Subsurface Fault Research Articles

The July 2019 Ridgecrest, California, Earthquake Sequence: Kinematics of Slip and Stressing in Cross‐Fault Ruptures

AbstractThe July 2019 Ridgecrest, California, earthquake sequence produced cross‐fault ruptures from a Mw6.4 left‐lateral foreshock and a Mw7.1 right‐lateral mainshock. We use interferometric synthetic aperture radar and satellite optical imagery to characterize the surface displacements and subsurface fault slip characteristics of the sequence. We document ~46 km of surface rupture and peak slip values of ~5 m associated with the Mw7.1 and evidence that the two ruptures crossed each other. We additionally document evidence of triggered creep along 20–25 km of the central Garlock fault. Static stress change analysis shows that the foreshock sequence systematically promoted slip at the Mw7.1 hypocenter. Moreover, we find static stress changes promoted slip on the Garlock fault only in locations where we document surface creep, strongly indicating that the Garlock fault is sensitive to static stress changes. A potential rupture of the Garlock fault where slip was promoted could produce a Mw6.7–7.0 earthquake.

Appraisal of Potential Hydrocarbon Zones in Masila Oil Field, Yemen

Hydrocarbons serve the prime energy source of the world and the backbone to industrial development. Hydrocarbon potential is explored here using the spatial multi-criteria decision-making method. In order to demarcate the hydrocarbon potential zones within Masila oil field, Yemen, various thematic layers of multi-criteria factors affecting the hydrocarbons such as basement depth map (BDM), gravity anomaly map (GAM), magnetic anomaly map (MAM), subsurface faults system map (SFSM), upper syn-rift isopach map (USRIM), post-rift isopach map (PRIM), and lower syn-rift isopach map (LSRIM) were generated using the information extracted from both the geophysical and wells data available for the oil field. Geographical information system (GIS) techniques were used to generate various thematic maps that were subsequently converted to raster format and reclassified into various classes on the basis of their relative importance and the opinion of relevant experts. These thematic maps and their feature classes were assigned weights (eigenvalues) and rank by using the analytic hierarchy process (AHP) and following the theories of multi-criteria analysis (MCA). Furthermore, these maps were superimposed using the weighted linear combination (WLC) model on the GIS platform. The results of the integration of the thematic maps were classified into three classes representing the three hydrocarbon potential zones, namely, high potential zone (39.8%), moderate potential zone (49.1%), and low potential zone (11.1%). The cross plots of occurrence of the hydrocarbon reservoir, existing oil fields, and well locations were inferred for validation. The validation results revealed a good prospect for hydrocarbon potential in the study area. The novel quantitative approach used in the present study is capable of providing comprehensive results and assessment of hydrocarbons in the Masila oil field and other areas as well.

Results, problems and future tasks of palaeostress and fault-slip analyses in the Pannonian Basin: the Hungarian contribution

In the Pannonian Basin fault-slip analysis and palaeostress determinations started in 1982–1984 by the work of Françoise Bergerat, Jacques Geyssant and Claude Lepvrier. After F. Bergerat introduced some Hungarian researchers to field and laboratory techniques, fault-slip analysis expanded into the Pannonian Basin in Hungary and in some adjacent areas, and is still going on. This paper reviews the results achieved during this ~35 year time span from a historical perspective, by the compilation of the determined Cenozoic stress fields, their temporal boundaries, and uncertainties. The model for the evolution of the stress field became more complicated, and more and more details in the deformation history were realised, both in the Alcapa and Tisza–Dacia units of the Pannonian Basin. The integration of palaeomagnetic and stress data indicated that some changes in stress axes (and in fault orientation) are due to vertical-axis block rotation, and are not real rotation of the external stress field itself. With the development of new approaches, like tilt test, combination of surface and subsurface fault data, radiometric age determination of deformed rocks, fine separation of fracture sets can be achieved and the multiple recurrences of stress fields with identical principal axes were demonstrated. The refined methods permitted the demonstration of basin-wide inhomogeneity of the stress field for some phases. However, the precise timing, the number of phases, separation of events with similar stress axes, and the variability of the maximal stress axes of a given stress field remain the major questions to be solved in the future. Rheological conditions of the deforming media and physical parameters of the fracturing will be the other future steps in research, to be achieved by studies of deformation bands, fluid inclusions, isotopes, in order to better understand the connection of deformation, subsidence, diagenesis, and fluid flow.

