Changes In Seismic Wave Velocity Research Articles

Estimation of seismometer clock time offsets using Kalman Filter towards accurate seismic velocity change

SUMMARY Monitoring seismic velocity changes obtained from ambient noise correlations is widely used to understand changes in rock properties in response to earthquakes, volcanic activities and environmental changes. Since continuous seismic data have been accumulated, this method can estimate long-term changes in seismic velocity, such as crustal recovery after a major earthquake and temporal variations in seismic velocity related to long-term environmental change. Changes in seismic velocity can be estimated with a high temporal resolution by measuring the phase differences of ambient noise correlations based on a seismic interferometry method. Still, these phase differences are influenced not only by seismic wave velocity changes but also by errors in clock timing in seismometers. The clock drift occurs due to out-of-synchronization with the GPS clock and the drift of the internal clock. Therefore, to accurately monitor temporal changes in crustal structure by measuring the phase differences of noise correlations, it is crucial to evaluate the contribution of errors in clock timing to the phase differences. Recently, a method using an extended Kalman filter based on a state-space model was developed for reliable detection of temporal changes in the waveforms of ambient noise correlations, with the state-space model offering the advantage of flexible modelling of time-series data. In this study, we incorporated the time-shifts caused by clock time errors of the seismometer into the state-space model of the temporal changes in ambient noise correlations. We estimated seismic velocity changes, amplitude changes of noise correlations and clock time errors from 2010 April to 2021 September at seismic stations around the Shinmoe-dake volcano in Japan, which experienced eruptions in 2011 and 2018, respectively. Several stations exhibited clear clock time offsets, and the occurrence of clock time-shifts coincided with the dates when the data logger was turned off for seismic station maintenance or replacement of the seismometer. The proposed method provides stable estimations with respect to the signal-to-noise ratio of the waveform, and this stable estimation facilitates accurate timing of seismic recordings, enabling precise analysis of seismic phase arrival times.

Untangling the environmental and tectonic drivers of the Noto earthquake swarm in Japan.

The underlying mechanism of the ongoing seismic swarm in the Noto Peninsula, Japan, which generates earthquakes at 10 times the average regional rate, remains elusive. We capture the evolution of the subsurface stress state by monitoring changes in seismic wave velocities over an 11-year period. A sustained long-term increase in seismic velocity that is seasonally modulated drops before the earthquake swarm. We use a three-dimensional hydromechanical model to quantify environmentally driven variations in excess pore pressure, revealing its crucial role in governing the seasonal modulation with a stress sensitivity of 6 × 10-9 per pascal. The decrease in seismic velocity aligns with vertical surface uplift, suggesting potential fluid migration from a high-pore pressure zone at depth. Stress changes induced by abnormally intense snow falls contribute to initiating the swarm through subsequent perturbations to crustal pore pressure.

Structure and elasticity of CaC2O5 suggests carbonate contribution to the seismic anomalies of Earth’s mantle

Knowledge of carbonate compounds under high pressure inside Earth is key to understanding the internal structure of the Earth, the deep carbon cycle and major geological events. Here we use first-principles simulations to calculate the structure and elasticity of CaC2O5-minerals with different symmetries under high pressure. Our calculations show that CaC2O5-minerals represent a group of low-density low-seismic-wave velocity mantle minerals. Changes in seismic wave velocity caused by the phase transformation of CaC2O5-Cc to CaC2O5-I4¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\bar{4}$$\\end{document}2d (CaC2O5-C2-l) agree with wave velocity discontinuity at a depth of 660 km in the mantle transition zone. Moreover, when CaC2O5-Fdd2 transforms into CaC2O5-C2 under 70 GPa, its shear wave velocity decreases by 7.4%, and its density increases by 5.8%, which is consistent with the characteristics of large low-shear-velocity provinces (LLSVPs). Furthermore, the shear wave velocity of CaC2O5-I4¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\bar{4}$$\\end{document}2d is very similar to that of cubic Ca-perovskite, which is one of the main constituents of the previously detected LLSVPs. Therefore, we propose that CaC2O5 and its high-pressure polymorphs may be a main component of LLSVPs.

