California is a state located on the western coast of the United States, and it is famous for it’s well known fault systems. Geologically California is very complicated, once a subduction zone millions of years ago, and now a transform plate boundary, a place where the Pacific and North American plates grind past each other due to plate tectonics.
Patterns can be used to predict events in the future, but first the pattern must be known. For a thorough understanding of earthquake activity in the San Francisco Bay Area, different earthquakes in time must be known, as well as some additional geophysical information such as magnetics. We’ll start with historical large energy seismicity. What does large energy mean? Well the 1989 Loma Prieta earthquake was a M6.9 which released 2.75e^19 joules of energy, equal to the energy production of the United States. The Earth has energy systems far greater than those created by humanity. It’s important we understand them as best as possible to mitigate risk to humans and infrastructure.
To understand earthquake dynamics, one must understand patterns on a geologic timescale.
California Earthquake History
The San Andreas is a 1200 km long right-lateral transform fault, with the fault being nearly vertical along most of the 1200 km stretch. The longer a fault that ruptures, the larger the earthquake. Most famously the San Andreas ruptured in 1906 at an estimated magnitude of 7.9 off the coast of San Francisco, creating what became to be known as the 1906 San Francisco earthquake. Further back, the San Andreas Fault ruptured the year of 1857 with a est. M7.9 in central California near Fort Teton.
On August 8th 1989, a magnitude 5.4 earthquake ruptured in the Santa Cruz mountains near Loma Prieta at Lake Elsman. This was a foreshock, as two months later a larger M6.9, which became known as the 1989 Loma Prieta Earthquake, ruptured. The 1989 killed 63 individuals and is estimated to have cost 6 billion USD ($13 billion in 2020). Researchers from Stanford who were operating magnetic equipment near the fault observed “significant magnetic disturbances” preceding the earthquake. A anomalous dip in the 0.2-5 Hz range was registered a day before the rupture, and three hours before the rupture this changed to a exceptionally high level of activity in the 0.01-0.5 Hz range. Data from two magnetometers were used, one of the Stanford campus and the other magnetometer a low frequency 0.01-10Hz model, was located just 7km away from the epicenter. Antony C. Fraser-Smith of Stanford led the research.
To the far north of the San Francisco Bay Area, low magnitude 7+ earthquakes have occurred in Humbolt county. These happened 1923, 1980, and 1992 respectively, all during the wet season. Located westwards off the Mendocino Cape is the Mendocino Triple Junction, a three-way transform fault intersection where the San Andreas Fault ends.
Most recently, and of particular importance to the Napa Valley Seismic Project, were the two most recent Napa, CA earthquakes. On September 3rd 2000, a magnitude 4.9 earthquake ruptured along the West Napa Fault near the town of Yountville. Yountville being situated on small volcanic hills, wasn’t affected by the surface waves of the 2000 Yountville Earthquake much, but the city of Napa 9-18 kilometers away experienced significant shaking.
On August 24th 2014, the West Napa Fault ruptured with a M6.0 earthquake. The 2014 South Napa Earthquake’s epicenter was located just above the Napa delta, 20 kilometers to the south of the 2000 Yountville Earthquake, and was an order of magnitude larger. One person was killed, and damages from the South Napa earthquake exceeded $1 billion dollars.
More on Magnetics
Sheldon Breiner, founder of Geometrics, also recorded magnetic disturbances after large earthquakes, notably the 1964 M9.2 Alaska Earthquake. His research through Stanford on the 1964 earthquake focused on the data from two Varium rubidium vapor magnetometers, one located in New Jersey and another at Stanford. The Stanford magnetometer recorded a higher gamma reading in the very low frequency band for hours after the earthquake compared to the New Jersey magnetometer.
Napa is located at the northern end of the SF Bay Area, where the three valleys of Marin, Sonoma, and Napa flow into San Pablo Bay flatlands.. The southern end of the SF Bay Area is the Santa Cruz Mountains, the location of the Loma Prieta earthquakes of 1988-1989, and the San Jose Basin. On May 14th 2002, a M4.9 earthquake ruptured outside of Gilroy, a town 50km south of San Jose. The 2002 Gilroy Earthquake had the same magnitude of 4.9 as the 2000 Yountville Earthquake.
