Seismic Changes Research Articles

Time‐lapse VSP integration and calibration of subsurface stress field utilizing machine learning approaches: A case study of the morrow B formation, FWU

AbstractThis study aims to develop a methodology for calibrating subsurface stress changes through time‐lapse vertical seismic profiling (VSP) integration. The selected study site is a region around the injector well located within Farnsworth field unit (FWU), where there is an ongoing CO2‐enhanced oil recovery (EOR) operation. In our study, a site‐specific rock physics model was created from extensive geological, geophysical, and geomechanical characterization through 3D seismic data, well logs, and core assessed as part of the 1D MEM conducted on the characterization well within the study area. The Biot‐Gassmann workflow was utilized to combine the rock physics and reservoir simulation outputs to determine the seismic velocity change due to fluid substitution. Modeled seismic velocities attributed to mean effective stress were determined from the geomechanical simulation outputs, and the stress‐velocity relationship developed from ultrasonic seismic velocity measurements. A machine learning‐assisted workflow comprised of an artificial neural network and a particle swarm optimizer (PSO) was utilized to minimize a penalty function created between the modeled seismic velocities and the observed time‐lapse VSP dataset. The successful execution of this workflow has affirmed the suitability of acoustic time‐lapse measurements for 4D‐VSP geomechanical stress calibration pending measurable stress sensitivities within the anticipated effective stress changes and the availability of suitable and reliable datasets for petroelastic modeling. © 2023 Society of Chemical Industry and John Wiley & Sons, Ltd.

Improving the Estimation of the Occurrence Time of an Impending Major Earthquake Using the Entropy Change of Seismicity in Natural Time Analysis

This article is focused on a new procedure concerning a more accurate identification of the occurrence time of an impending major earthquake (EQ). Specifically, we first recapitulate that, as was recently shown [P. Varotsos et al., Communications in Nonlinear Science and Numerical Simulation 125 (2023) 107370], natural time analysis of seismicity supplemented with the non-additive Tsallis entropy Sq leads to a shortening of the time window of an impending major EQ. This has been shown for the Tohoku mega-EQ of magnitude M9 that occurred in Japan on 11 March 2011, which is the largest event ever recorded in Japan. Here, we also show that such a shortening of the time window of an impending mainshock can be achieved for major, but smaller EQs, of the order of M8 and M7. In particular, the following EQs are treated: the Chiapas M8.2 EQ, which is Mexico’s largest EQ for more than a century that took place on 7 September 2017 near the coast of Chiapas state in Mexico, the 19 September 2017 M7.1 EQ that occurred within the Mexican flat slab, and the M7.1 Ridgecrest EQ on 6 July 2019 in California.

The absence of remotely triggered seismicity in Gujarat, NW India during the Nepal earthquake, 2015

Gujarat in the Northwestern Deccan Volcanic Province of India is among the most seismically earthquake-prone. The region may be susceptible to remote dynamic triggering, especially the Kachchh region, which recently hosted the Mw7.7 Bhuj earthquake in 2001. Its aftershocks continue because it is critically stressed and contains nucleation points more frequently close to failure. From waveforms and catalog data, we examine whether remote dynamic triggering occurs following 25 April 2015, Mw7.8 Nepal mainshock in the Gujarat region, Northwestern India. The 2015 Nepal event perturbed the Gujarat region with a peak dynamic stress of ∼53 kPa, much higher than the global lower limit of 1 kPa. Due to the large magnitude and high peak dynamic stress, ideally, the 2015 mainshock should have resulted in the triggered seismicity in the study region. To study the remote dynamic triggering in detail, we also have examined the other recent regional large earthquakes with comparable peak dynamic stresses (>50 kPa), namely, 16 April 2013, Mw7.7 Iran, 24 September 2013, Mw7.7 Pakistan, and 26 October 2015, Mw7.7 Afghanistan. Our result shows that despite their significant peak dynamic stress, there is no significant change in the local seismicity. The analysis suggests that the surface wave amplitude is not the only factor that governs the remote dynamic triggering. Our results also indicate that the faults were not critically stressed during the mainshock candidates in the study region.

