Constraining Paleoearthquakes by Combining Faulted Stratigraphy and Microgeomorphology: A Case Study on the Haiyuan Fault, Northwestern China

Abstract Surface-rupturing strong earthquakes will leave evidence distributed along fault zones. The combination of paleoearthquake trench excavation and faulted microgeomorphic analysis at the same site provides more comprehensive knowledge of paleoearthquakes than either method could accomplish alone. In this article, we report on our use of trench excavation and dating, together with a 5-cm resolution digital elevation model obtained from an unmanned aerial vehicle based on the structure from motion photogrammetry technology, to investigate the timing and size of strong paleoearthquake events in the Dashagou site near the west end of the Haiyuan fault, which ruptured in the 1920 Haiyuan earthquake. The result reveals that at least four strong paleoearthquake events with the same or even higher magnitude (including the 1920 Haiyuan earthquake) have occurred along the west end of the Haiyuan fault since the mid-Holocene. Event IV occurred shortly before 6.0 ka with a horizontal displacement of 4.27±1.50 m and a vertical displacement of 0.70±0.39 m. Event III occurred at approximately 4.65±0.45 ka with a horizontal displacement of 5.45±1.25 m and a vertical displacement of 0.38±0.23 m. Event II occurred at approximately 1.0 ka with a horizontal displacement of 3.86±0.90 m and a vertical displacement of 0.55±0.27 m. The most recent event was the 1920 Haiyuan earthquake, with a horizontal displacement of 2.15±0.82 m and a vertical displacement of 0.26±0.12 m. From the results of these four events, we can certainly conclude that the fault has mainly maintained the strike-slip kinematic pattern over the past 6 ka. These observations highlight the benefits of combining trench excavation and faulted microgeomorphology to gain a more complete understanding of paleoearthquakes.