I just saw this question while looking at Lusi 9th anniversary news. As an author of several studies on this topic, wanted to say that the answer provided by Matt is an excellent summary of the triggering debate.
Two updates though.
1) I published a detailed reexamination and processing of the petrophysical and drilling data from BJP-1. This includes a discussion on the specific errors in the petrophysical model used in the Lupi et al. study and corrigendum.
Max Rudolph et al. have now also published a study in Geophysical Research Letters that undertakes similar modelling to that done in the Lupi et al. study and corrigendum. In this new paper, the effect of Yogyakarta earthquake is modelled using both the Lupi et al. corrigendum velocity model and my fully reprocessed and corrected velocity model. The results of the modelling indicate that the errors in the Lupi et al. velocity model significantly overestimate the effects of the earthquake. Interestingly, these results do also suggest that structure helps amplify the earthquake effects, but that the biggest influence is the increase in seismic wave amplitude that occurs when waves pass upwards from the fast volcanic/volcaniclastic layer into the Kalibeng clays. The domed shape causes little amplification, in contrast to the conclusions of Lupi et al. Regardless, the models all indicate that the effect of the earthquake is still very small, and much less than the known influence of the drilling kick. This new study can be found here:
2) We have an article in Nature Geoscience published:
In this study, we use the drilling mud gas measurements from the BJP-1 borehole to directly test the hypothesis that an earthquake triggered liquefaction of the Kalibeng clays at Lusi (which is essential for the earthquake triggering hypothesis). Clay liquefaction is associated with release of gases and fluids. So, we examined the gas measurements coming from the BJP-1 well in the days before and after the Yogyakarta earthquake. Despite the BJP-1 well being open and exposed to almost the entire thickness of the clays, there is no increase in gas release in the 25 hours after the earthquake (the time period between the earthquake and the drilling kick that is alternatively argued to have triggered the disaster). This strongly indicates that the earthquake did not cause any liquefaction of clays at the Lusi location, and is the first actual direct measurements used to test the earthquake triggering hypothesis.
In addition, we examined the source of H2S from Lusi. H2S was observed coming out of the Lusi vent in its first few days of eruption. The earthquake triggering community has argued that this H2S comes from the Kalibeng Clays. In particular, the hypothesis is that the H2S has migrated from great depth into the Kalibeng clays via the Watukosek fault, with these deep hydrothermal fluids ‘priming’ the clays and making them prone to liquefaction. Yet, the gas measurements while drilling BJP-1 reported no H2S in the Kalibeng clays at all. H2S was first observed right at the bottom of the well, a few hours before the Yogyakarta earthquake. H2S was then observed during the drilling kick and then from the Lusi vent. Hence, the drilling gas data indicates that the H2S cannot have been sourced from the Kalibeng clays, and suggests that there is no evidence for deep hydrothermal ‘pre-charging’ of the clays. Rather, the H2S most likely comes from greater depths, near the bottom of the well (which is near or just in Carbonates, in which high H2S levels are commonly observed throughout the basin). This suggests that initial eruptive fluids from Lusi are essentially the same as those during the drilling kick 18 hours earlier, and that these fluids are tapping a deeper source than the Kalibeng clays. The drilling-trigger hypothesis is the only hypothesis that proposes such a deep initial fluid source, and the borehole makes a great pathway for deep fluids in the carbonates to suddenly escape through a 1000m thick sealing volcanic/volcaniclastic layer, pass through the Kalibeng clays and erupt at the surface.
Maybe this does not ‘close the argument’, but it does make the earthquake triggering hypothesis even more unlikely, and pose more questions that this hypothesis seems unable to answer (just like the question as to why the disaster would be triggered by the Yogyakarta quake, when there were many other larger quakes that had no effect). Furthermore, all these new measurements are fully in line with the drilling trigger hypothesis.
Like testing any competing hypotheses, you keep throwing data at them until they either fall down or keep standing. The drilling-trigger hypothesis is still standing strong; but the earthquake-trigger hypothesis has either fallen down, or is looking very shaky!