Source geometry from exceptionally high resolution long period event observations at Mt Etna during the 2008 eruption

During the second half of June, 2008, 50 broadband seismic stations were deployed on Mt Etna volcano in close proximity to the summit, allowing us to observe seismic activity with exceptionally high resolution. 129 long period events (LP) with dominant frequencies ranging between 0.3 and 1.2 Hz, were extracted from this dataset. These events form two families of similar waveforms with different temporal distributions. Event locations are performed by cross-correlating signals for all pairs of stations in a two-step scheme. In the first step, the absolute location of the centre of the clusters was found. In the second step, all events are located using this position. The hypocentres are found at shallow depths (20 to 700 m deep) below the summit craters. The very high location resolution allows us to detect the temporal migration of the events along a dike-like structure and 2 pipe shaped bodies, yielding an unprecedented view of some elements of the shallow plumbing system at Mount Etna. These events do not seem to be a direct indicator of the ongoing lava flow or magma upwelling

Helicopter Location and Tracking using Seismometer Recordings

We use frequency domain methods usually applied to volcanic tremor to analyse ground based seismic recordings of a helicopter. We preclude misinterpretations of tremor sources and show alternative applications of our seismological methods. On a volcano, the seismic source can consist of repeating, closely spaced, small earthquakes. Interestingly, similar signals are generated by helicopters, due to repeating pressure pulses from the rotor blades. In both cases the seismic signals are continuous and referred to as tremor. As frequency gliding is in this case merely caused by the Doppler effect, not a change in the source, we can use its shape to deduce properties of the helicopter and its flight path. We show in this analysis that the number of rotor blades, rotor revolutions per minute (RPM), helicopter speed, flight direction, altitude and location can be deduced from seismometer recordings. Access to GPS determined flight path data from the helicopter offers us a robust way to test our location method

Micrometre-scale deformation observations reveal fundamental controls on geological rifting

Many of the world’s largest volcanic eruptions are associated with geological rifting where major fractures open at the Earth’s surface, yet fundamental controls on the near-surface response to the rifting process are lacking. New high resolution observations gleaned from seismometer data during the 2014 Bárðarbunga basaltic dyke intrusion in Iceland allow us unprecedented access to the associated graben formation process on both sub-second and micrometre scales. We find that what appears as quasi steady-state near-surface rifting on lower resolution GPS observation comprises discrete staccatolike deformation steps as the upper crust unzips through repetitive low magnitude (MW < 0) failures on fracture patches estimated between 300 m2 and 1200 m2 in size. Stress drops for these events are one to two orders of magnitude smaller than expected for tectonic earthquakes, demonstrating that the uppermost crust in the rift zone is exceptionally weak

Tremor-rich shallow dyke formation followed by silent magma flow at Bárdarbunga in Iceland

The Bárdarbunga eruption in Iceland in 2014 and 2015 produced about 1.6 km3 of lava. Magma propagated away from Bárdarbunga to a distance of 48 km in the sub-surface beneath Vatnajökull glacier, emerging a few kilometres beyond the glacier's northern rim. A puzzling observation is the lack of shallow (<3 km deep), high-frequency earthquakes associated with shallow dyke formation near the subaerial and subglacial eruptive sites, suggesting that near-surface dyke formation is seismically quiet. However, seismic array observations and seismic full wavefield simulations reveal the presence and nature of shallow, pre-eruptive, long-duration seismic tremor activity. Here we use analyses of seismic data to constrain the relationships between seismicity, tremor, dyke propagation and magma flow during the Bárðarbunga eruption. We show that although tremor is usually associated with magma flow in volcanic settings, pre-eruptive tremor at Bárdarbunga was probably caused by swarms of microseismic events during dyke formation, and hence is directly associated with fracturing of the upper 2–3 km of the crust. Subsequent magma flow in the newly formed shallow dyke was seismically silent, with almost a complete absence of seismicity or tremor. Hence, we suggest that the transition from temporarily isolated, large, deep earthquakes to many smaller, shallower, temporally overlapping earthquakes (< magnitude 2) that appear as continuous tremor announces the arrival of a dyke opening in the shallow crust, forming a pathway for silent magma flow to the Earth's surface

