method has been explained in detail and examined for both intraplate and active regions. Seismotectonic data has been collected for both the regions, and seismic study area (SSA) map was generated for radii of 150, 300, and 500 km. The regional rupture character was established by considering percentage fault rupture (PFR), which is the ratio of subsurface rupture length (RLD) to total fault length (TFL). PFR is used to arrive RLD and is further used for the estimation of maximum magnitude for each seismic source. Maximum magnitude for both the regions was estimated and compared with the existing methods for determining M (max) values. The proposed method gives similar M (max) value irrespective of SSA radius and seismicity. Further seismicity parameters such as magnitude of completeness (M (c) ), ``a'' and ``aEuro parts per thousand b `` parameters and maximum observed magnitude (M (max) (obs) ) were determined for each SSA and used to estimate M (max) by considering all the existing methods. It is observed from the study that existing deterministic and probabilistic M (max) estimation methods are sensitive to SSA radius, M (c) , a and b parameters and M (max) (obs) values. However, M (max) determined from the proposed method is a function of rupture character instead of the seismicity parameters. It was also observed that intraplate region has less PFR when compared to active seismic region

Soil Void Ratio Correlation with Shear Wave Velocities and SPT N Values for Indo-Gangetic Basin

In this study attempt has been made to understand in-situ void ratio in Indo-Gangetic basin (IGB) and to form empirical relations between void ratio and shear wave velocity (V-s), N values considering subsoil investigation data. Multichannel analysis of surface wave (MASW) test and standard penetration test was carried out along with soil property measured at 25 locations. The general soil profile varied from silty sand to clay of low compressibility, ground water level fluctuated between 1-27 m, depth of borehole varied from 20-40 m. Regression analysis was conducted on 202 data sets of void ratio and shear wave velocity, 293 data sets of void ratio and SPT- N value, which resulted in inverse correlations between void ratio and V-s, SPT N value. The datas were segregated into fine, coarse grained data based on engineering classification and relations were developed separately. Until now, no studies have related in-situ void ratio to V-s and SPT N. These correlations will be useful to predict void ratio for sites having measured values of V-s and N value. These void ratios can be further used to assess liquefaction susceptibility

Correlation of densities with shear wave velocities and SPT N values

Site effects primarily depend on the shear modulus of subsurface layers, and this is generally estimated from the measured shear wave velocity (Vs) and assumed density. Very rarely, densities are measured for amplification estimation because drilling and sampling processes are time consuming and expensive. In this study, an attempt has been made to derive the correlation between the density (dry and wet density) and V-s/SPT (standard penetration test) N values using measured data. A total of 354 measured Vs and density data sets and 364 SPT N value and density data sets from 23 boreholes have been used in the study. Separate relations have been developed for all soil types as well as fine-grained and coarse-grained soil types. The correlations developed for bulk density were compared with the available data and it was found that the proposed relation matched well with the existing data. A graphical comparison and validation based on the consistency ratio and cumulative frequency curves was performed and the newly developed relations were found to demonstrate good prediction performance. An attempt has also been made to propose a relation between the bulk density and shear wave velocity applicable for a wide range of soil and rock by considering data from this study as well as that of previous studies. These correlations will be useful for predicting the density (bulk and dry) of sites having measured the shear wave velocity and SPT N values

Selection of Ground Motion Prediction Equations for Seismic Hazard Analysis of Peninsular India

Seismic hazard analysis provides an estimation of seismic hazard parameters like peak ground acceleration (PGA) or spectral acceleration (SA) for different periods. The extent of ground shaking and the hazard values at a particular region are estimated using ground-motion prediction equations (GMPEs)/attenuation equations. There are several GMPEs applicable for the region to estimate the PGA and SA values. These equations may result in higher or lower PGA and SA values than the region specific reported values, which are based on the parameters involved in the development of GMPEs. In this study, an attempt has been made to identify the best GMPEs for various parts of Peninsular India (PI) by performing an ``efficacy test,'' which make use of the average log likelihood value (LLH). Various intraplate earthquakes such as Coimbatore earthquake, Satpura earthquake, Anjar earthquake, Koyna earthquake, Bhadrachalam earthquake, Broach earthquake, Shimoga earthquake, Killari earthquake, Jabalpur earthquake, Pala earthquake, Kottayam earthquake, and Bhuj earthquake have been considered for the same. Macroseismic intensity maps of these earthquakes have been digitalized and European Macroseismic Scale (EMS) values at the surface have been synthesized. PGA value determined from each GMPE for known magnitude and hypocentral distances are then converted to EMS values. These calculated EMS values have been used to estimate LLH values which are further used to compute Data Support Index (DSI), rank and weights corresponding to a particular GMPE. Conventionally, LLH values are estimated for the entire distance range and GMPEs are ranked accordingly, but in this study, the LLH is calculated for the distance segments of 0-200 km and 200 km to maximum damaged distance in the region based on Isoseismal maps. If the maximum damaged distance is less than 200 km, a distance segment up to 200 km is adopted. Comparison between the rankings of the GMPEs in segments 0-200 km and 200-maximum damage distance is presented here. Segment-based GMPEs ranking shows different ranks, DSI and weights for each GMPE as compared to ranking considering entire distance. Finally, this study provides a list of GMPEs that perform best for the estimation of ground motion parameters in different parts of PI. This study shows that the GMPEs of HAHO-97, ATK-08, CAM-06, TOR-02, NDMA-10, and PEZA-11 perform better for the estimation of ground motion in most part of PI in the distance segment 0-200 km. The GMPEs of TOR-02, RAIY-07, and RAIY-07 (PI) perform best in the 200-maximum damage distance segment

