Characteristics Of Strong Ground Motion Of Thrusting Earthquakes

In comparison with earthquakes occurring on strike-slip and normal faults,thrust-type earthquake produces stronger surface effects and therefore potentiallycauses seismic disasters in more aggressive way. Rarely seen withinintra-continental tectonic environments, most extremely destructive thrust-typeearthquakes often occur on subducting boundary between oceanic plate andcontinental plate as well as on those between continental plates. As special cases,the1999M7.6Chi-Chi earthquake in Taiwan and the2008M8.0Wenchuanearthquake in Sichuan are both products of intra-continental thrust faults, theChelongpu fault and the Longmen Shan fault zone. More specially, the Wenchuanearthquake is generated on a high-angle listric reverse fault. These two events resultin severe seismic disasters and heavy casualties due to their great sizes and thedensely populated areas. As shown in previous studies, the majority of seismichazards result directly or indirectly from strong ground motions associated with theearthquakes. This causal relation indicates that the knowledge about earthquakestrong ground motion is critical to disaster reduction and understanding theearthquake mechanism itself. In recent20years, Taiwan and the mainland of Chinaconstantly developed earthquake strong ground motion network in great efforts. Asthe development of network, the abundant earthquake strong ground motion data wereontained during the1999Chi-Chi earthquake and the2008Wenchuan earthquake.The strong ground motion data of these two earthquakes are of great value andprovide good chance to study and compare the patterns of seismic hazards due tothrust-type earthquakes.This study compared the seismic hazards due to the1999Chi-Chi earthquakeand the2008Wenchuan earthquake respectively, and summarized the characteristic distributions of seismic hazards and their relations with the spatial variations ofearthquake strong ground motion during these two earthquakes. In order to preciselyanalyze the relation between the quantities of strong ground motion and the structuralfeatures of sources, the study comprehensively reviewed the methods of how tocorrect near field acceleration records and also discussed the integration drift withintroducing the method of integration correction. We corrected the earthquakestrong ground motion records for these two earthquakes, using improved integrationcorrection method developed by previous studies. On the basis of correction, weanalyzed and compared the features of earthquake strong ground motion in near-andfar-fields for these two earthquakes. With the consideration of the tectonics andrupture processes of sources, this study further discussed how thrust fault structuralfeatures controls on strong ground motion. Our results show that (1) it is morereasonable to use the distance to fault other than epicentral distance to describe thespatial attenuation of earthquake strong ground motion;(2) with the increase ofdistance to fault, the peak acceleration, peak velocity and peak displacement attenuateon both blocks of thrust fault but attenuate more slowly on the hanging wall;(3) thearea with intensive strong ground motion on the hanging wall during Chi-Chiearthquake is large and involves distant region far from the seismogenic Chelongpufault, which is in contrast to that the intensively striking area during Wenchuanearthquake concentrates within narrow belt along Longmenshan fault zone and thoseregion very close to Yingxiu-Beichuan fault;(4) for both earthquakes, the peakacceleration on hanging wall is stronger than that on footwall;(5) the horizontal peakacceleration is larger than corresponding vertical component during the Chi-Chiearthquake, on the contrary, the vertical component is the larger one during theWenchuan earthquake;(6) the horizontal peak acceleration attenuates more rapidlythan do horizontal components in the middle-field and this relation is reversed in far-field during Wenchuan earthquake;(7) the peak displacement is positively linearlyrelated to the static displacement in Chi-Chi earthquake and the linear relationship ismore significant on footwall;(8) the static displacement decreases nearly to zero asthe peak displacement is lower than some threshold value;(9) in Chi-Chi earthquake,the maximum differences in west-to-east horizontal transient displacement betweenhanging wall stations and footwall stations vary in a large range while that in othertwo components distributed concentratedly; the maximum displacement differencesdecrease from north to south, reflecting consistent variation that not happens to staticdisplacement differences;(10) in Chi-Chi earthquake, the hanging-wall-to-footwalldifferences in peak displacement are obviously smaller than that in maximumtransient displacement.This study suggests that the intensity of earthquake strong ground motion and itsspatial variation pattern are not only related to field condition but also receiving thecontrol from geometrics of source fault; hanging-wall-to-footwall difference inearthquake strong ground motion extensively happens to thrust fault, and as a wholethe strong ground motion is more intensive and attenuates slower with increase offault distance on hanging wall; those extreme-value parameters like the peak variableor static variable, e.g., hanging-wall-to-footwall difference in peak displacement orthat in static displacement, may possibly lead to underestimation of seismic hazardand the corresponding transient differences during earthquake process calls for moreadequate attentions.