Shallow M5.0 earthquake shakes West Java, Indonesia
A moderate event strikes beneath an active volcanic region - what was the source?
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On September 18, 2024 at 9:41 AM local time, a magnitude 5 earthquake struck beneath the active volcanic mountains of West Java, Indonesia. Shaking was felt for about 10 seconds in the surrounding region, with most reports coming from the nearby large city of Bandung. A brief event report from the Center for Volcanology and Geological Disaster Mitigation (PVMBG) (this link translates to English) indicates that some injuries and damage to buildings did occur.
The Indonesian Meteorology, Climatology and Geophysics Agency (BMKG) located the earthquake origin between two active volcanoes: Gunung Wayang-Windu and Gurung Kendang, at a shallow crustal depth. These are part of the Java Volcanic Arc, a line of volcanoes that have formed above the Indian slab, which is subducting northward at about 6.5 cm/yr.
The stratovolcanoes in this area are certainly of interest, as some of them have produced historic eruptions. A large and destructive eruption of Gunung Panandayan occurred in 1772, and a smaller explosive eruption occurred during November, 2002. Shallow earthquakes near Papandayan prompted a special report from PVMBG during the first week of March in 2023. Finally, Gunung Guntur had significant eruption activity during the 19th century.
So, it is natural to ask whether today's earthquake is related to the volcanoes, or not, or maybe. Let’s take a look at the data. We only have access to a high-quality earthquake catalog in this area (produced by BMKG) since about 2008. The map below, along with some cross sections and a timeline, shows only shallow (<20 km) earthquakes in the area since the beginning of the BMKG catalog. The earthquakes are colored by time, with blue-hued earthquakes occurring earlier, and red-hued earthquakes more recently.
There is apparently a lot of shallow crustal seismicity in this area! Today’s earthquake certainly stands out against the seismic background of the last fifteen years. It is the largest magnitude event recorded by the BMKG catalog within the shallow crust in this area since 2008, slightly beating out a 2019 earthquake that occurred beneath Gunung Guntur to the northeast.
Unfortunately, the reported depths in the BMKG catalog are mostly default values of 10 km. A default depth is usually assigned when the seismic data isn’t good enough to estimate the real depth accurately. That means it is hard to know whether these earthquakes are deep in the crust, or are actually shallow.
Fortunately, seismologists have done some more advanced work on these particular earthquakes, producing better depth estimates. Supendi et al. (2018) drew a cross section similar to our B-B’ section; their relocated earthquakes really do occur between 5 km and 25 km depth. This makes them more likely to be tectonic earthquakes, produced by slip on faults cutting through the crust, rather than volcanic earthquakes, which tend to be quite shallow.
The same authors also calculated a number of focal mechanisms for shallow crustal earthquakes across West Java. Today’s earthquake occurred near the northeastern end of a broad band of seismicity that reaches the coast in the south-southwest. Those earthquakes exhibit thrust and strike-slip mechanisms, which led them to interpret the presence of a strike-slip fault at depth. That fault does not appear at the surface — it either doesn’t reach the surface at all, or is hidden by the young volcanic deposits.
Based on these prior results, we would expect that today’s earthquake would have a strike-slip or thrust-type focal mechanism. Fortunately, two institutions did report focal mechanisms! This is a bit unexpected, because events lower than magnitude 5 often are not reported.
Here’s what BMKG reported:
This is a mostly strike-slip event. This is pretty similar to the previous results of Supendi et al. (2018). There was either right-lateral slip in a northeast-southwest oriented fault, or left-lateral slip on an east-west oriented fault. In both cases, the faults are not vertical, instead having a moderate dip.
The GEOFON project at GFZ also reported a focal mechanism. However, their results indicate a normal fault, quite different from the strike-slip fault inferred by BMKG.
The mechanisms are very different. This highlights the challenge of estimating focal mechanisms at lower magnitudes — it is easy to get things wrong.
So, which solution do we prefer? The strike-slip fault from BMKG clearly looks better. First, the BMKG solution matches the earlier published results. Second, it was created using a regional seismic network that can infer mechanisms for small events — i.e. the data are inherently better. Third, it makes more tectonic sense. Strike-slip faulting is common in Java, due to the oblique subduction of the Indian Ocean lithosphere northward beneath the island, which causes a kind of sideways yanking of the crust.
So, it seems most likely that the earthquake was produced by slip on a tectonic fault, one that may extend from the coast inland to the volcanoes.
Earthquakes near volcanoes always raise the question of whether an eruption is imminent. Seismic activity can certainly be a sign that a volcano is stirring. However, volcano monitoring agencies use seismicity as only one part of their hazard assessment program. There are many other observations, like gas emissions, temperature changes, surface deformations, and seismic tremor, that need to be considered. It takes a lot of experience and expertise to interpret volcano-related data! In this case, while an earthquake occurred near two volcanoes, it does not appear to be related to magmatic activity.
For those who are particularly interested in the potential volcanic implications of earthquakes in Indonesia, we recommend following the announcements of PVMBG.
References:
Supendi, P., Nugraha, A.D., Puspito, N.T., Widiyantoro, S. and Daryono, D., 2018. Identification of active faults in West Java, Indonesia, based on earthquake hypocenter determination, relocation, and focal mechanism analysis. Geoscience Letters, 5, pp.1-10. https://doi.org/10.1186/s40562-018-0130-y