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The thermochemical nature of the lower mantle, and the two Large Low Shear-wave Velocity Provinces (LLSVPs) in particular, remains a topic of active debate. Exclusively imaging seismic velocity anomalies only provides limited ability to distinguish between thermal and compositional origins. Here we use whole Earth oscillations, or normal modes, to study 3D variations in mantle shear wave- (vs), compressional wave- (vp) and bulk sound velocity (vc), density (rho) and shear attenuation (qmu). These observations provide new constraints on the presence of lower mantle chemical heterogeneity. Here, we compare the two-step splitting function inversion method to the less frequently used, computationally more expensive one-step direct spectrum inversion. In theory, the one-step inversion suffers less from non-uniqueness and only requires regularization once. In practice, we find that the average spectral misfits for the one-step inversion are always lower. The ratio between vs and vp anomalies obtained from their joint inversion, proposed to be an indicator of chemical heterogeneity when exceeding a threshold predicted by mineral physics, varies significantly between the two inversion methods. The method of computing the ratio is just as important. We obtain ratios exceeding the threshold in the lower mantle only when dividing the root mean square amplitudes of our vs and vp models, although lower than some previous studies suggest. However, by taking the median ratio from a grid at each depth, we barely exceed the threshold. Instead of relying on these 1D representatives of the ratio, we infer chemical heterogeneity in certain depth ranges based on a wide spread in distributions of vs, vp anomalies and their ratio. Another constraint on the presence of chemical heterogeneity comes from the anti-correlation of vs and vc structure, found in many previous studies. We find (de-)correlation in the lower mantle in joint vs and vc inversions, with only slightly negative correlation for the two-step inversion. We shift towards more negative correlation values when extracting vc from our previously obtained vs and vp models. Lower mantle density structure has remained elusive and controversial in recent decades. In joint inversions for vs, vp, rho and discontinuity topography, we show that a basal layer of excess density is located underneath part of the otherwise lighter-than-average LLSVPs, reconciling previous observations. This dense layer is more robust in the one-step inversion and does not result from high-amplitude ghost patterns that may have plagued earlier normal mode studies. Compositional variations such as iron enrichment may explain this dense but seismically slow layer. Finally, we obtain preliminary results for imaging lower mantle shear attenuation in joint vs and qmu one-step inversions. We first select the best way of inverting for 3D qmu in synthetic tests, before applying this method to real data. Mode selection appears to be crucial, and we also need to properly account for detailed elastic structure to minimize leakage into anelastic structure. The preliminary 3D qmu model shows low attenuation in LLSVPs and high attenuation in the surrounding lower mantle, possibly indicating a dominant role for grain size.

The article is devoted to the «tomographic» approach to the interpretation of geopotential fields. A brief excursus into the history of the development of interpretational (pseudo-) tomography is given. General issues concerning the creation of «effective» tomographic models are considered. Transformations based on the equivalent source technique are proposed as a key part of the tomography. This type of transformations permits the usage of elevation data of magnetic survey as input. When constructing an analytical approximation, usually a system of linear algebraic equations (SLAE) with an approximated right part is solved. Using a model example, some issues concerning the conditionality of SLAE for different depths of equivalent sources are studied. Two iterative methods for the solving of systems, the Seidel method and the quickest gradient descent, are compared. It is shown that the latter method makes it possible to achieve the required accuracy of the field approximation with a smaller number of iterations. The advantage in the calculation speed, when the matrix of coefficients of the transform operator (source function) is sparce, is emphasized. An example of using this approach to interpret magnetic field anomalies above the submarine volcano 3.18 within the Kuril Iceland arc is presented.

