Hearing the news about an earthquake in the end of May 2017 made me wondering. With some parameters such as magnitude above 5, location and depth, the earthquake was very large this time. The Meteorology Climatology and Geophysics Agency (BMKG) recorded the earthquake of magnitude 6.6 at a depth of 11 km and epicenter on land 38 km northwest of Poso, Central Sulawesi, occurred on Monday (29/05/2017). In a press release broadcasted, BMKG mentioned 12 aftershocks occurred with 3 earthquakes at magnitude above 5.
Having no reference at all regarding with earthquake conditions in the region, news and information were searched from several sources. An article written by Ahmad Arif in Kompas daily (31/5/2017) with a title Waspada Gempa Besar di Sulawesi opened knowledge related to the context of potential earthquake in Sulawesi region. In its article, the latest National Seismic Source Map 2017 mentions 48 fault or earthquake sources on the island of Sulawesi. The map compiled by Pusat Studi Gempa Nasional or the National of Earthquake Study Center experiences changes that previously identified 12 earthquake sources on the 2010 earthquake map.
The earthquake that occurred at the end of last month (29/5) has resulted in some damages and injuries. Until June 1, 2017, the Regional Disaster Management Agency of Poso Regency recorded 349 damaged buildings, of which 168 houses were severely damaged. Meanwhile, 4 people heavy severe injured and 21 minor injured.
According to BMKG, the impact of the earthquake based on shake map analysis shows several levels of impact in Central Sulawesi region, such as III – V MMI in Poso, Loeo, Palu, Kasongan and Toli – Toli. Palopo, Masamba and Balikpapan areas at level II MMI. The cause of this earthquake is shallow crustal earthquake activity in the Palolo Graben Zone. The quake did not trigger a potential tsunami because the epicenter was on land.
MMI or Modified Mercalli Intensity or Mercalli scale is an earthquake strength measuring unit symbolized by roman numerals. The higher the MMI value, for example the IX-XII indicates the earthquake causes most of the permanent building walls to collapse and the structure of the building is severely damaged. BMKG defines simply the rate of MMI with ‘heavy damage’.
Compared to the earthquake incident in the zone, Sulawesi region has a larger faults, namely Palu-Koro Fault. According to the Expedition Team of Palu-Koro, the earthquake in Poso was not surprising because the Sulawesi region crossed by a large active fault. Looking at the map of earthquake source in Sulawesi 2017, many points with magnitude potential above 6. In Kompas article (31/5/2017), Palu-Koro Fault is mentioned as the second longest land fault in Indonesia, about 250 km, after the big fault of Sumatera. Palu-Koro fault is the main in Central Sulawesi, extending from north to south, starting from Donggala at the end of Teluk Palu to Bone Bay.
The Expedition Team of Palu-Koro wrote that based on paleoseismology study conducted by M.R. Daryono, the seismic cycle of the 130-year-old Palu-Koro fault cycle. Recorded two large earthquakes in 1907 and 1909. A large earthquake is described by the fall of young coconuts from the tree. According to the release of this team, these years are the 130-year cycle of the Palu-Koro Fault active period. In the Earthquake Source Map in Sulawesi 2017, the potential for earthquake strength in this fault reaches magnitude 6.8. The Palu-Koro fault is quite intensive with frequent earthquakes such as the Donggala earthquake (1927), Bora earthquake (1938), Tambu earthquake (1968), Lawe earthquake (1985), Palolo earthquake (2005) (Marjiyono, et al , 2013).
The potential for earthquake hazard in Sulawesi is very real. Although acknowledged, the quake experts from LIPI Danny Hilman Natawidjaya difficult to know which segment in the path of an earthquake that can trigger a major earthquake in the near future (Kompas, 31/5/2017). Not to mention the incident of the Poso earthquake, residents do not know the potential of disaster from active faults. Understanding of threats and risks needs to be transferred to people in the region.
BNPB and BPS data are related to the number of exposed population to potential earthquake hazard of medium to high class category in Central Sulawesi, using Population Census 2010 data, amounting to 2,525,640 people or about 95.85%. Of these, about 436,697 or 5.44% are exposed to vulnerable populations. Meanwhile, based on exposed areas with potential earthquake hazard with high class category reached 877.986 ha or 14.35%, and medium category 5.015.319 ha or 81.95%.
Great efforts are needed to share knowledge as an educational and disaster-awareness campaign in the midst of the people of Central Sulawesi. Of course, this is a long process and needs a synergize effort in building culture of disaster awareness. Learning from the great earthquake that devastated the city of Kobe in Japan in 1995 known as Great Hanshin Awaji, the percentage of survivors by self-effort by 35%. The percentage is the highest compared to other factors, such as survivors because of family members, friends, neighbors and rescue teams. This shows that knowledge, understanding and skills become one of the important factors for everyone to respond to earthquake hazards.
On the other hand, concrete efforts related to building factors are also very important. The death toll after many earthquakes was caused by rubble of buildings and not because of the earthquake itself. Geological Agency provides recommendations related to building security in areas prone to earthquake. Here are some recommendations, especially after the earthquake in Poso, Central Sulawesi some time ago:
- Conducting relocation on the residents affected by minor surface rupture covers in RT 7, Dusun 3, Wuasa Village, Sub-district Lore Utara.
- For residents whose houses are located in liquefaction zone in RT 06 and RT 07, Dusun 3, Wuasa Village, can rebuild their houses using earthquake resistant construction or make wooden stage house.
- Building vital, strategic and inviting concentration of many people to be built following earthquake resistant building rules.
- Avoid building on swampy soil, rice fields and pile of land that do not meet the technical requirements, as they are prone to earthquake shocks.
- Avoid building on the bottom, and steep slopes that have been weathering and soil conditions loose because it will potentially occur when the earth / earthquake shaking earthquake.