Permeability is a key rock property important for scientific applications that require simulation of fluid flow. Although permeability is determined using core flooding experiments, recent advancements in micro-CT imaging and pore scale fluid flow simulations have made it possible to constrain permeability honoring pore scale rock structure. Previous studies have reported that complex association of pores and solid grains often results in preferential flow paths which influence the resulting velocity field and, hence, the upscaled permeability value. Additionally, the pore structure may change due to geochemical processes such as microbial growth, mineral precipitation and dissolution. This could result in a flow field which dynamically evolves spatially and temporally. It would require numerous experiments or full physics simulations to determine the resultant upscaled Darcy permeability for such dynamically changing systems. This study presents a graph theory-based approach to upscale permeability from pore-to-Darcy scale for changing pore structure. The method involves transforming a given micro-CT rock image to a graph network map followed by the identification of the least resistance path between the inlet and the outlet faces using Dijkstra’s algorithm where resistance is quantified as a function of pore sizes. The least resistance path is equivalent to the path of lowest resistance within the domain. The method was tested on micro-CT images of the samples of Sherwood Sandstone, Ketton Limestone and Estaillades Limestone. The three micro-CT images were used to generate 30 synthetic scenarios for geochemically induced pore structure changes covering a range of pore and solid grain growth. The least resistance value obtained from Dijkstra’s algorithm was observed to correlate with upscaled permeability value determined from full physics simulations, while improving computational efficiency by a factor of 250. This provides confidence in using graph theory method as a proxy for full physics simulations for determining effective permeability for samples with changing pore structure.

We undertook the design, development, and operation of a two-scale high-performance seismic network in SE Victoria to establish a baseline for natural seismicity around CarbonNet's Pelican Site. This is particularly important for providing assurance of the safety of the CO2 storage space and discriminating between natural and induced seismicity. The new high-precision earthquake catalogue developed from this work demonstrates a significant improvement in monitoring passive seismicity; a 77% average increase in annual event detections relative to pre-2017, magnitude of completeness lowered from 1.2 to 0.7, a minimum detection magnitude of-0.5, and location errors of & LE;-1 km within the network. Using the new earthquake catalogue, we also discovered the hitherto unknown deepest seismogenic zone in Australia in the offshore Gippsland Basin, with event depths exceeding 25 km. The spatial distribution of earthquakes indicates no seismic activity within a-10 km radius around the Pelican Site, providing confidence in its selection for potential CCS development. Together with the spatial distribution of earthquakes, newly estimated and more robust earthquake magnitude-frequency parameters enable the discrimination of natural versus induced seismicity with greater confidence. Continuous monitoring is recommended to overcome the limitations of the short monitoring time window.

ABSTRACT Understanding the modulatory influence of fault-scale geology on seismic behavior of earthquake faults is central to determining the physics of faulting and seismic hazard analysis. Although laboratory experiments predict that seismic parameters can be modulated by fault-scale geology, there is scant empirical evidence of this process at field scale due largely to a lack of shallow earthquakes of which causative faults can be mapped to known bedrock structure. The 20 May 2016 Mw 6 Petermann Ranges earthquake (PRE) is the best-recorded continental event in Australia to date, and it is an excellent candidate to investigate the possible link between seismic parameters and fault-scale geology as its causative fault has previously been linked to known bedrock structure using distributions of aftershocks, surface observations, and geophysical mapping. In this study, we analyze strain energy partitioning of PRE by determining seismic radiation efficiency (0.31) and apparent stress (0.34 MPa) together with previously estimated stress drop (2.2 MPa) and find that the combination of these macroseismic parameters deviates from that expected of a shallow immature fault in intraplate continental regions typically characterized by large recurrence intervals. It instead appears to have mimicked a mature fault, which we attribute to the characteristics of the causative fault confined to mechanically weaker, phyllosilicate-rich foliations of the bedrock that have anomalously lower fracture energy. Therefore, PRE rupture suggests the presence of a spectrum of shallow (<20 km) fault slip behavior modulated by fault-scale geology.