Reconstructing the details of subsurface structures deep beneath complex overburden structures, such as subsalt, remains a challenge for seismic imaging. Over the past few years, the Marchenko redatuming approach has proven to reliably retrieve full-wavefield information in the presence of complex overburden effects. When used for redatuming, current practical Marchenko schemes cannot make use of a priori subsurface models with sharp contrasts because of their requirements regarding initial focusing functions, which for sufficiently complex media can result in redatumed fields with significant waveform inaccuracies. Using a scattering framework, we evaluate an alternative form of the Marchenko representation that aims at retrieving only the unknown perturbations to focusing functions and redatumed fields. From this framework, we have developed a two-step practical focusing-based redatuming scheme that first solves an inverse problem for the background focusing functions, which are then used to estimate the perturbations to focusing functions and redatumed fields. In our scheme, initial focusing functions are significantly different from previous approaches because they contain complex waveforms encoding the full transmission response of the a priori model. Our goal is the handling of not only highly complex media but also realistic data — band-limited, unevenly sampled, free-surface-multiple contaminated data. To that end, we combine the versatility of Rayleigh-Marchenko redatuming with our scattering-based scheme allowing an extended version of the method able to handle single-sided band-limited multicomponent data. This scattering-Rayleigh-Marchenko strategy accurately retrieves wavefields while requiring minimum preprocessing of the data. In support of the new methods, we evaluate a comprehensive set of numerical tests using a complex 2D subsalt model. Our numerical results indicate that the scattering approaches retrieve accurate redatumed fields that appropriately account for the complexity of the a priori model. We find that the improvements in wavefield retrieval translate into measurable improvements in our subsalt images.

We present a novel approach for imaging global mantle discontinuities based on full-waveform inversion (FWI). Over the past decades, extensive research has been done on imaging mantle discontinuities at approximately 400 km and 670 km depth. Accurate knowledge of their topography can put strong constraints on thermal and compositional variations and hence geodynamic modelling. So far, however, there is little consensus on their topography. We present an approach based on adjoint tomography, which has the advantage that Fréchet derivatives for discontinuities and measurements, to be inverted for, are fully consistent. Rather than working with real data, we focus on synthetic tests, where the answer is known in order to be able to evaluate the performance of the developed method. All calculations are based on the community code SPECFEM3D_GLOBE. We generate data in fixed 1-D or 3-D elastic background models of mantle velocity. Our ‘data’ to be inverted contain topography along the 400 km and 670 km mantle discontinuities. To investigate the approach, we perform several tests: (i) In a situation where we know the elastic background model 1-D or 3-D, we recover the target topography fast and accurately, (ii) The exact misfit is not of great importance here, except in terms of convergence speed, similar to a different inverse algorithm, (iii) In a situation where the background model is not known, the convergence is markedly slower, but there is reasonable convergence towards the correct target model of discontinuity topography. It has to be noted that our synthetic test is idealised and in a real data situation, the convergence to and uncertainty of the inferred model is bound to be larger. However, the use of data consistent with Fréchet kernels seems to pay off and might improve our consensus on the nature of mantle discontinuities. Our workflow could be incorporated in future FWI mantle models to adequately infer boundary interface topography., Geophysical Journal International, 226 (3), ISSN:0956-540X, ISSN:1365-246X

The Groningen gas field in the Netherlands is an ideal test bed for in-situ reservoir monitoring techniques because of the availability of both active and passive in-reservoir seismic data. In this study, we use deconvolution interferometry to estimate the reflection and transmission response using active and passive borehole data within the reservoir at ∼3-km depth and separate up- and downgoing P- and S-wave fields by f-k filtering. We validate the results using synthetic data of a 1D elastic model built from sonic logs recorded in the well. The estimated full-waveform reflection response for a virtual source at the top geophone is consistent with the synthetic response. For the virtual source at the bottom geophone, the reflection response appears to be phase delayed, though its arrivals are consistent with the local subsurface geology. Similarly, the first-order estimated local transmission response successfully approximates that of the P-wave velocity in the reservoir. The study shows that reliable subsurface information can be obtained from borehole interferometry without detailed knowledge of the medium parameters. In addition, the method could be used for passive reservoir monitoring to detect velocity, attenuation, and/or interface time-lapse variations.

