22 results found
Williams PR, Pancoast A, Treu T, et al., 2020, Space Telescope and Optical Reverberation Mapping Project. XII. Broad-line Region Modeling of NGC 5548, Astrophysical Journal, Vol: 902, ISSN: 0004-637X
We present geometric and dynamical modeling of the broad line region (BLR) for the multi-wavelength reverberation mapping campaign focused on NGC 5548 in 2014. The data set includes photometric and spectroscopic monitoring in the optical and ultraviolet, covering the Hβ, C iv, and Ly broad emission lines. We find an extended disk-like Hβ BLR with a mixture of near-circular and outflowing gas trajectories, while the C iv and Ly BLRs are much less extended and resemble shell-like structures. There is clear radial structure in the BLR, with C iv and Ly emission arising at smaller radii than the Hβ emission. Using the three lines, we make three independent black hole mass measurements, all of which are consistent. Combining these results gives a joint inference of . We examine the effect of using the V band instead of the UV continuum light curve on the results and find a size difference that is consistent with the measured UV-optical time lag, but the other structural and kinematic parameters remain unchanged, suggesting that the V band is a suitable proxy for the ionizing continuum when exploring the BLR structure and kinematics. Finally, we compare the Hβ results to similar models of data obtained in 2008 when the active galactic nucleus was at a lower luminosity state. We find that the size of the emitting region increased during this time period, but the geometry and black hole mass remained unchanged, which confirms that the BLR kinematics suitably gauge the gravitational field of the central black hole.
Hicks S, 2020, Review of the Sicily Seismic Lockdown by Cannata et al.
Lecocq T, Hicks SP, Van Noten K, et al., 2020, Global quieting of high-frequency seismic noise due to COVID-19 pandemic lockdown measures, Science, Vol: 369, Pages: 1338-1343, ISSN: 0036-8075
Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the COVID-19 pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. While the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of population dynamics.
Cooper GF, Macpherson CG, Blundy JD, et al., 2020, Variable water input controls evolution of the Lesser Antilles volcanic arc (vol 87, pg 931, 2020), Nature, Vol: 584, Pages: E36-E36, ISSN: 0028-0836
Hicks S, Okuwaki R, Steinberg A, et al., 2020, Back-propagating super-shear rupture in the 2016 Mw7.1 Romanche transform fault earthquake, Nature Geoscience, Vol: 13, Pages: 647-653, ISSN: 1752-0894
How an earthquake rupture propagates strongly influences potentially destructive ground shaking. Complex ruptures often involve slip along multiple faults, masking information on the frictional behaviour of fault zones. Geometrically smooth ocean transform fault plate boundaries offer a favourable environment to study fault dynamics, because strain is accommodated along a single, wide fault zone that offsets homogeneous geology. Here we present an analysis of the 2016 M7.1 earthquake on the Romanche fracture zone in the equatorial Atlantic, using data from both nearby seafloor seismometers and global seismic networks. We show that this rupture had two phases: (1) upward and eastward propagation towards a weaker region where the transform fault intersects the mid-ocean ridge, then (2) unusual back-propagation westwards at super-shear speed toward the centre of the fault. We suggest that deep rupture into weak fault segments facilitated greater seismic slip on shallow locked zones. This highlights that even earthquakes along a single distinct fault zone can be highly dynamic. Observations of back-propagating ruptures are sparse, and the possibility of reverse propagation is largely absent in rupture simulations and unaccounted for in hazard assessments.
Cooper GF, Macpherson CG, Blundy JD, et al., 2020, Variable water input controls evolution of the Lesser Antilles volcanic arc, Nature, Vol: 582, Pages: 525-529, ISSN: 0028-0836
Oceanic lithosphere carries volatiles, notably water, into the mantle through subduction at convergent plate boundaries. This subducted water exercises control on the production of magma, earthquakes, formation of continental crust and mineral resources. Identifying different potential fluid sources (sediments, crust and mantle lithosphere) and tracing fluids from their release to the surface has proved challenging1. Atlantic subduction zones are a valuable endmember when studying this deep water cycle because hydration in Atlantic lithosphere, produced by slow spreading, is expected to be highly non-uniform2. Here, as part of a multi-disciplinary project in the Lesser Antilles volcanic arc3, we studied boron trace element and isotopic fingerprints of melt inclusions. These reveal that serpentine—that is, hydrated mantle rather than crust or sediments—is a dominant supplier of subducted water to the central arc. This serpentine is most likely to reside in a set of major fracture zones subducted beneath the central arc over approximately the past ten million years. The current dehydration of these fracture zones coincides with the current locations of the highest rates of earthquakes and prominent low shear velocities, whereas the preceding history of dehydration is consistent with the locations of higher volcanic productivity and thicker arc crust. These combined geochemical and geophysical data indicate that the structure and hydration of the subducted plate are directly connected to the evolution of the arc and its associated seismic and volcanic hazards.
