Passive Seismic Research Articles

Understanding slow-moving landslide triggering processes using low-cost passive seismic and inclinometer monitoring

Landslides are a major natural hazard, threatening communities and infrastructures worldwide. Mitigation of these hazards relies on understanding their causes and triggering processes, which critically depend on subsurface characteristics and their variations over time. In this study, we present a novel approach combining passive seismic and low-cost inclinometer monitoring methods to improve the understanding of landslide activation mechanisms and their controls. We evaluate the efficiency of this approach on a shallow, slow-moving landslide directly endangering a road bridge, a bridge that is part of an important emergency response route. Results show the value of combining the two approaches for observing and monitoring landslide hazards. Passive seismic monitoring captures the variation in soil properties (rigidity and density) over time by sensing the variations of the seismic wave velocity (dV/V and its associated correlation coefficient). At the same time, novel low-cost inclinometers are monitoring subsurface deformation (from millimetric to pluricentimetric scale) and temperature. Seismic precursors detected at the bottom sensor a few hours prior to the reactivation are followed by the reactivation of the landslide toe, releasing stresses in the top part that lead to the reactivation of the whole landslide. This reactivation occurs during an episode of heavy rainfall following a 7-month drought. Meanwhile, temperature monitoring enables us to track water infiltration and to highlight its role in the landslide mechanisms. Overall, the combination of the two monitoring methods shows promise for quantifying the sliding mechanisms of landslide reactivations and for designing landslide early warning systems.

Real-time tunneling risk forecasting using vibrations from the working TBM

Tunnel Boring Machines (TBMs) are effective tools for excavating tunnels through various soil and rock strata, especially in complex urban areas. During excavating, hazards overlooked during site investigation, such as abrupt geological changes, buried obstacles like boulders and building foundations, fault zones, and water-bearing regions, significantly increase the risks of cutter damage and tunneling accidents. Geophysical methods have been proposed during tunnel excavation to predict these potential hazards ahead of the cutter face. With the existence of a metallic TBM and subsequently installed concrete tunnel lining, the performance of electromagnetic geophysical methods such as ground-penetrating radar and DC resistivity tomography are greatly hampered, despite their success in the traditional open tunnels. In this study, we present a passive seismic method to forecast the tunneling risks ahead of the TBM cutterhead. This method uses the working TBM as the source and predicts the velocity/density changes in front of the TBM by delineating the reflected seismic energy from the noisy recordings. The key to this technology is seismic interferometry, which transforms the chaotic ambient noise into interpretable seismic profiles. We utilize beamforming and semblance analysis to extract the traveltime information from the passive records. After velocity inversion and calibration, we map the arrival-time of a reflection event to a seismic impedance interface ahead of the cutterface. To test the proposed method, we demonstrate the process on a simulated passive seismic record after laying out its theoretical foundations. We present a field application of our method where geological interfaces are predicted in real time as the excavation progresses. The field data results match well with the muck density and TBM parameters measured after excavation and hence demonstrate the applicability of our method in complex urban environments for tunneling risk reduction.

Active and passive seismic methods for site characterization in Nuweiba, Gulf of Aqaba, Egypt

