Geophysical Journal International

Attias, Eric;Weitemeyer, Karen;Hölz, Sebastian;Naif, Samer;Minshull, Tim A;Best, Angus I;Haroon, Amir;Jegen-Kulcsar, Marion;Berndt, Christian

2018 Geophysical Journal International

doi: 10.1093/gji/ggy227

Summary We present high-resolution resistivity imaging of gas hydrate pipe-like structures, as derived from marine controlled-source electromagnetic (CSEM) inversions that combine towed and ocean-bottom electric field receiver data, acquired from the Nyegga region, offshore Norway. Two-dimensional CSEM inversions applied to the towed receiver data detected four new prominent vertical resistive features that are likely gas hydrate structures, located in proximity to a major gas hydrate pipe-like structure, known as the CNE03 pockmark. The resistivity model resulting from the CSEM data inversion resolved the CNE03 hydrate structure in high resolution, as inferred by comparison to seismically constrained inversions. Our results indicate that shallow gas hydrate vertical features can be delineated effectively by inverting both ocean-bottom and towed receiver CSEM data simultaneously. The approach applied here can be utilised to map and monitor seafloor mineralisation, freshwater reservoirs, CO2 sequestration sites and near-surface geothermal systems. CSEM, Gas and hydrate systems, Simultaneous inversion, Tomography © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

Vyas, J C;Mai, P M;Galis, M;Dunham, Eric M;Imperatori, W

2018 Geophysical Journal International

doi: 10.1093/gji/ggy219

Summary We investigate Mach wave coherence for kinematic supershear ruptures with spatially heterogeneous source parameters, embedded in 3D scattering media. We assess Mach wave coherence considering: 1) source heterogeneities in terms of variations in slip, rise time and rupture speed; 2) small-scale heterogeneities in Earth structure, parameterized from combinations of three correlation lengths and two standard deviations (assuming von Karman power spectral density with fixed Hurst exponent); and 3) joint effects of source and medium heterogeneities. Ground-motion simulations are conducted using a generalized finite-difference method, choosing a parameterization such that the highest resolved frequency is ∼5 Hz. We discover that Mach wave coherence is slightly diminished at near fault distances (< 10 km) due to spatially variable slip and rise time; beyond this distance the Mach wave coherence is more strongly reduced by wavefield scattering due to small-scale heterogeneities in Earth structure. Based on our numerical simulations and theoretical considerations we demonstrate that the standard deviation of medium heterogeneities controls the wavefield scattering, rather than the correlation length. In addition, we find that peak ground accelerations in the case of combined source and medium heterogeneities are consistent with empirical ground motion prediction equations for all distances, suggesting that in nature ground shaking amplitudes for supershear ruptures may not be elevated due to complexities in the rupture process and seismic wave-scattering. Mach wave, Kinematic rupture, 3D scattering media, Ground motion prediction equations © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

McGuire, Jeffrey J;Kaneko, Yoshihiro

2018 Geophysical Journal International

doi: 10.1093/gji/ggy201

Abstract The key kinematic earthquake source parameters: rupture velocity, duration and area, shed light on earthquake dynamics, provide direct constraints on stress-drop, and have implications for seismic hazard. However, for moderate and small earthquakes, these parameters are usually poorly constrained due to limitations of the standard analysis methods. Numerical experiments by Kaneko and Shearer [2014,2015] demonstrated that standard spectral fitting techniques can lead to roughly 1 order of magnitude variation in stress-drop estimates that do not reflect the actual rupture properties even for simple crack models. We utilize these models to explore an alternative approach where we estimate the rupture area directly. For the suite of models, the area averaged static stress drop is nearly constant for models with the same underlying friction law, yet corner frequency based stress-drop estimates vary by a factor of 5-10 even for noise free data. Alternatively, we simulated inversions for the rupture area as parameterized by the second moments of the slip distribution. A natural estimate for the rupture area derived from the second moments is A=πLcWc, where Lc and Wc are the characteristic rupture length and width. This definition yields estimates of stress drop that vary by only 10% between the models but are slightly larger than the true area-averaged values. We simulate inversions for the second moments for the various models and find that the area can be estimated well when there are at least 15 available measurements of apparent duration at a variety of take-off angles. The improvement compared to azimuthally-averaged corner-frequency based approaches results from the second moments accounting for directivity and removing the assumption of a circular rupture area, both of which bias the standard approach. We also develop a new method that determines the minimum and maximum values of rupture area that are consistent with a particular dataset at the 95% confidence level. For the Kaneko and Shearer models with 20+ randomly distributed observations and ∼10% noise levels, we find that the maximum and minimum bounds on rupture area typically vary by a factor of two and that the minimum stress drop is often more tightly constrained than the maximum. © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

