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Research of Earthquake Topographic Effect
Abstract
In this paper, the influence of topography on ground-motion intensity parameter, response spectrum, peak ground acceleration(PGA) and the ratio of response spectrum were studied based on the measured data from large scale shaking table test and observation stations in XiShan park for Wenchuan earthquake. The results show that: in the EW direction, the height do not affect PGD, SM_A, VSI and HI, while V_RMS decreases slightly at the middle of the slope, the other intensity parameters increase with the increase of height. In the EW direction, the height has no influence on PGD, V_RMS, SMA, SED, A_RMS, VSI and HI, while other parameters increase as the height increase. In the UD direction, the height has no influence on SMV, PGD, V_RMS, SMV, ASI, VSI and HI, while A_RMS decreases at the middle of the slope, and the other intensity parameters increase as the height increase. Ignoring the local topographic effect, the amplitude of response spectrum increases with the increase of height at the part of short period(T <= 1s), the part of long period(T>1s) is not effected by height. The ground motion will be amplified by local canyon topography, and the influence of local topography is larger than height. The research carried out in paper will deepen the understanding of topographic effect.
... This led researchers to turn their attention to the implementation of advanced numerical modelling techniques, which have been made possible thanks to advances in computational power (e.g. Bouchon et al. 1996;Komatitsch & Vilotte 1998;Paolucci 2002;Álvarez-Rubio et al. 2004;Paolucci & Morstabilini 2006;Lee et al. 2009;Miksat et al. 2010;Fan et al. 2014;Restrepo et al. 2016). In particular, in the last 20 yr, the spectral element method (SEM) has been widely and successfully implemented to simulate seismic wave propagation in 3-D earth models. ...
During earthquakes, structural damage is often related to soil conditions. Following the 01 April 2014 Mw 8.1 Iquique earthquake in Northern Chile, damage to infrastructure was reported in the cities of Iquique and Alto Hospicio. In this study, we investigate the causes of site amplification in the region by numerically analyzing the effects of topography and basins on observed waveforms in the frequency range 0.1–3.5 Hz using the spectral element method. We show that topography produces changes in the amplitude of the seismic waves (amplification factors up to 2.2 in the frequency range 0.1–3.5 Hz) recorded by stations located in steep areas such as the ca. 1 km-high coastal scarp, a remarkable geomorphological feature that runs north–south, that is, parallel to the coast and the trench. The modeling also shows that secondary waves—probably related to reflections from the coastal scarp—propagate inland and offshore, augmenting the duration of the ground motion and the energy of the waveforms by up to a factor of three. Additionally, we find that, as expected, basins have a considerable effect on ground motion amplification at stations located within basins and in the surrounding areas. This can be attributed to the generation of multiple reflected waves in the basins, which increase both the amplitude and the duration of the ground motion, with an amplification factor of up to 3.9 for frequencies between 1.0 and 2.0 Hz. Comparisons between real and synthetic seismic waveforms accounting for the effects of topography and of basins show a good agreement in the frequency range between 0.1 and 0.5 Hz. However, for higher frequencies, the fit progressively deteriorates, especially for stations located in or near to areas of steep topography, basin areas, or sites with superficial soft sediments. The poor data misfit at high frequencies is most likely due to the effects of shallow, small-scale 3D velocity heterogeneity, which is not yet resolved in seismic images of our study region.
Based on a typically simply-supported girder bridge in Wenchuan, a set of finite element model samples of bridges were provided by considering the uncertainty of bridge parameters. Nonlinear dynamic transient analysis was preformed using the Wenchuan earthquake ground motion record,in which seismic peak responses of bridge components were included. The seismic demands of bridge components were analyzed by principal component method for reduction of dimensions, and the composite seismic demand of bridge system was obtained. By using the regression analysis method, a probabilistic relationship between the ground motion intensity and the composite seismic demand was established. On this basis, the different ground motion intensity measures were suggested and compared. The results show that the velocity-based ground motion intensity measures have preferable efficiency, proficiency and sufficiency, and could be used for the seismic vulnerability analysis and seismic risk assessment further.