An anomalous seismic sequence of five small (MW < 3) felt earthquakes occurred between 29 August 2020 and 05 December 2020 around the Victoria Reservoir in the central highlands of Sri Lanka that clearly exceeded the established national background seismic rate. Using seismic waveform template-matching and a newly developed single-station earthquake location method based on travel-time back-projection, we detected an additional co-located 23 microseismic events, of which 18 occurred within the same period as the felt events. This hitherto undetected seismic swarm defines a seismogenic zone beneath the western flank of the reservoir between 1.5 and 3 km depths. The reservoir-induced peak stresses, resolved on E-W striking faults, predicted from the poroelastic theory that include both drained and undrained crustal responses are ~ 15 kPa in an area overlapping the seismogenic zone, which, together with the physical and spatio-temporal characteristics of the seismic swarm, establish a causal link between reservoir-induced stresses and the earthquake swarm with implications to seismic hazard. This is the first record of induced seismicity in Sri Lanka. The newly developed efficient computational workflows with minimal operational costs described in our study provide a blueprint for monitoring reservoir-induced seismicity in developing countries with severe resource limitations.

The CarbonNet Project is operated by the state of Victoria, and jointly funded and managed by State and Federal governments. The Pelican storage site is situated in the shallow marine environment, between 1 to 15 km offshore from the iconic Ninety Mile Beach in Victoria, Australia. The site has been appraised to demonstrate safe and environmentally-benign permanent storage of CO2 at commercial scale. A 125 million tonne storage site has been confirmed using new high-resolution 3D seismic data covering the 3D volume of future plume travel. A new appraisal well (data well) has confirmed pressure isolation across the main seal interval, and excellent sandstone reservoirs for storage. Open-file petroleum data consists of over 1,500 exploration and development wells in this basin and “wall-to-wall” 3D seismic data offshore, supplemented by extensive onshore 2D seismic data. In addition, numerous onshore brown coal boreholes allow an excellent onshore-offshore stratigraphic correlation and detailed interpretation of depositional environments and facies geometries.The topseal at the Pelican site is a sequence of intraformational shales with interbedded coals and minor sands/silts. These are proven by nearby hydrocarbon traps and by the pressure and salinity data from numerous local wells, including the recent appraisal well, Gular-1. The state of geomechanical stress is well-characterised by the well. The excellent reservoirs are supported by a world-class aquifer with strong pressure buffering and dissipation capacity which will limit any local pressure increase due to injection. Aquifer performance is quantified by basin-wide pressure responses to a 60-year production history in this prolific petroleum basin.This shallow-water marine site offers unique monitoring challenges in terms of ecological sensitivity, operational visibility, and a high-energy marine environment. These challenges strongly affect the choices of effective monitoring technologies and the CarbonNet project has participated closely in field trials of a suite of surface, water column, atmospheric, and subsurface sensing technologies in a range of projects conducted by the Commonwealth Scientific and Industrial Research Organization (CSIRO), and leading Australian Universities. These projects have been commissioned and managed by ANLEC R&D, with operational and permitting co-ordination by the CO2CRC.Marine projects have allowed detailed characterization of shallow coastal marine environment the nature of the modern seabed and its sediment transport processes with extensive marine surveys which have included; seafloor mapping, diver and Remotely Operated Vehicles (ROV) faunal sampling, water column bio-acoustics characterization and sea surface/atmospheric chemistry sampling. These surveys have been augmented by data collected moored surface buoys, seabed landers, and unmanned surface Saildrone vehicles (USVs). As a result of these studies significant advance has been made in understanding the dynamics and ecology of the local seabed in the coastal waters overlying the Pelican site.Seabed conditions during storm events were extreme and some of the instruments have required redesign to improve reliability, given sediment turbidity and strong currents. In-field sensors and data download systems have been validated and sensor platform deployments tested. The key challenge remains in detection of moderate to small signals within a very well-mixed marine water body, that already contains natural variation including the interaction of waters from the Bass Strait and Tasman Sea. The results of these studies are helping identify a subset of suitable technologies for future marine monitoring and verification activities. .Seismology projects are aimed at improved signal detection and location accuracy for low-energy natural events and offering insights into future seismic design for an offshore storage site. Trials have extended the existing onshore hard-rock seismometer network to a closer network of coastal locations, often in traditionally sub-optimal substrates. Some offshore bedrock islands have proven to be excellent sites. Trials of a shallow-water OBS have also been completed with significant learnings for deployment, recovery, and data analysis. Improved methods for data analysis show that although coastal noise levels are higher than at inland sites, the noise is well-characterised and the signal:noise level is acceptable. Both location accuracy and completeness magnitude have been improved – the latter to ML 0.5 in the entire local area, around ML 0.2 at the proposed storage site, and as low as ML -0.5 in favourable locations. This network is now one of the best continuously monitoring regional passive seismic network in operation in Australia. New insights into the spatial distribution of seismicity are emerging. Most events are onshore and associated with Proterozoic/Palaeozoic basement contacts. A few offshore events are smaller in magnitude and associated with known east-west basement fault trends, principally at the margins of the Gippsland Basin. No events have been recorded within at least a 5 km radius from the prospective storage site in the last 18 months. Future improvements in detection at the storage site will require downhole sensors, perhaps using DAS fibre.Atmospheric projects have aimed to trial technologies for measurement of potential offshore sources of CO2, from an onshore sensor position. Developments in instrumentation have been significant with open-path FTIR ranges increased from a few hundred metres to 1.5 km, and methods developed to accurately calibrate these long paths using mobile instruments and “air cores” (coiled sampling tubes that are steadily filled from one end as the sampler transects the optical path). Instrumentation limits are not the constraining factor for this site.The uncertainties induced by other fluxes in the region make it difficult to detect signals from the injection site, given restrictions on sampling geometry and meteorology. In tests, direct detection of known signals is adequate, but we are unlikely to characterise external sources well enough to allow reliable detection. Signal detection is limited by the intrinsic atmospheric variability (including external sources) and our ability to represent that on relevant scales. We therefore do not recommend this technique as a first-line monitoring system for this project. Methods allowing sampling closer to the injection site would likely allow reliable detection and these may include the sail drones trialled by CSIRO in the marine monitoring project.In summary, the range of atmospheric, marine, and subsurface technologies that has been tested and validated for pre-commercial use at this shallow-water location has laid the foundation for a viable and cost-effective monitoring program to be developed. The MMV program will offers useful insights into storage security and provide assurance to stakeholders that appropriate environmental measurements are being recorded and proactive management of storage risks is ongoing.