Lacassin R, Devès M, Hicks SP, et al., 2020, Rapid collaborative knowledge building via Twitter after significant geohazard events, Geoscience Communication, Vol: 3, Pages: 129-146, ISSN: 2569-7110
Twitter is an established social media platform valued by scholars as an open way to disseminate scientific information and to publicly discuss research results. Scientific discussions are widely viewed by the media who can then pass on information to the wider public. Here, we take the example of two 2018 earthquake-related events which were widely commented on Twitter by geoscientists: the Palu &lt;i&gt;M&lt;/i&gt;&lt;sub&gt;w&lt;/sub&gt;&amp;#8201;7.5 earthquake and tsunami in Indonesia and the long-duration Mayotte island seismo-volcanic crisis. We build our study on a content and contextual analysis of selected Twitter threads about the geophysical characteristics of these events. From the analysis of these two examples, we show that Twitter promotes very rapid building of knowledge &amp;#8211; in the minutes to hours and days following an event &amp;#8211; via an efficient exchange of information and active discussion between the scientists themselves and with the public. We discuss the advantages and potential pitfalls of this relatively novel way to make scientific information accessible to scholarly peers and to lay people. We argue that scientific discussion on Twitter breaks down the traditional &lt;q&gt;ivory towers&lt;/q&gt; of academia, following growing trends towards open science, and may help people to understand how science is developed, and, in the case of natural/environmental hazards, to better understand their risks
Bie L, Rietbrock A, Hicks S, et al., 2020, Along‐arc heterogeneity in local seismicity across the lesser antilles subduction zone from a dense ocean‐bottom seismometer network, Seismological Research Letters, Vol: 91, Pages: 237-247, ISSN: 0895-0695
The Lesser Antilles arc is only one of two subduction zones where slow‐spreading Atlantic lithosphere is consumed. Slow‐spreading may result in the Atlantic lithosphere being more pervasively and heterogeneously hydrated than fast‐spreading Pacific lithosphere, thus affecting the flux of fluids into the deep mantle. Understanding the distribution of seismicity can help unravel the effect of fluids on geodynamic and seismogenic processes. However, a detailed view of local seismicity across the whole Lesser Antilles subduction zone is lacking. Using a temporary ocean‐bottom seismic network we invert for hypocenters and 1D velocity model. A systematic search yields a 27 km thick crust, reflecting average arc and back‐arc structures. We find abundant intraslab seismicity beneath Martinique and Dominica, which may relate to the subducted Marathon and/or Mercurius Fracture Zones. Pervasive seismicity in the cold mantle wedge corner and thrust seismicity deep on the subducting plate interface suggest an unusually wide megathrust seismogenic zone reaching ∼65km∼65 km depth. Our results provide an excellent framework for future understanding of regional seismic hazard in eastern Caribbean and the volatile cycling beneath the Lesser Antilles arc.
Hicks S, Verdon J, Baptie B, et al., 2019, A Shallow Earthquake Swarm Close to Hydrocarbon Activities: Discriminating between Natural and Induced Causes for the 2018–2019 Surrey, United Kingdom, Earthquake Sequence, Seismological Research Letters, Vol: 90, Pages: 2095-2110, ISSN: 0895-0695
Earthquakes induced by subsurface industrial activities are a globally emotive issue, with a growing catalogue of induced earthquake sequences. However, attempts at discriminating between natural and induced causes, particularly for anomalously shallow seismicity, can be challenging. An earthquake swarm during 2018–19 in south-east England with a maximum magnitude of ML 3.2 received great public and media attention because of its proximity to operating oilfields. It is therefore vital and timely to provide a detailed characterisation of the earthquake sequence at present, and to decide based on current evidence, whether the earthquakes were likely natural or induced. We detected 168 low-magnitude earthquakes and computed detailed source parameters of these events. Most earthquakes occurred at a shallow depth of 2.3 km, >1 km deeper than the geological formations targeted by the oilfields, and laterally >3 km away from the drill-sites. We combine the east-west trending cluster of the seismicity with 2-D seismic reflection profiles to find the causative fault system for the earthquakes. A b-value close to unity and strike-slip faulting mechanisms are consistent with tectonic reactivation along a pre-existing fault. Overall, we find no indicators in the earthquake parameters that would strongly suggest an induced source. Nor do we find any clear trends between seismicity and drilling activities based on operational logs provided by the operators. Injected volumes are near-zero and monthly production amounts are many orders of magnitude smaller than other reported cases of extraction-induced seismicity. On balance, and based on the available evidence, we find it currently unlikely that nearby industrial activities induced the seismic swarm. Most likely, the Surrey earthquakes offer a uniquely detailed insight into shallow seismicity within sedimentary basins. Nevertheless, self-reporting of injection and production times and volumes by operators, and the lac
Hicks SP, 2019, Geoscience analysis on Twitter, Nature Geoscience, Vol: 12, Pages: 585-586, ISSN: 1752-0894
Social media is increasingly being used to share near-real-time analysis of emergent and sometimes hazardous geological events. Such open discussion can drive new research directions and collaborations for geoscientists.
