The potential of detecting flaws in an experimental dam at Älvkarleby, Sweden, using P-wave traveltime tomography

2020 ◽  
Author(s):  
Silvia Salas-Romero ◽  
Christopher Juhlin ◽  
Christian Bernstone
Keyword(s):  
Asset Management ◽  
Seismic Data ◽  
Geophysical Methods ◽  
Structure Constant ◽  
P Wave ◽  
Internal Erosion ◽  
Management Tool ◽  
Low Velocity ◽  
Traveltime Tomography ◽  
Defect Position

<p>A large number of dams located in Sweden, built in the second half of 20<sup>th </sup>century, are earth embankment dams. Seepages and internal erosion represent safety issues, which are difficult to detect and predict. There are indirect methods to detect seepages, but these do not provide their location. The hydropower operator Vattenfall has initiated a research project to assess geophysical methods as a decision support and asset management tool for this type of structure. The project consists of detecting built-in flaws in the core of a 20 m long and 4 m high experimental dam using geophysical (seismic and resistivity) and temperature measurements taken at the top of and inside the dam structure for a period of approximately 18 months. The behaviour of the dam itself will be monitored by geotechnical instrumentation.</p><p>This work focuses on testing P-wave traveltime tomography for detecting defects and supporting the interpretation of P-wave reflection seismic data. Synthetic traveltime studies were performed using the dam structure, constant P-wave velocities for each material, and the seismic acquisition design. Five parallel lines of hydrophones were used, three at the top and two at the bottom of the dam. The central hydrophone line at the top of the dam coincides with the position of the seismic sources. In addition, four boreholes to 4 m depth are positioned on each side of the central hydrophone line in both edges of the dam. Within these boreholes shots and receivers were positioned at every 0.5 m depth. The initial velocity model of the dam considers that the dam is filled with water up to a height of 3.5 m. A series of defects (low velocity zones with varying size and position) were inserted. Other factors, like noise or error in the acquisition geometry, were also considered. The defects may be cavities or permeable/loose layers.</p><p>Preliminary results show, in general, that the defect position can be identified by tomography. The velocity and size of the defects, however, are not well recovered by the method. Recovery of the defects using traveltime tomography is greatly influenced by the defect position, as the seismic ray coverage is limited in some parts, such as the central lower part of the dam. In the case of a defect located closer to the top hydrophone lines or one of larger size, the anomalies are better identified. We note that the amplitudes of the anomalies are very small, which may complicate identifying defects using real data. The anomaly signatures depend on the shape of the defect, for example a cubic defect compared to a horizontal permeable layer, which could help to identify and characterize the defect. Although the primary focus lies on identifying the presence of defects, information about their dimension and type is also important.</p><p>Future work will be focused on processing repeated seismic fieldwork campaigns at the experimental dam, in order to investigate the dam integrity using time-lapse seismic measurements, including comparing the seismic data with other types of data.</p>

scholarly journals Integrated magnetotelluric and seismic investigation of Cenozoic graben structure near Obrzycko, Poland

2021 ◽  
Author(s):  
Adam Cygal ◽  
Michał Stefaniuk ◽  
Anna Kret
Keyword(s):  
Seismic Data ◽  
Geophysical Methods ◽  
Geological Structure ◽  
Data Sets ◽  
Low Velocity ◽  
Shallow Subsurface ◽  
Geophysical Model ◽  
Novel Approach ◽  
Loose Deposits ◽  
Static Corrections

AbstractThis article presents the results of an integrated interpretation of measurements made using Audio-Magnetotellurics and Seismic Reflection geophysical methods. The obtained results were used to build an integrated geophysical model of shallow subsurface cover consisting of Cenozoic deposits, which then formed the basis for a detailed lithological and tectonic interpretation of deeper Mesozoic sediments. Such shallow covers, consisting mainly of glacial Pleistocene deposits, are typical for central and northern Poland. This investigation concentrated on delineating the accurate geometry of Obrzycko Cenozoic graben structure filled with loose deposits, as it was of great importance to the acquisition, processing and interpretation of seismic data that was to reveal the tectonic structure of the Cretaceous and Jurassic sediments which underly the study area. Previously, some problems with estimation of seismic static corrections over similar grabens filled with more recent, low-velocity deposits were encountered. Therefore, a novel approach to estimating the exact thickness of such shallow cover consisting of low-velocity deposits was applied in the presented investigation. The study shows that some alternative geophysical data sets (such as magnetotellurics) can be used to significantly improve the imaging of geological structure in areas where seismic data are very distorted or too noisy to be used alone

