The Calaveras Fault is the eastern branch of the San Andreas system in the San Francisco Bay Area, splaying off the San Andreas near Hollister and running northwest through the Santa Clara Valley and the East Bay to Danville. It is one of the world's best-studied creeping faults: its central and southern segments slip steadily — up to ~16 mm/yr, the fastest continuous surface creep measured in California — so they build up little strain. The defining paradox: it moves the most continuously yet threatens the least large-quake hazard on those creeping segments. The real headline risk only entered the science in 2015.
The USGS hazard model splits this fault into 4 sections, each with its own geometry and a geologic slip rate. Each rate says whether it rests on site slip-rate data in the USGS earthquake-geology database or only on the fault’s mapped slip-rate class.
| Section | Movement | Slip rate | Geometry |
|---|---|---|---|
| Calaveras (center) CA | Right-lateral strike-slip | 15 mm/yr (9–20)rate class only | dips 77° E · 0–11 km deep |
| Calaveras (north) CA | Right-lateral strike-slip | 6 mm/yr (3–8)site data | dips 80° E · 0–14 km deep |
| Calaveras (south) CA | Right-lateral strike-slip | 15 mm/yr (10–20)site data | dips 85° E · 0–9.5 km deep |
| Calaveras (south, Paicines) CA | Right-lateral strike-slip | 10 mm/yr (5–15)site data | dips 77° E · 0–13 km deep |
Source: USGS NSHM23 fault sections database v3 and western US geologic deformation model v1.0 (on-plane slip rates, preferred with low–high range; site data = measured sites in the NSHM23 earthquake-geology database, rate class only = the fault’s mapped slip-rate category); central US and Cascadia sources from the model’s source files (code.usgs.gov/ghsc/nshmp/nshms/nshm-conus).
The creeping central segment is visibly tearing the town of Hollister apart — offset curbs and streets at Dunne Park and at 6th and 7th Streets creep at roughly 7–12 mm/yr. Because creep accounts for nearly all the slip, the segment releases its strain in moderate (M5–6.2) earthquakes; mainshocks nucleate on small "stuck patches" embedded in the creeping zone. The northern segment, by contrast, creeps little (1.7–3.5 mm/yr) and is more locked, with larger-quake potential on longer timescales.
The most recent large earthquake was the April 24, 1984, M6.2 Morgan Hill earthquake on the central Calaveras east of San Jose (no deaths, ~$7.5–8M damage). It closely resembles the 1911 Calaveras earthquake (M~6.2–6.6) on nearly the same segment — the only M6+ in the Bay Area in the 75 years after 1906. Other notable events: the 1861 San Ramon Valley M~5.8, the 1979 Coyote Lake M5.9, and the 2007 Alum Rock M5.4–5.6 (the largest Bay Area quake since 1989).
In 2015, researchers discovered that the Hayward and Calaveras faults are directly connected at depth, east of San Jose (Chaussard et al., 2015). A rupture could propagate across both, reaching up to ~M7.3 — materially raising the East Bay's worst-case scenario beyond what earlier segment-by-segment models assumed. The USGS official all-segments scenario for the Calaveras alone is M7.0.
The Calaveras runs through or beneath San Jose (~1 million people, the Bay Area's largest city), Fremont, Pleasanton, San Ramon, Danville, Morgan Hill, Gilroy, and Hollister. See the Hayward Fault, the California page, and our seismic risk lookup.