Pressure increase at the magma-hydrothermal interface at Krafla caldera, North-Iceland, 2018–2020: Magmatic processes or hydrothermal changes?

Following volcano deflation since 1989, the deformation pattern in Krafla caldera, North Iceland, changed in 2018. Geodetic measurements reveal a difference in surface velocity fields for 2015–2018 and 2018–2020 periods, reflecting a change in the deformation pattern. The difference velocity field broadly fits deformation caused by a spherical pressure source within a uniform elastic half-space, with a volume change of 2.6–3.8 × 105 m3/yr and centre depth of 2.1–2.5 km, which is close to the brittle-ductile boundary in the area, at a depth of 1.8 to 2.2 km. Potential processes causing the deformation change are evaluated: magmatic processes such as magma inflow or accumulation of volcanic gas, changes in the geothermal area because of change in geothermal production, or a combination of these. In particular, we evaluate if the change in deformation may relate to about 0.1 MPa/yr pressure increase in the geothermal system as measurements in monitoring well KG-10 indicate, eventually due to changes in the geothermal exploitation strategy at the Krafla power plant. Modelling shows that inferred volume change may be due to a spherical source with 1.4 km radius with 0.1 MPa/yr pressure change if the surrounding crust has a Young's modulus E of about 7 GPa. However, the average regional Young's modulus for the upper crust in Iceland has been estimated to be 30 GPa. We use the Finite Element Method (FEM) to assess the influence on the displacement due to the presence of a local crustal volume 5 × 5 × 4 km (horizontal dimensions × depth), which envelops the source (within the Krafla caldera), with E = 7 GPa in the central area and 30 GPa in the far field, in a three-dimensional model. Such a model can reproduce significant features of the observed deformation. There are no changes in seismicity in 2018. In late 2019, the earthquake rate increases following a slight decrease over few months. The seismic moment release is relatively steady until the end of 2019, when the rate increases and is relatively constant in 2020. Gravity measurements in late 2019, when compared to limited measurements in 2018, are inconclusive regarding the nature of the deformation, but useful for further monitoring. No noticeable changes have been observed in the chemical composition of fumarole discharge in the Krafla field that relate to new intrusions.

Evaluating the impact of 4D seismic data artefacts in data assimilation

Time-lapse or 4D seismic data are important constraints in reservoir studies because they enable monitoring of saturation and pressure changes that result from hydrocarbon production. 4D seismic data have been quantitatively added, along with production data, in history matching or data assimilation procedures to reduce uncertainty and improve production forecasts. Before performing quantitative studies, it is important to ensure that the 4D seismic data are reliable, with minimal artefacts such as side-lobe effects that can disturb the identification of anomalies. In this work, we propose different ways of treating 4D seismic data in data assimilation for a real reservoir. Explicitly, we evaluated the impact on data assimilation results when considering different amounts of 4D information and three treatments to the identified artefacts. The treatments were: ignoring them, excluding them from data assimilation or defining no seismic changes at their locations. The results show that well and seismic matches are improved when 4D seismic data are assimilated, also improving the production predictions. Despite being a thin reservoir, assimilating two single-layer maps allowed us to predict relevant observed dynamic behaviour, such as the evolved gas trapped in the lower interval. Furthermore, when a treatment was applied to the artefacts, they produced better models than using a single two-layer map (with lower production errors and visually closer impedances to the observed data). Our recommendation is the assimilation of well and 4D seismic data, with the exclusion of unreliable information, for better life-cycle decisions.