Study of diurnal and seasonal variations in the time series data of soil 222Rn gas

ABSTRACT The diurnal and seasonal variation study has been performed for soil radon time series (RTS) data obtained using RTM-1688 radon monitor (SARAD GmbH, Dresden, Germany). Data were collected from radon station installed at latitude 34.39621 and longitude 73.47347 on fault line passing beneath the Muzaffarabad city, for a period of 1 year. RTS data were analysed based upon their monthly recorded values followed by an interpretation of season-wise data, based upon 10-day measurements, in order to get more insight into radon variations. Minimum value of 1.3553 × 104 Bq m−3 was recorded for the first 10 days of the month of March 2017, whilst maximum value 2.7902 × 104 Bq m−3 was recorded from 8th to 17th day of August 2017. Overall mean value for RTS data were found to be (2.1024 ± 0.2088) × 104 Bq m−3. Data obtained for the current study show that the variations observed in the season of spring are comparatively smaller in magnitude than the other three seasons. Month-based correlation between soil radon gas with environmental parameters has also been investigated for the entire study period. For the months starting from March to September, a positive correlation of radon in soil with the relative humidity and pressure has been observed while no significant correlation was found between radon in soil and temperature for these months. For October to December 2017, the soil radon gas was mainly influenced by temperature variations and almost the same pattern was observed for the months of January and February with additional impact coming from pressure. The current study findings can be used as an indicator for some applications, viz., seismic activity, location of subsurface faults, indoor radon concentration and uranium exploration.

Spatial distribution of gas hydrate deposits in Krishna-Godavari offshore basin, Bay of Bengal

To delineate the occurrences of gas hydrates and free gas in the Krishna-Godavari (KG) basin, Bay of Bengal, we used 3D seismic data to construct a velocity model of high spatial density. We used an automated version of migration velocity analysis to estimate P-wave velocities using Kirchoff prestack time migration. We manually checked all common image gathers (CIGs) to assess the reliability of the auto-picking algorithm. The velocity model was validated against available downhole information from drilling Sites NGHP-01-02, NGHP-01-03, and NGHP-01-10. The model shows a) anomalously high velocities at Site NGHP-01-10 where significant gas hydrates were reported and b) background velocities at Sites NGHP-01-02 and NGHP-01-03 where a negligible amount of hydrate was recovered. A bottom simulating reflector (BSR) is identified in the 3D seismic data, and an average velocity map is prepared above the BSR. Background velocities of marine sediments in the KG basin vary from 1.43 km/s to 1.6 km/s; velocities higher than 1.65 km/s indicate gas hydrate bearing sediments while velocities lower than 1.43 km/s indicate free gas bearing sediments. High velocities observed below seafloor mounds and ridges suggest the presence of gas hydrate in a structurally deformed area. Slope basins are generally devoid of gas hydrates. Free gas is observed at depths of up to several hundred meters below the BSR suggesting the migration of deeper fluid/gas into the gas hydrate stability zone (GHSZ). Zones of gas hydrates and free gas are preferentially oriented along major faults indicating the role of faults in the transportation of free gas and formation of gas hydrates deposits. We identify a free gas bearing horizon at depths (>300 m) below the BSR, which might contribute to the lateral migration of free gas. Gas hydrate deposits in KG basin are classified into two categories: i) leaky deposits which occur in the areas of active seafloor faults, and are characterized by heterogeneous distribution of gas hydrates, and ii) non-leaky deposits which are formed in areas of subsurface faults, and show a relatively homogeneous distribution of gas hydrates. Site NGHP-01-10 is a good example of leaky gas hydrate deposits above a deep-seated gas reservoir.

