East Java Earthquake, an Alarm for Us
By realizing earthquake and tsunami disaster mitigation measures, we can still live safely and comfortably even in disaster-prone areas.
In the midst of handling the impact of the destructive Seroja Cyclone in East Nusa Tenggara (NTT), we were hit by a strong earthquake in East Java.
The earthquake, which had a magnitude of 6.1, occurred on Saturday, April 10, 2021, at 02:00:16 a.m. Western Indonesia Standard Time, had an epicenter at the coordinates of 8.83 south latitude and 112.5 east longitude.
Precisely it was in the sea at a distance of 96 kilometers south of Malang, East Java, with a depth of 80 kilometers.
Benioff zone
By paying attention to the location of the epicenter and the depth of the hypocenter, the earthquake was of medium depth and triggered by rock deformation in the Benioff zone. The Benioff zone is a subducted part of the Indo-Australian Oceanic Plate and dips down the island of Java under the megathrust zone. Because the epicenter of the earthquake was at medium depth, the earthquake was not considered a megathrust earthquake.
The results of an analysis of the source mechanism show that the earthquake had a thrust fault mechanism. The mechanism of the source of such ascending fault is actually sensitive to potential tsunamis if the hypocenter of the earthquake is shallow and the magnitude is large. The earthquake not strong enough to disturb the sea water column and did not trigger a tsunami.
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The impact of the earthquake in several areas in East Java reached the maximum intensity scale of V-VI MMI, which was potentially damaging. The estimate of the shake map from the Meteorology, Climatology and Geophysics Agency (BMKG), which was informed 15 minutes after the earthquake, was considered accurate because the earthquake that occurred was proven to have damaged thousands of houses spreading across 17 regencies/cities in East Java.
The earthquake also killed nine people and injured more than 84 people.
Apart from causing damage, the East Java earthquake shook a wide area, including Lombok and West Java. The occurrence of a broad spectrum of shocks was associated with a hypocenter of an earthquake at medium depth.
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According to experts, the occurrence of such a destructive earthquake in East Java is not a strange thing. Tectonically, the southern region of Malang is known as a seismically active and complex area. History shows that Malang and its surroundings have been hit by several destructive earthquakes, namely in 1896, 1937, 1958, 1962, 1963, 1967, 1972 and 1998.
The location of the East Java earthquake hypocenter is inside the subducted Indo-Australian plate. Earthquakes centered within such a plate are referred to as "intra-slab earthquakes". The Bali earthquake on March 22, 2017 with a magnitude of 5.6, the Tasikmalaya earthquake on Dec. 15, 2017 with a magnitude of 6.9, and the Banten earthquake on Jan. 23, 2018 with a magnitude of 6.1 are examples of intra-slab earthquakes in southern Java.
Even though the hypocenter of an intra-slab earthquake is quite deep, the shock can be felt very broadly as the accumulated strain energy is converted into seismic waves during the earthquake. As a result, the earthquake will radiate shocks with a very significantly high frequency.
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Based on the time curve of the fracture process that occurred at the source time function, it was clear that the amount of energy radiating from the fracture source of the East Java earthquake was very small. The duration of the fracturing process was very short, only in 2.5 seconds, but it was able to radiate a very significant shock, which damaged thousands of houses with widespread shocks.
BMKG monitoring results as of Thursday, April 15, 2021, there were 13 aftershocks with magnitudes ranging from 2.9 to 5.3. By observing the activity of the aftershocks, it appears that there the productivity of aftershocks was small. Earthquakes in the oceanic intra-slab of medium depth generally have less severe aftershocks.
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The low productivity of aftershocks is a reflection of the sudden large drop in stress during the main earthquake. Knowing that the East Java earthquake had a high stress drop leads to the notion that this earthquake was very unlikely to be followed by the next big earthquake. Conversely, the tectonic condition in the earthquake zone will soon stabilize and return to normal.
Alarming for us
The devastating earthquake reminds us that the potential threat of an earthquake in the south of Java Island should not be ignored.
Tectonically, in the southern region of Java there are several earthquake source zones, such as (1) earthquake sources outside the subduction zone (outer rise), (2) plate subduction zones that can trigger inter-plate earthquakes, (3) Benioff zone which can trigger intra-slab earthquakes, and (4) active faults on the seabed that can trigger intra-plate earthquakes.
With regard to a part of these earthquake sources, a strong earthquake with the potential for a tsunami can occur at any time unpredictably.
The BMKG earthquake catalog recorded that from 1840 to 2009 in southern Java there had been more than 12 large earthquakes with magnitudes between 7.0 and 8.0, while tsunamis on the south coast of Java had occurred eight times. To anticipate a repeat of a strong earthquake and tsunami, experts have several times warned of the potential for an earthquake and tsunami in southern Java.
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The results of the latest study show that in the megathrust zone of southern Java, there are several slip deficit areas which indicate a locked inter-plate contact area. In these areas, it is suspected that there is a stress field accumulation process that one day could potentially cause a strong earthquake.
The results of this study produce an estimation of the worst case scenario of earthquake shocks and tsunami which can be used as a reference for earthquake and tsunami mitigation.
Apart from reminding us of the potential threat of an earthquake and tsunami in southern Java, the East Java earthquake also reminded us to be more serious in disaster mitigation efforts by preparing concrete steps to minimize the impact of losses of life and property due to the earthquake and tsunami.
The victims who died and injured were not actually caused by the earthquake, but by the collapsed buildings that hit the occupants. The many damaged houses in East Java during the Malang earthquake is a reflection of our weak structural mitigation. Therefore, structural mitigation efforts by building earthquake-resistant buildings must be realized.
If not, at any time if a strong earthquake occurs, the number of casualties will continue to rise because the most important solution in earthquake mitigation is to build earthquake-resistant buildings.
By realizing structural mitigation, we can minimize the losses of life due to earthquakes.
If some are not able to build earthquake-resistant buildings, there are other alternatives by building houses with lightweight materials made of wood and bamboo, which are designed to be attractive. By realizing structural mitigation, we can minimize the losses of life due to earthquakes.
To anticipate tsunamis, we need to increase the dissemination of information on tsunami mitigation, understand the concept of self-evacuation through early warnings, create evacuation route maps, install evacuation signs, hold evacuation drills and construct temporary evacuation sites.
Apart from that, there needs to be serious efforts to arrange the coastal space based on tsunami risk, as well as to improve the performance of the tsunami early warning system. By realizing earthquake and tsunami disaster mitigation measures, we can still live safely and comfortably even in disaster-prone areas.
Daryono, Researcher at Meteorology, Climatology and Geophysics Agency (BMKG) and Member of National Center for Earthquake Studies (PuSGeN)
This article was translated by Hyginus Hardoyo.