ALBERT

Description: The Taupo Volcanic Zone (TVZ) is one of the world's most productive regions of rhyolitic volcanism and contains the highly active Okataina Volcanic Centre (OVC). Within the TVZ, intra-arc extension is expressed as normal faulting within a zone known as the Taupo Rift. The OVC is located within a complex part of the rift, where volcanism and deformation is considered influenced by rift structure and kinematics. There has been significant research on the structural, volcanic and geophysical properties of the rift and OVC, but less focus on deformation using geodetic data. The limited studies that have utilized geodetic data do not clearly resolve the distribution of deformation and strain rates within the rift and OVC. This is essential to ensure that deformation signals from volcanic processes at the OVC are correctly identified and distinguished from those related to regional tectonic or local hydrothermal processes within the rift. In this paper, we present a picture of contemporary deformation at the OVC and within the surrounding rift in detail, using existing and new GPS campaign and continuous GPS (cGPS) data collected between 1998 and 2011. The results show a highly heterogeneous deformation and strain rate field (both extension and shortening) through the study area, partitioned into different parts of the rift. Our results agree well with earlier geodetic studies, as well as identify new features, but some deformation patterns conflict with long-term geological observations. In the OVC, we observe a locally rotated horizontal velocity field, significant vertical deformation and variable strain rates across the caldera. In the Tarawera Rift, we identify elevated extension and shear rates, which may have significant implications for volcanism there. A shortening pattern is identified through the central rift, which is unexpected in an intra-arc rifting environment. We attempt to explain the source/s of shortening and extension and discuss their implications for geodetic monitoring efforts in the OVC.

Description: We develop an approach for simulating acousto-elastic wave phenomena, including scattering from fluid–solid boundaries, where the solid is allowed to be anisotropic, with the discontinuous Galerkin method. We use a coupled first-order elastic strain-velocity, acoustic velocity–pressure formulation, and append penalty terms based on interior boundary continuity conditions to the numerical (central) flux so that the consistency condition holds for the discretized discontinuous Galerkin weak formulation. We incorporate the fluid–solid boundaries through these penalty terms and obtain a stable algorithm. Our approach avoids the diagonalization into polarized wave constituents such as in the approach based on solving elementwise Riemann problems.

Description: The mechanical damage characteristics of sandstone subjected to cyclic loading is very significant to evaluate the stability and safety of deep excavation damage zones. However to date, there are very few triaxial experimental studies of sandstone under cyclic loading. Moreover, few X-ray micro-computed tomography (micro-CT) observations have been adopted to reveal the damage mechanism of sandstone under triaxial cyclic loading. Therefore, in this research, a series of triaxial cyclic loading tests and X-ray micro-CT observations were conducted to analyse the mechanical damage characteristics of sandstone with respect to different confining pressures. The results indicated that at lower confining pressures, the triaxial strength of sandstone specimens under cyclic loading is higher than that under monotonic loading; whereas at confining pressures above 20 MPa, the triaxial strength of sandstone under cyclic loading is approximately equal to that under monotonic loading. With the increase of cycle number, the crack damage threshold of sandstone first increases, and then significantly decreases and finally remains constant. Based on the damage evolution of irreversible deformation, it appears that the axial damage value of sandstone is all higher than the radial damage value before the peak strength; whereas the radial damage value is higher than the axial damage value after the peak strength. The evolution of Young's modulus and Poisson's ratio of sandstone can be characterized as having four stages: (i) Stage I: material strengthening; (ii) Stage II: material degradation; (iii) Stage III: material failure and (iv) Stage IV: structure slippage. X-ray micro-CT observations demonstrated that the CT scanning surface images of sandstone specimens are consistent with actual surface crack photographs. The analysis of the cross-sections of sandstone supports that the system of crack planes under triaxial cyclic loading is much more complicated than that under triaxial monotonic loading. More axial and lateral tensile cracks were observed in the specimens under cyclic loading than under monotonic loading.