scholarly journals No mafic layer in 80 km thick Tibetan crust

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Gaochun Wang ◽  
Hans Thybo ◽  
Irina M. Artemieva
Keyword(s):  
Seismic Data ◽  
P Wave ◽  
Indian Plate ◽  
Low Velocity ◽  
Crustal Earthquakes ◽  
Tectonic Processes ◽  
P Wave Velocity ◽  
Juvenile Crust ◽  
Mafic Lower Crust ◽  
Thick Crust

AbstractAll models of the magmatic and plate tectonic processes that create continental crust predict the presence of a mafic lower crust. Earlier proposed crustal doubling in Tibet and the Himalayas by underthrusting of the Indian plate requires the presence of a mafic layer with high seismic P-wave velocity (Vp > 7.0 km/s) above the Moho. Our new seismic data demonstrates that some of the thickest crust on Earth in the middle Lhasa Terrane has exceptionally low velocity (Vp < 6.7 km/s) throughout the whole 80 km thick crust. Observed deep crustal earthquakes throughout the crustal column and thick lithosphere from seismic tomography imply low temperature crust. Therefore, the whole crust must consist of felsic rocks as any mafic layer would have high velocity unless the temperature of the crust were high. Our results form basis for alternative models for the formation of extremely thick juvenile crust with predominantly felsic composition in continental collision zones.

A reality check on full-wave inversion applied to land seismic data for near-surface modeling

2022 ◽  
Vol 41 (1) ◽  
pp. 40-46
Author(s):  
Öz Yilmaz ◽  
Kai Gao ◽  
Milos Delic ◽  
Jianghai Xia ◽  
Lianjie Huang ◽  
...  
Keyword(s):  
Wave Velocity ◽  
Seismic Data ◽  
Surface Modeling ◽  
P Wave ◽  
Borehole Data ◽  
Near Surface ◽  
Traveltime Tomography ◽  
Wave Inversion ◽  
Full Wave ◽  
Elastic Inversion

We evaluate the performance of traveltime tomography and full-wave inversion (FWI) for near-surface modeling using the data from a shallow seismic field experiment. Eight boreholes up to 20-m depth have been drilled along the seismic line traverse to verify the accuracy of the P-wave velocity-depth model estimated by seismic inversion. The velocity-depth model of the soil column estimated by traveltime tomography is in good agreement with the borehole data. We used the traveltime tomography model as an initial model and performed FWI. Full-wave acoustic and elastic inversions, however, have failed to converge to a velocity-depth model that desirably should be a high-resolution version of the model estimated by traveltime tomography. Moreover, there are significant discrepancies between the estimated models and the borehole data. It is understandable why full-wave acoustic inversion would fail — land seismic data inherently are elastic wavefields. The question is: Why does full-wave elastic inversion also fail? The strategy to prevent full-wave elastic inversion of vertical-component geophone data trapped in a local minimum that results in a physically implausible near-surface model may be cascaded inversion. Specifically, we perform traveltime tomography to estimate a P-wave velocity-depth model for the near-surface and Rayleigh-wave inversion to estimate an S-wave velocity-depth model for the near-surface, then use the resulting pairs of models as the initial models for the subsequent full-wave elastic inversion. Nonetheless, as demonstrated by the field data example here, the elastic-wave inversion yields a near-surface solution that still is not in agreement with the borehole data. Here, we investigate the limitations of FWI applied to land seismic data for near-surface modeling.

Iterative P-wave velocity inversion using Markov chain Monte Carlo method

2020 ◽  
Author(s):  
Hyunggu Jun ◽  
Hyeong-Tae Jou ◽  
Han-Joon Kim ◽  
Sang Hoon Lee
Keyword(s):  
Wave Velocity ◽  
Seismic Data ◽  
Velocity Structure ◽  
Low Frequency ◽  
P Wave ◽  
Velocity Analysis ◽  
Seismic Section ◽  
Traveltime Tomography ◽  
P Wave Velocity ◽  
Frequency Components

