"Offsets of Late Quaternary morphology, rate of slip, and recurrence of large earthquakes on the Chang Ma Fault (Gansu, China)""

Abstract

The Chang Ma fault, in the western Qilian Shan, Gansu province, shows evidence of consistent left‐lateral displacements despite a complex en échelon geometry. We have measured short‐ and long‐term offsets on two N70°–80°E striking segments of the fault, along the southern edge of the Ta Quen Kou basin, where it cuts late Quaternary terraces, transverse ridges, and drainage channels. To analyze the offsets of such structures at one site in the eastern part of the basin, we ran topographic profiles parallel and perpendicular to the fault over a distance of about 5 km. Spectral analysis and comparison of longitudinal profiles north and south of the fault allowed us to separate horizontal offsets of structures of different wavelengths, apparent on air photos and satellite images. Transverse profiles were used to determine the vertical throw, whose variations appear to result from changes in the fault strike. Small horizontal offsets of ridge crests and valley slopes are about 55 m. Along the N70°E striking segment of the fault, this displacement corresponds to an uplift of the northern block of about 27 m implying that the fault dips about 73° to the South. The measurements also constrain horizontal projection of the slip vector to about N78°E. A larger horizontal offset (≈500 m) of morphological swells of longer wavelength (≈1.6 km) is visible on the longitudinal profiles. Assuming that periglacial erosion has reshaped the surface of the lateral slopes and tops of ridges during the Würm glacial period, the smallest offsets of adjacent ridge crests may postdate the begining of the Holocene, yielding an average rate of left‐lateral slip on the fault of 5.5±2.2 mm/yr in the last 10±2 kyr. In the western part of the Ta Quen Kou basin, a somewhat slower rate of Holocene slip (4.6±.4 mm/yr) is obtained. Measurement of the horizontal displacement due to the 1932 earthquake there (6.2±0.6 m) would imply that the recurrence interval for similar earthquakes at this second site is 1350±540 years.