Gates, Hinges and Multi-Segment Ruptures

On June 24, Venezuela experienced a rare and destructive earthquake doublet: a 7.2 foreshock closely followed by a 7.5 mainshock. These were the strongest earthquakes to strike Venezuela in over a century and caused widespread injuries and destruction.

Both earthquakes originated near the junction of the Boconó and San Sebastián faults, and the eastward propagation directed the seismic energy towards Caracas. 

Britannica

In addition to the damage caused, stress-transfer models suggest the doublet has also increased stresses on unruptured segments of San Sebastián beneath Caracas, creating the potential for further large earthquakes in the future.

Open & Closed Gates

The San Andreas and San Jacinto faults intersect at the Cajon Pass, a junction that acts like an “earthquake gate”. Sometimes it blocks a rupture (closed gate). Other times it allows the rupture to propagate across multiple segments (open gate). Which depends on stress conditions and geometry, but the earthquakes that have passed through the gate produce longer, more damaging ruptures. The recent Venezuela doublet appears to have behaved like an “open-gate” event, jumping across the Boconó/San Sebastián junction and spreading east.

The Southern San Andreas ShakeOut Scenario (USGS Circular 1324)

In 2008 the USGS (in collaboration with the Southern California Earthquake Center and other partners), modeled the implications of a hypothetical but realistic 7.8 earthquake on the southern San Andreas Fault, starting near the Salton Sea and moving northwest towards the Los Angeles basis. As a multi-segment rupture, it would cause widespread destruction and injury and (as of today) would cost an estimated ~$1 trillion of economic losses and ~$160 billion of insured losses.

Takeaways

Segmented faults do not always stop earthquake rupture. Under the right stress conditions, the seismic activity cascades across multiple segments causing larger, more damaging events.
High stress junctions need to be closely monitored as we seek to improve our understanding of fault interactions and stress transfer.

The key hazard is not the presence of large faults – it is how those faults interact to amplify impact.

Please contact Haley Sisk, Research Analyst with any questions or comments on this topic.

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