Preliminary Engineering Seismology Report From Strong Motion Records For Malang Earthquake-East Java, Indonesia 10th, April 2021 Sigit Pramonoa),Fani Habibaha),Furqon Aa),Ardian Oa), Audi K, Dwikorita Karnawatia),M.Sadlya),Rahmat Ta),Dadang Permanaa),Fajri Syukura) On Saturday, 10th April 2021 had been occurred devastating earthquake at 07:00:02 UTC with moment magnitude (Mw) updated 6.1, earthquake epicenter located 8.83 °S - 112.50 °E at southern part of Java Island in depth 80 km. Meteorologycal Climatologycal and Geophysics Agency has committed for developing earthquake ground motions accelerometer sensor in Indonesia since 2004. This report presents characteristics ground motion records of East Java related with the potential damage area close to epicenter used ground motion recorded which have been detected from Indonesia National Strong Motion Network. More than 50 accelerometer sensors had detected during that earthquake at the epicenter distance less than 1000 km. GEJI accelerometer station located is closest to earthquake source with the epicenter distance 64.4 km to epicenter. As an early report that accordance to SNI 1710-2019 GEJI accelerometer station as classified soil class D, it showed maximum peak ground acceleration of GEJI accelerometer station is 223.08 gals and maximum spectral acceleration 642.5 gals at 0.2 second. It has estimated impact ground shaking V-VI MMI. Three accelerometers which have the large motion with PGA more than 100gals have been identified, they showed that the horizontal shaking is larger than vertical at the PGA, short period Ss and long period spectra S1. It has associated with the directional wave that showed peak direction horizontal E-W was most dominant. While we compared the spectral from GEJI site to SNI (Indonesia National Standard) design, it showed well proportional between of them, it means that the spectral accelerometer at short period Ss still has width range from SNI 1726-2019 design. All strong motion records data attenuate with distance at rate generally consistent with modified next generation attenuation (NGA) from Zhao equation. aIndonesia Agency of Meteorology Climatology and Geophysics,Jakarta,10620, [email protected] INTRODUCTION GEOLOGICAL SETTING OF EASTERN JAVA ISLAND INDONESIA Within the framework of Indonesia Strong motion Network involved the considering of potential impact area after the earthquake occurred in Indonesia. Malang City is located a part of Java Island, detailly in east of Java Island. Geo-morphology presents any complex topography at eastern of Java Island. The time and mechanism of shifting from Paleogene- Neogene magmatic belts in the Southern Mountains of Java into Recent volcanic belts is still poorly understood. New geochronological data of intrusive and volcanic rocks are therefore collected from transitional areas to reveal new understanding regarding with this evolution (Akmaludin,2005). Beside of this, in the middle along the east to west found the sedimentary medium. Malang earthquake which occurred on Saturday, 10th April 2021 has caused the destruction affected in eastern part of Java Island. It was detected by Indonesia strong motion Network along the Java and Bali Island. The affected area in eastern part of java Island weather any related with the geological regime eastern part of Java Island or any others contributions, it would be evaluated in this report. According to Van Bemmelen (1994), Eastern Java region was divided into six Physiographic Zones, in the order from northern to southern regions as follow : Alluvial Plains Zone of Northern Java, Rembang – Madura Anticlinorium Zone Central Depression of Randublatung Zone, Kendeng Anticlinorium Zone, Java Central Depression Zone (Solo Zone), Quarternary, Volcanic Arc Zone, Southern Mountains Zone. The subduction activities located in the southern part of Java Island are giving the main influence to the seismicity of the Eastern Java Region, especially the subduction segmentation of Central Java, East Java and Bali. The activities frequency and earthquake magnitude of the Java subduction zone is significantly lower compared to the Sumatra subduction zone. Even so, there have also been several large earthquakes occurred at off the coast of southern Java, especially along the megathrust that generated tsunami wave such as 1994 Mw 7.8 East Java earthquake and 2006 Mw 7.8 Pangandaran earthquake. Figure 1. Physiographic zone division of Eastern Java region according to van Bemmelen (1949) Among numerous earthquakes that occurred in Java subduction, none have reached Mw 8. Tectonic plate’s relative movement in the Java subduction zone is hypothesized to be accommodated dominantly by aseismic movement without energy accumulation. This assumption is based on the age information of the oceanic plate that subducts beneath Java Island, which is relatively older (120-130 Ma) compared to the plate that subducts beneath Sumatera island (46 Ma). Therefore, the older oceanic plate becomes colder and heavier. As the consequences of this condition, the dip angle of slab becomes larger and the contact between the oceanic plate and continental plate rarely takes place. It is also indicated that the slab-pull force is stronger than the slab-push force in this region, proven by numerous normal faulting events distribution around the fore-arc region of the Java Island. (Abercrombie et al., 2001). Generally, in term of active geological structure, there is no dominant structure developed in Java Island like the one formed in Sumatera Island through the Sumatera Fault System. In central and eastern region of Java Island, the identified dominant geological structure is the thrust fault structure (Kendeng Fault Zone). Whereas in eastern region of Java Island, there are also some minor normal fault structures (Pasuruan Fault, Probolinggo Fault and Wongsorejo Fault). Figure 1 presents regarding from field reported the destruction of houses and the others buildings which have location with the background of different regional geological regime. It showed the area with close to epicenter which located in southern mountain didn’t found destruction report, like in area in close to Gedangan area which have the similar location with the accelerometer sensors in southern part of east java region. However, to complete this reported based on the different geological background still having the plan to do field survey in that area, which have been probably to find the destruction houses and others building caused by the Malang earthquake on Saturday 10 April 2021. Figure 2. MMI distribution in affected area THE HISTORY OF DESTRUCTIVE EARTHQUAKE IN EASTERN JAVA REGION Reviewing the Catalogue of Indonesian Significant and Destructive Earthquake (1821 – 2018), there is a long history of destructive earthquakes that hit the eastern region of Java Island. The first destructive earthquake occurred in eastern region of Java Island, that ever noted in history was on 22 March 1836 around Mojokerto and Jombang region that felt in the scale of VII – VIII MMI. Then, in August 1896 there were a couple of events in the scale of VII MMI felt in Wlingi and Tulungagung region. On 1 December 1915, a VIII MMI earthquake shock Madiun, Maospati and Magetan region causing massive destruction to an infamous sugar factory. On 1 March 1936, a VII MMI earthquake felt in some part of Central and Eastern region of Java Island. On 11 August 1939, a VII MMI earthquake felt in Lamongan region. On 19 June 1950, a VII MMI earthquake felt in Gresik Region. Recorded in 20 October 1958, a magnitude 6.7 earthquake with the depth of 100 km occurred in East Java and felt in the scale of VII – VIII MMI in Malang region. On 10 October 1960, a VI – VII MMI earthquake felt in Tulungagung region. An important earthquake event of magnitude 6.2 with depth of 80 km was felt around VIII – IX MMI in Dampit, Southern Part of Malang on 19 February 1967, causing massive casualties and damage in Dampit, Gondang and Trenggalek.The largest earthquake that ever occurred in this region and triggered tsunami wave that hit southern coast of Java island is the 2 June 1994 Banyuwangi earthquake with magnitude 7.8 and 18 km depth, that only felt III – IV MMI but causing hundreds of casualties due to the tsunami wave. On 28 September 1998, a magnitude 6.3 earthquake with the depth of 177 km occurred in East Java and felt in the scale of V – VI MMI in Blitar, Bantur and Malang region. On 9 September 2007, a magnitude 5.3 earthquake with the depth of 10 km occurred in East Java and felt in the scale of V – VI MMI in Situbondo region. On 8 July 2003, a magnitude 5.9 earthquake with the depth of 9.4 km occurred in East Java and felt in the scale of III MMI in Malang, Probolinggo and Nganjuk region, causing building damage in Malang and Jember Region. On 16 November 2016, a magnitude 6.2 earthquake with the depth of 69 km occurred in East Java and felt in the scale of III – IV MMI in Malang region, causing building damage in Malang Region. On 10 October 2018, a magnitude 6.1 earthquake with the depth of 6 km occurred in East Java and felt in the scale of III – IV MMI in Sumenep, Jember, Situbondo, Banyuwangi, Lumajang, Probolinggo, Bondowoso and Pasuruan region, causing building damage in Probolinggo, Jember, Situbondo and Sumenep Region. Figure 3. East Java, Indonesia earthquake caused building collapsed (Source: BPBD of Malang District) Figure 3 presented the affected the earthquake related with kind of building in Java and East Java, Indonesia Island area. A magnitude 6.5 causes damage building, many cracks in a part of the building until collapsed. BNPB mentioned numerous buldings, public facilities, private building, government and university buildings and some fructuring was seen on major bridge in East Java, Indonesia City. Site classification of soil which refers to SNI 1726 : 2019.