ALBERT

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 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: 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.

Description: In this study, information carried by the ambient seismic field is exploited to extract impulse response functions between two seismic stations using one as a ‘virtual’ source. Interferometry by deconvolution method is used and validated by comparing the extracted ambient noise impulse response waveforms with records of moderate magnitude earthquakes (from M w 4 to 5.8) that occurred close to the virtual source station in Japan. As the information is only available at low frequencies (less than 0.25 Hz), the ambient seismic field approach is coupled to a non-stationary stochastic model to simulate time domain accelerograms up to 50 Hz. This coupling allows the predicted ground motion to have both the deterministic part at low frequencies coming from the source and the crust structure and the high-frequency random contribution from the seismic waves scattering. The resulting combined accelerograms for an M w 5.8 event show a good agreement with observed ground motions from a real earthquake.

Description: On 2012 May 19, an m b  = 4 earthquake shook the town of Montes Claros, Brazil in the middle of the São Francisco Craton. Because of the scarce seismicity in the area, an event like this could provide valuable information to characterize the governing seismotectonics and stress field for the region. Here, we present the results of more than 1 yr of local seismic monitoring after the main shock. We found that the seismicity originated at approximately 1-km depth in an NNW-oriented blind reverse fault, dipping to the E. The magnitude of the main shock was 4 m b , with aftershocks reaching up to 3.6 m b . Focal mechanisms from first motion polarities and waveform moment tensor inversions indicate a reverse faulting in agreement with the orientation of the aftershock locations. In addition, we derived a new 1-D local velocity model using a simultaneous inversion of hypocentres and velocity layers. The results indicate P -wave velocities of 4.5 km s –1 for the upper layer of carbonate rocks and 5.23 and 5.69 km s –1 for the lower fractured and compact crystalline basement layers, respectively. Higher Vp / Vs ratios were obtained for the upper two layers compared to the lowermost layer, possibly indicating presence of rock fracturing and percolated water. The calculated stress drop for the main event is 0.33 MPa, which is a relatively low value for an intraplate earthquake but still within the observed range. The inversion of the main shock focal mechanism and previously published focal mechanisms suggests a compressional stress regime in the central part of the São Francisco Craton, which is different from the strike-slip regime in the southern part, although both have an EW-oriented 1. On the other hand, focal mechanisms of events located to the west of the craton indicate an NW–SE oriented 1 for central Brazil. This variability highlights the importance of local sources of stresses (e.g. flexural stresses) in mid-plate South America, unlike other mid-plate areas of the world, such as central and east North America, where a more uniform stress field is observed.

Description: The Paradox Valley Unit (PVU), a salinity control project in southwest Colorado, disposes of brine in a single deep injection well. Since the initiation of injection at the PVU in 1991, earthquakes have been repeatedly induced. PVU closely monitors all seismicity in the Paradox Valley region with a dense surface seismic network. A key factor for understanding the seismic hazard from PVU injection is the maximum magnitude earthquake that can be induced. The estimate of maximum magnitude of induced earthquakes is difficult to constrain as, unlike naturally occurring earthquakes, the maximum magnitude of induced earthquakes changes over time and is affected by injection parameters. We investigate temporal variations in maximum magnitudes of induced earthquakes at the PVU using two methods. First, we consider the relationship between the total cumulative injected volume and the history of observed largest earthquakes at the PVU. Second, we explore the relationship between maximum magnitude and the geometry of individual seismicity clusters. Under the assumptions that: (i) elevated pore pressures must be distributed over an entire fault surface to initiate rupture and (ii) the location of induced events delineates volumes of sufficiently high pore-pressure to induce rupture, we calculate the largest allowable vertical penny-shaped faults, and investigate the potential earthquake magnitudes represented by their rupture. Results from both the injection volume and geometrical methods suggest that the PVU has the potential to induce events up to roughly M W 5 in the region directly surrounding the well; however, the largest observed earthquake to date has been about a magnitude unit smaller than this predicted maximum. In the seismicity cluster surrounding the injection well, the maximum potential earthquake size estimated by these methods and the observed maximum magnitudes have remained steady since the mid-2000s. These observations suggest that either these methods overpredict maximum magnitude for this area or that long time delays are required for sufficient pore-pressure diffusion to occur to cause rupture along an entire fault segment. We note that earthquake clusters can initiate and grow rapidly over the course of 1 or 2 yr, thus making it difficult to predict maximum earthquake magnitudes far into the future. The abrupt onset of seismicity with injection indicates that pore-pressure increases near the well have been sufficient to trigger earthquakes under pre-existing tectonic stresses. However, we do not observe remote triggering from large teleseismic earthquakes, which suggests that the stress perturbations generated from those events are too small to trigger rupture, even with the increased pore pressures.

Description: P and S receiver functions (PRF and SRF) from 19 seismograph stations in the Gibraltar Arc and the Iberian Massif reveal new details of the regional deep structure. Within the high-velocity mantle body below southern Spain the 660-km discontinuity is depressed by at least 20 km. The Ps phase from the 410-km discontinuity is missing at most stations in the Gibraltar Arc. A thin (~50 km) low- S -velocity layer atop the 410-km discontinuity is found under the Atlantic margin. At most stations the S410p phase in the SRFs arrives 1.0–2.5 s earlier than predicted by IASP91 model, but, for the propagation paths through the upper mantle below southern Spain, the arrivals of S410p are delayed by up to +1.5 s. The early arrivals can be explained by elevated Vp / Vs ratio in the upper mantle or by a depressed 410-km discontinuity. The positive residuals are indicative of a low (~1.7 versus ~ 1.8 in IASP91) Vp / Vs ratio. Previously, the low ratio was found in depleted lithosphere of Precambrian cratons. From simultaneous inversion of the PRFs and SRFs we recognize two types of the mantle: ‘continental’ and ‘oceanic’. In the ‘continental’ upper mantle the S -wave velocity in the high-velocity lid is 4.4–4.5 km s –1 , the S -velocity contrast between the lid and the underlying mantle is often near the limit of resolution (0.1 km s –1 ), and the bottom of the lid is at a depth reaching 90–100 km. In the ‘oceanic’ domain, the S -wave velocities in the lid and the underlying mantle are typically 4.2–4.3 and ~ 4.0 km s –1 , respectively. The bottom of the lid is at a shallow depth (around 50 km), and at some locations the lid is replaced by a low S -wave velocity layer. The narrow S–N-oriented band of earthquakes at depths from 70 to 120 km in the Alboran Sea is in the ‘continental’ domain, near the boundary between the ‘continental’ and ‘oceanic’ domains, and the intermediate seismicity may be an effect of ongoing destruction of the continental lithosphere.