Determination of seismic site classification of seismic recording stations in the Himalayan region using HVSR method

An attempt has been made to classify seismic stations installed along the Himalayan belt and in adjoining regions using recorded strong-motion data and different empirical methods. For all recorded data, HVSRs (horizontal-to-vertical spectral ratios) were computed using pseudo-response spectral acceleration (PSA) values. Five empirical techniques based on HVSRs and PSA were used to classify the stations. The first and second methods are based on the predominant period of the site and relationship between lino and parameters of HVSR. The third and fourth methods compute the correlation between the HVSR curve of a station and standard HVSR curves. Fifth used the PSA and PGA (peak ground acceleration) to identify the site as rock and soil. Conclusively, the site class which had the highest frequency of occurrence amongst the five methods was determined to be the final class for a given station. The final site class recommended is matched with the existing available site classification and also with available field test results

Selection of representative shear modulus reduction and damping curves for rock, gravel and sand sites from the KiK-Net downhole array

Representative computation of ground response parameters requires accurate information about nonlinear dynamic behavior of the soil column, commonly incorporated in site response analysis through shear modulus reduction and damping curves which are functions of the strain level. Most site response studies are carried out by considering a set of existing shear modulus and damping curves, without knowing its suitability for the in situ soil type. In this study, an attempt has been made to identify suitable shear modulus and damping curves for soil grouped into different classes viz. sand, gravel and rock. Soil profiles of sites having surface and bedrock motion recordings are selected from the KiK-Net downhole array database and equivalent linear and total stress nonlinear site response analysis has been carried out by varying the shear modulus and damping curves for different sites. Estimated surface response spectra for each set of shear modulus and damping curves are compared with the observed response spectra at each site, and a detailed analysis is made to find out which set of curves gives a best match with the recorded data. Based on this study, representative property curves for rock, gravel and sand are suggested, which could be used for further site response studies in the region. This study shows that only a set of shear modulus and damping curves ensure a compatible spectrum with the recorded data from the KiK-Net downhole array sites, among the many available shear modulus and damping curves in the literature

Seismic site classification and amplification of shallow bedrock sites

This study attempts to develop empirical correlations between average penetration resistance ((NSPT-R) over bar), averaged velocities over depth up to bedrock depth ((VS-R) over bar) and 30 m ((V-S30) over bar) for shallow depth sites (having bedrock at a depth less than 25 m). A total of 63 shallow sites were assessed for penetration resistance values up to the bedrock from Standard Penetration Tests (SPT) and dynamic soil property analysis, i.e., Shear Wave Velocity (V-S) from Multichannel Analysis of Surface Waves. The study shows that 30 m averaged shear wave velocities are more than the average velocity up to bedrock depth in shallow bedrock sites because of inclusion of rock site velocity. Furthermore, averaged SPT-N((NSPT-R) over bar) and average V-S ((VS-R) over bar) up to bedrock depth were correlated with the 30 m average((V-S30) over bar) values. This is the first attempt in developing empirical relationships of this kind for seismic site classification. These correlations can be made useful for seismic site classification of sites in regions with Standard Penetration Test (N-SPT) values and limited V-S values. Further surface and bedrock motion recordings of 12 selected KiK-net shallow depth sites were collected and amplifications were estimated with the respective peak ground acceleration, spectral acceleration and thereby related to the average shear wave velocity up to bedrock and 30 m. The results show that the amplification is better correlated to the (VS-R) over bar than (V-S30) over bar for shallow depth sites, and more data can be added to strengthen this correlation