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Maximum acceleration recording from Lushan earthquake on April 20, 2013
Abstract
The Lushan earthquake on April 20, 2013 is another destructive event after the Wenchuan earthquake in 2008. More than 100 strong motion stations were triggered in the main shock. The 51BXD station located in Baoxing Country recorded a peak ground acceleration (PGA) of more than 1g, that is 1.026g (-1005.3 gal) in the EW direction. It has been the first time that more than 1g is recorded at free field site in China and it should be a milestone event in Chinese strong motion observation history. However, in the released official Lushan intensity map, the around area was classified to intensity VII, which raises an interesting question why such a great PGA leads to a low intensity. During the post-earthquake field survey, it was found that this station was located on a steep slope, not a real "free-field" station, so the recording could be affected by the topography. By comparing with 3 other nearby stations with the next generation attenuation models, it is shown that the earthquake response spectrum less than 0.2 s was amplified significantly. To support this evidence, a background noise test was performed around this area, and predominant period and horizontal acceleration amplification factor were calculated with the traditional spectral ratio method. The calculation results clearly show that from the bottom of the valley to the top, the predominant frequency increases and the amplification factors also raise with the predominant frequency increasing. In addition, the response spectrum of 51BXD recording was calculated and compared with some others from the Wenchuan earthquake to interpret why there were so slight damages to its around buildings. The preliminary conclusion is that the topography is the root cause for 1g peak ground acceleration recording in the Lushan earthquake.
... However, stations 51BXM, 51LSF, and 51BXD are located either on river terraces or on the slope of steep hills, implying that the topography may have an effect on the observed ground motions. In particular, station 51BXD, which recorded the largest PGA, is located on the midslope of a small hill (a detailed description was provided by Wen et al. [2013] and Ren, Ji, et al. [2013]). ▴ Figure 9. Comparisons of the horizontal PGAs observed at hanging-wall and footwall sites in the Lushan earthquake with those predicted by four Next Generation Attenuation (NGA) empirical GMPEs. ...
... The predicted values were calculated with (a) V S30 350 m=s and (b) V S30 760 m=s. Wen et al. (2013) calculated H/V spectral ratios from ambient noise recordings acquired along the slope of a hill and observed site amplification for a wide range of frequencies (3-20 Hz) in the middle of the slope. The amplitudes of the H/V peaks are approximately 4.0, 2.5, and 1.5 for frequencies of 15, 7, and 3 Hz for sites on the foothill, midhill, and uphill, respectively. ...
... The simulation results of most stations match well with the observations, but a few stations exhibit significant deviations. For instance, the simulated results for station 51BXD are systematically low, probably due to the station is located on a ridge and affected by the complex local topography (Wen et al., 2013). In addition, the simulated results for stations 51CDZ, 51LSF, 51PJD, and 51PJW notably deviate from the recorded data. ...
The stochastic EXtended finite‐fault ground‐motion SIMulation algorithm (EXSIM) has been widely applied in simulating and predicting broadband strong ground‐motion. However, an increasingly number of researchers have found that EXSIM may overestimate ground‐motions at low frequencies for some large‐magnitude earthquakes and/or thrust earthquakes, for which the far‐field source model has been explained by a double‐corner‐frequency model. Despite controversy, the double‐corner‐frequency model is now being accepted as one of the main categories of the far‐field source model. This study demonstrated the limited applicability of EXSIM to earthquakes explained by the double‐corner‐frequency source model, by presenting the equivalence between motions generated by EXSIM and those generated by EXSIM's point‐source version, SMSIM, which adopts the ω‐square single‐corner‐frequency model. Furthermore, two improvements to EXSIM have been proposed: (a) the incorporation of the asperity‐distributed stress‐drop compound faults model and (b) the hybrid application of EXSIM with the proposed model. The effects of the two improvements have been verified by comparing EXSIM‐generating motions with recorded ground‐motions for the 2013 Mw 6.7 Lushan thrust earthquake. Significantly, consistent simulation accuracy has been achieved across high‐ and low‐frequency bands as well as in far‐ and near‐fields. The consistent accuracy of the improved EXSIM in simulating high‐ and low‐frequency ground motions enables its direct and independent application to broadband ground motion simulations. Moreover, the first validation of this consistent accuracy in both near‐ and far‐field scenarios further enhances its application in earthquake engineering practices.
