Earthquakes originating from the meeting of two crusts between the oceanic crust and the continental crust occur in subduction zones (for example in the south of Java Island (Figure 1)). On August 28, 2024, the Central Java Provincial Government issued a circular on the threat of a megathrust earthquake. The threat of this earthquake originates from the southern sea of Java Island. This circular letter began with a warning from BMKG (Meteorology, Climatology, and Geophysics Agency), where the warning began with a 7 Magnitude megathrust earthquake located in Kyushu, Japan which occurred in early August 2024. Although the Kyushu earthquake in terms of tectonics has nothing to do with the Java megathrust earthquake, the megathrust earthquakes in Java and Japan have geological similarities, namely that they are both megathrust earthquakes that occur in subduction zones. People wonder what a megathrust is? The issue of the megathrust earthquake has caused panic in the community both on social media, television, and others.
Figure 1. Java Island subduction zone and cross-section A as an illustration of the meeting of continental crust and oceanic crust.
Megathrust earthquakes are earthquakes that occur due to the release of energy by megathrust faults that occur at shallow depths (<15 km) in the prime accretion zone of the subduction zone. Megathrust earthquakes occur at the plate interface (Figure 1) (the boundary between the oceanic crust and the continental crust which is called the shear zone). In the seismic records in southern Java, there have been megathrust earthquakes with a magnitude of <8 M, namely the earthquakes of June 2, 1994 (7.8 M) and July 17, 2006 (7.7 M). The earthquake triggered the deaths of more than 250-800 people due to a tsunami as high as 8-15 m in southern Java. With this soul newspaper, it is only natural that our society needs to be aware of megathrust earthquakes. Although earthquakes cannot be predicted when they will occur, earthquake technology is developing rapidly, especially in Japan.
Figure 2. Subduction zone in the Nankai Trench, Japan and cross-section B as an illustration of the shear zone as a source of tremor events.
One of the areas studied for megathrust earthquakes in Japan is located in the Nankai Trench. They studied the shear zone (Figure 2) in cross-section B. Small-scale earthquake vibrations (tremors) with magnitudes <4 occurred in the shear zone. The number of tremors is measured every day by installing DONET in the sea around the Nankai Trench. DONET is a geophysical tool for recording tremors developed in Japan. Do you still remember the two major earthquakes? Namely the Aceh earthquake in Indonesia in 2004 which claimed 200 thousand lives and the Tohoku earthquake in Japan which claimed 10 thousand lives. The megathrust earthquake that triggered the tsunami during the Tohoku earthquake 9.0 in 2011. The height of the tsunami reached 35 m on the coast of Tohoku. Before the earthquake occurred, it was preceded by an increase in tremor events in the shear zone. So that tremors can be used as a prediction tool for megathrust earthquakes. Therefore, an earthquake in southern Java could occur in the future, so vigilance is needed in order to mitigate megathrust earthquakes.
Article by Fahrudin, ST., MT., Ph.D.
