Numerical simulation of fault development along NE-SW Himalayan profiles in Nepal

We examined the state of stress in and around the Himalayan nappes via 2D finite element method using elastic rheology under plane strain condition. This paper describes how we used advanced numerical modelling technique, the finite element method to compute stress and fault as a function of rock layer properties, convergent displacement and boundary condition in the convergent tectonic environment. Interpretation of the calculated results remains somewhat ambiguous because of the limitation of elastic modelling, however, the results are still comparable with geological and geophysical data. Some interesting features of our models are: (1) compressive state of stress  in Himalaya; (2) effect of geometry of MHT on stress orientation; (3) the diffuse zone of failure elements along the flat-ramp-flat regions of the Main Himalayan Thrust (MHT); (4) normal and thrust faults pattern in the vicinity of Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT); (5) initiation of faults at depth and their propagation toward south under increasing convergent dis placement,  which is consistent with the sequence of thrusting in Himalaya; and (6) direct correlation of simulated fault patterns with geological evidences. Thus overall features of the numerical models are able to conclude that the mid-crustal ramp, MBT and MFT are the most active structures in the present day plate kinematics.</jats:p

Numerical simulation of fault development in fold-and-thrust belt of Nepal Himalaya

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 111</jats:p

Numerical simulation of site effects in the upper aterno valley array during the aftershock sequence of the 2009 L'Aquila earthquake

On April 6th, 2009 a Mw=6.3 earthquake jolted the Abruzzo region of Central Italy, very close to the urban center of L'Aquila. Availability of high-quality recordings of the mainshock along with several aftershocks makes the seismic sequence the best recordednear-source events in Italy. In the present study, attention is devoted to the strong motion recordings of the upper Aterno River Valley array, which is part of the Italian Accelerometric Network (Rete Accelerometrica Nazionale, RAN), deployed NW of L'Aquila. These data provide a better understanding of the role played by site effects in the seismic response of the valley in epicentral area. This was accomplished by comparing recordings with the results of 1D and 2D site response analyses. The subsoil model of the Aterno Valley passing through the accelerometric stations was assumed from a previous study and was integrated with the results of dynamic tests carried out on reconstituted samples of coarse materials frequently encountered in the subsoil. First, the ground surface motion computed by assuming linear soil behavior was compared to the small-magnitude (ML=3-3.5) aftershocks recordings. It was found that 2D modeling provides a satisfactory understanding of the amplification phenomena in the array. Moreover, 2D analyses performed slightly better than 1D predictions. Based on this calibration study, further site response analyses were carried out and the computed ground motion was compared with the aftershock recordings of moderate magnitude (MW=4-5.6). In contrast, the results from these events do not show the analogous performance as obtained in the linear range. More specifically, shape of acceleration response spectra is generally satisfactorily simulated whereas discrepancies are observed in terms of PGA as well as maximum spectral amplitude. It is speculated on the possible explanations of these discrepancies

Some remarks on the cyclic response of non-plastic and high-plasticity natural silty soils of the Kathmandu valley (Nepal)

This note presents the results of constant-volume equivalent-undrained cyclic simple shear tests performed on saturated natural silty soils obtained from two different locations in the Kathmandu valley (Nepal) and characterized by different plasticity. Two simple shear devices were employed: the DSDSS (Double Specimen Cyclic Simple Shear) device and a modified NGI-type DSS device. The above two types of tests complemented each other and jointly covered a wide range of cyclic shear strains amplitudes. Nonlinear soil behavior (i.e. variation of shear stiffness and damping with shear strain level) and liquefaction resistance of the non-plastic silty soil were investigated. The results of the experimental investigation presented in this paper, albeit limited, increase the knowledge on the cyclic response of natural silty soils, especially those of very low plasticity, and provide first geotechnical data on the soft fluvio-lacustrine deposits of the Kathmandu Valley, responsible for site effects and liquefaction phenomena during the 2015 Mw=7.8 Gorkha earthquake

