Atikul Haque Farazi

Seismic ambient noise (SAN) energy can potentially blur regional and teleseismic arrivals as well as various microearthquakes at specific frequencies. Therefore, quantification of the SAN energy level in a region is required to optimize seismic station distribution for seismological investigations. Moreover, evaluation of station performance and noise source is possible from observation of SAN energy levels. The SAN energy distribution from seismic stations in the Bengal Basin (BB), Bangladesh has not yet been estimated. At the same time, this tectonically active and complex region is less studied using seismic methods. This study aims to quantify SAN energy and characterize its diurnal variation along with evaluating station performance at 11 seismic stations, which were temporarily installed in the deeper portion of the BB. Herein, the daily SAN energy level was determined within the period range of 0.02-30 s by estimating the power spectral density (PSD) of seismic data for 7 continuous days. SAN energy and its variation over time were observed using the probability density functions (PDFs) of PSDs and spectrograms, respectively. The sources of SAN energies at different period bands were also investigated by comparing the PSDs with daily variations in human activities, nearby noise sources, local meteorological factors (i.e., air temperature and precipitation), and sea level height. The insights from this study could be useful for the future deployment of seismic networks as well as seismological studies in the BB.

The S-wave velocity (VS) in the Bengal Basin, Bangladesh has not been resolved from the ground surface to an intermediate depth in a regional context despite its importance for seismic hazard and risk evaluation. For this reason, we estimated VS profiles beneath 19 seismic stations in Bangladesh to a depth approximately 2800 m by employing full horizontal to vertical spectral ratio (HVSR) curve inversion under the diffuse field theory for the noise wavefield. The seismic stations are concentrated in three tectonic zones within the basin: the Surma basin (SB, Zone 1), Bengal Foredeep (BF, Zone 2), and Chittagong Tripura Fold Belt (CTFB, Zone 3). Full HVSR analysis (from 0.2 to 10 Hz) allowed us to obtain deep profiles with combined insights from shallow geotechnical boreholes and deep P-wave velocity (VP) information from active seismic surveys. From the resultant VS profiles, engineering bedrock (VS > 760 m/s) depths were also identified throughout the study area for the first time. The VS profiles within the Holocene to Miocene sedimentary sequences showed rapid variations from location to location. This is due to the highly variable near-surface geology caused by the dense and complex river network and tectonic deformation in Bangladesh. Except for three stations, the engineering bedrock depth exceeded 30 m at all stations. These results indicate the existence of deep soft soil in the study area, where VS30 based site characterization is inappropriate. Furthermore, seismic site response was estimated at a station (DHAK) by simulating a subduction zone earthquake. The resulting response spectrum (RS) exhibited ground motion amplification over a longer period, suggesting that multistory buildings at the site may be at risk if subjected to large earthquakes. The outcomes of this study can serve as useful guidelines for seismic risk reduction planning in Bangladesh.

Summary This study presents the shear-wave velocity (VS) structures of sedimentary sequences and a section of the upper crustal layer in the Fukushima forearc region of the Japan Trench subduction zone, which were obtained by analysing the horizontal-to-vertical (H/V) spectral ratios of ambient vibration records. The H/V curves were derived using 31 days of continuous seismic data from 3 broadband and 16 short-period ocean bottom seismometer (OBS) stations. Using the broadband data, H/V ratios from 0.01 to 10 Hz were derived, but the ratios below 0.1 Hz frequencies were unusually large and temporally unstable. Characterization of seismic noise energy from ∼1 year of seismic data of three broadband OBSs revealed variable and elevated energy conditions below 0.1 Hz due to typical long-period oceanic noise; we link these observations with the unstable H/V ratios below this frequency. Therefore, H/V analysis was performed in the frequency range of 0.1–10 Hz for both broadband and short-...

Liquefaction can intensify the destruction caused by an earthquake; thus, a region with high liquefaction potential could be more disastrous. Bangladesh is surrounded by the Indo-Burma Folded Belt in the east, the Dauki Fault and Himalayan Syntaxis in the north that are known to have occurred high magnitude earthquakes (e.g., Mw > 7) in the past. Therefore, assigning seismic hazards in the regions that are economically growing fast is of great interest. Among many other hazard assessment parameters, soil liquefaction potential index (LPI) can be used to assess seismic hazards. In this study, we have assessed the seismic hazard potential for a small town (Moulvibazar) in the northeast Bangladesh documenting liquefaction potential indices for different surface geological units using an earthquake of moment magnitude Mw 8 having a peak horizontal ground acceleration (PGA) of 0.36 g. Twenty-five standard penetration test (SPT) boreholes were completed within the study area to obtain SPT-N values for two surface geological units: (1) Holo–Pleistocene low elevated terrace deposits (Zone 1) and (2) Holocene flood plain deposits (Zone 2). Using the SPT-N values, the LPI values have been calculated for the soil profile of each borehole. The LPI values in the town vary from 0 to 42.33, whereas values from 1.42 to 7.52 are in Zone 1 and values from 0 to 42.34 are in Zone 2. It has been predicted that 42% and 78% areas of Zone 1 and Zone 2, respectively, might exhibit surface manifestation of liquefaction. The results of this study can be used for seismic risk management of Moulvibazar town.