&lt;p&gt;Imaging the subsurface structure through seismic data needs various information and one of the most important information is the subsurface P-wave velocity. The P-wave velocity structure mainly influences on the location of the reflectors during the subsurface imaging, thus many algorithms has been developed to invert the accurate P-wave velocity such as conventional velocity analysis, traveltime tomography, migration velocity analysis (MVA) and full waveform inversion (FWI). Among those methods, conventional velocity analysis and MVA can be widely applied to the seismic data but generate the velocity with low resolution. On the other hands, the traveltime tomography and FWI can invert relatively accurate velocity structure, but they essentially need long offset seismic data containing sufficiently low frequency components. Recently, the stochastic method such as Markov chain Monte Carlo (McMC) inversion was applied to invert the accurate P-wave velocity with the seismic data without long offset or low frequency components. This method uses global optimization instead of local optimization and poststack seismic data instead of prestack seismic data. Therefore, it can avoid the problem of the local minima and limitation of the offset. However, the accuracy of the poststack seismic section directly affects the McMC inversion result. In this study, we tried to overcome the dependency of the McMC inversion on the poststack seismic section and iterative workflow was applied to the McMC inversion to invert the accurate P-wave velocity from the simple background velocity and inaccurate poststack seismic section. The numerical test showed that the suggested method could successfully invert the subsurface P-wave velocity.&lt;/p&gt;

Rapid map and inversion of P-SV waves

Geophysics ◽  
1991 ◽  
Vol 56 (6) ◽  
pp. 859-862 ◽  
Author(s):  
Robert R. Stewart
Keyword(s):  
Spatial Resolution ◽  
Seismic Data ◽  
P Wave ◽  
Rock Properties ◽  
Low Velocity ◽  
P Waves ◽  
Near Surface ◽  
Seismic Profiling ◽  
Vertical Seismic Profiling ◽  
And Inversion

Multicomponent seismic recordings are currently being analyzed in an attempt to improve conventional P‐wave sections and to find and use rock properties associated with shear waves (e.g. Dohr, 1985; Danbom and Dominico, 1986). Mode‐converted (P-SV) waves hold a special interest for several reasons: They are generated by conventional P‐wave sources and have only a one‐way travel path as a shear wave through the typically low velocity and attenuative near surface. For a given frequency, they will have a shorter wavelength than the original P wave, and thus offer higher spatial resolution; this has been observed in several vertical seismic profiling (VSP) cases (e.g., Geis et al., 1990). However, for surface seismic data, converted waves are often found to be of lower frequency than P-P waves (e.g., Eaton et al., 1991).

scholarly journals Shallow Seismic Investigation of the Teton Fault

2014 ◽  
Vol 37 ◽  
pp. 2-10
Author(s):  
Glenn Thackray ◽  
Mark Zellman ◽  
Jason Altekruse ◽  
Bruno Protti ◽  
Harrison Colandera
Keyword(s):  
Seismic Data ◽  
Velocity Structure ◽  
Hanging Wall ◽  
Normal Fault ◽  
Fault Scarp ◽  
P Wave ◽  
Low Velocity ◽  
Wave Refraction ◽  
Shallow Seismic ◽  
Teton Range

