Seismic Velocity Variations Research Articles

Seismic refraction tracks porosity generation and possible CO2 production at depth under a headwater catchment

In weathered bedrock aquifers, groundwater is stored in pores and fractures that open as rocks are exhumed and minerals interact with meteoric fluids. Little is known about this storage because geochemical and geophysical observations are limited to pits, boreholes, or outcrops or to inferences based on indirect measurements between these sites. We trained a rock physics model to borehole observations in a well-constrained ridge and valley landscape and then interpreted spatial variations in seismic refraction velocities. We discovered that P-wave velocities track where a porosity-generating reaction initiates in shale in three boreholes across the landscape. Specifically, velocities of 2.7 ± 0.2 km/s correspond with growth of porosity from dissolution of chlorite, the most reactive of the abundant minerals in the shale. In addition, sonic velocities are consistent with the presence of gas bubbles beneath the water table under valley and ridge. We attribute this gas largely to CO2 produced by 1) microbial respiration in soils as meteoric waters recharge into the subsurface and 2) the coupled carbonate dissolution and pyrite oxidation at depth in the rock. Bubbles may nucleate below the water table because waters depressurize as they flow from ridge to valley and because pores have dilated as the deep rock has been exhumed by erosion. Many of these observations are likely to also describe the weathering and flow path patterns in other headwater landscapes. Such combined geophysical and geochemical observations will help constrain models predicting flow, storage, and reaction of groundwater in bedrock systems.

Cross-Correlation Analysis of Long-Term Ambient Seismic-Noise Recordings in the Caribbean Netherlands to Monitor the Volcanoes on Saba and St. Eustatius

ABSTRACTThe continuous recordings of broadband seismometers on Saba and St. Eustatius in the Lesser Antilles provide a unique and long data set to measure temporal seismic velocity variations (dv/v) at two active but quiescent volcanoes (Mt. Scenery and The Quill). We compare results from single-station cross-component (SC) correlations with cross-station cross-component (CC) correlations and achieve the best similarities within the frequency band 1.3–2.1 Hz, with average correlations of 0.82 for Saba and 0.36 for St. Eustatius, justifying the use of SC as proxy for CC at these frequencies. Temporal dv/v variations derived from 13 yr of data show different characteristics at both islands. At St. Eustatius dv/v highly correlates (0.72) with air temperature and can be modeled by a simple sine wave with a period of 1 yr. Remaining residuals reveal cohurricane dv/v drops, thus at times of the passage of a hurricane. At Saba, subsurface velocity variations show temporal coseismic changes, up to −0.49% compared with −0.19% at St. Eustatius, and thus show a higher sensitivity to ground shaking. Our data set, although limited, shows a linear relation (correlation 0.78) between the coseismic dv/v drop and peak ground velocity at Saba around 1.3 Hz. We model the associated seismic velocity recovery with an exponential decay function and we estimate the recovery time at 2 yr. After subtracting the coseismic drop and recovery model, dv/v at Saba obtained from CC data correlates with the sine model (correlation 0.71). SC may be an appealing alternative for CC for monitoring purposes; however, the use of a small network is preferred to reduce the variance in dv/v (at St. Eustatius from 0.12% to 0.05%) and to detect dv/v variations unrelated to volcanic activity (e.g., hurricane). We continue work on the implementation of CC in the daily monitoring for Mt. Scenery and The Quill.

Seismic velocity variations associated with the 2018 lower East Rift Zone eruption of K\u012blauea, Hawai\u02bbi

