Volcanology and Geological Disaster Mitigation Center head Surono said the volcano shot ejecta 3.5 kilometers into the sky as thundering sound were heard and felt up to 12 kilometers away.

“Our team is now examining if any of the clouds reached more than 10 kilometers down the volcano slope. Only then we can decide whether we need to widen the no-go area,” Surono said in Yogyakarta on Saturday.

Since the volcano first erupted on Tuesday — a day after it was placed on the highest alert — the center has called for the evacuation of people living within the main 10-kilometer danger zone around the crater.

Surono said the initial ash clouds, which lasted seven minutes, were heading to the Lamat, Senowo and Krasak rivers. The next clouds, which were spewed out several minutes later, lasted 22 minutes.

”The energy [of Saturday’s eruptions] is three times larger than previous eruptions in 1997, 2001 and 2006,” Surono said.

The volcano, which has killed 36 residents since Tuesday’s eruption, showed a significant drop in activity, with scientists observing only seven volcanic earthquakes and 34 multiphase ones on Wednesday.

But it then showed a sudden rise in volcanic activity, with scientists recording 33 and 68 volcanic earthquakes and 129 and 233 multiphase ones on Thursday and Friday respectively.

“When the frequency and intensity of the volcanic and multiphase earthquakes increased, the explosive eruptions followed,” Surono said.

The head of the Volcanic Technology Development and Research Agency (BPPTK) in Yogyakarta, Subandrio, said that the agency at present could only rely on seismographs to monitor volcanic activities at Merapi as 16 reflectors installed to monitor the volcano’s peak were covered by ash following the eruptions.

“We’re going to run into problems when the solar panel that charges the seismometer battery, which is installed 500 meters from the peak, is also covered by ash,” he said, adding that the battery could last around a month if not recharged.

In a related development, National Disaster Mitigation Agency (BNPB) head Syamsul Maarif said his agency would set up more shelters due to reports of overcrowded evacuation centers.

“So far we have no problem with food supplies and logistics,” Syamsul said during a visit to BPPTK offices in Yogyakarta on Saturday.

Authorities have allocated Rp 500 million (US$56,000) to each of the four affected regencies in Yogyakarta and Central Java: Magelang, Boyolali and Klaten in Central Java, and Sleman in Yogyakarta.
Saturday’s eruption also triggered ash clouds over surrounding cities, forcing authorities to temporarily close nearby tourism sites and Yogyakarta’s Adisucipto airport after the runway had been covered by thick volcanic ash.

“We had to temporarily close the airport from 6 a.m. to 7:20 a.m. for the safety of passengers and flights,” Halendra, the operational manager of airport management company PT Angkasa Pura I Adisucipto, said.

The temporarily closure forced the cancellation of five departures while an aircraft arriving from
Bali was diverted to Surabaya’s Juanda Airport.

Saturday’s eruptions also led to panic among hundreds of residents at the Umbulharjo shelter in Cangkringan district in Sleman regency, who fled toward the city.

“The situation was frightening. The thunderous sounds coming from Merapi scared me and I left the shelter with my daughter,” resident Anik Lestari said.

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