Nuweiba is located on the western coast of the Gulf of Aqaba (GoA), which represents the southern part of the Gulf of Aqaba - Dead Sea transform fault (GoA-DST). The GoA is characterized by high seismic activity and is the source of the November 22, 1995, Nuweiba earthquake, with a moment magnitude of Mw 7.3. This seismic unrest caused extensive damage in the city of Nuweiba due to the strong shaking caused by the earthquake. In addition, soil amplification triggered by the thick soft-sediments of the Wadi Watir delta beneath the city amplified the destructive power of the event. Therefore, an assessment of the effect of the Wadi Watir sediments on the ground motion at Nuweiba is very important for future construction, seismic hazard assessment, and seismic risk mitigation. Accordingly, the goal of the current study is to define the amplification characteristics of the Wadi Watir deposits using microtremor and surface wave data sets. The horizontal-to-vertical spectral ratio (HVSR) technique is applied to the microtremor data and Multichannel Analysis of Surface Waves (MASW) is applied to the surface wave data. The data are processed to obtain the distribution of the fundamental resonance frequency (F0), the depth of the geophysical bedrock using joint inversion of the Rayleigh wave dispersion curve and ellipticity peak, the shear wave velocity structure, the time-averaged shear-wave velocity in the topmost 30 m (Vs30), and the shear wave transfer function. The results of the HVSR show that F0 in the northern part of the city is 4.2–5.2 Hz and 0.5–0.8 Hz in the middle of the city. Bedrock depth is found to be in the range of 27 and 199 m. The one-dimensional velocity structure obtained from the MASW measurements has resolved two sedimentary layers within the upper 15 m. The Vs30 ranges between 207 and 556 m/s, according to the Vs30 the soil in the study area is categorized into soil classes B and C according to Eurocode 8. The obtained shear wave transfer functions show that the highest amplification factors are observed in the middle part of the study area.

Imaging an Enigmatic Paleovalley with Passive Seismic Methods (Unaweep Canyon, Colorado, United States)

Abstract High-resolution passive seismic imaging of shallow subsurface structures is often challenged by the scarcity of coherent body-wave energy in ambient noise recorded at surface stations. We show that the autocorrelation (AC) of teleseismic P-wave coda extracted from just one month of continuous recording at 5 Hz geophones can overcome this limitation. We apply this method to investigate the longitudinal subsurface bedrock structure of Unaweep Canyon—a paleovalley in western Colorado (United States) with complex evolution. Both fluvial and glacial processes have been proposed to explain the canyon’s genesis and morphology. The teleseismic P-wave coda AC retrieves zero-offset reflections from the shallow (200–500 m depth) basement interface at 120 stations along a 5 km long profile. In addition, we invert interferometrically retrieved surface-wave dispersion for the shear-wave structure of the sedimentary fill. Combined interpretation of these results and other geophysical and well data suggests an overdeepened basement geometry most consistent with glacial processes.

Joint passive seismic imaging based on surface wave inversion and reflection wavefield retrieval: A case study in the sedimentary basin areas adjacent to Well Songke-2

Owing to their cost-effectiveness, environmental friendliness, and high-efficiency, passive seismic methods are widely used in geophysical exploration. However, field passive seismic data suffer from low signal-to-noise ratio, as well as the fact that different types of seismic waves are naturally mixed together, which can affect the accuracy of subsurface imaging results and the subsequent geological interpretation. In this paper, we demonstrate that these problems can be addressed and present a case study on passive seismic detection in sedimentary basin areas adjacent to Well Songke-2 in Songliao Basin. To obtain accurate and reliable imaging results of the sedimentary strata, a detailed practical scheme is proposed, where fundamental and higher mode surface wave dispersion curves are inverted for obtaining the near-surface S-wave velocity profile, and the body wave component of ambient noise is utilized to retrieve the reflection wavefield information. The obtained profiles from surface wave inversion and reflection wavefield retrieval illustrate similar underground structures. The marker boundaries T2 (1.5 s) and T4 (2.1 s) are well demonstrated, and a low velocity stratum (0.4 s) is detected at a shallow depth of around 400–600 m. Further, the results are highly consistent with the data obtained from borehole logging of Well Songke-2 and the deep reflection seismic profile adjacent to this area, which indicates that the surface wave and body wave in passive seismic data can be utilized together to contribute to a detailed and accurate subsurface imaging and interpretation. Overall, this study investigated and validated the reliability and accuracy of the combination use of passive seismic methods for geological structure exploration, which can further boost their applications for geological interpretation in sedimentary basin areas.

