2006 Pangandaran earthquake and tsunami

In tsunami prone regions, strong earthquakes serve as familiar warnings, and this is especially true for earthquakes in Indonesia. Previous estimates of the tsunami hazard for the Java coastline may have minimized the risk to the area, and to the northwest along the Sumatran coast, the risk is substantially higher for tsunami, especially near Padang. Previous events along the coast of Java in 1921 and again in 1994 illustrate the need for an accurate assessment of the threat. The July 2006 earthquake had an unusually slow rupture velocity which resulted in minor shaking on land for around three minutes, but the intensity was very light relative to the size of the tsunami that followed.[7]

The earthquake produced shaking at Pangandaran (where the M6.3 2006 Yogyakarta earthquake was felt more strongly) of intensity III–IV (Weak–Light), intensity III at Cianjur, and II (Weak) at Yogyakarta. Further inland and farther from the epicenter, intensity IV shaking made tall buildings sway in Jakarta, but at some coastal villages where many of the casualties occurred, the shaking was not felt as strong. An informal survey of 67 people that were present at the time revealed that in at least eight cases, individuals stated that they did not feel the earthquake at all (a typical M7.7 earthquake would have been distinctly noticed at those distances). The unusually low felt intensities, along with the short period body wave magnitudes, were components of the event that narrowed its classification into that of a tsunami earthquake.[6]

A tsunami surge of 5.98 m (19.6 ft) occurred near Pameungpeuk (2008 image)

Tsunami earthquakes can be influenced by both the presence of (and lack of) sediment at the subduction zone, and can be categorized as either aftershocks of megathrust earthquakes, like the M7 June 22, 1932 Cuyutlán event in Mexico, or as standalone events that occur near the upper portion of a plate interface. Northwestern University professor Emile Okal imparts that in the aftershock scenario, they can occur as a result of stress transfer from a mainshock to an accretionary wedge or a similar environment with "deficient mechanical properties", and as standalone events they can occur in the presence of irregular contacts at the plate interface in a zone that lacks sediment.[11]

One of the initial characterizations of tsunami earthquakes came from seismologist Hiroo Kanamori in the early 1970s, and additional clarity materialized following the 1992 Nicaragua earthquake and tsunami, which was evaluated to have a surface wave magnitude of 7.0 when analyzing short period seismic signals. When longer period signals of around 250 seconds were investigated, the shock was reevaluated to have a moment magnitude of 7.6, with a hypothesis that the slow nature of the slip of the event may have concealed its substantial extent. Sediment was thought to have contributed to a slower rupture, due to a lubrication effect at the plate interface, with the result being an earthquake signature that had abundant long period seismic signals, which could be an important factor in the tsunami-generation process.[4]

A tsunami warning system was not in operation at the time of the shock, but the Pacific Tsunami Warning Center (operated by the National Oceanic and Atmospheric Administration in Hawaii) and the Japan Meteorological Agency posted a tsunami watch, based on the occurrence of a M7.2 earthquake. The bulletin came within 30 minutes of the shock, but there was no means to transmit the warning to the people on the coast that needed to know. Many of those who felt the earthquake responded by moving away from the shore, but not with any urgency. The withdrawal of the sea that exposed an additional 5–10 meters (16–33 ft) of beach created an even more significant warning sign, but in some locations wind waves on the sea effectively concealed the withdrawal that signalled the approach of the tsunami.[12][13]

A 300 km (190 mi) portion of the southwest and south-central Java coast was affected by the tsunami, and resulted in around 600 fatalities, with a high concentration in Pangandaran. Two thousand kilometers (1,200 mi) to the southeast at the Steep Point area of western Australia, a runup of 2 m (6 ft 7 in) was measured, which was comparable to a similar runup in northern Oman from the 2004 Indian Ocean earthquake and tsunami, though in that case it was at a much greater distance of 5,000 kilometers (3,100 mi). Within three weeks of the event, scientists from five different countries were on the ground in Java performing a survey of the affected areas, including gathering runup (height above normal sea level) and inundation (distance the surge moved inland from the shore) measurements.[13]