Computational methods applied to earthen historical structures

Earthen structures have an important representation among the UNESCO World Heritage List sites as well as among the built environment in general. Unfortunately, earthen heritage structures are also numerous within the UNESCO List of World Heritage in Danger whereas other existing common earthen structures are extremely vulnerable to seismic and climate change events. Within the field of heritage conservation, structural analysis contributes to the safety evaluation of the structure, the diagnosis of the causes of damage and decay, and to the validation of interventions. Thus, the need to develop effective and accurate computational methods suitable for the study of both monumental and vernacular earthen structures becomes evident. This paper compiles, summarizes, and highlights the latest developments and implementations of computational methods for the study of such structure typologies. The literature has been explored following the PRISMA-S checklist methodology and a narrative synthesis was used for the presentation of results. Finally, future trends, opportunities, and challenges are discussed.

Homohysteria

This article uses a feminist lens to critique and repurpose the concept of homohysteria. Eric Anderson (2005) created the term homohysteria to explain seismic changes in social behavior of men, particularly noting how a society’s relationship with homosexuality will explain sanctions enacted on men’s behavior. I argue that while Anderson and his adherents developed a novel concept to discuss shifts in male behavior, the all-encompassing nature of a macro-concept, applies too broad a stroke in terms of explaining modifications in relations with manhood. Instead of dismissing the theory, I assert Anderson’s original scope was limited, and that homohysteria is best understood at a meso-level of implementation and change. Further, I argue the concept of effeminophobia must be central to homohysteria, given the fear of the effeminate in a masculine form (Allan 2018). This research is pivotal in demonstrating wider use for Anderson’s concept by expanding its relevance.

Innovative 3D modeling of an old oil field for sustainable production: Case study of Katin-Barbes oil field (KBOF), SE Anatolia- Turkey

The goal of this study is to establish a workflow for the re-interpretation of almost depleted fields targeting the long-term sustainable oil production; in particular, for the oil fields in Turkey and the neighboring Middle Eastern countries located on the fold and thrust belts of the Zagros Mountains. It also fills some of the gaps in our understanding of the northern part of the Arabian Platform by describing the seismic characteristics of the Cretaceous reservoirs that were deposited during the Aptian to Turonian. The Katin-Barbes oil field (KBOF) in SE-Turkey was used as a case study. In this area, 3D seismic data were used for structural interpretation and re-modeling of Cretaceous carbonates sedimented in the complex tectonic region. The well logs from 55 wells in the field were used to create a compilation of formation tops and were used as reference points for two separate sets of 3-D seismic data, acquired in 1991 and 2017. The quality of seismic data was improved with interpretation filters. The structural model was obtained by using various qualifiers from the seismic cubes and seismic facies changes were identified by analyzing a number of seismic attributes. Therefore, seismic data and velocities from the borehole measurements were combined to form a velocity model in building a structural model. Seismic attributes and well logs were used to create a porosity model. Consequently, top and base of two reservoir units; the Derdere and Sabunsuyu Formations have been clarified and re-defined, and potential new well locations were identified. Depending on the results of this investigation, 9 new wells were drilled in the potential areas in the KBOF, recently. Except the last one drilled on the NE boundary of the northern block, all wells have been completed as the “oil producing wells” and top of the reservoir units were encountered at almost the same depths in our depth model. Therefore, results and proposed methods in this research are confirmed by the real, borehole data. This research will be an examplary study for the re-evaluation of older and/or almost depleted oil fields, either in Turkey or in the other Middle Eastern countries.

