The Meteorology, Climatology, and Geophysics Agency (BMKG) has said the earthquake that shook the south of Yogyakarta, with a magnitude of 5.3 on Monday 28th June at 5:15am, was not a megathrust earthquake.
“The earthquake that occurred was not a megathrust earthquake, because it didn’t originate in the contact area between the Indo-Australian Plate and the Eurasian Plate, but the hypocentre of this earthquake was a bit deep into the Benioff Zone,” said BMKG Earthquake and Tsunami Mitigation Coordinator, Daryono.
He explained that the megathrust zone is an earthquake source zone at the subduction of a sloping plate and is still shallow. Meanwhile, the Benioff Zone is an earthquake source zone in a plate slab that is subducted deeper and has started to dip.
BMKG’s analysis shows that the epicentre of the earthquake was located at coordinates 8.56 south and 110.58 east, at sea, at a distance of 66km south of Wonosari in Gunungkidul Regency, at a depth of 61 km.
According to Daryono, taking into account the location of the epicentre and its depth, the southern Yogyakarta earthquake, with an updated magnitude of 5.1, was an intermediate type of earthquake. It occurred due to deformation or faults in the Indo-Australian Plate which is subducted under Java Island with an oblique thrust source mechanism.
The impact of the earthquake was felt in Bantul and Gunungkidul at III-IV on the Modified Mercalli Intensity (MMI) scale, whilst in Purworejo, Trenggalek, Tulungagung, Blitar, Nganjuk it was experienced at II. In Sleman and Yogyakarta the quake was at II-III and in Klaten, Cilacap, Kebumen, Banjarnegara, Malang, and Solo it was experienced at level II.
So far, there have been no reports of damage caused by the earthquake. The modelling results show that the earthquake has no potential for a tsunami. As of 5:50am, the results of BKMG’s monitoring have not shown any aftershock activity in the south of Yogyakarta.
“Seeing the very strong ground motion, even though the magnitude of the earthquake was relatively small, with a broad spectrum of shocks, the earthquake south of Yogyakarta seemed to be centred within the plate at medium depth, similar to the ‘intraslab earthquake’ on the south of Malang on 10th April and 21st May 2021,” added Daryono.