Anisotropic seismic velocity variations in response to different orientations of tidal deformations

SUMMARY Microcracks or micropores in rocks cause the elastic moduli to change with the applied strain owing to the nonlinear elasticity of the geomaterial, which causes temporal changes in the seismic wave velocity. Thus, variations in seismic wave velocity can be used as a proxy for understanding the strain or stress variations in the crust, which are crucial for figuring out the dynamics of the fault zones and volcanic domains. According to the theory of nonlinear elasticity, the second- and third-order elastic constants and strain tensors contribute to the strain derivative of seismic wave velocity changes. Although laboratory experiments have estimated third-order elastic constants for rock samples, the in situ values of those constants for the crust are difficult to obtain. In this study, seismic velocity changes were investigated in different directions of tidal deformations to provide constraints on the third-order elastic constants in the shallow crust by applying a seismic interferometry method to ambient noise records. We observed that negative velocity changes were of larger magnitude in the station-pair direction parallel to the tidal strain’s direction. Nonlinear elasticity in shallow crust may cause anisotropic velocity variations in response to tidal deformations. Our results highlight the use of velocity change measurements in different directions of tidal strain to constrain nonlinear elastic parameters on a field scale.

Monitoring the Ambient Seismic Field to Track Groundwater at a Mountain–Front Recharge Zone

The heterogeneity of the fractured-basalt and interbedded-sediment aquifer along the eastern margin of the Columbia Plateau Regional Aquifer System has presented challenges to resource managers in quantifying recharge and estimating sustainable withdrawals. Previous studies indicated recharge pathways in alluvial sediments atop a mountain–front interface upgradient of the basalt flows. In this sedimentary zone, six seismic stations were deployed for one year to detect velocity changes in low-frequency seismic waves that could be correlated to changes in groundwater recorded by a well transducer near the center of the seismic station network. Waveforms in the 1−5 Hz range were recorded at each station to determine changes in wave velocities between station pairs and correlate these velocity changes to changes in groundwater levels. The velocity–groundwater relation allowed for estimation of daily groundwater levels beneath the seismic station network. Existing hydrogeologic information was used to estimate hydraulic gradients and hydraulic conductivities, which allowed for the calculation of the daily volume of recharge passing beneath the seismic stations and into the confined aquifer system. The daily recharge volumes across the seismic station network were summed for comparison of the total annual recharge calculated from the change in seismic wave velocities (154,660 m3) to a flow model calculation of recharge based on areal precipitation and infiltration (26,250 m3). The 6× greater recharge estimated from the seismic wave velocity changes for this portion of the recharge zone is attributed to preferential pathways of high hydraulic conductivity and greater depth associated with paleochannels beneath the seismic station network.

Seismic Scattering Property Changes Correlate With Ground Deformation at Suwanosejima Volcano, Japan

AbstractContinuous estimation of changes in seismic velocity and seismic scattering property by passive interferometry using seismic ambient noise is a promising tool for monitoring volcanoes. To improve the usefulness of this method, it is necessary not only to detect subsurface structural changes but also to quantitatively compare the estimated changes in seismic wave velocity and seismic wave scattering property with other observations, such as ground deformation. We applied passive interferometry to continuous seismic records from the Suwanosejima volcano, Japan, recorded between April 2017 and December 2021. We detected repeated significant waveform decorrelations in seismic ambient noise cross‐correlation functions (CCFs), indicating changes in seismic scattering property in the shallow areas of the volcano. These decorrelations were observed from 2 weeks to a few days before the increase in the number of explosions, suggesting that seismic scattering properties changed significantly during that period. We found that the timing of the decorrelation in seismic ambient noise CCFs and tilt changes related to magma accumulation and injection beneath Suwanosejima were well synchronized. The high correlation between the amounts of decorrelation and the tilt change during the magma accumulation period possibly suggests that a large volume of accumulated magma caused great changes in the scattering property. These results provide a significant first step toward a quantitative comparison of the amount of changes in the scattering property with the amount of magma accumulation beneath volcanoes.

Earthquake‐Derived Seismic Velocity Changes During the 2018 Caldera Collapse of Kīlauea Volcano

AbstractThe 2018 Kīlauea caldera collapse produced extraordinary sequences of seismicity and deformation, with 62 episodic collapse events which significantly altered the landscape of the summit region. Despite decades of focused scientific studies at Kīlauea, detailed information about the internal structure of the volcano is limited. Recently developed techniques in seismic interferometry can be used to monitor the internal structure of an active volcano more directly by detecting subtle spatiotemporal changes in seismic wave velocity, but their utility relies on accurate interpretations of the underlying phenomena causing those velocity changes. Here, we retrospectively apply repeating‐earthquake‐based seismic interferometry to the 2018 Kīlauea eruption sequence. We find that seismic velocities changed over two distinct time scales: a sudden increase followed by a slower decrease in velocity in the hours following each collapse event, and a gradual, long‐term decrease in velocity over several weeks that ceased approximately 1 month prior to the end of the eruption. Modeling suggests that short‐term changes can be explained by magma reservoir pressurization which specifically closed vertical ring fractures. Long‐term changes are related to subsidence of the caldera and likely include the influence of inelastic strain from the formation of new fractures. These observations provide new insights into the evolution of Kīlauea during its progressive collapse and will inform future interpretations for near‐real‐time monitoring at hazardous volcanoes around the world using similar techniques, especially where a dominant fracture orientation is present.

