Abstract

The 22 May 1960 Mw 9.5 Valdivia megathrust earthquake, with a rupture length close to 1000 km in the central-south Chile, is the largest recorded earthquake in the modern times. The city of Valdivia is located about 300 km south of the northern boundary of the rupture in front of one of the largest asperities of the earthquake. In this article, we analyze the geology of the city and results from geophysical exploration methods that could explain the observed pattern of structural earthquake-induced damage. Surface waves methods results indicate that the soils in Valdivia have shear-wave velocity in the upper 30 m VS30 ranging from 150 to 300 m=s, whereas horizontal-to-vertical spectral ratios (HVSRs) calculated from ambient seismic noise show predominant vibration periods between 0.6 and 1.4 s. The housing stock in Valdivia at the time of the earthquake mainly consisted of one- and two-story wooden buildings, and fewer masonry and reinforcedconcrete buildings. Our reinterpretation of the data indicates that despite the large seismic demand and the low shear-wave velocities, well-designed, well-constructed, and founded structures were barely damaged. Most of the structural damage concentrated in hybrid structural systems, poorly designed and constructed structures, and structures with deficient foundations build over uncontrolled backfills that experienced lateral movement and differential settlement. The predominant vibration periods from the HVSR do not correlate with the most damaged areas, but it seems to correlate with the depth of the soil deposit. The reduced damage of the larger structures in the city at the time of the earthquake may be partially explained because their vibration periods did not resonate with the predominant vibration period of the soil, in addition to their large structural redundancy.