Pure and Applied Geophysics

Chorozoglou, D.; Iliopoulos, A.; Kourouklas, C.; Mangira, O.; Papadimitriou, E.

doi: 10.1007/s00024-019-02253-wpmid: N/A

Seismic hazard assessment is one of the main targets of seismological research, aiming to contribute to reducing the catastrophic consequences of strong earthquakes (e.g., $$ M \ge 6.0 $$ M ≥ 6.0 ). From the early stage of seismological research, both purely seismological and statistical methods were adopted for seismic hazard assessment. An approach towards this target was attempted by means of network theory, aiming to provide insight into the complex physical mechanisms that cause earthquakes and whether the occurrence of strong earthquakes can be predicted to some extent. Application of network theory in different areas of the world with intense seismic activity, such as Japan, California, Italy, Greece, Iran, and Chile, has yielded promising results that have negligible probability of being obtained by purely random guessing.

Prasath, R.; Paul, Ajay; Singh, Sandeep

doi: 10.1007/s00024-019-02239-8pmid: N/A

The historical and the present seismicity catalogues of the Garhwal Himalaya have been studied for their spatio-temporal variations and their implications on the seismotectonics of the region. The Micro-Seismicity, Fractal dimensions (Dc) and Frequency Magnitude Distribution (b-value) coupled with the available literature on geology, geomorphology and geophysics have been used to derive the seismotectonics and stress level changes in the region. The seismic cross sections for the relocated micro-seismicity, focal mechanisms and the swath profiles (for the presence of Physiographic Transition 2 (PT2) at the foothill of the Higher Himalaya) indicate the constant presence of the Mid-Crustal Ramp (MCR) in the detachment plane and its active seismogenic nature. The comparison of this scenario suggests the constant presence of seismogenically active MCR structure throughout the Central Seismic Gap. The seismic cross sections further reveal that the sensu stricto Main Central Thrust (Munsiari Thrust) is also a site of generation of the micro-seismicity in few segments due to its reactivation by thrusting along the MCR. The high fractal dimension value (Dc = 1.47) suggests the heterogeneous nature of the region, owing to the presence of local faults and transverse tectonics. The high stress accumulation in the Garhwal Himalaya with low b value (b = 0.70) suggests the high probability of occurrence of a larger or greater earthquake in the near-future. Further, the study also reveals that the 2011 Chamoli earthquake of M 5.0, preceded by a quiescence period of nearly a year shows different stress levels before and after its occurrence, which is well constrained with the increased moderate earthquake activity around the Chamoli region. This increased seismic activity and stress conditions in the Chamoli region suggest the high possibility of the occurrence of major earthquakes, hence the study recommend for a detailed seismic hazard evaluation of the region.

Sandeep, ; Joshi, A.; Sah, S.; Kumar, Parveen; Lal, Sohan; Devi, Sonia; Monika,

doi: 10.1007/s00024-019-02232-1pmid: N/A

Kumaon and Garhwal regions are the chief terrains of Uttarakhand Himalaya. The present article simulates the strong ground motion of the 2011 IndoNepal earthquake in the Kumaon region using modified semi empirical technique (MSET). Acceleration records at ten stations in the near field region have been simulated which validates well with actual records and therefore confirms the reliability of MSET. In addition, MSET has been used to simulate strong motion records of future scenario earthquakes (Mw 7.0 and Mw 8.0) in Kumaon region by assuming the earthquake location same as that of 2011 IndoNepal earthquake. Isoacceleration maps are also provided, which reveals more than 400 gal value of PGA at epicentral distances less than 25 kms for an earthquake of magnitude 8.0. The comparison of isoacceleration map of future scenario earthquake (Mw 7.0) in Kumaon region has been done with isoacceleration map of 1991 Uttarkashi earthquake (Mw 6.8) in Garhwal region which suggests distinct attenuation characteristics of these two regions.

Białoń, Wojciech; Lizurek, Grzegorz; Dec, Jerzy; Cichostępski, Kamil; Pietsch, Kaja

