HISTORIC EARTHQUAKES: effects in Ventura County

By

F. Harold Weber. Jr., and Edmund W. Kiessling, Geologists

California Division of Mines and Geology

Taken from Chapter II, "General features of seismic hazards of Ventura County, California " by F. Harold Weber, Jr., and Edmund W. Kiessling, California Division of Mines and Geology Open-File Report 76-5 LA, Seismic hazards study of Ventura County, California, 1975 (revised 1976), by F. Harold Weber, Jr., Edmund W. Kiessling, Edward C. Sprotte, Jeffrey A. Johnson, Roger W. Sherburne, and George B. Cleveland. The entire report is on file at district offices of the California Division of Mines and Geology.

The documented history of earthquakes in California spans a very short period of time, especially when contrasted with the similar history of China, which extends back several thousand years. The first earthquake in California to be documented occurred in 1769, only 209 years ago. Members of the Spanish expedition led by Gaspar de Pórtola experienced strong shaking from this earthquake while camped near present-day Santa Ana and lesser shaking from aftershocks while enroute northwest to the future site of the San Gabriel Mission. The effect of the 1769 event on the land area of present-day Ventura County is wholly unknown.

The next documented large earthquake occurred on December 8, 1812. Damage caused by this event included the collapse of the roof of the San Juan Capistrano Mission in San Diego County, killing 40 people. This earthquake produced moderate shaking at Ventura, judging from reports of the Spanish padres at the Ventura (San Buenaventura) mission.

Ironically, only two weeks later on December 21, 1812, a large earthquake occurred in the Santa Barbara region, resulting in severe damage to the Santa Barbara, Ventura and other missions (figures 1, 2 and 3).

SEE FIGURE 1

Figure 1. Historical earthquakes during the period 1812-1973 which were of damaging intensity in coastal Ventura County. Earthquakes of 1872, 1906, and 1927 caused only very slight damage.

The coastal Ventura area began to attract more settlers after California became a state in 1850. By 1857, when the first documented great earthquake occurred in California near Fort Tejon, one to two thousand people probably lived rather primitively in the Ventura-Colonia (Oxnard) area. The San Buenaventura Mission, which had been damaged in the 1812 earthquakes, was again severely damaged by the 1857 earthquake. In 1872 the great but distant Owens Valley earthquake was felt in Ventura but it caused only very minor damage.

When Ventura County was chartered and separated from Santa Barbara County on January 1, 1873, it had an estimated population of 3,500. Production of oil had begun, a shipping port had been established at Hueneme, and the first citrus production soon followed.

By 1906 the population of the county had reached about 15,000, in contrast to today's population of about 450,000. The 1906 San Francisco earthquake, the third great California earthquake in 50 years, caused only minor damage in Ventura. A small local earthquake in 1912 caused intense shaking at the Ocean View school.

SEE FIGURE 2

Figure 2. Areas of southern Ventura County effected by historical earthquakes. Only selected faults are shown. Locations of earthquakes (except 1919 and 1926), are shown on figure 1.

1. Mission San Buenaventura: December 21, 1812; Mission seriously damaged; Bell Tower partially collapsed; Padre Señan, head of Mission wrote "-sea was greatly stirred up by the tremors in such a way that the waters might flood the Mission-" January 9, 1857: Roof caved. June 29, 1925: Exterior cracks developed.

2. April 18, 1906: Near the train depot, one of the large oil tanks was "shook up," causing loss of nearly 100 barrels of oil.

3. Ventura. June 25, 1925: A few chimneys fell, plate glass broke, and interior plaster fell in office buildings; bricks loosened and fell from De Leon Hotel.

4. January 9, 1857: "Long," northwest trending cracks as wide as 6-8 inches formed in bed of river; "On either side of cracks lay a ridge of wet sand" near mouth of river cracks were wider.

5. Oxnard: March 10, 1933: "Some cars swayed as if they would turn over," and utility polls swayed; plaster loosened. Bricks fell from older buildings, power installations damaged.

6. March 26, 1872: "Colonia" (Oxnard Plain area): Deepest, 192 foot "artesian" water well, flow increased for at least several days, and for 24-hours "great quantities of sand and gravel were ejected, with occasional pebbles the size of hen's eggs." Similar effect also reported in other wells.

7. December 14, 1912: Ocean View School; "overturned desks and wrecked two barns nearby."

8. February 16, 1919: Large boulders (reported) rolled down steep slopes north of the Hermitage Ranch. This earthquake occurred to the north, near Maricopa.