This study aims at evaluation of seismic soil liquefaction hazard potential at Probashi Palli Abasan Project area of Tongi, Gazipur, exploiting standard penetration test (SPT) data of 15 boreholes, following Simplified Procedure. Liquefaction potential index (LPI) of each borehole was determined and then cumulative frequency distribution of clustered LPI values of each surface geology unit was determined assuming cumulative frequency at LPI = 5 as the threshold value for liquefaction initiation. By means of geotechnical investigation two surface geological units—Holocene flood plain deposits, and Pleistocene terrace deposits were identified in the study area. We predicted that 14% and 24% area of zones topped by Pleistocene terrace deposits and zones topped by Holocene flood plain deposits, respectively, would exhibit surface manifestation of liquefaction as a result of 7 magnitude earthquake. The engendered hazard map also depicts site specific liquefaction intensity through LPI values of respective boreholes, and color index, which was delineated by mapping with ArcGIS software. Very low to low, and low to high liquefaction potential, respectively, was found in the areas covered by Pleistocene terrace deposits and Holocene flood plain deposits. LPI values of both units are such that sand boils could be generated where LPI > 5.

Rainfall induced landslide has become a common phenomenon in Bangladesh causing huge loss of life and properties almost every year. As landslide phenomenon in Chittagong City commonly takes place in the rainy season, it is worthy to find out other factors that affect slope stability rather focusing solely on rainfall. We investigated some engineering parameters like grain size, cohesion, and angle of internal friction of slope materials where the devastating landslides in 2007 and 2008 took place. Grain size analysis revealed that the slope material is composed of 83-98% sand, 2-17% silt and little or no clay particle. Results from direct shear test showed that cohesion of slope materials is very low ranging from 0.0459 to 0.0801 kg/cm 2 , whereas internal friction angle varies from 26.83 o to 34.30 o. But, original slope angle of that particular sites are much higher extending up to 84 o. Hence, higher percentage of sandy materials, low cohesion value and greater original slope angle than internal friction angle were the main attributes that severely affected slope stability that, in turn, triggered landslides in the study area. Moreover, different human activities like hill cutting that make the slopes more vulnerable were also responsible for landslide hazard in Chittagong City in 2007 and 2008.

Tertiary sandstones collected from southwest Sarawak, Malaysia, were analyzed to decipher their provenance, weathering, and tectonic setting. The studied sandstones have a sublitharenite composition and are dominantly composed quartz with little mica and feldspar, and a small amount of volcanic fragments. These sandstones were generally derived from quartz-rich recycled orogenic sources. They have relatively high SiO2 content with low Na2O, CaO, MnO, and MgO contents. Values of Chemical Index of Alteration (CIA) of these rock samples vary from 71 to 93, with an average of 81, implying intense chemical alteration during weathering. A felsic igneous source is suggested by a low concentration of TiO2 compared to CIA, enrichment of Light Rare Earth Elements, depletion of Heavy Rare Earth Elements, and negative Eu anomalies. A felsic origin is further supported by a Eu/Eu* range of 0.65–0.85 and high Th/Sc, La/Sc, La/Co, and Th/Co ratios. This work presents the first reported geochemical data of Tertiary sandstones of the Sarawak Basin. These data led us to conclude that the sandstones were dislodged from recycled orogenic sources and deposited in a slowly subsiding rifted basin in a passive continental tectonic setting.

"""Khalashpir coal field, the 3rd largest coal field of Bangladesh where proved reserve of about 143 million tons and probable reserve of about 685 million tons is estimated, has been decided for underground mining and artificial ground freezing (AGF) technique has been proposed as shaft sinking method after feasibility study. This method has been used in mining industry over the past 125 years for support of shaft sinking, tunneling and foundation excavation. To run the process brine solution is chilled between -25oC and -35oC in large refrigerant plant. The brine is then circulated through freezing tubes in required pattern to remove hit from the ground and thus freeze the soil to achieve soil strength and decrease the mobility of liquid water. This process results in a frozen earth barrier which gives strength to loose soil and bars water from inrushing excavation work for sinking a shaft. In this paper structural design of frozen ground works has been proposed to achieve a vertical cylindrical ice-wall for shaft sinking in Khalashpir coal field.
""""

""Abstract : Khalashpir coal field is the 3rd largest coal field in Bangladesh, where coal occurs at depths of
257m to 483m below the surface. Considering the Geological, Geo-environment and other related geoengineering
information, underground mining have been selected there to extract the deposit. In this paper, our
concern is about shaft sinking method for underground mining. Depths of the coal seams reveal the necessity of
a vertical shaft underground which again needs deep excavation. The problem arises with the excavation
because of nearly 138m thick Dupitila Sandstone Formation just 6m below the surface in the area. It is loose,
water bearing, containing dominantly porous and permeable sandstone and experiences massive water flow. So,
the major concern is that any excavation through this will readily collapse and suffer massive water inrush. This
will totally disturb the whole mining work progression and cause economic loss as well. By analyzing the
ground condition of the Khalashpir cola field, artificial ground freezing has been identified most appropriate as
shaft sinking method to control the ground water and to stabilize the loose soil during excavation. Lawfulness of
the method and reason of neglecting other two common shaft sinking methods has been pointed out in this
paper.""

