Investigating triggering relations among typhoon, landslides and earthquakes in Central Taiwan

Abstract:

Central Taiwan is a seismic active region that also experiences landslides and typhoons. 5 devastating earthquakes, the 2009 M = 6.2 Nantou, 2010 M = 6.4 Jiashian, 2013 M = 6.2 Nantou in March, 2013 M = 6.5 Nantou in June and 2016 M = 6.6 Meinong occurred in central mountainous area after (3 - 78 months) the 2009 Typhoon Morokat hit the same area. The identified close proximity in space and time between the 2009 Typhoon Morokat and earthquakes suggest the occurrence of cascading hazards. We propose to test the hypothesis that a large amount of precipitation during the 2009 Typhoon Morokat induced landsliding and sediment erosion, resulted in surface unloading which may promote the failure of surrounding active fault systems and then lead to the occurrence of earthquakes eventually. We also probe the contribution of other sources of erosions, including long-term surface erosion during the interseismic period and large earthquake induced erosion to earthquake triggering.

We studied the suggested earthquake triggering due to surface erosion in central Taiwan using observed suspended sediment concentrations, which are indicative of erosion rates, a 3D elastic finite element model, and Coulomb Failure Stress change (∆CFS) calculations. Based on interpolated erosion rates and chosen 30% bedload fraction, we calculated ∆CFSs on rupture planes of 5 mainshocks (M > 6) from 2009 to 2017 and 5 active fault planes. Our results indicate that: (1) At hypocenters of the 2010 and 2016 mainshocks, ∆CFSs imposed by typhoon induced erosion can be 2 – 3 times higher than the changes induced by long-term surface erosion during the interseismic period; (2) Significant stress perturbations (up to 0.13 bar) were found in surrounding fault planes located within 40 – 60 km from the hypocenters of mainshocks; (3) Cumulative effect from all sources of erosion promoted the rupture propagation of the June 2013 Nantou mainshock and future failure on the Chukou fault considering threshold of ∆CFS as 0.1 bar. Uncertainties in the estimated erosion rates, variation of the bedload fraction and their impact on stress change estimates, earthquake triggering effects from the 1999 M = 7.3 Chi-Chi earthquake induced erosion will also be discussed.