Source Modeling of the 2009 Fengpin–Hualien Earthquake Sequence, Taiwan, Inferred From Static Strain Measurements

We analyze high-resolution borehole strainmeter signals to constrain the fault plane solutions and locations of the 2009 Fengpin-Hualien earthquakes, in eastern Taiwan. The December Mw\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$M_w$$\end{document} 6.6 earthquake has ruptured a narrow section of the Collision Seismic Zone (within depth of 34 to 44 km), an active structure located offshore northeast Taiwan. The October Mw\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$M_w$$\end{document} 6.3 event has ruptured the deep section of the Longitudinal Valley fault (25–32 km depth) located between Fengpin and Fanglin. Coulomb stress changes induced by the October event in the hypocenter region of the December event are low (< +30 kPa), which suggests that the latter event was not induced by static stress transfer. The December event is characterized by an abundant seismicity for about 2.5 days following the mainshock. The temporal evolution between the aftershock rate and geodetic strain suggests that seismicity could have been controlled by a short-lived frictional afterslip. Besides, we infer that significant positive static Coulomb stress changes (> +100 kPa) induced by this sequence at shallow to moderate depths in the Longitudinal Valley (0–12 km) may have influenced the occurrence of large mainshocks in eastern Taiwan during the past decade. Conversely, limited stress (< +30 kPa) is transferred to the shallow, locked section of the southernmost Ryukyu trench. This study emphasizes the importance of borehole strain measurements in active regions to enhance detection of seismic and aseismic events unresolved by surface geodesy.