Paleoseismic study of Milun fault activated by the 2018 Mw 6.4 Hualien earthquake rupture in Eastern Taiwan.

The active Milun Fault is the northernmost segment of the suture between Philippine Seaplate and Eurasian plate on the Taiwan Island. Historically, it was linked to two destructiveearthquakes on October 22nd, 1951, and February 6th, 2018, respectively. The 1951 ML7.3earthquake and the 2018 MW6.4 earthquake both produced surface ruptures along the MilunFault, causing casualties and damage on buildings in the Hualien city, which is the mostpopulated area in eastern Taiwan. Although the Milun Fault is surely intimidating, thesubsurface configuration and history of this active fault remain little exposed. The purpose ofthis study is to uncover the fault at shallow depth, reason the activity and seismic history ofthe Milun Fault and estimate the recurrence interval. Two trenches along with four boreholeswere carried out six months after the shock at the study site, located ∼ 2.6 kmnorth of downtown Hualien, where surface ruptures were reported during the 2018earthquake. The walls of one trench revealed a high dip angle of 80˚ to the east forthe main fault of the Milun Fault and its branch faults dip 36˚, 56˚, and 70˚ tothe east, respectively. The growth strata in the footwall of the branch faults werefolded. Distinctive sedimentary features such as the colluvial-wedge deposits andsoil liquefaction were also observed on the trench walls. By compiling the resultsof restoration and stripping of strata in conjunction with the age constraints, fivepaleo-earthquake events associated with the Milun Fault are resolved: (1) the 2018 event andthe earlier ones occurred at (2) 380-0 yr BP (3) 686-380 yr BP (4) 2184-1396 yrBP (5) 3232-2630 yr BP. Although the recurrence interval was estimated as ca.70 yrs earlier, our data suggest that the recurrence interval could be longer thanpreviously expected. However, due to the limited records of stratigraphy and possibleabsence of paleo-earthquakes, further investigation will be needed to generate a moresophisticated estimate of the recurrence interval. In addition, based on the borehole data,the slip rate of the Milun fault is about 6.0 to 6.5 mm/yr within the past 5000 yrs.These new findings shall benefit the earthquake hazard assessment for the region. [ABSTRACT FROM AUTHOR]