Mount Merapi, located on the border of Yogyakarta and Central Java provinces, showed increased activity on Thursday as it emitted volcanic gas, rumbled, generated minor earthquakes and sprayed ash and gravel.
The increased activity was short-lived, lasting only four minutes from 01:12 p.m. to 01:16 p.m. The alert status of one of Indonesia's most active volcanoes remained unchanged on Thursday.
'The flurry of seismic activity was merely caused by the release of volcanic gas from Mt. Merapi, which keeps lots of gas,' head of the Geological Disaster Technology Development and Research Center ( BPPTKG ) Yogyakarta, Subandriyo, said.
He added that the phenomenon could have been triggered by Monday's deep earthquake that occurred around 4 kilometers from the peak of Mt. Merapi and might have shifted the magma inside the volcano.
Subandriyo went on to say that thundering sounds coming from Mt. Merapi on Thursday could be heard within a 7-km radius from the volcano's peak. Ash and gravel rained down on the southern and southeastern slopes.
'We have yet to find out the height of the volcanic gas plume as clouds shrouded the volcano and limited our visibility,' Subandriyo went on.
He said that after the devastating 2010 eruptions, Mt. Merapi had often showed increased volcanic
activity. Yet, he said, that did not indicate the volcano was going to erupt anytime soon.
'The gravel was leftover from the 2010 eruptions,' Subandriyo added.
He stressed that the volcano was still safe for hikers, but he warned climbers not summit the crater. People are allowed to go as far as the Pasar Bubrah area, some 200 meters from the peak of the volcano.
Meanwhile, National Disaster Mitigation Agency ( BNPB ) spokesman Sutopo Purwo Nugroho called on residents living on the slopes of Merapi to remain calm despite the increased activity.
Head of Klaten Regency Disaster Mitigation Agency ( BPBD ) logistics section, Subarko Utomo, said that his agency had distributed 8,000 dust masks to affected residents in Klaten, Central Java.
Rapid response teams, he said, had also been deployed to the areas. 'I had the chance to meet with the people in the affected areas. They had gone to safer ground when we arrived,' Subarko said.
Sukiman, coordinator of Pasak Merapi radio community, said that residents had panicked and immediately headed to safer areas after hearing the thundering noises and feeling the shaking ground.
He said the people returned to their homes not long after the volcanic gas was detected, although the volcanic ash and gravel was still coming down.
Separately in Semarang, thousands of volunteers comprising military and police personnel, scouts, paramedics and Search and Rescue ( SAR ) team members attended a simulation to get themselves prepared should Mt. Slamet have a major eruption.
'These days, it [the volcano] erupts between one to five times a day, spewing volcanic ash. If something bad happens, we, rescuers, have to be ready to assist the affected residents,' Budiman, one of the volunteers, said.
Mt. Slamet is located in the middle of Central Java's five regencies ' Brebes, Banyumas, Purbalingga, Pemalang and Tegal.
Suherdjoko in Semarang contributed to this story