Ovid and the Liberty of Speech in Shakespeare’s England. By Heather James

Heather James, by examining a number of Tudor- and Stuart-era authors through the twin lenses of Ovidian reception and early modern concerns about autocratic pressure on literary expression, offers an intriguing perspective on the period’s restless ferment of poetry and political philosophy. Through a series of close readings of selected works by Spenser, Marlowe, Shakespeare, and Jonson, James focuses on Ovid as both these authors’ inspiration and, more daringly, the springboard for their own potentially perilous verses about speech and silence in a wider world inescapably overshadowed by the authority of the Crown. The towering status of Ovid in early modern English literature is of course well-established in scholarship; what James does is argue for a better appreciation of Ovid’s importance for the political ramifications of poetic work. After a somewhat desultory introduction, James considers Spenser in her first chapter, “Flower Power: Political Discontents in Spenser’s Flowerbeds.” She begins to build her theme of Ovidian influence not by choosing an overtly political work, but by taking “the counterintuitive position that Spenser is in fact deeply political when he invites his readers to wallow and get lost in lush flowerbeds of poetry” (20). James posits that when Spenser indulges in long, descriptive floral catalogues that seem to have no relation to external politics, he is in fact offering an escape from such politics with a “poetic aromatherapy” (21) of blooms “all flagrantly irrelevant to moral use or political encomium” (27). The heart of the chapter is a close reading of Muiopotmos, or the Fate of the Butterflie: in it, the butterfly Clarion flutters through a lush garden, but is killed by the spider Aragnoll, who embodies “the darker passions of the court and Crown, namely Envy” (40)—for in Spenser, Aragnoll is the son of Arachne. In a “seismic change” (43), however, Spenser’s Arachne deviates from her Ovidian predecessor by spontaneously becoming a spider due to her envy of Athena’s art. Spenser’s confection of a poem becomes in James’s reading a metaphor for reading for pleasure and “an ideal of imaginative and expressive freedoms” that is ultimately “a gorgeous but doomed fantasy” (50).

Conceptualising headteachers’ leadership of technology in schools across the United Kingdom during early coronavirus disease 2019

The coronavirus disease 2019 pandemic brought seismic changes to schooling which few could have anticipated. Across the four countries of the UK, (England, Scotland, Wales and Northern Ireland) urgent changes to the locus and focus of education were required. This paper conceptualises how headteachers led their schools in the use of digital technology as they adjusted to teaching and learning in the early stages of a UK wide school ‘lockdown’. The study uses data from extensive interviews with the headteachers of 12 schools across all phases of learning in schools in all four countries in the UK. The analysis uses elements of Cashman's et al.'s (2014) ‘leadership by convening’ lenses to conduct a threefold analysis of the transcripts: the direction of communications (internal/external); the timeframes (synchronous/asynchronous); and headteachers’ perceptions of how their digital technology helped them navigate challenges. These challenges are classified as: (a) technical – The solution required technology or technical expertise; (b) adaptive – The solution required a change in behaviour by at least one party in the communication process (internal or external); (c) organisational – solutions to these challenges contained both a technological and a behavioural component. A new model of leadership with technology is suggested.

Influence of rainfall in a short-term seismic velocity monitoring at an urban landslide site in Minas Gerais, Brazil

Most high-magnitude landslides in Brazil are triggered by rainfall. The partial or total liquefaction of the soil mass is directly related to an increase in the pore pressure, which can happen after or during heavy and sustained rainfalls. Traditional near-surface geophysical methods for saturation and rigidity assessment usually involve short-duration data acquisition campaigns, reporting results representative of a specific moment. On the other hand, the cross-correlation of the ambient noise wavefield registered at different locations in continuous data streams has emerged as a promising alternative for monitoring seismic velocity variations in time. In the present study, we evaluate if the observed seismic velocity perturbations can satisfactorily relate to rainfall periods on a local scale and within minutes of resolution at a slope in Itabirito, state of Minas Gerais, Brazil, where landslide activity occurred. We calculate velocity changes in the tail portion (coda) from Green's functions retrieved from cross-correlation of seismic noise time series registered at pairs of stations from September 2022 to November 2022. In the 50 days observation period, the monitored area remained stable. We identified clear time intervals where the relative seismic velocity decreases after or during rainfall, which indicates modification in the saturation conditions of the soil. The results demonstrated that seismic velocity changes obtained from the coda of cross correlations of the ambient noise wavefield could satisfactorily constrain rainfall periods and indicate possible changes in the saturation conditions of the soil at a small local scale with a high temporal resolution.