9. March 10, 1933: Piru; "Several noticeable mountain slides."

10. February 18, 1926: Simi; "Damage to telephone equipment."

11. February 9, 1971: Fillmore: old buildings damaged.

Sources of data include Townley & Allen (1939); newspaper accounts and various reports.

Relatively large earthquakes outside the county, which caused damage ranging from minor to locally severe damage within the county, occurred in 1925 (Santa Barbara), 1927 (Point Arguello), 1933 (Long Beach), 1941 (Santa Barbara), 1952 (Arvin-Tehachapi), and 1971 (San Fernando). Damaging earthquakes within the county region occurred in 1950 (north of Ojai), 1957 (Hueneme), 1963 (Camarillo), and 1973 (Point Mugu). The 1973 event was by far the strongest of this latter group. Fortunately the epicenter was about 10 miles E - SE of the Oxnard area, where moderate damage occurred.

EARTHQUAKES OF THE 1800s

December 21, 1812: Santa Barbara Region

This seismic event apparently consisted of two individual earthquakes about 15 minutes apart, near 10:00 a.m. on Monday December 21. The event caused very serious damage to the Missions La Purisima, Santa Inez, Santa Barbara, San Buenaventura, and San Fernando (figure 3). As La Purisima and San Fernando Missions are 116 miles apart, the implications of such widespread damage are that either or both of these individual earthquakes were very large, even considering that the construction of the missions was primitive.

SEE FIGURE 3

Figure 3. Known effects of earthquakes of December 8 and 21, 1812 on southern California.

Comment on the December earthquakes was given by Fray Jose Señan in his biennial report of April 8, 1813 from Mission San Buenaventura. Fray Señan's report was translated by Maynard Geiger, O. F. M., Old Mission Santa Barbara, who is a noted authority on the history of the missions. Fray Señan stated that "These conditions have been rendered even more severe by the horrible tremblors or earthquakes that have been experienced in this province and which will constitute a special epoch in it because of the great resulting damages. The violence of these occurrences have been notable and their continuance have been extraordinary for since they began on December 8th of the past year they have continued until the past February with considerable violence and even since then they have been experienced as sufficiently strong though they have diminished in their force and frequency."

"As a result of these ruinous events we have to build anew the churches of Missions San Fernando and Santa Barbara. In the latter case also other buildings have suffered damage which will require a long time to repair. Mission San Gabriel suffered somewhat. At Missions Santa Inez and San Buenaventura quite some time will be required to repair the damage which I consider annoying to describe in detail. Concerning the last named mission I will say only that the tower partially fell and that the wall of the sanctuary was cracked from top to bottom. At present the tower is being torn down while the rest is being repaired although with very considerable labor and the sanctuary wall is being rebuilt."

The natural effects of the December 21 earthquake must have been very severe, as can be gleaned from reports from various sources. For example, Holden (1898), apparently quoting from the "Martinez" diary, stated that "Several asphaltum springs formed in the mountains and Tulares; gaps in the Sierra; the 'shore volcano' has more openings, and another is reported behind the Sierra de los Pinos."

Geiger (1965) states, "Jose Arguello, the local (Presidio) commander, reported that shocks were still continuing on January 14, 1813, and the earth had opened at several places causing 'sulphur-spewing volcanoes' to appear. In late March the land still quakes, the latest shock occurring on the eighteenth of that month."

Several accounts included reports of a tsunami (Holden, 1898): "P. Gil [father in charge of the mission] reported a huge wave at sea. A stick with a pendant ball was set up at the Mission (Santa Barbara), and the ball vibrated continually for 8 days, and later, at intervals for 15 days. A ship at Refugio was carried up a cañon by the wave and returned to sea."

Run-up heights reported for the tsunami were: 50 feet at Gaviota, 30-35 feet at Santa Barbara, and 15 feet or more at Ventura (Wood and Heck, 1966). These figures are attributed to unpublished notes of the late Professor G. D. Louderback, University of California, Berkeley. From the evidence available it is doubtful that the run-up heights were nearly so high (Marine Advisors, Inc., 1965).

In a letter dated May 23, 1975, to the California Division of Mines and Geology, Father Geiger wrote the following with regard to a tsunami report by Father Señan: ". . . Señan states in the baptismal register for January 3-9, 1813 that baptisms were being administered in a Jacal in a place in the 'interior' because of the earthquake and 'a threatened tidal wave.' On the 9th he wrote that "the sea was greatly stirred up by the tremors in such a way that we feared that the waters might flood the Mission [photo 1]." This is the only evidence of a tsunami at Ventura that Geiger has found in his research on the missions.