doi: 10.1007/s00024-019-02249-6pmid: N/A

Seismic activity of Podhale (Poland) and Spiš (Slovakia) regions has been recognized for years. The first information about tremors from this area comes from the XVIII century. Four earthquakes, with intensity over VI in the MSK scale, were reported in 1643, 1724, 1840 and 1901. Since 1960’s an instrumental measurements have been conducted. However, until 2011 no stronger tremors have been recorded in the area of artificial water reservoir Czorsztyn lake located in extremely complex geotechnical condition, between border zone of Inner and Outer Carpathians separated by Pieniny Klippen Belt. Before Czorsztyn 2D seismic survey, knowledge of tectonic boundaries and velocities was very limited. The seismic survey confirmed assumptions of a flower type faults system and provided the first 3D velocity model for this area. After a series of earthquakes, in 2013 a SENTINELS network started its operation and since that time it recorded almost 200 events. The focal mechanisms were calculated for around 20 of them. Both location and moment tensor are crucial in the investigation of the origins of seismogenic process related to industrial operations. Therefore the relocation of the events and validation of the moment tensor solutions for the SENTINELS network were conducted with the use of a 3D velocity model. The validation of mechanisms was conducted with the use of the synthetic tests based on the 1D velocity model derived from the 3D velocity model, taking into account its lateral velocity anisotropy. It was based on the synthetic amplitudes generated with the assumed normal and strike-slip faulting type, similar to the obtained solutions. The validation proved that focal mechanisms are reliable even in a sparse focal coverage and noise not exceeding 40% of the P-wave amplitude. Most of the events are normal or strike-slip with nodal planes striking NW–SE or NNE-SSW. The latter ones are in agreement with the orientation of the main discontinuities in this area.

Ali, Sherif; Badreldin, Hazem

doi: 10.1007/s00024-019-02262-9pmid: N/A

The present-day stress field in Egypt has been investigated on the basis of updated earthquake focal mechanism catalog covering the period from 1951 to 2017. Our catalog contains 234 focal mechanisms compiled from previous studies in addition to 22 new source mechanism solutions achieved in this study. According to the distribution of the recent earthquake epicentres, Egypt is divided into nine seismotectonic regions. The available fault plane solutions in Egypt demonstrate a spatial variability of source mechanisms, which categorize the study area into three groups. The first group includes Dahshour, Beni Suef, Cairo-Suez district, Northern-Central Gulf of Suez and Southern Gulf of Suez, which characterized by pure normal faulting mechanism to normal faulting with strike-slip component. Pure strike-slip faulting has clearly characterised Aswan and Gulf of Aqaba regions in the second group. However, the third group, which contains Abu Dabbab and the northern Egyptian continental margin, is characterized by thrust and strike-slip faulting. To calculate the orientation of the principle stress axes and the shape ratio we have applied the stress inversion technique. The present-day stress regime shows a variability of the tectonic stresses including extensional tectonic, transtensional and strike-slip. The transtensional stress regime with a maximum horizontal extensional NNE stress axis represents the dominant stress field pattern in Egypt. The results exhibit a good agreement with the tectonic settings and recent deformations in Egypt.

Kumar, Sanjay; Kumar, Prakash

doi: 10.1007/s00024-019-02264-7pmid: N/A

In this paper we investigated the feasibility of using the higher-frequency waveforms to characterize the seismicity in the Koyna–Warna region, globally well-known for its reservoir-triggered earthquakes. The local seismicity and source parameters have been extensively explored by many researchers using various types of data, e.g. analog data, broadband seismological data, and deep seismic sounding data. However, here we have utilized the seismological data recorded by 97 temporary 4.5-Hz geophones to locate the local seismicity in the region. We first investigated an appropriate 1-D P-wave velocity model through the inversion of P- and S-times. In order to compensate for the thick and laterally heterogeneous basalt, we incorporated the station corrections in the 1-D velocity inversion process. The station correction varies from + 0.09 to − 0.20 s for the P-waves, with clear demarcations of positive and negative station corrections corresponding to the northern and southern regions, respectively. The patterns of positive and negative station corrections suggested that the northern part is characterized by lower velocity, while slighter higher velocities present in the southern part of the study area are relative to the estimated 1-D velocity model. The earthquake locations are further improved by using the double-difference (hypoDD) approach, which enabled us to accurately map shallow subsurface seismogenic faults in the region. The data comprises 10,062 P-wave and 9049 S-wave catalog differential times from 348 earthquakes (1.0 ≤ M L ≤ 4.5). The precisely relocated earthquakes (ERH < 0.05 km ERZ < 0.5 km and rms < 0.02 s) form the three clusters of earthquakes in the NW–SE direction along the lineaments close to Warna Reservoir. The observed seismicity trends on the surface could be due to the vertical manifestations of faults or fractures. Their alignments are confirmed by the analysis of focal mechanism which exhibits a predominant normal type of faults with one nodal plane consistent with the main alignment of the seismic events.

