A segment of the San Andreas fault, known as Parkfield in Central California, where earthquakes occur regularly, might be showing preliminary signs preceding an earthquake, according to recent research. This area typically experiences tremors approximately every 22 years, with its last significant quake happening in 2004, indicating a potential impending earthquake. However, the current signals are not consistent with those observed before the previous rupture, suggesting either a delayed occurrence or a different epicenter location. Luca Malagnini, the lead author of the study and the director of research at the National Institute of Geophysics and Volcanology in Italy, emphasizes the uncertainty until the actual event unfolds.
The San Andreas fault delineates the boundary between the Pacific and North American tectonic plates. South of Parkfield, the fault remains locked, while north of it, the plates move freely, albeit slowly. Parkfield serves as a transitional zone between these contrasting plate movements. When seismic activity occurs in this region, it typically results in earthquakes of around magnitude 6, which, due to the area’s remote location, pose minimal threats to life or property directly. Nevertheless, such quakes can influence stress levels on nearby faults, underscoring the importance of monitoring Parkfield closely for predictive insights.
Researchers aspire to identify reliable precursors to earthquakes, such as rock strain or changes in subsurface permeability, to enhance early warning systems. Parkfield’s recurrent seismic events make it a promising site for such investigations, although definitive clues remain elusive thus far.
In their recent study, Malagnini and colleagues examined seismic wave attenuation, which relates to how sound waves lose energy traversing Earth’s crust. Before the 2004 earthquake, the attenuation of low-frequency waves increased while high-frequency waves decreased in the six weeks preceding the event. This phenomenon reflects the stress on rocks as the Pacific plate moves against the North American plate, resulting in the opening of long cracks in the subsurface, which absorb strain, while shorter cracks close up. However, current data suggests that while Parkfield may be nearing the end of its seismic lull, there’s no clear evidence of the same attenuation patterns observed prior to the 2004 quake. Malagnini speculates that an earthquake may occur this year but anticipates differences in its characteristics compared to the previous event.
Malagnini refrains from pinpointing the exact timing of the next quake but anticipates that post-event analysis will yield valuable insights for future predictions. He expresses readiness to observe the forthcoming earthquake and glean lessons for future monitoring endeavors.