... Since the natural frequency of the main building structures in China's small and medium-sized cities and towns is 1-3 Hz [22,27], the corresponding natural vibration period is 0.3-1 s. Within this range, except that the response spectrum value of station 51JZZ is higher than the design spectrum of the earthquake fortification, the acceleration response spectra recorded by the other four stations are far less than the design spectrum of the earthquake fortification. ...
Eighty-eight free-field strong ground motion observation datasets obtained by the National Strong Motion Observation Network System of China for the Jiuzhaigou Ms7.0 earthquake were used to study the characteristics of the strong ground motion and the site response. In this study, we calculated the VS30 of the near field station and compared the observed values of the horizontal peak acceleration and peak velocity with the CB2014 ground motion prediction models developed by Campbell et al. (2014). The results indicate that the observed values of the horizontal peak acceleration and peak velocity of the Jiuzhaigou earthquake are both smaller than the predicted values obtained from the ground motion prediction equations. By regressing the spatial variation curves of the D
SR (5–95%) and D
SR (5–75%) ground motion durations, and comparing them with Bommer’s (2009) ground motion duration prediction curve, it was found that the duration of the Jiuzhaigou earthquake was greater than that obtained from the global empirical prediction equation. The scope of the source duration corresponding to the D
SR (5–75%) duration is 2.76–4.28 s, and the scope of the source duration corresponding to the D
SR (5–95%) duration is 8.88–10.36 s, which are close to the peak time and completion time of the seismic moment release during the source rupture process. The linear elastic acceleration response spectra of 11 stations within 100 km of the fault were calculated for comparison with the design spectrum. It was concluded that the range of the predominant period of the response spectrum was 0.05–0.26 s, which is less than the natural vibration period of the local multistory building. However, the response spectrum value recorded by station 51JZZ is greater than the design spectrum of the 8° rare earthquake and the observation values of the nearby strong ground motion stations. Through analysis of the H/V spectrum ratio and the case of station 51JZZ, which is closest to the epicenter, this phenomenon was concluded to be related to the magnification effect of the near-surface soil layer and the nonlinear response of the site under the action of strong earthquakes.
... In 2013, a M s = 7.0 earthquake struck Lushan County in Yaan City, Sichuan Province. For the first time, a PGA value larger than 1.0 g was recorded in China (Wen et al., 2013). The DGA maps (Fig. 14) demonstrate DGA values around Lushan are as high as 1.2 g. ...
Many devastating earthquakes inflicted heavy casualties and property losses in the seismically active China Seismic Experimental Site area (CSES: 97.5° ~ 105.5°E, 21° ~ 32°N). We performed a first-order seismic zoning based on Neo-deterministic Seismic Hazard Assessment (NDSHA) in the study area delimited by 94° ~ 108°E and 19° ~ 35°N, containing the South-East margin of the Tibetan Plateau and the Sichuan-Yunnan region. The seismic hazard is expressed by maps of peak ground displacement (PGD), peak ground velocity (PGV) and design ground acceleration (DGA) values, extracted from synthetic seismograms computed at a regional scale and mapped on a regular grid of 0.2° × 0.2° over the study area. For the computation of synthetic seismograms, we considered and updated all the available geophysical-geological-tectonic information, including historical and instrumental earthquake catalogues, seismogenic zones, seismogenic nodes, focal mechanisms, and geophysical structural models. We tested the performance of our assessments with available data (i.e., after the Great Wenchuan (2008, May 12, Ms = 8.0) and Lushan (2013, April 20, Ms = 7.0) earthquakes) and verified the negligible influence of large events located “far” from the study area. The results indicate the high seismic hazard of the region, with a particular attention (i.e., where DGA > 0.6 g) to the areas located around the main fault zones, e.g., the Longmenshan, Anninghe and Zemuhe Fault Zones. These first-order NDSHA zoning findings may serve as a knowledge basis to support both large- to mid-range preparedness actions and (multi-scenario) site-specific studies.