NBC105: 2019 Seismic design of buildings in Nepal: New provisions in the code

The NBC 105: 2019 Seismic Design of Buildings in Nepal is the revised version of the original code for seismic design first published in 1994. The code has never been reviewed and updated since then till the moment. Recognizing the development in research and technology and new knowledge learnt from various large earthquakes in last 25 years, the Government of Nepal decided to initiate the first revision of the seismic design code. The objective of this revised standard is to provide designers with general procedures and criteria for the structural design of buildings prevalent in Nepal. This paper presents the basic features of the revision and the principles adopted in the standard. A new seismic hazard map of Nepal was proposed at the outset based on probabilistic format. Accordingly the PGA values for various locations of Nepal were revised. The performance requirements have been introduced precisely in terms of collapse prevention and damage limitation; there is a further recommendation to verify the performance requirements checking the ultimate limit state and serviceability limit state. It is proposed to check life safety and damage limitation performance requirements. Two different spectra are proposed for seismic coefficient method and modal response spectrum method. Four types of sub soil category are proposed. Very soft soil category is added in addition to previous three categories. This new soil category represents a very deep soft soil found in Kathmandu valley. Research has indicated that hard soil should have greater acceleration demand at smaller periods. This issue is rarely addressed in design codes internationally and the revised version of the NBC 105 is one of the first codes to accommodate this in practice. The revised code, retaining the linear analysis, introduces the non-linear methods of analysis. The empirical formulae for determination of fundamental translation period have been revised. Other principal changes include the importance classes and importance factors, load combinations and load factors. The Performance factor (K), which was used in the earlier version to obtain seismic coefficient, does not reflect the modern seismic design philosophy of reducing the elastic seismic forces. The response reduction factors (Ductility factor, R and Overstrength factor, ) are introduced. The horizontal base shear coefficient will be determined separately for ultimate limit state and serviceability limit state. The horizontal design spectrum for the modal response spectrum method has been given different for ultimate limit state and for serviceability limit state. A separate section on structural irregularity has been added. The revised code now requires checking the inter-story drift for both serviceability limit state and ultimate limit state. The standard has been developed in a new format considering the recent development in the research and technology as well as the lessons from the recent earthquakes. The whole document has been spread over 10 sections with 2 annexes separately for ductile detailing of structural concrete and structural steel

Probabilistic seismic hazard assessment of Nepal for revision of national building code NBC105

Being located in seismically active Himalayan mountain belt, Nepal has been the locus of many devastating earthquakes. The Mw 8.4 Bihar-Nepal earthquake of 1934 AD was the biggest earthquake disaster in Nepal that had highlighted the need of a building seismic design code for safer construction. Though the necessity was realised earlier, Nepal developed its first National Building Code (NBC-105) only in 1994 after the 1988 Mw 6.9Udayapur earthquake in eastern Nepal. In April 2015, central Nepal witnessed the Mw 7.8 Gorkha earthquake, which had epicentre at Barpak village of Gorkha district, about 75 km west of Kathmandu. The ground mtions recorded at soft soil sites in Kathmandu Valley clearly show strong site effect resulting in high energy in long period, i.e. at 3s to 5s. A comparative study has revealed that, at least in Kathmandu Valley, the observed ground motions exceeded the seismic design demand proposed by NBC-105 for some period ranges. Unsurprisingly, the earthquake caused extensive damage to buildings and infrastructures in 14 districts(mostly towards east of the epicentre due to further ruptured directivity effect) and killed 8,970 people. This earthquake also triggered revision of the existing national building code (known as NBC- 105)by the Government of Nepal.A key feature of the revision of NBC-105 has been re-assessment of national seismic hazard by adopting a probabilistic approach. Since the development of NBC-105 in 1994, a large number of studies have been carried on seismo-tectonics, active fault, paleoseismology, seismicity, geodesy etc, which have significantly increased the level of knowledge on seismic sources in the central Himalayas. In addition, after the 2015 Gorkha earthquake, much knowledge is gained on the geometry of the main seismogenic fault, the Main Himalayan Thrust (MHT) also called the Main Frontal Thrust (MFT) at the surface of the Himalayan front. Based on recent researches, in contrast to seismic sources adopted in 1994, a fault source (MHT) and area sources, i.e. northern garbens in Tibet, strike-slip event dominant sources in eastern and western Nepal and a source south of MHT are considered for seismic hazard analysis. As there is no specific Ground Motion Prediction Equation (GMPE) for the Himalayas, based on seismo-tectonics, GMPEs are adopted including Next Generation Attenuation laws. More than two GMPES are used for each source using the logic tree approach. Seismic hazard is computed for 2%, and 10% probability of exceedence in 50 year. In contrast to hazard map of 1994, the zones of relatively higher Peak Ground Acceleration (PGA) i.e. 0.36g to 0.46g are, for 10% probability of exceedence in 50 year, concentrated just above the locked portion of MHT throughout the country. The PGA values gradually decrease towards the north and south of MFT. This pattern of PGA distribution is consistent with the coupling nature of the MHT in the Himalayas

