Comparative Study On Relative Strength Of Intrinsic And Scattering Attenuation For Different Tectonic Regions

Q is an important parameter describing wave attenuation and physical and chemical state of the materials in the Earth. According to the attenuation mechanism, there are two main attenuation, named as intrinsic attenuation and scattering attenuation. In this thesis, the intrinsic attenuation factor and scattering attenuation factor are separated first, and the relative strength of intrinsic and scattering attenuation calculated then, which is quite meaningful to study physical and chemical state of the Earth, and the attenuation mechanism of the seismic wave.In the thesis, the intrinsic and scattering attenuation of S coda wave is studied. Comparing different coda attenuation models from China or international research, we choose the coda energy flux model to separate the two kinds of attenuation.In order to obtain the difference between relative strength of intrinsic and scattering attenuation for different tectonic regions of China, the following events are selected:events with epicenter≤50km,2.0≤M1≤5.0, recorded by eight stations in Yunnan province, events occurred before and after Wenchuan M8.0, with epicenter≤50km,2.0≤M1≤5.0, recorded by two stations in Sichuan province, and seismic swarms occurred in Qingdao, with epicenter＜50km, recorded by two stations in Qingdao, Shandong province. The beginning of coda wave is set as 1.5 times of S wave travel time. The coda wave is sampled by three kinds of coda time window as 6sec, 10sec, and 15sec respectively. The focus frequencies of the octave Butterworth filters are 1.5Hz,3.0Hz, 6.0Hz, and 12.0Hz. On the basis of coda energy flux model and the new developed GEFM separating method, the attenuation are calculated at the mentioned 12 stations in different tectonic regions. Furthermore, Bo, which can show the relative strength between the two attenuation, is obtained. In details:1 New developed GEFM method, separating intrinsic and scattering attenuationAccording to coda energy flux model (Frankel& Wennerberg,1987), the GEFM method of calculating intrinsic attenuation Qi and scattering attenuation Qs using Genetics method is developed. The formula, steps, comparative between forward and inverse calculations, and the credibility of the method are discussed in details. Furthermore, the data selection, the filtering, the judging principles of the credibility, and the reasonable selection of the 1.5 times S wave travel time are proved in the thesis.2 At the non-active volcanic area, with low velocity magma pocket, the scattering attenuation is stronger than the intrinsic attenuationIn Tengchong volcanic area, used to erupt in history, the albedo Bo at different frequency band and coda time window is usually larger than 0.50, and it increases with depth. The Bo in Tengchong volcanic area is different from it is in Hawaii active volcanic area, but similar with Campi Flegrei and Long Valley volcanic areas, and the above two volcanic areas are the possible volcanic eruption areas. Furthermore, Tengchong, Campi Flegrei, and Long valley volcanic areas are all shown that there are low velocity body in the crust. It hints that the volcanic areas with stronger scattering attenuation than intrinsic attenuation at present, will probably have volcanic eruption events in the future.3 The regions with different tectonic movement and seismic activity show different attenuationThe coda of 6sec time window mainly travels in the crust in 2 stations in Qingdao area, and the coda of 6sec time window travels all in the crust of the 10 stations in other regions. The Bo is about 0.20 at 2 stations in Qingdao, similar with the low Bo in southern Korean peninsula, but they are all lower than that in Yuannan and Longmenshan area. It is regarded that the low level seismic activity at Korean peninsula and Qingdao area is the main reason why Bo is much lower. The intrinsic attenuation maybe control the seismic wave attenuation in low tectonic movement and seismic activity region.4 The relative strength of intrinsic and scattering attenuation is related to the present seismic activity courseAt 9 stations distributing in Yunnan area and Longmenshan fault, the Bo with 6sec coda time window can be sorter into two types:in one aspect, there are 4 stations in Tonghai-Chuxiong region and Youzhaping, Zhongjiagou region after Wenchuan M8.0 show Bo≥0.40 at all frequency band. Especially, the 2 stations in Longmenshan area show Bo＞0.50 at some frequency band, and the average shows Bo is nearly 0.50. In another aspect, another 5 stations (Lijiang, Yongsheng, Heqing, Zhongdian, Simao), with Youzhaping, Zhongjiagou before Wenchuan M8.0, show 0.30≤Bo≤0.40 at all frequency band.The 2 stations at Longmenshan fault area, the coda time window and Bo at all frequency band increase after the Wenchuan M8.0. Even for 6sec coda time window, the Bo is nearly 0.50. The Bo in Tonghai and Chuxiong station are larger than 0.40, which is probably related to the Tonghai M7.7 quake in 1970. The Bo in Yongsheng and Heqing station are between 0.30 and 0.40. An M=73/4 quake occurred in 1515, and nearly 500+years passed. The observations prove that the relative strength if intrinsic attenuation and scattering attenuation probably is related to strong seismic activity course. An M≥7.7 shock, with great energy releasing and large aftershocks, can result in material cracking in 100km length, and cause apparent increase in scattering attenuation. After great shocks, the fault plane will experience a long time on reunion, thus the Bo could be different at different periods after the shock.