The 1892 Vacaville–Winters earthquakes occurred in northern California as a large doublet on April 19 and April 21. Measured on a

seismic scale Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake. These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking (quaking) caused by an earthquake at ...

that is based on an

isoseismal map In seismology, an isoseismal map is used to show lines of equally felt seismic intensity, generally measured on the Modified Mercalli scale. Such maps help to identify earthquake epicenters, particularly where no instrumental records exist, such a ...

or the event's felt area, the 6.4 and 6.2 events were assigned a maximum Mercalli intensity of IX (''Violent''), and affected the North Bay and Central Valley areas. The total damage was estimated to be between $225,000 and 250,000 and one person was killed. No evidence of fault movement on the surface of the ground was observed as a result of either of the strong shocks. Both occurred in the domain of the San Andreas strike-slip system of faults, but their focal mechanism is uncertain.

Tectonic setting

The primary tectonic feature of the region is the strike-slip San Andreas system of faults (that move laterally alongside one another). From west to east, the Hayward–Rogers Creek Fault Zone and the Concord–Green Valley Faults are the closest to Vacaville and Winters. A 1999 forecast stated that the Hayward and Rogers Creek Faults have been given a 32% chance of a M6.7 or greater shock before 2030, and the Concord and Green Valley Faults have been assigned a lower probability of 6% each. Vacaville and Winters both lie to the east of these faults along the low foothills. Numerous

geoscientists A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althoug ...

have remarked on the potential for blind thrust earthquakes in this area. These professionals specifically labelled it the Coast Range–Sierran Block Boundary Zone in a sequence of published articles from the 1980s and 1990s and stated that the

1983 Coalinga earthquake The 1983 Coalinga earthquake struck at 4:42 p.m. Monday, May 2 of that year, in Coalinga, California. The shock was felt from the Greater Los Angeles Area north to Susanville in Lassen County, and between the Pacific Coast and western Nevad ...

(a blind thrust event) occurred within the zone.

Earthquakes

Each large shock in the doublet, one on April 19 and the other two days later on April 21, had a high intensity and was felt over roughly the same geographical area. The area over which the initial shock was felt with an intensity of VIII (''Severe'') was about 1,100 km² and the area for the second shock was about 890 km². The furthest extent that they were felt was from Redding in the north to Salinas and

Fresno Fresno () is a major city in the San Joaquin Valley of California, United States. It is the county seat of Fresno County and the largest city in the greater Central Valley region. It covers about and had a population of 542,107 in 2020, maki ...

in the south and to

Virginia City, Nevada Virginia City is a census-designated place (CDP) that is the county seat of Storey County, Nevada, and the largest community in the county. The city is a part of the Reno– Sparks Metropolitan Statistical Area. Virginia City developed as a boom ...

in the east. Investigators completed a study of the area that included a three dimensional crustal velocity model. Proprietary

seismic reflection Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflection (physics), reflected seismic waves. The method require ...

data were also inspected in an attempt to establish a source for the sequence. Observations of the range front and modelling of strong ground motions led to the presentation of a rupture on the west- dipping Gordon Valley blind thrust fault that could have produced the felt intensities that were specific to the April 19 event.

Damage

The shocks affected Solano and Yolo Counties along the border of the Sacramento Valley. In the north, the communities of

Esparto Esparto, halfah grass, or esparto grass is a fiber produced from two species of perennial grasses of north Africa, Spain and Portugal. It is used for crafts, such as cords, basketry, and espadrilles. '' Stipa tenacissima'' and '' Lygeum spart ...

and Capay both experienced Mercalli intensities of VIII (''Severe'')—which is associated with damage—and in the south, Vacaville and Fairfield both experienced the same. Work by seismologist Toussan Toppozada, that was published in a 1981 report, further clarified that Vacaville was at the center of the destruction. Damage also occurred at Dixon as well as the area of the highest reported intensity of IX (''Violent''), midway between Winters and Vacaville, at Allendale. There, some buildings were damaged, some collapsed, and long ground

fissure A fissure is a long, narrow crack opening along the surface of Earth. The term is derived from the Latin word , which means 'cleft' or 'crack'. Fissures emerge in Earth's crust, on ice sheets and glaciers, and on volcanoes. Ground fissure ...

s were observed. These widespread and north-trending cracks were very narrow, from in width. Despite these surface cracks and other offset drainages and other

lineament ''See also Line (geometry)'' A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault. Typically a lineament will appear as a fault-aligned valley, a series of fault or fold-ali ...

s, no surface faulting occurred. This was confirmed by trench excavations where these features were found. Other ground effects occurred though. Reports of rockslides and dislodged boulders were written about in the newspapers, but the most common type of damage were the many chimneys that were damaged or destroyed. Total damage was estimated between $225,000 and $250,000, and one person was reported dead.

Intensity

for the April 19 event shows an asymmetrical and slightly elongated pattern of isoseismal lines (of equal intensity) on the north–south axis. There are high intensities east of the Napa–Solano county border. No locations in Napa county reached any higher than intensity VI (''Strong''), indicating that the shock originated east of the

Vaca Mountains The Vaca Mountains are a mountain range in Napa and Solano Counties, California that is one of the California Coast Ranges. They represent the easternmost of the Inner Coast Ranges in north−central California, and divide the Suisun Valley on ...

. The April 21 event differed slightly, in that the strongest effects were shifted slightly to the northwest.

References

{{DEFAULTSORT:Vacaville-Winters earthquakes

1892 Events January–March * January 1 – Ellis Island begins accommodating immigrants to the United States. * February 1 - The historic Enterprise Bar and Grill was established in Rico, Colorado. * February 27 – Rudolf Diesel applies fo ...

1892 earthquakes 1892 in California Earthquakes Earthquakes History of Solano County, California History of Yolo County, California April 1892 events Buried rupture earthquakes