Cordell D, Unsworth MJ, Diaz D, et al., 2019, Fluid and melt pathways in the central Chilean subduction zone near the 2010 Maule earthquake (35° ‐ 36° S) as inferred from magnetotelluric data, Geochemistry, Geophysics, Geosystems, Vol: 20, Pages: 1818-1835, ISSN: 1525-2027
The subduction zone of central Chile (36° S) has produced some of the world's largest earthquakes and significant volcanic eruptions. Understanding the fluid fluxes and structure of the subducting slab and over‐riding plate can provide insight into the tectonic processes responsible for both seismicity and magmatism. Broadband and long‐period magnetotelluric data were collected along a 350 km profile in central Chile and Argentina and show a regional geoelectric strike of 15±19° east of north. The preferred two‐dimensional inversion model included the geometry of the subducting Nazca plate as a constraint. On the upper surface of the Nazca plate, conductors were interpreted as fluids expelled from the down‐going slab via compaction at shallow depth (C1) and metamorphic reactions at depths of 40‐90 km (C2 and C3). At greater depths (130 km), a conductor (C7) is interpreted as a region of partial melt related to de‐serpentinization in the back‐arc. A resistor on the slab interface (R1) is coincident with a high‐velocity anomaly which was interpreted as a strong asperity which may affect the co‐seismic slip behavior of large megathrust earthquakes at this latitude. Correlations with seismicity suggest slab fluids alter the forearc mantle and define the down‐dip limit of the seismogenic zone. Beneath the volcanic arc, several upper crustal conductors (C4 and C5) represent partial melt beneath the Tatara‐San Pedro Volcano and the Laguna del Maule Volcanic Field. A deeper lower‐crustal conductor (C6) underlies both volcanoes and suggests a connected network of melt in a thermally‐mature lower crust.
Bie L, Hicks S, Garth T, et al., 2018, ‘Two go together’: near-simultaneous moment release of two asperities during the 2016 Mw 6.6 Muji, China earthquake, Earth and Planetary Science Letters, Vol: 491, Pages: 34-42, ISSN: 0012-821X
On 25 November 2016, a Mw 6.6 earthquake ruptured the Muji fault in western Xinjiang, China. We investigate the earthquake rupture independently using geodetic observations from Interferometric Synthetic Aperture Radar (InSAR) and regional seismic recordings. To constrain the fault geometry and slip distribution, we test different combinations of fault dip and slip direction to reproduce InSAR observations. Both InSAR observations and optimal distributed slip model suggest buried rupture of two asperities separated by a gap of greater than 5 km. Additional seismic gaps exist at the end of both asperities that failed in the 2016 earthquake. To reveal the dynamic history of asperity failure, we inverted regional seismic waveforms for multiple centroid moment tensors and construct a moment rate function. The results show a small centroid time gap of 2.6 s between the two sub-events. Considering the >5 km gap between the two asperities and short time interval, we propose that the two asperities failed near-simultaneously, rather than in a cascading rupture propagation style. The second sub-event locates ∼39 km to the east of the epicenter and the centroid time is at 10.7 s. It leads to an estimate of average velocity of 3.7 km/s as an upper bound, consistent with upper crust shear wave velocity in this region. We interpret that the rupture front is propagating at sub-shear wave velocities, but that the second sub-event has a reduced or asymmetric rupture time, leading to the apparent near-simultaneous moment release of the two asperities.