Probing environmental and tectonic changes underneath Mexico City with the urban seismic field

Abstract. The sediments underneath Mexico City have unique mechanical properties that give rise to strong site effects. We investigated temporal changes in the seismic velocity at strong-motion and broadband seismic stations throughout Mexico City, including sites with different geologic characteristics ranging from city center locations situated on lacustrine clay to hillside locations on volcanic bedrock. We used autocorrelations of urban seismic noise, enhanced by waveform clustering, to extract subtle seismic velocity changes by coda wave interferometry. We observed and modeled seasonal, co- and post-seismic changes, as well as a long-term linear trend in seismic velocity. Seasonal variations can be explained by self-consistent models of thermoelastic and poroelastic changes in the subsurface shear wave velocity. Overall, sites on lacustrine clay-rich sediments appear to be more sensitive to seasonal surface temperature changes, whereas sites on alluvial and volcaniclastic sediments and on bedrock are sensitive to precipitation. The 2017 Mw 7.1 Puebla and 2020 Mw 7.4 Oaxaca earthquakes both caused a clear drop in seismic velocity, followed by a time-logarithmic recovery that may still be ongoing for the 2017 event at several sites or that may remain incomplete. The slope of the linear trend in seismic velocity is correlated with the downward vertical displacement of the ground measured by interferometric synthetic aperture radar, suggesting a causative relationship and supporting earlier studies on changes in the resonance frequency of sites in the Mexico City basin due to groundwater extraction. Our findings show how sensitively shallow seismic velocity and, in consequence, site effects react to environmental, tectonic and anthropogenic processes. They also demonstrate that urban strong-motion stations provide useful data for coda wave monitoring given sufficiently high-amplitude urban seismic noise.

An African American Woman Considers Transcendence via Psychoanalysis

ABSTRACT As a recent graduate in psychoanalysis, I assert that psychoanalysis has been responsible for a significant transformation in my life – indeed a transcendence over the intended constraints posed by my designated race, gender, and social class. I review my biography in the context of the seismic changes our social structures have continued to experience. My biography is analyzed using eclectic psychoanalytic theories to demonstrate the use of psychoanalysis in the service of personal liberation. Mentalization, as well as, Developmental and Attachment psychoanalytic theories may be used in an emancipatory effort by those currently oppressed or marginalized in our society when trained psychoanalysts are accessible. I believe the history of anti-Black racism is relevant to the psychoanalysis of patients regardless of race. The concept of “Racial Battle Fatigue” will be described and applied to the analysis of myself, a person of African descent.

Tidal Response of Seismic Wave Velocity at Shallow Crust in Japan

AbstractMicrocracks in geomaterials cause variations in the elastic moduli under applied strain, thereby creating seismic wave velocity variations. These are crucial for understanding the dynamic processes of the crust, such as fault‐zone damage, healing, and volcanic activities. Solid earth tides have been used to detect seismic velocity changes responding to crustal‐scale deformations. However, no prior research has explored the characteristics of the seismic velocity variations caused by large‐scale tidal deformation. To systematically evaluate the tidal response to velocity variations, we developed a new method that utilized the flexibility of a state‐space model. The tidal response was derived from hourly stacked noise autocorrelations using a seismic interferometry method throughout Japan. In particular, large tide‐induced seismic velocity changes were observed in the low S‐wave velocity region of the shallow crust. Overall, the tidal responses of velocity variations can provide new insights into the response mechanisms of the shallow crust to applied strain.

Petrophysical Acoustic Characterization of Weathered Granite: A Case Study of Mesozoic Granites in the Coastal Area of Fujian Province, China

