San Andreas May Be a 'Zipper' Fault

The San Andreas Fault, a 1,200-kilometer tectonic boundary in California, has long been studied for its role in earthquake activity. A new theory suggests it may function as a "zipper fault," where intersecting faults merge rather than offset. This hypothesis, still under evaluation, could alter our understanding of fault dynamics and seismic hazards. Here, we explore this concept and its potential implications.

Understanding the Zipper Fault Theory

Introduced by geologists John Platt and Cees Passchier at the 2015 American Geophysical Union meeting, the zipper fault theory proposes that when two faults intersect—especially those with opposing slip directions—they may combine instead of displacing each other. This merging resembles a zipper’s action, either joining or separating based on the faults’ orientation and movement.

The theory originated from observations at Cap de Creus, Spain, where rock shear patterns contradicted traditional fault models. After developing 27 variations, the researchers suggested this mechanism could apply broadly, including to the San Andreas Fault, offering a new perspective on fault interactions.

Application to the San Andreas Fault

The San Andreas Fault, a right-lateral strike-slip fault, defines the boundary between the Pacific and North American plates. It intersects with other faults, such as the left-lateral Garlock Fault near Gorman, approximately 60 miles north of Los Angeles. Conventionally, one fault would be expected to offset the other at such junctions. However, the zipper fault theory posits that these faults merge, potentially affecting their slip behavior.

At the San Andreas/Garlock intersection, the theory predicts a lower slip rate north of the junction compared to the south. Researchers have proposed using GPS measurements to test this hypothesis, which could clarify whether this merging occurs and how it influences fault movement. This may also relate to the fault’s varied behavior—creeping steadily in its central segment between Parkfield and Hollister, while remaining locked in the northern and southern sections.

Implications for Earthquake Risk Assessment

If the San Andreas Fault exhibits zipper-like behavior, it could impact earthquake risk assessment. A merging junction might redistribute tectonic stress differently than an offsetting one, potentially influencing rupture patterns or quake magnitude. This could help explain the contrast between the fault’s creeping and locked segments, offering insights into stress transfer.

Current Research and Evidence

The zipper fault theory has generated interest but limited progress. Initial discussions highlighted its potential to explain fault behavior globally, from Turkey’s Anatolian faults to Tibet’s Karakoram system. Yet, as of now, few follow-up studies have emerged. The proposed GPS testing at the San Andreas/Garlock junction indicates ongoing investigation, but conclusive data is lacking, leaving the theory in an early, unverified stage.

San Andreas and Garlock: A Key Intersection

The intersection of the San Andreas and Garlock faults near Gorman is central to this theory. Platt noted this location as a critical example, where merging could account for observed fault interactions. If validated, this could redefine how we interpret fault junctions, not only in California but in diverse tectonic settings worldwide, such as thrust belts or core complexes.

A Theory in Progress

The possibility that the San Andreas Fault operates as a zipper fault presents an intriguing hypothesis. It suggests a new mechanism for fault interaction that could enhance our understanding of seismic activity.