A new study in the Journal of Geophysical Research says stress on San Andreas Fault lines has reached 1,000-year highs at the Cajon Pass fault junction, about 50 miles from downtown Los Angeles. The junction is where the Mojave South segment and the North San Bernardino segment of the San Andreas Fault meet the San Jacinto Bernardino segment of the San Jacinto Fault.
The model estimated stress levels of 2.8 on the Mojave South segment, 1.8 on the North San Bernardino segment and 3.6 on the San Jacinto Bernardino segment. The San Jacinto Bernardino segment was the most heavily loaded portion of the Cajon Pass fault junction.
Cajon Pass fault junction
The study used computer simulations of the last 1,000 years of rupture history. It found that the two fault systems may interact when their stress levels become equal, and it describes the Cajon Pass as an earthquake gate that can influence how ruptures spread.
That matters for Southern California because the junction sits near Los Angeles and can affect whether a rupture stays on one fault or moves across connected fault segments. The study does not put a date on any quake, but it does point to a system that has not released a major earthquake in over 100 years.
San Andreas and San Jacinto faults
Over the past 1,000 years, the San Andreas and San Jacinto fault systems produced at least 36 earthquakes of magnitude 6.4 or greater. In October 1987, Pasadena police officers inspected damage after an earthquake in Southern California, a reminder that strong shaking in the region has repeatedly drawn attention to these fault systems.
The new research was published this month, so the practical question for people in Southern California is not whether the stress exists — the model says it does — but how the next rupture will behave if it starts at or near Cajon Pass. The study leaves that boundary issue open, which is the part that matters most for how far shaking could travel.
Los Angeles and Southern California
People in Southern California are the audience most directly affected by the finding, especially those near Los Angeles, San Bernardino and Pasadena. Heavy traffic along I-15 South in the Cajon Pass has also shown how closely the corridor links daily movement to the fault junction’s location.
The study makes the clearest case for watching the Cajon Pass fault junction itself, not just the individual faults on either side of it. The pressure there is already at a level the model ties to a long rupture history, and the open question is when that stored stress will move into a real earthquake.