The 2018 lower East Rift Zone eruption of Kīlauea (Hawai‘i) marked a dramatic change in the volcano’s 35-year-long rift zone eruption. The collapse of the middle East Rift Zone vent Pu‘u ‘Ō‘ō was followed by one of the volcano’s most voluminous eruptions in 500 years. Over the course of this 3-month eruption, the draining of summit-stored magma led to near-daily collapses of a portion of the caldera and ultimately up to 500 m of summit subsidence. While deformation data indicated that the summit and middle East Rift Zone were inflating for the previous several years, why Pu‘u ‘Ō‘ō collapsed and what initiated down-rift dike propagation remains unclear. Using ambient noise seismic interferometry, we show that a Ml5.3 decollement earthquake beneath Kīlauea’s south flank in June 2017 induced a coseismic decrease of up to 0.30% in seismic velocity throughout the volcano. This velocity decrease may have been caused by dynamic stress–induced shallow crustal fracture, i.e., weakening to dilatant crack growth, and was greatest near Pu‘u ‘Ō‘ō. Additionally, we verify a pre-eruptive increase in seismic velocity, consistent with increasing pressurization in the volcano’s shallow summit magma reservoir. This velocity increase occurred coincident with the first in a series of lower-crustal earthquake swarms, 6 days before a 2-month period of rapid summit and middle East Rift Zone inflation. The increase in up-rift magma-static pressure, combined with the pre-existing weakness from the June 2017 earthquake, may have facilitated down-rift dike propagation and the devastating 2018 eruption.

Rainfall-Induced Variation of Seismic Waves Velocity in Soil and Implications for Soil Response: What the ARGONET (Cephalonia, Greece) Vertical Array Data Reveal

ABSTRACTWe apply interferometry by deconvolution to compute the shear-wave velocity in shallow sediments (0–83.4 m) based on earthquake records from a vertical accelerometric array (ARGOstoli Network [ARGONET]) on Cephalonia Island, Greece. Analysis of the time variation of measured values reveals a cyclical pattern, which correlates negatively to rainfall and a soil moisture proxy. The pattern includes a sharp reduction in velocity at the beginning of rainy seasons and a gradual rise toward dry periods, the overall variation being around 20%–25% within the shallowest depth interval examined (0–5.6 m) and estimated to reach 40% within the top 2 m. The variation itself and its amplitude are verified by surface-wave dispersion analysis, using ambient vibration data. Synthetic standard spectral ratios suggest that this seasonal effect leaves an imprint on soil response, causing differences in the level of high-frequency ground motion between dry and rainy seasons, and this is verified by earthquake records. Furthermore, the near-surface velocity decrease due to soil saturation can be of the same order of magnitude as the nonlinear coseismic variation, masking the physical process of the nonlinear response of the site due to weak-to-strong-motion shaking. Thus, seasonal variations of seismic-wave velocities in shallow sediments may be important for a number of site-effect related topics, such as high-frequency ground-motion variability, soil anisotropy, kappa measurements, nonlinear site response, and so on.

Seismic monitoring in the Gugla rock glacier (Switzerland): ambient noise correlation, microseismicity and modelling

SUMMARY A network of seismometers has been installed on the Gugla rock glacier since October 2015 to estimate seismic velocity changes and detect microseismicity. These two processes are related to mechanical and structural variations occurring within the rock glacier. Seismic monitoring thus allows a better understanding of the dynamics of rock glaciers throughout the year. We observed seasonal variations in seismic wave velocity and microseismic activity over the 3 yr of the study. In the first part of our analysis, we used ambient noise correlations to compute daily changes of surface wave velocity. In winter, seismic wave velocities were higher, probably due to refreezing of the permafrost active layer and cooling of the uppermost permafrost layers, leading to increased overall rigidity of the medium. This assumption was verified using a seismic model of wave propagation that estimates the depth of P- and S-wave velocity changes from 0 down to 10 m. During melting periods, both a sudden velocity decrease and a decorrelation of the seismic responses were observed. These effects can probably be explained by the increased water content of the active layer. In the second part of our study, we focused on detecting microseismic signals generated in and around the rock glacier. This seismic activity (microquakes and rockfalls) also exhibits seasonal variations, with a maximum in spring and summer, which correlates principally with an exacerbated post-winter erosional phase of the front and a faster rock glacier displacement rate. In addition, we observed short bursts of microseismicity, both during snowfall and during rapid melting periods, probably due to pore pressure increase.

