Northern Sumatra Earthquake

First Posting | Summary
Posting Date: July 3, 2013, 11:14:00 AM

On the afternoon of Tuesday, July 2, a M6.1 earthquake struck Indonesia’s Aceh province on the western tip of Sumatra. The epicenter was located 55 km (34 miles) west of Bireun, a town of 26,000. According to the United States Geological Survey (USGS), the earthquake occurred at 2:37 p.m. local time (7:37, UTC) and struck at a shallow depth of only 10 km (6 miles). The main shock was followed by at least two aftershocks of M5.2 and M5.3.

Exposure at Risk and Reported Damage

The USGS reported that the greatest shaking intensity experienced in northern Sumatra was VI-VIII on the Modified Mercalli Intensity Scale, or moderate to strong shaking. In addition, buildings in the affected region are quite susceptible to earthquake ground motion. They often have an irregular floor shape, which, in mid- to high-rise buildings, can exhibit asymmetrical torsion rigidity that has led to heavy damage during past earthquakes. Furthermore, poor detailing and workmanship, inadequate materials, and a lack of rigorous inspection and quality control procedures for building construction tend to exacerbate damage in this region.

A rescue and recovery effort, including police, soldiers, and volunteers, is currently well under way across western Sumatra. The two districts hit hardest were Bener Meriah and Central Aceh. There have been reports of at least 29 fatalities in these two districts. A landslide and collapsed buildings killed 12 and injured 70 in Bener Meriah district, according to the National Disaster Mitigation Agency. In addition, about 600 houses and buildings were damaged there, and many residents are living in tents outside their homes. In the next district, Central Aceh, 17 people were killed and about 350 were injured, according to the local mitigation agency. About 1,500 houses and buildings were damaged, and hundreds were evacuated.

Tectonic Setting

This event occurred in northern Sumatra just 30 km east of the great Sumatra Fault, which runs the entire 1,900 km length of Sumatra Island. This fault is one of the largest intraplate strike-slip faults in the world, and accommodates some of the 6 to 6.5 cm/year plate motion between the Indo-Australia and Sunda plates and has an average slip rate of about 2.5 cm/year. The Sumatra Fault runs parallel to the Sumatra Subduction Zone, which has generated some of the largest megathrust earthquakes in the world, including a magnitude 9.0 earthquake in 1833 and the M9.3 mega-earthquake in December 2004, which generated one of the most devastating tsunamis in recent history.

The Sumatra Fault is the location of frequent historic earthquakes. More than 20 disastrous earthquakes have occurred along various segments of this fault since the end of the 19th century. Yesterday’s earthquake ruptured about 30 km east of the northern segment of the Sumatra Fault. The preliminary faulting mechanism of this event indicates it occurred on a strike-slip fault sub-parallel to the main fault. While most major crustal earthquakes in central and southern Sumatra have occurred along the main fault trace, the historic events in northern Sumatra typically do not. The diffusive distribution of significant historic earthquakes in northern Sumatra may be attributed to the branching out of the Sumatra Fault in the north as it enters into the ocean.

Due to the location of this earthquake and the low penetration of earthquake insurance in this region, AIR does not expect significant insured losses from this event. 

First Posting | Downloads
Posting Date: July 3, 2013, 11:14:00 AM