Knock, knock, knocking on your door – the Julian earthquake in southern California issues reminder to be prepared

Minimal damage from the Julian earthquake should result in maximum earthquake preparedness efforts.
 

By Debi Kilb (Scripps Institution of Oceanography at UCSD), Alice-Agnes Gabriel (Scripps Institution of Oceanography at UCSD & LMU Munich), John Rekoske (Scripps Institution of Oceanography at UCSD), and Duncan Agnew (Scripps Institution of Oceanography at UCSD)
 

Citation: Kilb, D., Gabriel, A.-A., Rekoske, J. and Agnew, D., 2025, Knock, knock, knocking on your door – the Julian earthquake in southern California issues reminder to be prepared, Temblor, http://doi.org/10.32858/temblor.362
 

Those who live in California know it is earthquake country, and they need to be prepared. But how prepared? Should you plan to rely on your neighbor’s generator and stocked garage freezer for food? Do you really need three or more days of water? Trust us, you don’t want to shake your head after a large quake, saying ‘if only I had …’.

The April 14 magnitude 5.2 earthquake near Julian, a small town about 40 miles northeast of San Diego, was a good reminder for southern California residents to be prepared for earthquakes (Figure 1). This quake packed quite a wallop and was felt from Santa Barbara to Ensenada. More than 40,000 people logged on to the USGS ‘Did You Feel It’ webpage to report what they experienced. Luckily, this earthquake caused minimal damage, but it’s a good reminder to get your earthquake response supply kits ready.

In Southern California, the last 10 to 12 years have been relatively seismically quiet, aside from the 2019 Ridgecrest earthquake sequence. Over the 92.25 years of recorded seismic network data, a year with no or only one magnitude 5 or larger earthquake is not at all unusual: 45% of the years fall into this category. But 80% have at least one earthquake that’s magnitude 5 or higher, so a magnitude 5.2 earthquake, such as the one that occurred near Julian, is not considered unusual either (Table 1).
 

 

The why and the where

The 2025 Julian earthquake occurred within the Elsinore fault system, which is one of a triad of major fault systems in southern California – Elsinore, San Jacinto, and San Andreas. The Elsinore fault has an estimated slip rate of between 4 and 5 millimeters per year, much less than the 20 to 25 millimeters per year of the southern San Andreas Fault. The Elsinore system is also less active than the San Jacinto fault system.

The Julian earthquake occurred in a region with relatively low seismicity rates. The 190-mile-long strike-slip fault, consisting of 7 named sections (Working Group on California Earthquake Probabilities, 1995), extends from near the U.S.-Mexico border, through San Diego County, to the northern end of the Santa Ana Mountains near Los Angeles. This fault is capable of producing a quake as powerful as magnitude 7.5 and may connect to the Whittier fault (Rockwell, 1989).

The Julian section in the southeast part of the Elsinore Fault Zone cuts diagonally across various Peninsular Range batholithic and pre-batholithic metamorphic terrane. The southeastern extension of the Elsinore Fault Zone, the Laguna Salada fault, ruptured in 1892 in a magnitude 7 earthquake (Hough and Elliot, 2004), but the main trace of the Elsinore Fault Zone has only seen one historical event greater than magnitude 5.2 — the earthquake of 1910, a magnitude 6 shock near Temescal Valley that produced no known surface rupture and did little damage.
 

 

There’s a movie for that

Immediately after the Julian earthquake, two of us, John Rekoske and Alice-Agnes Gabriel, seismologists from Scripps Institution of Oceanography, used a supercomputer (called Super-MUC located in Munich, Germany) to generate a simulation of the expected ground motions from the earthquake.

They numerically simulated the 3D propagation of seismic waves using earthquake source information that was rapidly available from the USGS. This information includes how the fault moved (i.e., strike, dip, rake, and hypocentral depth of the earthquake). The model consists of a 3D community subsurface velocity model for Southern California (SCEC CVM-H v15.1.0; Shaw et al., 2015), made available by the Statewide California Earthquake Center, including high-resolution topography. This simulation used the open-source software SeisSol and required 14 minutes of run-time on 768 cores, the equivalent of more than one week on just one core. The output data were turned into a movie depicting the temporal evolution of the seismic wavefield in terms of the ground’s up-and-down motions (see Figure 2 for a snapshot from the movie).
 

 

We can learn a lot from this movie. First, notice how the seismic waves travel from Julian to the Los Angeles region (about 100 miles) in only 31 seconds, equivalent to speeds over 12,000 miles per hour, about 20 times faster than a commercial jet. Also of note is the tail end of the movie, where the only remaining ground motions are near Los Angeles. Now that’s odd. Why would Los Angeles still be shaking while there is no longer motion near the mainshock?
 

 

We can learn why by thinking about Jell-O. What happens if you have a big bowl of Jell-O and give it a quick and hard shake? It wiggles back and forth for a relatively long time after the initial jolt. Nothing would happen if you did the same thing with a bowl of concrete. The difference is that Jello-O has minimal rigidity, a measure of how much force you need to change the shape of a material in a sideways manner. The Los Angeles basin, which is about 10 kilometers deep at its maximum, contains low rigidity unconsolidated sediments, similar to the Jello-O scenario. When seismic waves come whizzing through, reverberations of the seismic energy are created within the bounding basin.
 

