Small earthquakes can have a big impact on the movements of major faults

Seismic symphonies of minor earthquakes may affect grand movements on major faults.

Small and distant earthquakes can disrupt the growth of slow-slip events — gradual fault movements that can release tremendous amounts of energy at gentle tempos, a new analysis of seismic data suggests. Reported in the May 16 Science Advances, the research shows that the more frequently small earthquakes occur near a fault’s slow-slip zone, the less synchronized the slipping becomes.

Megathrust faults — massive fractures where one tectonic plate pushes under another — are notorious for hatching Earth’s most devastating temblors. These same faults can also slide steadily for days or weeks in slow-slip events while emitting faint vibrations called tectonic tremor. First identified around two decades ago, these softly humming events can shift stress on faults and may influence the timing of large, destructive earthquakes. But it’s not clear why these enigmatic events can grow for hundreds of kilometers along some faults while remaining restricted to small parts of others.

The new study shows that minor quakes can impact these big faults from tens of kilometers away, at distances where any shaking from those quakes would probably be unnoticeable, says geophysicist Heidi Houston of the University of Southern California in Los Angeles, who was not involved in the work. “They’re looking at earthquakes within 50 kilometers of tremor … and most of these earthquakes are quite small.”

Slow-slip events, sometimes called slow earthquakes, typically occur near the edges of the part of a megathrust fault that births the largest temblors. Monitoring such events can help reveal where the active edges of this volatile zone are, says geophysicist Gaspard Farge. “We need anything that we can get in the quiet intervals between the earthquakes to try to understand what the system does.”

For the new study, Farge and geophysicist Emily Brodsky, both of the University of California, Santa Cruz, analyzed records of earthquake activity and slow-slip events from megathrust faults in Japan and Cascadia, as well as fault zones in Alaska, New Zealand, California and Taiwan.

For each fault, they measured how often earthquakes larger than magnitude 2.2 struck within 50 kilometers of the fault. Then they compared that information to how much of the fault slipped at the same time during slow-slip events, assessing how synchronously the fault behaved.

“We see that the higher the rate of small earthquakes around the slow-slip region, the less synchronized the activity is,” Farge says. At the distances involved, the quakes probably do not directly move the rock on the fault. Instead, seismic waves generated by the quakes may alter stresses on the fault as they pass by.

This effect may be analogous to how dawn disrupts the synchronized flashing of fireflies, Farge says. At night, each firefly can see more of its neighbors flashing in the dark, but the arrival of dawn makes it harder to discern those flashes. Likewise, he says, when many small quakes occur, different parts of a fault may experience varying stimuli and become less likely to move in tandem.

“It’s a pretty reasonable idea, and I think that they show that it works,” Houston says. Scientists have shown that faults with slow-slip events are sensitive enough to be influenced by lunar tides. Given that, she says, perhaps it’s no big surprise that little quakes in large numbers could bump things out of sync.