Experimental studies on relationship between P-wave velocity and porosity of fault rocks from the rupture of the 2008 Wenchuan earthquake

The seismogenesis fault rocks are characterized by high porosities which causes significant deviation in elastic wave velocities between them and crystallized rocks. Experimental studies on the relationship between the velocities and pores and mineral contents can provide effective constraint on interpretation of logging and seismic survey data. The P-wave velocity (V-p) and total porosity (phi(t)) of fault rocks from the rupture zone of the 2008 Wenchuan earthquake, China, were measured at pressures ranging from 10MPa to 600 MPa and room temperature. The results indicate that compressional wave velocity rises logarithmically with increasing pressure and the rate of velocity increasing becomes smaller and smaller with pressurization, partial derivative V-p(p)/partial derivative P = a(v)/p. Meanwhile, the total porosity drops logarithmically with pressurization. An interesting phenomenon is that pores can survive in fault rock specimens even when pressure reaches 600 MPa. Compressional wave velocity of the fault rocks is inversely associated with total porosity and clay mineral contents. The results provide insight into fluid flowing in deep crust, understanding of formation of clay minerals within fault zone and element migration along fault belt, and interpretation of trapped wave in Wenchuan earthquake region.