Time-lapse tomographic images of the Irpinia Fault System (Southern Italy) reveal Vp/Vs ratio changes that correlate with micro-seismicity production and evolution

2020 ◽  
Author(s):  
Grazia De Landro ◽  
Raffaella Esposito ◽  
Amoroso Ortensia ◽  
Aldo Zollo
Keyword(s):  
Pore Pressure ◽  
Seismic Velocity ◽  
Physical Modelling ◽  
Seismic Attenuation ◽  
Fault System ◽  
Earthquake Activity ◽  
Earthquake Triggering ◽  
Seismicity Rate ◽  
Velocity Models ◽  
Velocity Changes

<p>This study focuses on the the active fault system that caused the 1980 M_S 6.9 Irpinia earthquake (Irpinia fault zone (IFZ)) that is presently interested by a continuous and frequent micro-earthquake activity occurring within the volume in the volume enclosed by two antithetic faults. It is therefore important to improve the knowledge of the IFZ dynamics, with reference to potential future occurrence of moderate to large earthquakes, especially in terms of earthquake triggering mechanisms. Several previous works evaluated the spatial distribution of elastic/anelastic fault-embedded medium properties and related rock physical micro-parameters in connection with the seismicity rate. These studies showed a spatial correlation between high Vp/Vs, low seismic attenuation in rock volumes where most of seismicity occurs, suggesting that fluid-driven pore-pressure changes may plays a key role in controlling the seismicity production at the IFZ.<br>Here we reconstruct accurate 4D seismic velocity images of the volume embedding IFZ which allows to detect and track space-time changes of medium elastic properties possibly induced by fluid pore pressure migration and investigate the related seismicity production.<br>We analyzed the arrival time phase catalogue of about ten years (2005-2016) of Mw < 3.1 events recorded by the ISNet (Irpinia Seismic Network) and INGV network. We divided the catalog in 5 not-overlapping epochs by selecting in each of them , approximately the same number of events and an uniform volume coverage, in order to ensure that the 3D P and S velocity models could be equally well resolved for each epoch. By comparing the Vp, Vs and Vp/Vs images at each epoch in the equally resolved volume, we are able to detect medium velocity changes. Some regions, in the first 6 km of depth of NE part, do not show velocity changes with time, which is interpreted as the main effect of unperturbed lithology mainly controlling the average seismic velocity. In other regions, in the central part of the model at about 8-10 km depth, we clearly detect velocity changes causing an up to 10% Vp/Vs variation between different epochs. Based on the rock physical modelling, we associate the time-varying Vp/Vs and the observed amplitude of variation to fluid-driven changes in rock physical properties related to their spatial migration or pore-pressure induced changes. The regions where large Vp/Vs changes occur appear correlated with the largest seismicity production volumes, suggesting a direct link between the physical processes associated with fluid mobility and/or pore pressure migration and earthquake generation at the IFZ.</p>

scholarly journals Physics-Based Relationship for Pore Pressure and Vertical Stress Monitoring Using Seismic Velocity Variations

2021 ◽  
Vol 13 (14) ◽  
pp. 2684
Author(s):  
Eldert Fokker ◽  
Elmer Ruigrok ◽  
Rhys Hawkins ◽  
Jeannot Trampert
Keyword(s):  
Pore Pressure ◽  
Seismic Velocity ◽  
Pressure Head ◽  
Groundwater Table ◽  
Surface Velocity ◽  
Seismic Velocities ◽  
Stress Monitoring ◽  
Near Surface ◽  
Velocity Variations ◽  
Velocity Changes

Previous studies examining the relationship between the groundwater table and seismic velocities have been guided by empirical relationships only. Here, we develop a physics-based model relating fluctuations in groundwater table and pore pressure with seismic velocity variations through changes in effective stress. This model justifies the use of seismic velocity variations for monitoring of the pore pressure. Using a subset of the Groningen seismic network, near-surface velocity changes are estimated over a four-year period, using passive image interferometry. The same velocity changes are predicted by applying the newly derived theory to pressure-head recordings. It is demonstrated that the theory provides a close match of the observed seismic velocity changes.

