An Earthquake with a Magnitude of 5.1 Shook Gunungkidul This Morning, Not a Megathrust Earthquake But an Intraslab Earthquake

PUBLISHED,- An earthquake with a magnitude of 5.3 which was later updated to 5.1 occurred on Monday (28/06/2021) at 05.15 WIB. This earthquake rocked the Gunungkidul area, Yogyakarta Special Region. Based on the observations of the Meteorology, Climatology and Geophysics Agency (BMKG), the location of the earthquake was at coordinates 8.49 South Latitude and 110.59 East Longitude, the epicenter was at sea 55 km southwest of Gunungkidul.

This earthquake was felt in the Bantul and Gunungkidul areas with a strength of III-IV MMI, in the areas of Purworejo, Blitar, Trenggalek, Tulung Agung and Nganjuk it was felt with a strength of III MMI, Sleman and Yogyakarta was felt with a strength of II-III MMI.

Through his personal twitter, Daryono, the Head of the BMKG Earthquake and Tsunami Mitigation Division, explained that the earthquake in southern Yogyakarta this morning appeared to be centered in the plate at a moderate depth, similar to the earthquake south of Malang on April 10 and May 21, 2021. In his tweet, Daryono stated, “The South Yogyakarta Mag 5.1 earthquake this morning was not a megathrust earthquake, because it did not originate in the contact area between the Indo-Australian Plate and the Eurasian Plate (shallow-sloping subduction), but the hypocenter was slightly deeper into the Benioff Zone (subduction began to swoop).”

According to Daryono, this morning’s Gunungkidul earthquake was suspected to be an intrasalab earthquake or an earthquake originating within the (Indo-Australian) plate, this was proven by a transverse incision of the seismicity of Java Island in a north-south direction.

Gambar 1. Lokasi dan kedalaman Gempa Gunungkidul 28/06/2021, sumber:

Intraslab earthquakes are usually caused when the subducting oceanic plate breaks, cracks or breaks. One of them, caused by the dehydration of rocks in the earth. Intraslab earthquakes have the source of the earthquake in the interior of the subduction zone.

Intraslab earthquakes have the characteristics of spreading vibrations and the location of the source is relatively deep and does not have the potential to cause a tsunami. In addition, intraslab earthquakes tend not to be followed by aftershocks. This is because the oceanic plates are tougher so it is easier to return to their initial position. In contrast to megathrust, which involves small shallow faults, more frequent aftershocks of less strength are followed.

Gambar 2. Kerusakan pada atap rumah di Giripurwo, Purwosari, Gunungkidul, Daerah Istimewa Yogyakarta, Sumber:
Gambar 3. Kerusakan pada atap rumah di Giripurwo, Purwosari, Gunungkidul, Daerah Istimewa Yogyakarta, Sumber:

Furthermore, Daryono also uploaded some of the damage caused by the earthquake. You can see a house with a typical Javanese roof, the roof is damaged and in another photo you can see the condition of the house filled with collapsed ceilings scattered about. This damage was reported to have occurred in Giripurwo, Purwosari, Gunungkidul, Yogyakarta Special Region. Until now there has been no official report regarding the damage caused by the Gunungkidul earthquake this morning. (LS)