Detecting and locating earthquakes relies on seismic events being recorded by a number of deployed seismometers. To detect earthquakes effectively and accurately, seismologists must design and install a network of seismometers that can capture small seismic events in the sub-surface. A major challenge when deploying an array of seismometers (seismic array) is predicting the smallest earthquake that could be detected and located by that network. Varying the spacing and number of seismometers dramatically affects network sensitivity and location precision and is very important when researchers are investigating small-magnitude local earthquakes. For cost reasons, it is important to optimise network design before deploying seismometers in the field. In doing so, seismologists must accurately account for parameters such as station locations, site-specific noise levels, earthquake source parameters, seismic velocity and attenuation in the wave propagation medium, signal-to-noise ratios, and the minimum number of stations required to compute high-quality locations. AuScope AVRE Engage Program team has worked with researchers from the seismology team at the University of Melbourne to better understand their solution for optimising seismic array design to date: an analytical method called SENSI that has been developed by Tramelli et al. (2013) to design seismic networks, including the GipNet array deployed to monitor seismicity in the Gippsland region in Victoria, Australia. The underlying physics and mechanics of the method are straightforward, and when applied sensibly, can be used as a basis for the design of seismic networks anywhere in the world. Our engineers have built an application leveraging a previously developed Geophysical Processing Toolkit (GPT) as an application platform and harnessed the scalability of a Cloud environment provided by the EASI Hub, which minimised the overall development time. The GPT application platform provided the groundwork for a web-based application interface and enabled interactive visualisations to facilitate human-computer interaction and experimentation.