The Analysis of Triaxial Passive Seismic Signal Data Using Python Scripting Language

The local geological and the physical conditions of earth’s ground defines its vibration characteristics. Such vibrations can be detected using conventional passive seismic methods, i.e., a microtremor recording method, which acquires ground vibration data in a suitable sampling time. The method utilizes natural seismic sources, originating from both natural and artificial sources. In this work we present the algorithm to process passive seismic signals, recorded from a laboratory developed triaxial seismic logging device. We use python scripting language to perform the signal processing work. We analyzed the Horizontal to Vertical Spectral Ratio (HVSR) of the signal to determine the on-site natural frequency. The results were compared to the reference data from a high sensitivity microtremor apparatus, at the exact same location. The results show a fair agreement between the low sensitivity and the high sensitivity seismometer, on the sites with low environmental disturbance. However, the values fluctuate significantly at noisy sites. Either way, our results measure the correct dominant frequencies as expected from the geological condition.

Shear wave velocity and site period measurements for the western portion of the Mexico City Basin following the Mw7.1 2017 Puebla–Morelos, Mexico, earthquake

Dynamic site characterization was performed at 25 sites located on the western portion of the Mexico City Basin that were severely damaged during the Mw7.1 2017 Puebla–Morelos, Mexico, earthquake. Testing was conducted using active and passive seismic surface wave methods and the microtremor horizontal-to-vertical spectral ratio method to determine site periods and develop one-dimensional (1D) shear wave velocity ( Vs) profiles for the first 60 m of the subsoil. The measured site periods were compared to site period maps developed in 2004 and 2020 along with values computed using the Design Seismic Actions System (SASID) software following the 2020 version of the Complementary Technical Norms for Seismic Design (NTC-DS). The most noticeable biases in the predictions from the 2004 site period map were observed between the boundary of Zone II and Zone IIIa, at which site periods are overestimated. These estimates were improved upon in the 2020 site period map and showed a close similarity with SASID computed site period values. The Vs, depth, and thickness of the lacustrine clay layer were also determined to be quite variable within the basin. The softest sites are located between the lakebeds with a Vs between 45 and 57 m/s. Sites located toward the outer rim of the North lakebed have a higher Vs between 80 and 100 m/s. The thickness of the clay layer varies significantly in the western side of the Basin with values ranging between approximately 3 and 34 m. Overall, the results of this study indicate good agreement with the model embedded in the SASID software. The results (1) emphasize the need to regularly monitor changes that occur over time in the lacustrine clay layer, (2) complement the development of models that improve our understanding of wave propagation within the Basin, and (3) update and improve Mexico City’s Norms.

Effects of Lunar Near‐Surface Geology on Moonquakes Ground Motion Amplification

AbstractLunar seismology has focused on exploring the Moon's seismic activity and its internal structure. However, less attention has been given to site specific, near‐surface geology‐related amplification of seismic waves on the Moon. Here, we quantified ground‐motion amplifications of seismic waves due to local site conditions at the Apollo landing sites. We analyzed Apollo Passive Seismic Experiments seismometer recordings of 30 artificial and natural impacts and estimated the mid‐period (0.1–3 Hz) ground‐motion amplification at each site. We applied techniques commonly used in engineering seismology to understand the local site effects, such as the Horizontal over Vertical Spectral Ratio (HVSR) and the Reference Site Method (RSM) on lunar seismic data. The HVSR and the RSM show convergent spectral ratios specific for each landing site. To correlate these empirical results estimated from lunar artificial and meteoroid impact events with the local site geology, we simulated ground motion amplification using 2D finite difference modeling of lunar geological models and estimated the best geological setting capable of producing a similar frequency response to those observed at each Apollo landing site. Correlation results from the empirical methods and the simulated models show similarities ranging between 79% and 92%, a correlation more than 90% between the RSM and the HVSR results, and similarity exceeding 96% between RSM estimations using different reference sites. These results demonstrate the prominence of a local site geology dependent amplification for frequencies between 0.1 and 3 Hz.