Preceeding the 2002 Gilroy Earthquake by 18 years was the 1984 Morgan Hill Earthquake, a M6.2 rupture along the Calaveras Fault. More recently, in 2007 the Calaveras Fault ruptured in a M5.5 earthquake near Alum Rock, a volcanic area on the eastern side of San Jose.
Surface Wave Patterns
According to data from nearby CSMIP Carquinez Bridge Geotechnical Arrays, surface waves generated from the 2014 Napa Earthquake had a peak frequency of 4-6 Hz. Aftershocks were generated the strongest surface waves at 10 Hz. If you experienced this earthquake, you would have experienced a strong jolt every 0.2 seconds, and anecdotal reports from locals agree. Shaking for both the 2000 and 2014 earthquakes was stronger than expected for the magnitude of each earthquake. HVSR and gravity data indicates that the city of Napa is centered over a ~2km deep basin. In a basin, wave trapping and the basin-edge effect can amplify surface-waves. Koichi Hayashi, a main contributor to the NVSP, and Craig Mitchell of CSUEB observed that deep geologic basins 1+ kilometers in depth can increase the amplitude of waves with a frequency of 0.5 – 5 Hz. They postulate that west of the Hayward Fault, the basin-edge effect will amplify low frequency (0.5 – 5 Hz) ground motion.
Napa Earthquake Patterns
Situated 70 km NW of Napa near Cobb Mountain, The Geysers are the world’s largest geothermal field, supplying California with approximately 20% of its renewable energy. Volcanism arrived in the Napa area 8 or 9 million years ago, and youngest volcanics date to 2.8 million years old. This volcanism was created by the impact of the Pacific and North American plates where mass amounts of energy are created by the slow grinding between these two massive plates.
Tufts, andesite, and rhyolites are common on the slopes of the Napa basin, and these rocks belong one volcanic eruption or another throughout Napa’s explosive past. Now, Cobb Mountain is the youngest volcano, at about 10,000 years old, and Clear Lake also is a volcanic caldera. Under Cobb Mountain, vast amounts of volcanic energy is still present in the subsurface, and some of this energy is released through the geysers of the area. Some of the energy is also released as small magnitude earthquakes, occurring frequency in swarms as magma shifts underground. The seismic energy from these earthquake swarms propagate away from the epicenters, radiating along fault networks connected to the area. The Geysers release seismic energy south into SF Bay Area fault systems.
The 2000 Yountville Earthquake released significant amounts of energy, with the most energy being released in the City of Napa, and then in 2014 the South Napa Earthquake occurred even further south, releasing energy felt throughout the Bay Area. Depending on the current stress level of a fault, it could either be close to empty, as the West Napa Fault likely is after teh 2014 earthquake, or it could be at critically high levels. As more energy is added to a fault structure from earthquakes nearby and from around the globe, stress will increase until a breaking point is reached.
SF Bay Area Fault Patterns
With the 2000 and 2014 Napa Earthquakes to the north, and the 1984 Morgan Hill, 1989 Loma Prieta, 2002 Gilroy, and 2007 Alum Rock Earthquakes from the south transferring energy into the heart of the SF Bay Area, the risk of a major fault rupture in Northern California increases. The largest faults of the San Francisco Bay Area are the San Gregorio, San Andreas, Calaveras, and Hayward faults, and geophysicists are most worried about the risk from the Hayward Fault. The Hayward Fault last ruptured in 1868 as a M6.8 earthquake, and since then stress has been building to what we now believe are critical levels. Earthquake magnitude is correlated to the length of the fault that ruptures, and multiple faults rupturing simultaneously also increases the total magnitude of a seismic event.
HVSR peaks along the Hayward Fault are less than 1 Hz, typically at 0.5 Hz, and with the basin-edge effect, any future earthquake to rupture in the East Bay will cause significant destruction along those peak horizontal frequencies of 0.2 – 1 Hz.