SEE PHOTO 1

Photo 1. View south in 1880 from hills north of western part of Ventura shows mission in right foreground and marshy coastal area (now filled). Earthquake of December 21, 1812, probably in the western Santa Barbara Channel, is said to have caused a tsunami which may have extended at least as far east as the Ventura area as a potentially damaging wave. The scene shown here probably was somewhat similar to the one observed by Father Señan, in charge of the mission, during the period when he wrote on January 9, 1813 with regard to the earthquake that "--the sea was greatly stirred by the tremors in such a way that we feared that the water might flood the mission." Photo from collection of Los Angeles Title and Trust Company.

The December 21, 1812 earthquake was classified by Richter (1958, p. 472) as "outstanding", placing it in rank with the 1857 and 1872 earthquakes. He stated that it was "probably offshore," and shows the epicenter on a small-scale map in the western part of the Santa Barbara Channel. James A. Hileman, California Institute of Technology (personal communication, May 31, 1975) estimated that the December 21, 1812 earthquake had a magnitude of between 7 and 7¼, based partly on mathematical curves developed by his colleague M. D. Trifunac. Hamilton and others (1969) estimated that it had a magnitude of 7.

January 9, 1857: Fort Tejon

The earthquake occurred about 8:30 a.m. on a Friday, and the intensity at Ventura was estimated at IX on the Rossi-Forel scale, (Townley and Allen, 1959, p. 35). The roof of the San Buenaventura Mission caved-in; houses were reported to have fallen in San Fernando, Los Angeles County.

One detailed account of the effects of the earthquake in the area that is now Ventura County was made by a U. S. Government Survey party encamped in Sycamore Canyon near the coast. The survey party leader, Johnson, actually traveled to Ventura the day of the earthquake. He, wrote: "We forded the Santa Clara River 6 miles from its mouth. The stream in itself is insignificant, but is from a half to three-quarters of a mile wide from bank to bank; and here I met with the first evidences of the terrible power exerted by the convulsion of nature so recently felt. Long cracks were visible in the bed of the river, many of them being 6 or 8 inches across, and extending in a direction southeast and northwest. These openings must have been at first considerably wider, for many of them had evidently been filled with water from the river, as on either side of the cracks lay a ridge of wet sand."

"These appearances were visible as far as I could see up and down the bed of the river. Near the mouth of the river the cracks were longer and wider. Persons residing within a mile of the entrance say that the water was thrown out from the cracks as high as 6 feet, and that large blocks of earth sank several feet below the former level, and there remain."

Researchers have estimated the magnitude of this earthquake at 8 or 8¼ based on (1) the length of ground rupture along the San Andreas fault --- at least 150 miles and probably 200 miles, and (2) the intensity of ground disturbance as far away as the Los Angeles area.

March 26, 1872; Owens Valley

The Owens Valley earthquake occurred at 2:20 a.m. and had an estimated magnitude of 8¼. The Ventura Signal of March 30, reported on the local area: "The only effect worthy of note was on the artesian wells of Colonia. James Fenlon, whose well is the deepest one there, being 192 feet says that the flow of water was very materially increased, and for 24 hours after the shock great quantities of sand and gravel were ejected, with occasional pebbles the size of hen's eggs. The flow of sand and gravel has ceased, but the unusual flow of water continues. James Leonard informs us that the flow has also materially increased in his well, and we hear that such was the effect on others."

The strong effect on the wells of Colonia (present-day Oxnard area) probably was a result of differential settling and compaction. As the Oxnard area is nearly 200 miles from the earthquake epicenter, the significant ground response occurred because the area is underlain by water saturated or marshy, unconsolidated to poorly consolidated sediments.

GROUND RESPONSE PATTERNS

Distinctive and unusual patterns of ground shaking in the alluvial areas of coastal Ventura County may have occurred historically, as interpreted from reports of the varying response of the region to different earthquakes. For example, the 1925 Santa Barbara and 1933 Long Beach earthquakes were of similar magnitudes (M~6.3) and both occurred offshore within a few miles of the coast. The Santa Barbara earthquake was about 25 miles west-northwest of Oxnard, whereas the Long Beach earthquake was 80 miles to the east-southeast. According to press and historical accounts of damage and other features of surface response, the overall effect on the Oxnard Plain of the 1933 Long Beach event may have been greater than the effects of the 1925 Santa Barbara earthquake. Even the 1941 Santa Barbara earthquake (M 5.9) seemed to have a greater effect on the Oxnard area than the 1925 Santa Barbara event.

