Ravi S. Jakka

Site Characterization of Strong‐Motion Recording Stations of Delhi Using Joint Inversion of Phase Velocity Dispersion and H/V Curve

Site characterization is one of the most important aspects of any strong‐motion instrumentation. Nowadays it has become common practice to provide the characterization details up to bedrock level. Without proper site characterization, strong‐motion records of any station cannot be fully utilized. In India, strong‐motion instrumentation sites were classified in three categories as per V S 30 values. These V S 30 values were estimated using the Borcherdt (1994) methodology, in which physical properties of visible soil layer are used for V S 30 estimation. Because this methodology does not use any field testing, the probability of getting erroneous results is very high. Hence, to get accurate assessment of site characteristics, we conducted field testing at 19 strong‐motion instrumentation sites in Delhi, India. The site characteristics assessed here are determined using joint inversion of multichannel analysis of surface waves and horizontal‐to‐vertical spectral ratio (HVSR) from ambient noise results simultaneously to estimate shear‐wave velocity profiles. The benefit of using this method is that it provides site characteristics assessed through shear‐wave velocity profiles up to much deeper soil strata. The results obtained from this analysis are further validated using ground response analysis from recorded ground motions. Further, the profiles obtained are studied for uncertainties using the computer program STRATA. Transfer functions obtained from STRATA are then compared with HVSR of ambient vibration records as well as HVSR from weak‐motion earthquake records available for the sites. These curves are found to well matching with each other in this study. Site characterization carried out here will be very useful for studies related to seismic‐hazard assessment of the Delhi region and studies related to attenuation models.

Suitable triggering algorithms for detecting strong ground motions using MEMS accelerometers

With the recent development of digital Micro Electro Mechanical System (MEMS) sensors, the cost of monitoring and detecting seismic events in real time can be greatly reduced. Ability of MEMS accelerograph to record a seismic event depends upon the efficiency of triggering algorithm, apart from the sensor’s sensitivity. There are several classic triggering algorithms developed to detect seismic events, ranging from basic amplitude threshold to more sophisticated pattern recognition. Algorithms based on STA/LTA are reported to be computationally efficient for real time monitoring. In this paper, we analyzed several STA/LTA algorithms to check their efficiency and suitability using data obtained from the Quake Catcher Network (network of MEMS accelerometer stations). We found that most of the STA/LTA algorithms are suitable for use with MEMS accelerometer data to accurately detect seismic events. However, the efficiency of any particular algorithm is found to be dependent on the parameter set used (i.e., window width of STA, LTA and threshold level).

Study of Local Site Effects for Strong Motion Recording Stations of Delhi

Effects of local site conditions on strong ground motion characteristics are a well-known phenomenon studied by many researchers. Similar studies were also carried out in past for Delhi to understand the amplification of ground motion in various locations using different methods. In this study, an attempt is made to study the amplifications in eight locations of Delhi where strong motion recording stations are located. For this study, shear wave velocity profiles were used for performing ground response analysis using equivalent linear approach. For input motion, response spectra compatible motion was used, compatible to response spectra for Rock sites as per IS 1893:2002. Transfer functions from available earthquake records and ambient vibration records were also calculated for comparison. The results obtained were presented in the form of response spectra and amplification functions for all the sites. The results suggest an amplification of the order of 2–3 with respect to actual motion at sites coming under site class D of NEHRP. No amplification was found at IMD site which suggest that it can be used as a reference site for Delhi. Other two sites with NEHRP site class C found to have amplification of the order of 1–2. Similarly, output response spectra, at surface, for all the sites except IMD and Kalkaji were found to be amplified with respect to the corresponding response spectra of IS 1893:2002.

Uncertainties in Site Characterization Using Surface Wave Techniques and Their Effects on Seismic Ground Response

Surface wave methods which utilize the dispersion property of Rayleigh waves are widely used for subsurface site characterization. As a non-invasive method of site characterization, it has many advantages over the invasive methods of geotechnical site characterization. Surface wave methods determine the small strain shear modulus of near-surface materials, and this shear modulus is the key input in the evaluation of the soil response under dynamic/seismic loading. So, the accuracy of testing is very important, otherwise it may lead to significant consequences on the seismic hazard studies. There are different uncertainties associated with surface wave methods. These uncertainties can be broadly classified into three categories: Model-based uncertainty, Data measurement uncertainty, and Inversion uncertainty. Model-based uncertainty basically contains the near-field effects which lead to the underestimation of Rayleigh wave phase velocity. Data measurement uncertainty is another major source of uncertainty, which arises while conducting the surface wave tests due to the noise present in the surroundings in the form of continuous or transient signals. Noise results in a scatter in the measured dispersion curve and this scatter in the dispersion curve may provide different velocity profiles, which are falling in the range of measured data variation. Inversion uncertainty deals with non-unique solution of inversion. Non-unique solution may results into several equivalent velocity profiles, with a good fit with the experimental dispersion curve. Now, the consequence of this data measurement and inversion uncertainty may show significant variation on ground response analysis.

Effect of uncertainty in \(V_{\mathrm{S}}{-}N\) correlations on seismic site response analysis

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Seismic Slope Stability of Embankments Constructed with Pond Ash

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