ABSTRACT The 20 May 2016 surface-rupturing intraplate earthquake in the Petermann Ranges is the largest onshore earthquake to occur in the Australian continent in 19 yr. We use in situ and Interferometric Synthetic Aperture Radar surface observations, aftershock distribution, and the fitting of P-wave source spectra to determine source properties of the Petermann earthquake. Surface observations reveal a 21-km-long surface rupture trace (strike=294°±29°) with heterogeneous vertical displacements (<0.1–0.96 m). Aftershock arrays suggest a triangular-shaped rupture plane (dip ≈ 30°) that intersects the subsurface projection of the major geophysical structure (Woodroffe thrust [WT]) proximal to the preferred location of the mainshock hypocenter, suggesting the mainshock nucleated at a fault junction. Footwall seismicity includes apparent southwest-dipping Riedel-type alignments, including possible activation of the deep segment of the WT. We estimate a moment magnitude (Mw) of 6.0 and a corner frequency (fc) of 0.2 Hz, respectively, from spectral fitting of source spectra in the 0.02–2 Hz frequency band. These translate into a fault area of 124 km2 and an average slip of 0.36 m. The estimated stress drop of 2.2 MPa is low for an intraplate earthquake; we attribute this to low-frictional slip (effective coefficient of friction >0.015) along rupture-parallel phyllosilicate-rich surfaces within the host rock fabric with possible additional contributions from elevated pore-fluid pressures.

SUMMARYDetermining the crustal structure of ocean island volcanoes is important to understand the formation and tectonic evolution of the oceanic lithosphere and tectonic swells in marine settings, and to assess seismic hazard in the islands. The Azores Archipelago is located near a triple junction system and is possibly under the influence of a mantle plume, being at the locus of a wide range of geodynamic processes. However, its crustal structure is still poorly constrained and debated due to the limited seismic coverage of the region and the peculiar linear geometry of the islands. To address these limitations, in this study we invert teleseismic Rayleigh wave ellipticity measurements for 1-D shear wave speed (VS) crustal models of the Azores Archipelago. Moreover, we test the reliability of these new models by using them in independent moment tensor inversions of local seismic data and demonstrate that our models improve the waveform fit compared to previous models. We find that data from the westernmost seismic stations used in this study require a shallower Moho depth (∼10 km) than data from stations in the eastern part of the archipelago (∼13–16 km). This apparent increase in the Moho depth with increasing distance from the mid-Atlantic ridge (MAR) is expected. However, the rate at which Moho deepens away from the MAR is greater than that predicted from a half-space cooling model, suggesting that local tectonic perturbations have modified crustal structure. The 1-D VS models obtained beneath the westernmost seismic stations also show higher wave speeds than for the easternmost stations, which correlates well with the ages of the islands except Santa Maria Island. We interpret the relatively low VS profile found beneath Santa Maria Island as resulting from underplating, which agrees with previous geological studies of the island. Compared to a recent receiver function study of the region, the shallow structure (top ∼2 km) in our models shows lower shear wave speed, which may have important implications for future hazard studies of the region. More generally, the new seismic crustal models we present in this study will be useful to better understand the tectonics, seismicity, moment tensors and strong ground motions in the region.

Meteorological and geophysical hazards will concur and interact with coronavirus disease (COVID-19) impacts in many regions on Earth. These interactions will challenge the resilience of societies and systems. A comparison of plausible COVID-19 epidemic trajectories with multi-hazard time-series curves enables delineation of multi-hazard scenarios for selected countries (United States, China, Australia, Bangladesh) and regions (Texas). In multi-hazard crises, governments and other responding agents may be required to make complex, highly compromised, hierarchical decisions aimed to balance COVID-19 risks and protocols with disaster response and recovery operations. Contemporary socioeconomic changes (e.g. reducing risk mitigation measures, lowering restrictions on human activity to stimulate economic recovery) may alter COVID-19 epidemiological dynamics and increase future risks relating to natural hazards and COVID-19 interactions. For example, the aggregation of evacuees into communal environments and increased demand on medical, economic, and infrastructural capacity associated with natural hazard impacts may increase COVID-19 exposure risks and vulnerabilities. COVID-19 epidemiologic conditions at the time of a natural hazard event might also influence the characteristics of emergency and humanitarian responses (e.g. evacuation and sheltering procedures, resource availability, implementation modalities, and assistance types). A simple epidemic phenomenological model with a concurrent disaster event predicts a greater infection rate following events during the pre-infection rate peak period compared with post-peak events, highlighting the need for enacting COVID-19 counter measures in advance of seasonal increases in natural hazards. Inclusion of natural hazard inputs into COVID-19 epidemiological models could enhance the evidence base for informing contemporary policy across diverse multi-hazard scenarios, defining and addressing gaps in disaster preparedness strategies and resourcing, and implementing a future-planning systems approach into contemporary COVID-19 mitigation strategies. Our recommendations may assist governments and their advisors to develop risk reduction strategies for natural and cascading hazards during the COVID-19 pandemic.