Landslide monitoring using seismic refraction tomography – The importance of incorporating topographic variations

Seismic refraction tomography provides images of the elastic properties of subsurface materials in landslide settings. Seismic velocities are sensitive to changes in moisture content, which is a triggering factor in the initiation of many landslides. However, the application of the method to long-term monitoring of landslides is rarely used, given the challenges in undertaking repeat surveys and in handling and minimizing the errors arising from processing time-lapse surveys. This work presents a simple method and workflow for producing a reliable time-series of inverted seismic velocity models. This method is tested using data acquired during a recent, novel, long-term seismic refraction monitoring campaign at an active landslide in the UK. Potential sources of error include those arising from inaccurate and inconsistent determination of first-arrival times, inaccurate receiver positioning, and selection of inappropriate inversion starting models. At our site, a comparative analysis of variations in seismic velocity to real-world variations in topography over time shows that topographic error alone can account for changes in seismic velocity of greater than ±10% in a significant proportion (23%) of the data acquired. The seismic velocity variations arising from real material property changes at the near-surface of the landslide, linked to other sources of environmental data, are demonstrated to be of a similar magnitude. Over the monitoring period we observe subtle variations in the bulk seismic velocity of the sliding layer that are demonstrably related to variations in moisture content. This highlights the need to incorporate accurate topographic information for each time-step in the monitoring time-series. The goal of the proposed workflow is to minimize the sources of potential errors, and to preserve the changes observed by real variations in the subsurface. Following the workflow produces spatially comparable, time-lapse velocity cross-sections formulated from disparate, discretely-acquired datasets. These practical steps aim to aid the use of the seismic refraction tomography method for the long-term monitoring of landslides prone to hydrological destabilization.

On the Variability of Pressure Propagation During Hydraulic Stimulation Based on Seismic Velocity Observations

AbstractPermeability enhancement of a reservoir through hydraulic stimulation is ever so often accompanied by potentially hazardous induced seismicity. Monitoring of in situ pressure propagation remains an important step in understanding the underlying seismo‐hydromechanical processes during hydraulic stimulation and mitigating hazardous induced seismicity. In an effort to monitor pressure propagation remotely, active seismic monitoring was performed during decameter‐scale hydraulic stimulation experiments at the Grimsel Test Site in Switzerland. Using cross‐correlation schemes, subtle traveltime variations that correlate well with the high‐pressure fluid injections during hydraulic stimulation become apparent. The 4‐D seismic tomograms obtained through time‐lapse differential traveltime inversion reveal transient changes in the seismic velocity within the stimulated volume that depend on the effective stress. In‐depth comparison of the seismic velocity variations and measured pressure data indicates that the seismic velocity variations can be used as a proxy for pressure propagation within an equilibrated system. Observations show that traveltime tomography is insensitive to isolated, high‐pressure variations such as a breakthrough within the fracture network. Joint interpretation of the 4‐D seismic tomograms with geological and hydromechanical data reveals fundamental differences between the different experiments due to a large variability in the rock mass response to the hydraulic stimulation. Using the observed velocity variations as a proxy for pressure propagation, we can infer characteristics of the shear zones and the rock volume at the Grimsel Test Site. The observed velocity variations provide not only information on the characteristics of the reservoir itself but also its response to different stimulation techniques.

A Reappraisal of Seismicity Recorded During the 1996 Gjálp Eruption, Iceland, in Light of the 2014–2015 Bárðarbunga–Holuhraun Lateral Dike Intrusion

This study performs a reanalysis of the seismicity recorded during the 1996 Gjalp eruption that occurred at NW Vatnajokull, Iceland. The seismicity was recorded by the temporary HOTSPOT network consisting of 30 three-component broadband stations. In total 301 events were identified between 29 September and 12 October and their phases were manually picked. A velocity model was estimated from P-phase travel times by using VELEST. Events were first located using the algorithm NONLINLOC in order to obtain absolute locations. Precise relative locations were obtained with HYPODD by utilizing catalog and cross-correlation differential travel times. Results show that events clustered first along the SW rim of the Barðarbunga caldera and later along the Gjalp fissure, with most hypocentral depths located between 3 and 8 km. Waveforms of the 10 largest events that followed the Barðarbunga earthquake were inverted in order to obtain moment tensors. For all events we found that the deviatoric moment tensor fits the data better than pure double-couple or full moment tensor solutions. Events along the Barðarbunga caldera exhibited reverse focal mechanisms, while those at the Gjalp fissure exhibited mostly strike-slip faulting. Seismic velocity variations calculated using ambient noise interferometry, point to the possibility that a small subglacial eruption occurred at Barðarbunga before the main earthquake. This removed melt from the magma chamber causing its roof to collapse, and also resulted in the lateral migration of magma towards the Gjalp fissure. The 2014–2015 Barðarbunga–Holuhraun eruption shares common characteristics with the 1996 Gjalp eruption, although the size of the latter was much smaller.

