一种利用Nyström离散与FFT快速褶积的散射地震波并行计算方法

利用数值方法解Lippermann-Schwinger (L-S)方程的主要困难在于系数矩阵存储和线性方程组求解.这主要是因为L-S方程的积分部分是一个空间褶积,在离散后将导致一个满秩矩阵,进而形成一个大型或超大型代数方程组.因此,在利用L-S解决地震波散射问题时,一般是利用散射级数法而非数值方法.然而,散射级数法的计算精度和收敛性强烈地依赖于速度扰动的强度,而克服这种依赖性的一个可能的途径就是对现有的数值方法进行改进或是建立新的数值求解方案.在这种思想指导下,首先对L-S方程进行改写,得到一个与原L-S方程等价的积分方程(等价L-S方程).然后,对等价L-S方程进行逐点归一化处理,并利用Nyström法对经归一化处理的等价L-S方程(归一化等价L-S方程)进行离散,并用FFT计算空间褶积.之所以这样选择是由于归一化等价L-S方程经Nyström法离散生成的系数阵为一个Toeplitz阵,可利用其Toeplitz性质降低存储空间;而FFT可以将矩矢空间褶积转化为乘积,且积分核部分只要计算一次即可.进一步,为节约正演计算时间,设计了进程级和线程级相结合的MPI+OpenMP并行模式.数值试验表明,与传统的积分方程数值算法相比,利用等价L-S方程、Nyström离散和FFT快速褶积的计算方案可极大地降低存储需求,进而在保证精度的同时提高计算效率.

     

地形强迫对偶极型阻塞形势的影响分析

本研究运用多尺度变换和摄动法简化具有地形强迫的正压准地转涡度方程,得到带有扰动项的非线性Shrödinger方程,在此基础上,采用孤立子直接微扰理论研究地形强迫对偶极型阻塞结构的作用。研究表明：地形强迫对偶极型阻塞发展具有增强和持续作用,其增长率与大地形山体的斜率成正比。     

Seismicity around the Hekla and Torfajökull volcanoes, Iceland, during a volcanically quiet period, 1991–1995

The volcano Hekla in south Iceland had its latest eruption in January–March 1991. The eruption was accompanied and followed by considerable seismic activity. This study examines the seismicity in the Hekla region (63°42′–64°18′N, 18°30′–20°12′W) during a period when the high activity related to the eruption had ceased, from July 1991 to October 1995. The aim is to define the level of the normal background seismicity of the area that can be compared to the eruption-related activity. The Hekla Volcano proper was generally aseismic during the study period. The most prominent earthquake cluster is in the neighbouring Torfajökull Volcano. The epicentres are concentrated in the western part of the caldera and west of it. The hypocentres are located at all depths from the surface down to 14?km, with highest activity at 5–12?km. Inside this cluster, in the northwest part of the caldera, is a spherical volume void of earthquakes, approximately 4?km in diameter and centred at 8?km depth. This is interpreted as a cooling magma body. Small, low-frequency events of volcanic origin were occasionally recorded at Torfajökull. This activity has mainly occurred in swarms and was most abundant during the first year of the study period, presumably reflecting some kind of connection to the 1991 Hekla eruption. Our study area also includes the easternmost section of the South Iceland seismic zone, a transform zone characterized by bookshelf faulting on transverse faults. Two lineaments of epicentres were identified, roughly corresponding to mapped faults of the South Iceland seismic zone. The hypocentres are relatively deep, mainly at 6–12?km, matching the general trend of hypocentral depth increasing toward the east. The seismicity is highest in the area of the mapped faults. However, the epicentres extend beyond them and indicate greater width of the South Iceland seismic zone, or 20–30?km rather than approximately 10?km as indicated by the length of the surface faults. The seismicity in the volcanic systems of Hekla and Vatnafjöll shows some characteristics of the South Iceland seismic zone. Epicentres are concentrated into two N–S lineaments, one of which coincides with the location of the 1987 Vatnafjöll earthquake (M_w=5.9), a strike-slip event on a N- to S-trending fault. The hypocentres of the Hekla–Vatnafjöll events are mainly at 8–13?km depth, which indicates a continuation of the depth trend of the earthquakes of the South Iceland seismic zone. The events located at Hekla proper and immediately north of it are all of low-frequency character, which can be held as an indication of volcanic origin. On the other hand, they show clear S arrivals at observing stations like normal high-frequency tectonic earthquakes.