Spatio–Temporal Variability of Multiple Deterministic Tsunami Scenarios in Denpasar City

The coastlines of Indonesia sit on the edge of a massive, restless tectonic plate boundary where the earth's crust is constantly grinding together. When this friction suddenly releases, it can trigger earthquakes powerful enough to displace huge volumes of ocean water, sending tsunamis racing toward shore. For communities living near the water, the difference between survival and catastrophe often comes down to two things: how fast the wave arrives and how much force it carries. While scientists have long known that the region is at risk, traditional maps often show only a single, average picture of what might happen. They might tell a city how far the water could reach, but they often miss the critical details of how fast the water moves or how the specific shape of the coastline might focus that energy into a deadly surge. Understanding these hidden dynamics is essential for anyone living in a coastal city, because the speed of the water determines whether a building stands or collapses, and the arrival time dictates whether people can run to safety before the water hits.

In Denpasar, the capital city of Bali, a researcher named Guruh Sukarno Putra set out to replace that single, static picture with a more dynamic and realistic view of the danger. Instead of guessing at one possible earthquake, he built a computer model to simulate nine different scenarios of a massive earthquake, each with a magnitude of 8.7, occurring along the fault line just south of the island. These scenarios were designed to test how the tsunami would behave if the earthquake's energy came from slightly different locations or if the ground slipped in different patterns. Using a sophisticated simulation tool known as the Method of Splitting Tsunami, the study tracked how these waves would travel from the deep ocean, crash onto the shore, and flood the streets of the Sanur and Serangan coastal districts. The goal was not just to see how deep the water would get, but to understand the full force of the event, combining the depth of the water with its speed to measure the true destructive potential.

The results of these simulations reveal a race against time that is far tighter than many might expect. In every scenario, the tsunami waves reached the coast of Denpasar incredibly quickly, with the first signs of the surge arriving in as little as twenty-three minutes. For the most severe scenarios, the waves hit the entire study area within thirty minutes. This leaves an extremely narrow window for evacuation, suggesting that relying on distant warning systems is not enough; the community must be ready to move immediately. The study also found that the most dangerous part of the tsunami is not always the first wave to hit the shore. In many of the simulated events, the largest and most destructive waves arrived as the second or third pulse, meaning that the initial calm after the first wave could be a deceptive trap for those who return to the water's edge.

When the researchers looked at the physical impact, the difference between a moderate event and a catastrophic one was stark. In the worst-case scenarios, the water did not just lap at the shore; it surged inland with a depth exceeding fourteen meters in some places. More importantly, the speed of the water was terrifying, reaching nearly five hundred centimeters per second. To put this in perspective, that is a flow fast enough to rip heavy structures from their foundations and carry away cars and debris with ease. By combining these two factors—how deep the water gets and how fast it moves—the study calculated a "damage likelihood" map. This map showed that in the most extreme scenarios, an area of nearly seventeen square kilometers could face a very high likelihood of catastrophic damage. This zone covers a significant portion of the coastal strip, including dense residential areas, tourist beaches, and critical infrastructure like ports and harbors.

The study also highlighted that the danger is not evenly distributed. While the entire coast is at risk, certain areas like the eastern beaches of Sanur and the low-lying lagoons of Serangan are particularly vulnerable to the highest speeds and deepest waters. The simulations showed that the shape of the coastline and the underwater terrain can act like a funnel, accelerating the water and focusing its energy into specific corridors. This means that a building just a few hundred meters away from another could face vastly different forces. The research explicitly notes that these findings are based on computer simulations of maximum-probable events and do not account for future sea-level rise or other compounding factors, but they provide a crucial baseline for understanding the worst that could happen. The data suggests that the current evacuation plans, which often rely on people running to higher ground, may be insufficient if the roads are flooded or blocked by debris within those first thirty minutes.

Ultimately, this work serves as a wake-up call for urban planners and disaster managers in Denpasar. The simulations indicate that the city's exposure to a near-field tsunami is far more severe and rapid than previously understood through single-scenario models. The findings suggest that the most effective strategy for saving lives will likely involve a shift toward vertical evacuation, where people seek refuge in the upper floors of sturdy, multi-story buildings that are specifically reinforced to withstand the immense force of the water. It also points to the urgent need for structural audits of existing hotels and public buildings to ensure they can survive the momentum of the waves. By mapping out exactly where the water will go and how hard it will hit, this study provides the detailed information needed to design better evacuation routes, place temporary shelters in safe zones, and retrofit buildings to survive the next great earthquake. The message is clear: the threat is real, the window to act is short, and the only way to prepare is to understand the specific, violent nature of the water that is coming.