Estimating temporal changes in seismic velocity using a Markov chain Monte Carlo approach

SUMMARYWe present a new method for estimating time-series of relative seismic velocity changes (dv/v) within the Earth. Our approach is a Markov chain Monte Carlo (MCMC) technique that seeks to construct the full posterior probability distribution of the dv/v variations. Our method provides a robust, computationally efficient way to compute dv/v time-series that can incorporate information about measurement uncertainty, and any prior constraints that may be available. We demonstrate the method with a synthetic experiment, and then apply the MCMC algorithm to three data examples. In the first two examples we reproduce dv/v time-series associated with the response to the 2010 M 7.2 El Mayor-Cucapah earthquake at two sites in southern California, that have been studied in previous literature. In the San Jacinto fault zone environment we reproduce the dv/v signature of a deep creep slip sequence triggered by the El Mayor-Cucapah event, that is superimposed on a strong seasonal signal. At the Salton Sea Geothermal Field we corroborate the previously observed drop-and-recovery in seismic velocity caused by ground shaking related to the El Mayor-Cucapah event. In a third, new example we compute a month long velocity change time-series at hourly resolution at Piñon Flat, California. We observe a low amplitude variation in seismic velocity with a dominant frequency of 1 cycle per day, as well as a second transient signal with a frequency of 1.93 cycles per day. We attribute the 1-d periodicity in the dv/v variation to the combined effects of the diurnal tide and solar heating. The frequency of the signal at 1.93 cycles per day matches that of the lunar (semi-diurnal) tide. Analysis of the uncertainties in the Piñon Flat time-series shows that the error contains a signal with a frequency of 1 cycle per day. We attribute this variation to seismic noise produced by freight trains operating within the Coachella Valley. By demonstrating the applicability of the MCMC method in these examples, we show that it is well suited to tackle problems involving large data volumes that are typically associated with modern seismic experiments.

Systematic Detection of Clustered Seismicity Beneath the Southwestern Alps

AbstractWe present a new automated earthquake detection and location method based on beamforming (or back projection) and template matching and apply it to study the seismicity of the Southwestern Alps. We use beamforming with prior knowledge of the 3‐D variations of seismic velocities as a first detection run to search for earthquakes that are used as templates in a subsequent matched‐filter search. Template matching allows us to detect low signal‐to‐noise ratio events and thus to obtain a high spatiotemporal resolution of the seismicity in the Southwestern Alps. We describe how we address the problem of false positives in energy‐based earthquake detection with supervised machine learning and how to best leverage template matching to iteratively refine the templates and the detection. We detected 18,754 earthquakes over 1 year (our catalog is available online) and observed temporal clustering of the earthquake occurrence in several regions. This statistical study of the collective behavior of earthquakes provides insights into the mechanisms of earthquake occurrence. Based on our observations, we infer the mechanisms responsible for the seismic activity in three regions of interest: the Ubaye valley, the Briançonnais, and the Dora Maira massif. Our conclusions point to the importance of fault interactions to explain the earthquake occurrence in the Briançonnais and the Dora Maira massif, whereas fluids seem to be the major driving mechanism in the Ubaye valley.

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

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.

Testing the hypothesis that temperature modulates 410 and 660 discontinuity topography beneath the eastern United States

