An earthquake of magnitude 7.5 is not a surprise for Indonesian region, which is located on the Pacific Ring of Fire. However, a tsunami triggered by Palu-Kora fault (a strike-slip fault where faults move parallel to each other), which travelled virtually unnoticed, is a surprise to the authorities and the scientific community. The earthquake of magnitude 7.5 hit the the city of Palu on September 28th. The epicenter of the earthquake is located in the northern region of Sulawesi Island, around 80 km north of Palu city. The causality from the earthquake is more than 1400.
Usually tsunami will be trigger by underwater subduction zone earthquakes. Surprisingly, in the case of Palu, the tsunami was triggered by a strike-slip earthquake. Since faults move parallel to each other during a strike-slip earthquake, the vertical slip at the surface will be very minimal and these types of earthquake will not trigger tsunami. Hence the tsunami triggered at Palu has baffled the scientific community. What can be the reason of this deadly tsunami triggered by a strike slip fault and why it went unnoticed?
The exact reason for the triggering of the tsunami is not yet known. Some of the scenarios, which could have caused the tsunami are
(i) Even though this a strike-slip earthquake, there can be some vertical movement, which is good enough to trigger a small (a localized) tsunami.
(ii) There can be huge subsea landslides at the time of the earthquakes and this might have triggered the tsunami.
(iii) There can be considerable change in submarine topography due to the earthquake.
(iv) A relatively small tsunami getting amplified due to the shallow and narrow bay area near Palu.
The tsunami generated by the earthquake was a localized one and once this was triggered, the Palu city, which is on the far end of a narrow horse shoe shaped bay, with no tsunami protection, was a sitting duck. When the Tsunami waves travel through deep sea, the wave height will be very less and when the waves reach shallow water, the wave height increases to dissipate the excess energy. In the case of Palu, the narrow bay confined the tsunami waves and amplified the tsunami wave heights.
Soon after the earthquake a tsunami warning was issued and this was withdrawn after some time. The initial warning indicated a tsunami wave height of around 0.5m at Palu, whereas the actual wave height which hit the region was around 5 m. When tsunami hit the coast, many people were gathered at the beach preparing for a beach festival and this also lead to higher causality.
The tsunami might have been the result of a "worst scenario" – the warning system failing, narrow bay area amplifying the tsunami wave height and people gathered at the beach to celebrate a festival etc. Still, the unnoticed (or unexpected) tsunami at Palu clearly shows the limitation of the current tsunami warning system in place. The nearest tsunami warning sensor was around 200 km away from Palu. It is high time to consider upgrading the tsunami warning system which will be capable of picking even localized tsunamis.
The immediate focus of the government is on rescue operations and making the Palu airport fully operational. in the next phase, the focus will shift to the reconstruction of Palu city and this will involve lots of investment. In the last few months, Indonesia was ravaged by many earthquakes and the economic impact of these events are substantial. In many of the emerging markets, like Indonesia, the insurance penetration rate is very low. This will mean that the affected people and the government will struggle to overcome the economic impact of a natural catastrophe. In such cases, re/insurance industry can help the communities to be more resilient.
Category: Climate/natural disasters: Disaster risk, Earthquakes, Resilience
Location: Palu, Central Sulawesi, Indonesia