Using a new seismic waveform data set, we locate 234 earthquakes and estimate source parameters (focal mechanisms, magnitude, and stress drop) of the two largest earthquakes (E1 and E2) in the 2012 seismic sequence in Thorpdale, Victoria. The focal mechanisms suggest thrust faulting, consistent with previous observations in southeast Australia. The estimated magnitudes are Mw 4.9 ± 0.14 (E1) and 4.3 ± 0.1 (E2). The estimates of stress drop 57 ± 7.4 MPa (E1) and 28 ± 2.4 MPa (E2) reflect strength of faults in an intraplate environment. By analyzing spatiotemporal distribution of aftershocks, we show that E1 and E2 reflect two separate seismic sequences about a month apart. E1 and E2 ruptured two adjacent faults with orientations 218°/78°/78° (Fault 1) and 134°/27°/171° (Fault 2), respectively, where angles indicate strike/dip/slip rake. To test causal mechanism of fault interaction, we performed Coulomb stress modeling, which shows very weak unloading of E2 by E1. Thus, we infer that fault interaction might instead reflect remotely triggered fluid diffusion. Also, a local rotation of the stress field proceeding E1 might have favorably reoriented Fault 2 for failure. Finally, we identify an apparent correlation between high heat flow and seismicity in southeastern Australia, suggesting a combination of mechanisms including transient stress perturbations and lithospheric thermal weakening associated with high heat flow as the principal factors localizing intraplate seismicity.

To evaluate the geospatial hazard relationships between recent (contemporary) rockfalls and their prehistoric predecessors, we compare the locations, physical characteristics, and lithologies of rockfall boulders deposited during the 2010–2011 Canterbury earthquake sequence (CES) (n=185) with those deposited prior to the CES (n=1093). Population ratios of pre-CES to CES boulders at two study sites vary spatially from ∼5:1 to 8.5:1. This is interpreted to reflect (i) variations in CES rockfall flux due to intra- and inter-event spatial differences in ground motions (e.g., directionality) and associated variations in source cliff responses; (ii) possible variations in the triggering mechanism(s), frequency, flux, record duration, boulder size distributions, and post-depositional mobilization of pre-CES rockfalls relative to CES rockfalls; and (iii) geological variations in the source cliffs of CES and pre-CES rockfalls. On interfluves, CES boulders traveled approximately 100 to 250 m further downslope than prehistoric (pre-CES) boulders. This is interpreted to reflect reduced resistance to CES rockfall transport due to preceding anthropogenic hillslope de-vegetation. Volcanic breccia boulders are more dimensionally equant and rounded, are larger, and traveled further downslope than coherent lava boulders, illustrating clear geological control on rockfall hazard. In valley bottoms, the furthest-traveled pre-CES boulders are situated further downslope than CES boulders due to (i) remobilization of pre-CES boulders by post-depositional processes such as debris flows and (ii) reduction of CES boulder velocities and travel distances by collisional impacts with pre-CES boulders. A considered earth-systems approach is required when using preserved distributions of rockfall deposits to predict the severity and extents of future rockfall events.

Standard Earth models assume a simple uniform inner core boundary (ICB) separating the liquid iron outer core from the solid iron inner core. Metallurgical and geodynamic experiments, however, predict lateral variations along this boundary originating from thermochemical and geodynamic instabilities during solidification. We search for evidence of this lateral heterogeneity by exploiting the sensitivity of antipodal PKIIKP waveforms to the shear wave velocity structure of the uppermost inner core beneath their reflection points on the underside of the ICB. Measuring PKIIKP/PKIKP energy ratios from 33 rare antipodal seismograms in the 178o to 180o distance range, we find this ratio varying between 0.1 and 1.1. Synthetic seismograms demonstrate that a laterally homogeneous liquid‐solid ICB cannot account for this variability. Observations instead support a spatially variable ICB transition consisting of either (1) gradients in seismic velocities and density in which they smoothly increase from those at the outer core to those in the bulk of the inner core over a maximum depth of 10 km or (2) a layered transition with localized double discontinuities in velocities and densities separated by 4–10 km. A layered transition can generate a coda following PKIKP if shear velocity is small (<2 km/s) in the transition. Our results imply that the ICB is not uniform and might appear patchy with lateral rigidity variations. Nonuniform small‐scale structural features that we infer to be present at the ICB are consistent with nonlinear solidification mechanisms driven by small‐scale outer core convection in the lowermost outer core.

