A peer-reviewed study published in the Journal of Geophysical Research: Solid Earth found that modelled stress on several fault segments around the pass is at or above the highest levels reached during a simulated 1,000-year earthquake history. The findings sharpen understanding of how large earthquakes could spread across Southern California but do not predict when one will occur.
Researchers led by geophysicist Liliane Burkhard built a four-dimensional earthquake-cycle model combining geological records of past ruptures with fault slip rates and geodetic constraints. The simulation tracked the accumulation, release and redistribution of Coulomb stress along the southern San Andreas Fault System over roughly a millennium.
By 2025, the model estimated mean Coulomb stress of 2.8 megapascals on the Mojave South segment of the San Andreas, 1.8 MPa on the North San Bernardino segment and 3.6 MPa on the San Jacinto Bernardino segment. The latter two estimates exceeded the characteristic pre-earthquake stress ranges identified for those segments in the model.
The researchers described Cajon Pass, north-east of Los Angeles, as a conditional “earthquake gate”. The junction links fault sections whose stress state may determine whether a rupture stops there or continues from one fault system into another.
“Our results show that stress levels on multiple fault segments are now at or above the highest values seen in the past millennium and that the region may be capable of a large through-going rupture involving both fault systems,” Burkhard said when the findings were released by the University of Hawaiʻi at Mānoa.
The study found that joint ruptures appear more plausible when stress levels on the Mojave South and San Jacinto Bernardino segments are closely aligned. For the modelled 1812 Wrightwood earthquake, which researchers treated as a rupture crossing the junction, the difference between stress on those segments was less than 0.2 MPa.
That pattern contrasts with the 1857 Fort Tejon earthquake, a magnitude 7.9 event that ruptured more than 330 kilometres of the San Andreas system but stopped north of Cajon Pass. The comparison supports the hypothesis that the junction can sometimes block rupture propagation and at other times allow it to continue.
The authors cautioned, however, that stress alone does not provide a clock for earthquakes. Their modelling found no single universal stress threshold that consistently preceded rupture. Each fault segment instead displayed its own characteristic range, while fault geometry, material properties and interactions among neighbouring segments can also affect how an earthquake develops.
The work therefore concerns earthquake potential rather than short-term prediction. Scientists cannot determine the date of a major earthquake from the calculated stress values, and a highly loaded fault may remain locked for an uncertain period.
Stress is accumulating fastest north of Cajon Pass, where the Mojave South segment gains about 1.8 MPa per century, according to the model. Rates south of the junction are lower, at roughly 1.0 to 1.5 MPa per century, reflecting differences in long-term slip rates.
The simulation was constrained by a 1,000-year paleoseismic record assembled from evidence at fault sites, including displaced sediments dated by radiocarbon methods and other geological indicators. It also incorporated continuing plate motion and the slower relaxation of the material beneath the elastic crust after earthquakes.
The San Andreas and San Jacinto systems together accommodate most of the relative motion between the Pacific and North American plates in Southern California. Their proximity around Cajon Pass makes the junction particularly important for assessing whether a large rupture could involve multiple fault sections.
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