We don’t usually link to videos showing tragedies like this, but Alaskans should know what last September’s Palu tsunami looked like. Many of our coastal communities are at risk for similar disasters.
The loss of life at Palu reinforces our standard advice for tsunami evacuation: If you live, work, or play in an inundation zone, the earthquake is the warning.
After the Palu tsunami, news stories focused on Indonesia's early warning system. Some questioned the decision to cancel the warning 30 minutes after the earthquake. Others focused on Indonesia's slowness to install a new type of system. All of this might have been moot. The earthquake had knocked out cell towers needed to deliver the warning texts and electricity needed to sound the sirens.
Were these apparent failures responsible for the high death toll? In a recent study, Matías Carvajal and his coauthors used 38 videos shot by residents and CCTV cameras to reconstruct what happened. They found that the first waves arrived just two minutes after the earthquake, and the devastating wave shown in the video arrived two or three minutes after that. They concluded that “the western sector of Palu city was likely devastated within 4-6 minutes of the mainshock.”
In other words, the tsunami had already happened when the warning went out.
Carvajal’s findings also support what many researchers already suspected: that the tsunami was caused by underwater landslides rather than ground motions from the earthquake itself. Like tectonic tsunamis, landslide tsunamis begin when large volumes of water get displaced rapidly. Unlike tectonic tsunamis, landslide tsunamis can follow any type or size of earthquake, or even no earthquake at all. And they usually arrive far sooner than tectonic tsunamis.
In Alaska, landslide tsunamis caused most of the deaths from the 1964 Great Alaska earthquake. In some towns, such as Valdez and Whittier, witnesses said the first waves arrived before the shaking had stopped. Alaska's coastline is serrated with narrow fjords, which can funnel tsunamis towards communites sited at their heads, increasing the height and power of the waves. This may have happened in Palu Bay as well. Notice how close it is in shape to Resurrection Bay, where tsunamis devastated Seward in 1964.
Because of their local nature, landslide tsunamis present special challenges for technology-based warning systems. If waves can arrive in under five minutes, how do you design a system that can deliver warnings fast enough to give people time to evacuate?
The National Tsunami Warning Center issues tsunami warnings based on earthquake detections. Because seismic waves travel so fast, earthquake-based warnings give people the most time to react. Unfortunately, seismic data can only tell us if an earthquake could have started a tsunami, not whether it really did. NTWC uses DART buoys and tide guages to confirm the existance of tsunamis, but this information is not available for several minutes or longer after the earthquake. If the first warnings relied on this data, they would be far too late for communities close to the source.
Even the initial warning, based solely on the earthquake detection, takes time. Seismic waves need to reach seismic stations. Data from seismic stations need to reach the tsunami warning center, and the warning center needs to analyze that data. Once a warning does go out, people need to receive, open, read, and understand the message.
What this means is that effective warning for landslide tsunamis might not even be possible.
That is, unless people in tsunami-prone communities understand that the earthquake itself is the warning, and they don't need to wait for official instructions. When people treat earthquakes as tsunami warnings instead of waiting for a text or a siren, they save precious minutes that could be the difference between life and death.
At the Earthquake Center, we work with the Alaska Division of Geological and Geophysical Surveys and the Alaska Division of Homeland Security and Emergency Management to help coastal communities understand their exposure to tsunamis. The collaborative Tsunami Mapping Project produces maps showing possible inundation from different tsunami scenarios. These maps help local emergency managers to identify evacuation routes and site tsunami shelters.
We have also produced pedestrian travel time maps for several communities. To make these maps, we use information such as steepness and surface type to model how long it would take people to reach safety on foot from different places in an inundation zone. This information helps communities to optimize evacuation routes and to identify places where people should head directly uphill rather than trying to evacuate by road.
We hope this work will save lives when the next Palu-style tsunami strikes Alaska. In the Palu video, it's clear that many people did know what to do after they felt the earthquake. Unfortunately, many others did not. If you live in a place that could be hit by a landslide tsunami, know where the inundation zone is and know your evacuation routes. Decisive, informed action could save your life.
If you are in a tsunami zone and feel very strong shaking or shaking that lasts for around 20 seconds or longer, move to higher ground without waiting for instructions. Once you're in a safe place, stay there until you've been told that it's safe to leave. The first wave may not be the last or the largest.
Palu Bay's shape may have helped to direct and amplify tsunami waves. Note its similarity to Resurrection Bay and other Alaska fjords.
Part of the tsunami inundation map for Atka, showing the maximum extent of tsunami inundation (red line) and maximum permanent flooding from ground subsidence (yellow line).
The pedestrian travel time map for the Homer Spit shows very long evacuation times (91-159 minutes for the red area) if people had to escape on foot.
Travel time modeling shows that evacuation times for the Homer Spit could be drastically reduced with the addition of a centrally located vertical evacuation structure (red star). In the Palu video, some residents are within the inundation zone but have safely evacuated vertically in a parking structure.