# 1933 Sumatra earthquake

The 1933 Sumatra earthquake or Liwa earthquake affected southern Sumatra, Indonesia, on 25 June at 04:54 WIB. The earthquake had an estimated surface-wave magnitude (M$_s$ ) of 7.5–7.7 and occurred at a shallow depth of 20 km (12 mi). It was caused by slip along a 130 km (81 mi) section of the seismically active Great Sumatran fault. The earthquake caused damage and hundreds of deaths in Kaur and Liwa. At least 788 people were reported killed, although the death toll may have been in the thousands. Aftershocks followed, including one that caused additional fatalities. The mainshock also triggered a nearby volcanic eruption at Suwoh two weeks later, killing many people.

## Tectonic setting

The west coast of Sumatra is dominated by the Sunda megathrust; a 5,500 km (3,400 mi) long convergent boundary where the Australian Plate subducts beneath the Burma Plate and Sunda Plate at a rate of 60 mm (2.4 in) per year. Convergence along this plate boundary is highly oblique, severely deforming the overriding Sunda Plate. This deformation is accommodated by strike-slip motion along the Great Sumatran fault; a 1,900 km (1,200 mi) long strike-slip fault system located on the island of Sumatra. The fault is divided into about 20 segments. Slip along the fault is not uniform; in northern Sumatra, it moves at 16–20 mm (0.63–0.79 in) per year; the central segments slip at 23 mm (0.91 in) per year, and in the south, is at 6 mm (0.24 in) per year. The Great Sumatran fault was the source of the 1994 Liwa and 1995 Kerinci earthquakes. It produced its largest earthquake during the 1943 Alahan Panjang sequence; measuring M$_s$  7.8.

## Earthquake

The earthquake occurred on 25 June at 04:54 WIB with a surface-wave magnitude of 7.5 or 7.7. It was caused by a strike-slip rupture on southern portion of the Great Sumatran Fault. Its epicenter was 50 km (31 mi) southeast of the Semangko segment, and 60 km (37 mi) north of the Kumering segment. Based on the distribution of damage, a considerable length of the southern end of the 150 km (93 mi) long Kumering segment ruptured during the earthquake. An analysis of the aftershocks suggest a 130 km (81 mi) long rupture directed northwestward from the epicenter at the southeastern termination of this segment. Such a length was consistent with the expected rupture extent for a similarly-sized earthquake. However, the rupture may have extended southwards and invovled the northern Semangko segment. Five aftershocks were recorded, two of which were located 100 km (62 mi) and 130 km (81 mi) northwest of the epicenter, respectively. A secondary branch away from the main strand of the Great Sumatran Fault may have also ruptured. The 1933 earthquake epicenter was located close to that of the M$_w$  6.8 earthquake in 1994.
Between 1933 and 1994, no earthquakes of magnitude 6.0 or larger were recorded on the aforementioned portions of the Great Sumatran Fault. Ground fissures were reported in Negeri Ratu and near Wai Uluhan stream, located 16 km (9.9 mi) apart. However remains unclear if these fissures were continuous or not. These fissures were located on the Kumering sement and may represent surface rupture.

## Damage

Destruction was widespread along an area between Lake Ranau and Suwoh, coinciding with the approximate length of the Kumering segment rupture. The earthquake had a maximum Modified Mercalli intensity of VIII–IX (Severe–Violent). Many buildings and other infrastructures in the western portion of southern Sumatra were damaged. Damage in Sebarus, a village in Lampung, was so severe that it forced all of its residents to leave. In Pasirah, every home was destroyed and rice fields were ruined, while inn Liwa and Banding Agung, all of its buildings were razed. Landslides were widespread, seen in the Barisan Mountains. Large fissures and ground subsidence were reported from Kota Agung to Makaka. On Lake Ranau, at the entrance of the Wai Warduk in Kotabatu, a "co-seismic flood wave" inundated ten paddy fields by up to 0.5 m (1 ft 8 in).
At least 424 deaths were reported in Kaur, South Bengkulu, while another 364 people died in Liwa. At least 50 people were badly injured in addition to 600 others with minor injuries. The total number of fatalities is not known and may be in the thousands, although the National Geophysical Data Center earthquake database only attributed 76 deaths with the event. An early statement by Reuters said at least 20 Europeans were among the 60 people dead which they later retracted. Officials took strong measures to ensure food, medication and hygiene was adequate.
An aftershock on June 26 also caused several fatalities in Bengkulu. Two weeks after the mainshock, geothermal activity at Suwoh increased, and a series of phreatic eruptions occurred, causing ash to fall on settlements. The eruption was rated a 4 on the Volcanic explosivity index and also caused many deaths. It ejected 0.2 km$^3$ (0.048 cu mi) of tephra which blanketed the region.