Ground motions higher than expected

The observed peak ground velocities produced by the Julian earthquake delivered an unexpected punch in some regions. These velocities appear higher than predicted by standard empirical ground motion models (Figure 3, left). The local geology may partly explain this discrepancy (Figure 3, right). Much of the region between Julian and the outskirts of Los Angeles consists of hard granitic bedrock, which transmits seismic waves more efficiently than softer sedimentary materials.

Think of it this way, you can run faster and not get as tired on a nice flat cement sidewalk than through soft sand. In the same way, the stronger granitic rocks exhibit lower intrinsic attenuation, allowing seismic energy to travel farther with less energy loss. As a result, strong ground motions reached areas well beyond the immediate vicinity of the earthquake’s starting point. This highlights the importance of
accounting for location-specific effects that can systematically influence how seismic waves travel and amplify. For example, using detailed 3D attenuation models and local site conditions helps account for these so-called nonergodic effects. In doing so, we can go beyond statistical averages and improve the accuracy of seismic hazard assessment.
 

 

Bingo – an exact match

Mother Earth does not always produce what we expect. However, one thing that (so far) was as expected was the size of the largest aftershock. Back in 1965, Marcus Båth proposed a rule of thumb that the largest aftershock is typically 1.2 magnitude units smaller than the mainshock. For this magnitude 5.2 quake, this rule suggests the largest aftershock would be magnitude 4.0. And, lo and behold, that’s what we have observed at the time of writing (Figure 4).
 

 

What’s next?

Californians should expect earthquakes; it’s a way of life for us. Consider subscribing to ShakeAlert, the earthquake early system for the West Coast (California, Oregon, and Washington). For Android users, Google will send you push notifications alerts when large ground shaking from earthquakes is expected at your location. However, iOS users will need to download the MyShake app from the App Store to receive alerts. These alert systems aim to issue alerts to the public a few seconds or tens of seconds before the arrival of strong shaking from earthquakes. For the Julian quake, initial estimates report that alerts were sent to over 700,000 smart phones via the MyShake app, and millions more were alerted via Google alerts to Android phones and wireless emergency alerts.

Your earthquake plan should not rely solely on the kindness of your neighbors and friends. Some easy actions you can take now include ensuring you always have at least half a tank of gas in your car, storing three days of water (1 gallon per person per day; FEMA P-2151), and having cash on hand in case the infrastructure supporting credit card purchases is not operable.

Take the time to go room-to-room in your home and ask yourself, if I felt an earthquake shaking, where would I go and what would I do? No matter what you have in your bookcases, bolt them to the walls, remove items hanging over your bed or locations where you commonly reside. Encourage the practice of drop-cover-hold-on drills. These simple steps can make a big difference, and we hope you take heed.
 

Science Editor: Dr. Alka Tripathy-Lang, Ph.D.
Reviewer: Dr. Gabrielle Tepp, Ph.D.
 

References

California Institute of Technology (Caltech) (1926): Southern California Seismic Network. International Federation of Digital Seismograph Networks. Other/Seismic Network. doi:10.7914/SN/CI

Chen, Y., & Liu, M. (2023). Long‐lived aftershocks in the New Madrid seismic zone and the rest of stable North America. Journal of Geophysical Research: Solid Earth, 128, https://doi.org/10.1029/2023JB026482

Federal Emergency Management Agency. (2021). Food and Water in an Emergency (FEMA P-2151). U.S. Department of Homeland Security.
https://www.fema.gov/sites/default/files/documents/fema_food-water-emergency.pdf

Hauksson, E., and P. M. Shearer. (2006). Attenuation models (QP and QS) in three dimensions of the southern California crust: Inferred fluid saturation at seismogenic depths, J. Geophys. Res., 111, B05302, doi:10.1029/2005JB003947.

Hough, S.E. and Elliot, A. (2004). Revisiting the 23 February 1892 Laguna Salada earthquake: Bulletin of the Seismological Society of America, v.94, no. 4, p 1571-1578.

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Rockwell, T.K. (1989). Behavior of individual fault segments along the Elsinore-Laguna Salada fault zone, southern California and northern Baja California—Implications for the characteristic earthquake model, in Schwartz, D.P., and Sibson, R.H., eds., Proceedings of Conference XLV—Fault segmentation and controls of rupture initiation and termination: U.S. Geological Survey Open-File Report 89-315, p. 288-308.

Ross, Z. E., & Cochran, E. S. (2021). Evidence for latent crustal fluid injection transients in Southern California from long‐duration earthquake swarms. Geophysical Research Letters, 48. https://doi.org/10.1029/2021GL092465.

Shaw, J. H., Plesch, A., Tape, C., Suess, M. P., Jordan, T. H., Ely, G., et al. (2015). Unified structural representation of the Southern California crust and upper mantle. Earth and Planetary Science Letters, 415, 1–15. https://doi.org/10.1016/j.epsl.2015.01.016.

Trugman, D. T., & Ross, Z. E. (2019). Pervasive foreshock activity across southern California. Geophysical Research Letters, 46(15), 8772-8781. https://doi.org/10.1029/2019GL083725.

Working Group on California Earthquake Probabilities. (1995). Seismic hazards in southern California—Probable earthquakes, 1994 to 2024: Bulletin of the Seismological Society of America, v. 85, no. 2, p. 379-439.

 

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