甘肃省老虎山断裂带区域地壳纵波速度的实验

EXPERIMENTAL STUDY ON THE P-WAVE VELOCITY OF THE CRUST IN THE LAOHUSHAN FAULT ZONE IN GANSU PROVINCE

前人研究表明,老虎山断裂带区域未来几十年内可能会密集发生微小地震。微小地震因震级低、信号主频率高、能量衰减快而具有明显的近场特点。近场地壳波速结构,尤其是浅部速度结构对高频微小地震的精确定位有极大的影响。为此,在室温和围压介于50~500MPa的条件下,系统地测量了该地区代表性岩石的P波速度(V_(P))。实验表明,在低压范围内,V_(P)随着压力的增加呈对数增加;在压力高于临界压力P_(c)时,V_(P)随着压力的增加呈线性趋势增加。P_(c)介于200~450MPa,平均值为262.5MPa。结晶岩的P_(c)平均值为250MPa,砂岩的P_(c)平均值为271.4MPa;经对比发现,砂岩的P_(c)明显高于结晶岩。这可归因于在低压范围内裂纹孔隙随着压力的增加而闭合,而在高压范围内裂纹孔隙基本完全闭合。因此,提高近场微小地震的定位精度应考虑弹性波波速随压力的非线性增加。此外,实验结果显示此区域岩石弹性波速度是深度的函数。

The previous research shows that dense small earthquakes may occur in the Laohushan fault zone in the coming decades.Micro-earthquakes have obvious near-field characteristics due to low earthquake magnitude,high main frequency and fast energy attenuation of signal.Near-field crustal wave velocity structure,especially the shallow velocity structure,has a great impact on the precise positioning of high-frequency micro-earthquakes.To this end,the P-wave velocity(V_(P))of representative rocks in this area was systematically measured at room temperature and ambient pressure of 50~500MPa.The experiment result shows that:(1)V_(P) shows a logarithmic increase with pressure in the low pressure range and a linear trend increase with the pressure when exceeding the critical pressure P_(c).P_(c) is in the range of 200~450MPa,and is 262.5MPa on average.The average P_(c) of crystalline rock is 250MPa and the average P_(c) of sandstones is 271.4MPa.The P_(c) of sandstones is significantly higher than that of crystalline rocks.This is attributed to the low pressure range,where the cracks and pores are closing with increasing pressure,and the cracks and pores are almost completely closed in the high pressure range.(2)Under ambient temperature,the velocity pressure derivative in nonlinear domain of velocity-pressure curve is much larger than that in linear domain.(3)The intrinsic velocity of intrusive rocks is greater than that of sandstone and metamorphic sandstone.Combined with previous geothermal survey data,seismic tomography and field geological survey results,the following conclusions are obtained:(1)Under the condition that temperature varies with depth,the influence of temperature on P-wave velocity is less than that of pressure on P-wave velocity in the depth corresponding to the pressure below the critical pressure P_(c) at the 500MPa pressure interval deduced by simple fitting.Temperature has a strong effect on P-wave velocity at depths where the pressure is higher than the critical pressure P_(c).The experimental measurements are higher than the actual formation velocity,the difference is about 0.2142km/s.(2)The lower limit and upper limit of mean P-wave velocities obtained by experimental measurements are smaller than the lower limit and greater than the upper limit of P-wave velocities obtained by previous inversions,respectively.It can be attributed to the high wave velocity of intrusive rocks and metamorphic rocks outcropped in the shallow part of the stratum,and the low wave velocity of sedimentary rocks in the deep part of the stratum.This is consistent with the geological survey results in this area.(3)The combination of velocity in nonlinear domain and velocity in linear domain can provide a more real reference of P-wave velocity structure model in this region.