Seismic constraints on rock damaging related to a failing mountain peak: the Hochvogel, Allgäu

AbstractLarge rock slope failures play a pivotal role in long‐term landscape evolution and are a major concern in land use planning and hazard aspects. While the failure phase and the time immediately prior to failure are increasingly well studied, the nature of the preparation phase remains enigmatic. This knowledge gap is due, to a large degree, to difficulties associated with instrumenting high mountain terrain and the local nature of classic monitoring methods, which does not allow integral observation of large rock volumes. Here, we analyse data from a small network of up to seven seismic sensors installed during July–October 2018 (with 43 days of data loss) at the summit of the Hochvogel, a 2592 m high Alpine peak. We develop proxy time series indicative of cyclic and progressive changes of the summit. Modal analysis, horizontal‐to‐vertical spectral ratio data and end‐member modelling analysis reveal diurnal cycles of increasing and decreasing coupling stiffness of a 260,000 m3 large, instable rock volume, due to thermal forcing. Relative seismic wave velocity changes also indicate diurnal accumulation and release of stress within the rock mass. At longer time scales, there is a systematic superimposed pattern of stress increased over multiple days and episodic stress release within a few days, expressed in an increased emission of short seismic pulses indicative of rock cracking. Our data provide essential first order information on the development of large‐scale slope instabilities towards catastrophic failure. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Monitoring of velocity changes based on seismic ambient noise: A brief review and perspective

Over the past two decades, the development of the ambient noise cross-correlation technology has spawned the exploration of underground structures. In addition, ambient noise-based monitoring has emerged because of the feasibility of reconstructing the continuous Green’s functions. Investigating the physical properties of a subsurface medium by tracking changes in seismic wave velocity that do not depend on the occurrence of earthquakes or the continuity of artificial sources dramatically increases the possibility of researching the evolution of crustal deformation. In this article, we outline some state-of-the-art techniques for noise-based monitoring, including moving-window cross-spectral analysis, the stretching method, dynamic time wrapping, wavelet cross-spectrum analysis, and a combination of these measurement methods, with either a Bayesian least-squares inversion or the Bayesian Markov chain Monte Carlo method. We briefly state the principles underlying the different methods and their pros and cons. By elaborating on some typical noise-based monitoring applications, we show how this technique can be widely applied in different scenarios and adapted to multiples scales. We list classical applications, such as following earthquake-related co- and postseismic velocity changes, forecasting volcanic eruptions, and tracking external environmental forcing-generated transient changes. By monitoring cases having different targets at different scales, we point out the applicability of this technology for disaster prediction and early warning of small-scale reservoirs, landslides, and so forth. Finally, we conclude with some possible developments of noise-based monitoring at present and summarize some prospective research directions. To improve the temporal and spatial resolution of passive-source noise monitoring, we propose integrating different methods and seismic sources. Further interdisciplinary collaboration is indispensable for comprehensively interpreting the observed changes.

Estimating changes in seismic wave velocity from a pneumatic source in an operational mine

Rock mass characteristics such as applied stress, pore pressure, and fracture density are coupled to the seismic wave propagation velocity. Therefore, measuring small relative changes of seismic wave velocities in operational mines has the potential for tracking changes in these important rock mass properties. To this end, we have conducted an experiment with the aim of measuring seismic body wave velocities in situ at Williams mine, Hemlo, Canada, using a pneumatic source and multiple receivers. We determine that the signal-to-noise ratio is improved by deconvolving the source signal from the signal recorded at the remotely grouted geophones. We evaluate the estimated relative changes in traveltimes and assume that these are due to changes in the body wave velocity caused by mining-related activities. We are able to successfully detect the source response at remote receivers in an operational mine at a distance of more than 430 m.