Helicopter vs. volcanic tremor: Characteristic features of seismic harmonic tremor on volcanoes

We recorded high-frequency (> 10 Hz) harmonic tremor with spectral gliding at Hekla Volcano in Iceland. Particle motion plots indicated a shallow tremor source. We observed up to two overtones beneath our Nyquist frequency of 50 Hz and could resolve a source of closely spaced pulses of very short duration (0.03-0.1 s) on zoomed seismograms. Volcanic tremor with fundamental frequencies above 5 Hz, frequency gliding and/or repetitive sources similar to our observations were observed on different volcanoes around the world. However, this frequency content, duration and occurrence of volcano-related tremor was not observed in the last 35 years of seismic observations at Hekla. Detailed analysis reveals that this tremor was related to helicopters passing the volcano. This study relates the GPS track of a helicopter with seismic recordings of the helicopter at various distances. We show the effect the distance, number of rotor blades and velocity of the helicopter has on the observed up and down glidings at up to 40 km distance. We highlight similarities and differences between volcano-related and helicopter tremor in order to help avoid misinterpretations.European Commission - Seventh Framework Programme (FP7

Efficacy of Seismic Interferometry in Removing Surface Waves from Active Seismic Records

The EGU General Assembly 2021 (EGU21), Virtual Conference, 19-30 April 2021While there are seismic techniques which make use of surface waves in imaging the subsurface, there are also those where these types of waves are considered coherent noise. Important examples where the surface waves may significantly degrade the obtained images include different types of reflection seismic surveys (e.g., shallow surveys for engineering, environmental and groundwater investigations, and deep surveys for imaging hydrocarbon reservoirs). In a strongly heterogeneous medium (encountered typically in onshore surveys), the conventional methods for attenuating these waves (such as f-k "velocity" filtering) often do not give satisfactory results.Science Foundation IrelandGeological Survey of Ireland (GSI)PIPCO RSG Ltd

Stress state and patterns at the upper plate of Hikurangi Subduction Margin

&amp;lt;p&amp;gt;Quantifying the contemporary stress state of the Earth&amp;amp;#8217;s crust is critical for developing a geomechanical understanding of the behavior of brittle deformation (fractures and faults).In this study we characterize the shallow contemporary stress state of the active Hikurangi Subduction Margin (HSM), New Zealand, to better understand how it affects and responds to variable deformation and slip behavior documented along this plate boundary. The HSM is characterized by along-strike variations in megathrust slip behavior, ranging from shallow slow slip events (SSEs) and creep at the northern and central HSM to interseismic locking and stress accumulation in the southern HSM. We estimate the state of stress across the HSM utilizing rock strength estimates from empirical relationships, leak-off test data, wireline logs and borehole geology, and measurement of borehole wall failures such as borehole breakouts and drilling&amp;amp;#8208;induced tensile fractures from eight boreholes. Stress magnitude constraints at depth intervals where BOs are observed indicate that the maximum principal stress (&amp;amp;#963;&amp;lt;sub&amp;gt;1&amp;lt;/sub&amp;gt;) is horizontal along the shallow (&amp;lt;3 km) HSM and the stress state is predominantly strike-slip or contractional (barring localized areas where an extensional stress state is determined). Our results reveal a NE-SW (margin-parallel) S&amp;lt;sub&amp;gt;Hmax&amp;lt;/sub&amp;gt; orientation in the shallow central HSM, which rotates to a WNW- ESE/NW-SE (margin-perpendicular) S&amp;lt;sub&amp;gt;Hmax&amp;lt;/sub&amp;gt; orientation in the shallow southern HSM. The central NE-SW S&amp;lt;sub&amp;gt;Hmax&amp;lt;/sub&amp;gt; orientation is inconsistent with active, km-scale, NE-SW striking contractional faults observed across the central HSM. Considering both stress magnitude and orientation patterns at the central HSM, we suggest that long-term clockwise rotation of the Hikurangi forearc, over time, may transform motion on these km-scale central HSM faults from contractional dip-slip to a more contemporary strike/oblique-slip. The southern shallow WNW- ESE/NW-SE S&amp;lt;sub&amp;gt;Hmax&amp;lt;/sub&amp;gt; orientation is nearly perpendicular to focal-mechanisms derived NE-SW S&amp;lt;sub&amp;gt;Hmax&amp;lt;/sub&amp;gt; orientations within the subducting slab. This, combined with observed strike-slip and contractional faulting in the region and the NW-SE convergence direction, implies the overriding plate in the southern HSM is in a contractional stress state, potentially as deep as the plate interface, which is decoupled from that experienced in the subducting slab. Observed localized extensional stress states across the HSM may occur as a result of local extensions or reflect uncertainties in our estimations of S&amp;lt;sub&amp;gt;Hmax&amp;lt;/sub&amp;gt; magnitude which are sensitive to the UCS values used (unconstrained by laboratory testing). This UCS uncertainty and the potential errors it can introduce into a stress model highlights the importance of developing robust empirical relationships for UCS in regions where stress is a critical geological consideration for hazard and resource management.&amp;lt;/p&amp;gt;</jats:p

