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Seismic Velocity Structure of the Irish Crust from Quarry Blasts
Full text linkTravel-time tomography uses the travel times of seismic waves between pairs of sources and receivers to constrain the elastic properties of the subsurface. However, the low rate of natural seismicity in Ireland limits the application of standard local earthquake tomography. This study uses seismic wave arrival times from controlled explosions generated during quarry and mining activities to refine the constraints on the velocity structure of the Irish crust.
Previous seismic studies have utilised (i) surface wave dispersion from teleseismic earthquakes, providing broad insights into the lithospheric structure, and (ii) spatially sparse seismic reflection and refraction profiles. While these studies have delineated major tectonic features, such as the late-Caledonian Leinster Granite and a crustal boundary linked to the closure of the Iapetus Ocean, the precise boundaries of these features remain unresolved. Subašić (2021) employed the FMTOMO package to compute a preliminary 3D travel-time tomography model of the Irish crust based on quarry blast data. FMTOMO (Rawlinson et al., 2006) uses a gradient-based subspace inversion scheme to derive a seismic velocity model from observed travel times. In this study, we re-evaluate and expand the input dataset used by Subašić (2021) and focus on optimising the regularisation parameters of the tomographic inversion.
Event classification into natural earthquakes and quarry explosions is performed using the spectral ratio method applied to S-wave trains, a procedure developed and routinely applied by the Irish National Seismic Network (INSN). The updated dataset includes 1,411 quarry blast events with P- and S-wave travel-time measurements from 2013–2014, a period of increased station density due to temporary seismic deployments. Quarry blasts, being surface explosions, are assumed to have well-constrained surface locations. A catalogue of 234 quarry sites in Ireland was initially compiled from satellite imagery by the INSN.
Hypocentre locations for each event are first calculated from phase arrival times and subsequently relocated to the nearest quarry. Given that quarry mines in Ireland typically range from hundreds of metres to a maximum of ~1.5 km in length, most events fall within the error margin of the initial locations. For events located beyond a 3 km radius of known quarries, additional searches for unrecorded sites were conducted. Satellite imagery inspections of these unclassified events identified 25 additional quarries. The operational status of these quarries during the study period was confirmed using historical imagery from Google Earth by comparing quarry areas before and after the analysed time frame
Improving passive reflection seismic imaging in complex geological settings through site effect reduction: application to Krafla volcano, Iceland
No full textReflection imaging at volcanoes presentssignificant challenges due to the highly heterogeneous
subsurface, which generates complex wavefields characterized by substantial wave scattering.
These scattered waves obscure coherent energy, such as reflections from geological structures
in the subsurface. In this study, we develop processing strategies to address the limitations
of high-frequency (5–20 Hz) passive reflection imaging at Krafla, a volcanic caldera in NE
Iceland. Krafla is among the few locations worldwide where magma has been encountered at
2.1 km depth when drilling the IDDP1 borehole. We analyse over 300 local microearthquakes
and industrial noise recorded during five weeksin the summer of 2022. We show that wavefields
lack coherency even between stations spaced at 30-m intervals due to the dominance of site
effects beneath the stations. However, data coherency improvesin the common-station domain,
where different earthquakes recorded by a fixed station are analysed, thereby stabilizing the
site effect. Spectral analyses in this domain reveal that site effects are partly due to resonances
at the stations, likely caused by lava flows and cavities in the heterogeneous near-surface.
By constructing a resonance removal filter, we successfully deconvolve resonance effects
from the data, revealing previously masked coherent energy. We further reduce site effects
by applying linear stacking of clustered earthquake traces and nonlinear amplitude weighting.
Our approach significantly enhances coherency between stations and enables the identification
of reflections in microearthquakes likely originating from the known magma–rock interface
beneath the IDDP1 borehole
A new subsurface temperature model for Ireland from joint geophysical–petrological inversion of seismic, surface heat flow and petrophysical data
No full textHigh-quality maps of subsurface temperature and the geothermal gradient are useful when assessing the geothermal potential of a region. However, determining geothermal potential is a challenge when direct measurements of in-situ temperature and thermal property information are sparse and indirect geophysical methods are sensitive to a range of parameters, not just temperature. Here, we produce subsurface temperature maps of Ireland using a joint geophysical–petrological inversion, where seismic and other geophysical and petrophysical data are inverted directly for temperature in 1-D columns and are collated into a pseudo 3-D temperature volume. Additionally, the inversion produces new models for Moho and LAB depth and for the average crustal radiogenic heat production.