Description: The relative seismic velocity variations possibly associated to large earthquakes can be readily monitored via cross-correlation of seismic noise. In a recently published study, more than 2 yr of continuous seismic records have been analysed from three stations surrounding the epicentre of the 2009 April 6, M w 6.1 L'Aquila earthquake, observing a clear decrease of seismic velocities likely corresponding to the co-seismic shaking. Here, we extend the analysis in space, including seismic stations within a radius of 60 km from the main shock epicentre, and in time, collecting 5 yr of data for the six stations within 40 km of it. Our aim is to investigate how far the crustal damage is visible through this technique, and to detect a potential post-seismic recovery of velocity variations. We find that the co-seismic drop in velocity variations extends up to 40 km from the epicentre, with spatial distribution (maximum around the fault and in the north–east direction from it) in agreement with the horizontal co-seismic displacement detected by global positioning system (GPS). In the first few months after L'Aquila earthquake, the crust's perturbation in terms of velocity variations displays a very unstable behaviour, followed by a slow linear recovery towards pre-earthquake conditions; by almost 4 yr after the event, the co-seismic drop of seismic velocity is not yet fully recovered. The strong oscillations of the velocity changes in the first months after the earthquake prevent to detect the fast exponential recovery seen by GPS data. A test of differently parametrized fitting curves demonstrate that the post-seismic recovery is best explained by a sum of a logarithmic and a linear term, suggesting that processes like viscoelastic relaxation, frictional afterlip and poroelastic rebound may be acting concurrently.

Description: Computationally efficient 3-D frequency-domain full waveform inversion (FWI) is applied to ocean-bottom cable data from the Valhall oil field in the visco-acoustic vertical transverse isotropic (VTI) approximation. Frequency-domain seismic modelling is performed with a parallel sparse direct solver on a limited number of computer nodes. A multiscale imaging is performed by successive inversions of single frequencies in the 3.5–10 Hz frequency band. The vertical wave speed is updated during FWI while density, quality factor Q P and anisotropic Thomsen's parameters and are kept fixed to their initial values. The final FWI model shows the resolution improvement that was achieved compared to the initial model that was built by reflection traveltime tomography. This FWI model shows a glacial channel system at 175 m depth, the footprint of drifting icebergs on the palaeo-seafloor at 500 m depth, a detailed view of a gas cloud at 1 km depth and the base cretaceous reflector at 3.5 km depth. The relevance of the FWI model is assessed by frequency-domain and time-domain seismic modelling and source wavelet estimation. The agreement between the modelled and recorded data in the frequency domain is excellent up to 10 Hz although amplitudes of modelled wavefields propagating across the gas cloud are overestimated. This might highlight the footprint of attenuation, whose absorption effects are underestimated by the homogeneous background Q P model ( Q P = 200). The match between recorded and modelled time-domain seismograms suggests that the inversion was not significantly hampered by cycle skipping. However, late arrivals in the synthetic seismograms, computed without attenuation and with a source wavelet estimated from short-offset early arrivals, arrive 40 ms earlier than the recorded seismograms. This might result from dispersion effects related to attenuation. The repeatability of the source wavelets inferred from data that are weighted by a linear gain with offset is dramatically improved when they are estimated in the FWI model rather than in the smooth initial model. The two source wavelets, estimated in the FWI model from data with and without offset gain, show a 40 ms time-shift, which is consistent with the previous analysis of the time-domain seismograms. The computational efficiency of our frequency-domain approach is assessed against a recent time-domain FWI case study performed in a similar geological environment. This analysis highlights the efficiency of the frequency-domain approach to process a large number of sources and receivers with limited computational resources, thanks to the efficiency of the substitution step performed by the direct solver. This efficiency can be further improved by using a block-low rank version of the multifrontal solver and by exploiting the sparsity of the source vectors during the substitution step. Future work will aim to update attenuation and density at the same time of the vertical wave speed.

