Environment

Major tsunami hits Indonesia after shallow M7.5 earthquake


A major tsunami struck parts of Indonesia after powerful M7.5 earthquake hit the island of Sulawesi at 10:02 UTC (17:02 local time). The quake followed M6.1 earthquake at 07:00 UTC in which at least 1 person lost its life and 10 others were injured. Numerous moderate to strong aftershocks shook the region over the next few hours. 

Tsunami waves up to 3 m (10 feet) hit the city of Palu, a growing tourist resort with a population of about 350 000 and the nearby fishing town of Donggala (population 280 000), both located some 80 km (50 miles) from the epicenter, after Indonesian authorities lifted the tsunami alert.

Sutopo Purwo Nugroho, a spokesperson of the National Disaster Mitigation Agency, said communications had been cut off in both cities, adding that officials hope to be able to gauge the scale of the damage at daybreak. 

Unofficial reports received by 18:00 UTC today (01:00 local time) mention at least 6 deaths, including 1 caused by earlier M6.1, and dozens injured. 

“The second quake was felt very strongly, we expect more damage and more victims,” Nugroho said.

Numerous buildings have been destroyed and damaged as well as roads, bridges and other infrastructure.

Some 20 000 people are estimated to have felt severe shaking, 73 000 very strong, 604 000 strong, 1 374 000 moderate, 12 047 000 light and 9 259 000 weak.


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The USGS issued a yellow alert for shaking-related fatalities and economic losses. Some casualties and damage are possible and the impact should be relatively localized. Past yellow alerts have required a local or regional level response.

Overall, the population in this region resides in structures that are vulnerable to earthquake shaking, though resistant structures exist. The predominant vulnerable building types are unreinforced brick with concrete floor and precast concrete frame with wall construction.

The quake occurred as a result of strike-slip faulting at shallow depths within the interior of the Molucca Sea microplate, part of the broader Sunda tectonic plate. Focal mechanism solutions for the earthquake indicate rupture occurred on either a left-lateral north-south striking fault, or along a right-lateral east-west striking fault.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Strike-slip events of the size of the September 28, 2018 earthquake are typically about 120×20 km in size (length x width); modeling of this earthquake implies dimensions of ~80×30 km, predominantly down-dip and south of the hypocenter.

Shallow earthquakes of this size can often have a deadly impact on nearby communities. Historically, this region has hosted several large earthquakes, with fifteen events of M6.5 and larger within 250 km of the September 28th earthquake over the preceding century.

The largest of these was a M7.9 earthquake in January 1996, about 100 km (62 miles) to the north of today’s event. The 1996 earthquake – a shallow thrust faulting earthquake likely to have occurred on the regional subduction zone system at depth beneath the shallow crust – resulted in approximately 10 fatalities, over 60 injuries, and significant building damage in the local region.

Image credit: Google, USGS, TW

Today’s M7.5 earthquake was preceded by a series of small-to-moderate sized earthquakes over the hours leading up to this event; the USGS located 4 other earthquakes of M4.9 and larger in the epicentral region, beginning with a M6.1 earthquake three hours earlier and just to the south of the M7.5 event. There has also been an active aftershock sequence, with ten events of M4.7 and larger in the three hours following this earthquake. The largest aftershock in this timeframe was M5.8, about 12 minutes after the M7.5 earthquake.

Note: This tsunami report will be updated when new information becomes available. 

Featured image: Tsunami wave at Palu, Indonesia on September 28, 2018





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