Wang, Sheng;Li, Zhiwei

2018 Geophysical Journal International

doi: 10.1093/gji/ggy223

Summary S-wave velocity and attenuation structures of shallow sediments play important roles in accurate prediction of strong ground motion. However, it is more difficult to investigate the attenuation than velocity structures. In this study, we developed a new approach for estimating frequency-dependent S-wave attenuation ($$Q_S^{ - 1}$$) structures of shallow sediments based on multiple time window analysis of borehole seismograms from local earthquakes. Multiple time windows for separating direct and surface-reflected S-waves in local earthquake waveforms at borehole stations are selected with a global optimization scheme. With respect to different time windows, the transfer functions between direct and surface-reflected S-waves are achieved with a weighted averaging scheme, based on which frequency dependent $$Q_S^{ - 1}$$ values are obtained. Synthetic tests suggest that the proposed method can restore robust and reliable$$Q_S^{ - 1}$$ values, especially when the dataset of local earthquakes is not abundant. We utilize this method for local earthquake waveforms at 14 borehole seismic stations in the North China basin, and obtain $$Q_S^{ - 1}$$ values in 2 ∼ 10 Hz frequency band, as well as average $${V_P}$$, $${V_S}$$ and $${V_P}/{\rm{\;}}{V_S}$$ ratio for shallow sediments deep to a few hundred meters. Results suggest that $$Q_S^{ - 1}$$ values are to 0.01∼0.06, and generally decrease with frequency. The average attenuation structure of shallow sediments within the depth of a few hundred meters beneath 14 borehole stations in the North China basin can be modeled as $$Q_S^{ - 1} = 0.056{f^{ - 0.61}}$$. It is generally consistent with the attenuation structure of sedimentary basins in other areas, such as Mississippi Embayment sediments in the United States and Sendai basin in Japan. Seismic attenuation, Earthquake ground motions, Sedimentary basin processes, Time series analysis © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

Tan, Yuyang;Zhang, Haijiang;Li, Junlun;Yin, Chen;Wu, Furong

2018 Geophysical Journal International

doi: 10.1093/gji/ggy224

Summary Induced seismicity is widely detected during hydraulic fracture stimulation. To better understand the fracturing process, a thorough knowledge of the source mechanism is required. In this study, we develop a new method to determine the focal mechanism for induced seismicity. Three misfit functions are used in our method to measure the differences between observed and modeled data from different aspects, including the waveform, P wave polarity and S/P amplitude ratio. We minimize these misfit functions simultaneously using the neighbourhood algorithm. Through synthetic data tests, we show the ability of our method to yield reliable focal mechanism solutions and study the effect of velocity inaccuracy and location error on the solutions. To mitigate the impact of the uncertainties, we develop a joint inversion method to find the optimal source depth and focal mechanism simultaneously. Using the proposed method, we determine the focal mechanisms of 40 stimulation induced seismic events in an oil/gas field in Oman. By investigating the results, we find that the reactivation of pre-existing faults is the main cause of the induced seismicity in the monitored area. Other observations obtained from the focal mechanism solutions are also consistent with earlier studies in the same area. Induced seismicity, Downhole method, Waveform inversion © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

Sulaiman, Aseem;Elawadi, Eslam;Mogren, Saad

2018 Geophysical Journal International

doi: 10.1093/gji/ggy220

Abstract This study provides interpretation and modeling of gravity survey data to map the subsurface basement relief and controlling structures of a coastal area in the southwestern part of Saudi Arabia as an aid to groundwater potential assessment. The gravity survey data were filtered and analyzed using different edge detection and depth estimation techniques and concluded by 2-D modeling conducted along representative profiles to obtain the topography and depth variations of the basement surface in the area. The basement rocks are exposed in the eastern part of the area but dip westward beneath a sedimentary cover to depths of up to 2200 m in the west, while showing repeated topographic expressions related to a tilted fault-block structure that is dominant in the Red Sea rift zone. Two fault systems were recognized in the area. The first is a normal fault system trending in the NNW–SSE direction that is related to the Red Sea rift, and the second is a cross-cutting oblique fault system trending in the NE–SW direction. The interaction between these two fault systems resulted in the formation of a set of closed basins elongated in the NNW–SSE direction and terminated by the NE–SW fault system. The geomorphology and sedimentary sequences of these basins qualify them as potential regions of groundwater accumulation. Numerical modelling, Gravity anomalies and Earth structure, Structure of the Earth © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

