Identifying active fault zones can help scientists determine whether an area is prone to surface rupture; finding features associated with ancient volcanoes may help.
By Thystere Matondo Bantidi, Ph.D., Temblor science writing extern (@Thysterebantidi)
Citation: Bantidi, T., 2023, Old dike swarms could influence future fault ruptures, Temblor, http://doi.org/10.32858/temblor.323
On July 4, 2019, at 10:34 a.m. local time, a magnitude-6.4 earthquake startled Southern California. Even more surprising was the magnitude-7.1 mainshock that followed nearly 34 hours later. These events — part of the Ridgecrest earthquake sequence — occurred on largely unmapped faults. The mainshock was the largest earthquake to occur in southern California in 20 years since the 1999 magnitude-7.1 Hector Mine earthquake.
These earthquakes result from the relative motion between the Pacific and North American plates, primarily accommodated by the San Andreas Fault System, which trends in a northwest direction. Most of the remaining motion occurs on similarly trending fault zones further inland, like the Eastern California Shear Zone, which hosted the Ridgecrest sequence. However, the geometry of the Ridgecrest earthquakes was complicated, with a network of interlaced faults rupturing orthogonal (or perpendicular) to one another.
Scientists are grappling with the question of how to explain the complexity of this fault geometry, which presents a challenge in assessing regional seismic hazards. A recent study published in Nature Communications led by U.S. Geological Survey scientist Johanna M. Nevitt explores the connection between active faults and much older igneous dikes in southeastern California — where the Ridgecrest earthquake sequence struck four years ago.
When a crack cuts through rock and fills with magma, that cross-cutting feature is called a dike. When a collection of dikes intrudes into continental crust, scientists call this a dike swarm. California’s Independence dike swarm is a large group of northwest-trending dikes that formed 148 million years ago. This ancient feature forms a significant fabric in Earth’s crust, and could be a zone of weakness.
“Large earthquakes in southeastern California have occurred within the footprint of the Independence dike swarm,” says Nevitt. Understanding how the dikes influence the mechanical properties of the crust could be important in trying to understand the occurrence of these large earthquakes, she says, which also include the 1872 magnitude-7.4 Owens Valley earthquake and the 1992 magnitude-7.3 Landers earthquake.
Using remote sensing data, field observations, and mechanical modeling of the 2019 magnitude-7.1 Ridgecrest earthquake, Nevitt and colleagues explored whether the geological history of southeastern California played a role in controlling the slip distribution — where and how much the Earth’s surface broke during this event. In particular, they looked at factors like rock type, basin thickness and structural complexity, finding that none of these played a clear role.
Instead, they found that the slip distribution was primarily controlled by the geometry of the fault and how stress is oriented in the crust. Fault geometry closely follows the surrounding Independence dike swarm. The team suggests that the dikes and their associated fractures serve as weakened zones that have now become a series of faults.
This finding may help explain the origin of orthogonal faulting in the Eastern California Shear Zone, which includes the area that ruptured in the 2019 Ridgecrest earthquake sequence. “Previous studies have shown that anisotropy can lead to orthogonal faulting,” says Nevitt. In the case of Ridgecrest, the magnitude-6.4 foreshock struck on a fault oriented in a northeast direction, whereas the magnitude-7.1 mainshock ruptured in a northwest direction perpendicular to the foreshock, and largely in line with the Independence Dike Swarm. The dikes provide the anisotropy.
“If individual structures [like dikes and fractures within the Independence dike swarm] have the potential to reactivate,” says Nevitt, then faults forming along these existing zones of weakness might make sense. “These zones provide the planes that are ready to slip.”
However, “such causal relationship between dikes, rotation of faults and regional seismicity may not be straightforward and additional geological investigations could be necessary to nurture discussion,” says Masayuki Kano, an assistant professor of geophysics at Tohoku University who was not involved with the study.
Earthquakes wreak havoc in part because seismic waves cause the ground to shake. They also cause damage when Earth’s surface breaks, which doesn’t happen in all earthquakes. Significant surface rupture can cause damage to infrastructure, buildings, and homes. Though it’s not possible to predict exactly when an earthquake will strike or how large a future quake might be, identifying active fault zones can help scientists anticipate areas prone to surface rupture.
When existing crustal structures are repeatedly activated, they tell scientists about the likelihood of where future structures may move — like faults rupturing in major earthquakes. By investigating geological and geophysical features of these reactivated crustal structures, scientists would have a better idea of where surface faulting might be expected prior to an earthquake, Kano says. Such information can be valuable in mitigating seismic risks. For instance, if infrastructure like nuclear power plants, dams, or high-rise buildings exist near a potentially active fault zone — or if such infrastructure is slated to be built — scientists can evaluate ground motions based on the assumed surface faulting, and take proactive measures, he says.
Seismic hazard and risk mitigation has great interest among both the public and emergency management officials. Although not yet tested broadly, the findings of this study can conceptually be applied now. Future work will test whether crustal weakness caused by dikes can explain faulting, both in Ridgecrest and elsewhere, Nevitt says.
Nevitt, J. M., Brooks, B. A., Hardebeck, J. L., & Aagaard, B. T. (2023). 2019 M7.1 Ridgecrest earthquake slip distribution controlled by fault geometry inherited from Independence dike swarm. Nature Communications, 14(1), 1546.
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