The densely populated Bangladesh occupies most of the part of the Bengal Basin; the basin is located just above the subduction margin extended with an N-S alignment between the Indian and the Burmese Plates. The subduction tectonics along this Indo-Burmese plate boundary has put this locality to high seismic risk, which is also supported by the historical earthquake records. Moreover, being in the foothill of the Himalayan Mountains and the Indo-Burmese Ranges, respectively, to the North and the East, this country has become extremely riverine to be filled by sediments. Soft sedimentary layers over geophysical bedrock, with shear-wave velocity, VS > 760 m/s, can significantly amplify earthquake ground motion to cause damage to infrastructure. In addition, bedrock depth significantly controls the phenomena of soil-infrastructure vibration resonance. That is why, for seismic risk evaluation, it is essential to have adequate information on soft sediment thickness or depth to the sediment-bedrock interface.

The continuously subsiding Sylhet Basin (SB, Zone 1), being a sub-basin within the northern limit of the Bengal Basin, is the flexural depocenter in the northeastern Bangladesh with possibly the thickest (~ 25 km) sedimentary successions (Bürgi et. al. 2021). The active Dauki Fault demarcates the northern limit of the Sylhet Basin as well as the Bengal basin, along which the Shillong Plateau has been uplifted.

In this work, we present VS velocity up to 3000 m beneath three seismic stations in the Sylhet Basin, namely JAML, SUST and JAFL, data of which are available in the Incorporated Research Institutes for Seismology (IRIS) website. Here, subsurface VS profile is estimated by inversion of single-station horizontal-to-vertical (H/V) spectral ratio curve within 0.2 to 10 Hz. The H/V curves at three stations are obtained from 15 days continuous recordings of seismic ambient noise data. We use HV-Inv software (García-Jerez et. al. 2016) for the inversion, in which the H/V ratio is interpreted under the diffuse filed assumption (Sánchez-Sesma et. al. 2011) for full H/V inversion considering contribution from the full noise wavefield. The inversion process is constrained using the existing general lithological information as well as unpublishable VP data from active seismic surveys of Bangladesh Petroleum Exploration Company Ltd. (BAPEX).

From this analysis, we find that geophysical bedrock depth is approximately at 180 m, 220 m and 160 m, respectively, below the stations JAML, SUST and JAFL. To the best of our knowledge, neither the current approach of VS estimation was applied nor such high-resolution VSinformation of sedimentary successions was reported in the study area previously. The presented velocity information could be crucial for engineering development, seismic hazard mitigation, and exploration purpose in the Sylhet Basin.

Ocean bottom seismometers (OBS) are widely in use since recent past to monitor seismicity of slow earthquakes as well as that of ordinary earthquakes. Seismic velocity structures, especially of S-wave are essential to estimate hypocenters of them with accuracy. Here we focus on spatial and temporal stability of ambient noise horizontal to vertical spectral ratio (H/V) spectra calculated from ocean bottom seismometers, as the first step toward future application of ambient noise H/V to estimate S-wave velocity structure. We aim to use the Nakamura’s method (1989) for ambient noise H/V spectra using a 3-component OBS array in the Japan Trench, to image deep structure above the plate interface near the trench. To achieve the imaging, it is necessary to examine spatial and temporal stability of the derived H/V spectra from these seismometers. First, we split each 24-hours record into 1-hour windows after removing the instrumental response, Then, Fourier amplitude spectra of each component is taken and smoothed using Konno and Ohmachi (1998) method, with applying downsampling, mean and trend removal, and tapering to each window. Finally, a 1-hour H/V spectral ratio is calculated with taking quadratic mean of two horizontal components. However, a total of 21 OBS, 3 broadband and 18 short-period, stations have been used in this study. A daily variation and stability of the H/V spectra are examined along with comparing them spatially from one station to another. Stability of the H/V spectra from OBS is promising for carrying out our future endevour of deeper observation using the ambient noise H/V method.

Ocean bottom seismometers (OBS) are widely in use since recent past to monitor seismicity of slow earthquakes as well as that of ordinary earthquakes. Seismic velocity structures, especially of S-waves, are essential to estimate hypocenters of them with accuracy. The horizontal to vertical spectral ratio (HVSR) method, originally proposed by Nogoshi and Igarashi (1971) and familiarized by Nakamura (1989), is a good technique to get S-wave velocity structure by inversion of the HVSR curve. However, we aim to utilize the ambient noise (possibly <0.5Hz) HVSR approach utilizing OBS data. From this perspective, here we focus on the stability of HVSR spectra of ambient noise calculated from OBS data as the first step toward future application of this method to estimate S-wave velocity structure deploying OBS stations. We had followed the Nakamura's method (1989) for obtaining HVSR spectra and then compared their shapes for stability analysis.