USC Civil Engineering Department

Description of the Event:

An earthquake of magnitude Mw 7.8 occurred on the just off the north coast of the eastern part of Flores Island, Indonesia, at 05:29 GMT (13:29 local time) on December 12, 1992. This shock was felt on the island of Bali, 700 km to the west. It set off a series of tsunamis, which arrived on the shores of Flores as shortly as two minutes after the initial shock, and which reached every part of the north shore within five minutes. The epicenter was located approximately 35 km NW of Maumere, which is the largest city on the island. The fault which produced the quake lies between the epicenter near the Cape of Batumanuk and the Cape of Bunga, on the northeastern tip of the island. The length of the fault is approximately 110 km, and the width is about 35 km. Over 1,000 aftershocks were recorded by a field survey team from Japan during the week-long period from December 30th to January 5th. The shore to the west of Cape Batumanak was uplifted, displacement ranging from .5 - 1.1m. Subsidence occurred on the east side, reaching 1.6m at Kolisia village (located 25 km NW of Maumere).

The town of Wuring (see pictures 10-15) is located about 3 km NW of Maumere, on a spit of length 650 m, width about 200 which runs NNE. Runup height was about 2.9 m above mean sea level; however, since the ground height is only 1.8 - 2.1 m above sea level, the spit was completely inundated. The wave speed was estimated as 2.7 - 3.6 m/s by Matsutomi (1993). Most houses were destroyed, and those that remained leaned towards the southwest, indicating flow from the northwest. About 1,400 people lived on the spit, of which 87 perished. Runup heights along the coast from Wuring to the epicenter varied from 2 - 5.2 m, peaking in Kolisia village, also the location of maximum subsidence.

The island of Babi (see pictures) is located about 40 km NE of Maumere city. It is a round island of diameter 2.5 km, surrounded by a coral reef, with a mountain of elevation 351 m rising in the center. There is no flat land on the coast, with the exception of the south coast which faces Flores Island, 5 km to the south. Two villages are located on this flat land: a Muslim village to the west and a Christian village to the east. The population of the island is 1,093, of which 263 were victims of the tsunami. Maximum runup on the island was measured at 5.6 m, in the Christian village. Witnesses stated that the tsunami arrived 3 min after the earthquake. Some witnesses indicated that the waves originated from the south, suggesting that waves reflected from Flores Island had more serious effects.

The village of Nebe is located on Flores Island, facing Babi Island. The residential area is located within a palm tree forest; the ground elevation ranges from 1.7 - 2.6 m. The tsunami height was measured at 4.0 and 4.6 m. Most houses were brick, and nearly all the residences were completely destroyed. However, only two of 150 inhabitants were killed, which suggests that the palm tree forest dispelled some of the tsunami energy.

The village of Riang-Kroko is located on the Cape of Watupajung, at the extreme NE end of the island. The coast here faces west. This village was hit by a huge tsunami, with a maximum surge of 26.2 m above sea level. In the residential area the ground elevation is 3.6 m. Here, 69 families totaling 406 persons lived in more than 200 houses. After the wave swept through, no evidence of human life remained. Here 137 people lost their lives. The extreme height of the wave in comparison to the values for the rest of the island suggest that landslides in nearby areas may have contributed to the magnitude.

In total, 1690 Flores Islanders died, and the tsunami destroyed approximately 18,000 houses. Two interesting points arose from this event. The number of women (175) killed on Babi island was more than twice that of men (88). This statistical pattern has been mirrored in many other events, and can be at least partially attributed to the tendency of men, especially in these low-development areas, to take refuge for themselves, while women tend to attempt to protect the old and the young. Secondly, it seems that a layer of vegetation can be effective in protecting coastal residential areas. A greenbelt, or zone of arranged trees or other relatively strong plants, can dissipate some of the energy of the incident waves. While this will not prevent sea water from flowing into the area, it can reduce runup. Many villages subject to this event had a single line of palm trees as a greenbelt, which was not effective, as palm trees have few low branches. Just about any other type of tree would be better, ideally varieties with many low branches and high density of leaves. Finally, a wide coral reef is also an effective dissipater of tsunami energy.