Body wave observations of the Earth's inner core show that it contains strong seismic heterogeneity, both laterally and radially. Models of inner core structure generated using body wave data are often limited by their parameterisation. Thus, it is difficult to determine whether features such as anisotropic hemispheres or an innermost inner core truly exist with their simple shapes, or result only from the chosen parameterisation and are in fact more complex features. To overcome this limitation, we conduct seismic tomography using transdimensional Markov Chain Monte Carlo on a high quality dataset of 5296 differential and 2344 absolute P-wave travel times. In a transdimensional approach, the data defines the model space parameterisation, providing us with both the mean value of each model parameter and its probability distribution, allowing us to identify well versus poorly constrained regions. We robustly recover many first order observations found in previous studies without the imposition of a priori fixed geometry including an isotropic top layer (with anisotropy less than 1%) which is between 60 and 170 km thick, and separated into hemispheres with a slow west and a faster east. Strong anisotropy (with a maximum of 7.2%) is found mainly in the west, with much weaker anisotropy in the east. We observe for the first time that the western anisotropic zone is largely confined to the northern hemisphere, a property which would not be recognised in models assuming a simple hemispherical parameterisation. We further find that the inner most inner core, in which the slowest anisotropic velocity direction is tilted relative to Earth's axis of rotation (ζ=55∘±16∘), is offset by 400 km from the centre of the inner core and is restricted to the eastern hemisphere. We propose that this anomalous anisotropy might indicate the presence of a different phase of iron (either bcc or fcc) compared to the rest of the inner core (hcp). Published version This research was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 681535 - ATUNE) and a Vici award number 016.160.310/526 from the Netherlands Organisation for Scientific Research (NWO). LW acknowledges support from a Discovery Early Career Research Award (project number DE170100329), funded by the Australian Government.

Volcano Young Sheveluch is one of the most active volcanoes in Kamchatka. Since August 1980 to the present time, an extrusive dome has been growing in the crater of the volcano. Plastic lavas in the form of large ribbons were noted on the dome already in 1980–1981, the first lava flow was observed after a paroxysmal eruption on May 9, 2004. The next large block of lava started to extrude in the end of April – beginning of May 2020. By June, 11th, it rose above the dome on 50–80 m, and up to 120 m by the end of October. On September 28, it was noted that the lava block acquired smooth surfaces. Portions of lava squeezed out from inside of the volcano in August – September were plastic. By December 8, the block had collapsed. In February 2021, a new block of plastic lava began to rise from the destroyed block. In March its height above the dome exceeded 50–60 m, and 200 m in June.

By studying the processes taking place at the operating geothermal stations it is possible to model and understand some features of formation of minerals of geothermal origin, processes of transfer by hydrothermal solutions and concentration of some metals by analogy with the ore-forming processes at the formed deposits. During the release of steam-water mixture and outpouring of water from the wells silica gel is deposited, a number of elements (including noble metals) migrate and accumulate in the sediments. A great role during exploitation of wells, operation of turbines, separators and other structural elements of geothermal stations is played by the chemical composition of the coolant. Because of its high mineralization deposition on various units and parts of equipment occurs, what may complicate the operation of a geothermal plant. We describe the composition of chemogenic deposits on various technological elements of the Mutnovskaya geothermal power plant. Data on sediments sampled in boreholes, on turbine blades and separator are presented. We have revealed a wide variety of such deposits, both oxide compounds and ore phases (from magnetite, chalcopyrite, minerals of faint ore group to native gold, tellurides, selenides and sulfides of gold and silver). We suppose that the processes currently taking place at the Mutnovskoye steam hydrothermal field are similar to the hydrothermal processes that contributed to the formation of vein zones of the gold-silver-polymetallic deposit of the same name.

This report provides a brief description of the field work on the Kuril Islands. It was performed within the framework of the R&D theme, projects of the RSF and the RFFR, which are realized in the laboratory of postmagmatic processes of the Institute of Volcanology and Seismology FEB RAS. Hydrological and hydrochemical works were performed on the rivers draining the slopes and thermal fields of the Sinarka, Kuntomintar volcanic massifs (Shiashkotan Island), and the Vernadsky and Karpinsky Ridges (Paramushir Island). The study of the chemical erosion of volcanic islands and the assessment of the hydrothermal export of magmatic volatiles are the goals of this work. Infrared photography was taken and the total flux of volcanic SO2 and diffusion flux of CO2 were measured on thermal fields in the caldera of Golovnin volcano. A detailed hydrogeochemical survey was made on the thermal fields of the Ebeko volcano to study the relationship of volcanic and hydrothermal activity of the volcano. For further analytical work, a large number of water and gas samples were taken and a representative collection of rocks and sediments was collected during the expedition.