Preliminary results from seismic data collected at two sites on the Teton fault reveal shallow sub-surface fault structure and a basis for evaluating the post-glacial faulting record in greater detail. These new data include high-resolution shallow 2D seismic refraction and Interferometric Multi-Channel Analysis of Surface Waves (IMASW) (O’Connell and Turner 2010) depth-averaged shear wave velocity (Vs). The Teton fault, a down-to-the east normal fault, is expressed as a distinct topographic escarpment along the base of the eastern front of the Teton Range in Wyoming. The average fault scarp height cut into deglacial surfaces in several similar valleys and an assumed 14,000 yr BP deglaciation indicates an average postglacial offset rate of 0.82 m/ka (Thackray and Staley, in review). Because the fault is located almost entirely within Grand Teton National Park (GTNP), and in terrain that is remote and difficult to access, very few subsurface studies have been used to evaluate the fault. As a result, many uncertainties exist in the present characterization of along-strike slip rate, down-dip geometry, and rupture history, among other parameters. Additionally, questions remain about the fault dip at depth. Shallow seismic data were collected at two locations on the Teton fault scarp to (1) use a non-destructive, highly portable and cost-effective data collection system to image and characterize the Teton fault, (2) use the data to estimate vertical offsets of faulted bedrock and sediment, and (3) estimate fault dip in the shallow subsurface. Vs data were also collected at three GTNP facility structures to provide measured 30 m depth-averaged Vs (Vs30) for each site. Seismic data were collected using highly portable equipment packed into each site on foot. The system utilizes a sensor line 92 m long that includes 24 geophones (channels) at 4 m intervals. At both the Taggart Lake and String Lake sites, P-wave refraction data were collected spanning the fault scarp and perpendicular to local fault strike, as well as IMASW Vs seismic lines positioned on the hanging wall to provide Vs vs. Depth profiles crossing and perpendicular to the refraction survey lines. The Taggart Lake and String Lake 2D P-wave refraction profile and IMASW Vs plots reveal buried velocity structure that is vertically offset by the Teton fault. At Taggart Lake, we interpret the velocity horizon to be the top of dense glacial sediment (possibly compacted till), which is overlain by younger, slower, sediments. This surface is offset ~13 m (down-to-the-east) across the Teton fault. The vertical offset is in agreement with the measured height of the corresponding topographic scarp (~12 - 15 m). Geomorphic analysis of EarthScope (2008) LiDAR reveals small terraces, slope inflections and an abandoned channel on the footwall side of the scarp. At String Lake, the shallow buried velocity structure is inferred as unconsolidated alluvium (till, colluvium, alluvium); this relatively low velocity zone (

Using 2D long-streamer seismic data waveform tomography to decipher sedimentary record of fault activity

2020 ◽  
Author(s):  
Amin Kahrizi ◽  
Matthias Delescluse ◽  
Mathieu Rodriguez ◽  
Pierre-Henri Roche ◽  
Anne Becel ◽  
...  
Keyword(s):  
Mass Transport ◽  
Seismic Data ◽  
Velocity Model ◽  
P Wave ◽  
Fault System ◽  
Fault Activity ◽  
Traveltime Tomography ◽  
P Wave Velocity ◽  
Waveform Tomography ◽  
Mass Transport Deposits

&lt;p&gt;Acoustic full-waveform inversion (FWI), or waveform tomography, involves use of both phase and amplitude of the recorded compressional waves to obtain a high-resolution P-wave velocity model of the propagation medium. Recent theoretical and computing advances now allow the application of this highly non-linear technique to field data. This led to common use of the FWI for industrial purposes related to reservoir imaging, physical properties of rocks, and fluid flow. Application of FWI in the academic domain has, so far, been limited, mostly because of the lack of adequate seismic data. While refraction seismic datasets include large source-receiver offsets that are useful to find a suitable starting velocity model through traveltime tomography, these acquisitions rarely reach the high density of receivers necessary for waveform tomography. On the other hand, multichannel seismic (MCS) reflection data acquisition has a dense receiver spacing but only modern long-streamer data have offsets that, in some cases, enable constraining subsurface velocities at a significant enough depth to be useful for structural or tectonic purposes.&lt;/p&gt;&lt;p&gt;In this study, we show how FWI can help decipher the record of a fault activity through time at the Shumagin Gap in Alaska. The MCS data were acquired on RV Marcus G. Langseth during the ALEUT cruise in the summer of 2011 using two 8-km-long seismic streamers and a 6600 cu. in. tuned airgun array. One of the most noticeable reflection features imaged on two profiles is a large, landward-dipping normal fault in the overriding plate; a structural configuration making the area prone to generating both transoceanic and local tsunamis, including from landslides. This fault dips ~40&amp;#176;- 45&amp;#176;, cuts the entire crust and connects to the plate boundary fault at ~35 km depth, near the intersection of the megathrust with the forearc mantle wedge. The fault system reaches the surface at the shelf edge 75 km from the trench, forming the Sanak basin where the record of the recent activity of the fault is not clear. Indeed, contouritic currents tend to be trapped by the topography created by faults, even after they are no longer active.&amp;#160; Erosion surfaces and onlaps from contouritic processes as well as gravity collapses and mass transport deposits results in complex structures that make it challenging to evaluate the fault activity. The long streamers used facilitated recording of refraction arrivals in the target continental slope area, which permitted running streamer traveltime tomography followed by FWI to produce coincident detailed velocity profiles to complement the reflection sections. FWI imaging of the Sanak basin reveals low velocities of mass transport deposits and velocity inversions indicate mechanically weak layers linking some faults to gravity sliding on a d&amp;#233;collement. These details question previous interpretation of a present-day active fault. Our goal is to further analyze the behavior of the fault system using the P-wave velocity models from FWI to quantitatively detect fluids and constrain sediment properties.&lt;/p&gt;

scholarly journals Imaging structure and geometry of slabs in the greater Alpine area – A P-wave traveltime tomography using AlpArray Seismic Network data

