Review of morphological changes of Anak Krakatau before and after 2018 eruption using Aerial photogrammetry

We carried out drones photogrammetry on Anak Krakatau in 2013, 2019, 2020 and 2021. Changes in the morphology of Anak Krakatau occurred so quickly as its volcanic activity never stopped. Its pyroclastic cone in 2013 reached a height of 334 meters with a coastline circumference of 6.8 km. The circumference of the pyroclastic cone calculated from the traces of the old crater wall has a volume of 31971 m3. During the 2018 eruption, a large part of the cone was pulverised, resulting in a large crater of 400 m in diameter. Paradoxically, while the top of the volcano has dropped to 156 m, the island-volcano has grown relatively large, with an additional circumference of 7.2 km. The body of Anak Karakatu has experienced an addition to the coastline, especially on the South, East and North sides. This happened due to the eruption and tsunami accompanied by avalanches during the December 2018 eruption. During the 2020 and 2021 eruption periods, the coastline has not changed. This period was marked by the growth of a pyroclastic cone in the crater that had formed previously during the 2019 eruption. The pyroclastic cone was opened to the southwest by lava flows that flowed westward as far as 600 meters.


Introduction
The use of aerial photogrammetry methods has been widely used to examine volcanic activity.Some of which have been successfully carried out such as research on monitoring of lava dome growth in Redoubt volcano (Diefenbach et al., 2013), geomorphological analysis and growth of lava dome at Molodoy Sviveluch Volcano (Shevchenko et al., 2015), analysis of precise volume of volcanic ashplume to understand the dynamic of gas emission at Yasur Volcano in Vanuatu (Gomez & Kennedy, 2018), assesment of changes in morphology and structur at lava dome in Merapi Volcano (Darmawan et al, 2018), assesment of emplaced lava with gas composition and emision rate in Agung volcano (Syahbana et al., 2019), lava hazard forecasting in Kilauea (Diefenbach et al., 2018), and lava dome changes at Agung volcano (Andaru & Rau, 2019).Some important points from the results of the above research studies include preparation that's understanding the characteristics of the volcano to be studied, including knowing the morphology, peak height, and orientation of the crater opening.Knowing the object or volcanic activity that will be carried out to collect aerial photography data, then planning a flight path, and it can be done starting from the slope of the mountain, to collect data on the slopes, then to the top to collect situation data inside the crater.And for high mountains the flight path can be made to circle vertically from starting point to the level with the height of the top mountain, and then flown horizontally to the crater peak.This can prevent the uav from falling due to wind and battery drain.After knowing morphological aspects, peak height, made a flight path, then determine the type of drone can be used.

2
Anak Krakatau volcano located is Sunda Strait (Figure 1a), has a low slope and topography, we can fly from the southern slopes (Figure 1b), on this side the coastline and morphology are good for a starting point for flying drones.To deal with strong winds and crater smoke, we waited for the wind to calm down, then took aerial photography inside the crater.Morphological changes in the Anak Krakatau Complex are strongly influenced by volcanic activity and rock resistance, as well as seawater abrasion (Ismail et al., 2020), in 2019 a crater lake was formed (Figure 1c), and until now its eruption activity has formed a pyroclastic cone (cinder cone) and lava flows stretching westward (Figure 1d).
Research on the history of the eruption of Karakatau has been carried out.According to (Self, 1992) the 1883 eruption of Krakatau was affected by overpressure in the magma chamber by saturation of volatiles which then triggered the eruption, and this catastrophic eruption spewed around 12.5 km 3 of magma with composition in the magma chamber composed of rhyodacit, the zoning underneath is Dacit magma, then in the lowest zone in the magma chamber is an andesitic magma (Mandeville et al., 1996).The death toll in this eruption is estimated at 36,417 people (Auker et al., 2013).Anak Krakatau is the third evolution of the Krakatau volcano, which has grown since 1929, with a rest period between 1-8 years, with the intensity of eruptions occuring 1-6 times a year (Sutawidjaja, 2006).According to Abdurrachman et al. (2018) Anak Krakatau growth was affected by the partial melting of the subduction system between Java and Sumatra, and mantle upwelling that infiltrate beneath Mount Anak Krakatau.
Meanwhile, on 22 December 2018 an eruption accompanied by an avalanche triggered a tsunami in the Sunda Strait causing 430 fatalities along the densely populated coasts of Banten and Lampung (Borrero et al., 2020).According to (Grilli et al., 2021) with numerical method, estimates of the volume of landslides ranging from 0.175 to 0.313 km 3 and the geometry of the body of Mount Anak Krakatau which collapsed is the peak part of up to 50% of the body volume below sea level, and after the 2018 eruption activity, the potential danger of the eruption is vulcanian and strombolian eruptions which can be accompanied by ejection of incandescent rocks and heavy ash rain and lava flows within a radius of 2 km from the center of the crater (Kristianto et al., 2021).
In this study, we will present data from morphological observations from field observations and also drone data to estimate the volume of pyroclastic cones and lava flows for the 2013, 2019, 2020 and 2021 eruption periods.