A GIS-based tool for automatic extraction of drainage anomalies. A case study of enfidha (North-East of Tunisia)

It is particularly useful to analyze drainage anomalies for morphostructural interpretation of low relief areas. In basins and sub-basins, these anomalies may be due to local changes resulting from lithological discontinuities or tectonic activity caused by subsidence or uplift. Indeed, drainage anomalies are the most revealing geomorphological evidence of active faults. We present a new method to extract the drainage anomalies in watershed. The adapted methodology is therefore based on the drainage anomalies extraction in the Enfidha plain, indicating that the area is potentially tectonically active. Our objective in this article is to develop an automatic extraction tool, using the model builder technique in the ArcGIS environment, to extract drainage anomalies. To use this tool, a Digital Elevation Model (DEM) and drainage networks are required. In order to obtain the desired results, several tools are used: filtering of the DEM, generation of the summit surface, extraction of the slope orientation, calculation of the stream direction, etc. Based on the Automatic Drainage Anomalies Extraction Tool (ADAET), the relationship between drainage anomalies and tectonics was studied in the Enfidha sector of north-eastern Tunisia. The sub-watershed selected for this study is drained by numerous rivers and occupies the northeastern part of the Enfidha plain. This plain is known for its relatively high seismicity. Therefore, the method of automatic extraction of drainage anomalies in a GIS environment based on a global DEM (SRTM), detailed in this paper, appears very promising and can be of great utility in morpho-structural and morpho-neotectonic studies. This method allows to discover a high concentration of drainage anomalies on the Oued El Boul river and in the plain. Correlation with existing tectonic models shows that parts of the selected drains are located near the subsurface fault. Therefore, the drainage anomalies prove the existence of neotectonic deformation in the terrain. The activity of this fault has been proven by several authors. In this region, the extraction of drainage anomalies is most often used and reflects neotectonic activities.

Seismic damage zone and width\u2013throw scaling along the strike-slip faults in the Ordovician carbonates in the Tarim Basin

Understanding the scaling relation of damage zone width with displacement of faults is important for predicting subsurface faulting mechanisms and fluid flow processes. The understanding of this scaling relationship is influenced by the accuracy of the methods and types of data utilized to investigate faults. In this study, seismic reflection data are used to investigate the throw and damage zone width of five strike-slip faults affecting Ordovician carbonates of the Tarim intracraton basin, NW China. The results indicate that fault slips with a throw less than 200 m had formed wide damage zones up to 3000 m in width. Also, damage zone width is found to have both a positive correlation and a power-law relation with throw of two orders of magnitude, with a ratio of these values varying in a range of 2–15. The relationship between throw and damage zone width is not a simple power-law and changes its slope from small to larger size faults. The results indicate that throw scales well with damage zone width for the studied faults, and hence these can be used to predict fault geometries in the Tarim Basin. The study of the wide carbonate damage zones presented here provides new insights into scaling of large-size faults, which involve multiple faulting stages.

An assessment of geomorphometric anomalies and their significance on the seismotectonic activity through geoinformatics

The natural system of the drainage basin is interrupted by sub-surface faults and active tectonic processes. The anomalous nature of the drainage network and its basic morphological characteristics produce significant geomorphometric anomalies of the drainage basins which are an indicator of seismotectonic activity. In recent years, the increasing frequency of earthquakes and the subsequent damage in the Indian sub-continent necessitates a study of the seismotectonic significance with special reference to morphometric analysis. Hence, the present research is focused on studying the geomorphometric anomalies at a micro basin level and their impact on seismotectonic activity using geoinformatics. The study area is the lower Tista sub-basin which belongs to the Himalayan mountain in the northern part, whereas the southern part represents a peneplain surface. The quantitative databases of various morphometric parameters have been generated using digital elevation model data and satellite images to identify the geomorphic anomalies at the micro basin level which have been compared with past seismological databases to understand the impact on seismotectonic activity. The result shows that the drainage bifurcation, anomalous drainage network, drainage compression along the mountain front, narrow steep valleys in the high resistance surface, neotectonic deformation and the asymmetric valley shape are probable geomorphometric anomalies which coincide with past seismic activity in the study region. The present study also shows that the tectonic tilt, compressed meanders in the uplifted surface and the linear valley in the alluvial surface are also equivalently significant for past earthquake occurrences in the foothill zone. It is also observed that the elongated and linear micro basins and the presence of a strike–slip fault with major lineaments in the Quaternary surface are great indicators of geomorphometric anomalies which have no resemblance with any significant earthquake in the past and are equally important signatures of seismotectonic activity. The study concludes that the geomorphometric anomaly is an important parameter of seismotectonic activity which can be used for studying seismic hazards in the future.