Shirzad, Taghi; Naghavi, Mojtaba; Afra, Mahsa; YaminiFard, Farzam

doi: 10.1007/s00024-019-02269-2pmid: N/A

Tehran region is located at the southern foothills of central Alborz Mountains which is one of the most seismically active regions due to the convergence between the Arabian and the Eurasian plates. In this study, 3-D crustal velocity structure of Tehran region was investigated using first P-wave arrival times of events with magnitudes less than 4. Data used was recorded by the stations from Tehran Disaster Management and Mitigation Organization, Iranian Seismological Center and International Institute of Earthquake Engineering and Seismology from 2004 to 2018. In order to obtain the crustal velocity structure of Tehran, iterative, damped least-squares method was used to solve/obtain earthquake parameters and velocity structures, simultaneously. Final results show three tectonic characteristics in the study area including high velocity anomalies in northern and eastern Tehran Mountains, and low velocity anomaly in triangular Tehran basin, so that the separation of the eastern Tehran Mountains and Tehran basin is especially sharp in cross-section profiles. Moreover, some low velocity anomalies are appeared around major faults in the study area (e.g. North Tehran Fault). The tomographic maps also show that the Tehran region contains of an upper crust layer with the thickness of 6 km which is located over middle crust layer bedrock with constant velocity up to 15 km. This upper crust layer consists of a sedimentary layer (~ 3 km thickness) overlaid a basement layer. Moreover, the thick and thin upper crust layers are separated by North Tehran and Kosar/Ghasr-Firooz faultsystems from the Tehran basin, respectively.

Wu, L.; Li, S.; Sun, P.; Huang, R.; Li, B.

doi: 10.1007/s00024-019-02257-6pmid: N/A

All rocks undergo creep when under a long-term load, and shear sliding is a major failure mode of natural slopes. However, the mechanical properties of fractured mudstone differ from those of intact rock. To investigate these properties and the influence of normal stress and pre-cut crack length on mudstone shear creep characteristics, we performed shear creep tests on fractured mudstone. The experimental results show that axial load magnitude and crack length have a marked influence on the rock’s shear creep behavior. The larger the axial compressive stress, the smaller the shear creep deformation. The longer the crack, the more significant the shear creep deformation. When stress is low, existing creep models can reflect the creep properties of mudstone well. However, most published creep models cannot accurately portray the nonlinear creep behavior of mudstone in a tertiary creep stage. We propose an improved time-dependent creep model that overcomes the shortcomings of the traditional models in describing nonlinear creep. The new model has fewer parameters and can be applied to practical engineering problems involving soft rock. During the shear creep tests on soft argillaceous samples under different loads, shear failure zones in fractured mudstone exhibited both gradual and sudden failure. The shear time-dependent model for mudstone developed in this study can effectively explain mudstone instability.

Thabet, Mostafa

doi: 10.1007/s00024-019-02256-7pmid: N/A

It is commonly accepted that the horizontal-to-vertical spectral ratio (HVSR) technique enables the detection of the fundamental resonance frequency ( $$f_{\text{HVSR}}$$ f HVSR ) of a given site. The utility of this $$f_{\text{HVSR}}$$ f HVSR is analyzed using the nonlinear regression relationships between $$f_{\text{HVSR}}$$ f HVSR and bedrock depth ( $$h$$ h ). The derived relationships are mostly site-specific, so that the present paper consists of two main parts. The first is a literature review for the available empirical relationships between $$f_{\text{HVSR}}$$ f HVSR and $$h$$ h . The aim of this part is to highlight the practical limitations of these established relationships and to make fair comparisons. The second is to generate new relationships, taking advantage of the very wide range of available lithological, geophysical, and geotechnical borehole drilling data of the 697 KiK-NET seismic stations in Japan. For this purpose, HVSR are calculated using 10,000 weak earthquakes or linear events recorded at KiK-NET stations to determine the $$f_{\text{HVSR}}$$ f HVSR and correlate it with the corresponding $$h$$ h . The overlying layers/bedrock interface falling within sedimentary, igneous, or metamorphic layers significantly affect the derived frequency–depth relationships. In addition, these relationships are strongly reproduced by the $$V_{\text{p}} /V_{\text{s}}$$ V p / V s ratio of the bedrock in the range of 1.6–2.2. Interestingly, it is found that $$f_{\text{HVSR}}$$ f HVSR less than 1 Hz corresponding to $$h$$ h more than 100 m leads the trend of the overall frequency–depth relationship.

Arisbaya, Ilham; Handayani, Lina; Mukti, Maruf; Sudrajat, Yayat; Grandis, Hendra; Sumintadireja, Prihadi

doi: 10.1007/s00024-019-02241-0pmid: N/A

We present an interpretation of newly acquired audio-magnetotelluric data to reveal the subsurface geometry of Cimandiri Fault Zone, one of the major active faults in the western part of Java. The line section is 25 km long in a nearly north–south direction across the axes of CFZ with 24 stations of 750–1200 m spacing intervals. The 2D AMT inversion model shows two conductive zones in the southern part that may be associated to the Miocene rocks of the Southern Mountains, and a conductive zone in the northern part that is likely to be associated with Gunung Walat fold-belt. The subsurface structures of the Southern Mountains are dominated by south-dipping thrusts that may uplift the shallow marine sediments. Shallow seismicity occurred around CFZ indicating the activity of these blind thrusts.