Our guide Nur, our bursar Mauar, and our driver Noval picked us up at 8:30 am, excited about spending a day in the field. I was hoping we would be able to see one of the active faults, get a better understanding of the timing and mechanism of the tsunami, and take a look at the tsunami deposits. I am not sure we actually succeeded at any of these goals, but we certainly gave it a good try.
To set the stage, Palu Bay reminds me of San Francisco Bay: To the west there is a peninsula along which runs one of the major faults (like the San Andreas but with left-lateral movement), at the south end of the bay there is the city of Palu (like San Jose), and east of the bay you have a continental mass with a couple of faults related to the first one (like the Hayward and the Calaveras). In this framework the September 28, 2018 earthquake was triggered at the north end of the easternmost fault (like at the north end of the Calaveras fault). Palu Bay is very deep, 600 to 800 m, so its submerged slopes are very steep.
To start at the end, we first visited the east side of the bay, where a 4-m tall "dock" gives a nice look of the bay. Turns out this is not a dock, but the foundation of a building that stood 50 m inland. During the shaking the submerged slope failed and a scoop of the land moved into the water as a submarine landslide. This in turn generated a tsunami wave that quickly slammed against the base of the bay (and presumably also against the west side). A half-dozen other submarine slope failures happened over the following few minutes, making for a complex tsunami pattern. I think it would be fun to look at Sentinel images pre- and post-disaster to identify the areas where submarine landslides took place.
The tsunami flooded the water front to a depth of over 10 meters, to judge from buildings that got stripped and hollowed by the water (for example, stairs got ripped apart). Unfortunately when the earthquake struck, at 6 pm, there was a music concert going in the water front, and the folks there didn't have time to react when the first of three tsunami waves hit them three minutes later. The loss of life was horrendous. The Golden Arches (yes, it looked like the MacDonald's Golden Arches) bridge across the Palu River collapsed, and the floating mosque was set adrift and careened against the water front.
Puzzling about this event is the fact that the tsunami left no sediments behind. Yes, seven years have passed and some washout could be expected, but we looked in nooks and crannies and found nothing. Maybe because the tsunami was generated so close to Palu the waves didn't have a chance to pick up much sediment, but I find its total absence a mystery.
As to the fault, we looked at the western fault, which has a dramatic relief, but could not put our finger on the fault plane (way too much vegetation on the way). But then again, the west fault was not the one that broke. On the other hand, Irvan found an article where the fault break, which starts on the east, propagates under the bay and breaks to surface through the flat west side of the bay (like the break propagating south from the Calaveras, under San Francisco Bay, and breaking the ground in south San Jose). If that is the case, we were looking for the fault break in the wrong place! Tomorrow we are going to the area of Balaroa to look for liquefaction deposits, so maybe we will get another chance to look for the fault break there.
Kaleb tells me that there is an ongoing volcanic eruption on the east of the island of Flores (Mount Lewotobi Laki-Laki), and in four days we will be flying to the west end of Flores. If the prevailing wind is to the west they might cancel the flight, which will be another obstacle thrown by fate across my path.
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