Geological and Geophysical Observations to Determine the Gas Seepage Source of Titas Gas Field Region, Bangladesh

Titas Gas Field, the largest gas field in Bangladesh, has been encountering gas seepages in numerous points at the surface in an area of about 7 sq. km. at the southeastern part of the field since 2006. Gas has been seeping through the water wells, small and large holes in the fields including agricultural lands, in the river and through the cracks in the ground. The present research attempts to point out the source of the gas seepages based on the field studies, wireline log analyses and other available borehole data. A reconnaissance resistivity survey has been carried out around the high seepages area to detect any evidence of shallow subsurface fault. No fault was detected by resistivity survey in the shallow depth in the seepage area and thus fault as a conduit for the seepage could not be confirmed. Primarily, all wells of Titas Well Location (TWL-C) (Titas-06, 08, 09 and 10) were suspected as possible source wells as the surface distribution of seepages generally clusters around TWL-C. Titas-06 and Titas-08 were taken out of suspect list as the gas seepage distributions do not follow well path and cement bonding against reservoir sands including ‘A’ sand in these two wells are also good. Gas seepages follow the well trajectories of Titas-09 and 10 wells but cement bonding against ‘A’ gas sand in Titas-09 also discarded the well as a probable source of the gas seepage. Also, the suggestion that Titas well-03 could be a source of gas seep is also ruled out because of the fact that it is located 3 km away from the seepage area, there is no evidence of any seepage in between the well 3 and the seepage area, and the gas sands are also well protected. The above evidences turned the whole focus onto Titas-10. Gamma ray, resistivity, density, sonic and neutron log signatures recorded initially in the Titas-10 well strongly indicated a major gas sand and has been designated ‘A1’ gas sand. On the basis of various logs including CBL/VDL log it is evident that about 23 meters (3157-3180-meter MD) gas sand at the top which is not protected by cement. It is most likely source of gas seeps in Titas gas field is the gap in the cement protection at the top of A sand in Titas-10 well.
 Asiat. Soc. Bangladesh, Sci. 45(1): 93-109, June 2019

Delineation of Damage Zones From 3 km Downhole Geophysical Logs in the Koyna Seismogenic Zone, Western India

AbstractDelineation of subsurface faults and damage zones is a major goal of scientific drilling projects in seismically active areas. Geophysical logs acquired in a 3‐km deep scientific borehole KFD1 in the Koyna seismogenic zone, a site of recurrent reservoir triggered seismicity over the past 55 years, provide an unprecedented opportunity to investigate the rock properties and delineate the fault zones. KFD1 passed through 1,247‐m thick Deccan traps and continued for 1,767 m into the underlying granitic basement rocks that host the seismic activity in the depth range 2–10 km. We have studied the physical properties and acoustic behavior of basement granitoids from the analysis of geophysical logs from 1,500 to 3,000 m. Salient results are as follows. (1) Seven anomalous zones are identified below 2,100‐m depth based on electrical resistivity, caliper, density, neutron porosity, self‐potential, and sonic data. (2) The anomalous zones are characterized by significant shear wave velocity anisotropy (up to 25%), as revealed by cross‐dipole sonic data. (3) Dispersion analysis of dipole flexural modes confirms that the anisotropy is primarily stress induced; fast polarized shear wave azimuth (FSA) therefore indicates the orientation of maximum horizontal compressive stress SHmax. (4) Comparison of FSA with independent estimates of SHmax orientations obtained from drilling induced tensile fractures and strike of inclined fractures in the anisotropic zones shows that FSA is controlled mainly by the stress regime. Therefore, the stress rotations inferred from anisotropy analyses in the anomalous zones indicate their association with subsurface fault damage zones.