(1) CERIS, Instituto Superior Te ́cnico, Av. Rovisco Pais, 1, 1049-001 Lisbon, Portugal, (2) University College London, Gower Street, London WC1E 6BT, United Kingdom, (3) Universita`di Bologna, Via Zamboni, 33 40126 Bologna, Italy, (4) Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Via Donato Creti 12, 40128 7 Bologna, Italy, (5) University of Münster, Institute for Geophysics, Münster, Germany

Up until now, Portugal lacked a countrywide shear velocity model sampling short length-scale crustal structure, which limits interpretations of seismicity and tectonics, and predictions of strong ground motion. In turn, such interpretations and predictions are important to help mitigate risk of destruction from future large on- and offshore earthquakes similar to those that Portugal has experienced in the past (e.g. the Mw 8.5–8.7 tsunamigenic event in 1755). In this study, we measured teleseismic Rayleigh Wave Ellipticity (RWE) from 33 permanent and temporary seismic stations in Portugal with wave periods between 15s and 60s, and inverted it for 1-D models of shear wave velocity (Vs) structure beneath each station using a fully non-linear Monte Carlo method. Because RWE is strongly sensitive to the uppermost few kilometres of the crust, both RWE measurements and Vs models are spatially correlated with surface geology in Portugal. For instance, we find that sedimentary basins produced by rifting that had begun in the Mesozoic such as the Lusitanian Basin (LB) and the Lower Tagus-Sado Basin (LTSB) are characterised by higher RWE (lower Vs). Interestingly, we observe similar RWE (and Vs) values in the interior of the Central Iberian Zone (CIZ), which is a metamorphic belt of Paleozoic age. Together with reduced crustal thickness previously estimated for the same parts of the CIZ, this suggests that the CIZ might have experienced an episode of extension possibly simultaneous to Mesozoic rifting. The Galicia-Tras-os-Montes-Zone (GTMZ) that has undergone polyphased deformation since the Paleozoic is characterised by the lowest RWE (highest Vs) in Portugal. Ossa Morena Zone and the South Portuguese Zone exhibit intermediate Vs values when compared to that of basins and the GTMZ. Our crustal Vs model can be used to provide new insights into the tectonics, seismicity and strong ground motion in Portugal.

The February 22nd 2006 M-w = 7 Machaze earthquake is one of the largest, if not the largest, earthquakes reported since 1900 within Continental Africa. This large continental intraplate event has important implications to our understanding of tectonics and strong ground motion prediction locally and in the global context. Thus, accurate estimates of source parameters of this earthquake are important. In this study, we inverted the complete azimuthally distributed high frequency (0.05-2 Hz) P waveform dataset available for a best -fitting point source model and obtained stress drop estimates assuming different theoretical rupture models from spectral fitting. Our best -fitting point source model confirms steep normal faulting, has strike = 173 (309), dip = 73 (23), rake = 72 (-132), and shows a 12%-4% improvement in waveform fit compared to previous models, which translates into an error minimization. We attribute this improvement to higher order reverberations near the source region that we took in to account and the excellent azimuthal coverage of the dataset. Preferred stress drop estimates assuming a rupture velocity = 0.9 x shear wave velocity (Vs) are between 11 and 15 MPa though, even higher stress drop estimates are possible for rupture velocities lower than 0.9Vs. The estimated stress drop is significantly higher than the global stress drop average of intraplate earthquakes, but is consistent with stress drop estimated for some intra-continental earthquakes elsewhere. The detection of a new active structure that appears to terminate in Machaze, its step -like geometry, and lithospheric strength all favors a hypothesis of stress concentration in the source region, which is likely the cause of this event and the higher than average stress drop. (C) 2016 Elsevier Ltd. All rights reserved.