To assess the robustness of the resulting temperature model, an uncertainty analysis has been performed by inverting all of the 1-D columns for a range of reasonable input parameters applicable to the Irish crust (rather than the ‘best’ input parameters). The resulting uncertainty model suggests temperature estimates at 2 km depth in our model could vary by ± 2 to 5 °C with an average of 3.5 °C in most locations. The uncertainty model can be used to assess confidence in different regions of the temperature model. In addition, 3-D forward modelling was performed to assess the lateral heat flow variations when compared to the purely 1-D inversion. The upper-crustal geothermal gradient ranges from 20 to 40 °C km−1 indicating a higher geothermal gradient for Ireland than previously reported with subsurface temperatures at 2 km depth > 60 °C everywhere, sufficient for residential and industrial heating purposes. The temperature gradient is typically higher in areas with thinner lithosphere. However, in some locations, the observed geotherms are elevated further due to high radiogenic heat production in granitic rocks. In Northern Ireland, a thin lithosphere, coupled with a weakly conductive basalt layer overlying warm crust, results in elevated temperatures. These are the first temperature maps for Ireland that include uncertainty estimates, providing ranges for the subsurface temperature values, and demonstrate that the maps are comparable to direct independent borehole temperature measurements, which are observed to fall within the model uncertainty. Our new methodology provides workflows for determining the geothermal potential in areas with limited direct temperature measurements. The final temperature model with uncertainty provides useful constraints for geothermal exploration and utilization on the island of Ireland
How to turn a supernova into a PeVatron
No full textContext. It is important to determine which Galactic cosmic-ray (CR) sources have the ability to accelerate particles to the knee of the CR spectrum at a few peta-electronvolt (PeV). In particular, we need to consider whether supernova remnants (SNRs) could also be contributors to this process. Current models for particle acceleration in very young remnants assume the circumstellar material (CSM) consists of smooth, freely expanding winds. There is strong evidence that some supernovae (SNs) expand into much denser CSM, including dense shells ejected by eruptions shortly before explosion.
Aims. We investigate the effects of dense circumstellar shells on particle acceleration in SN shocks during the first few years post-explosion to quantify whether SNs resulting from interactions may act as PeVatrons.
Methods. We used the PION code to model the CSM around luminous blue variables (LBVs) after having a brief episode with a mass-loss rate of up to 2 Solar Masses per year. Consequently, we performed spherically symmetric 1D simulations using our time-dependent acceleration code RATPAC, where we simultaneously solved the transport equations for CRs, magnetic turbulence, and the hydrodynamical flow of the thermal plasma in the test-particle limit.
Results. We find that the interaction with the circumstellar shells can significantly boost the maximum energy by enhancing particle escape during the onset of the shock-shell interaction, followed by the reacceleration of the shock propagating into a medium with a preamplified field. Early interactions boost the maximum energy to a greater degree and interactions within the first five months after explosion can increase Emax to levels over 1 PeV
A multi-ion non-equilibrium solver for ionised astrophysical plasmas with arbitrary elemental abundances
No full textContext. While many astrophysical plasmas can be modelled successfully assuming ionisation and thermal equilibrium, in some cases this is not appropriate and a non-equilibrium approach is required. In nebulae around evolved stars, the local elemental abundances may also strongly vary in space and time.
Aims. Here we present a non-equilibrium multi-ion module developed for the fluid-dynamics code PION, describing the physical processes included and demonstrating its capabilities with some test calculations.
Methods. A non-equilibrium ionisation solver is developed that allows arbitrary elemental abundances for neutral and ionised (but not molecular) gas, for the elements H, He, C, N, O, Ne, Si, S, and Fe. Collisional ionisation and recombination, photoionisation and charge-exchange reactions are included, and ion-by-ion non-equilibrium radiative cooling is calculated based on the instantaneous ion fractions of each element. Element and ion mass-fractions are advected using passive scalars, operator-split from the microphysical processes.
Results. The module is validated by comparing with equilibrium and non-equilibrium calculations in the literature. Effects of charge exchange on ion abundances in cooling plasmas are discussed. Application to modelling shocks and photo-ionised H II regions is demonstrated. The time-dependent expansion of a WR nebula is studied, including photoionisation and collisional processes, and spectral-line luminosities calculated for non-equilibrium and equilibrium plasma states.