Radiometric Surveys for Detection of Uranium in Dhofar Region, Sultanate of Oman

The Proterozoic basement of the Dhofar region of the Sultanate of Oman might be considered as a potential source of Uranium mineralization like other Proterozoic basins of the world, which collectively contribute 30%-40% to the global production. Uranium exploration assumes great importance in this part of the world and therefore geochemical and geophysical exploration for Uranium was initiated. Initial, random sampling of the study area with Gamma ray spectrometry as well as in situ XRF (Xray Florescence) analysis indicated the presence of around 30-40 ppm of Uranium. Since Gamma ray spectrometry results in the calculation of the heat produced during radioactive decay of Potassium (K), Uranium (U) and Thorium (Th) within the rocks in the study area, measurements were acquired along eleven traverses of varying length from 100-250 m at a traverse interval of 20 m and sampling interval of 10 m. Individual plots of the concentrations of radioelements such as U, K and Th have shown favorable anomalous radioactive sources. Besides this, Uranium to Thorium (eU/eTh) and Uranium to Potassium (eU/K) ratio plots have been presented and the alteration zones associated with Uranium mineralization through all traverses have been identified. Further, the generated composite ternary image based on the combination of K, Th and U from Gamma ray spectrometry determines different relative amounts of radioelements and paves the way for deciphering the level of radioactivity in the study area. However, there seems to be no presence of strong Uranium anomalies in the near surface of the limited study area. As the study area is heavily faulted, the depth to concealed subsurface fault structures is estimated to be around 64 m based on Hartley spectral analysis of total magnetic anomaly (line-6).

Accounting for uncertain fault geometry in earthquake source inversions – II: application to the Mw 6.2 Amatrice earthquake, central Italy

SUMMARY Our understanding of earthquake sources is limited by the availability and the quality of observations and the fidelity of our physical models. Uncertainties in our physical models will naturally bias our inferences of subsurface fault slip. These uncertainties will always persist to some level as we will never have a perfect knowledge of the Earth’s interior. The choice of the forward physics is thus ambiguous, with the frequent need to fix the value of several parameters such as crustal properties or fault geometry. Here, we explore the impact of uncertainties related to the choice of both fault geometry and elastic structure, as applied to the 2016 Mw 6.2 Amatrice earthquake, central Italy. This event, well instrumented and characterized by a relatively simple fault morphology, allows us to explore the role of uncertainty in basic fault parameters, such as fault dip and position. We show that introducing uncertainties in fault geometry in a static inversion reduces the sensitivity of inferred models to different geometric assumptions. Accounting for uncertainties thus helps infer more realistic and robust slip models. We also show that uncertainties in fault geometry and Earth’s elastic structure significantly impact estimated source models, particularly if near-fault observations are available.

3D Imaging of Geothermal Faults from a Vertical DAS Fiber at Brady Hot Spring, NV USA

In March 2016, arguably the most ambitious 4D (3D space + over time) active-source seismic survey for geothermal exploration in the U.S. was acquired at Brady Natural Laboratory, outside Fernley, Nevada. The four-week experiment included 191 vibroseis source locations, and approximately 130 m of distributed acoustic sensing (DAS) in a vertical well, located at the southern end of the survey area. The imaging of the geothermal faults is done with reverse time migration of the DAS data for both P-P and P-S events in order to generate 3D models of reflectivity, which can identify subsurface fault locations. Three scenarios of receiver data are explored to investigate the reliability of the reflectivity models obtained: (1) Migration of synthetic P-P and P-S DAS data, (2) migration of the observed field DAS data and (3) migration of pure random noise to better assess the validity of our results. The comparisons of the 3D reflectivity models from these three scenarios confirm that sections of three known faults at Brady produce reflected energy observed by the DAS. Two faults that are imaged are ~1 km away from the DAS well; one of these faults (middle west-dipping) is well-constructed for over 400 m along the fault’s strike, and 300 m in depth. These results confirm that the DAS data, together with an imaging engine such as reverse time migration, can be used to position important geothermal features such as faults.

A geophysical correlation between near-surface radioactivity and subsurface faults detected by gravity method for a region located in the western desert of Iraq

This research study deals with the processing and interpretation of the total Bouguer anomaly and the total count iso-radiation data for a region located in the western desert of Iraq. The research aims to delineate the approximate locations and the effective depth of faults and how does rocks radioactivity act nearby the faults. The graphical method adopted to separate the residual fields from regional’s, in order to be processed later by applying the horizontal gradient filter. The results displayed as maps and profile section which delineate qualitatively the approximate locations of faults and quantitatively the high radio counts and major faults effective depth within the region. The results of this study showed that the major faults within the region extend with the directions NW-SE, with effective fault depth ranges of 1.7 – 4.9 Km. High radiometric anomalies detected near or between fault locations as they detected after conducting a gravity-radiometric profile section across the region. Peaks of high radiation total counts are located over the elevated blocks which produced by faulting action. The detected faults penetrate the main groundwater aquifer in the region and this considered responsible for transporting radioactive elements to the surface over and nearby the detected fault locations to form spots of accumulated radioactive deposits.

