ALBERT

Description: Rupture properties, such as rupture direction, length, propagation speed and source duration, provide important insights into earthquake mechanisms. One approach to estimate these properties is to investigate the body-wave duration that depends upon the relative location of the station with respect to the rupture direction. Under the assumption that the propagation is unilateral, the duration can be expressed as a function of the dip and azimuth of the rupture. Examination of duration measurements with respect to both the take-off angle and the azimuth is crucial to obtain robust estimates of rupture parameters, especially for nearly vertical rupture propagation. Moreover, limited data coverage, such as using only teleseismic data, can bias the source duration estimate for dipping ruptures, and this bias can map into estimates of other source properties such as rupture extent and rupture speed. Based upon this framework, we introduce an inversion scheme that uses the duration measurements to obtain four parameters: the source duration, a measure of the rupture extent and speed, and dip and azimuth of the rupture propagation. The method is applied to two deep-focus events in the Sea of Okhotsk region, an M w 7.7 event that occurred on 2012 August 14 and an M w 8.3 event from 2013 May 24. The source durations are 26 ± 1 and 37 ± 1 s, and rupture speeds are 49 ± 4 per cent and 26 ± 3 per cent of shear wave speed for the M w 7.7 and 8.3 events, respectively. The azimuths of the two ruptures are parallel to the trench, but are in opposite directions. The dips of the M w 7.7 and 8.3 events are constrained to be 48° ± 8° downdip and 19° ± 8° updip, respectively. The fit to the data is significantly poorer for the M w 8.3 event than the M w 7.7 event, suggesting that the unilateral rupture may not be a good assumption. The analysis is expanded into a multi-episode model, and a secondary episode is determined for the M w 8.3 event in the southeast direction. The two-episode model gives a better fit to the data than the unilateral model and is compatible with the back-projection analysis, demonstrating that the rupture propagation of the M w 8.3 event is complex.

Description: We take account of the effect of Earth's surface topography in quasi-dynamic earthquake cycle simulations using a boundary integral equation method. While we have so far assumed a homogeneous elastic half-space medium with a flat free surface, Earth's actual surface topography is complicated. Here, we constructed new slip response functions in half-space with an arbitrarily shaped surface topography in which we used slip response functions in full-space by introducing imaginary free surface cells in addition to embedded fault ones. By comparing analytical slip response functions in the case of a flat surface overlying half-space with the new ones, we developed a computationally efficient method for setting the Earth's surface region, which was divided into cells with the appropriate sizes depending on the fault source cell depth to maintain the computational accuracy. With these new slip response functions, we simulated simple interplate earthquake cycles in the region close to the Japan Trench, off Miyagi, Tohoku, in northeast Japan, which has the amplitude of 7 km in depth. Compared with the case where the flat surface level was set at the trench depth, the slip response functions for the case where actual seafloor topography was used had smaller amplitudes. Hence, the actual topography produces smaller recurrence times for earthquake cycles than that for the flat surface case. These effects of the actual Earth's surface topography mainly come from changes in the distance between the surface and the fault compared with the flat surface case. Changes in the slip response function also represent changes in the fault stiffness of the system. Considering the actual topography of the Earth's surface to be convex upwards as opposed to the flat, the fault stiffness becomes larger compared to the case of the flat Earth's surface. This leads to a change in the frictional instability, and sometime leads to the change in the way of rupture.

Description: The fate of subducted slabs is enigmatic, yet intriguing. We analyse seismic arrivals at ~20–50 s after the direct P wave in an array in northeast China (NECESSArray) recordings of four deep earthquakes occurring beneath the west-central Pacific subduction zones (from the eastern Indonesia to Tonga region). We employ the array analysing techniques of fourth root vespagram and beam-forming analysis to constrain the slowness and backazimuth of later arrivals. Our analyses reveal that these arrivals have a slightly lower slowness value than the direct P wave and the backazimuth deviates slightly from the great circle direction. Along with calculation of 1-D synthetic seismograms, we conclude that the later arrival is corresponding to an energy of S -to- P converted at a scatterer below the sources. Total five scatterers are detected at depths varying from ~700 to 1110 km in the study region. The past subducted oceanic crust most likely accounts for the seismic scatterers trapped in the mid-mantle beneath the west-central subduction zones. Our observation in turn reflects that oceanic crust at least partly separated from subducted oceanic lithosphere and may be trapped substantially in the mid-mantle surrounding subduction zones, in particular in the western Pacific subduction zones.