Deep structure of the Verkhnetovskaya kimberlite pipe in the Arkhangelsk diamondiferous province according to passive seismic and radiological methods

AbstractThe exploration of kimberlite pipes is difficult. However, information about their deep structure is necessary for tasks such as prospecting, exploration and development of kimberlite pipes. Therefore, it is necessary to increase the efficiency of deep structure research. In addition, pipes in different territories differ in their properties. The latter requires the study of the peculiarities of the manifestation of pipes in various areas. The Verkhnetovskaya pipe is the only kimberlite object currently known in the Chernoozerskaya area of the Arkhangelsk diamondiferous province. This indicates the need for a comprehensive study of this pipe. The main reason it is difficult to study the pipes of the Arkhangelsk diamondiferous province is the large overburden thickness. In order to increase the efficiency of prospecting, we used a set of methods: microseismic sounding method, passive seismic interferometry, H/V method, gamma spectrometry and emanation mapping. The analysis showed that a new field measurement technique was tested in this research, resulting in more stable data. The main improvement in field measurements is the determination of the optimal accumulation time of microseism when implementing the microseismic sounding method. In addition, the location of the stations in the implementation of passive interferometry made it possible to minimize the influence of the azimuthal distribution of ambient noise sources. The studies made it possible to construct the geophysical image of the investigated pipe and the surrounding medium. As a result, it was shown that the northwestern side has a vertical structure, while according to the drilling data, the side has a uniform slope. In particular, the presence of previously unknown lateral channels was shown. The proposed methodology made it possible to obtain important information with minimal time and technical costs, confirming the applicability of the proposed methodology in the Chernoozerskaya area.

Passive seismic interferometry imaging: An example from the Ketzin pilot CO2 geological storage site

Ketzin is a well-known onshore CO2 geological storage test site. Injection started in June 2008 and ended in August 2013. About 67 kt of CO2 was injected into a saline aquifer reservoir. Numerous geophysical surveys have been conducted at the site, including passive seismic ones. Passive seismic interferometry can be applied in many seismic exploration investigations to image the underground structures. This study compares several passive seismic interferometry imaging methods to a data set acquired at Ketzin. The passive data set consist of 5 lines with 20 h of recordings in total. First, we apply the autocorrelation method to obtain zero offset stacked sections directly. Secondly, we test different interferometry methods to generate virtual shot gathers. These virtual shot gathers are then processed to obtain CDP stacked sections. Results from these methods are compared with an active data set recorded at the site. The processed passive sections have relatively less coherent reflections compared with the active stacked sections. However, the passive sections obtained by autocorrelation show good agreement with the active stacked sections. The stacked sections obtained by processing virtual shot gathers using crosscorrelation show similar features to the active stacked sections, but with poorer coherence in the reflections and are noisier than the autocorrelation sections.

Buried bedrock valleys revealed in Michigan’s central Upper Peninsula using the horizontal-to-vertical spectral ratio passive seismic method

A network of north–south linear troughs occupies the eastern portion of the Lake Superior basin. Morphology suggests these features are tunnel valleys that formed through erosion as subglacial conduits and conveyed meltwater to a former glacial margin. We demonstrate the southern continuation of these valleys onshore into the central Upper Peninsula of Michigan by mapping bedrock topography using the Horizontal-to-Vertical Spectral Ratio (HVSR) passive seismic method. This is a single location, noninvasive seismic technique that records ambient signal. HVSR data are used to estimate subsurface elevation of the sediment-bedrock contact at each site. Regional bedrock topography was estimated using HVSR data from 347 locations. We delineated, for the first time, eight buried (or partially buried) bedrock valleys with geometric characteristics (width, depth, orientation, and continuity) suggesting a southward extension of the Lake Superior tunnel valley network. The largest valleys terminate near the crest of the Munising/Grand Marais upland. This association suggests the upland was constructed by deposition of sediment-laden meltwater at the terminal ends of these tunnel valleys as fans (or heads of outwash). Episodic erosion of the largest valleys likely began during the Two Rivers phase (∼13 ka) and were again active during the Marquette phase (∼11 ka). The smallest valleys developed later during final regional deglaciation following the Marquette phase.

