The 2022 Chihshang, Taiwan, Earthquake: Initial GEER Team Observations

We recently returned from a post-earthquake reconnaissance trip to Taiwan sponsored by the National Science Foundation–funded Geoengineering Extreme Events Reconnaissance (GEER) Association. We studied the effects of the September 18, 2022 M_W6.9 Chihshang, Taiwan earthquake. The earthquake occurred on the Central Range strike-slip fault, with the rupture direction extending north from the epicenter. Nearfield seismic stations measured peak ground accelerations (PGAs) exceeding $0.5\mathrm{g}$ along the fault. Peak ground velocities (PGVs) increased in the direction of the rupture with average intensities of $8\mathrm{cm}/\mathrm{s}$ near the epicenter, increasing along the fault to $89\mathrm{cm}/\mathrm{s}$ at the northern terminus. The ground motion recordings of the east (approximately fault parallel) component indicated strong velocity pulses in the direction of the rupture (Fig. 1).

We found eight damaged bridges within 2 km of the fault, all with multiple spans of simply supported concrete deck girders founded on concrete piers: two bridges had multispan and foundation pier collapses, three bridges were closed for repair, and the remaining bridges were operational with reductions in service. Six bridges, including the two collapses, had longitudinal orientations perpendicular to the fault. All but one bridge spanned an $\sim 800\text{-}\mathrm{m}$-wide braided river channel, with foundation elements supported by alluvial soils.

We initially assumed that earthquake-induced liquefaction caused the collapses; however, we did not find any surface manifestations—an observation echoed by our Taiwanese colleagues who were in the field immediately after the earthquake. The bridges also did not have major construction defects or signs of deterioration, suggesting that the seismic forces from the pulselike ground motion and fling effects may have been the primary cause of damage to the bridges. To lend further credence to our interpretation, the bridges at the northern end of the fault sustained more damage; the pulselike ground motions in this area were stronger.