Spatial Evaluation of Pore Pressure Variations Related to Rainfall from Seismic Velocity Changes

2021 ◽  
Author(s):  
Rezkia Dewi Andajani ◽  
Takeshi Tsuji ◽  
Roel Snieder ◽  
Tatsunori Ikeda
Keyword(s):  
Pore Pressure ◽  
Negative Correlation ◽  
Ambient Noise ◽  
Seismic Velocity ◽  
Diffusion Mechanism ◽  
Velocity Change ◽  
Diffusion Parameters ◽  
The Status ◽  
Seismic Velocity Changes ◽  
Velocity Changes

<p>Crustal pore pressure, which could trigger seismicity and volcanic activity, varies with fluid invasion. Various studies have discussed the potential of using seismic velocity changes from ambient noise to evaluate pore pressure conditions, especially due to rainfall perturbations. Although the influence of rainfall on seismic velocity changes has been reported, consideration of the spatial influence on rainfall towards seismic velocity and its mechanism have not been well understood. We investigated the mechanism of rainfall-induced pore pressure diffusion in southwestern Japan, using seismic velocity change (Vs) inferred from ambient noise. We modeled pore pressure changes from rainfall data based on a diffusion mechanism at the locations where infiltration is indicated. By calculating the correlation between Vs changes and the modeled pore pressure with various hydraulic diffusion parameters, the optimum hydraulic diffusion parameter was obtained. We estimated the diffusion parameters with the highest negative correlation between pore pressure and Vs change because a negative correlation indicates pore pressure increase due to diffusion induced by groundwater load. Furthermore, the spatial variation of the hydraulic diffusivity infers the heterogeneity of the rocks in different locations. This finding suggests that the response of pore pressure induced by rainfall percolation depends on location.  We show that seismic velocity monitoring can be used to evaluate the status of pore pressure at different locations, which is useful for fluid injection, CO<sub>2</sub> wellbore storage, and geothermal development.</p>

High-resolution seismic velocity analysis by sign-based weighted semblance

Geophysics ◽  
2021 ◽  
pp. 1-35
Author(s):  
M. Javad Khoshnavaz
Keyword(s):  
Seismic Velocity ◽  
Resolution Enhancement ◽  
Synthetic Data ◽  
Correlation Coefficients ◽  
Velocity Model ◽  
Seismic Attenuation ◽  
Velocity Analysis ◽  
Velocity Models ◽  
Amplitude Versus Offset ◽  
Amplitude Versus

Building an accurate velocity model plays a vital role in routine seismic imaging workflows. Normal-moveout-based seismic velocity analysis is a popular method to make the velocity models. However, traditional velocity analysis methodologies are not generally capable of handling amplitude variations across moveout curves, specifically polarity reversals caused by amplitude-versus-offset anomalies. I present a normal-moveout-based velocity analysis approach that circumvents this shortcoming by modifying the conventional semblance function to include polarity and amplitude correction terms computed using correlation coefficients of seismic traces in the velocity analysis scanning window with a reference trace. Thus, the proposed workflow is suitable for any class of amplitude-versus-offset effects. The approach is demonstrated to four synthetic data examples of different conditions and a field data consisting a common-midpoint gather. Lateral resolution enhancement using the proposed workflow is evaluated by comparison between the results from the workflow and the results obtained by the application of conventional semblance and three semblance-based velocity analysis algorithms developed to circumvent the challenges associated with amplitude variations across moveout curves, caused by seismic attenuation and class II amplitude-versus-offset anomalies. According to the obtained results, the proposed workflow is superior to all the presented workflows in handling such anomalies.

Passive seismic velocity monitoring of natural faults: The FaultScan project

2020 ◽  
Author(s):  
Florent Brenguier ◽  
Aurelien Mordret ◽  
Yehuda Ben-Zion ◽  
Frank Vernon ◽  
Pierre Boué ◽  
...  
Keyword(s):  
Seismic Velocity ◽  
Fault Zones ◽  
Body Waves ◽  
Fault System ◽  
Seismic Arrays ◽  
San Jacinto Fault ◽  
Highway Network ◽  
Seismic Velocity Changes ◽  
Passive Seismic ◽  
Velocity Changes