Active and Passive Seismic Imaging of the Central Abitibi Greenstone Belt, Larder Lake, Ontario

AbstractPassive seismic methods are considered cost‐effective and environmentally friendly alternatives to active (reflection) seismic methods. We have acquired colocated active and passive seismic surveys over a metal‐endowed Archean granite‐greenstone terrane in the Larder Lake area to investigate the reliability of estimated elastic properties using passive seismic methods. The passive seismic data were processed using two different data processing approaches, ambient noise surface‐wave tomography (ANSWT) and receiver function analysis, to generate shear‐wave velocity and P to S wave (P‐S) convertibility profiles of the subsurface, respectively. The Cadillac‐Larder Lake Fault (CLLF) was imaged as a south‐dipping subvertical zone of weak reflectivity in the reflection seismic profile. To the north of the CLLF, a package of north‐dipping reflections in the upper crust (at depths of 5–10 km) resides on the boundary of high (on the top) and low (on the bottom) shear‐wave velocity zones estimated using the ANSWT method. This package of reflections is most likely caused by overlaying mafic volcanic and underlying felsic intrusive rocks. The P‐S convertibility profile imaged the Moho boundary at ∼40‐km depth as well as a south‐dipping feature that penetrates the mantle, which is interpreted to be either caused by the delamination of the lower crust or a possible deeper extension of the Porcupine‐Destor Fault. Overall, the reflectivity, shear‐wave velocity, and P‐S convertibility profiles exhibit a good correlation and provided a detailed image of the subsurface lithological structure to a depth of 10 km.

Postseismic Survey of a Historic Masonry Tower and Monitoring of Its Dynamic Behavior in the Aftermath of Le Teil Earthquake (Ardèche, France)

ABSTRACTOn 11 November 2019, an Mw 4.9 earthquake struck the middle Rhône valley (South-East France) producing moderate to severe damage in the town of Le Teil and its surroundings. This unexpected event stressed the vulnerability of the French cultural built heritage to a moderate seismic hazard. Commonly applied to modern civil engineering structures, passive seismic methods are still lacking on historic constructions to understand properly the different factors driving their dynamic behavior. In this article, the results of a two-month seismic monitoring survey carried out shortly after the Le Teil mainshock in a historic masonry tower are presented and discussed. Located only 5 km south of the epicenter, the Gate Tower of Viviers (eleventh century) was instrumented with four highly sensitive seismic nodes. Ambient vibrations, as well as aftershocks and quarry blasts from the nearby Le Teil quarry, were recorded and used in the analysis. Through vibration-based analysis, the article addresses three relevant aspects of the dynamic response of ancient masonry structures. We discuss first the differences in the building’s response induced by the three reported types of vibrations, focusing on the particular signal characteristics of shallow aftershocks and quarry blasts. Then, we apply the Random Decrement Technique (RDT) to track the dynamic behavior variations over two months and to discuss the role of the environmental conditions in the slight fluctuations of the structural modal parameters (natural frequencies, damping coefficients) of unreinforced masonry structures. We also show evidence of the nonlinear elastic behavior under both weak seismic and atmospheric loadings. The correlation between the presence of heterogeneities in the construction materials and the nonlinear threshold supports the relevance of such types of monitoring surveys as a valuable tool for future modeling works and conservation efforts.

Surface and underground seismic characterization at Terziet in Limburg—the Euregio Meuse–Rhine candidate site for Einstein Telescope