The leading hypothesis to explain 410 and 660 km discontinuity topography and coincident velocity variations is the thermal hypothesis stated as: temperature variations are the primary modulator of discontinuity topography and seismic velocity variations. To test the thermal hypothesis, discontinuity topography maps are correlated with coincident P- and S-velocity variations for the eastern half of the United States sampled by IRIS-EarthScope USArray seismic data. The discontinuity topography maps were made via common-conversion point migration of P-wave receiver functions. The receiver functions were made using a multi-event and multi-station deconvolution method. Fundamental to our results is the choice of three-dimensional P- and S-velocity models, which are used as migration velocity models and for correlation analysis. Two three-dimensional velocity models are used in our analysis: the MITS-model of Golos et al. (2018) and the SL-model of Schmandt and Lin (2014). The Pearson correlation coefficient is used to estimate the degree of linearity between the discontinuity topography and coincident velocity variations. A bivariate regression of discontinuity topography versus coincident velocity variations (termed the mineral physics slope) is performed and compared to a range of slopes constrained by published velocity-temperature derivatives and Clapeyron slopes. Using spatially binning, the discontinuity topography and coincident velocity variations, spatial maps of the correlation coefficient and mineral physics slope are made. Most of the discontinuity sampling area has reasonable correlation values (≥0.4) and plausible mineral physics slope values. The veracity of the thermal hypothesis is assessed by integrating the probability density functions of the mineral physics slopes over a domain defined by the published range of 410 and 660 Clapeyron slopes. At the 410, the MITS-model and SL-model thermal hypothesis probabilities are 52% and 51%, respectively, and the seismic Clapeyron slope estimates are 2.7 and 1.3 MPa/K, respectively. At the 660, the MITS-model and SL-model thermal hypothesis probabilities are 54% and 75%, respectively and the seismic Clapeyron slope estimates are −1.1 and −1.7 MPa/K, respectively. These Clapeyron slopes estimates are in the middle of plausible Clapeyron slope ranges. Using these Clapeyron slopes, temperature maps show a ±300 K range at the 410 and a ±600 K range at the 660. For regions that are inconsistent with the thermal hypothesis, we suggest that the leading explanations are uncertainties in the velocity models used and secondarily, hydration effects.

Interferometric time-lapse velocity analysis: application to a salt-water disposal well in British Columbia, Canada

SUMMARY We investigate the possibility of passive monitoring of a salt-water disposal well in British Columbia, Canada, using continuously recorded ambient seismic noise. We find seismic velocity variations induced by a reduction of injection pressure in an effort to mitigate an elevated level of seismicity, most likely associated with the disposal of salt water. The relative velocity variations are derived from time-shifts measured between consecutive cross-correlation functions for each station pair in a surface array composed of five broad-band seismometers. The probable driving mechanisms responsible for the velocity changes are reduced pore pressures and/or lowered poroelastic stresses beyond the injection wellbore, respectively. Hydrologic data (e.g. snow and rainfall), noise energy trends and fluctuations in the incident direction of dominant noise sources do not correlate with the estimated relative velocity variations. Velocity variations are detected ahead of the zone of induced seismicity, thus indicating that seismic interferometry may aid in mitigation efforts to reduce the risk of induced seismicity by (1) providing verifiable and repeatable measurements of physical changes within the surrounding area and (2) providing hard constraints for modelling efforts to constrain how and where pore-pressure fronts change.

Constraining crustal structure in the presence of sediment: a multiple converted wave approach

SUMMARYReceiver functions are sensitive to sharp seismic velocity variations with depth and are commonly used to constrain crustal thickness. The H–κ stacking method of Zhu & Kanamori is often used to constrain both the crustal thickness (H) and ${V_P}$/${V_S}$ ratio ($\kappa $) beneath a seismic station using P-to-s converted waves (Ps). However, traditional H–κ stacks require an assumption of average crustal velocity (usually ${V_P}$). Additionally, large amplitude reverberations from low velocity shallow layers, such as sedimentary basins, can overprint sought-after crustal signals, rendering traditional H–$\ \kappa $ stacking uninterpretable. We overcome these difficulties in two ways. When S-wave reverberations from sediment are present, they are removed by applying a resonance removal filter allowing crustal signals to be clarified and interpreted. We also combine complementary Ps receiver functions, Sp receiver functions, and the post-critical P-wave reflection from the Moho (SPmp) to remove the dependence on an assumed average crustal ${V_P}$. By correcting for sediment and combining multiple data sets, the crustal thickness, average crustal P-wave velocity and crustal ${V_P}$/${V_S}$ ratio is constrained in geological regions where traditional H–$\ \kappa $ stacking fails, without making an initial P-wave velocity assumption or suffering from contamination by sedimentary reverberations.