<p>Laboratory experiments report that detectable seismic velocity changes should occur in the vicinity of fault zones prior to earthquakes. However, operating permanent active seismic sources to monitor natural faults at seismogenic depth has been nearly impossible to achieve. The FaultScan project (Univ. Grenoble Alpes, Univ. Cal. San Diego, Univ. South. Cal.) aims at leveraging permanent cultural sources of ambient seismic noise to continuously probe fault zones at a few kilometers depth with seismic interferometry. Results of an exploratory seismic experiment in Southern California demonstrate that correlations of train-generated seismic signals allow daily reconstruction of direct P body-waves probing the San Jacinto Fault down to 4 km depth. In order to study long-term earthquake preparation processes we will monitor the San Jacinto Fault using such approach for at least two years by deploying dense seismic arrays in the San Jacinto Fault region. The outcome of this project may facilitate monitoring the entire San Andreas Fault system using the railway and highway network of California. We acknowledge support from the European Research Council under grant No.~817803, FAULTSCAN.</p>

scholarly journals Spatial and temporal influence of rainfall on crustal pore pressure based on seismic velocity monitoring 

2020 ◽  
Author(s):  
Rezkia Dewi Andajani ◽  
Takeshi Tsuji ◽  
Roel Snieder ◽  
Tatsunori Ikeda
Keyword(s):  
Pore Pressure ◽  
Negative Correlation ◽  
Seismic Velocity ◽  
Rainfall Variability ◽  
Diffusion Mechanism ◽  
Band Pass Filter ◽  
Earthquake Triggering ◽  
Mountain Areas ◽  
Strong Negative Correlation ◽  
Pass Filter

Abstract Crustal pore pressure, which controls the activities of earthquakes and volcanoes, varies in response to rainfall. The status of pore pressure can be inferred from observed changes in seismic velocity. In this study, we investigate the response of crustal pore pressure to rainfall in southwestern Japan based on time series of seismic velocity derived from ambient noise seismic interferometry. To consider the heterogeneity of the area, rainfall and seismic velocity obtained at each location were directly compared. We used a band-pass filter to distinguish the rainfall variability from sea level and atmospheric pressure, and then calculated the cross-correlation between rainfall and variations in S-wave velocity (Vs). A mostly negative correlation between rainfall and Vs changes indicates groundwater recharge by rainfall, which increases pore pressure. The correlations differ between locations, where most of the observation stations with clear negative cross-correlations were located in areas of granite. On the other hand, we could not observe clear correlations in steep mountain areas, possibly because water flows through river without percolation. This finding suggests that geographical features contribute to the imprint of rainfall on deep formation pore pressure. We further modelled pore pressure change due to rainfall based on diffusion mechanism. A strong negative correlation between pore pressure estimated from rainfall and Vs indicates that the Vs variations are triggered by pore pressure diffusion in the deep formation. Our modelling results show a spatial variation of diffusion parameter which controls the pore pressure in deep formation. By linking the variations in seismic velocity and crustal pore pressure spatially, this study shows that seismic monitoring may be useful in evaluating earthquake triggering processes or volcanic activity.

Revealing seasonal crustal seismic velocity variations in Taiwan with single-station cross-component analysis 

2021 ◽  
Author(s):  
Kuan-Fu Feng ◽  
Hsin-Hua Huang ◽  
Ya-Ju Hsu ◽  
Yih-Min Wu
Keyword(s):  
Pore Pressure ◽  
Seasonal Variations ◽  
Seismic Velocity ◽  
Pressure Change ◽  
Temporal Variations ◽  
Air Pressure ◽  
Weather Data ◽  
Single Station ◽  
Seismic Velocity Changes ◽  
Velocity Changes

<p>Ambient noise interferometry is a promising technique for studying crustal behaviors, providing continuous measurements of seismic velocity changes (dv/v) in relation to physical processes in the crust over time. In addition to the tectonic-driven dv/v changes, dv/v is also known to be affected by environmental factors through rainfall-induced pore-pressure changes, air pressure loading changes, thermoelastic effects, and so forth. In this study, benefiting from the long-term continuous data of Broadband Array in Taiwan for Seismology (BATS) that has been operated since 1994, we analyze continuous seismic data from 1998 to 2019 by applying single-station cross-component (SC) technique to investigate the temporal variations of crust on seismic velocity. We process the continuous waveforms of BATS stations, construct the empirical Green’s functions, and compute daily seismic velocity changes by the stretching technique in a frequency band of 0.1 to 0.9 Hz. We observe co-seismic velocity drops associated with the inland moderate earthquakes. Furthermore, clear seasonal cycles, with a period of near one-year, are also revealed at most stations, but with different characteristics. Systematic spectral and time-series analyses with the weather data are conducted and show that the rainfall-induced pore-pressure change is likely the main cause to the seasonal variations with high correlations. The strong site-dependency of these seasonal variations also precludes air pressure and temperature which varies smoothly in space from being dominant sources and suggests spatially-varying complex hydro-mechanical interaction across the orogenic belt in Taiwan.</p>

