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1

Site condition investigation using horizontal to vertical spectral ratios of Iran strong motion data

Eshaghi Attieh, Hassani Behzad, Shahvar Mohammad Pourmohammad, Farzanegan Esmail

Acta Geophysica

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2023

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Vol. 71, no. 5

2049--2064

EN

In this study, horizontal to vertical spectral ratios (HVSR) have been calculated for strong ground motion data recorded by the Iran Strong Motion Network (ISMN). Then, a uniform and specific procedure is applied for selecting the peak amplitude (Apeak) and peak frequency (ƒpeak) of the average HVSR for each station in the database. Based on this procedure, Apeak and ƒpeak values were estimated at 502 stations of ISMN. Also, the relationships between HVSR parameters and time-averaged shear wave velocity in the upper 30 m (VS30) have been investigated and these relationships are presented for Iran. The results and uncertainties of these models are compared with other related studies conducted in Japan, North and Central America, as well as previous studies for Iran. Compared to other datasets, the VS30 model presented for Iran shows the highest VS30 values among all models. The equation developed in this study can be used to estimate VS30 at ISMN stations where there is no other site characteristics available. Therefore, the soil type can be classified according to the Standard No. 2800, which is useful in seismic design codes and other related studies.

2

Generation of seismic hazard maps for Assam region and incorporation of the site effects

Bandyopadhyay Srijit, Parulekar Yogita M., Sengupta Aniruddha

Acta Geophysica

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2022

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Vol. 70, no 5

1957--1977

EN

Probabilistic seismic hazard assessment (PSHA), including a site-specific amplification study, is crucial to evaluate site-specific spectra of soil sites to better understand the behavioural patterns of the soils under earthquake excitation. This paper represents the results of the PSHA for Assam state, located in the highest seismic zone of India ever delineated to date. In that sense, the study area is divided into ten areal zones concerning seismicity source modelling to represent the seismo-genesis of the Assam state in detail. The earthquake recurrence parameters of each zone are obtained from Gutenberg–Richter (G–R) recurrence relation with updated homogenized and de-clustered earthquake catalogue from 1735 up to 2021. Earthquakes with magnitude greater than 4 (M>4) are considered using eight attenuation relationships for continental active shallow crust region, subduction zone and intraplate region. Hazard curves are obtained using a logic tree structure thus minimizing the epistemic uncertainty. The peak ground acceleration (PGA) value obtained at the rock outcrop of the Assam state for 10, 5, 2, and 0.5% probability of exceedance in 50 years with return periods such as 475, 975, 2475, and 9975 years lies between 0.24 and 0.34 g, 0.3 and 0.44 g, 0.42 and 0.59 g, and 0.56 and 0.91 g, respectively. The estimated PGA value at rock outcrop level is comparatively higher than that reported in the codal provisions. Site-specific response spectra at bedrock level (Vs=1100 m/s) for major cities (Jorhat, Tezpur, Silchar, Dibrugarh, Guwahati, Nagaon) of Assam state have been proposed for different earthquake return periods of 475,2475, and 9975 years. Finally, site amplification study is performed for Guwahati city and surface level 5% damped response spectra with PGA of 0.696 and 0.924 g are obtained for earthquake a return period of 2475 and 9975 years, respectively.

3

Investigation of fault zone induced site effect in the İzmit basin, Turkey

Firtana-Elcomert Karolin, Kocaoğlu Argun

Acta Geophysica

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2021

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Vol. 69, no. 6

2069--2083

EN

The seismic hazard in the İzmit basin, located in Marmara region of western Turkey, is high due to the northern branch of the North Anatolian Fault (NAF) and the potential ground motion amplification that may be caused by local site conditions, sedimentary basin effect as well as fault zone (FZ) induced site effect resulting from the generation of guided waves. In this study, we elaborate the relevance of the FZ-induced site effect in the İzmit basin along a 16.5-km-long N-S profile across the basin and perpendicular to the NAF by time and frequency domain analysis of waveforms obtained from two-dimensional (2D) simulations of viscoelastic wave propagation for a double-couple source at 14 km depth using a reference (basin-only) model and three basin-with-fault models: shallow (6 km), intermediate (12 km) and deep (19 km) FZ models. Our results show that the FZ-induced site effect within and near the northern branch of NAF in the İzmit basin can be very prominent with amplification of about 5–10 in the frequency range of 0.05–4 Hz and about 20 at frequencies above 2 Hz, respectively. We obtain the most dramatic results for the deep FZ model causing shear- and surface-wave amplification of about 15 at frequencies higher than 2.5 Hz for the distances between 6 and 13 km.