Description: Joint analysis of the seismic velocities and geoid, gravity and gravity gradients are used to constrain the viscosity profile within the mantle as well as the lateral density variations. Recent ESA's Gravity field and steady-state Ocean Circulation Explorer measurements of the second-order derivatives of the Earth's gravity potential give new possibilities to determine these mantle properties. Using a simple mantle model and seismic tomography results, we investigate how the gravitational potential, the three components of the gravity vector and the gravity gradients can bring information on the radial viscosity profile and on the mantle mass anomalies. We start with lateral density variations in the Earth's mantle based either on slab history or deduced from seismic tomography. The main uncertainties are: for the latter case, the relationship between seismic velocity and density—the so-called density/velocity scaling factor—and for the former case, the variation with depth of the density contrast between the cold slabs and the surrounding mantle. We perform a Monte Carlo search for the viscosity and the density/velocity scaling factor profiles within the mantle, which allows to fit the observed geoid, gravity and gradients of gravity. We compute the posterior probability distribution of the unknown parameters, and find that the gravity gradients improve the estimate of the scaling factor within the upper mantle, because of their sensitivity to the masses within the upper mantle, whereas the geoid and the gravity better constrain the scaling factor in the lower mantle. In the upper mantle, it is less than 0.02 in the upper part and about 0.08–0.14 in the lower part, and it is significantly larger for depths greater than 1200 km (about 0.32–0.34). In any case, the density/velocity scaling factor between 670 and 1150 km depth is not well constrained. We show that the viscosity of the upper part of the mantle is strongly correlated with the viscosity of the lower part of the mantle and that the viscosity profile is characterized by a decrease in the lower part of the upper mantle (about 10 20 –2  x  10 20 Pa s) and by an increase (about 10 23 –2  x  10 23 Pa s) at the top of the lower mantle (between 670 and 1150 km). The viscosity of the mantle below 1150 km depth is well estimated in our Monte Carlo search and is about 10 22 –4  x  10 22 Pa s.

Description: In this study we derive a spectral model describing the source, propagation and site characteristics of S waves recorded in central Italy. To this end, we compile and analyse a high-quality data set composed of more than 9000 acceleration and velocity waveforms in the local magnitude ( M l ) range 3.0–5.8 recorded at epicentral distances smaller than 120 km. The data set spans the time period from 2008 January 1 to 2013 May 31, and includes also the 2009 L'Aquila (moment magnitude M w 6.1, M l = 5.8) sequence. This data set is suitable for the application of data-driven approaches to derive the empirical functions for source, attenuation and site terms. Therefore, we apply a non-parametric inversion scheme to the acceleration Fourier spectra of the S waves of 261 earthquakes recorded at 129 stations. In a second step, with the aim of defining spectral models suitable for the implementation in numerical simulation codes, we represent the obtained non-parametric source and propagation terms by fitting standard parametric models. The frequency-dependent attenuation with distance r shows a complex trend that we parametrize in terms of geometrical spreading, anelastic attenuation and high-frequency decay parameter k. The geometrical spreading term is described by a piecewise linear model with crossover distances at 10 and 70 km: in the first segment, the spectral ordinates decay as 〈 tex – mathid = " IM 0001" 〉 r – 1.01 while in the second as 〈 tex – mathid = " IM 0002" 〉 r – 1.68 . Beyond 70 km, the attenuation decreases and the spectral amplitude attenuate as 〈 tex – mathid = " IM 0003" 〉 r – 0.64 . The quality factor Q ( f ) and the high-frequency attenuation parameter k , are 〈 tex – mathid = " IM 0004" 〉 Q ( f ) = 290 f 0.16 and k = 0.012 s, respectively, the latter being applied only for frequencies higher than 10 Hz. The source spectra are well described by 2 models, from which seismic moment and stress drops of 231 earthquakes are estimated. We calibrate a new regional relationship between seismic moment and local magnitude that improves the existing ones and extends the validity range to 3.0–5.8. We find a significant stress drop increase with seismic moment for events with M w larger than 3.75, with so-called scaling parameter  close to 1.5. We also observe that the overall offset of the stress-drop scaling is controlled by earthquake depth. We evaluate the performance of the proposed parametric models through the residual analysis of the Fourier spectra in the frequency range 0.5–25 Hz. The results show that the considered stress-drop scaling with magnitude and depth reduces, on average, the standard deviation by 18 per cent with respect to a constant stress-drop model. The overall quality of fit (standard deviation between 0.20 and 0.27, in the frequency range 1–20 Hz) indicates that the spectral model calibrated in this study can be used to predict ground motion in the L'Aquila region.