Coseismic changes in subsurface structure associated with the 2018 Hokkaido Eastern Iburi Earthquake detected using autocorrelation analysis of ambient seismic noise

Autocorrelation analysis using ambient noise is a useful method to detect temporal changes in wave velocity and scattering property. In this study, we investigated the temporal changes in seismic wave velocity and scattering property in the focal region of the 2018 Hokkaido Eastern Iburi Earthquake. The autocorrelation function (ACF) was calculated by processing with bandpass filters to enhance 1–2 Hz frequency range, with aftershock removal, and applying the one-bit correlation technique. The stretching method was used to detect the seismic wave velocity change. After the mainshock, seismic velocity reductions were observed at many stations. At N.AMAH and ATSUMA, which are located close to the mainshock, we detected 2–3% decreases in seismic wave velocity. We compared parameters indicating strong ground motion and showed the possibility of correlations with peak dynamic strain and seismic velocity reduction. We also investigated the relationship between waveform correlation and lag time, using ACFs from before and after the mainshock, and detected distortion of the ACF waveform. The source of the waveform decorrelation was estimated to be located near the maximum coseismic slip, at around 30 km depth. Thus, distortion of the ACF waveform may reflect the formation of cracks, due to faulting at approximately 30 km depth.

High Temporal Resolution Monitoring of Small Variations in Crustal Strain by Dense Seismic Arrays

AbstractWe demonstrate the feasibility of detecting very weak deformation in the shallow crust with high temporal resolution by monitoring the relative changes in seismic wave velocity (dv/v) using dense arrays of seismometers. We show that the dv/v variations are consistent between independent measurements from two seismic arrays. Dominant peaks in the observed dv/v spectrum suggest that tides and temperature changes are the major causes of daily and subdaily velocity changes, in accordance with theoretical strain modeling. Our analysis illustrates that dv/v perturbations of the order of 10−4, corresponding to crustal strain changes of the order of 10−8, can be measured from ambient seismic noise with a temporal resolution of 1 hr. This represents a low‐cost technique for high precision and high time‐resolution monitoring of crustal deformation that is complementary to existing geodetic measurements and is instrumental in both the detection and understanding of low‐amplitude precursory processes of natural catastrophic events.

Effects of Fe and Al incorporations on the bridgmanite–postperovskite coexistence domain

The postperovskite phase transition of Fe and Al-bearing MgSiO3 bridgmanite, the most aboundant mineral in the Earth's lower mantle, is believed to be a key to understanding seismological observations in the D″ layer, e.g., the discontinuous changes in seismic wave velocities. Experimentally reported phase transition boundaries of Fe and Al-bearing bridgmanite are currently largely controversial and generally suggest wide two-phase coexistence domains. Theoretical simulations ignoring temperature effects cannot evaluate correctly two-phase coexistence domains under high-temperature. We show high-pressure and high-temperature phase transition boundaries for various compositions with geophysically relevant impurities of Fe2+SiO3, Fe3+Fe3+O3, Fe3+Al3+O3, and Al3+Al3+O3 derived from the ab initio finite-temperature free energies calculated combining the internally consistent LSDA+U method and a lattice dynamics approach. We found that at ∼2500K, incorporations accompanied by Fe3+ expand the two-phase coexistence domains distinctly, implying that D″ seismic discontinuities likely arise from the phase transition of Fe2+-bearing bridgmanite.

Monitoring seismic wave velocity changes associated with the Mw 7.9 Wenchuan earthquake: increasing the temporal resolution using curvelet filters

The aim of this study is to improve the temporal resolution of seismic wave velocity variations measured using ambient noise correlations. We first reproduce the result obtained by Chen et al. using a network of 21 broad-band stations ideally located around the fault system activated during the Wenchuan earthquake.We measure a velocity drop of 0.07 per cent that was associated with the main shock, with a temporal resolution of 30 days. To determine whether this velocity drop is co-seismic or post-seismic, we attempt to increase the temporal resolution of our observations. By taking advantage of the properties of the curvelet transform, we increase the signal-to-noise ratio of the daily correlations computed between each station pair. It is then possible to measure the velocity drop associated with the Wenchuan earthquake with a temporal resolution of 1 day. This shows that the velocity drop started on 2008 May 12, which was the day of the earthquake, and the velocity reached its lowest value 2 days after the main shock. Moreover, there was a second velocity drop on 2008 May 27, which might relate to strong aftershocks.

