In the future, weaker earthquakes will give rise to more destructive tsunamis

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The Chinese territory of Macau is reasonably safe from earthquake-induced tsunamis – for now. But if sea level rises by half a meter (a plausible outcome by 2060) Macau’s tsunami risk will double, researchers reported last week in the journal Science Advances.

The relationship between sea level rise and coastal cities’ vulnerability to flooding from storm surges is well known. But it turns out that sea level rise can also magnify the consequences of natural disasters that aren’t directly connected to the climate system.

Researchers used computer models to simulate earthquakes along the Manila Trench, which runs from the southern tip of Taiwan to Luzon, the Philippines. They modeled the tsunamis that could result from earthquakes of varying magnitude, and mapped the waves’ inundation of Macau, a densely populated city-state on the northern shore of the South China Sea.

They performed this analysis for current sea level, 0.5 meters of sea level rise, and 1 meter of sea level rise (the latter could occur by 2100) – a total of more than 5,000 tsunami simulations overall.

At the current sea level, a Manila Trench earthquake has to reach 8.8 in magnitude before the resulting tsunami is likely to have much effect on Macau. But with 1 meter of sea level rise, earthquakes with a magnitude of 8.2 to 8.4 could produce tsunamis that cause significant flooding, the analysis shows.

In the future, then, weaker earthquakes will give rise to more destructive tsunamis. Or, to put it another way, smaller waves will have bigger impacts.

“As sea level rises, Manila Trench earthquakes that do not pose a tsunami threat at current sea level become potentially damaging as the same tsunami waves combined with higher sea level lead to inundation that is deeper and extends further inland,” the researchers write. “The implication is that, Macau, though relatively safe from tsunami today, will rapidly become tsunami-prone.”

The study doesn’t fully capture the effects of the daily tidal cycle, nor how buildings would modify tsunami flow. Still, the results suggest that Macau’s city planners should take tsunami risk into account when mapping out the future of the city.

More than 60% of Macau is built on reclaimed land, and additional artificial islands are under construction. The researchers calculated that if two of the planned islands are built at an elevation of 3 meters above current sea level, by 2100 they will be vulnerable to a tsunami generated by an earthquake of 8.6 magnitude. But if they are built at 5 meters above current sea level, they will remain out of harm’s way.

Moreover, Macau is merely a case study, the researchers say. Sea level rise is also likely to increase tsunami risks for other coastal city-states and densely populated islands, like Hong Kong and the Maldives. As sea level rise accelerates, large swaths of coastline may be entering the tsunami hazard zone.

Source: Li L. et al. “A modest 0.5-m rise in sea level will double the tsunami hazard in Macau.” Science Advances. 2018.

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