Structure of the northwestern North Anatolian Fault Zone imaged via teleseismic scattering tomography
Sebastian RostG. A. HousemanA. W. FrederiksenDavid G. CornwellMetin KahramanSelda Altuncu PoyrazU. TeomanD. A. ThompsonN. TürkelliLevent GülenMurat UtkucuTim Wright
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SUMMARY Information on fault zone structure is essential for our understanding of earthquake mechanics, continental deformation and seismic hazard. We use the scattered seismic wavefield to study the subsurface structure of the North Anatolian Fault Zone (NAFZ) in the region of the 1999 İzmit and Düzce ruptures using data from an 18-month dense deployment of seismometers with a nominal station spacing of 7 km. Using the forward- and back-scattered energy that follows the direct P-wave arrival from teleseismic earthquakes, we apply a scattered wave inversion approach and are able to resolve changes in lithospheric structure on a scale of 10 km or less in an area of about 130 km by 100 km across the NAFZ. We find several crustal interfaces that are laterally incoherent beneath the surface strands of the NAFZ and evidence for contrasting crustal structures either side of the NAFZ, consistent with the presence of juxtaposed crustal blocks and ancient suture zones. Although the two strands of the NAFZ in the study region strike roughly east–west, we detect strong variations in structure both north–south, across boundaries of the major blocks, and east–west, parallel to the strike of the NAFZ. The surface expression of the two strands of the NAFZ is coincident with changes on main interfaces and interface terminations throughout the crust and into the upper mantle in the tomographic sections. We show that a dense passive network of seismometers is able to capture information from the scattered seismic wavefield and, using a tomographic approach, to resolve the fine scale structure of crust and lithospheric mantle even in geologically complex regions. Our results show that major shear zones exist beneath the NAFZ throughout the crust and into the lithospheric mantle, suggesting a strong coupling of strain at these depths.Keywords:
North Anatolian Fault
Seismometer
Passive seismic
Seismic Tomography
Abstract Lunar seismology has always suffered from the limited number of seismic stations and limited coverage of the seismic network. Additional seismic data are necessary to probe the lunar interior in depth. Instead of a costly new deployment of seismometers, the aim of this study is to investigate the possibility of using the Apollo 17 Lunar Surface Gravimeter (LSG) as a lunar seismometer. The LSG was designed to detect gravitational waves (associated to change in the curvature of spacetime) and tidal ground motion on the Moon, but the data were not investigated for seismic use partially because of a malfunction of the instrument. We first evaluated the influence of the malfunction through comparison with other Apollo seismic data and found that the effect of the malfunction is small, and the LSG detected seismic signals in a manner that was consistent with those of the other Apollo seismometers. Then we carried out source location with the additional station of the LSG. We relocated previously located deep moonquake nests to evaluate the influence of the LSG data, which are generally noisier than other Apollo seismic data. Then we located deep moonquake nests that were previously unlocatable. Forty deep moonquake nests were examined, and we located five new nests. One newly located nest, A284, was most likely to be located on the farside. This series of analyses indicates that the LSG functioned as a lunar seismometer, and that its data are useful for improving seismic analyses with the previous seismic data set of the Moon.