2021 ◽  
Author(s):  
Marcel Paffrath ◽  
Wolfgang Friederich ◽  
Keyword(s):  
Alpine Region ◽  
Seismic Network ◽  
P Wave ◽  
Fault System ◽  
Earth Model ◽  
Low Velocity ◽  
European Origin ◽  
Traveltime Tomography ◽  
Model Domain ◽  
Asthenospheric Upwelling

Abstract. We perform a teleseismic P-wave traveltime tomography to examine the geometry and structure of subducted lithosphere in the upper mantle beneath the Alpine orogen. The tomography is based on waveforms recorded at over 600 temporary and permanent broadband stations of the dense AlpArray Seismic Network deployed by 24 different European institutions in the greater Alpine region, reaching from the Massif Central to the Pannonian Basin and from the Po plain to the river Main. Teleseismic traveltimes and traveltime residuals of direct teleseismic P-waves from 331 teleseismic events of magnitude 5.5 and higher recorded between 2015 and 2019 by the AlpArray Seismic Network are extracted from the recorded waveforms using a combination of automatic picking, beamforming and cross-correlation. The resulting database contains over 162.000 highly accurate absolute P-wave traveltimes and traveltime residuals. For tomographic inversion, we define a model domain encompassing the entire Alpine region down to a depth of 600 km. Outside this domain, a laterally homogeneous standard earth model is assumed. Predictions of traveltimes are computed in a hybrid way applying a fast Tau-P method outside the model domain and continuing the wavefronts into the model domain using a fast marching method. For teleseismic inversion, we iteratively invert demeaned traveltime residuals for P-wave velocities in the model domain using a regular discretization with an average lateral spacing of about 25 km and a vertical spacing of 15 km. The inversion is regularized towards an initial model constructed from an a priori model of the crust and uppermost mantle and a standard earth model beneath. The resulting model provides a detailed image of slab configuration beneath the Alpine and Apenninic orogens. Major features are an overturned Adriatic slab beneath the Apennines reaching down to 400 km depth still attached in its northern part to the crust but exhibiting detachment towards the southeast. A fast anomaly beneath the western Alps indicates a short western Alpine slab that ends at about 100 km depth close to the Penninic front. Further to the east and following the arcuate shape of the western Periadriatic Fault System, a deep-reaching coherent fast anomaly with complex interior stucture generally dipping to the SE down to about 400 km suggests a slab of European origin extending eastward to the Giudicarie fault. This slab is detached from overlying lithosphere at its eastern end below a depth of about 100 km. Further to the east, well-separated from the slab beneath the western and central Alps, another deep-reaching, nearly vertically dipping high-velocity anomaly suggests the existence of a slab beneath the Eastern Alps of presumably European origin which is completely detached from the orogenic root. Our image of this slab does not require a polarity switch because of its nearly vertical dip and full detachment from the overlying lithosphere. Fast anomalies beneath the Dinarides are weak and concentrated to the northernmost part and shallow depths. Low-velocity regions surrounding the fast anomalies beneath the Alps to the west and northwest follow the same dipping trend as the overlying fast ones, indicating a kinematically coherent subducting tectosphere in this region. In contrast, low-velocity anomalies to the east suggest asthenospheric upwelling presumably driven by retreat of the Carpathian slab and extrusion of eastern Alpine lithosphere towards the east while low velocities to the south are presumably evidence of asthenospheric upwelling and mantle hydration due to the backarc position behind the European slab.

scholarly journals Delimiting the Neoproterozoic São Francisco Paleocontinental Block with P-wave traveltime tomography

2019 ◽  
Vol 219 (1) ◽  
pp. 633-644 ◽  
Author(s):  
Marcelo Peres Rocha ◽  
Paulo Araújo de Azevedo ◽  
Marcelo Assumpção ◽  
Antônio Carlos Pedrosa-Soares ◽  
Reinhardt Fuck ◽  
...  
Keyword(s):  
P Wave ◽  
Rift System ◽  
Mantle Flow ◽  
Low Velocity ◽  
Traveltime Tomography ◽  
Velocity Anomaly ◽  
Velocity Anomalies ◽  
Orogenic Belts ◽  
Synthetic Models ◽  
Araçuaí Orogen