Near-salt stress-induced seismic velocity changes and seismic anisotropy and their impacts on salt imaging: A case study in the Kuqa depression, Tarim Basin, China

2020 ◽  
Vol 8 (3) ◽  
pp. T487-T499
Author(s):  
Yunqiang Sun ◽  
Gang Luo ◽  
Yaxing Li ◽  
Mingwen Wang ◽  
Xiaofeng Jia ◽  
...  
Keyword(s):  
Salt Stress ◽  
Seismic Anisotropy ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Seismic Velocities ◽  
Kuqa Depression ◽  
Velocity Models ◽  
Salt Structure ◽  
Seismic Velocity Changes ◽  
Velocity Changes

It has been recognized that stress perturbations in sediments induced by salt bodies can cause elastic-wave velocity (seismic velocity) changes and seismic anisotropy through changing their elastic parameters, thus leading to difficulties in salt imaging. To investigate seismic velocity changes and seismic anisotropy by near-salt stress perturbations and their impacts on salt imaging, taking the Kuqa depression as an example, we have applied a 2D plane-strain static geomechanical finite-element model to simulate stress perturbations and calculate the associated seismic velocity changes and seismic anisotropy; then we used the reverse time migration and imaging method to image the salt structure by excluding and including the stress-induced seismic velocity changes. Our model results indicate that near-salt stresses are largely perturbed due to salt stress relaxation, and the stress perturbations lead to significant changes of the seismic velocities and seismic anisotropy near the salt structure: The maximum seismic velocity changes can reach approximately 20% and the maximum seismic anisotropy can reach approximately 10%. The significant changes of seismic velocities due to stress perturbations largely impact salt imaging: The salt imaging is unclear, distorted, or even failed if we exclude near-salt seismic velocity changes from the preliminary velocity structure, but the salt can be better imaged if the preliminary velocity structure is modified by near-salt seismic velocity changes. We find that the locations where salt imaging tends to fail usually occur where large seismic velocity changes happen, and these locations are clearly related to the geometric characteristics of salt bodies. To accurately image the salt, people need to integrate results of geomechanical models and stress-induced seismic velocity changes into the imaging approach. The results provide petroleum geologists with scientific insights into the link between near-salt stress perturbations and their induced seismic velocity changes and help exploration geophysicists build better seismic velocity models in salt basins and image salt accurately.

Detection of temporal change in near-source attenuation during intense fluid-driven seismicity following the 2011 Tohoku-Oki earthquake

2020 ◽  
Vol 224 (1) ◽  
pp. 138-150
Author(s):  
Keisuke Yoshida
Keyword(s):  
Pore Pressure ◽  
Seismic Waves ◽  
Temporal Change ◽  
Seismic Attenuation ◽  
Border Region ◽  
Focal Region ◽  
Earthquake Occurrence ◽  
Fluid Movement ◽  
Seismicity Rate ◽  
Fault Strength

SUMMARY The behaviour of fluids in the crust is key to understanding earthquake occurrence as fluids decrease fault strength. The attenuation of seismic waves may be locally high in fault zones as fluids are intensely distributed in these zones. This study uses a novel, simple approach to examine near-source attenuation in the focal region of intense swarm activity in the Yamagata–Fukushima border region, Japan, which is believed to be triggered by fluid movement following the 2011 Tohoku-Oki earthquake. Near-source attenuation was estimated by determining the decay of amplitude ratios of nearby earthquake pairs with traveltime differences precisely quantified using a waveform correlation. In the initial ∼50 d, ${{\boldsymbol{Q}}^{ - 1}}$ was high, then it significantly decreased to become almost constant for the subsequent period. This pattern is similar to those independently observed for background seismicity rate, b-value, stress drop and fault strength. These patterns can be attributed to the hypothesis that the swarm was triggered by fluid movement following the 2011 Tohoku-Oki earthquake; the source and seismicity characteristics and the seismic attenuation were altogether affected by the temporal change in pore pressure. The method demonstrated in this study may be a useful tool to detect high pore pressure anomaly at depth and understand its relationship with earthquake occurrence.