Subsurface structure and spatial segmentation of the Longmen Shan fault zone at the eastern margin of Tibetan Plateau: Evidence from focal mechanism solutions and stress field inversion

As the eastern margin of the Tibetan Plateau, the Longmen Shan fault zone (LMSFZ) is characterized by a complex structural style and strong seismicity, such as the 2008 Wenchuan Ms8.0 and 2013 Lushan Ms7.0 earthquakes. In order to better understand the subsurface geometry of the LMSFZ we inverted 391 focal mechanism solutions (FMS) of earthquakes (M ≥ 3.5) that occurred between 2009 and 2016 in the Longmen Shan region using seismic waveforms. Subsequently, we calculated the stress field based on the FMSs. The results show that the LMSFZ is dominated by a thrust faulting stress regime with subhorizontal NW-SE-trending maximum principal stress σ1 and subvertical minimum principal stress σ3, indicating a lateral compression due to the southeastward movement of the Bayan Har block. Changes in fault dips at ~ 10 km depth are inferred as the result of a shallow level detachment layer, which may also promote shortening of the upper crust and uplift of the Longmen Shan. Based on differences in subsurface fault geometry, stress field and geomorphological evidence, we divide the LMSFZ into two segments by Beichuan County: the southwestern segment has a listric fault geometry, while the northeastern segment is dominated by subvertical transpressional strike-slip faults. According to the FMSs results, we suggest that the Lixian fault is a tear fault accommodating different thrusting rates in the eastern margin of the Bayan Har block. The normal faulting earthquakes in the Lixian area are caused by local extension resulting from the sinistral shear movement of Lixian fault in a strike-slip faulting stress regime. It is suggested that under the continuous southeastward movement of the Bayan Har block and the obstruction of the rigid Sichuan basin, stress is accumulating along the southwestern segment of the LMSFZ, to which accordingly a high seismic hazard is ascribed and which should be carefully monitored.

Aeromagnetic anomalies interpretation based on improved bi-dimensional empirical mode decomposition (BEMD) and RGB composition

In this study, the bi-dimensional empirical mode decomposition (BEMD) technique is improved by morphological extrema reconstruction, kriging interpolation and self-adaptive termination condition to make the decomposed components more robust. The decomposed components is a self-adaptive and data-driven dyadic filter bank. The high pass filter BIMF1 is about shallow or subsurface faults, it is an indirect indication to polymetallic deposits, especially tectonic-hydrothermal ones. The band pass filter BIMF2 is about regional anomalies from the middle-shallow or mantle-crustal, it is especially indicative to contact-metasomatic deposits. The low pass filter BIMF3+Residue is the shape of deep source trend of mantle magmatic crystalline basement. As they are independent and the original magnetic data can be reconstructed by their summation, they are taken as R, G, and B component and composited as an image, which retains all characters of the filters and shows ore-prospecting areas. The BEMD-RGB algorithm in this way makes aeromagnetic anomalies easier to interpret and more geologically meaningful. The whole process is performed on MATLAB 2010b platform automatically.

Application of Electrical Resistivity Method in Mapping Underground River Channels: A Case Study of Kabatini Area in the Kenyan Rift Valley

Kabatini area is in Nakuru County which occurs in Kenya within the great Gregory rift valley. The area is bounded by longitudes 180900-181200 and latitudes 9971050-9971200 in UTM. This research aimed to investigate the possible occurrence of subsurface faults recharging Kabatini aquifer which is the primary source of groundwater for the residents of Nakuru town and its environs. Vertical electrical sounding and geological mapping methods were used to unravel this problem of subsurface fault occurrence in the area of study. Vertical Electrical Sounding data collected were analyzed using the computer-based inversion using EarthImager software, which allows the performance of automatic interpretation of the Schlumberger sounding curves to obtain the equivalent n-layer model from the apparent resistivity curve of each sounding. This procedure was used for all the sounding points collected in the study area, to obtain the equivalent apparent resistivity models of each sounding, their depths as well as thicknesses. The results indicate the occurrence of subsurface faults trending in the north-south direction which is proven by K-H-K, K-H-H, Q-H-A, Q-H-Q. Depth iso-resistivity analysis shows that the fault plane is deeper than 165 meters below the surface.