4

Modeling of the strong ground motion of 25th April 2015 Nepal earthquake using modified semi-empirical technique

Lal S., Joshi A., Sandeep, Tomer M., Kumar P., Kuo C.-H., Lin C.-M., Wen K.-L., Sharma M. L.

Acta Geophysica

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2018

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Vol. 66, no. 4

461--477

EN

On 25th April, 2015 a hazardous earthquake of moment magnitude 7.9 occurred in Nepal. Accelerographs were used to record the Nepal earthquake which is installed in the Kumaon region in the Himalayan state of Uttrakhand. The distance of the recorded stations in the Kumaon region from the epicenter of the earthquake is about 420–515 km. Modified semiempirical technique of modeling finite faults has been used in this paper to simulate strong earthquake at these stations. Source parameters of the Nepal aftershock have been also calculated using the Brune model in the present study which are used in the modeling of the Nepal main shock. The obtained value of the seismic moment and stress drop is 8.26 9 1025 dyn cm and 10.48 bar, respectively, for the aftershock from the Brune model .The simulated earthquake time series were compared with the observed records of the earthquake. The comparison of full waveform and its response spectra has been made to finalize the rupture parameters and its location. The rupture of the earthquake was propagated in the NE–SW direction from the hypocenter with the rupture velocity 3.0 km/s from a distance of 80 km from Kathmandu in NW direction at a depth of 12 km as per compared results.

5

Site Specific Ground Motion Modeling and Seismic Response Analysis for Microzonation of Baku, Azerbaijan

Babayev G., Telesca L.

Acta Geophysica

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2016

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Vol. 64, no. 6

2151--2170

EN

We investigated ground response for Baku (Azerbaijan) from two earthquakes of magnitude M6.3 occurred in Caspian Sea (characterized as a near event) and M7.5 in Shamakhi (characterized as a remote extreme event). S-wave velocity with the average shear wave velocity over the topmost 30 m of soil is obtained by experimental method from the VP values measured for the soils. The downtown part of Baku city is characterized by low VS30 values (< 250 m/s), related to sand, water-saturated sand, gravel-pebble, and limestone with clay. High surface PGA of 240 gal for the M7.5 event and of about 190 gal for the M6.3 event, and hence a high ground motion amplification, is observed in the shoreline area, through downtown, in the north-west, and in the east parts of Baku city with soft clays, loamy sands, gravel, sediments.

6

Application of the horizontal to vertical spectral ratio technique for estimating the site characteristics of ground motion caused by mining induced events

Olszewska D., Lasocki S.

Acta Geophysica Polonica

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2004

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Vol. 52, nr 3

301-318

EN

The work presents an attempt of application of the horizontal-to-vertical spectral ratio (HVSR) method for estimating the local amplification of ground motion caused by mining seismic events in the Legnica Głogów Copper District. Amplifying properties of the surface layer are assessed from the ratio of amplitude spectra of the horizontal and vertical components of ground acceleration, recorded at the surface. The location of a local maximum of the ratio in the frequency band up to 8 Hz assigns the resonant frequency of the surface layer; the maximal value estimates the amplification factor at the measurement point. The spectral ratio was evaluated for 219 ground acceleration records from ten recording stations. The HVSR curves for induced seismicity turned out to be similar to the typical HVSR-s for natural earthquakes. Amplification factors estimated by the HVSR method were used to reduce the observed peak ground accelerations (PGA-s) to the bedrock. The reduction significantly improved an agreement beween the PGA values order and the order of epicentral distances. The obtained results suggest that the HVSR method can be successfully used to evaluate the local influence of the surface layer also for induced seismicity, in spite of the fact that in this case the conditions for application of the method essentially differ from the conditions met in natural seismicity.