Can you forecast earthquakes? This is perhaps the question that most people ask me as an earthquake scientist. We are not there yet, but I’m actively working on it. In a recent publication, my colleagues and I looked at whether earthquakes give us a warning before a bigger one strikes. We’ve observed that many large earthquakes are preceded by smaller events, called foreshocks, but the difficulty is that foreshocks usually look exactly like ordinary earthquakes. Until now, scientists had no reliable way to distinguish between a harmless tremor and a signal that a major rupture was about to begin.
In our recent study published in Geophysical Research Letters, we found that the energy released during foreshocks has a specific pattern that sets them apart from regular earthquakes. We estimated this energy by looking at the velocity envelope of the ground shaking recorded by seismometers preceding major earthquakes. A case in point is the magnitude-7.3 earthquake that preceded the major magnitude-9.1 Tohoku earthquake that struck Japan in 2011. For the latter, this signal reaches its peak value then stays elevated for hours or even days. The plateau is because of the many earthquakes that followed the main event, called aftershocks, that keep the signal elevated. But when we examined the signal of the magnitude-7.3 earthquake, we saw something different. Instead of a stable, long-lasting plateau, the signal went up and down in sharp bursts, forming a jagged “sawtooth” pattern. This unusual behavior suggests that the fault was not releasing energy smoothly but through separate patches slipping one after another, a typical characteristic of foreshock activity.
Seismic signal recorded during the magnitude-7.3 earthquake that preceded the major magnitude 9.1 Tohoku earthquake that struck Japan in 2011. The transformed seismic signal of the foreshock (red line) shows sawtooth profile in contrast with the mainshock (black line) (Source: Giuseppe Petrillo/ Earth Observatory of Singapore).
We observed a similar pattern for foreshocks preceding other major earthquakes, such as the the 2014 magnitude-8.1 Iquique earthquake in Chile, the magnitude-7.0 Ridgecrest earthquake in California, the Amatrice-Norcia sequence in Italy, etc. This gives us confidence that our results can be applied for earthquakes around the world.
At the heart of our interpretation is the idea that earthquakes are collective phenomena. Faults are not uniform surfaces but patchworks of regions that either resist or give way under stress. As tectonic forces load the fault, some patches rupture early, producing small quakes. If surrounding areas are not yet close to failure, the sequence ends there. But if many patches are already critically stressed, one rupture can cascade into others, producing the characteristic sawtooth waveform we identified. This pattern may be the fingerprint of a fault on the verge of breaking.
A conceptual sketch illustrating the progression of the rupture during the foreshocks and main event: isolated patches failing (a), then cascades of interacting foreshocks (b), and finally the full rupture of a mainshock (c) (Source: Giuseppe Petrillo/ Earth Observatory of Singapore).
To test whether we could systematically detect foreshocks, we developed a simple measure, called the Q index, that quantifies how much a waveform deviates from the usual aftershock pattern (long-lasting plateau), using only the first 45 minutes of seismic data after an event. We then analysed 68 earthquakes of magnitude 6 or larger worldwide since 2011. Of these, eleven were followed within ten days by a bigger quake and were thus classified as foreshocks. Remarkably, ten of these eleven foreshocks showed anomalous Q values, while almost all the other earthquakes did not. This means that, with just a simple check of the waveform, we can often tell apart foreshocks from ordinary events.
While we still cannot predict earthquakes with certainty, our findings show that the shaking during and following earthquakes contains crucial information about what can come next. In particular, the sawtooth signal characteristics of foreshocks offer a glimpse of the processes unfolding deep underground, which could help prepare for subsequent larger events.
The lead author and a co-author on the study: Dr Giuseppe Petrillo (left), Research Fellow at the Earth Observatory of Singapore, and Assistant Professor Luca Dal Zilio from the Asian School of the Environment and Earth Observatory of Singapore at Nanyang Technological University Singapore (Source: Earth Observatory of Singapore).
This research contributes to the programme Integrating Volcano and Earthquake Science and Technology (InVEST) and is supported by the Ministry of Education, Singapore, under its MOE AcRF Tier 3 Award MOET32021-0002.