The island of Nusa Kambangan (30 km × 4 km (18.6 mi × 2.5 mi)) sits on the south coast of Java and is separated from the main island by a narrow strait. It is a large and mostly uninhabited nature reserve, and is referred to as the Alcatraz of Indonesia, due to the three high security prisons that are located at the town of Permisan. Of all the measurements taken during the post-tsunami survey, the highest runup heights (10–21 m (33–69 ft)) were seen on the island behind a beach, where hibiscus and pandanus plants, and large coconut trees were mangled and uprooted up to 1,500 m (4,900 ft) from the shore. The (sea floor) bathymetry in the area supported a proposition that a canyon slope failure or an underwater landslide may have contributed to or focused the tsunami energy at that location. Nineteen farmers and one prisoner were killed there, but the deep water port of Cilacap (just to the east) was protected by the island, although one large moored vessel made ground contact during the initial 1.5 m (4 ft 11 in) withdrawal.[13]

Since the earthquake caused only minor ground movement, and was only lightly felt, all the damage that occurred on the island was due to the tsunami. Types of buildings that were affected were timber/bamboo, brick traditional, and brick traditional with reinforced concrete. Semi-permanent timber or bamboo structures that were based on a wooden frame were the most economical style of construction that were assessed following the disaster. A tsunami flow depth of 2 m (6 ft 7 in) usually resulted in complete destruction of these types of structures. A group of scientists that evaluated the damage considered the unreinforced brick construction as weak, because the performance of homes constructed in that style did not fare much better than the timber/bamboo variety. Hotels and some houses and shops that were of reinforced brick construction were far better off, because units that were exposed to a flood depth of 3–4 meters (9.8–13.1 ft) were considered repairable.[8]

Many wooden cafes and shops within 20 meters (66 ft) of the shore were completely removed by the tsunami at Pangandaran, and severe damage still occurred to unreinforced masonry that was within several hundred meters, but some hotels that were constructed well held up better. The villages of Batu Hiu and Batu Kara, both to the west of Pangandaran, experienced similar damage. Other severe damage was seen at Marsawah village, Bulakbenda, where all buildings had been removed down to their foundation within 150 meters (490 ft) of the water line, and even 300–500 m (980–1,640 ft) further inland there were many buildings that were totally destroyed. Witnesses reported that waves were breaking several hundred meters inland at that location.[12]

A July 19 satellite image of the Pangandaran coast showing tsunami debris

Officials in Indonesia received information regarding the tsunami in the form of bulletins from the Pacific Tsunami Warning Center and the Japan Meteorological Agency, but wanted to avoid panic, and did not attempt to disseminate the advisories to the public. Virtually no time was available to make that sort of effort (had the intention been to communicate the danger with the public) because some community leaders were sent text messages with pertinent information only minutes prior to the arrival of the first waves. The tsunami affected the coast of Java comprising mostly fishing villages and beach resorts that were unscathed following the 2004 Indian Ocean tsunami, and was also only several hundred kilometers distant from the region that saw heavy destruction just several months prior during the 2006 Yogyakarta earthquake.[15]

Trained research teams were already on the ground on Java responding to the May earthquake and began a survey of more than one hundred Muslim farmers, plantation laborers, and fishermen (or those with fishing-related occupations) that were affected by the tsunami. Almost two thirds of the group reported that they lived in permanent structures made of wood, brick, or cement, while the remainder lived in semi-permanent facilities made from earth or stone. The government was cited as the first responder for water, relocation and medical assistance, and helping with the deceased. For rescue, shelter, clothing, and locating missing people, individuals were listed as the primary provider, but 100% of those surveyed replied that the government should be responsible for relief. Most of those requiring aid stated that they were given effective assistance within 48 hours and that they were satisfied with the help.[15]