Verdon JP, Kendall J-M, Hicks SP, et al., 2017, Using beamforming to maximise the detection capability of small, sparse seismometer arrays deployed to monitor oil field activities, Geophysical Prospecting, Vol: 65, Pages: 1582-1596, ISSN: 0016-8025
Like most other industrial activities that affect the subsurface, hydraulic fracturing carries the risk of reactivating pre‐existing faults and thereby causing induced seismicity. In some regions, regulators have responded to this risk by imposing traffic light scheme‐type regulations, where fracture stimulation programs must be amended or shut down if events larger than a given magnitude are induced. Some sites may be monitored with downhole arrays and/or dense near‐surface arrays, capable of detecting very small microseismic events. However, such monitoring arrangements will not be logistically or economically feasible at all sites. Instead, operators are using small, sparse arrays of surface seismometers to meet their monitoring obligations.The challenge we address in this paper is to maximise the detection thresholds of such small, sparse, surface arrays so that they are capable of robustly identifying small‐magnitude events whose signal‐to‐noise ratios may be close to 1. To do this, we develop a beamforming‐and‐stacking approach, computing running short‐term/long‐term average functions for each component of each recorded trace (P, SH, and SV), time‐shifting these functions by the expected travel times for a given location, and performing a stack.We assess the effectiveness of this approach with a case study using data from a small surface array that recorded a multi‐well, multi‐stage hydraulic fracture stimulation in Oklahoma over a period of 8 days. As a comparison, we initially used a conventional event‐detection algorithm to identify events, finding a total of 17 events. In contrast, the beamforming‐and‐stacking approach identified a total of 155 events during this period (including the 17 events detected by the conventional method). The events that were not detected by the conventional algorithm had low‐signal‐to‐noise ratios to the extent that, in some cases, they would be unlikely to be identified even by manual analysis of the seismograms. We conclude th
Verdon JP, Kendall JM, Hicks SP, et al., 2016, Using beam forming to maximise event detection using small broadband seismometer arrays
In response to cases of injection-induced seismicity, regulators have responded by applying traffic-light scheme type monitoring requirements. Such schemes may require operators to detect small-magnitude events. While this can be done with dense arrays of surface geophones, or by deploying instruments downhole, the cost of doing so may be prohibitive for every stage of every well. Instead, we seek to maximise what can be detected using small (10-20) arrays of surface stations. We do so by developing a beamforming/stacking algorithm where STA/LTA functions derived from the P, SV and SH components are combined to reveal events with low signal-to-noise ratio. We apply our method to a case study from North America where induced events were recorded on a network of 17 broadband seismometers placed around a hydraulic fracturing site. Where conventional event detection algorithms reveals 4 induced events during the monitoring period, our method reveals 20 such events, an improvement in event detection of 500%.
Hicks SP, Rietbrock A, 2015, Seismic slip on an upper-plate normal fault during a large subduction megathrust rupture, Nature Geoscience, Vol: 8, Pages: 955-960
Hicks SP, Rietbrock A, Ryder IMA, et al., 2014, Anatomy of a megathrust: The 2010 M8.8 Maule, Chile earthquake rupture zone imaged using seismic tomography, Earth and Planetary Science Letters, Vol: 405, Pages: 142-155
Hicks SP, Rietbrock A, Haberland CA, et al., 2012, The 2010 Mw 8.8 Maule, Chile earthquake: Nucleation and rupture propagation controlled by a subducted topographic high, Geophysical Research Letters, Vol: 39
Hicks SP, Nippress SEJ, Rietbrock A, 2012, Sub-slab mantle anisotropy beneath south-central Chile, Earth and Planetary Science Letters, Vol: 357-358, Pages: 203-213
Hicks S, Okuwaki R, Steinberg A, et al., Back-propagating super-shear rupture in the 2016 Mw7.1 Romanche transform fault earthquake
Leptokaropoulos K, Harmon N, Hicks SP, et al., Tidal Triggering of Microseismicity at the Equatorial Mid-Atlantic Ridge, Inferred from OBS Network
van den Ende M, Bruhat L, Funning G, et al., The Seismica initiative: a community-led Diamond Open Access journal for seismological research
<jats:p>&lt;p&gt;On 24 November 2020, the Springer Nature publishing group announced the introduction of Open Access (OA) journals in Nature and its sibling journals. The corresponding OA publication fee (charged directly to the authors) was set to &amp;#8364;9,500/$11,390/&amp;#163;8,290, an amount that may be well out of reach for researchers with limited financial means. This is especially a problem for researchers in developing countries, and for early-career researchers on small, personal fellowships. Funding agencies often demand that research be published under an OA license, forcing authors to accept the high publication fees.&lt;/p&gt;&lt;p&gt;The high cost of these and similar OA fees for other Earth science journals prompted a discussion among the seismological community on Twitter, during which the idea was raised to start a free-to-publish, free-to-read journal for seismological research. The concept of Diamond Open Access was previously adopted by Volcanica (www.jvolcanica.org) for volcanological research, providing a precedent and directives for similar initiatives (like Seismica, but also Tektonika for the structural geology community). Following the community discussion on Slack with over 100 participants, a small &quot;task force&quot; was formed to investigate in detail the possibility of starting a Diamond OA seismology journal, taking Volcanica as a model. In this contribution, we report the progress that has been made by the task force and the seismological community in the conceptualisation of the journal, and the steps that remain to be taken. Once the initiation of the journal is completed, Seismica will offer a platform for researchers to publish and access peer-reviewed work with no financial barriers, promoting seismological research in an inclusive manner. We invite all interested members of the seismological and earthquake community to participate in t
Schlaphorst D, Harmon N, Kendall J-M, et al., Variation in Upper Plate Crustal and Lithospheric Mantle Structure in the Greater and Lesser Antilles from Ambient Noise Tomography
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