Mt. Merapi Threat Level Raised, but Eruption not Imminent

Location of Mt. Merapi. Larger map: maps.google.com

In the wake of a raise in the threat level¹ on Wednesday, April 30, tensions remain high in the shadow of Mount Merapi. In a press release posted on YouTube², Center for Research and Technology Development of Geologic Disaster (Indonesian: Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi; BPPTKG)³ head Subiandro cited the increase in seismic activity over the past two days as the reason.

Earthquake activity beneath a volcano tends to increase before an eruption, as magma and volcanic gas is forced through underground passageways and fractures⁴. This causes further breaks and, consequently, vibrations—volcanic tremors or low-frequency earthquakes.

The implication, from both BPPTKG's raising of the threat level from normal to alert and the report of low-frequency earthquakes, is that an eruption is likely to occur. To this effect, Ganjar Pranowo, the Governor of Central Java, is calling⁵ for increased focus on the deprecated state of the region's evacuation routes. Quarries populate Mt. Merapi, its volcanic rock mined and hauled out everyday in large trucks. Over time, these trucks have warped and cracked the roads and bridges, reducing their efficacy. Travel along the volcano's upper regions is already impossible in anything but off-road vehicles.

Mt. Merapi Continues to Rumble

Location of Mt. Merapi. Larger map: maps.google.com

Mount Merapi showed some activity early on Sunday morning.

According to Triyono, the officer at the Mount Merapi Observation Post in Ngepos, Srumbung subdistrict, there was rumbling around 04:25–04:35 local time (22:25–22:35 BST; 17:25–17:35 ET).¹ Sri Sumarti, head of the Mt. Merapi division at the Center for Research and Technology Development of Geologic Disaster² (Indonesian: Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi; BPPTKG) in Jogja, reported tremors to have started minutes earlier.³
“Tremors were recorded for around 20 minutes at 04:21 local time on Sunday, and our observers began to hear thunder at 04:26,” she said.

Locals were startled by the rumbling, said Ismael, the village leader of Babadan, Dukun Subdistrict, who also claimed “red-coloured material spewed from the mounts [sic] peak” after the rumbling. Observations made from the Mount Merapi Observation Post in Sewukan, Dukun Subdistrict, found billowing smoke, whereas the post in Ngepos was unable to make any visual confirmation because of fog. There were no evacuations.

A reported 12–15 km (7.4–9.3 mile) radius was affected by the ejection of volcanic ash, particularly on the southwest, south, and southeast of the peak. Head of the Data and Information Center of the National Disaster Mitigation Agency (Indonesian: Badan Nasional Penanggulangan Bencana; BNPB) Sutopo Purwo Nugroho said ash fell in subdistricts that included Dukun, Srumbung, Muntilan, Sawangan, and Mungkid. In Jogja itself, however, it seems to have barely registered.