Conclusions. The multi-ion module enables simulation of ionised plasmas with spatially varying elemental abundances using self-consistent ion abundances and thermal evolution. This allows prediction of spectral lines in UV, optical, IR, and X-ray even in cases where the plasma is out of ionisation equilibrium
Detection of Lowermost Mantle Heterogeneity Using Seismic Migration of Diffracted S‐Waves
No full textThe bottom of Earth's mantle hosts strong seismic wave speed heterogeneities. These are
commonly detected via forward modeling of seismic waveforms, which can include time‐consuming waveform
synthesis and visual inspection. Furthermore, such imaging has been most commonly carried out with waves
that have limited global coverage. In this work, we investigate the efficacy of the diffracted S (Sdiff) wavefield,
which has global coverage to map core‐mantle boundary heterogeneity. We implement a Kirchhoff migration
algorithm to objectively investigate the presence or absence of postcursors to Sdiff, caused by ultralow velocity
zones (ULVZs) and other sharp velocity contrasts. Our approach makes use of the expected moveout of ULVZ‐
generated Sdiff postcursors as a function of distance from great‐circle path at the base of the mantle. We
investigate epicentral distances >95°, where Sdiff includes asymptotic S/ScS up to diffraction. We test the
algorithm using synthetic waveforms calculated for models that include lowermost mantle wavespeed
heterogeneity via a recently proposed hybrid simulation approach. Our results demonstrate that the migration
approach, when applied to a single event, can well resolve the location of heterogeneity structures in the
azimuthal direction, but is less accurate at constraining the along‐great circle path location. To locate ULVZ
structure accurately, heterogeneity maps from different earthquakes with crisscrossing raypaths are combined.
Lastly, we provide real‐data proof‐of‐concept examples which detect ULVZs with different sizes that have been
proposed in past work. These include the Hawaiian ULVZ, which is roughly 1,000 km across and a ULVZ
beneath the Himalayas with a lateral extent of only 200 km
Detection and localization of Bryde’s whale calls using machine learning and probabilistic back-projection
No full textPassive acoustic monitoring can inform our understanding of baleen whale behavior by recording and analyzing their vocalizations. Two crucial factors in the analysis of whale calls are their detection and localization. In this study, we first develop a machine learning method to detect Bryde’s whale calls observed by ocean-bottom instruments deployed in the Panama basin, and back-project the detected events to determine their localizations. Using previously identified Bryde’s whale calls, we apply data augmentation strategies to increase the size of our training dataset to ultimately obtain 890,214 training examples. Using an evaluation dataset, we determine which detection thresholds optimize false positives and negatives and apply these to continuously recorded hydrophone data. The detection resulted in 4,514 potential events, of which 899 were recorded by at least three instruments. The waveforms of these events were automatically extracted, cross-correlation probability envelopes were computed between hydrophones, and these were finally back-projected onto a 3D grid to obtain final event localizations. For this network, this procedure is shown to be robust to high noise levels, random time errors and systematic bias introduced by the velocity model. This approach has further advantages, such as being computationally efficient and requiring minimal manual intervention
EWOCS-IV: 1Ms ACIS Chandra observation of the supergiant B[e] star Wd1-9
No full textContext. Supergiant B[e] (sgB[e]) stars are exceptionally rare objects, with only a select number of confirmed examples in the Milky Way. The evolutionary pathways leading to the sgB[e] phase remain largely debated, highlighting the need for additional observations. The sgB[e] star Wd1-9, located in the massive cluster Westerlund 1 (Wd1), is enshrouded in a dusty cocoon – likely the result of past eruptive activity – leaving its true nature enigmatic.
Aims. We present the most detailed X-ray study of Wd1-9 to date, using X-rays that pierce through its cocoon with the aim of uncovering its nature and evolutionary state.
Methods. We utilised 36 Chandra observations of Wd1 from the ‘Extended Westerlund 1 and 2 Open Clusters Survey’ (EWOCS), plus eight archival datasets, totalling 1.1 Ms. We used this dataset to investigate long-term variability and periodicity in Wd1-9, and analysed X-ray colours and spectra over time to uncover patterns that shed light on its nature.
Results. Wd1-9 exhibits significant long-term X-ray variability, within which we identify a strong ∼14-day periodic signal. We interpret this as the orbital period, marking the first period determination for the system. The X-ray spectrum of Wd1-9 is thermal and hard (kT ∼ 3.0 keV), resembling the spectra of bright Wolf-Rayet (WR) binaries in Wd1, while a strong Fe emission line at 6.7 keV indicates hot plasma from a colliding-wind X-ray binary.
Conclusions. Wd1-9, with evidence of past mass loss, circumbinary material, a hard X-ray spectrum, and a newly detected 14-day period, displays all the hallmarks of a binary – likely a WR+OB – that recently underwent early Case B mass transfer. Its sgB[e] classification is likely phenomenological, reflecting emission from the dense circumbinary material. This places Wd1-9 in a rarely observed phase, possibly revealing a newly formed WN star, bridging the gap between immediate precursors and later evolutionary stages in Wd1