Effect of Chemistry on the Physical Properties of Perovskite and Post‐Perovskite

We use density functional theory to study the effect of Fe and Al on properties of MgSiO 3 perovskite and post-perovskite. The addition of Fe increases the compressibility and density of MgSiO 3 and considerably decreases the transition pressure between the two phases. MgSiO 3 perovskite transforms to post-perovskite at about 112 GPa. FeSiO 3 is stable as post-perovskite at all pressures relative to perovskite. We find ferrous iron to be in a high spin state over the whole mantle pressure range, and it partitions preferentially into the post-perovskite structure. Ferrous iron in MgSiO 3 decreases the seismic wave velocities and slightly decreases the seismic anisotropy. At 120 GPa, FeSiO 3 post-perovskite has Vp= 12.4 km/s and Vs = 6.3 km/s and MgSiO 3 post-perovskite has Vp = 14.2 km/s and Vs = 7.9 km/s. The seismic anisotropy of post-perovskite MgSiO 3 is 15% for Vp and 26% for Vs. Aluminum slightly decreases the density and increases the transition pressure. Pure alumina transforms from perovskite to post-perovskite at 120 GPa. Al 2 O 3 also increases the compressibility of perovskite and decreases that of post-perovskite. Al decreases the seismic wave velocities and considerably increases the seismic anisotropy of post-perovskite. At 120 GPa, post-perovskite Al 2 O 3 has V p = 13.8 km/s and Vs = 7.4 km/s and seismic anisotropy of 18% for Vp and 43% for Vs. For proposed mantle compositions such as pyrolite the changes in seismic wave velocities due to the transition from perovskite to post-perovskite, that is positive jumps in both Vp and Vs, can explain those observed at the top of the D layer.

Passive seismic tomography for three-dimensional time-lapse imaging of mining-induced rock mass changes

Progression of an immature block cave in proximity to a mature sublevel cave was monitored over an 18-month period using double-difference tomography. This method utilizes existing microseismic data to generate time-lapse images of seismic wave velocity changes within the rock mass. The velocity changes are caused by damage zones forming or local high-stress concentrations. In this study, images were generated on monthly intervals. An average of 2000 microseismic events recorded by 25 stations were used to generate each image. Results showed a development of the initial undercut followed by the growing cave and interaction with the adjacent sublevel cave. The goal of the research is to provide a tool that can augment both numerical modeling results and underground geotechnical measurements to allow the mine operator to produce in the safest and most efficient manner possible.

Temporal changes in seismic wave propagation towards the end of the 2002 Mt Etna eruption

SUMMARY Stress can undergo rapid temporal changes in volcanic environments, and this is particularly true during eruptions. We use two independent methods, coda wave interferometry (CWI) and shear wave splitting (SWS) analysis to track stress related wave propagation effects during the waning phase of the 2002 NE fissure eruption at Mt Etna. CWI is used to estimate temporal changes in seismic wave velocity, while SWS is employed to monitor changes in elastic anisotropy. We analyse seismic doublets, detecting temporal changes both in wave velocities and anisotropy, consistent with observed eruptive activity. In particular, syn-eruptive wave propagation changes indicate a depressurization of the system, heralding the termination of the eruption, which occurs three days later.

Coda wave interferometric detection of seismic velocity changes associated with the 1999 M = 3.6 event at Mt. Vesuvius

We detect seismic wave velocity changes at Mt. Vesuvius, using doublets and the Coda Wave Interferometry method. The high sensitivity of multiply scattered coda waves to temporal changes in the medium allows us to detect velocity variation smaller than 0.4%. We use 17 doublets, some of them grouped in families of multiplets, spanning January 1996 to December 1999. Data show a systematic increase in velocity from 1996 to end‐September 1999, followed by a rapid drop in velocity. This drop immediately precedes a sustained swarm of VT‐type earthquakes, including the 9th October 1999 M = 3.6 event, the largest in the region since at least 1972. We propose a long term fluid pressurization followed by influx as a possible causative mechanism.

Seismic wave velocity changes and stress build‐up in the crust of the Kanto‐Tokai Region

We show there are large temporal P‐wave velocity changes in the crust of the Kanto‐Tokai region in Japan from repeated observations of explosions. We find that the variation is the response of the crustal rocks to two components of stress change, the tidal stress and the secular stress accumulation. The inferred annual rate of fractional velocity change (Δν/ν/year) caused by the stress accumulation in the upper crust is of the order of 10−3/year. The results indicate that the build‐up of stress in the crust can be monitored by precise observations of seismic wave velocities.

地震波速度の時間的変化の新検出法について

地震波速度の時間的変化の新検出法について