Seismic imaging of the Olduvai Basin, Tanzania

A 5.6-km-long line of refraction and reflection seismic data spanning the Pliocene-Pleistocene fill of the Olduvai Basin, Tanzania is presented. The line is oriented along a northwest-southeast profile through the position of Olduvai Gorge Coring Project (OGCP) Borehole 2A. Our aims are to (1) delineate the geometry of the basin floor by tracing bedrock topography of the metaquartzitic and gneissic basement, (2) map synsedimentary normal faults and trace individual strata at depth, and (3) provide context for the sequence observed in OGCP cores. Results with refraction tomography and poststack migration show that the maximum basin depth is around 405 m (±25 m) in the deepest portion, which quadruples the thickness of the basin-fill previously known from outcrops. Variations in seismic velocities show the positions of lower density lake claystones and higher density well-cemented sedimentary sequences. The Bed I Basalt lava is a prominent marker in the refraction seismic results. Bottom-most sediments are dated to >2.2 Ma near where Borehole 2A bottoms out at the depth of 245 m. However, the seismic line shows that the basin-fill reaches a maximum stratigraphic thickness of around 380 m deep at Borehole 2A, in the western basin where the subsidence was greatest. This further suggests that potential hominin palaeoenvironments were available and preserved within the basin-fill possibly as far back as around 4 Ma, applying a temporal extrapolation using the average sediment accretion rate.

Detection of pre-eruptive seismic velocity variations at an andesitic volcano using ambient noise correlation on 3-component stations: Ubinas volcano, Peru, 2014

Volcano monitoring and eruption forecasting are based on the observation and joined interpretation of several precursory phenomena. It is thus important to detect new types of precursor and to study their relationship with forthcoming eruptions. In the last years, variations of seismic velocity have been observed in some volcanoes, mainly basaltic, before eruptions. In this paper, we look for velocity variations and waveform decorrelations before the 2014 eruptive sequence of the andesitic Ubinas volcano in Peru. We compute velocity changes by using seismic ambient noise cross-correlation (between pairs of stations) and cross-components correlation (between vertical and horizontal components of single stations), as well as coda wave interferometry of seismic multiplets. With these different approaches, we show that the major explosions that occurred from 13 to 19 April were preceded by a clear velocity decrease and waveform decorrelation. The amplitude of velocity change is generally larger on single-station cross-components correlation than on two-station cross-correlation in all the frequency ranges tested (between 0.1 and 8 Hz). We highlight an apparent anisotropy of velocity change in single-station cross-components correlation, with larger amplitudes when correlating vertical and tangential components than using vertical and radial components with respect to the crater. The Mw = 8.1 Iquique earthquake on 1 April 2014 produced also a marked co-seismic velocity drop detected in a high frequency range(3–5 Hz) in both single-station cross-components correlation and cross-correlations. We locate in the horizontal plane and in depth the velocity perturbation and the structural change related with decorrelation. During the main phase of eruptive activity, the velocity decrease at low frequency (0.1–1 Hz) appears to affect the whole edifice mainly at depth of about 1 to 3 km below the surface. The structural perturbation is more concentrated on the south flank of the volcano, a zone that corresponds to an ancient collapse. We suggest that the observed velocity variations are due to the dilatation of the edifice and to microfracturation induced by magma pressurization. The structural change may be locally enhanced by a possible zone of material weakness in the southern sector. The co-seismic velocity perturbation is located mostly in the southeast flank, at depth smaller than 0.5 to 1 km, and may be related to the presence of the hydrothermal system of the volcano.

Seismic velocity reduction and accelerated recovery due to earthquakes on the Longmenshan fault

Various studies report on temporal changes of seismic velocities in the crust and attempt to relate the observations to changes of stress and material properties around faults. Although there are growing numbers of observations on coseismic velocity reductions, generally there is a lack of detailed observations of the healing phases. Here we report on a pronounced coseismic reduction of velocities around two locked sections (asperities) of the Longmenshan fault with a large slip during the 2008 Mw 7.9 Wenchuan earthquake and subsequent healing of the velocities. The healing phase accelerated significantly at the southern asperity right after the nearby 2013 Mw 6.6 Lushan earthquake. The results were obtained by joint inversions of travel time data at four different periods across the Wenchuan and Lushan earthquakes. The rapid acceleration of healing in response to the Lushan earthquake provides unique evidence for the high sensitivity of seismic velocities to stress changes. We suggest that stress redistribution plays an important role in rebuilding fault strength. After a fault ruptures, the recovery of its strength may be accelerated by earthquakes that redistribute stress, according to an analysis of temporal variation in crustal seismic velocity on the Longmenshan fault in China.

