When people think about geological faults, they usually think about earthquakes. Yet faults do not only move during earthquakes. Sometimes they slip silently, without generating noticeable shaking, releasing stress over hours or days through slow fault movements that remain largely hidden from conventional monitoring systems. Scientists have long suspected that these silent motions are an important part of the earthquake cycle. However, because they produce only subtle signals, they are notoriously difficult to detect. As a result, many questions remain unanswered: How often do they occur? Where do they happen? And can they influence subsequent seismic activity?

A research team led by Dr Zahra Zali (GFZ Helmholtz Centre for Geosciences), together with Prof. Patricia Martínez-Garzón (GFZ), Dr David Mencin (EarthScope), and Prof. Gregory C. Beroza (Stanford University), has uncovered a previously hidden population of so-called slow slip events beneath the Parkfield section of California’s San Andreas Fault. Using artificial intelligence and highly sensitive strainmeter observations, the researchers identified dozens of short-duration slow slip events and showed that these silent fault movements are systematically followed by increased low-frequency earthquake activity. The study has been published in Nature Communications.

Searching for hidden fault activity

Parkfield occupies a special place in earthquake science. Located on the San Andreas Fault, it is one of the most intensively monitored fault zones in the world. For decades, researchers have used Parkfield as a natural laboratory to investigate how faults accumulate and release stress. Despite the extensive monitoring network, some fault processes have remained remarkably difficult to observe.

“Faults can move in ways that do not generate strong seismic waves and therefore escape traditional earthquake detection methods,” says lead author Dr Zahra Zali. “We wanted to know whether important fault slip processes might be hidden within years of continuous deformation measurements.”

To search for such hidden signals, the team analysed continuous observations from borehole strainmeters. These instruments are capable of detecting extremely slow and small deformations in the Earth’s crust and are among the most sensitive tools available for monitoring active faults. The challenge, however, is that strainmeters produce enormous volumes of continuous data. Subtle transient signals can easily remain unnoticed among long-term deformation trends, environmental influences, and instrumental noise.

Artificial intelligence discovers what traditional methods missed

To address this challenge, the researchers developed a deep-learning approach capable of automatically identifying characteristic patterns associated with slow fault slip. Rather than searching for predefined signals, the artificial intelligence system learned directly from the continuous strain data and grouped similar deformation patterns together. This allowed the researchers to detect previously unrecognized short-duration slow slip events that release stress in a few hours. 

“These events are difficult to identify by conventional methods because they are small and often hidden within complex background signals,” explains Zali. “Artificial intelligence allowed us to recognize their patterns that would otherwise have gone unnoticed.”

The analysis resulted in the first catalogue of short-duration slow slip events at Parkfield derived directly from continuous strainmeter observations. Independent observations from nearby creepmeters further supported the existence of these events. By estimating the location and direction of slip, the researchers found that the events occurred at shallow depth and were consistent with the right-lateral motion of the San Andreas Fault.

Silent slip and seismicity are connected

The discovery became even more intriguing when the researchers compared the timing of the newly detected slow slip events with low-frequency earthquakes (LFEs), a special class of weak seismic signals associated with fault slip processes. They found that low-frequency earthquake activity increases following the occurrence of slow slip events.

This observation suggests that even small episodes of aseismic fault motion can modify local stress conditions and influence subsequent seismic activity. “Our results show that these slow fault movements are not isolated phenomena,” says Prof. Patricia Martínez-Garzón, Working Group Leader in GFZ-Section 4.2 “Geomechanics and Scientific Drilling” and professor at RWTH Aachen University, who supervised the project. “They appear to be linked to changes in seismic activity, which suggests that slow slip may play an important role in how stress evolves along active faults.”

Filling a missing gap in the earthquake Science

Slow slip events have been studied extensively in subduction zones, where one tectonic plate dives beneath another. However, comparable observations in transform fault systems such as the San Andreas Fault have remained limited, particularly for short-duration events. The new study helps fill this observational gap in slow earthquake research. The researchers also found that the detected events follow the same relationship between event size (seismic moment) and event duration as observed for regular earthquakes. In other words, the way the size of these slow slip events scales with how long they last is similar to that of earthquakes. Together, these findings support the growing view that fault slip occurs across a continuum of behaviors ranging from silent deformation to destructive earthquakes.

A new window into hidden fault processes

The findings highlight the increasing role of Artificial Intelligence in Earth science and demonstrate how machine-learning approaches can reveal previously hidden signals within large geophysical datasets.

The researchers expect that similar short-duration slow slip events may exist on other faults worldwide and that future studies using dense geodetic monitoring networks could uncover additional examples.

“Many important fault processes occur without producing damaging earthquakes,” says Zali. “By detecting these hidden signals, we can gain a more complete picture of how faults behave between earthquakes and how stress is transferred through the Earth’s crust.” 

Original study: 

Zali, Z., Martínez-Garzón, P., Mencin, D. et al.: „Slow slip modulates low-frequency seismicity on the Parkfield segment of the San Andreas Fault.“ Nat Commun 17, 5137 (2026). https://doi.org/10.1038/s41467-026-74095-9