Engineering Properties Of Strong Ground Motions With Double-plateau In Envelope

The 2008 Wenchuan earthquake(Mw7.9) in China and 2011 Tohoku earthquake(Mw9.0) in Japan caused severe casualties,loss of property and destroy of natural environment. Plenty of valuable strong ground motions were recorded in the such earthquakes, after the preliminary analysis of which, we discovered the phenomenon that a considerable volume of strong ground motions with double-plateau in envelope occured in each earthquake, which has never been clearly revealed before, and these events were associated with common characteristics contained in the two mega earthquakes that huge energy released, multiple rupture, large zone and long duration of rupture. Therefore, it is a noteworthy problem that difference of engineering properties and influence on the structure derived from such kind of ground motions versus the ground motions with single-plateau in envelope.This thesis studies the above-mentioned issues based on the strong ground motions from 2011 Tohoku earthquake in Japan, due to the quantity, quality and completeness of ground motion records and station information, by exploring spatial distribution of ground motions with double-plateau in envelope, the attenuation relation and difference of traditional characteristics in intensity, frequency and duration, envelope model and instantaneous frequency parameters between such two kinds of ground motions. The main work and research findings of this paper are summarized as follows:Firstly, spatial distribution characteristics of ground motions with double-plateau. This kind of ground motion was mainly caused by multiple rupture fault, which distributed primarily in a certain area of the main fracture reverse direction. showing obvious directivity.Secondly, the traditional characteristics and attenuation relation of intensity and frequency. It is found that peak ground acceleration of ground motions with double-plateau in value is basically greater than which with single-plateau, the ratio between them ranges from 2 to 1 with the increase of rupture distance; and the ratio between pre-post peak of ground motion with double-plateau ranges from 0.8 to 1.2. Accordingly, the arias intensity ratio of the two types of ground motions decreases from 3.5 to 1.5 or so with the increase of fault distance; and the arias ratio between pre-post peak of ground motion with double-plateau gradually increases from 0.5 to 0.8; suggesting that the energy carried by the former is significantly versus the latter. As for the response spectrum, the platform of acceleration with single-plateau is longer, and their feature period is bigger; inversely the platform value of acceleration with double-plateau is larger; it is proven to exhibit significant difference that the composition of frequency between pre-post peak of ground motion due to the doppler effect, high frequency component of pre-peak is outstanding, the smooth low frequency component of post-peak is rich.Thirdly, the non-stationary characteristics of intensity and frequency. The envelope of pre-post peak of ground motion in outline are both higher than which of ground motion with single-plateau in the same rupture distance; what’s more, the time interval between pre-post peak gradually diminished, and the shape of their envelope changes from high and thin to stout with the increase of rupture distance, smoothing gradually to ground motion with single-plateau. For instantaneous frequency, the initial frequency of ground motion with double-plateau is greater than which with single-plateau, and attenuation of the former is slower with time, inversely attenuation of the latter is more rapid over time.Fourthly, the seismic response of multistory frame structure motivated by ground motion with double-plateau is 10~30% more than which motivated by ground motion with single-plateau in the same conditions, and the elastic seismic response is more severe than elastic-plastic response.To sum up, the strong ground motion with double-plateau presents the uniqueness owing to they are generated in mega earthquake of multiple rupture and large zone of rupture, there is visible difference in engineering properties between such ground motion and the common ground motion with single-plateau, and seismic influence on multilayer structure exerted by the former is more significant. Such kind of ground motion should be taked into account when we design multistoried building in high intensity seismic region.Finally, the paper analyzes the deficiency of current research, and gives the suggestions for future research.