We present a detailed characterization of surface and underground seismic noise measured at Limburg in the south of the Netherlands. This location is the Euregio Meuse–Rhine candidate for hosting Einstein Telescope, a future observatory for gravitational waves. Seismic noise measurements were performed with an array of seismometers installed on the surface. Passive seismic methods like beamforming were used to extract the propagation wave types of ambient seismic noise and the Rayleigh-wave dispersion in the region. Subsurface shear-wave models sensitive to depths of 300 m were derived by using the Rayleigh-wave dispersion and ellipticity. Subsurface P-wave velocities to depths of 200 m were obtained from an active seismic survey. Wavepath Eikonal tomography was used on the source-receiver refracted-wave travel-times to obtain a subsurface P-wave velocity model. Both the passive and the active seismic data analysis point to the presence of a layered geology with a soft-soil to hard-rock transition occurring at a shallow depth of about 25 to 40 m. The surface arrays are complemented by two permanent tri-axial seismometers installed on the surface and in a borehole at 250 m depth. Their data are used to interpret the surface-wave and body-wave contributions to the observed seismic noise. We use a cross-correlation analysis and compute the theoretical surface-wave eigenfunctions to understand the contributions of the different wave types at different frequencies. We observe that below 4 Hz in the horizontal component and 9 Hz in the vertical component, the seismic noise at depth is dominantly due to surface waves. Above these frequencies a significant contribution can be attributed to both nearby and far-away body-wave sources. At a depth of 250 m we find that the surface noise power has been damped by up to a factor 104 above about 2 Hz. The Limburg geology with soft-soil on top of hard-rock efficiently damps the anthropogenic noise produced at the surface. This implies that Einstein Telescope’s test masses are shielded from anthropogenic seismic noise and construction at greater depth will not bring significant further improvements in this regard. A body-wave background has been identified that contributes about half of the total underground seismic noise at 250 m depth for frequencies above 4 Hz. It remains to be studied if subtraction schemes for Newtonian noise originating from this body-wave background will be necessary. Finally, we estimate an interferometer downtime of about 3% due to regional and teleseismic earthquakes. We believe this is acceptable as it is comparable to current experience at the LIGO and Virgo interferometer sites.

Stability Assessment and Geomorphological Evolution of Sea Natural Arches by Geophysical Measurement: The Case Study of Wied Il-Mielah Window (Gozo, Malta)

The Wied il-Mielaħ Window (Gozo–Malta) is a limestone natural arch on the north-western coast of the island of Gozo in Malta. It is located at the end of the Wied il-Mielaħ valley north of the village of Għarb. This natural arch is less well known than the Azure Window, which collapsed in March 2017 following a heavy storm, but notwithstanding, it is an imposing and important natural monument too. In the past, the Wied il-Mielah valley was responsible for discharging wastewater from the surrounding localities to the Mediterranean directly at the Wied il-Mielah Window. The sewage flag was often clearly visible underneath the archway into the open sea. The natural features of the arch provide an outstanding touristic attraction. To avoid what happened to the Azure Window, a methodology for the evaluation of the collapse hazard, combining passive seismic, ground penetrating radar (GPR), geological/geomorphological surveys and mine engineering methods, is here proposed. In this study, a methodological approach was applied, based on the following: (i) passive seismic method to study the physical–mechanical characteristics of the rock mass that constitutes the window; (ii) GPR method in order to demonstrate the conservation state (i.e., the intensity of fracturing); (iii) geological/geomorphological surveys in order to obtain a crack pattern; and (iv) scaled span empirical analysis in order to evaluate the stability of the arch. The calculation of the safety factor, with a static method, gave a value equal to 3.75 with a probability of collapse of the marine arch within 50 and 100 years.

Forward Modelling of Bouguer Anomalies along a transect of the Southern Apennines and the Southern Tyrrhenian Sea (Italy)

In the present study, we perform a gravity modelling at crustal scale along the trace of the CROP-04 (on-shore) and M-6B (off-shore) deep seismic reflection profiles, crossing the Southern Apennines and the Southern Tyrrhenian Sea (Italy). Along the 321 km-long modelled profile, we investigate the crustal-scale sources for the observed gravity anomalies through a simplified model of the crust and upper mantle across both onshore and offshore areas.After a compelling review of the published Moho geometries in the area, that were retrieved from either active or passive seismic methods, we test them in the observed gravity field through forward modelling of the Bouguer gravity anomalies. The comparison between the different Moho interpretations highlights the major contributors to the observed Bouguer gravity at the crustal scale, defining a set of starting values of these parameters for our final model.The proposed model locates the westward flexure of the Adriatic Moho, mimicking the subduction of the Adriatic lithosphere beneath the Peri-Tyrrhenian block and locates the step between the western (Tyrrhenian) and the eastern (Adriatic) Moho beneath the Apennines range providing a valuable geometrical and compositional model at the crustal scale. The model depicts a typical oceanic-to-continental crust transition in the Tyrrhenian domain and represents a solid starting base for further detailed modelling across the area.

