Yesterday, April 30, 2019, at 10:10 am, a small, magnitude 3.4 earthquake rocked the seafloor 25 kilometers northwest of San Simeon. That’s just offshore from where the Monterey/San Luis Obispo County line meets the coast; between Salmon Creek and Ragged Point. Local media reported it took place on the San Simeon Fault.
In other words, the quake took place immediately adjacent to some of the most unstable and landslide prone slopes in Big Sur. It was fortunate that this quake was so small, because larger – much larger – earthquakes are distinctly possible along the Big Sur coast.
The epicenter of yesterday’s earthquake was just off this segment of the southern Big Sur coast. The cattle grazing in Kozy Kove Meadow make a peaceful, bucolic scene, but it’s worth remembering that the flat is, in fact, the top of an enormous pile of rubble; the remnants of a massive landslide that very recently (in geologic time) slid into the ocean in this location.
A little history:
On October 22, 1926, Monterey experienced an unusual double earthquake. Two strong shocks of magnitude 6.1 and magnitude 6.3 occurred almost exactly an hour apart, at 4:35 and 5:35 in the morning. They were the strongest jolts felt in Monterey since the great Northern California Earthquake of 1906 and, due to the closeness of the epicenter, it is likely that Monterey has not been shaken as strongly since.
The event was unusual not only because there were two quakes and they were both so strong, but also because, unlike most earthquakes felt on the Monterey Peninsula, these quakes did not occur on the San Andreas Fault. Instead, they were located out at sea; probably somewhere a bit to the north of Cypress Point.
Although further from the epicenter, the shaking in Santa Cruz, and on the coast north of Santa Cruz, was worse; suggesting that, once started, the rupture may have traveled northward; focusing the released energy in that direction. While there was little damage on the Monterey Peninsula, plate glass windows shattered along Pacific Avenue in Santa Cruz, chimneys fell, plaster cracked, and unreinforced brick buildings sustained damage. Up the coast, a water main was severed at Laguna Creek, groceries were thrown from the shelves of the Davenport store and the lighthouse on Año Nuevo Island was damaged.
The quakes were strong enough to be felt over 100,000 square miles, from Healdsburg, in Sonoma County, to Lompoc, in Santa Barbara County, and east to the Sierra.
While little was known of the mechanisms responsible for California earthquakes in 1926, and still less was known in 1906, we now know that the 1906 earthquake was caused by the sudden release of strain accumulated as the Pacific plate moves steadily northward relative to the North American plate at a rate of about two inches per year. Specifically, the 1906 earthquake occurred along the San Andreas Fault, the zone where the rock overlying the moving plates most often ruptures; allowing everything west of the fault to continue riding north on the Pacific Plate.
At 5:12 am, on Wednesday, April 18, 1906, the accumulated strain overcame the forces locking the sides of the San Andreas Fault together at a spot that was probably just offshore of San Francisco’s Ocean Beach. This failure, contained at first to a small deeply buried patch of rock, allowed the sides of the fault to scrape suddenly past one another. If the rock surrounding this rupture had been locked tightly enough to withstand the force of the jolt, the zone of rupture might have only been a few square yards. If this had happened, the magnitude of the earthquake would have been less than 2; an unremarkable event that happens dozens of times a day on faults throughout the world. If the expanding rupture front had traveled for a mile along the fault before being stopped by tightly locked rock, the quake would have had a magnitude closer to 5. It would have awakened sleepers in San Francisco and been a topic of conversation, but would have done little damage.
But the rupture front, traveling both north and south along the fault, did not stop after a yard or a mile. Traveling at an average speed of 8,300 miles per hour, it ran north all the way to Cape Mendocino and, traveling at an average speed of 6,300 miles per hour, it ran south all the way to San Juan Bautista. In all, 296 miles of the San Andreas Fault ruptured in less than a minute; much of it rupturing all the way to the surface, where it offset fences, walls and roads by anywhere from two to thirty-two feet. The magnitude was somewhere between 7.7 and 7.9. As many as 3,000 people may have died and, by the time the fires were out, 80 percent of the city of San Francisco was destroyed.
By comparison, the 1989 Loma Prieta earthquake, the largest earthquake most current residents of the San Francisco and Monterey Bay areas have ever experienced, ruptured 25 miles of fault and had a magnitude of 6.9. That’s about 30 times less powerful.
A year after the Monterey earthquakes, on November 4, 1927, a much larger offshore quake took place nearly 150 miles to the south. This earthquake, centered somewhere west of Point Arguello, had a magnitude of around 7.3 and still ranks as one of the strongest earthquakes to strike California since 1906.
The quake, which hit at 5:51 am, was strong enough that people standing near Point Arguello were thrown to the ground and at least one person was reported thrown out of bed. Large landslides occurred throughout the area and sand and water fountained from the ground. The closest population center, Lompoc, was severely damaged, with numerous people fleeing into the streets in their pajamas. The jolt was felt by ships at sea and a six-foot tsunami (confirmed by tide gauges in San Francisco and San Diego) struck the Pismo Beach area, damaging the main rail line and inundating the train station in the small community of Surf (now part of Vandenberg Air Force Base). Later that morning, a ship off Point Arguello reported large numbers of dead or stunned fish floating on the surface.