Methods
Aerial photography data collection at Anak Krakatau can be done in a day, with 2 to 4 flights.When flying the Phantom 4 drone at Anak Krakatau, we used manual flight mode with the DJI Go application, and the drone's flying situation was monitored, both through the application and the drone's situation at altitude, and the morphology situation.Monitoring is observing the crater smoke and wind direction.The first to second flights were focused on the south and east sides (Fig. 1c, d).The 3rd and 4th flights from the summit of Anak Krakatau, to take the situation of the crater and also the morphology of the western and northern parts (Fig. 5a,b), (Fig. 7c).
Specifically for 2013 data, aerial photography data was carried out using a helicopter with a Canon EOS 5D Mark III camera that has been integrated with GPS as a coordinate reference.Flying altitude of 200 m from the top of Anak Krakatau.Field photo data as many as 105 photos.
During the 2019-2021 aerial photography data collection period, we used the Phantom 4 drone, which is a quadcopter drone that is easy to carry and use on every flight path.It has a stable camera gimbal for taking aerial photos perpendicular with 20 Megapixel camera specifications, and has been integrated with the GPS and GLONASS satellite systems as coordinate references.Data collection, vehicles used, and altitude can be seen in Table 1.Field photo data for each collected and then selected based on the quality of the photos.Oblique and blurred photos are not used for processing.The selected photos are then processed using the Agisoft Metashape Professional application to generate 3D spatial data (DEM).After generating the DEM, we use    3) shows that the lost body mass of Anak Krakatau reached 0.245 km 3 , and the additional volcanic mass on the east side due to eruptions and avalanches reached 0.124 km 3 .The east side of the body also experienced an increase in volume of 0.084 km 3 .This mass is thought to be the result of an eruption followed by an avalanche and then caused a tsunami on 22 December 2018.Reconstruction of the collapse of the southwest side slope using SAR satellite imagery for the period 19 November 2018 to 2 February 2019 by (Williams et al., 2019) shows that the peak of Anak Krakatau on 22 December 2018 is still intact but collapsed in 28 December 2018 , so that the volume of Anak Krakatau's body that avalanche does not include its peak.This means that the volume of the southwestern slope which failed was relatively small.In construction he estimated about ∼0.1 km 3 , and this volume was much less than the volume estimated by (Giachetti et al., 2012) of 0.28 km 3 .
The delineation that we do (Figure 4), apart from using the DEM comparison, we also use the results of our field observations and we found a crack in the slope which is currently the highest peak of Anak Krakatau (Figure 5a), and we predict that this fracture zone will become a path of west slope instability, which will then trigger avalanches, if Anak Krakatau's cone continues to grow.The addition of the coastline has occurred significantly (Figure 6), and this is directly propotional to the addition of Anak Krakatau's body volume since the 2019 period, and the coastline has not changed in the 2020-2021 period.