Spatio-temporal seismic velocity variations associated to the 2016–2017 central Italy seismic sequence from noise cross-correlation

2019 ◽  
Vol 219 (3) ◽  
pp. 2165-2173
Author(s):  
Gaia Soldati ◽  
Lucia Zaccarelli ◽  
Licia Faenza
Keyword(s):  
Seismic Waves ◽  
Seismic Velocity ◽  
Linear Trend ◽  
Central Italy ◽  
Fault System ◽  
Seismic Sequence ◽  
Sedimentary Deposits ◽  
Main Shocks ◽  
Spatio Temporal ◽  
Velocity Changes

SUMMARY We investigate the temporal changes of crustal velocity associated to the seismic sequence of 2016–2017, which struck central Italy with a series of moderate to large earthquakes. We cross-correlate continuous recordings of 2 yr of ambient seismic noise from a network of 28 stations within a radius of 90 km around Amatrice town. We then map the spatio-temporal evolution of the velocity perturbations under the effect of subsequent earthquakes. Coinciding with each of the three main shocks of the sequence we observe a sudden drop of seismic velocity which tends to quickly recover in the short term. After the end of the strongest activity of the sequence, the coseismic velocity changes display gradual healing towards pre-earthquake conditions following a quasi-linear trend, such that by the end of 2017 about 75 per cent of the perturbation is recovered. The spatial distribution of the velocity drop fluctuates with time, and the area that shows the most intense variations beyond the ruptured fault system elongates in the NE direction. This zone roughly corresponds to a region of foredeep sedimentary deposits consisting of highly hydrated and porous sandstones, which respond to the passage of seismic waves with increased pore pressure and crack number, leading to a reduction of the effective relative velocity.

scholarly journals Spatial and temporal influence of rainfall on crustal pore pressure based on seismic velocity monitoring

2020 ◽  
Vol 72 (1) ◽  
Author(s):  
Rezkia Dewi Andajani ◽  
Takeshi Tsuji ◽  
Roel Snieder ◽  
Tatsunori Ikeda
Keyword(s):  
Pore Pressure ◽  
Negative Correlation ◽  
Seismic Velocity ◽  
Rainfall Variability ◽  
Diffusion Mechanism ◽  
Band Pass Filter ◽  
Earthquake Triggering ◽  
Mountain Areas ◽  
Strong Negative Correlation ◽  
Pass Filter

AbstractCrustal pore pressure, which controls the activities of earthquakes and volcanoes, varies in response to rainfall. The status of pore pressure can be inferred from observed changes in seismic velocity. In this study, we investigate the response of crustal pore pressure to rainfall in southwestern Japan based on time series of seismic velocity derived from ambient noise seismic interferometry. To consider the heterogeneity of the area, rainfall and seismic velocity obtained at each location were directly compared. We used a band-pass filter to distinguish the rainfall variability from sea level and atmospheric pressure, and then calculated the cross-correlation between rainfall and variations in S-wave velocity (Vs). A mostly negative correlation between rainfall and Vs changes indicates groundwater recharge by rainfall, which increases pore pressure. The correlations differ between locations, where most of the observation stations with clear negative cross-correlations were located in areas of granite. On the other hand, we could not observe clear correlations in steep mountain areas, possibly because water flows through river without percolation. This finding suggests that geographical features contribute to the imprint of rainfall on deep formation pore pressure. We further modelled pore pressure change due to rainfall based on diffusion mechanism. A strong negative correlation between pore pressure estimated from rainfall and Vs indicates that the Vs variations are triggered by pore pressure diffusion in the deep formation. Our modelling results show a spatial variation of diffusion parameter which controls the pore pressure in deep formation. By linking the variations in seismic velocity and crustal pore pressure spatially, this study shows that seismic monitoring may be useful in evaluating earthquake triggering processes or volcanic activity.

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