Large paleoearthquake timing and displacement near Damak in eastern Nepal on the Himalayan Frontal Thrust

An excavation across the Himalayan Frontal Thrust near Damak in eastern Nepal shows displacement on a fault plane dipping similar to 22 degrees has produced vertical separation across a scarp equal to 5.5m. Stratigraphic, structural, geometrical, and radiocarbon observations are interpreted to indicate that the displacement is the result of a single earthquake of 11.33.5m of dip-slip displacement that occurred 1146-1256A.D. Empirical scaling laws indicate that thrust earthquakes characterized by average displacements of this size may produce rupture lengths of 450 to >800km and moment magnitudes M-w of 8.6 to >9. Sufficient strain has accumulated along this portion of the Himalayan arc during the roughly 800years since the 1146-1256A.D. earthquake to produce another earthquake displacement of similar size. Plain Language Summary The densely populated country of Nepal sits above the Himalayan Frontal Thrust fault. It is repeated displacements on this fault that are responsible for the uplift of the Himalaya mountains and considered capable of producing great earthquakes. Here we excavate a trench across the fault to show a great earthquake occurred 1146 -1256 AD in eastern Nepal. It has been a sufficiently long time since then that stresses have accumulated to a level capable of producing another such great earthquake

Assessment of site effects in the Kathmandu valley, Nepal, during the 2015 Mw 7.8 Gorkha earthquake sequence using 1D and 2D numerical modelling

The paper reports on the results of 1D and 2D site response analyses carried out in the Kathmandu Valley, Nepal, in order to investigate how site effects influenced the seismic response during the 2015 Mw 7.8 Gorkha earthquake sequence. The mainshock and a Mw 6.6 aftershock, for which recordings at both rock and soil sites are available, were considered. First, 1D analyses were carried out for the Pulchowk soft soil site, where a borehole was drilled. The shear wave velocity profile was defined using several 2D seismic array surveys carried out in the valley and constrained by noise measurements at Pulchowk site; the nonlinear soil behavior was characterized by means of cyclic simple shear tests carried out on undisturbed soil samples. Both equivalent and nonlinear approaches were adopted in the 1D analyses. Overall, the 1D model was capable to capture some relevant features shown by mainshock recordings such as the de-amplification of medium-to-high frequencies. On the contrary, the unusual high spectral amplification at long periods (3–6 s) recorded during the mainshock was better captured by the 2D finite element analyses carried out on a 20 km-large cross section of the entire valley, thus supporting the hypothesis of the occurrence of basin effects. The paper contributes to the understanding of site effects in Kathmandu Valley for the implementation of seismic risk mitigation strategies in the area