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Passive seismic plays an important role in the investigation of the interior structure of the Earth. Passive seismic is a 3-D seismic imaging of the target geology without using artificial surface sources. It uses multi-component seismic receivers to take advantage of shear wave energy generated by the microearthquakes thereby delivering a shear wave (Vs) velocity distribution estimate of the subsurface in addition to the conventional compressional (Vp) image. Recently, the passive seismic tomography surveys became an essential tool for the oil industry and modern reservoir management. The passive seismic technology is applied to investigate the relatively shallow depths that lie in hydrocarbon exploration window. In addition, some of the problems that are encountered in the conventional seismic explorations, for example salt domes effects, are solved using this technique. Passive Seismic Method constitutes the passive seismic transmission tomography in which 3-D images are created using the observed travel time of seismic signals originating from micro-earthquakes occurring below the target; and passive seismic emission tomography where the micro-seismic activity itself becomes the imaging target. The most straight-forward approach is to observe and record the direct arrivals of the seismic waves from these events and to map the distribution of hypocenter locations. Passive seismic technology, as an imaging and processing technique, challenges the following issues:<br>1. Identification of anisotropic flow and well targeting.<br>2. Determination of the three-dimensional VP and VP/VS velocity structure.<br>3. Analyzing the seismicity.<br>4. Getting under salt formations.<br>5. Description of the deformation processes of the reservoir.<br>6. Delineation of leaky fault structures, mapping active and conductive fractures of faults, at an<br>intermediate scale between borehole imaging and 3-D seismic imaging.<br>7. Predictive reservoir models thus Reducing uncertainty.<br>The Gulf of Suez, Egypt, is characterized by its high hydrocarbon potentialities where most of Egypt oil production comes from. The basic problems in exploration at the Gulf of Suez come from its complex geologic structural setting as well as the presence of anhydrites that mask the structures below. Therefore, Passive seismic transmission tomography (PSTT) creates 3-D images using the observed travel time of seismic signals originating from micro-earthquakes occurring below the so masked structures. The cost/benefit justification of 3D seismic applies to Passive Seismic. Deeper pool tests drilled with this coverage will have a much higher success rate. Coverage will provide risk-reducing information content. For example: new interpretation could prevent drilling of unsuccessful step-out wells ($1 MM savings per well). Additionally, PSTT may be the only viable seismic option for certain areas. One of the most important parts of the passive tomography investigation is the quality control of the results. This can be done using many different procedures and their correlation can lead to safe conclusions about the resolution power of the dataset and therefore the quality of the tomographic inversion results. The method used does not only verify the estimation of their accuracy, but also points out the areas of higher and lower analysis precision, thus making it easier to control the interpretation of the results. This paper represents the passive seismic technology as an alternative to the conventional seismic exploration for delineating the structures that are masked by salt domes and Anhydrites in the Gulf of Suez and other regions, as well.
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P065 PASSIVE SEISMIC TOMOGRAPHY A COMPLEMENTARY GEOPHYSICAL METHOD – SUCCESSFUL CASE STUDY Abstract 1 Passive Seismic Tomographic Inversion a method that used P and S-wave travel times from natural micro-earthquakes to accurately estimate 3D Vp (structural) and Vp/Vs (lithologic) information of the subsurface and its potential is presented here. The objective of the project was to use this complementary geophysical technique to aide in the imaging of the subsurface volume in the region in terms of hydrocarbon exploration and/or delineation. The results of this case study will show the very good agreement between the predicted estimates of passive versus the
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A new three-dimensional delay traveltime tomography is performed to image the intermediate structure of the western Gulf of Corinth. A large data set, collected in 1991 during a two-month passive tomographic experiment, has been reanalysed for the reconstruction of detailed Vp and Vs images. An improved tomography method, based on an accurate traveltime computation, is applied to invert simultaneously delayed P and S first-arrival traveltimes for both velocity and hypocentre parameters. We perform different synthetic tests to analyse the sensitivity of tomography results to the model parametrization and to the starting 1-D model selection. The analysis of the retrieved Vp and Vs models as well as deduced Vp/Vs and Vp·Vs images allows us to interpret and delineate the distribution of lithological variation, porosity/crack content and fluid saturation in the upper 9–11 km of the crust beneath the gulf. The tomographic models image a rather complex crustal structure, which is characterized by a vertical change in both velocity features and seismicity distribution. We identify a shallower zone of the crust (0–5 km depth), in which velocity distributions seem to be controlled by the still active N–S extensional regime and a deeper zone (7–11 km depth), which matches the seismogenic zone. The correlation between this latter and a specific unit of the Hellenic mountain structure (the Pyllite–Quartzite series) allows us to suggest a possible explanation for seismicity concentration in a narrow band at 7–9 km depth. Finally, the occurrence of clusters showing low-angle normal fault mechanisms in areas characterized by high Vp/Vs values indicates a possible role of fluids in triggering brittle creep along the identified low-angle normal faults.
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In this study, a method for characterizing ambient seismic noise in an urban park using a pair of Tromino3G+ seismographs simultaneously recording high-gain velocity along two axes (north-south and east-west) is presented. The motivation for this study is to provide design parameters for seismic surveys conducted at a site prior to the installation of long-term permanent seismographs. Ambient seismic noise refers to the coherent component of the measured signal that comes from uncontrolled, or passive sources (natural and anthropogenic). Applications of interest include geotechnical studies, modeling the seismic response of infrastructure, surface monitoring, noise mitigation, and urban activity monitoring, which may exploit the use of well-distributed seismograph stations within an area of interest, recording on a days-to-years scale. An ideal well-distributed array of seismographs may not be feasible for all sites and therefore, it is important to identify means for characterizing the ambient seismic noise in urban environments and limitations imposed with a reduced spatial distribution of stations, herein two stations. The developed workflow involves a continuous wavelet transform, peak detection, and event characterization. Events are classified by amplitude, frequency, occurrence time, source azimuth relative to the seismograph, duration, and bandwidth. Depending on the applications, results can guide seismograph selection (sampling frequency and sensitivity) and seismograph placement within the area of interest.