SummaryThe São Francisco Paleocontinental Block (SFPB) represents part of the Congo-São Francisco Paleocontinent (CSFP), amalgamated around 2 Ga. In the Neoproterozoic, a branched continental rift system evolved to ocean basins around most edges of the SFPB that remained only partially linked to the Congo Paleocontinent by means of the Bahia-Gabon Continental Bridge. After the Brasiliano—Pan-African orogeny, two relatively preserved CSFP sectors formed the São Francisco and Congo cratons, surrounded by Neoproterozoic orogenic belts. Recent results of upper mantle P-wave seismic tomography allowed us to suggest a delimitation in lithospheric depths of the Neoproterozoic SFPB, which comprise the São Francisco Craton, and that this would have been connected with the Congo Paleocontinent along the Araçuaí Belt. It is characterized by high-velocity anomalies and its boundaries with other blocks are marked by low-velocity anomalies at lithospheric depths. We tested the resolution of the tomographic results through synthetic models obtained by a ray tracing scheme using the observed ray configuration. We observe that the lateral resolution is adequate, but the method used was not able to set the depth reached by the SFPB. Our results indicate that the SFPB area in lithospheric depths is larger than the surface area ascribed to the São Francisco craton, and thus, the SFPB basement deeply extends beneath neighboring orogenic regions, suggesting that these Neoproterozoic mobile belts, such as Araçuaí Orogen and the Brasilia Fold Belt, reworked the continental crust. We observe a low-velocity anomaly in the SFPB central region, corresponding to the Pirapora aulacogen. Our results have a good spatial correspondence with the low Bouguer anomalies used to define the SFPB in previous studies. The limits of the SFPB are consistent with deviation of the mantle flow, as suggested by SKS fast polarization.

The Study of Crustal Velocity Structure Characteristics in Yangjiang Area of South China

2020 ◽  
Author(s):  
Xiaona Wang ◽  
Zhihui Deng ◽  
Xiuwei Ye ◽  
Liwei Wang
Keyword(s):  
South China ◽  
Seismic Data ◽  
Velocity Structure ◽  
Seismic Network ◽  
P Wave ◽  
Adjacent Area ◽  
Low Velocity ◽  
Arrival Times ◽  
P Wave Velocity ◽  
Buried Fault

&lt;p&gt;This paper collects 43,225 absolute first arrival P wave arrival times and 422,956 high quality relative P arrival times of 6,390 events occurred in Yangjiang and its adjacent area from Jan, 1990 to Aug, 2019, these seismic data is recorded by 49 stations from Guangdong seismic network, Guangxi seismic network and Hainan seismic network. Based on the seismic data above, we simultaneously determine the crustal 3D P wave velocity structure and the hypocenter parameters of 6255 events in Yangjiang and its adjacent area by applying Double-Difference seismic tomography. The result shows that, shallow P wave velocity in Yangjiang area is higher due to the thinner sedimentary layer and widely exposed Yanshanian granite, Indosinian granite and Cambrian metamorphic rocks. There are obvious correspondences between the distribution of shallow velocity and fault structure as well as geological structure. A wide range of low velocity anomaly exists in 20km depth, which verifies the low velocity layer in the middle crust at Yangjiang area of South China continent. The velocity image from land to ocean in 30km depth shows low velocity in NW side and high velocity in SE side, which verifies the characteristic of crust thinning in South China coastal continent. The NEE seismic belt from Yangbianhai to Pinggang is speculated to locate in a buried fault of southwest segment of Pinggang fault. The buried thrust fault is a N78&amp;#176;E strike fault, dip to NW with a dip angle of 85 &amp;#176;. In addition, the buried fault locates in the abnormal junction of high velocity on the NW side and low velocity on the SE side, which reflects the tectonic activity characteristic of NW plate uplifting and SE plate declining from Miocene period. The characteristic of activity in the buried fault shows thrust movement with a small strike slip component, which is consistent with the focal mechanism of M4.9 earthquake occurred in 2004.&lt;/p&gt;

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