The 1 October 2009 Bengkulu Quake Field Report

The 1 October 2009 Bengkulu Quake Field Report

The following field report was prepared by Dr. Danny Hilman Natawidjaja of the Geoteknologi Division of the Indonesian Institute of Science (LIPI). He and three colleagues from LIPI and the Meteorological, Climatological and Geophysical Agency of Indonesia (BMKG) travelled to Sumatra to scientifically survey details of the earthquake that struck near Bengkulu on 1 Oct. 2009. The quake, which the U.S. Geological Survey recorded of 6.6 magnitude, appears to have been caused by a rupture on the Sumatran Fault, which runs north-south along the western side of Sumatra.

On 7 Oct. 2009 Benny and I flew to Jambi. There we met Mudrik and Purna, who helped arrange a rental van for our trip. Together we all travelled on to Bangko, 100 km east of Kerinci Lake. The next day we reached the earthquake-affected areas in Gunung Raya District, south of Kerinci Lake. Overall, the damage was not as great as we had expected. Things seemed under control in the local villages, which are in the wide valley of the pull-apart basin between two sections of the Sumatran fault: the Seblat segment (on the east side of the lake) and the Dikit segment (on the west side of the lake). We found almost no surface ruptures or ground cracks except for a single ground crack in the asphalt road in the village of Lempur, which resulted in a badly damaged house. Most of houses were constructed of wood, so they could generally stand the shaking.

The Dikit section was where we suspected the fault rupture occurred. We were not able to approach that area, however, since it was about 5 km to the forest,  without road access from the village.

We later changed course and by the end of the week were headed to the village of Danau Pauh (Pauh Lake), near the southern part of the Dikit segment, where an emergency-response camp had been established. The trip was along very broken and bumpy asphalt roads. In the camp, we learned that the only village that suffered major devastation, with about 80% of the houses damaged, was Ranah Kemumu, about 45 km NW from Danau Pauh. Ranah Kemumu is located right on the fault zone.

(Ranah Kemumu, by the way, is located just inside the boundary of the Kerinci-Seblat National Park. Only forest trails, no roads, connect Ranah Kemumu and the Gunung Raya District. Some local people call the area “the house of the big cats".)

About 30 km from Danau Pauh was another base camp for emergency-response teams, at Tanjung Kasri. It can be accessed only by 4WD car, only in dry weather, and the last 15 km can be reached only on foot or by motorcycle. The Vice Bupati (head of the sub-province), who was in charge of the emergency response, provided assistance with his 4WD vehicle.

Upon leaving Danau Pauh, we found a big U-shaped ground crack cutting the road. If it slides, its head scarp will completely prevent access to Tanjung Kasri and Ranah Kemumu. Not much damage was evident in villages along the way to Tanjung Kasri. The two most obviously affected buildings we saw were a brick-walled house in Tanjung Kermas, nearly destroyed, and a badly damaged large mosque in Tanjung Kasri, also constructed of brick. In both cases the damage was apparently related to the ground cracks. We did not see any fault ruptures en route to Tanjung Kasri. Local people reported a big ground crack on the hill near the village, but we didn’t have time to check it out.

On the following Saturday, Benny and I left Danau Pauh to return to Jambi. Two Mudrik and Purna stayed behind to proceed on to Ranah Kemumu. Later that day they reached the area with a relief team and reported seeing fault surface ruptures and 100-cm offset. They were due to return to Bandung on Sunday. The next day they hadn’t arrived, and I suspected they were still trapped in Ranah Kemumu since I had heard on the TV news the night before that heavy rain had caused a big mudslide in the area, blocking the road between Tanjung Kasri and Ranah Kemumu. I learned by phone from people at the Danau Pauh relief camp that heavy equipment was being used to try to clear the road after the mudslide. It was three days before Mudrik and Purna were able to get out. 

I plotted the location of the 100-cm offset and also a sand-hot water burst (up to several meters) reported by many people in the Gruoh area. I was able to confirm that Mudrik and Purna found the surface fault rupture and the associated 105-cm offset (photo 1).

A few ponds were formed between fault strands, showing up to 20-cm subsidence (photo 2). In an image I had sent earlier, I saw an image of the L-band ALOS PALSAR from Yasuo Awata of the Geological Survey of Japanshowing (vertical) differential movement, relative to the satellites, of up to about 20 cm along the 20-30 suspected rupture zone. (Awata planned to put the image on GSJ’s website.) 

A few kilometers south of Ranah Kemumu, Mudrik and Purna found the hot spring in the river along the fault zone. According to locals, some hot fountains up to 5 meters had been lowered about 2-3 meters after the earthquake.

Upon entering Ranah Kemumu, Mudrik and Purna were surprised to find that the damage was not as bad as had been reported, even though the village lies right above the fault. All of the houses were constructed of wood, so about the only damage they suffered was being shaken from their weak foundations (photo 3).

No deaths from the earthquake were reported. But the local people said the shaking was so great that they and all other living things -- including chickens, horses and cows – were thrown to the ground. By the time Mudrik and Purna arrived, the villagers had begun restoring homes to their foundations.

People of the village seemed to be relatively wealthy. The agriculture system was good and there was electricity from a hydro-electric power plant. Almost every house had a parabolic TV antenna. There was no food shortage; people seemed to have enough food even without support from the relief team. Still, they were grateful for the sympathy, food and medical supply the emergency response team was finally able to provide. Their well-being was a relief to those in the response camp at Danau Pauh, who had heard rumors of a rice shortage in Ranah Kemumu and were very concerned about not being able to deliver food. Mudrik and Purna later reported, in fact, that the people of Ranah Kemumu had fed them well, and treated them to the best coffee they’d ever tasted!