In recent years, with the exploration and development of granite buried-hill oil and gas reservoirs, petrophysics research has played an important role in the study of reservoir characteristics and fluid identification. Through analysis of the relationship between the fluid-bearing petrophysical parameters and the reservoir, the seismic response changes caused by reservoir fluid changes can be determined. Mesozoic granites in the coastal area of Fujian Province in eastern China were investigated as the research object of this project. The mineral composition, density, porosity, P-wave velocity, and S-wave velocity of the granite were measured and analyzed by X-ray diffraction, rock density, rock porosity, and rock acoustics methods. Therefore, the granite’s petrophysical properties, fluid response characteristics, and gas sensitivity parameters were analyzed. The result of the study shows that the granite is predominantly monzogranite. According to the type of reservoir space assemblage, the samples can be divided into two types: those containing fracture-dissolution pores and those containing only dissolution pores. All the samples were characterized by medium to high densities and low to extra-low porosity. There was a linear correlation between the P-wave velocity and S-wave velocity under gas and water-saturated conditions. Factors such as P-wave to S-wave velocity ratio, Poisson’s ratio, Lame coefficient, and other parameters of the samples were analyzed, and the threshold values that distinguished the water and gas-saturated states of the samples were measured and determined. In addition, there were negative correlations between the P- and S-wave velocities and porosity. The sensitivities of the petrophysical parameters to the gas capacity from high to low are Ip2 − 2.03 Is2, λ − 0.03 μ, λ, λ/μ, E − 2.03 μ, σ, K/μ, K, Ip, Vp/Vs, Vp, E, μ, Vs, and Is. For granite-buried hill reservoirs, the variation ranges of the parameters, such as the density, porosity, and P-wave velocity, of the fracture-dissolution pore granite samples were larger than those of the dissolution pore samples. The bulk parameters (Ip, Vp, K, λ) and combination parameters (Ip2 − 2.03 Is2, K/μ, λ− 0.03 μ, E − 2.03 μ, λ/μ) of the dissolution pore samples were more sensitive to the gas capacity. The results of this study provide a basis for the geophysical identification of granite-buried hill reservoirs.

Interaction of Air Pressure and Groundwater as Main Cause of Sub‐Daily Relative Seismic Velocity Changes

AbstractOur study investigates variations of seismic velocity that occur over short time scales of hours. We use coda wave interferometry to inspect 5 months of continuous data from a seismological array in southern Germany. This results in relative seismic velocities (dv/v) that show temporal variations on the order of 10−4. Spectra of the velocity time series contain strong daily and sub‐daily behavior indicating that the daily and sub‐daily changes in the seismic velocity are primarily caused by atmospheric tides. We also note the influence of temperature changes on daily variations, but as a second‐order effect. The explanatory model focuses on depth variations of the groundwater table, linking atmospheric pressure (loading and de‐loading the Earth's surface) to variations in seismic velocity. Our results highlight an important environmental influence on seismic velocity that needs to be considered before seismic velocity variations can be used for inspecting fluid and stress variations in situ.

Telemedicine in Vascular Surgery During COVID-19 Pandemic: A Systematic Review and Narrative Synthesis

The COVID-19 pandemic resulted in seismic changes to healthcare service delivery. The use of telemedicine was widely adopted during the pandemic, although its value in the safe care of vascular patients is unknown. A systematic review was undertaken to identify studies that described outcomes or patient/clinician views of telemedicine (telephone or video) services in vascular surgery during or after the pandemic. Two reviewers independently searched medical databases, selected studies, extracted data and undertook a narrative synthesis. Twelve studies were included. Most studies reported increased telemedicine use during the pandemic. Most patients (80.6%-100%) were satisfied with telephone or video consultation. >90% of the patients felt that telemedicine was a good substitute during the pandemic to avoid travelling and reduce transmission risk. Three studies showed patients had a strong preference for continuing telemedicine consultations post-pandemic. Two studies evaluating patients with arterial ulceration and venous diseases reported no significant difference in clinical outcome between patients reviewed face-to-face and those seen remotely. One study showed clinicians preferred face-to-face consultations. No study conducted cost analysis. Patients and clinicians viewed telemedicine favorably as an alternative to face-to-face clinics during the pandemic and included studies did not identify any safety concerns. Its role post-pandemic is not clearly defined, although these data suggest a significant proportion of patients would appreciate, and be suitable for, such consultations in the future.