Effects of both soil layer and irregular topography on ground motion are relatively serious, their amplification effects on ground motion must be considered in site selection and seismic fortification of buildings to prevent or mitigate seismic hazard of buildings. Utilizing acceleration time histories of the Wenchuan earthquake' s mainshock recorded by Zigong topographic array located in Xishan park, Zigong city, the amplification effects of both soil and site topography on ground motions are analyzed and studied by traditional spectral ratio method. The results are as follows. (1) The amplification effect of topography at low frequency is not obvious, and the maximal amplification factor is 1. 24. And the amplification effect of topography at high frequency is significant. In the frequency range from 1 Hz to 10 Hz, the amplification factor of hilltop is maximal, the maximal amplification factors of EW, NS and UD ground motions on hilltop are 4. 15, 3. 61 and 2. 41 respectively, the corresponding frequencies are 5.72 Hz, 6. 46 Hz and 7. 44 Hz respectively. In the frequency range from 10 Hz to 20 Hz, the amplification factor of some components of ground motions on ridges in the vicinity of hilltop is maximal, the maximal amplification factors of EW (74) and UD (74$) ground motions of ridges in the vicinity of hilltop are 9. 10, 5. 56 and 2. 52 respectively, corresponding frequencies are 16. 97 Hz, 16. 91 Hz and 17. 91 Hz respectively. (2) The maximal topographic amplification factors of ground motions tend to increase with elevation of the hill, and the trend in frequency range from 0. 1 Hz to 10 Hz is more obvious. (3) The amplification effect of horizontal ground motion at soil sites is obvious when frequency is more than 2 Hz, and that of vertical ground motion is obvious when frequency is more than 4 Hz. The maximal amplification factors of EW, NS and UD ground motions of soil site are 13.4, 12. 168 and 6.0 respectively, corresponding frequencies are 6. 94 Hz, 7. 55 Hz and 10. 8 Hz respectively. (4) The maximal amplification factors of soil site are more than ones of topographic sites. For horizontal ground motions, the former is 3 times the latter at lowest For vertical ground motions, the former is 2. 5 times the latter at lowest (5) For both topographic site and soil site, the maximal amplification factors of horizontal ground motions are more than the ones of vertical ground motions.
Differences were measured in seismograms recorded at eight proximate hardrock sites: five in tunnels and three on outcrops on hills. Tunnel sites affected the amplitude of incident signals by as much as a factor of three, at frequencies inversely proportional to the depth of the site into the tunnel. For example, the maximum effect of a site 12 m deep was at 25 Hz while that of a site 85 m deep was at 11 Hz. Most of the observed tunnel effects could be explained in terms of interference between the incident and surface-reflected waves. Rock outcrop sites on hills affected the amplitude of incident signals by as much as a factor of eight, at frequencies inversely proportional to the dimension of the hill, and more for horizontally than for vertically polarized signals. Hard-rock site effects typically varied by no more than a factor of three for input signals from different earthquakes, having widely different azimuth and incident angles.
Integrated ground velocities and displacements calculated from the accelerogram recorded at the Pacoima dam site indicate that the strong ground motion was predominately in the vertical and NS direction, in general agreement with the mechanism of faulting as inferred from aftershock studies, and with fault displacements observed in the field. High-frequency peak accelerations of 1.25 g were recorded in two horizontal directions, these being the highest ground accelerations so far recorded for earthquakes. Response spectrum curves calculated from the accelerograms do not show unusual features, and the numerical values are consistent with past experience. The high-frequency, high-amplitude impulsive ground motion associated with the highest peak accelerations did not contribute significantly to the over-all response spectrum values. The presence of the high-frequency motions in the recorded accelerograms is presumably the consequence of the proximity of the recording site to the fault dislocation.
Field experiments were performed to obtain data needed for studying the effects of topography on seismic motion. L-7 instruments deployed at the crest and base of Kagel Mountain and Josephine Peak, California, recorded several aftershocks of the February 9, 1971, San Fernando earthquake. L-7 instruments deployed at the crest, middle, and base of Butler Mountain near Tonopah, Nevada, recorded the seismic signal generated by the cavity collapse following the Nevada Test Site detonation, ALGODONES. Frequency-dependent amplification of the motion at the crest relative to the motion at the base was observed at all three mountains. Factors as large as 30 in the frequency domain were seen at Kagel Mountain with lesser amounts at the other two locations. The three mountains are similar in shape, but different in size. The effect of shape is not clear. The smallest mountain amplified the motion in a narrow range of periods (peaking around 0.3 to 0.5 sec), the medium-size mountain showed amplification over a slightly broader range (peaking at periods of 0.4 to 0.5 sec), whereas the largest mountain showed less amplification, but it occurred over a broader range of periods. Effects in the time domain are generally comparable to, or less than, the effects in the frequency domain. The ratios of peak amplitudes (crest to base) are usually less than the spectral ratios. It is concluded that topography plays a significant role and is an important consideration in determining the seismic motion that a particular site receives.
Finite difference calculations for SH-motion, as well as field observations, show that topography can have significant effects on seismic waves when the incident wavelengths are comparable to the size of the topographic features and the topographic slopes are relatively steep. These effects are frequency-dependent and can range from amplification to deamplifieation at a given site. For the models considered in this paper, amplifications of 75 per cent as compared to motions produced in regions of no topography were found. A qualitative discussion of the strong-motion accelerogram recorded at Pacoima Dam from the San Fernando earthquake of February 9, 1971, suggests that the recorded accelerations could, at any period, be expected to differ by 25 to 50 per cent from those that would have been recorded if no topographic features were present. The complex terrain precludes a prediction of the actual magnitude or sign of the difference. The present results are probably most significant from the viewpoint of engineering seismology, for the conditions required for a significant topographic effect are most likely to occur in the period range of engineering interest.