Eruption of Mount Ile Lewotolok on November 29, 2020: A historical volcanic and tectonic seismicity assessment

2002) . Abstract. An eruption of volcano is related to the past volcanic and tectonic seismicity. Recently, on November 29, 2020, a 1423 m Mount Ile Lewotolok in Lembata Island has erupted. In here, this paper aimed to assess the volcanic and tectonic seismicity as determinant factor and precursor of recent Mt Ile Lewotolok eruption. The assessment shows that Mt Ile Lewotolok volcanic activities were characterized by both tectonic and deep volcanic seismicity. Since 2010, mean tectonic quake magnitudes of M 4.133 (95%CI:M 3.205-5.062) have occurred at mean depth of 13.500 km (95%CI:8.201-18.799 km) within a distance of 3-4 km from the summit. Tectonic quake has occurred frequently in southwest of Ile Lewotolok and this has contributed to the past eruptions in 2012 and 2017. Recent eruption has been influenced by deep volcanic seismicity rather than local tectonic. Based on November 2020 record, mean of deep volcanic seismicity frequency was 2.190 events/day (95%CI:1.1363.243 events/day km) that has outnumbered (ttest=2.665, P=0.013) the events of local tectonic quakes (mean 0.621 events/day; 95%CI:0.1421.099events/day). Prior to the eruption there was significant increase of deep volcanic seismicity (P=0.023) while local tectonic quake was not showing an increasing trend (P=0.764). This result confirms that the deep volcanic seismicity frequency is a precursor that may trigger the eruption and deep volcanic seismicity data can be used as indicator of volcanic activities.

Ile Lewotolok (1423 m above sea level) is one of active volcano located in semiarid ecosystem of Lembata island in south east of Indonesia archipelago. This volcanic mountain has extensive history of eruption. A record on this volcanic activity was dated back to 17 th century. In 1660, an eruption was observed in the central crater. Another eruption was recorded in 1819 and repeated again on October 6, 1849. First record on the impacts of this volcano eruption was on October 5 and 6, 1852. In this time, a solfatara complex has been reported on the eastern slope. Eruption was repeated again on 1864 ad 1889 with an eruption column has been observed. In 1920, another eruption has happened with eruption column.
An increase in volcanic activity has been observed from January 6 to February 3 and June 1939 and repeated again on December 15, 1951. Increasing seismic activity with tremors has been observed on January 2, 2012. Mount (Mt) Ile Lewotolok has an increase in volcanic activity in the form of significant seismicity since October 7, 2017. Local tectonic earthquakes, deep volcanic and shallow volcanic have been recorded since mid-September 2017. Recently, on November 29, 2020, Mt Ile Lewotolok has erupted again at 0845h. Considering this situation, this paper aimed to assess the volcanic and tectonic seismicity as determinant factors of recent Mt Ile Lewotolok eruption.

Materials and Methods
The aerial visualization of Mt Ile Lewotolok (Figure 1) was obtained based on Sentinel 2 L2A and Landsat 8 OLI imageries. The satellite image then adjusted to natural color band combination to retrieve the eruption column. The volcanic and tectonic seismicity data were obtained from archive and latest records. Seismic monitoring activity to obtain the data was using Kinemetric seismic recorder with a seismometer sensor installed in the field. The recorded seismic data included deep volcanic, local tectonic and eruption quakes.

3.Results and Discussion
The eruption column resulted from Mt Ile Lewotolok eruption on Sunday, November 29, 2020, at 0845h was observed to be 4000 m above the peak (5423 m above sea level). The eruption column was observed to be gray with thick intensity towards the east and west. Seismograph recorded this eruption has a maximum amplitude of 35 mm with 600 second duration. From above, the column has an increase in size (Figure 2). At the initial of eruption, the longest column diameter was 14 km. The columns then increased in diameter almost 3 folds and elongated to south east directions of the island (Figure 2).
The eruption of Ile Lewotolok was characterized by several pre deep volcanic and local tectonic quakes. Figure 3 presents the frequency of pre deep volcanic and local tectonic quakes recorded 1 month before the eruption. The recording period was from November 1, 2020 to November 29, 2020 when eruption happened. It is apparent that there is a significant increase of deep volcanic quake events prior to the eruptions. Local tectonic quakes were also increasing whereas the frequency was declining prior to the eruption. The frequencies of deep volcanic quakes have outnumbered the local tectonic quakes. Eruption quakes were never occurred within the recording period whereas it has the highest values observed on the eruption day. Deep volcanic quake frequencies were observed having negative correlation with local tectonic quakes. An increase in deep volcanic quake was followed by a decrease in local tectonic quakes (Figure 4). Pattern showing an increase of deep volcanic quake can also be observed in Tambora eruption. Prior to the eruption of Tambora in August, deep volcanic quake frequency was increasing from 200 to 300. Local tectonic quakes were also increasing from 30 to 90.
Deep volcanic seismicity is occurred at depth of 10 to 60 km beneath volcanoes in a range of tectonic settings and this type of quakes has contributed to magma transport in the mid-to-lower crust and uppermost mantle (Chouet & Matoza 2013, Matoza 2020. This seismicity extends down to mantle   be found on local, regional and global scales. Eggert and Walter (2009) stated that the time between a tectonic earthquake and a volcanic eruption may vary from a few seconds to years with the distance between the events can reach up to thousands of kilometers, but is most evident in the near-field. In Mt Ile Lewotolok, the quakes were occurred within the distances of 3-4 km.
An assessment on historical quake records as determinant factors of eruption is still limited in Indonesia as a country that has numerous volcanoes. Whereas a recent study has assessed the determinant factors of Mount Sinabung eruption in 2010. In agreement to this study, the Mt Sinabung eruption was preceded by earthquakes. A large tectonic earthquake may amplify the probability of unrest at Sinabung and facilitate the intrusion of new magma (Kriswati et al. 2019).

4.Conclusion
Volcano eruption is not a stand alone geological process. Instead of depending on a single factor, an eruption is resulted from combined spatio-temporal geological process including tectonic and volcanic quakes. This study has provided empirical evidences of how series of spatially and temporally distributed quakes have affected the recent Mt Ile Lewotolok eruption. The results confirm that the occurrence of repeated tectonic quakes at depth of 10 km may contribute to the past eruption in 2012 and 2017. In contrary to the effects of tectonic quakes, current eruption on November 2020 was affected by increasing frequencies of deep volcano seismicity prior to the eruption time.