The post-tsunami survey team measured tsunami parameters at 64 coastal points in Lampung and Banten from 26 to 30 December 2018. Tsunami flow depth was measured from ground level at locations where reliable tsunami traces were found. Tsunami runup was measured by using automatic laser rangefinders referenced to the IOC post-tsunami survey guideline (UNESCO 2014). The surveyed runup heights were later adjusted to account for the tidal levels at the time of the tsunami event.
Beach morphology at the surveyed areas consists of two different types (see boxes in Fig. 1): the first type includes the surveyed areas in Lampung and the northern part of Banten coast in the latitude range of 6.17°S–6.49°S. These areas are characterized by flat topography, where the beach slope is less than 10o and the difference between ground elevation and mean sea level is usually less than 5 m within the distance of 100 m from the coast. Geologically, these areas were composed of loose sand with gray-white to brownish color and quaternary coral limestone at some area with grain size of moderate to coarse sand. The second type is the cliff-type beaches at the southern part of Banten in 6.50°S–6.65°S, where ground levels > 10 m (from sea level) can be found in a relatively shorter distance from the coast. Moderate to high relief characterizes this type of coast, composed of tertiary volcanic rock including lava, volcanic breccia and tuff.
Tsunami Runups and Flow Depths
In Lampung Province, tsunami traces were measured at 15 points where flow depth reached 2.9 m and maximum tsunami inundation distance was up to 150 m at Kunjir Village, Rajabasa Subdistrict (Fig. 2). Measurements in Lampung were mostly made in Rajabasa Subdistrict since tsunami traces were less visible in other places. In this region, the average tsunami flow depth was 1.5 m and it inundated coastal areas as far as 50 m from the coast, on the average (Tables 1, 2).
In Banten coast (east to southeast side of the AK volcano), different levels of tsunami inundation depths and runups were observed. Coastal locations from 6.1°S to the north suffered less tsunami destruction, where small wooden stores located very close to the shoreline survived without any significant damage (Fig. 3a). Tsunami traces were not visible in this location, and condition of grasses and sands did not indicate that a tsunami inundated the area. However, the Ciwandan tide gauge station, located in this region, recorded maximum tsunami amplitude of 0.5 m arriving 46 min after the first tsunami peak (Table 3). In addition to the fact that the tsunami that arrived in this area was not significant, fewer tsunami traces available here may be attributed to the existence of coastal defense structures which were built to protect the seaside parts of resorts and villas. Such protecting structures must have prevented the tsunami from overflowing and penetrating further inland (Fig. 3b).
Moving southward to the latitude range of 6.17°S–6.37°S, the average flow depth, runup and inundation distance were 2.2 m, 4.4 m and 131.2 m, respectively. Here, tsunami traces were clearly observed at the walls of the damaged buildings located close to the shore. Maximum runup was 5.8 m (location 6.20°S, 105.84°E) and maximum inundation distance was up to 330 m (location 6.26°S, 105.83°E). Longer inundation distances observed here could be attributed to the higher flow depths, in addition to flat topography which helped the tsunami to penetrate further inland once it overtopped the coastal structures.
Towards the south at a narrow bay named Lada Bay located at 6.4°S–6.53°S and 105.68°E–105.82°E (see location in Fig. 2), almost no tsunami trace was observed, although such a narrow bay has the potential to amplify tsunami heights. One reason that the area inside the bay was less affected by the tsunami is the fact that the bay is relatively sheltered by Tanjung Lesung peninsula, a most attractive tourism destination in the region, which was itself heavily damaged by the tsunami with at least 52 casualties. At the time of the tsunami, the Tanjung Lesung peninsula resort area was packed with visitors for the end-of-the-year festivals.