Description: We examine spatio-temporal patterns of microseismicity recorded during one month in an underground mine by addressing three key questions: (1) where does the seismicity occur? (2) Why does it occur in these locations? and (3) what triggers it? To obtain accurate locations, we perform a multiplet analysis and use a modified version of the double-difference (DD) relocation method. This approach leads to highly accurate relative event locations and requires groups of multiplets only. Most of the 281 relocated events are close to the main shaft and tunnels; thus we postulate seismicity is facilitated by stresses associated with the potential for subsidence in addition to the hoop stresses acting on the two vertical shafts. Most events occurred during certain hours of the day and there is a 68 per cent correlation with reported rock removal; therefore, it is likely they were triggered by static and dynamic stress perturbations caused by the transportation of debris along tunnels instead of our initial guess that blasting was the principal causative mechanism. Given that seismicity is present around the main shaft but absent close to the second one, we conclude that for seismicity to occur both a favourable stress state and additional external perturbing forces must exist, thus leading to dynamic event triggering in an initially stable stress situation. This analysis provides more insight into anthropogenic processes that might trigger seismicity, thereby facilitating identification of hazardous and potential damage areas in mine settings.

Description: We present synthetic tests of 2-D adjoint tomography of surface wave traveltimes obtained by the ambient noise cross-correlation analysis across the Czech Republic. The data coverage may be considered perfect for tomography due to the density of the station distribution. Nevertheless, artefacts in the inferred velocity models arising from the data noise may be still observed when weak regularization (Gaussian smoothing of the misfit gradient) or too many iterations are considered. To examine the effect of the regularization and iteration number on the performance of the tomography in more detail we performed extensive synthetic tests. Instead of the typically used (although criticized) checkerboard test, we propose to carry out the tests with two different target models—simple smooth and complex realistic models. The first test reveals the sensitivity of the result on the data noise, while the second helps to analyse the resolving power of the data set. For various noise and Gaussian smoothing levels, we analysed the convergence towards (or divergence from) the target model with increasing number of iterations. Based on the tests we identified the optimal regularization, which we then employed in the inversion of 16 and 20 s Love-wave group traveltimes.

Description: Least-squares migration (LSM) is a linearized inversion technique for subsurface reflectivity estimation. Compared to conventional migration algorithms, it can improve spatial resolution significantly with a few iterative calculations. There are three key steps in LSM, (1) calculate data residuals between observed data and demigrated data using the inverted reflectivity model; (2) migrate data residuals to form reflectivity gradient and (3) update reflectivity model using optimization methods. In order to obtain an accurate and high-resolution inversion result, the good estimation of inverse Hessian matrix plays a crucial role. However, due to the large size of Hessian matrix, the inverse matrix calculation is always a tough task. The limited-memory BFGS (L-BFGS) method can evaluate the Hessian matrix indirectly using a limited amount of computer memory which only maintains a history of the past m gradients (often m 〈 10). We combine the L-BFGS method with least-squares pre-stack Kirchhoff depth migration. Then, we validate the introduced approach by the 2-D Marmousi synthetic data set and a 2-D marine data set. The results show that the introduced method can effectively obtain reflectivity model and has a faster convergence rate with two comparison gradient methods. It might be significant for general complex subsurface imaging.

Description: We present a method for high-resolution imaging of lithospheric structures based on full waveform inversion of teleseismic waveforms. We model the propagation of seismic waves using our recently developed direct solution method/spectral-element method hybrid technique, which allows us to simulate the propagation of short-period teleseismic waves through a regional 3-D model. We implement an iterative quasi-Newton method based upon the L-BFGS algorithm, where the gradient of the misfit function is computed using the adjoint-state method. Compared to gradient or conjugate-gradient methods, the L-BFGS algorithm has a much faster convergence rate. We illustrate the potential of this method on a synthetic test case that consists of a crustal model with a crustal discontinuity at 25 km depth and a sharp Moho jump. This model contains short- and long-wavelength heterogeneities along the lateral and vertical directions. The iterative inversion starts from a smooth 1-D model derived from the IASP91 reference Earth model. We invert both radial and vertical component waveforms, starting from long-period signals filtered at 10 s and gradually decreasing the cut-off period down to 1.25 s. This multiscale algorithm quickly converges towards a model that is very close to the true model, in contrast to inversions involving short-period waveforms only, which always get trapped into a local minimum of the cost function.