Description: During arc-continent collision, buoyant sections of sediments and rifted continental crust from a subducting plate will accrete to the forearc of the upper plate as long as this backstop remains intact. Deformation of the oceanic arc and forearc block may ultimately lead to accretion of these mafic rock units to the new orogen. The Taiwan mountain belt, which formed at ~6.5 Ma by oblique convergence between the Eurasian passive margin and the overriding Luzon arc in northern Taiwan, offers important insight in this process, since the collision is more advanced in the north than in the south. The incipient stage of arc-collision can be studied in southern Taiwan, while the northern portion of the orogen is presently undergoing collapse due to a flip in the subduction polarity between the Eurasian Plate and the Philippine Sea Plate. In this study, we seismically image the structure of the northern section of the mountain belt with a tomographic inversion. We present marine and land-based seismic refraction data, as well as local earthquake data, from transect T6 of the Taiwan Integrated Geodynamic Research (TAIGER) program across the Taiwan mountain belt and the adjacent Ryukyu arc. Our 2-D compressional seismic velocity model for this transect, which is based on a tomographic inversion of 10 213 P -wave arrival times, shows that the Eurasian crystalline continental crust thickens from ~24 km in the Taiwan Strait to ~40 km beneath the eastern Central Range of Taiwan. The detailed seismic velocity structure of the Taiwan mountain belt shows vertical continuity in the upper 15 km, which suggests that rocks are exhumed to the surface here from the middle crust in a near-vertical path. The continental crust of the westernmost Ryukyu arc is almost as thick (~40 km) as in the adjacent northern Central Range of Taiwan, and it appears to override the leading edge of the Philippine Sea Plate offshore northeastern Taiwan. If we assume that the western Ryukyu arc crust also thickened in the collision, then the mountain belt is wider and less thick in northern Taiwan than in central Taiwan (~50 km), which may be the result of post-collisional extension in the north.

Description: In this study we derive a spectral model describing the source, propagation and site characteristics of S waves recorded in central Italy. To this end, we compile and analyse a high-quality data set composed of more than 9000 acceleration and velocity waveforms in the local magnitude ( M l ) range 3.0–5.8 recorded at epicentral distances smaller than 120 km. The data set spans the time period from 2008 January 1 to 2013 May 31, and includes also the 2009 L'Aquila (moment magnitude M w 6.1, M l = 5.8) sequence. This data set is suitable for the application of data-driven approaches to derive the empirical functions for source, attenuation and site terms. Therefore, we apply a non-parametric inversion scheme to the acceleration Fourier spectra of the S waves of 261 earthquakes recorded at 129 stations. In a second step, with the aim of defining spectral models suitable for the implementation in numerical simulation codes, we represent the obtained non-parametric source and propagation terms by fitting standard parametric models. The frequency-dependent attenuation with distance r shows a complex trend that we parametrize in terms of geometrical spreading, anelastic attenuation and high-frequency decay parameter k. The geometrical spreading term is described by a piecewise linear model with crossover distances at 10 and 70 km: in the first segment, the spectral ordinates decay as 〈 tex – mathid = " IM 0001" 〉 r – 1.01 while in the second as 〈 tex – mathid = " IM 0002" 〉 r – 1.68 . Beyond 70 km, the attenuation decreases and the spectral amplitude attenuate as 〈 tex – mathid = " IM 0003" 〉 r – 0.64 . The quality factor Q ( f ) and the high-frequency attenuation parameter k , are 〈 tex – mathid = " IM 0004" 〉 Q ( f ) = 290 f 0.16 and k = 0.012 s, respectively, the latter being applied only for frequencies higher than 10 Hz. The source spectra are well described by 2 models, from which seismic moment and stress drops of 231 earthquakes are estimated. We calibrate a new regional relationship between seismic moment and local magnitude that improves the existing ones and extends the validity range to 3.0–5.8. We find a significant stress drop increase with seismic moment for events with M w larger than 3.75, with so-called scaling parameter  close to 1.5. We also observe that the overall offset of the stress-drop scaling is controlled by earthquake depth. We evaluate the performance of the proposed parametric models through the residual analysis of the Fourier spectra in the frequency range 0.5–25 Hz. The results show that the considered stress-drop scaling with magnitude and depth reduces, on average, the standard deviation by 18 per cent with respect to a constant stress-drop model. The overall quality of fit (standard deviation between 0.20 and 0.27, in the frequency range 1–20 Hz) indicates that the spectral model calibrated in this study can be used to predict ground motion in the L'Aquila region.