Paap, Bob;Kraaijpoel, Dirk;Bakker, Marcel;Gharti, Hom Nath

2018 Geophysical Journal International

doi: 10.1093/gji/ggy225

Summary Gas extraction from the Groningen natural gas field, situated in the Netherlands, frequently induces earthquakes in the reservoir that cause damage to buildings and pose a safety hazard and a nuisance to the local population. Due to the dependence of the national heating infrastructure on Groningen gas, the short-term mitigation measures are mostly limited to a combination of spatiotemporal redistribution of gas production and strengthening measures for buildings. All options become more effective with a better understanding of both source processes and seismic wave propagation. Detailed wave propagation simulations improve both the inference of source processes from observed ground motions and the forecast of ground motions as input for hazard studies and seismic network design. The velocity structure at the Groningen site is relatively complex, including both deep high-velocity and shallow low-velocity deposits showing significant thickness variations over relatively small spatial extents. We performed a detailed three-dimensional wave propagation modelling study for an induced earthquake in the Groningen natural gas field using the spectral-element method. We considered an earthquake that nucleated along a normal fault with local magnitude of $${{\rm{M}}_{\rm{L}}} = 3$$. We created a dense mesh with element size varying from 12 to 96 m, and used a source frequency of 7 Hz, such that frequencies generated during the simulation were accurately sampled up to 10 Hz. The velocity/density model is constructed using a three-dimensional geological model of the area, including both deep high-velocity salt deposits overlying the source region and shallow low-velocity sediments present in a deep but narrow tunnel valley. The results show that the three-dimensional density/velocity model in the Groningen area clearly play a large role in the wave propagation and resulting surface ground motions. The 3d structure results in significant lateral variations in site response. The high-velocity salt deposits have a dispersive effect on the radiated wavefield, reducing the seismic energy reaching the surface near the epicentre. In turn, the presence of low-velocity tunnel valley deposits can locally cause a significant increase in peak ground acceleration. Here we study induced seismicity on a local scale and use SPECFEM3D to conduct full waveform simulations and show how local velocity variations can affect seismic records. Computational seismology, Wave propagation, Induced seismicity, numerical modelling © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

Provenzano, Giuseppe;Vardy, Mark E;Henstock, Timothy J

2018 Geophysical Journal International

doi: 10.1093/gji/ggy221

Summary Characterisation of the top 10-50 m of the subseabed is key for landslide hazard assessment, offshore structure engineering design and underground gas-storage monitoring. In this paper, we present a methodology for the stochastic inversion of ultra-high-frequency (UHF, 0.2-4.0 kHz) pre-stack seismic reflection waveforms, designed to obtain a decimetric-resolution remote elastic characterisation of the shallow sediments with minimal pre-processing and little a-priori information. We use a genetic algorithm in which the space of possible solutions is sampled by explicitly decoupling the short and long wavelengths of the P-wave velocity model. This approach, combined with an objective function robust to cycle skipping, outperforms a conventional model parametrisation when the ground-truth is offset from the centre of the search domain. The robust P-wave velocity model is used to precondition the width of the search range of the multi-parameter elastic inversion, thereby improving the efficiency in high dimensional parametrizations. Multiple independent runs provide a set of independent results from which the reproducibility of the solution can be estimated. In a real dataset acquired in Finneidfjord, Norway, we also demonstrate the sensitivity of UHF seismic inversion to shallow subseabed anomalies that play a role in submarine slope stability. Thus, the methodology has the potential to become an important practical tool for marine ground model building in spatially heterogeneous areas, reducing the reliance on expensive and time-consuming coring campaigns for geohazard mitigation in marine areas. Inverse theory, Controlled source seismology, Acoustic properties, Geomechanics © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)