Effect of a simple mountain range on underground seismic motion (original) (raw)
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COMPLEX" SITE EFFECTS IN EARTHQUAKE GROUND MOTION, INCLUDING TOPOGRAPHY
In the first part, evidence of complex site effects in 2D geological configurations is illustrated, and the related difficulties in interpreting ground motion observations is discussed, using weak motion data recorded in three Alpine valleys in Northern Italy. While, on one hand, such valleys are mostly located in regions of low or moderate seismic hazard, where ground motion data are scarce, on the other hand investment in buildings and infrastructure has been substantial and may not always have been protected by adequate earthquake resistant provisions and zoning criteria. Moreover, experience and data obtained in regions hit by destructive earthquakes cannot be easily transferred to such a different context. The combined influence of 1D propagation effects on the dominant frequencies of motion, and of 2D effects on the amplification level and significant frequency band, are highlighted. For what concerns the influence of the surface relief, the incidence of topographic amplification is investigated (in the perspective of ground motion scenarios) for one historical earthquakes in a mountainous region, i.e. the destructive 1887 Western Liguria (Italy) earthquake which represents a relevant case in point.
Geophysical Journal of The Royal Astronomical Society, 2006
This paper presents a numerical simulation of the effects of an underground ridge (UGR) on seismically-induced ground motion characteristics using 2.5-D modelling with variable grid size. The responses of models with and without an UGR, estimated group velocity of later arrivals, horizontal differential ground motion, snapshots and spectral analysis of surface waves have confirmed high frequency surface wave generation near the ridge. Exponential decay of surface wave amplitude with depth was obtained in the vertical component but not in the radial and transverse components. Changes of focal mechanism parameters mainly affected the amplitude but not the spectral shape or bandwidth of UGR-induced surface waves. We predict that anomalous macroseismic intensity due to UGR-induced surface waves might arise in a zone of width of about 2.2 km parallel to a ridge and at a distance of about 0.5 km from the ridge edge, for the model geometry and the soil parameters considered. The UGR-induced surface waves were observed even when a UGR was overlain by a soil deposit. Increased depth or changes in the slope of the flanks of the UGR causes decrease of spectral amplitudes but does not affect the spectral shape, bandwidth or dominant frequency of induced surface waves.The dominant frequency of Rayleigh waves appeared to be twice that the fundamental frequency (f0) of first soft soil layer. For Love waves, the dominant frequency was twice that of f0 for f0≤ 0.63 Hz and was more than twice f0, when f0 > 0.63 Hz. The lower cut-off frequency for surface wave generation was approximately f0. Only spectral amplitudes were affected by a change of S-wave velocity in first soil layer but not the spectral bandwidth, spectral shape or dominant frequency when f0 was maintained constant. An increase of spectral bandwidth with decrease of f0 of the first soil layer was found. We obtained a decrease of surface wave amplitude in accordance with the decrease of amplitude of body waves with angle of incidence. The amplitude of surface waves on the side of incidence of body waves was more than the opposite side. Further, it was inferred that dominant frequency and spectral bandwidth were unaffected but spectral shape was highly dependent on the angle of incidence of body waves. The simulations revealed that UGR effects deserve particular attention for earthquake-resistant design and seismic microzonation.
The effect of underground structure on seismic motions of the ground surface
1982
Two-dimensional finite element modelling of underground structural anomalies at shallow depths has been done to obtain the response at the ground surface to damped, vertically incident, SH-waves. Power spectral ratios are examined to determine what effects the position, shape, depth and size of the anomaly have on the surface seismograms. Based on the results gathered from a number of models, inferences are made with respect to the inverse problem: given the seismic motion of the ground surface, what can be said about the underground structure?
Experimental Investigation of the Topographic Modification of Earthquake Ground Motion
2017
Experimental Investigation of the Topographic Modification of Earthquake Ground Motion Jacob Dafni Chair of the Supervisory Committee: Chair Joseph Wartman Civil and Environmental Engineering Topographic modification of earthquake ground motion can significantly impact ground motion amplitude and frequency content. While previous studies have investigated topographic modification of ground motion, also called “topographic effect”, there are discrepancies between the results of field and numerical investigations. A new experimental approach involving physical modeling in a geotechnical centrifuge was used to study topographic effects. The centrifuge captures the complexity of a physical process and shares many of the advantages of a numerical model (e.g., material properties, instrumentation location, and ground motions can be controlled). Experimental results show that topographic amplification can exceed amplification due to subsurface geology (i.e., “site amplification”). The resu...
MODELLING OF STRONG GROUND MOTION OF THE JULY 2004, M W 5.2 EARTHQUAKE IN KRN MOUNTAINS
The effects of the seismic source and of local geology on ground motion are studied in the Upper Soča Valley (western Slovenia), which was struck by two earthquakes, on 12 April 1998 (M W =5.6) and on 12 July 2004 (M W =5.2). The paper focuses on the latter event, while the former was thoroughly investigated in . 2D numerical modelling is applied, together with a recently improved technique of 'sub-structuring' (source and site effect) of the problem at study (DRM approach). Numerical modelling, used for computing site effects, is thereby coupled with an efficient mathematical method used for source modelling. The combined influence of 1D propagation effects on the dominant frequencies of motion, and of 2D effects on the amplification level and significant frequency band, are highlighted. Ground amplification is high in the frequency range of building vulnerability (1-7 Hz), consistent with findings. Comparison with Eurocode 8 spectra shows that the latter may not be conservative for periods up to 0.8 s. The relatively large response at such periods is related to the geological configuration of the valley, which appear to require a reasonably accurate definition of its 2D geometry.
