青藏高原东南部地区瑞雷波相速度层析成像

本研究收集了"中国地震科学探测台阵-南北地震带南段"项目325个流动宽频带台站于2011年8月至2012年9月记录的远震垂直向资料,利用双台法测得了3594条独立路径上的瑞雷波相速度频散曲线,反演得到了青藏高原东南部地区周期10~60s瑞雷波的相速度分布图像.空间分辨尺度图表明,在台站覆盖范围内的绝大部分地区横向分辨率达到50km.2D相速度分布图显示,青藏高原东南部地区地壳上地幔S波速度结构存在较明显的横向非均匀性.短周期(如10s)的相速度分布主要受地表沉积层厚度的影响.绝大多数地震发生在周期15s相速度图上的低速区或高低速的陡变梯度带附近,充分说明该区的强震活动与中上地壳速度结构的变化有直接关系.中等周期(如20~30s)的相速度分布主要与中下地壳速度结构、地壳厚度密切相关,小江断裂、松潘—甘孜块体呈现最显著的低速,可能暗示这两处的中、下地壳存在低速层.较长周期(如40~60s)的相速度分布与上地幔顶部热状态和构造活动(如岩浆作用)有关.滇西南地区表现为大范围的显著低速,可能暗示滇西南地区上地幔顶部物质存在部分熔融.不同构造块体下方的频散曲线,具有不同的相速度特征.腾冲火山下方的频散曲线在10~60s一直为较低的速度,尤其是到40s以后,相速度随周期的变大增速明显放缓,至60s比其他任何块体速度都低,暗示腾冲火山区下方的低速至少来自上地幔顶部(约100km).

Phase velocity maps of Rayleigh waves in the southeast Tibetan plateau

The southeast Tibetan plateau is located in the transition zone between the uplift plateau and the stable Yangtze block. Many volcanoes, deep and huge faults, strong earthquakes are developed in this region. The crustal and upper mantle structure is very complex here. Investigating the detail velocity structure beneath this region is very important when studying the deformation mechanism of intra-plate volcano, the seismicity in the Yunnan-Sichuan area, the breeding environment of strong earthquakes and the dynamics of Tibet plateau uplift.#br#The ballistic surface wave tomography is a classic method of studying the crustal and upper mantle structure. By collecting the vertical component of teleseismic data recorded by 325 broadband stations of the portable seismic array under the project "China Seismic Array-Phase 1" during Aug. 2011 and Sep. 2012, a total number of 3594 inter-station phase velocity dispersion curves of Rayleigh waves were determined by a frequency-time analysis method based on a continuous wavelet transform. In addition, all of dispersion curves were arranged into a tomographic inversion scheme, called Ditmar & Yanovskaya, to obtain the phase velocity maps ranging from 10 s to 60 s in the southeast Tibetan plateau. #br#The spatial resolution maps show that the lateral resolution is about 50 km. Our tomographic results reveal that the S-wave structure of the crust and upper mantle beneath the SE Tibet is obviously laterally heterogeneous. At short periods (e.g., 10 s), the distribution of phase velocity has good relationship with the sediment layer thickness on the surface. Most of strong earthquakes occurred in low-speed regions or the evident gradient belts where the velocities change from high to low values on the map of 15 s, revealing that the strong earthquakes are directly related to the velocity structure of middle and upper crust. At the intermediate periods (e.g., 20 s, 30 s), the distribution of phase velocity is related well with the crustal thickness. The obvious low-speed zone imaged beneath the Xiaojiang fault and Songpan-Ganzi block on the maps of intermediate periods may reveal there are low-velocity layers in the middle-lower crust which are also found by previous studies. At the longer periods (e.g., 40 s, 60 s), the distribution of phase velocity is related well with the thermal structure and tectonic activity (e.g. magmatic activity) in the upper mantle. A large low-velocity zone is imaged in west Yunnan province, indicating partial melts might exist in the upper mantle beneath this area. #br#It has different velocity characteristics on the phase velocity maps beneath different tectonic units. It is common that low-velocity zones between 10 s and 60 s are present beneath the Tengchong volcano, especially, it is much slower for the phase velocity speeding up as periods increasing after 40 s, and it becomes the lowest velocity zone for the Tengchong volcano than any other blocks, indicating that the low-velocity layer may extend down to the upper mantle (~100 km) at least beneath the Tengchong volcano.