Simulation of High-Frequency Rotational Motion in a Two-Dimensional Laterally Heterogeneous Half-Space

The EGU General Assembly 2021 (vEGU21), Virtual Conference, 19-30 April 2021The seismic waves responsible for vibrating civil engineering structures undergo interference, focusing, scattering, and diffraction by the inhomogeneous medium encountered along the sourceto-site propagation path. The subsurface heterogeneities at a site can particularly alter the local seismic wave field and amplify the ground rotations, thereby increasing the seismic hazard. The conventional techniques to carry out full wave field simulations (such as finite-difference or spectral finite element methods) at high frequencies (e.g., 15 Hz) are computationally expensive, particularly when the size of the heterogeneities is small (e.g., <100 m). This study proposes an alternative technique that is based on the first-order perturbation theory for wave propagation. In this technique, the total wave field due to a particular source is obtained as a superposition of the ‘mean’ and ‘scattered’ wave fields. Whereas the ‘mean’ wave field is the response of the background (i.e., heterogeneity-free) medium due to the given source, the ‘scattered’ wave is the response of the background medium excited by fictitious body forces. For a two-dimensional laterally heterogeneous elastic medium, these body forces can be conveniently evaluated as a function of the material properties of the heterogeneities and the mean wave field. Since the problem of simulating high-frequency rotations in a laterally heterogeneous medium reduces to that of calculating rotations in the background medium subjected to the (1) given seismic source and (2) body forces that mathematically replace the small-scale heterogeneities, the original problem can be easily solved in a computationally accurate and efficient manner by using the classical (analytical) wavenumber-integration method. The workflow is illustrated for the case of a laterally heterogenous layer embedded in a homogeneous half-space excited by plane bodywaves.Geological Survey of Ireland (GSI

Classification of long-term very long period (VLP) volcanic earthquakes at Whakaari/White Island volcano, New Zealand

We have observed very long period earthquakes (VLPs) over the period 2007 to the end of 2019 at Whakaari/White Island volcano, New Zealand. The earthquakes exhibit similitude between waveforms which suggests repeating source locations and processes. VLPs recorded at two permanent stations were detected using waveform semblance and were then classified into two main families (F1 and F2) using a clustering analysis. The two families are characterized by ‘mirror image’ reverse waveform polarity suggesting that they are genetically related, but occur during different evolutionary phases of volcanic activity. F1 events occurred throughout the observation period, while F2 events mainly occurred as swarms that mark the onset of volcanic unrest. A detailed cluster analysis reveals possible sub-families implying slight temporal evolutions within a family. Our results add to our understanding of the volcanic magma–hydrothermal system at Whakaari/White Island indicating that relatively stable VLP sources may be exploited to improve monitoring for future unrest.[Figure not available: see fulltext.]New Zealand Natural Hazards Research PlatformMinistry of Business, Innovation and EmploymentResilience to Nature’s Challenge