The timing of large shallow earthquakes follows a mathematical pattern called the "Devil's staircase" or sometimes called the Cantor function where clusters of earthquake events are separated by long but irregular intervals of seismic quiet.
This finding is different from the pattern predicted by the classical earthquake modeling that suggests cycles of build-up and release of tectonic stress would make earthquakes occur periodically or quasi-periodically.
Large earthquake sequences are "burstier"
Periodic large shallow earthquake sequences are somewhat rare, according to Yuxuan Chen of the University of Missouri in Columbia.
Scientists found that their large results could have implications in the assessment of seismic hazards as they found that large earthquakes sequences with magnitude 6.0 or greater are "burstier" than first expected. This means that there is a higher probability that seismic events will be repeated soon after a large earthquake because of the clustering of earthquakes.
It is more difficult now to predict an average recurrence time between big earthquakes because of the irregular gap between event bursts.
The catalogs of seismologists for large earthquakes in a region might include too little earthquakes over too short a time. Chen and his colleagues noted that it is hard to determine whether few events in a catalog happened within an earthquake cluster or spanned both clusters and inactive intervals.
In addition, they also said that, "for this same reason, we need to be cautious when assessing an event is 'overdue' just because the time measured from the previous event has passed some 'mean recurrence time' based an incomplete catalog."
They published their findings in the Bulletin of the Seismological Society of America.
Sometimes called a Cantor function, Devil's staircase is a fractal -or an irregular curve for which any suitably chosen part when magnified or reduced is similar in shape- demonstrated by a nonlinear dynamic system.
Examples of this pattern can be found in sedimentation sequences, changes in uplift and rates of erosion and reversals in the magnetic field of the Earth.
Chen's Ph. D. advisor Mian Liu had first encountered the Devil's Staircase in an unusual way. He recalled stumbling upon it while reading about two UCLA researchers' study of Andrei Chikatilo, a temporal notorious serial-killer who killed at least 52 people from 1979 to 1990 in the former Soviet Union.
The UCLA researchers are trying to understand how criminal minds work, how many neurons stimulate each other inside the brain at the time and realized that the pattern of Chikatilo's killings is a Devil's staircase. Liu became intrigued because he realized that earthquakes work similarly, that fault rupture could activate activity on other faults by stress transfer.
Moreover, he added that many large earthquakes that involve the rupture of multiple and variable fault segments in each rupture do not follow the basic assumption of repeated accumulation and release of energy on a given fault plane called the periodic earthquakes model.
Gang Luo of Wuhan University said that the factors that control the clustered events are complex. It involves stress that activates an earthquake, changes in frictional properties and stress transfer between faults or fault segments during a rupture.