Description: We analyse the physics and geometry of trade-offs between Earth structure and noise sources in interstation noise correlations. Our approach is based on the computation of off-diagonal Hessian elements that describe the extent to which variations in noise sources can compensate for variations in Earth structure without changing the misfit beyond the measurement uncertainty. Despite the fact that all ambient noise inverse problems are special in terms of their receiver configuration and data, some general statements concerning source-structure trade-offs can be made: (i) While source-structure trade-offs may be reduced to some extent by clever measurement design, there are inherent trade-offs that can generally not be avoided. These inherent trade-offs may lead to a mispositioning of structural heterogeneities when the noise source distribution is unknown. (ii) When attenuation is weak, source-structure trade-offs in ambient noise correlations are a global phenomenon, meaning that there is no noise source perturbation that does not trade-off with some Earth structure, and vice versa. (iii) The most significant source-structure trade-offs occur within two elliptically shaped regions connecting a potential noise source perturbation to each one of the receivers. (iv) Far from these elliptical regions, only small-scale structure can trade off against changes in the noise source. (v) While source-structure trade-offs mostly decay with increasing attenuation, they are nearly unaffected by attenuation when the noise source perturbation is located near the receiver-receiver line. This work is intended to contribute to the development of joint source-structure inversions of ambient noise correlations, and in particular to an understanding of the extent to which source-structure trade-offs may be reduced. It furthermore establishes the foundation of future resolution analyses that properly quantify trade-offs between noise sources and Earth structure.

Description: Reactive flow at depth (either related to underground activities, like enhancement of hydrocarbon recovery and CO 2 storage, or to natural flow like in hydrothermal zones) can alter fractures’ topography, which might in turn change their seismic responses. Depending on the flow and reaction rates, instability of the dissolution front can lead to a wormhole-like pronounced erosion pattern. In a fractal structure of rupture process, we question how the perturbation related to well-spaced long channels alters rupture propagation initiated on a weak plane and eventually the statistical feature of rupture appearance in frequency–magnitude distribution (FMD). Contrary to intuition, a spatially uniform dissolution is not the most remarkable case, since it affects all the events proportionally to their sizes leading to a downward translation of FMD: the slope of FMD ( b -value) remains unchanged. The parameter–space study shows that the increase of b -value (of 0.08) is statistically significant for optimum characteristics of the erosion pattern with spacing to length ratio of the order of ~1/40: large-magnitude events are more significantly affected leading to an imbalanced distribution in the magnitude bins of the FMD. The larger the spacing, the lower the channel's influence. Besides, a spatial analysis shows that the local seismicity anomaly concentrates in a limited zone around the channels: this opens perspective for detecting these eroded regions through high-resolution imaging surveys.

Description: Earth's free oscillations excited by a mega-thrust earthquake were observed by a continent-scale array of groundwater monitoring sites for the first time. After the occurrence of the 2011 Tohoku M w 9.0 earthquake, water level records at 43 out of 216 wells in the China mainland revealed long-period free oscillation signals. In the time domain, these free oscillations exhibit globe circling Rayleigh surface waves. In some single wells, even the globe-circling Rayleigh wave R7 was visible, which travels three times around the Earth after the first arrival and appears about 10 hr after the earthquake occurrence in the present case. The spectral analysis shows that the principal oscillatory fluctuations seen in the water level records correspond to the spheroidal modes 0 S l ( l  = 2–31 for frequencies between 0.3 and 5.0 mHz) of the Earth's free oscillation. Especially at quiet sites, the spheroidal modes at very low frequencies (〈1.5 mHz) can be identified with high signal-to-noise ratios. Using signal enhancement methods (product spectrum over 43 wells), even the gravest modes of these oscillations can be detected. The results suggest that groundwater level arrays can be considered as a low-cost complementary tool to study the Earth's free oscillations excited by great earthquakes. Additionally, the site-specific aquifer response may provide further insight into local hydrogeological conditions.