Two-dimensional assessment of topographical site effects on earthquake ground response
4th International FLAC Symposium on Numerical Modeling in Geomechanics, Paper: 04-08
Amplification factors resulting from earthquakes were assessed for different sites of two real cases: the earthquakes in Colombia (1999) and El Salvador (2001). Major damage was reported concentrated towards the top of slopes and ridges. In El Salvador, a big landslide was triggered during the earthquake in a combined failure mechanism. The geological materials in both cases are medium to highly plastic, and consist of pyroclasts, epyclasts, paleosoils, and residual soils of volcanic origin. From both areas example sites were numerically modeled with the real acceleration records (frequency-independent) and with artificial sinusoidal functions with different frequencies. Results show higher amplification patterns along the top of the hills with an irregular amplification/deamplification trend within narrow ranges. Along the slopes, the amplification generally decreases towards the base with an about exponential trend, and in depth decrease linearly. The results of the landslide section were similar to the failed slope.
Ground motion at mountains and sedimentary basins with vertical seismic velocity gradient
Geophysical Journal International, 1994
The boundary integral-Gaussian beam method (Benites & Aki 1989) is applied to study the ground motion in 2-D structures that exhibit irregular topography and interface, and whose shear wave velocity varies linearly with depth, fQr incident plane SH waves. In our first example of application the model is a half-space whose free-surface topography is a ridge of cosine shape, with vertical shear wave velocity gradient. In the second, the model is a semi-cylindrical sedimentary basin in a homogeneous half-space, in which the shear wave velocity of the sediments increases with depth. Our results for the case of the mountain show that the amplification on its top, predicted by the 2-D modelling when the velocity is constant, is enhanced when the velocity gradient is present, for all frequencies and by a factor up to 3. In the case of the basin, results show that the velocity gradient; (1) enhances the amplification at the edges of the valley, (2) makes the reverberations due to 2-D resonance have largeJ amplitudes and shorter intervals between arrivals, (3) shortens the total duration of the seismograms at all stations within the basin.
Chinese Science Bulletin, 2004
Influences on the ground motion simulations by soil amplification effects and multiple seismic wave interferences in the heterogeneous medium are investigated. Detailed velocity structure obtained from the microtremor array survey is adopted in the ground motion simulation. Analyses for amplification ratios of core samples of ten drill holes with 40 m deep in the sedimentary layers show that the soil amplification ratio influences nonlinearly the seismic ground motion. Based on the above analysis results, the ground motion in the heavily damaged zone in the Japanese Kobe earthquake of 1995 is simulated in a digital SH seismic wave model by using the pseudospectral method with the staggered grid RFFT differentiation (SGRFFTD). The simulated results suggest that the heterogeneous velocity structure results in a complicated distribution of the maximum amplitudes of acceleration waveforms with multiple peaks at the surface. Spatial distribution of the maximum amplitudes coincides well with that of collapse ratios of buildings in Kobe. The dual peaks of the collapse ratios away from the earthquake fault coincide well with the double peak amplitudes of simulated seismic acceleration waves also. The cause for the first peak amplitude of the ground motion is attributable to the interference of the secondary surface wave from the bedrock propagating horizontally along the surface sedimentary layer and the body wave from the basin bottom according to analyses of wave snapshots propagating in inhomogeneous structure of the Osaka group layers. The second peak amplitude of the ground motion may be attributive to the interference of the secondary surface wave from the tunneling waves in the shallow sediments and the body wave. It is important for the study on complicated distributions of earthquake damages to investigate influences on the ground motion by soil amplification effects and multiple seismic wave interferences due to the structure. Explorations of the structure to the bedrock are necessary for the urban mitigation disaster. Seismic wave simulations are valid for aseismatic study.
Analysis of Local Site Effects on Seismic Ground Response under Various Earthquakes
2018
By assessment of induced damages to structures and major infrastructures, seismic geotechnical researchers have concluded that the site conditions significantly influence on the failure distribution in urban and rural areas. Consequently, to determine the characteristics of seismic motions of the ground, it is essential to study the effective geotechnical factors. In this study influence of local site effects and soil conditions on the intensity of ground motion are investigated with two methods (non-linear and equivalent linear methods) based on one dimensional shear wave propagation in soil layer theory. In this regard, some series of site response analyses which consider various input motions, geotechnical parameters of site and non-linear properties were performed. The comparisons demonstrated that non-linear method provides a more accurate characterization of the true non-linear soil behavior compared to the equivalent-linear procedures. The earthquakes with Peak Ground Acceleration (PGA)less than 0.1 g have the most increase in horizontal acceleration at the surface in comparison with the earthquakes with greater peak accelerations.