Until recently, scientists were puzzled why an earthquake of only 7.5 magnitude caused a devastating tsunami that killed over 2,000 people on the island of Sulawesi in Indonesia in September of 2018. The earthquake didn’t even trigger the tsunami early warning system as normally a tsunami of the size (wave height up to 11m or 36 ft) would require an 8.4 or higher magnitude earthquake. A new study published in the journal Pure and Applied Geophysics has found that the cause of the tsunami was actually a marine landslide in Palu Bay on the west coast of Sulawesi. A combination of computer modeling and observation of the actual sea level data from the event was used to reach their conclusion. They observed a wave period that was very short, consistent with the 3 to 4 minutes expected based on the modeling of a marine landslide compared to the 15 to 60 minute periods for an earthquake-induced tsunami.
On March 21, 2008 a landslide caused the closure of Arizona State Route 87 between Payson and Phoenix near mile post 224. So far there are no official estimates as to the size, but based on aerial photos, it appears to be at least 50,000 sq-ft in plan. The offset at the head scarps was approximately 2-meters (6.6-ft) according to AZGS Geologists on site. It appears to have been a rotational slide as the toe of the landslide heaved the southbound roadway up by as much as 1-meter (3.3 ft). There was an existing soil-nail retaining wall on the slope that was destroyed by the slide as well. Lateral deformations can be seen in photos of the median barrier and the roadway striping. The deformations extended into the northbound lanes as well. More photos and links after the break. (Photo by ADOT)
For those of you following the landslide in residential La Jolla California known as the Soledad Mountain Road landslide, it made the news again a couple days ago when a section of the slide re-activated and slid 10-ft down the hill. Doesn’t sound like too big of a deal to me, no additional houses damaged. I’m curious how it related to the shear pins they installed a couple of months ago.
Additionally, the San Diego City Attorney has released a report detailing the sequence of events leading up to the landslide. One interesting note is that the Mayor of San Diego is in hot water with the City Attorney’s office for hiring outside legal council to handle the legal aspects of the landslide. According to the City Attorney’s office, he wasn’t allowed to do that, but so far the City Attorney hasn’t stepped in to force the outside attorney(s) to drop the case saying it would open the City up to even more liability.