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[1] We studied ionospheric disturbances associated with the two large earthquakes in Sumatra, Indonesia, namely, the 2007 Bengkulu and the 2005 Nias earthquakes, by measuring the total electron contents (TEC) using a regional network of global positioning system (GPS) receivers. We first focus on coseismic ionospheric disturbances (CIDs) of the Bengkulu earthquake (M w 8.5). They appeared 11-16 min after the earthquake and propagated northward as fast as~0.7 km/s, consistent with the sound speed at the ionospheric F layer height. Resonant oscillation of TEC with a frequency of~5 mHz continued for at least 30 min after the earthquake. The largest aftershock (M w 7.9) also showed clear CIDs similar to the main shock. A CID propagating with the Rayleigh wave velocity was not observed, possibly because the station distribution did not favor the radiation pattern of the surface waves. This earthquake, which occurred during a period of quiet geomagnetic activity, also showed clear preseismic TEC anomalies similar to those before the 2011 Tohoku-Oki earthquake. The positive and negative anomalies started 30-60 min before the earthquake to the north and the south of the fault region, respectively. On the other hand, we did not find any long-term TEC anomalies within 4-5 days before the earthquake. Co-and preseismic ionospheric anomalies of the 2005 Nias earthquake (M w 8.6) were, however, masked by strong plasma bubble signatures, and we could not even discuss the presence or absence of CIDs and preseismic TEC changes for this earthquake.

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Using the ionospheric total electron content (TEC) data from ground-based Global Navigation Satellite System (GNSS) receivers in Japan, we compared ionospheric responses to five explosive volcanic eruptions 2004–2015 of the Asama, Shin-Moe, Sakurajima, and Kuchinoerabu-jima volcanoes. The TEC records show N-shaped disturbances with a period ~ 80 s propagating outward with the acoustic wave speed in the F region of the ionosphere. The amplitudes of these TEC disturbances are a few percent of the background absolute vertical TEC. We propose to use such relative amplitudes as a new index for the intensity of volcanic explosions. Graphical Abstract

The study of ionospheric disturbances associated with the two large strike-slip earthquakes in Indonesia was investigated, which are West Sumatra on 2 March 2016 (Mw = 7.8), and Palu on 28 September 2018 (Mw = 7.5). The anomalies were observed by measuring co-seismic ionospheric disturbances (CIDs) using the Global Navigation Satellite System (GNSS). The results show positive and negative CIDs polarization changes for the 2016 West Sumatra earthquake, depending on the position of the satellite line-of-sight, while the 2018 Palu earthquake shows negative changes only due to differences in co-seismic vertical crustal displacement. The 2016 West Sumatra earthquake caused uplift and subsidence, while the 2018 Palu earthquake was dominated by subsidence. TEC anomalies occurred about 10 to 15 min after the two earthquakes with amplitude of 2.9 TECU and 0.4 TECU, respectively. The TEC anomaly amplitude was also affected by the magnitude of the earthquake moment. The disturbance signal propagated with a velocity of ~1–1.72 km s−1 for the 2016 West Sumatra earthquake and ~0.97–1.08 km s−1 for the 2018 Palu mainshock earthquake, which are consistent with acoustic waves. The wave also caused an oscillation signal of ∼4 mHz, and their azimuthal asymmetry of propagation confirmed the phenomena in the Southern Hemisphere. The CID signal could be identified at a distance of around 400–1500 km from the epicenter in the southwestern direction.

Preliminary research analyzed the Coseismic Ionospheric Disturbances (CIDs) of the strike-slip earthquake that occurred in Palu on September 28, 2018 (Mw = 7.5) and the materialization of a TEC anomaly with an amplitude of 0.4 TECU approximately 10–15 min later. The TEC anomaly amplitude is also affected by the magnitude of the earthquake moment; therefore, 3D analysis is needed to determine the spatial distribution of the ionospheric disturbances. This research aims to analyze the ionospheric disturbance of an earthquake in 3D using the Global Navigation Satellite System (GNSS) from the Geospatial Information Agency (BIG) or InaCORS stations spread over Sulawesi, Kalimantan, West Nusa Tenggara, East Nusa Tenggara, Bali, and Java with a 30 s sampling interval using GLONASS and GPS satellites. The checkerboard accuracy test was also carried out to evaluate the reliability of the 3D tomography model. The result showed that CIDs occur to the north and south of the epicenter around the equator, following the N-S Asymmetry theory. Furthermore, the tomography results indicate the presence of dominant and positive anomaly values at an altitude of 300–500 km. This follows the characteristics of variations in the ionosphere layer, where an altitude of 300–500 km is included in the F layer. The dominant anomaly at an altitude of 300 km is in accordance with the theory of the ionosphere’s height, which experiences maximum ionization at an altitude of ∼300 km (F layer) by Chapman’s profile. We also conducted preseismic studies of ionospheric anomalies before the earthquake as an additional analysis.

The COVID-19 pandemic has caused most waste recycling activities to be terminated due to several factors, such as concerns about the spread of coronavirus through the collected solid waste. This study investigates the socio-economic impact of the situation of the closed-loop system of solid waste recycling. Several recommendations for tackling this problem are presented in this research. Primary data collection for the waste bank and informal recycling sector was carried out in the eastern part of Surabaya during large-scale social restrictions. In-depth interviews were conducted with waste bank customers, waste bank unit representatives and the informal recycling sector to understand the pandemic’s socio-economic impact on the closed-loop system. Results show that this pandemic has significant impacts on individuals and stakeholders engaged in waste recycling activities. Customers of waste banks, who mostly belong to low-income communities, mentioned that the waste bank closure gave rise to social and economic problems, such as increasing unmanaged solid waste and decreasing income. This result also applied to the informal recycling sector. The government can use the recommendations in this study to generate related policies, such as enforcing the health protocol within solid waste management to keep the recycling system in place and the business alive.