... Firstly, PGA of Lushan earthquake are generally much larger than other earthquakes with similar magnitude. The EW component of PGA reaches 1005.4 gal at 51BXD station which has the first recording with PGA exceeding 1.0 g in China, according to Wen's research [Wen et al., 2013]. Consequently, higher observed values might lead to the result that more than half of IM Obv /IM SY ratios exceed the line of e σlny . ...
This manuscript presents a new model, named SY model, to predict horizontal peak ground acceleration and 5% damped response spectrum for shallow crustal earthquakes in Sichuan-Yunnan region of China. SY model considers the influences of magnitude, rupture distance, fault types, site amplification and hanging-wall scaling on ground motions. The random effect regression analysis and residual analysis are used to fit SY model based on a strong-motion database which is established to summarize the recordings of Sichuan-Yunnan region shallow crustal earthquakes. Prediction results are provided by SY model and subsequently compared with observations and those of other GMPEs for validation.
... The station 51BXZ is located on the relatively flat ground, not like the station 51BXD on a steep hillside. This gives us a hint that the short-period ground motion at station 51BXD is possibly amplified by the topographic effect [10].The hill could amplify the high-frequency ground motion from the foothill to uphill, that has been verified by using the strong motion data from Zigong topographic array during the 2008 Wenchuan earthquake [11]. ...
A Ms7.0 earthquake occurred on April 20, 2013 in Lushan Country, Sichuan Province, China, with the maximum seismic intensity of IX. It caused 196 death, 21 missing and 11470 injured. During the main shock, 114 strong motion recordings were accumulated by the National Strong Motion Observation Network System (NSMONS) of China and 63 recordings by the Local Earthquake Intensity Reporting Network of Chengdu City. The recordings show that the PGAs of ground motion in Lushan Earthquake are in accordance with the equation commonly used in China, but larger than the one used in fifth-generation ground motion zoning map, while the PGVs match well. The acceleration response spectra with 5% damping ratio of typical large recordings all have the peak periods around 0.1-0.2s where the spectral acceleration is considerably larger than GMPEs, and notably smaller when the period larger than 0.3s. The damage that relatively slight in Lushan Earthquake could be inferred by that the PGVs are small and the spectral acceleration is drastically smaller at the periods 0.3-1.0s corresponding to the natural period band of the 3-10 stories common buildings in the moderate and small cities and towns in China.
Current stochastic finite-fault simulation approaches do not fully consider the influence of slip distribution characteristics on the corner frequency and scaling factor. Using data from the 2013 Lushan earthquake (Mw 6.7), we developed a modified finite-fault model. This model was established by incorporating the corner frequency, the rupture velocity, and a scaling factor based on a nonuniform slip distribution. In this model, the radiated energy of the entire rupture fault is distributed to each subfault based on the weighted contribution of each subfault to the total slip of the rupture fault, thus generating an improved scaling factor. The path duration and kappa coefficient are obtained using Lushan event data, while other parameters, such as the stress drop, quality factor, geometric attenuation factor, and site amplification, are selected from previous studies. To verify the reliability of the improved model, the acceleration records taken during the Lushan earthquake from 17 near-field stations with less than 100 km from the fault were synthetized by random and inverted sources, and the simulated pseudospectral acceleration, Fourier acceleration spectrum, and model bias were obtained. The peak ground accelerations of 1681 virtual points were also predicted using the modified model. It was found that the modified model can effectively improve the modelling accuracy in short periods (T < 1 s).