To establish a consistent framework for seismic wave propagation that accommodates the effects of stress changes, it is critical to take into account the different definitions of stress and their corresponding effects on seismic quantities (e.g., wave speeds) as dictated by continuum mechanics. Revisiting this fundamental theoretical foundation, we first emphasize the role of stress within various forms of the wave equation resulting from different choices of stress definitions. Subsequently, using this basis, we investigate connections among existing theories that describe the variation of elastic moduli as a function of changes in stress. We find that there is a direct connection between predicting stress-induced elastic changes with the well-known third-order elasticity tensor and the recently proposed adiabatic pressure derivatives of elastic moduli. However, each of these approaches has different merits and drawbacks in terms of experimental validation as well as in their use. In addition, we investigate the connection with another general approach that relies on micromechanical structures (e.g., cracks and pores). Although this can be done algebraically, it remains unclear as to which definition of stress and which corresponding constitutive relationship should be considered in practical scenarios. We support our analysis with validations using previously published benchmark experimental data.

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

The results of monitoring at high-temperature fumarolic fields (Eastern and Western) of Avachinsky volcano in 2013–2020 are presented. Those were visual observations, temperature measurement and gas sampling, supplemented with video surveillance data. In order to identify the relationship between fumarolic activity and deep processes, the analysis of the volcano area seismicity for the specified period was carried out. Two periods of increased seismic activity have been identified. The first period, November 2014 – January 2015, is associated with changes in the character of gas emission, morphology and temperature of Western Fumarole observed in the field, reflected in the ratio of gas fluxes of the two high-temperature fumarolic fields. The second seismic increase, in October – December 2019, was accompanied in the dark by glowing, sometimes by bright flashes in the area of Western Fumarole, and as a result of ground survey in 2020 further changes in its morphology were revealed.

Growth of explosive activity of Ebeko volcano in April–June 2020

The paper presents the results of integrated geophysical observations of the Ebeko Volcano’s activity in late 2018 and early 2019. The instrument complex for observation was located at the Severo-Kurilsk seismic station at a distance of 7.2 km far from the volcanic crater. Three types of response in the vertical component of the electric field of the atmosphere have been distinguished during the drift of eruptive clouds from the Ebeko Volcano’s explosions, which gives evidence for various mechanisms of their occurrence. The registration of infrasonic acoustic signals in the near zone made it possible to estimate the trinitrotoluene equivalent for the strongest explosions, which was calculated to be about 100 kg of trinitrotoluene. The authors have revealed certain regularities in the dynamics of the volumetric activity of radon, associated with variations in meteorological values and hydrological features of the registration point. A long period of its activity and the proximity of the observation point to the crater give reason to suggest Ebeko Volcano to be a natural laboratory for studying the mechanisms of eruptions.

Isotope-geochemical material for Pliocene-Quaternary volcanoes of the Kamchatka region is generalized on a cartographic basis. The Sr-isotope anomalies of moderate and elevated radiogenicity, geochemically confirmed, are spatially conjugated. This made it possible to interpret these anomalies not only as a reflection of mantle plume material in the composition of volcanic rocks, but also of its hybrid environment, as a consequence of plum-lithosphere remobilization. The presence of multi-directional geochemical trends made it possible to propose the concept of moving boundary values for the composition of indicator rocks of the intraplate type and adakites, which significantly expanded the possibilities of their diagnostics. The isotope-geochemical heterogeneity of basaltoids of the region is generally determined by the peculiarities of concentration of rocks with intraplate and adakite geochemical characteristics, which allows considering the asthenospheric diapirism as the main factor of petrogenesis of Pliocene-Quaternary volcanism in Kamchatka.

In the area of the city of Petropavlovsk-Kamchatsky, field work was carried out, which included routes through hard-to-reach bays along the coastal cliffs of Avachinsky Gulf. Agates and chalcedony were collected in a boulder and pebble placer southwest of Cape Vertikalny. Their source is volcanic strata of the Zavoikovsky Complex of the Miocene age. Ornamental stones are observed among the pebbles and boulders of volcanics. Gray monochromatic chalcedony and low-contrast agates prevail among them, but there are also agates with a contrasting and complex pattern, as well as «moss» and «landscape» agates. The length of the coastal placer, separated by rocky cliffs, is about 2 km. Some agates have inclusions of native copper, from a fraction of mm and rarely up to 5 mm in size, which is of interest for mineral collectors.