At the cliff-type beach areas, tsunami energy is converted more into runup height rather than inundation distance (Kakinuma et al. 2012). In Tanjung Jaya and Banyuasih subdistrict, the area with cliff-type beaches located at the latitude range of 6.48°S–6.55°S, a larger tsunami was observed with a maximum runup of 13.5 m, maximum flow depth of 5.4 m and inundation distance of up to 159 m. Most of the beach areas were completely swept away and coastal forest was heavily damaged (Fig. 4a). Coral boulders were drifted up to 100 m to inland with sizes of up to as big as a medium truck (Fig. 4b) indicating the extreme powers of the tsunami flows.
Tsunami Impacts from Airborne Survey
At the islands surrounding the AK volcano, tsunami impacts are visible from the damage of coastal forests. At Sertung Island, dense vegetation at the northern part disappeared and only a tree was left at the sand spit of about 200 m length (Fig. 5a). At the southern part of the island, we observed the creation of an immense outcrop with height of up to 40 m after the tsunami impact (Fig. 5b). However, this outcrop might have been generated by mechanisms other than the direct tsunami impacts because of its symmetric shape and also because of the absence of tree debris on its top boundary. This may favour the hypothesis of material sliding, which could have been tsunamigenic. Loss of vegetation is also visible at the inner side of Rakata Island, with an estimated height of up to 15 m above sea level (Fig. 5c).
Further tsunami impacts were observed in the natural area designated for the conservation of rhinoceros at the Ujung Kulon Peninsula located approximately 60 km south of the AK volcano. Tsunami severely impacted the 4.5-km stretch coastline where the coastal forest was completely swept away to a distance of up to 800 m inland (Fig. 6a). Similar damage is also seen on an island located in the north of Ujung Kulon called Panaitan Island. Here, coastal forest located on the northwest of the peninsula was shattered by the tsunami, which washed away trees and vegetation to a distance of 200 m inland (Fig. 6b). These two locations are uninhabited and the vegetation in this area is at least 100 years old. Therefore, the tsunami that attacked these areas should have been higher than the one that hit Lampung and Banten coasts in mainland Indonesia.
Although most of the coastal forests observed through the airborne and land-based post-tsunami surveys were damaged by the tsunami, the case in Banyuasih Village (located at 6.57°S and 105.62°E) showed the protective role of vegetation in reducing tsunami forces for tsunamis of less than 5 m in height. Here, a tsunami flow depth of 3.6 m at the coast was reduced to 0.4 m due to the protective effect of a 200-m thick coastal forest (Fig. 6c).
The Naval Hydrographic and Oceanographic Center of Indonesia (http://www.pushidrosal.id) conducted a rapid bathymetric survey from 25 to 30 December 2019 by using multibeam sonar equipment in response to the AK volcano eruption (PUSHIDROSAL–Naval Hydrographic and Oceanographic Center 2019). Previously, they conducted a bathymetric survey by using similar equipment at the surrounding AK volcano in 2016 (PUSHIDROSAL–Naval Hydrographic and Oceanographic Center 2016). Having those time series data allowed the comparison between the bathymetric data before and after the eruption. Since the mechanism of caldera collapse is unclear, analysis of bathymetric changes can only draw upon the volume of materials involved during the caldera collapse, regardless of the mechanism. The surveyed area of the new bathymetry, however, covered only a portion of the area in front of the horseshoe caldera opening because approaching the AK volcano during the survey period was not allowed due to the eruption activities (Fig. 7).
In order to reveal the notable bathymetric changes through making cross-sections of the bathymetric data, we extracted the bathymetric data of the 2016 and 2018 surveys along the cross-section line a’–b’ in Fig. 7 (see the 2016 and 2018 survey traces in Figs. S2–S4). This cross-section line has water depths of 150 m in the north and 255 m in the south based on the 2016 bathymetric data. By analyzing the post-eruption bathymetry data, we observed an increase in the seafloor elevation towards south, indicating that the collapse material spread out not only to the west (as indicated by the visible horseshoe caldera opening of the AK) but also to the south part of the AK volcano (Fig. 7).