The San Gregorio-Hosgri Fault System:
While, at the time, no one could have imagined anything connecting the Monterey earthquakes of 1926 to the distant Point Arguello earthquake of 1927, many seismologists now think, not that there was a causal relationship between the two, but that they both most likely occurred on the same fault. One large, mostly offshore, fault zone stretching around 250 miles; all the way from Point Conception, in Santa Barbara County, to near Bolinas, in Marin County. A distance sufficient to, should an earthquake with vertical as well as horizontal movement ever rupture it from end to end, produce a quake with a magnitude in the neighborhood of 8.5. This fault zone runs directly along the Big Sur coast, coming ashore briefly near San Simeon, and again between Coastlands and Rocky Creek, cutting straight down, and responsible for creating, the Big Sur Valley itself.
The onshore segments of this fault are unmistakable, but were long considered to be separate, shorter faults. From north to south these segments carry names like San Gregorio Fault (in San Mateo County), Sur Fault (coming ashore at Rocky Creek, running up the Big Sur Valley, over Post Summit, and entering the ocean again near the Henry Miller Library), San Simeon Fault (the scene of yesterday’s earthquake), and Hosgri Fault (the bane of the Diablo Canyon nuclear power plant). Today, thanks to better seafloor mapping providing evidence of linkage, most seismologists consider all these faults to be, in fact, a single, much more potentially dangerous fault. Collectively, these segments are now most often referred to as the San Gregorio-Hosgri Fault zone.
USGS map of the San Gregorio-Hosgri Fault system
Research suggests that the San Gregorio-Hosgri Fault may have formed about 11 million years ago and that, in its youth, it may have accommodated almost half the movement between the Pacific and North American plates. Over time, it appears, movement along the central portion of the San Andreas Fault has accelerated, allowing movement on the San Gregorio-Hosgri Fault to slow. But it has not stopped.
One way of envisioning the relationship between the San Gregorio-Hosgri and San Andreas Faults is to imagine everything west of the San Gregorio-Hosgri moving north at the full two inches per year traveled by the Pacific plate. Meanwhile, the Santa Lucia Mountains, the Monterey Peninsula, the Salinas Valley, Morro Bay, and everything else stuck between the San Gregorio-Hosgri and the San Andreas is also moving north relative to the North American plate, but not as quickly. The combined movement on the San Gregorio and central San Andreas Faults equals the full two inches per year.
A 2018 study found evidence that the offshore portion of the fault between Piedras Blancas and Point Sur is currently moving at about 3.3 millimeters per year (it may once have moved at 25 millimeters per year or more), with a vertical component to its movement pushing the east side of the fault (i.e.the Santa Lucia Mountains) up at about .8 millimeters per year. Most of the time, of course, it is not actually moving at all, but is simply accumulating stress at this rate. Stress that will be released when the fault eventually ruptures; moving inches or feet in a fraction of a second.
Motion measured in millimeters, or fractions of millimeters, per year may not sound like much, but on a timescale of millions of years it adds up. Rock formations at Point Lobos, on the east side of the fault have been matched with formations at Marin County’s Point Reyes, on the west side, and formations at Point Sur, on the west side, have been matched to east side formations at Point San Luis, near Avila in San Luis Obispo County; suggesting an offset of nearly 100 miles. It is not an accident that this fault lies directly along the base of the steeply rising oceanfront ridges of Big Sur’s South Coast. It was the fault that created that spectacular rise; thrusting the mountains upward more quickly than erosion could wear them down.
A major strand of the San Gregorio-Hosgri Fault comes ashore at Rocky Creek, tearing through the ridge between Rocky and Bixby Creeks and creating the low saddle that makes Division Knoll a landmark. The slow motion of the fault has moved the mouths of Bixby Creek and the Big Sur River north over time, causing both streams to make large northward bends along the fault before entering the sea. The mouth of the Big Sur River may once have been located at the latitude of Pfeiffer Beach, but the fault has slowly pushed Pfeiffer Ridge into its path, forcing it gradually northward.
So will the San Gregorio-Hosgri Fault rupture end to end producing an epic 8+ magnitude earthquake? Probably not. But it will certainly continue to regularly produce earthquakes with magnitudes greater than 6, and sometimes even 7. The USGS calculates the risk of a large, 6.7 or greater, magnitude quake on the northern portion of the fault before 2032 to be 10%. As 2032 is not that far away, this is not an inconsiderable risk.
This short video presents an analysis of what a 7.5 magnitude earthquake, rupturing the San Gregorio Fault from the Big Sur Valley north into Monterey Bay might look like. Such a quake could result in more than 100,000 injuries and fatalities and cause more than $200 billion in damages.
But when an earthquake strikes the fault segments immediately adjacent to the Big Sur coast, it won’t need to be 7.5, or even 6.7, magnitude to cause catastrophic damage to Highway One.
An earthquake large enough to trigger multiple large landslides and leave numerous portions of the coast inaccessible by vehicle for an extended period of time is probably inevitable. It’s just very unlikely in any given year. Compared to immediate disasters, like climate change, it shouldn’t rank near the top of anyone’s list of worries. But, on the Big Sur coast, being prepared for self sufficiency is never a bad idea.
This post is dedicated to the memory of my good friend, Lew Rosenberg, whose geological detective work contributed so much to unlocking the secrets of the San Gregorio-Hosgri Fault zone and the Santa Lucia Range.