A small, local earthquake on December 14, 1912 severely shook the Ocean View School; the local earthquake of March 18, 1957 caused relatively great damage although it was only a magnitude 4.7. Damage was even caused in Camarillo on June 21, 1963 by a magnitude 3.7 earthquake. In contrast, the great, 185-mile distant Owens Valley earthquake of March 26, 1872 took place when the Ventura region was relatively unpopulated, but caused considerable ground response, judging from reports of its effect on water wells.

Analysis of the effects of past earthquakes on some structures in Ventura County, such as the former County Courthouse (now Ventura City Hall) and the Mission San Buenaventura, might give clues to the differing ground response to various earthquakes. Perhaps the study of comparable earthquakes in the future will disclose distinctive ground response patterns caused by geologic, geographic, and other factors not presently understood.

POSTULATED EFFECTS OF 1812 AND 1857 EARTHQUAKES

It can only be imagined what the possible maximum intensity of ground shaking, the property damage, and loss of life the 1812 and 1857 earthquakes could have caused in the region as it is known today. The earthquake effects may have included (1) severe liquefaction in portions of the Oxnard Plain where the ground water table was near the surface, causing ground cracks and lateral spreading types of landslides; (2) differential compaction, perhaps causing near-shore ponding of tidal waters; and (3) tsunamis which might have had the strength to damage severely the harbor facilities, beach front dwellings, and other developments along the coast.

The 1857 earthquake may have been intense enough in portions of the county region to have reactivated some of the many bedrock landslides. Oil fields, electrical power transmission towers, and water storage tanks exist in this region today. Very large boulders loosened by shaking may have rolled down long steep slopes, and undoubtedly rockfalls off steep cliffs were common in both the 1812 and 1857 earthquakes. Some of the aftershocks of the 1857 earthquake could have been large enough to cause intense and widespread ground shaking. Perhaps intense shaking caused ground breakage to occur secondarily along faults, in addition to the San Andreas fault zone, and large aftershocks may have activated faults which are not presently suspect of having moved.

CONCLUSIONS

The coastal region of Ventura County, including the areas of the cities of Ventura, Oxnard, and Camarillo, has experienced periodic but relatively minor damage from earthquakes since the beginning of its populated history. Fortunately, the earthquakes of December 21, 1812 and January 9, 1857 occurred when the area had a small population and development was very slight and basic. Although the Ventura County region has been one of the most tectonically active areas in southern California during Quaternary time, no large earthquake (M 6 +) has yet occurred in the populated region of Ventura County.

The projected growth of the population and development of the Oxnard Plain of Ventura County emphasizes the need of continuing geologic studies (1) to determine the exact location and character of faults capable of causing large earthquakes, and (2) to understand the difference in ground response in the Ventura area for small to moderate local earthquakes, and large distant events.

REFERENCES

Geiger, Maynard, 1965, Mission Santa Barbara, 1782-1965: Santa Barbara Mission, Santa Barbara, California, 286 p.

Hamilton, R. M., Yerkes, R. F., Brown, R. D., Jr., Burford, R. O., and DeNoyer, J. M., 1969, Seismicity and associated effects, Santa Barbara region, in Geology. petroleum development, and seismicity of the Santa Barbara channel region, California: U. S. Geological Survey Professional Paper 679-D, p. 47-68.

Holden, E. S., 1898, Catalog of earthquakes on the Pacific Coast 1769 to 1897, reprinted from the Smithsonian Miscellaneous Collections no. 1087, for distribution by the Lick Observatory, v. 37, no. 5, 253 p.

Marine Advisers, Inc., 1965, An examination of the evidence for the reported Santa Barbara coast tsunami of December 1812: Unpublished report submitted to Southern California Edison Company.

Richter, C. F., 1958, Elementary seismology: W. H. Freeman and Company Inc., 768 p.

Townley, S. D., and Allen, M. W., 1939, Descriptive catalog of earthquakes of the Pacific Coast of the United States, 1769 to 1928: Seismological Society of America Bulletin, v. 29, no. 1, 297 p.

Wood, H. O., 1916, Earthquakes in California: Seismological Society of America Bulletin, v. 6, nos. 2 and 3, p. 108-109.

Wood, H. O., and Heck, N. H., 1966, Earthquake history of the United States, Part II, Stronger earthquakes of California and western Nevada: U. S. Coast and Geodetic Survey.