Radon and carbon dioxide emissions from the surface rupture zone produced by the 1920 Haiyuan M8.5 earthquake in Northwest China

Abstract

A M8.5 earthquake struck the Haiyuan area in 1920 and resulted in a 240-km long surface rupture zone along the Haiyuan fault (HYF). In this study, to determine the spatial relationship between soil gases and fault activity on a regional scale, radon (Rn) and carbon dioxide (CO2) concentrations in soil gas were measured in situ with a grid spacing of 5 ∼ 10 km along the Haiyuan surface rupture zone (HYSRZ). Ordinary Kriging (OK) and Inverse Distance Weight (IDW) interpolation were used to investigate spatial variations in Rn and CO2 concentrations. Synchronous soil gas anomalies were identified in southern Haiyuan County and at the southern end of the HYSRZ, and were correlated with higher slip rate, larger deformation, and larger degree rupture size than other segments. Moreover, soil gas anomalies were spatially highly coupled with seismically active zones and low b value areas, suggesting that the permeability and porosity of gas emissions are enhanced by higher stress. Two seismic velocity profiles across the gas anomaly area confirmed that low-velocity bodies exist at 0 ∼ 10 km below the surface, as revealed by magnetotelluric (MT) sounding results, indicating that gas emissions are associated with the migration of deep fluids. Our results provide new insight into the source of fluids in the HYSRZ and offer support for the design of a continuous geochemical measurement network in this area.

Keywords

• Radon (Rn)
• carbon dioxide (CO2)
• spatial interpolation
• b value
• crustal velocity structure
• spatial variation