Intersecting orthogonal strike‐slip faults with opposite senses of slip pose the question of what allows rupture to propagate through the junction and through both faults versus confining rupture to ...a single fault. I conduct dynamic rupture simulations on simplified orthogonal strike‐slip fault systems, to determine which conditions produce rupture on both component faults. In models with uniform initial tractions on both faults, slip on the first fault must reduce normal stress on the second fault for it to rupture. If the first fault ends at the cross fault, a stopping phase causes the cross fault to rupture. In models where I resolve a uniform regional stress field on the faults, only a narrow range of stress orientations allow multifault ruptures. These results will be helpful for evaluating hazard near orthogonal strike‐slip faults.
Plain Language Summary
There are many examples around the world where two strike‐slip earthquake faults cross each other at nearly 90° angles. This is not remarkable when only one of the faults in a pair causes an earthquake, but it becomes notable when two or more crossing faults move at the same time. This raises the question of what causes the second fault to get involved, or not. To address this question, I use computer simulations of the physics of the earthquake process to test dozens of different fault configurations and earthquake starting points. I find that the location where the earthquake starts on the first fault controls whether the second fault is made stronger versus weaker, and therefore whether both faults can move together in one earthquake. These results can help us understand earthquake hazard around crossing faults.
Key Points
Nucleation location effectively controls whether multifault rupture occurs on orthogonal strike‐slip fault systems
A stopping phase from rupture reaching the end of one fault is often required to initiate rupture on the cross fault
Only a narrow range of regional stress orientations allows both cross‐faults to rupture
The largest earthquakes of the 2019 Ridgecrest, California, sequence were a M6.4 left‐lateral rupture followed 34 hr later by a M7.1 on a perpendicular right‐lateral fault. We use dynamic rupture ...modeling to address the questions of why the first earthquake did not propagate through the right‐lateral fault in one larger event, whether stress changes from the M6.4 were necessary for the M7.1 to occur, and how the Ridgecrest earthquakes affected the nearby Garlock Fault. We find that dynamic clamping and shear stress reduction confined surface rupture in the M6.4 to the left‐lateral fault. We also find that stress changes from the M6.4 were not necessary to allow a M7.1 on the right‐lateral fault but that they affected the slip and likely accelerated the timing of the M7.1. Lastly, we find that the Ridgecrest earthquakes may have brought the central Garlock Fault closer to failure.
Plain Language Summary
The M6.4 and M7.1 Ridgecrest, California, earthquakes of July 2019 occurred 34 hr apart, on two faults that cross each other. We used physics‐based computer simulations of the earthquake process to investigate why both faults did not move together in one bigger earthquake and whether the second earthquake only happened due to effects from the first. We found that the fault movement in the first earthquake compressed the second fault, which prevented it from moving at the same time. We also found that the second fault could have had a M7.1 earthquake on its own, without the influence of the M6.4 on the previous day but that the first earthquake affected the details of the second and likely made the second one happen sooner than it would have otherwise. This has meaning both for understanding why the Ridgecrest earthquakes happened this way and also for understanding possible earthquake behaviors on other crossing faults. We also looked at whether the Ridgecrest earthquakes brought the nearby Garlock Fault, which is capable of a M8 earthquake, closer to having a big earthquake, and we found that this is possible but not certain.
Key Points
Dynamic stress changes from the first Ridgecrest earthquake were insufficient to immediately trigger the second earthquake
Although the first earthquake did not create the conditions necessary for the second to occur, it affected the slip distribution and timing
Stress changes from the Ridgecrest ruptures explain the location of creep on the Garlock Fault and may have brought it closer to failure
Abstract
The largest earthquakes of the 2019 Ridgecrest, California, sequence were a
M
6.4 left‐lateral rupture followed 34 hr later by a
M
7.1 on a perpendicular right‐lateral fault. We use dynamic ...rupture modeling to address the questions of why the first earthquake did not propagate through the right‐lateral fault in one larger event, whether stress changes from the
M
6.4 were necessary for the
M
7.1 to occur, and how the Ridgecrest earthquakes affected the nearby Garlock Fault. We find that dynamic clamping and shear stress reduction confined surface rupture in the
M
6.4 to the left‐lateral fault. We also find that stress changes from the
M
6.4 were not necessary to allow a
M
7.1 on the right‐lateral fault but that they affected the slip and likely accelerated the timing of the
M
7.1. Lastly, we find that the Ridgecrest earthquakes may have brought the central Garlock Fault closer to failure.
Plain Language Summary
The
M
6.4 and
M
7.1 Ridgecrest, California, earthquakes of July 2019 occurred 34 hr apart, on two faults that cross each other. We used physics‐based computer simulations of the earthquake process to investigate why both faults did not move together in one bigger earthquake and whether the second earthquake only happened due to effects from the first. We found that the fault movement in the first earthquake compressed the second fault, which prevented it from moving at the same time. We also found that the second fault could have had a
M
7.1 earthquake on its own, without the influence of the
M
6.4 on the previous day but that the first earthquake affected the details of the second and likely made the second one happen sooner than it would have otherwise. This has meaning both for understanding why the Ridgecrest earthquakes happened this way and also for understanding possible earthquake behaviors on other crossing faults. We also looked at whether the Ridgecrest earthquakes brought the nearby Garlock Fault, which is capable of a
M
8 earthquake, closer to having a big earthquake, and we found that this is possible but not certain.
Key Points
Dynamic stress changes from the first Ridgecrest earthquake were insufficient to immediately trigger the second earthquake
Although the first earthquake did not create the conditions necessary for the second to occur, it affected the slip distribution and timing
Stress changes from the Ridgecrest ruptures explain the location of creep on the Garlock Fault and may have brought it closer to failure
The Bartlett Springs Fault (BSF), the easternmost branch of the northern San Andreas Fault system, creeps along much of its length. Geodetic data for the BSF are sparse, and surface creep rates are ...generally poorly constrained. The two existing geodetic slip rate inversions resolve at least one locked patch within the creeping zones. We use the 3‐D finite element code FaultMod to conduct dynamic rupture models based on both geodetic inversions, in order to determine the ability of rupture to propagate into the creeping regions, as well as to assess possible magnitudes for BSF ruptures. For both sets of models, we find that the distribution of aseismic creep limits the extent of coseismic rupture, due to the contrast in frictional properties between the locked and creeping regions.
Key Points
Rupture on the BSF is confined to locked patches
The BSF can still produce strong earthquakes
Geodetic inversions can be used to inform rupture model setup
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