Gao Meng-tan

Seismic motion attenuation relations in Sichuan and adjacent areas

The Sichuan and adjacent areas is divided into southwest China region (SWCR) and Sichuan Basin region (SCBR) according to tectonic backgrounds and seismic damage distribution features. 96 modern destructive earthquakes in SWCR and 40 in SCBR are gathered respectively. All their magnitude parameters are checked. Based on the statistic relations between epicentral intensity and magnitude as well as relation between sensible radius and magnitude, the near and far field seismic intensity attenuation features are represented and controlled. And then the seismic intensity attenuation relations along major axis, minor axis and mean axis are established separately. The systematic deviations of surface wave magnitude between China seismograph network and U.S. seismograph network are considered in this paper. By making use of the new attenuation relations of bedrock horizontal ground acceleration response spectrum in west U.S., the attenuation relations of bedrock horizontal ground acceleration response spectrum in SWCR and SCBR are digital transformed based on the attenuation model considering acceleration saturation of distance and magnitude in near field.

Effects of the hanging wall and footwall on peak acceleration during the Jiji (Chi-Chi), Taiwan Province, earthquake

TheM=7.6 Jiji (Chi-Chi) earthquake, Taiwan Province, on September 21, 1999 (local time) is a thrust fault style earthquake. The empirical attenuation relations of the horizontal and vertical peak ground accelerations (PGA) for the Jiji (Chi-Chi) earthquake are developed by regression method. By examining the residuals from the Jiji (Chi-Chi) earthquake-specific peak acceleration attenuation relations, it is found that there are systematic differences between PGA on the hanging-wall and footwall. The recorded peak accelerations are higher on the hanging-wall and lower on the footwall. The clear asymmetry of PGA distribution to the surface rupture trace can also be seen from the PGA contour map. These evidences indicate that the PGA attenuates faster on the hanging-wall than on the footwall. In the study of near-source strong motion, seismic hazard assessment, scenario earthquake and seismic disaster prediction, the style-of-faulting must be considered in order that the attenuation model can reflect the characteristic of ground motion in various seismic environmental regions.

New national seismic zoning map of China

A new set of seismic zoning maps were published in August 1, 2001. It includes two maps, one is the seismic zoning map of peak acceleration, and the other is the zoning map of the characteristic period of the response spectrum. The exceeding probability of the map is 10% within 50 years. The scale of the map is 1:4 000 000. These maps serve as the national standard. The background of this project, technical approach and key scientific measures, the basic feature and the application of the maps are introduced in this paper.

Seismic data compression based on integer wavelet transform

Due to the particularity of the seismic data, they must be treated by lossless compression algorithm in some cases. In the paper, based on the integer wavelet transform, the lossless compression algorithm is studied. Comparing with the traditional algorithm, it can better improve the compression rate. CDF (2, n) biorthogonal wavelet family can lead to better compression ratio than other CDF family, SWE and CRF, which is owe to its capability in canceling data redundancies and focusing data characteristics. CDF (2, n) family is suitable as the wavelet function of the lossless compression seismic data.

A Hazard Assessment Method for Potential Earthquake-Induced Landslides – A Case Study in Huaxian County, Shaanxi Province

The hazard assessment of potential earthquake-induced landslides is an important aspect of the study of earthquake-induced landslides. In this study, we assessed the hazard of potential earthquake-induced landslides in Huaxian County with a new hazard assessment method. This method is based on probabilistic seismic hazard analysis and the Newmark cumulative displacement assessment model. The model considers a comprehensive suite of information, including the seismic activities and engineering geological conditions in the study area, and simulates the uncertainty of the intensity parameters of the engineering geological rock groups using the Monte Carlo method. Unlike previous assessment studies on ground motions with a given exceedance probability level, the hazard of earthquake-induced landslides obtained by the method presented in this study allows for the possibility of earthquake-induced landslides in different parts of the study area in the future. The assessment of the hazard of earthquake-induced landslides in this study showed good agreement with the historical distribution of earthquake-induced landslides. This indicates that the assessment properly reflects the macroscopic rules for the development of earthquake-induced landslides in the study area, and can provide a reference framework for the management of the risk of earthquakeinduced landslides and land planning.

Potential rupture surface model and its application on probabilistic seismic hazard analysis

Potential sources are simplified as point sources or linear sources in current probabilistic seismic hazard analysis (PSHA) methods. Focus size of large earthquakes is considerable, and fault rupture attitudes may have great influence upon the seismic hazard of a site which is near the source. Under this circumstance, it is unreasonable to use the simplified potential source models in the PSHA, so a potential rupture surface model is proposed in this paper. Adopting this model, we analyze the seismic hazard near the Chelungpu fault that generated the Chi-Chi (Jiji) earthquake with magnitude 7.6 and the following conclusions are reached. ① This model is reasonable on the base of focal mechanism, especially for sites near potential earthquakes with large magnitude; ② The attitudes of potential rupture surfaces have great influence on the results of probabilistic seismic hazard analysis and seismic zoning.

