Why does the Sumatra subduction zone host more giant earthquakes than Java? Thermal structure and mineralogical insights

The episodic stick-slip behavior of megathrust faults in subduction zones can lead to severe earthquakes and tsunamis that pose a catastrophic threat to coastal populations. It is therefore of great importance to study their seismogenic conditions and earthquake activity. The subduction zones of Sumatra and Java, located seaward of the Sunda Arc, are geographically neighboring, but their earthquake phenomena differ significantly. The Sumatra subduction zone has been the scene of numerous very strong earthquakes (Mw > 8), including the 2004 Sumatra earthquake of magnitude Mw 9.1, while the Java subduction zone has experienced only a limited number of large earthquakes, with a maximum magnitude of less than Mw 8. So far, the underlying mechanism explaining this seismological mismatch between these two margins remains enigmatic. To determine the possible cause, we first calculated the 2D steady-state subduction zone thermal model using the finite element method off the coast of Sumatra and Java based on regional tectonic settings. We then extracted the pressure-temperature (P-T) conditions of the megathrusts and analyzed their relationship to the mineral composition of the megathrust shear zone, the serpentine metamorphic reaction, the frictional behavior, and the characteristics of historical earthquakes. Based on the modeling results, we found that the Sumatra megathrust can trigger large earthquakes in both the crustal and mantle regions from the seafloor to a depth of 60 km, covering a seismogenic zone of about 200 km in width. In contrast, the predicted seismogenic zone off the coast of Java is mainly restricted to the crustal region (< 20 km depth), with a width of less than 50 km. The main reason for this discrepancy is primarily related to the age of the subducted oceanic crust and the depth of the arc crustal Moho. The very hot oceanic crust off the coast of Sumatra enhances the metamorphic reaction of the serpentine minerals below the shallow part of the mantle wedge from the velocity-strengthening minerals lizardite/chrysotile to the velocity-weakening mineral antigorite, which facilitates the generation and rupture of earthquakes. In contrast, the P-T condition of the Java subduction is too cold to promote such a metamorphic reaction and thus facilitate earthquake rupture below the mantle wedge.