The Mw6.7 Lushan earthquake on April 20, 2013 provided the opportunity to validate the applicability of American Next Generation Attenuation Relationship(NGA-West 1 and NGA-West 2 model). A total of 42 free-field strong-motion recordings within the rupture distance of 200 km were used in this study. Firstly we established an empirical attenuation relationship for the peak ground acceleration and spectral acceleration. The residuals analysis shows that the model can properly reflect the attenuation tendency of the parameters. It was found that the NGA models underestimate the parameters slightly in short period less than 0.2 s, overestimate significantly in long period over 1 s. Compared with the NGA-West 1 model, the NGA-West 2 model not only performs better for the discreteness of short period spectral acceleration, but also reduces the residuals in long period
Field investigation of the engineering damages of the Lushan earthquake revealed some new and to much extent unique damage phenomena. The performance of the engineering facilities as well as the lifeline system also provide an opportunity to evaluate the post-Wenchuan-earthquake reconstruction, which will be interesting not only for the local people but also for other places subject to similar seismic risk.
The generalized inversion of S-wave amplitude spectra from the free-field strong motion recordings of the China National Strong Motion Observation Network System (NSMONS) are used to evaluate the site effects in the Wenchuan area. In this regard, a total of 602 recordings from 96 aftershocks of the Wenchuan earthquake with magnitudes of M3.7-M6.5 were selected as a dataset. These recordings were obtained from 28 stations at a hypocenter distance ranging from 30 km to 150 km. The inversion results have been verified as reliable by comparing the site response at station 62WUD using the Generalized Inversion Technique (GIT) and the Standard Spectral Ratio method (SSR). For all 28 stations, the site predominant frequency F
p and the average site amplification in different frequency bands of 1.0–5.0 Hz, 5.0–10.0 Hz and 1.0–10.0 Hz have been calculated based on the inversion results. Compared with the results from the horizontal-to-vertical spectral ratio (HVSR) method, it shows that the HVSR method can reasonably estimate the site predominant frequency but underestimates the site amplification. The linear fitting between the average site amplification for each frequency band and the V
s20 (the average uppermost-20 m shear wave velocity) shows good correlation. A distance measurement called the asperity distance D
Aspt is proposed to reasonably characterize the source-to-site distance for large earthquakes. Finally, the inversed site response is used to identify the soil nonlinearity in the main shock and aftershocks of Wenchuan earthquake. In ten of the 28 stations analyzed in the main shock, the soil behaved nonlinearly, where the ground motion level is apparently beyond a threshold of PGA > 300 cm/s2 or PGV > 20 cm/s, and only one station coded 51SFB has evidence of soil nonlinear behavior in the aftershocks.
Local site conditions play an important role in the effective application of strong motion recordings. In the China National Strong Motion Observation Network System (NSMONS), some of the stations do not provide borehole information, and correspondingly, do not assign the site classes yet. In this paper, site classification methodologies for free-field strong motion stations are reviewed and the limitations and uncertainties of the horizontal-to-vertical spectral ratio (HVSR) methods are discussed. Then, a new method for site classification based on the entropy weight theory is proposed. The proposed method avoids the head or tail joggle phenomenon by providing the objective and subjective weights. The method was applied to aftershock recordings from the 2008 Wenchuan earthquake, and 54 free-field NSMONS stations were selected for site classification and the mean HVSRs were calculated. The results show that the improved HVSR method proposed in this paper has a higher success rate and could be adopted in NSMONS.
The National Strong Motion Observation Network System (NSMONS) of China is briefly introduced in this paper. The NSMONS consists
of permanent free-field stations, special observation arrays, mobile observatories and a network management system. During
the Wenchuan Earthquake, over 1,400 components of acceleration records were obtained from 460 permanent free-field stations
and three arrays for topographical effect and structural response observation in the network system from the main shock, and
over 20,000 components of acceleration records from strong aftershocks occurred before August 1, 2008 were also obtained by
permanent free-field stations of the NSMONS and 59 mobile instruments quickly deployed after the main shock. The strong motion
recordings from the main shock and strong aftershocks are summarized in this paper. In the ground motion recordings, there
are over 560 components with peak ground acceleration (PGA) over 10 Gal, the largest being 957.7 Gal. The largest PGA recorded
during the aftershock exceeds 300 Gal.