Study on the characteristics of earthquake’s impact on cities

Firstly, the impact of historical earthquakes on 34 China province-level capital cities is evaluated by using historical earthquake catalog. The distribution of affected intensity shows, about 53% of cities have even not been affected by earthquake intensity VI, and 44% of cities have been hit by earthquake intensity VII to IX. For most of the cities, occurrence frequency of affected intensity VI is usually higher than that of affected intensity larger than VI, and the value of affected intensity with maximal occurrence frequency may be very different among cities. So both the maximal affected intensity and the affected intensity with maximal occurrence frequency should be taken into account when the prevention seismic intensity needs to be determined. Secondly, considering the incompleteness of records of historical earthquakes, a method of earthquake catalog computer simulation is introduced to study the features of affected intensity of big cities. 69 county-level cities of Fujian Province are selected to be statistical objects. The statistical result shows, for different risk levels the seismic intensity changes greatly among cities, the seismic intensity of 2% probability of exceedance in 50 years can be regarded as the characteristic affected intensity of city, and can be the basis of determining the city special earthquake prevention level and a proper indicator of future earthquake’s impact on cities.Firstly, the impact of historical earthquakes on 34 China province-level capital cities is evaluated by using historical earthquake catalog. The distribution of affected intensity shows, about 53% of cities have even not been affected by earthquake intensity VI, and 44% of cities have been hit by earthquake intensity VII to IX. For most of the cities, occurrence frequency of affected intensity VI is usually higher than that of affected intensity larger than VI, and the value of affected intensity with maximal occurrence frequency may be very different among cities. So both the maximal affected intensity and the affected intensity with maximal occurrence frequency should be taken into account when the prevention seismic intensity needs to be determined. Secondly, considering the incompleteness of records of historical earthquakes, a method of earthquake catalog computer simulation is introduced to study the features of affected intensity of big cities. 69 county-level cities of Fujian Province are selected to be statistical objects. The statistical result shows, for different risk levels the seismic intensity changes greatly among cities, the seismic intensity of 2% probability of exceedance in 50 years can be regarded as the characteristic affected intensity of city, and can be the basis of determining the city special earthquake prevention level and a proper indicator of future earthquake’s impact on cities.

Spatial and temporal rupture process of the January 26, 2001, Gujarat, India, M S=7.8 earthquake

The source parameters, such as moment tensor, focal mechanism, source time function (STF) and temporal-spatial rupture process, were obtained for the January 26, 2001, India, MS=7.8 earthquake by inverting waveform data of 27 GDSN stations with epicentral distances less than 90°. Firstly, combining the moment tensor inversion, the spatial distribution of intensity, disaster and aftershocks and the orientation of the fault where the earthquake lies, the strike, dip and rake of the seismogenic fault were determined to be 92°, 58° and 62°, respectively. That is, this earthquake was a mainly thrust faulting with the strike of near west-east and the dipping direction to south. The seismic moment released was 3.5×1020 Nm, accordingly, the moment magnitude MW was calculated to be 7.6. And then, 27 P-STFs, 22 S-STFs and the averaged STFs of them were determined respectively using the technique of spectra division in frequency domain and the synthetic seismogram as Green’s functions. The analysis of the STFs suggested that the earthquake was a continuous event with the duration time of 19 s, starting rapidly and ending slowly. Finally, the temporal-spatial distribution of the slip on the fault plane was imaged from the obtained P-STFs and S-STFs using an time domain inversion technique. The maximum slip amplitude on the fault plane was about 7 m. The maximum stress drop was 30 MPa, and the average one over the whole rupture area was 7 MPa. The rupture area was about 85 km long in the strike direction and about 60 km wide in the down-dip direction, which, equally, was 51 km deep in the depth direction. The rupture propagated 50 km eastwards and 35 km westwards. The main portion of the rupture area, which has the slip amplitude greater than 0.5 m, was of the shape of an ellipse, its major axis oriented in the slip direction of the fault, which indicated that the rupture propagation direction was in accordance with the fault slip direction. This phenomenon is popular for strike-slip faulting, but rather rare for thrust faulting. The eastern portion of the rupture area above the initiation point was larger than the western portion below the initiation point, which was indicative of the asymmetrical rupture. In other words, the rupturing was kind of unilateral from west to east and from down to up. From the snapshots of the slip-rate variation with time and space, the slip rate reached the largest at the 4th second, that was 0.2 m/s, and the rupture in this period occurred only around the initiation point. At the 6th second, the rupture around the initiation point nearly stopped, and started moving outwards. The velocity of the westward rupture was smaller than that of the eastward rupture. Such rupture behavior like a circle mostly stopped near the 15th second. After the 16th second, only some patches of rupture distributed in the outer region. From the snapshots of the slip variation with time and space, the rupture started at the initiation point and propagated outwards. The main rupture on the area with the slip amplitude greater than 5 m extended unilaterally from west to east and from down to up between the 6th and the 10th seconds, and the western segment extended a bit westwards and downwards between the 11th and the 13th seconds. The whole process lasted about 19 s. The rupture velocity over the whole rupture process was estimated to be 3.3 km/s.

S wave inelastic attenuation and site effect in the northern Tianshan area

In this paper, according to the relationship of observation spectrum with source spectrum, travel-path attenuation factor and site effect, we use the digital seismic data of moderate-small earthquakes to invert the soft rock site effect and S wave inelastic attenuation under the constraint of site effect, and determine the parameters of path and site. The parameters obtained by this method exclude the topographic effect of seismic stations, and truly reflect the seismic effect of free soft rock sites and the S wave inelastic attenuation in the studied region. The average soft rock site effect is about 1.5 times in the frequency domain of 2∼4 Hz and the S wave quality factor is obtained as Qs=278f0.346, which can be directly applied to the stochastic modeling of ground motions in the studied region.