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We present results from a low-frequency passive seismic (LFPS) survey that was carried out over an oil field in southern Germany. The LFPS method analyses the spectrum of the ambient seismic noise recorded with broadband seismometers in order to test for
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<p>The North Anatolian Fault Zone (NAFZ) extends for about 1500 km in the Eastern Mediterranean region, from eastern Anatolia to the northern Aegean. The NAFZ is characterized by strong and frequent seismic activity, increasing the seismic hazard in the region. In the Sea of Marmara area (NW Turkey), the North Anatolian Fault splits into three main branches. The northern branch of the fault, the Main Marmara Fault (MMF), has produced several major earthquakes (M7+) in the past, with a recurrence time of about 250 years. At present, there is a 150 km seismic gap along the MMF which has not ruptured since 1766. The observed fault segmentation, with creeping and locked segments, is indicative of along-strike variability in the fault strength along the seismic gap.</p><p>Previous modeling studies in the Sea of Marmara have revealed how crustal heterogeneities effectively affect the thermal and mechanical states of the lithosphere and can likely explain the observed fault segmentation in the area. Therefore, constraining the 3D structure of the deeper crust and upper mantle below the Sea of Marmara is crucial to better assess the mechanical stability of the fault and the possible seismic hazards in the area. In this study, we make use of seismic tomography models and forward gravity modelling to gain insights into the 3D lithospheric structure below the Sea of Marmara. Two tomographic models are used to compute a 3D density model of the area relying on two distinct approaches for the crust and the lithospheric mantle. The results showcase a heterogeneous and rather complex crustal density distribution in the study area[m1]&#160;. The 3D density distributions are used in a second step to forward model the gravity response. The results from this new tomography-constrained 3D gravity modelling are then compared to published gravity data and iteratively corrected to fit the overall gravity signals. The final 3D lithospheric-scale density model of the study area will be the basis for thermo-mechanical modeling experiments aimed at improving our current understanding of the present-day geomechanical state of the Sea of Marmara and the MMF and its implications for the seismic hazard of the region.</p>
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We present a new global model for the Earth's lithosphere and upper mantle (LithoRef18) obtained through a formal joint inversion of 3-D gravity anomalies, geoid height, satellite-derived gravity gradients and absolute elevation complemented with seismic, thermal and petrological prior information. The model includes crustal thickness, average crustal density, lithospheric thickness, depth-dependent density of the lithospheric mantle, lithospheric geotherms, and average density of the sublithospheric mantle down to 410 km depth with a surface discretization of 2° × 2°. Our results for lithospheric thickness and sublithospheric density structure are in excellent agreement with estimates from recent seismic tomography models. A comparison with higher resolution regional studies in a number of regions around the world indicates that our values of crustal thickness and density are an improvement over a number of previous global crustal models. Given the strong similarity with recent tomography models down to 410 km depth, LithoRef18 can be readily merged with these seismic models to include seismic velocities as part of the reference model. We include several analyses of robustness and reliability of input data, method and results. We also provide easy-to-use codes to interrogate the model and use its predictions for the development of higher-resolution models. Considering the model's features and data fitting statistics, LithoRef18 will be useful in a wide range of geophysical and geochemical applications by serving as a reference or initial lithospheric model for (i) higher-resolution gravity, seismological and/or integrated geophysical studies of the lithosphere and upper mantle, (ii) including far-field effects in gravity-based regional studies, (iii) global circulation/convection models that link the lithosphere with the deep Earth, (iv) estimating residual, static and dynamic topography, (v) thermal modelling of sedimentary basins and (vi) studying the links between the lithosphere and the deep Earth, among others. Several avenues for improving the reliability of LithoRef18's predictions are also discussed. Finally, the inversion methodology presented in this work can be applied in other planets for which potential field data sets are either the only or major constraints to their internal structures (e.g. Moon, Venus, etc.).
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