The Palu earthquake has further cemented its fame in earthquake history because in addition to the damage due to seismic shaking, and the associated tsunami waves, it also caused extensive soil failure in three populated areas. At each of the sites the problem seems to have started with liquefaction, but was followed by formation of earth flows that slid off the gentle slopes down toward the center of the valley. Photos taken after the earthquake showed prominent sand boils, which is a clear evidence of liquefaction, and rafts and hummocks of soil on top of which houses, schools, and even tall radio antennas moved past each other "like ships in the night". These earth flows have been described as "mudflows", but the truth is that there is very little clay on the sediments that form the gentle slopes of the valley; they are clean sands and gravelly sands, interbedded with angular talus deposits.
I was interested on seeing if I could figure out, seven years after, that this piece of prime real estate in the middle of the city had been part of an earth flow. The city had removed all the structures, construction debris, and asphalt, so for all one could tell this was virgin ground where cows placidly grazed. No, I am afraid that by simple inspection of the ground I would have never suspected that I was looking at one of the most disastrous of earth movements (several thousand people were buried together with their houses). The three areas are in fact dedicated in perpetuity as the resting place of the lost lives.
The clue came in the form of numerous seeps throughout the grassy slopes, and a fringe of reeds at the downside end of the area (a veritable swamp when we tried to cross it). The initial liquefaction happened on clean sand horizons that were saturated with water, and once the sand liquefied it was able to carry floating on top of it rafts of the vadose zone soils and the structures resting on top of them.
Now that we know the soils in the area are susceptible to liquefaction, how could the city manage the hazard? For starters I would start measuring water levels across the city, and designating any area where the depth to water table is less than 2 m as potentially hazardous. The next step would be to sample the soils in the potentially hazardous areas, looking for clean, lose sands. Measuring some basic soil strength properties would be very useful. One could then use standard slope stability analysis to assess the factor of safety for critical slopes. Finally, in those areas that were most susceptible one could start draining the water table. A pretty large order of business for a small city, but once hit they know all too well that it can happen again.
The final accomplishment was visiting the places where the ground was broken by the fault on September 28. It was a bit of an anti-climax, because when the city repaired the broken roads it painted a broad band on the asphalt with the label "Palu Fault". Once we had found a couple of spots we were able to extrapolate its position using Google Maps, and successfully found a third point in the line. Need to take advantage of every triumph we get. There are no geomorphic hints that a fault has broken repeatedly along this alignment, but I guess there is always a first time for every fault break.
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