Seismologists try to understand where and when earthquakes might happen in the future by studying recent earthquake sequences. Typically, they rely on the principle that stress accumulates slowly along a fault and is periodically released as an earthquake in a repeating, cyclical pattern.
But two near-identical earthquakes that struck the Ibaraki-Fukushima region of Japan in 2011 and 2016 broke the typical trend — occurring in relatively quick succession. The two earthquakes, which both registered a magnitude of 6, surprised scientists because it normally takes hundreds or even thousands of years for earthquakes of this size to strike on the same fault again.
Natalie Forrest spent her Masters at Cambridge studying this unusual earthquake activity taking place on the other side of the world during two lockdowns, using satellite data to estimate how the stresses built up on the fault between these two rare earthquakes.
The results, recently published in the Geophysical Journal International, show that the Mochiyama Fault was only reloaded by half the amount of the stress released in the 2011 earthquake. This implies that the fault became weaker over time, and that when it broke again in 2016 less stress was needed to initiate an earthquake. The study is one of the first to document a fault changing strength over the timescale of just a few years.
The findings challenge how we think of the earthquake cycle, said Forrest, who is now a PhD student at the School of Earth and Environment (SEE), University of Leeds. “We’ve shown that there isn’t always constant, slow stress accumulation on faults of constant strength — this means that seismic hazard in a particular area may vary over time, and in a way that is hard for current model predictions to account for.” Crucially, faults which have recently hosted a devastating earthquake may rupture again in a very short time period, increasing the seismic hazard in already exposed and vulnerable communities.
Cambridge Earth Sciences’ Sam Wimpenny and Alex Copley were also involved in the research. “Many seismic hazard models assume that strain accumulates slowly on faults and that the repeat time between them is on the order of hundreds of years,” said Wimpenny, who led the study and is now also based at SEE, University of Leeds. “This might not be the case though, after just a few years faults can build up enough strain and weaken such that they rupture again.”
Forrest used data from satellites to track how the surface around the Mochiyama Fault wasdeforming over time, to then infer how stresses accumulated on the fault at depth. “There are a range of ways we can study earthquakes remotely,” said Forrest, “And the accessibility of remote-sensing data means we can track earthquake processes in regions which are otherwise difficult to reach.”
Forrest and Wimpenny also built a model of a simplified fault plane to understand how it could have been reloaded with stress so soon after the 2011 earthquake. They explored potential mechanisms, such as forces due to nearby earthquakes, and whether the transfer of stress to other parts of the fault zone following the initial earthquake might have contributed to the second earthquake.
But, when they ran the models, they found that neither of these two mechanisms could have provided the conditions needed for a repeat earthquake in 2016, because the stress that built up over this time was less than half the magnitude of that which caused the initial earthquake.
That points to the fault plane becoming weaker over time, said Forrest, “The threshold governing how much stress the fault could take changed, so it broke in a second large earthquake sooner than expected.”
The cause for the reduction in fault strength still needs to be explored, said the authors, but one of their theories is that fluids travelling along the fault plane may affect the fault strength through time, by changing the effective friction of the fault. Fault weakening may be related to increased fluid pressure in the fault zone, or due to the dissolution of hydrothermal minerals, however the exact fault-weakening mechanism remains elusive.
During her PhD at Leeds, Forrest will continue to investigate the mechanisms related to the earthquake cycle, using newly-available InSAR data to analyse a range of faults across the world, looking for intriguing casestudies that show similar changes in fault behaviour to the Mochiyama Fault. According to Forrest, the ability of faults to reduce in strength over such a short period of time should be incorporated into new seismic hazard models, in order to better understand where future destructive earthquakes may occur.
Wimpenny, S., Forrest, N. and Copley, A., 2022. Time-dependent decrease in fault strength in the 2011–2016 Ibaraki–Fukushima earthquake sequence. Geophysical Journal International, 232(2), pp.788-809.