Field work in 2020 on the Alaid volcano located on the Atlasov Island in the Kuril island arc was carried out in the coastal zone from Cape Devyatka to Cape Kudryavy. Base camp was located in the site of the former fishing base Devyatka. At the time of the research, the Alaid volcano performed only fumarolic activity. We sampled rocks and fresh water. We also sampled copper, found, as in other slopes of Alaid Volcano, in cracks or in a form of solid covers. We also measured magnetic susceptibility of rocks in their natural manifestations. The unexpected but important outcome of field work was the discovery of a new colony of snails from the Bradybaenidae family, what allows us to confidently say that these snails have survived the eruptions of Alaid Volcano in the XX and XXI centuries.

We present a 3D shear-wave velocity model of the southern African upper mantle developed using 30–200 s period Rayleigh waves recorded on regional seismic networks spanning the subcontinent. The model shows high velocities (∼4.7–4.8 km/s) at depths of 50–250 km beneath the Archean nucleus and several surrounding Paleoproterozoic and Mesoproterozoic terranes, placing the margin of the greater Kalahari Craton along the southern boundary of the Damara Belt and the eastern boundaries of the Gariep and Namaqua-Natal belts. At depths ≥250 km, there is little difference in velocities beneath the craton and off-craton regions, suggesting that the cratonic lithosphere extends to depths of about 200–250 km. Upper mantle velocities beneath uplifted areas of southern Africa are higher than the global average and significantly higher than beneath eastern Africa, indicating there that is little thermal modification of the upper mantle present today beneath the Southern African Plateau.

The Marchenko integral, key to inverse scattering problems across many disciplines, is a long-standing equation that relates single-sided reflection data and Green's functions for virtual source locations inside of an inaccessible, one-dimensional volume. The concept was later expanded to two and three dimensions, yielding important advances in imaging complex media, particularly in the context of geophysics. However, this expansion is based on a set of coupled Marchenko equations which requires up and down decomposition of the wave fields at both the level of the measurement surface and the level of the virtual source of the desired Green's function. The underlying theory implies that the recently developed Marchenko relations, while enabling novel applications, carry intrinsic limitations. For example, this scheme cannot incorporate evanescent or refracted waves, and in turn practical implementations must discard data to meet such requirements. We present a derivation that circumvents these limitations, thereby yielding a Marchenko integral akin to those in recent advances, but that is more general than previously assumed. We set up a wave equation based framework to describe the physical concept of focusing functions by introducing homogeneous Green's functions of the second kind. Based on this, we derive integral representations for both closed and open boundary volumes. Owing to our perspective on the integral formalism, we present an inverse scattering approach for retrieving Green's functions from single-sided reflection data—with the same practical applicability of recent methods but without any limitations due to one-way decomposition. Finally, we illustrate the capability of the Marchenko method to obtain the full wave field, including evanescent and refracted waves, within an unknown scattering medium by means of a numerical example.

International audience; A new submarine volcano has been discovered offshore Mayotte, a part of the Comoros volcanic archipelago located between Africa and Madagascar. The edifice arose from the sea-floor following a seismo-volcanic crisis that started in May 2018. This seismo-volcanic activity highlights very deep magma reservoirs and dykes in the East Mayotte volcanic system. Since the crisis, the region has experienced >2000 earthquakes with magnitude ≥3.5 and activity continues today (August 17, 2021). The earthquakes are unusually deep and distributed into two swarms: one 5–15 km east of Petite-Terre at 25–55 km depth and a second 25 km away at 30–50 km depth. Significant subsidence of Mayotte to the East has been assigned to the drainage of a deep magma chamber, inferred to be located 30 km from the coast. However, at present, the earthquake locations and geodetic observations have not been sufficient to image entirely the structure of the volcanic plumbing system. In this study, we construct Vp, Vs, dVp, dVs and Vp/Vs 3D velocity models to assess the deeper structure of the young volcano plumbing system, offshore and East of Mayotte. Using >3000 earthquakes from an ongoing monitoring effort, and a 1D velocity model determined onboard, we jointly inverted for velocity structures, earthquake locations, origin times, and station corrections using LOTOS software. The calculated 3D velocity models highlight a complex volcanic system down to 40 km depth. Specifically, we image 3 interpreted reservoirs, more or less consolidated/old. The main reservoir is located at about 30 km depth and deeper, making it one of the deepest magmatic chamber imaged. The reservoirs are connected by several old crystallized conduits, whose existence could have been influenced by the presence of an old fracture zone, globally oriented N130°, due to a regional strike-slip motion of the lithosphere. Moreover, gas-saturated rock may be present below the currently degassing Horse Shoe structure. We were unable to image connections between the new volcanic edifice and reservoirs or conduits due to a lack of resolution in that part of the study area.