A nonlinear inversion of InSAR-observed coseismic surface deformation for estimating variable fault dips in the 2008 Wenchuan earthquake

Satellite Interferometric Synthetic Aperture Radar (InSAR) is playing an increasingly important role in the observation of coseismic surface deformation caused by earthquakes, and has been used to invert for subsurface fault structure and reveal earthquake source mechanisms. However, the mapping of complex non-planar or curved (e.g., listric-shaped) faults still remains a challenging task due to variable dips along the underground depth and the impenetrability of the deep crust. Here, we develop a set of new inversion algorithms to determine the listric fault geometry with InSAR- and GPS-observed surface deformation as the significant constraints. The fault surface with variable dip angles is discretized into consecutive sub-fault layers along the down-dip direction. A nonlinear iteration algorithm is used to minimize the objective function to determine the dip angle for each sub-fault layer. The proposed method is first tested using synthetic data to show its effectiveness for retrieval of varying fault geometry dips, and then applied to the 2008 Mw 7.9 Wenchuan earthquake that ruptured the Yingxiu-Beichuan fault for over 320 km along the southwest-northeast strike. The inversion shows that the dip angle of the seismogenic fault is up to 76° near the surface layer, and gradually decreases along the down-dip direction. A significant decrease in dip occurs within the depths of 6–15 km with a dip of 32° at a depth of 15 km. The dip angle decreases to 2° at a depth of 20 km, and finally merges with the subparallel PengGuan fault, which is basically consistent with geological investigations and seismic waveform data inversion. Using the inferred fault geometry, the slip model associated with the event is estimated. Five high-slip concentrations along the strike of the Yingxiu-Beichuan fault are recognized. The inversion misfit of InSAR data is reduced to 7.1 cm with a significant improvement compared to previous studies.

Subsurface fault investigation in Chiang Rai province, northern Thailand by integrated geophysical surveys

A magnitude 6.3 earthquake, the biggest instrumentally recorded earthquake in Thailand, occurred in Chiang Rai Province on 5 May 2014 and caused large damages in the affected area. The earthquake generated numerous aftershocks that could portray the location of the fault plane beneath the ground. In this work, we conducted integrated geophysical surveys consisting of 2D seismic reflection and 2D resistivity imaging surveys to explore for active faults that could have caused this earthquake. A seismic reflection survey line with a total length of 3,750 metres and resistivity survey with a total length of 1,975 metres were conducted along the Chiang Rai earthquake’s aftershock locations. The subsurface fault geometry was imaged from this integrated geophysical survey. Numerous subsurface discontinuities detected from both seismic reflection and 2D resistivity imaging survey were interpreted as potential faults along the survey line with depths from a few meters to around 500 metres. These subsurface discontinuities correspond well with the aftershock locations which could suggest a fault rupture plane.

Geophysical and geochemical investigation for structures favorable to uranium mineralization in Dhofar region, Sultanate of Oman

The very low frequency electromagnetic (VLF-EM) method is a rapid and reliable geophysical method was used in the investigation for uranium bearing structures in the Proterozoic basement of the Dhofar region that may be a potential source and trap for uranium and associated minerals in Sultanate of Oman. Accordingly, the in-phase and out of phase components of VLF-EM field along 11 traverses were recorded at two different transmitter frequencies viz. 16 kHz and 16.4 kHz at an approximate line interval of 20 m with a measurement interval of 10 m. These measured components were subjected to Fraser and Karous-Hjelt filtering in order to decipher the nature and depth to the sub-surface conductor. The Fraser filtered in-phase VLF-EM components yield a strong amplitude almost over the top of the conductor. On the other hand, the Hjelt filtered responses yield the depth to the source in the range 10–80 m. Spectral analysis of selected a few traverses result the depth to source in the range 31–39 m. Analytical signal approach of total magnetic field of line-I substantiates this depth. The peaks of Fraser filtered stacked profiles delineate approximately the trend of the sub-surface fault which mimic the trend inferred from the combined interpretation of VLF-EM and magnetic anomalies. The results of Fraser and Karous-Hjelt filter output suggest a mineralized conductive sub-surface fault aligned along NW-SE. It may be noted that the random sampling of gamma ray spectrometer has recorded a maximum uranium concentration of up to 30–40 ppm prior to VLF-EM survey.