Owing to convenience and economical advantages, the microtremor methods for site dynamic characteristic evaluation attract considerable interests. Now a lot of approaches have been brought forward and applied to practice all over the world. But due to lack of knowledge of the latest advance in international microtremor research, there are also some widely different opinions about the research and applications in Chinese earthquake engineering field. To promote the microtremor research and application in China, in this paper the latest advances in microtremor array methods and H/V methods are introduced and their features and application conditions are pointed out.
The influence of local topography on the seismic ground motion has been the hot spot of the earthquake engineering. Selecting the ridge topography with Wenchuan earthquake strong motion records in Zigong topographic array and Beichuan county river valley with typical earthquake disaster, this article analyzes the topographic amplification effect through earthquake observation records, code specifications and numerical simulation. Results show that ; (1) Code specifications could basically meet the requirements of general project sites ; (2) two-dimensional or three-dimensional numerical simulation should be performed on densely populated area higher than 60m and steep topography for topographic effect analysis. The conclusions could provide some basic data for code specification amendment.
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.
Over the history of instrumental strong-motion recording, the largest amplitudes of ground motions recorded to date have had a significant impact on the perception of the largest amplitudes of ground motion considered physically realizable. However, the length of the instrumental recording history is comparatively short, and instrumental recording networks are relatively sparse, which raises the issue of whether the full range of ground motions has been captured in the current global holdings of strong-motion data. Because the answer to this question is quite obviously negative, a more difficult question then arises: How much greater than the largest currently available observation could future ground motions be? The present article explores this issue, drawing on empirical observations, results from numerical simulations, and a statistical exercise involving the sampling of spatially correlated stochastic ground-motion fields.
This study investigates the characteristics of the free-field strong-motion records that have yielded the 100 largest peak accelerations and the 100 largest peak velocities in a relatively complete database of accelerograms recorded through the summer of 2007. The peak is defined as the maximum zero-to-peak amplitude of the acceleration or velocity vector during the strong shaking. This compilation includes 35 records with peak acceleration greater than 1g ( 980 cm/sec(2)) and 40 records with peak velocities greater than 100 cm/sec. The results sample an estimated 150,000 instrument-years of strong-motion recordings. The geometric mean of the two horizontal components of acceleration or velocity, as used in many ground-motion prediction equations, is typically 0.76 times the magnitude of this vector peak. Accelerations in the top 100 come from earthquakes as small as magnitude 4.8, while velocities in the top 100 all come from earthquakes with magnitude 5.7 or larger. These records are dominated by crustal earthquakes with thrust, oblique-thrust, or strike-slip mechanisms. Normal faulting mechanisms in crustal earthquakes constitute under 5% of the records in the databases searched and an even smaller percentage of the 100 largest acceleration or velocity records. All National Earthquake Hazard Reduction Program site categories have contributed exceptional records, in proportions similar to the extent that they are represented in larger databases.
In this paper we analyse the two‐dimensional scattering and diffraction of plane SH waves by a semi‐elliptical canyon. The exact series solution of the problem, for general angle of incidence of the plane SH waves, has been used to examine the dependence of surface amplifications inside and near the canyon. The nature of ground motion has been found to depend on two key parameters: (a) The angle of incidence. (b) The ratio of the canyon width to the wave length of incident SH waves.
For short incident waves surface displacement amplitudes change rapidly from one point to another, while for the long waves and shallow canyons displacement amplitudes display only minor departure from the uniform half‐space amplification of 2. For shallow canyons and long incident waves, the angle of incidence introduces only minor changes into the overall behaviour of surface amplitudes. For deep canyons and nearly grazing incidences, a prominent shadow zone is realized behind the canyon.