Temporal Changes of Seismic Velocity Caused by Volcanic Activity at Mt. Etna Revealed by the Autocorrelation of Ambient Seismic Noise

On active volcanoes, ambient noise-based seismic interferometry, able to detect very slight variations in seismic velocity associated with magma transport towards the surface, can be a very useful monitoring tool. In this work, we performed the autocorrelation of ambient seismic noise recorded at Mt. Etna volcano, by three stations located close to the active summit craters, during April 2013 - October 2014. Such an interval was chosen because of the number and variety of eruptions. The method implemented to perform autocorrelation was the phase cross-correlation, which does not require normalization of the signals. The detected seismic velocity variations were very consistent for all three stations throughout the study period, mainly ranging between 0.3 and -0.2%, and were time-related to both sequences of paroxysmal eruptions and more effusive activities. In particular, we observed seismic velocity decreases accompanying paroxysmal eruptions, suggesting an intense pressurization within the plumbing system, which created an area of extensional strain with crack openings. It is worth noting that classical cross-station approach failed to detect seismic velocity changes related to volcano activity. In addition, seismic velocity variations over time were integrated with ground deformation data recorded by GPS stations and volcanic tremor centroid locations. Finally, we showed that, although the investigated frequency band (1-2 Hz) contains most of the volcanic tremor energy, our results did not indicate a particular contamination of seismic velocity variation measurements by variations of tremor sources.

Spatio-temporal changes in the seismic velocity induced by the 2011 Tohoku-Oki earthquake and slow slip event revealed from seismic interferometry, using ocean bottom seismometer\u2019s records

Seismic interferometry is one of the most effective techniques for detecting temporal variations in seismic velocity caused by large earthquakes. Before the 2011 Tohoku-Oki earthquake (Mw9.0) near the Japan Trench, a slow slip event (SSE, Mw7.0) and low-frequency tremors were observed near the trench. Here, we applied a seismic interferometry technique using ambient noise to data from 17 ocean bottom seismometers (OBSs) installed above the focal region before the main shock. We used our technique to detect temporal variations in seismic velocity caused by the main shock, SSE, and low-frequency tremors. In the region above the large coseismic slip area, we detected a 1–2% seismic velocity decrease after the main shock. In addition, we observed very small temporal increases in seismic velocity near the SSE fault during the initial SSE stage. Moreover, for most of the OBSs, we observed temporal variations in the autocorrelation functions (ACFs) during the low-frequency tremors. These may have been caused by temporal variations in the ambient noise source distributions, resulting from low-frequency tremors. These results suggest the possibility of detecting low-frequency tremors using ACF monitoring.

Using Multiazimuth Seismic Data for Anisotropy Estimation in an Unconventional Reservoir

Productivity of a well in an unconventional reservoir is governed by various static and dynamic reservoir characteristics. Many of these characteristics have proxies among pre and post stack seismic attributes that can be derived from Multiazimuth 3D Seismic Data. The task of a geoscientist in this kind of reservoir is to understand these proxies to predict production behaviour. Natural fracture density and azimuth, as well as horizontal stress azimuth are the key attributes that seismic can help predict. Seismic Velocity and Amplitude variations with azimuth can be used to predict fracture strike, relative fracture density and define potential structural sweet spots. Azimuthal data from a Multiazimuth 3D seismic survey in the Nappamerri Trough of Cooper Basin has been interpreted to estimate fracture intensity and orientation. Co-rendered structural maps are used to create stress maps for different interval of interest. Stress maps help to identify areas of higher anisotropy and areas of lower minimum horizontal stress and so facilitate optimised well placement. To test the geological significance of these maps, correlation of stress vectors against well Image log and cross dipole sonic data was completed. This ground truth validates the prediction of direction and distribution of reservoir fractures based on full azimuth seismic data in this area.