Study of Deep Structure of the Kimberlite Pipe Named After M. Lomonosov of the Arkhangelsk Diamondiferous Province Obtained by Joint Using of Passive Seismic and Radiometric Methods

Study of Deep Structure of the Kimberlite Pipe Named After M. Lomonosov of the Arkhangelsk Diamondiferous Province Obtained by Joint Using of Passive Seismic and Radiometric Methods

Preliminary Result : Classification of Soil Using Fundamental Frequency and Vs30m

Geologically, Ngamprah – Padalarang is located above Lembang Fault which stretched from east to west of Bandung, with a length of 29 Km. therefore, to understand disaster risk around the area, a seismic microzonation study is needed. Microtremor measurement has been done in 21 locations around Ngamprah, West Bandung. Microtremor measurement is a passive seismic method that utilizes natural vibration so it can provide a fundamental frequency of microtremor data and can be applied to understand the lithology of the local site area. Furthermore, the fundamental frequency can be used to obtain Vs30m and correlate it to Eurocode 8 for classification of soil type. The result has shown, the fundamental frequency at Ngamprah around 3 – 4 Hz, meanwhile, we found an anomaly of high frequency around 9 – 11 Hz at Cimeta River. Low frequencies correlated to deep sedimentation meanwhile high frequencies may correlated to Lembang Fault Segment Cimeta which cut through Cimeta River and also indicated as slow sedimentation. In addition to that, VS30m obtained from fundamental frequency show around Cimeta river has high velocity (550 m/s) meanwhile at Ngamprah area is low velocity < 400 m/s . High velocity is indicated as very dense sand or very stiff clay meanwhile low velocity indicates deep deposits of gravel or stiff clay with thickness several tens to hundreds of meters.

Passive seismic data processing methods to identify contrast intrasalt interlayers in the geological section of the Astrakhan Arch

The article examines the problem of processing microseismic noise (MN) to identify and evaluate occurrence depth of contrasting geological objects – intersalt interlayers with a potentially high formation pressure. If it is impossible to use artificial wave sources, statistic processing of passive seismic data becomes critical. Due to the accumulation of power spectral density (PSD) during a long-term recording of MN the deterministic medium effect on a random signal spectrum is identified. PSD modulation when the surface or the bottom of the layered medium is exposed to white noise is expressed in terms of the Green’s function (GF) of a wave equation. Relevant GF variations corresponding to the layers form the basis for accumulated PSD approximation, and indicate the depth and contrast of the target features.

Integrated Seismic Hazard Analysis Of Southwest Penang Island Through Horizontal-To-Vertical Spectral Ratio And Probabilistic Seismic Hazard Assessment

Peninsular Malaysia is susceptible to large magnitude earthquakes induced by the regional sources as it is surrounded by countries that are known for their active seismicity. Tremors were felt in Penang Island of Pulau Pinang due to earthquake events in Sumatra, Indonesia in 2005 and 2009. Presence of cracks on buildings in the island was reported caused by the earthquake on 2nd November 2002. The tsunami that hit the island on 26 December 2004 was the aftermath of the Great Sumatra-Andaman earthquake with magnitude 9.1. The investigation of earthquake risks ensures that the effect of earthquake disasters in the inclined region can be reduced effectively. This paper aims to provide a comprehensive seismic hazard assessment in Penang Island by analysing the predominant natural frequency distribution in Balik Pulau through a passive seismic survey method known as horizontal-to-vertical spectral ratio (HVSR) and evaluating the ground motion throughout the island using probabilistic seismic hazard assessment (PSHA) approach. The natural frequencies of Balik Pulau mostly falls in the range of 3 to 4 Hz which is associated with loose deposits and stiff soil layer. The amplification factor extracted from the HVSR curves ranges approximately 4 to 5. The minimum ground motions estimated for a fixed intensity in 50 years for Penang Island is 0.006 g1 and can reach up to 0.025 g. While the minimum ground motions for a fixed return period of 98 years in 50 years is 0.016 g with maximum of 0.035 g.