The European Plate Observing System - Netherlands (EPOS-NL) is the Dutch research infrastructure for solid Earth sciences. EPOS-NL is a cluster of large-scale geophysical facilities for research on georesources and geohazards. It is a partnership between Delft University of Technology (TU Delft), the Royal Netherlands Meteorological Institute (KNMI) and Utrecht University (UU) and is funded by NWO, as part of the national roadmap for large-scale research infrastructure. EPOS-NL facilities include 1) The Earth Simulation Lab at UU, 2) The Groningen gas field seismological network and the ORFEUS Data Centre at KNMI, 3) The deep geothermal (DAP-)well to be installed on the TU Delft campus, and 4) A distributed facility for multi-scale imaging and tomography (MINT) at UU and TU Delft. EPOS-NL aims to further develop the infrastructure for solid Earth scientific research. It also makes cutting-edge research facilities and data available to (inter)national researchers, aiming to address key geo-societal challenges, notably: • Exploration for (renewable) geo-energy resources • Storage of fuels, CO2 and wastewater in the sub-surface, and • Hazards such as induced or natural earthquakes Addressing these challenges requires a multi-physics, multi-scale approach, and open access to state-of-the-art research facilities and data. EPOS-NL contributes to addressing these needs.

Marchenko methods can retrieve Green’s functions and focusing functions from single-sided reflection data and a smooth velocity model, as essential components of a redatuming process. Recent studies also indicate that a modified Marchenko scheme can reconstruct primary-only reflection responses directly from reflection data without requiring a priori model information. To provide insight into the artifacts that arise when input data are not ideally sampled, we study the effects of subsampling in both types of Marchenko methods in 2D earth and data — by analyzing the behavior of Marchenko-based results on synthetic data subsampled in sources or receivers. With a layered model, we find that for Marchenko redatuming, subsampling effects jointly depend on the choice of integration variable and the subsampling dimension, originated from the integrand gather in the multidimensional convolution process. When reflection data are subsampled in a single dimension, integrating on the other yields spatial gaps together with artifacts, whereas integrating on the subsampled dimension produces aliasing artifacts but without spatial gaps. Our complex subsalt model indicates that the subsampling may lead to very strong artifacts, which can be further complicated by having limited apertures. For Marchenko-based primary estimation (MPE), subsampling below a certain fraction of the fully sampled data can cause MPE iterations to diverge, which can be mitigated to some extent by using more robust iterative solvers, such as least-squares QR. Our results, covering redatuming and primary estimation in a range of subsampling scenarios, provide insights that can inform acquisition sampling choices as well as processing parameterization and quality control, e.g., to set up appropriate data filters and scaling to accommodate the effects of dipole fields, or to help ensuring that the data interpolation achieves the desired levels of reconstruction quality that minimize subsampling artifacts in Marchenko-derived fields and images.

Probabilistic seismic hazard analysis (PSHA) is the standard method used for designing earthquake-resistant infrastructure. In recent years, several unexpected and destructive earthquakes have sparked criticism of the PSHA methodology. The seismological part of the problem is the true frequency-magnitude distribution of regional seismicity. Two major models exist, the Gutenberg-Richter (G-R) and the Characteristic Earthquake (CE) model, but it is difficult to choose between them. That is because the instrumental, historical, and paleoseimological data available are limited in many regions of interest. Here, we demonstrate how a friction experiment on aggregates of glass beads can produce both regular (CE equivalent) and irregular (G-R equivalent) stick-slip. Using a new rotary shear apparatus we produced and analyzed large catalogs of acoustic emission (AE) events related to stick-slip. The distributions of AE sizes, interevent times, and interevent distances were found to be sensitive to particle size and the applied normal stress, and, to a lesser degree, the stiffness of the loading apparatus. More importantly, the system spontaneously switched behavior for short periods of time. In the context of PSHA, if faults are able to switch behavior as our experimental system does, then justifying the choice of either the CE or the G-R model is impossible based on existing observations.