San Onofre Loop to Cristianitos Fault

The Cristianitos Fault is a major earthquake fault line located less than one mile from the San Onofre nuclear power plant. This hike takes you on a 3.8 mile loop through San Onofre State Beach to the site of the fault line. Along the way, you’ll see the Monterey Formation, which holds most of the oil and gas deposits known in the LA basin and off the coast, as well as the abrupt end of the San Mateo formation at the fault line. It’s a hike and a geology lesson rolled into one!

According to About.com, “the Monterey Formation is a widespread, thick body of silica-rich rock laid down in Miocene time, approximately 16 to 6 million years ago. It’s easily recognized throughout the Coast Ranges by its rhythmic bedding and cherty nature, although parts of it are shaley and full of fossils.” You’ll see this formation, as well as the distinctive sand-colored band of the San Mateo formation on either side of the Cristianitos fault.

The bluffs along San Onofre State Beach are marked by six trails that lead from the parking areas down to the beach. Each trails is numbered 1-6 from north to south. Trail 6 is the last trailhead to the beach, and the area is famous for nude sunbathing. Despite signs warning that this is not permitted, it still goes on today.

For this trip, I took Trail 3 down to the beach, then worked my way north towards the Cristianitos fault. The composition of the beach changes over time. Sometimes it may be sand; other times covered in well-worn stones. This particular trip most of the beach was covered in stones, making walking more difficult.

Heading north along the beach was a slog through the large, loose stones. I almost wished I wore boots. As you can see in the next photo, the bluffs are constantly changing due to erosion, so stick to the marked trails where possible and be prepared for the unexpected… like this drop at the bottom of Trail 2:

As you continue north, keep your eyes open for the distinctive striated shale of the Monterey Formation.

At about the two mile mark, you reach a gap in the bluff that marks the Cristianitos Fault.

There is no signage, but you’ll see that the bluff to the north has a distinctive sand-colored band known as the San Mateo Formation. This band abruptly ends at the gap — marking the location of the Cristianitos Fault.

This light-colored band in this photo is that San Mateo Formation.

At north end of this band is a gap; this is the Cristianitos fault.

You can scramble up the hill (with caution) and actually touch the strike plate of the fault.

Geologists have determined the last activity of the fault by the layer of rubble immediately above the San Mateo formation, which has remained relatively undisturbed for about 120 thousand years. By definition, an “inactive” fault has had no major activity for 100 thousand years, so the Cristianitos fault clearly falls into the inactive category.

After visiting the fault, we retraced our steps south, taking Trail 1 up the bluff and following the bluff back to our car.

The Geology of the Cristianitos Fault Explained in Video

My friend John is a geologist by education, and graciously shared some insights into the Cristianitos fault in these videos.

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Trail Map and Elevation Profile

You can lengthen or shorten this loop by starting at a different trail. Trail 1 is the closest to the fault; Trail 6 is the furthest. The map below shows our route, which began at Trail 3.

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Essential San Onofre State Beach Resources

• Official San Onofre State Beach website
• SoCal Hiker’s Inside San Onofre State Beach – Coming soon!
• EveryTrail Guide to San Onofre State Beach – Coming soon!