Experimental Study On Compressional Wave Velocity In "Tianzhu Seismic Gap" Of LaoHushan Fault Zone In Gansu Province

Rock elastic wave velocity research in the laboratory contributes to oil and gas exploration,seismic positioning,and seismic tomography,and is a bridge connecting geology and seismic depth measurement.The difference in elastic wave velocity between different rocks is attributed to the mineral composition,porosity,temperature and pressure and rock density of the rock.The Lao Hushan fault zone is located on the northeast edge of the Qinghai-Tibet Plateau,with Gulang 8.0 magnitude earthquake on the west side and Haiyuan 8.5magnitude earthquake on its eastern side.In nearly a hundred years,between the Gulang fault zone and the Haiyuan fault zone,a major earthquake has not occurred,so the area is called the"Tianzhu seismic gap"of the big earthquake,and the Lao Hushan fault zone is just located in the"Tianzhu seismic gap".Previous studies have shown that small earthquakes may occur intensively in the Lao Hushan fault zone in the coming decades.Minor earthquakes(not greater than level 3)are characterized by low magnitude,high main frequency,dense occurrence and fast attenuation.Due to the rapid energy decay,the elastic wave propagation distance of small earthquakes is limited,and the source distance of the surface detection seismometer receiving is limited.Near-field crust wave velocity structure,especially the shallow velocity structure,has a great influence on improving the precise positioning of small earthquakes.Meanwhile,accurate positioning of a large number of small earthquakes can help identify active fault shock location,study seismic gestation and trigger process.In order to improve the small seismic positioning precision of the Lao Hushan fault zone,and then understand the fault activity,the elastic wave velocity measurement of the representative rock in this area is required.The research objects are representative rocks-sandstone,diorite,tonalite,metababite and metamorphic sandstone of Lao Hushan fault zone,with a total of 11rock samples.Before the experiment,12 cores were drilled from 11 samples to 20mm diameter;the core length was cut and cut to 40.000±01mm into a cylinder parallel to the bottom.The length,diameter,and mass of the 12 experimental samples were systematically measured,and the density was calculated.The experimental equipment used is a piston cylinder compressor of the National Key Laboratory of Earthquake Dynamics.The P-wave velocity(V_p)and sample length variation of the twelve rock samples were systematically measured at room temperature and ambient pressure of50MPa?500MPa.The experimental protocol is for pressure at 50MPa?200MPa at25MPa,to better record the change of speed with pressure and 50 MPa.at200MPa?500MPa In the process of measuring wave velocity,the displacement change of both compressor is measured to estimate the effect of the sample on the longitudinal wave velocity.The experiments show that in the 500MPa range,the sample length shortening of less than 833?m,is less than 400?m,and concentrated in the low pressure range.Due to the recorded sample length reduction under low pressure,the longitudinal wave velocity calculated the initial wave length.In the low pressure range,V_pshows a logarithmic with the surrounding pressure;V_pshows a linear trend with the surrounding pressure.This is attributed to the low pressure range with a nearly close closure associated with the increased pressure.To construct the speed model,a fitting of the experimental data is required.Except for GYF01-1 sandstone samples,all samples have a fitting variance greater than or equal to 0.93.Meanwhile,the mean value of the critical pressure P_cis between200MPa?450MPa,is 262.5MPa;the average pc mean value of the crystalline rock is250MPa,sandstone is 271.4MPa.The contrast found that the P_cof the sandstone is significantly higher than the crystalline rock.Within the low pressure range,the response parameter a_vof measure V_pto pressure has a change range of 0.2389km s^-1MPa^-1?0.6369km s^-1MPa^-1;the mean pressure bias derivative of V_pis 2.963×10^-4km s^-1MPa^-1?10.787x10^-4km s^-1MPa^-1.Therefore,the change rate of P-wave velocity-pressure under low pressure is much greater than that of P-wave velocity-pressure under high pressure.The velocity-pressure derivative calculated using the fitting parameters shows that the pressure strongly affects the change in the compressional wave velocity in the low pressure range.Meanwhile,it is found that increasing temperature under low pressure reduces the longitudinal wave velocity by about 0.1071km/s.The constructed velocity-depth model shows that the longitudinal wave velocity of the intrusive rock as a whole is greater than that of the metamorphic sandstone at the same pressure and that of the sandstone.Due to the limited signal propagation distance of small earthquakes,all small earthquakes that can be monitored by surface seismometers are near-field earthquakes.At the same time,the elastic wave length of small earthquakes is more sensitive to the scale of the velocity structure.Therefore,combining the nonlinear changing part of the p-wave velocity with pressure with the linear changing part,constructing the p-wave velocity profile in the study area will help to improve the near-field small seismic localization accuracy.