Discussion
The use of drones to support monitoring of volcanism activity and changes in volcanic morphology is a technological breakthrough, especially its use in eruption crises.The selection of the drone that will be used needs to be adjusted to the location of the volcano that will be used as the object of data collection.Preparation for flight, and flight execution to retrieve visual data requires accuracy so as to produce good visual data.The quality of the resulting DEM is influenced by the type of camera used, flying altitude, and data representation.
Based on drone visual data and field observations, in the 2019 period the body of Mount Anak Krakatau experienced an increase in area in the eastern part, which was caused by an avalanche followed by a tsunami.Then throughout the 2019 period, sea water on the west and south sides tends to be orange brown, this is thought to be related to magma activity which indicates a supply from depth, near surface activity, and surface activity in the form of emissions.The energy supply from this depth reaches its energy release in the 2020 period with an eruption in April 2020 in the form of a volcanic and Strombolian eruption, and also accompanied by an explosion.The eruption products of this period produce pyroclastic cones and lava flows.During the 2019-2021 eruption period, it contributed greatly to increasing the height of the peak and also the distribution of tephra that scattered on the peak, north and east (Figure 7c).

Conclusion
The use of drones for monitoring volcanic activity, especially for Mount Anak Krakatau, needs to be improved in technology.With Krakatau's high level of volcanic activity, drones are needed that can be flown remotely, especially from the Volcano Observation Post in Pasauran.Gas detection equipment and infrared thermal can also be additional components to support monitoring activities.
Taking into account the rapid changes in the morphology of Anak Krakatau, it shows that Anak Krakatau is still growing.The periodic use of the photogrammetric method using drones to monitor Anak Krakatau's volcanism is absolutely necessary as a mitigation effort, especially in monitoring the accelerated growth of its volcanic cone which can lead to a repeat of the December 2018 tsunami.

Figure 1 .
Figure 1.The research area is located on Mount Anak Krakatau, Sunda Strait (a).Photos (b) taken with hellicopter in 2013, Photos (c), and (d) were taken using a drone from the south.Situation of the cone of Mount Anak Krakatau in 2013 (b).Crater Lake with a diameter of 400 m in 2019 (c), and Crater Lake filled with pyroclastic cone and lava flow in 2020 (d).
Arcgis 10.8 to process and analyze DEM to calculated the volume of the avalanche by comparison between the avalanches on the southwest side of Anak Krakatau based on DEM 2013 and DEM 2019 data, and calculate the volume of pyroclastic cones and lava flows based on 2019, 2020, and 2021 DEM.

Figure 3 .
Figure 3.Comparison between body shapes that experience the addition and reduction of Mount Anak Krakatau.The colored part is DEM 2013 data, and the relief is DEM 2019.From the results of a comparison of the 2013 DEM and the 2019 DEM as well as the delineation on the west side which experienced avalanches (Figure 4), the estimated volume about 0.107 km 3 .The addition of the coastline to the south and east of Mount Anak Krakatau was more affected by the landslide and tsunami of December 22 2018.The mass slide of Mount Anak Krakatau entered the sea water, then was blown back by sea waves and deposited on the coast line on the south and east side of Anak Krakatau.A comparison of the 2019, 2020 and 2021 DEMs provides an estimated volume of a pyroclastic cone (cinder cone) of 0.001 km 3 , and a volume of lava flows of 0.017 km 3 .

Figure 4 .
Figure 4. Delineation of the landslide area of Mount Anak Krakatau based on the 2013 DEM and 2019 DEM.
Figure 5. the crack path at the top of Anak Krakatau (a), Crack path at the top of Anak Krakatau with visuals using drones (b), crater front situation (c), and (d).Photo taken in March 2019.

Figure 6 .
Figure 6.Changes in the coastline of Mount Anak Krakatau, showing an increase in area and circumference in the east.

Figure 7 .
Figure 7. Delineation of volcanic products from the eruption of Mount Anak Krakatau in the form of pyroclastic cones and lava flows (A).The 2020 eruption lava flows to the west (B), and the pyroclastic cone of Anak Krakatau which has started to grow since the 2020 eruption, photo in 2021 (C).

Table 1 .
Period aerial photographs of Anak Krakatau

Table 2 .
Morphological changes of Anak Krakatau evidenced by DEMs the volume of the 2013 DEM of Mount Anak Krakatau with the 2019 DEM (Figure