Deciphering the Whisper of Volcanoes: Monitoring Velocity Changes at Kamchatka's Klyuchevskoy Group With Fluctuating Noise Fields

AbstractVolcanic inflation and deflation often precede eruptions and can lead to seismic velocity changes () in the subsurface. Recently, interferometry on the coda of ambient noise‐cross‐correlation functions yielded encouraging results in detecting these changes at active volcanoes. Here, we analyze seismic data recorded at the Klyuchevskoy Volcanic Group in Kamchatka, Russia, between summer of 2015 and summer of 2016 to study signals related to volcanic activity. However, ubiquitous volcanic tremors introduce distortions in the noise wavefield that cause artifacts in the estimates masking the impact of physical mechanisms. To avoid such instabilities, we propose a new technique called time‐segmented passive image interferometry. In this technique, we employ a hierarchical clustering algorithm to find periods in which the wavefield can be considered stationary. For these periods, we perform separate noise interferometry studies. To further increase the temporal resolution of our results, we use an AI‐driven approach to find stations with similar responses and apply a spatial stack. The impacts of snow load and precipitation dominate the resulting time series, as we demonstrate with the help of a simple model. In February 2016, we observe an abrupt velocity drop due to the M7.2 Zhupanov earthquake. Shortly after, we register a gradual velocity increase of about 0.3% at Bezymianny Volcano coinciding with surface deformation observed using remote sensing techniques. We suggest that the inflation of a shallow reservoir related to the beginning of Bezymianny's 2016/2017 eruptive cycle could have caused this local velocity increase and a decorrelation of the correlation function coda.

The Mid‐Lithospheric Discontinuity Caused by Channel Flow in Proto‐Cratonic Mantle

AbstractGlobal geophysical observations show the presence of the enigmatic mid‐lithospheric discontinuity (MLD) at depths of ca. 80–150 km which may question the stability and internal structure of the continental lithosphere. While various mechanisms may explain the MLD, the dynamic processes leading to the seismic observations are unclear. Here we present a physical mechanism for the origin of MLD by channel flow in the cratonic mantle lithosphere, triggered by convective instabilities at cratonic margins in the Archean when the mantle was hot. Our numerical modeling shows that the top of the frozen‐in channel flow creates a shear zone at a depth comparable to the globally observed seismic MLD. Grain size reduction in the shear zone and accumulation of percolated melts or fluids along the channel top may reduce seismic wave speeds as observed below the MLD, while the channel flow itself may explain radial anisotropy of seismic wave speeds and change in direction of the seismic anisotropic fast axis. The proposed mechanism is valid for a broad range of physically realistic parameters and that MLD may have been preserved since its formation in the Archean. The intensity of the channel flow ceased with time due to secular cooling of the Earth's interior. The new mechanism may reshape our understanding of the evolution and stability of cratonic lithosphere.

Capturing seismic velocity changes in receiver functions with optimal transport

SUMMARY Temporal changes in seismic velocities are an important tool for tracking structural changes within the crust during transient deformation. Although many geophysical processes span the crust, including volcanic unrest and large-magnitude earthquakes, existing methods for seismic monitoring are limited to the shallow subsurface. We present an approach for deep seismic monitoring based on teleseismic receiver functions, which illuminate the crustal velocity structure from the bottom-up. Using synthetic waveform modelling, we show that receiver functions are uniformly sensitive to velocity changes throughout the crust and can locate the depth of the perturbation. We introduce a novel method based on optimal transport for measuring the non-linear time–amplitude signal variations characteristic of receiver function monitoring. We show that optimal transport enables comparison of full waveform distributions rather than relying on representative stacked waveforms. We further study a linearized version of optimal transport that renders time-warping signal variations into simple Euclidean perturbations, and use this capability to perform blind source separation in the space of waveform variations. This disentangles the effects of changes in the source–receiver path from changes in subsurface velocities. Collectively, these methods extend the reach of seismic monitoring to deep geophysical processes, and provide a tool that can be used to study heterogeneous velocity changes with different spatial extents and temporal dynamics.