Structural and physical property characterization in the Wenchuan earthquake Fault Scientific Drilling project — hole 1 (WFSD-1)

Abstract

The Wenchuan earthquake Fault Scientific Drilling project (WFSD) started right after the 2008 Mw 7.9 Wenchuan earthquake to investigate its faulting mechanism. Hole 1 (WFSD-1) reached the Yingxiu–Beichuan fault (YBF), and coresamples were recoveredfrom 32to 1201.15 m-depth. Coreinvestigation and a suite of geo- physical downhole logs (including P-wave velocity, natural gamma ray, self-potential, resistivity, density, poros- ity, temperature, magnetic susceptibility and ultrasound borehole images) were acquired in WFSD-1. Integrated studiesofcoresandlogsfacilitatequalitativeandquantitativecomparisonofthestructuresandphysicalproperties of rocks. Logging data revealedthat the geothermal gradient of the volcanicPengguan complex (above 585.75 m) is1.85 °C/100 m,whilethatofthesedimentaryXujiaheFormation(below585.75 m)is2.15 °C/100 m.Ingeneral, natural gammaray, resistivity, density, porosity, P-wave velocity and magneticsusceptibility primarilydepend on therocklithology.Allmajorfaultzonesarecharacterizedbyhighmagneticsusceptibility,lowdensityandhighpo- rosity, with mostly low resistivity, high natural gamma ray and sound wave velocity. The high magnetic suscepti- bility values most likely result from the transformation of magnetic minerals by frictional heating due to the earthquake. The YBF exposed in WFSD-1 can be subdivided into five different parts based on different logging re- sponses,each ofthemcorrespondingtocertainfault-rocks.Thehighgamma radiation,porosity andP-waveveloc- ity, as well as low resistivity and temperature anomalies indicate that the Wenchuan earthquake fault zone is located at 585.75–594.5 m-depth, with an average inclination and dip angle of N305° and 71°, respectively. The fact that the fracture directions in the hanging wall and footwall are different suggests that their stress fielddirec- tion is completely different, implying that the upper Pengguan complex may not be local.