Unearthing the buried Palu–Koro Fault and the pattern of damage caused by the 2018 Sulawesi Earthquake using HVSR inversion

A systematic pattern of destruction was observed caused by the Mw 7.5 Central Sulawesi Earthquake. In brief, the quake ruptured 180 km of Palu–Koro Fault and led to massive destruction of residential buildings in Palu city, Donggala and Sigi Regencies. In Palu city, the damage was concentrated only in Balaroa and Petobo neighbourhoods where at least 930 and 1255 houses, respectively, collapsed. Microtremor time series recorded prior to the earthquake were used to analyse the subsurface structure of Balaroa and surroundings. Surprisingly, inversion of horizontal-to-vertical spectral ratio curves was able to locate the subsurface fault crossing the Balaroa and its dipping direction. Furthermore, the velocity profile deduced from this inversion points out the importance of local geology in the massive destruction in Balaroa. The existence of a subsurface pond led to an extremely water-saturated sandy soil underneath Balaroa. A combination of high level of water saturation and high peak ground acceleration (0.5–0.7 g) was strongly suspected to be the cause of the mega-liquefaction. The high degree of ground motion (>1.54 g) of spectral acceleration 0.2 s resulted in severe damage in elevated areas in Donggala, while in the Palu Basin, ground motion as high as 1.30g of spectral acceleration 0.5 s led to the destruction of four-storey or higher buildings.