Landslide Hazards at La Conchita, California

By Randall W. Jibson

Open-File Report 2005-1067

U.S. Department of the Interior

U.S. Geological Survey
Any use of trade, firm, or product names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

This report has not been reviewed for stratigraphic nomenclature.
3

Introduction

On January 10, 2005, a landslide struck the community of La 

Conchita in Ventura County, California, destroying or seriously 

damaging 36 houses and killing 10 people. This was not the 

first destructive landslide to damage this community, nor is it 

likely to be the last. On January 14, 2005, at the invitation of 

the California Geological Survey, I visited La Conchita with 

James O’Tousa, contract geologist for Ventura County, and 

Pamela Irvine, Janis Hernandez, and Terry Jones, all from the 

California Geological Survey. This report describes my field 

observations and provides a description of the La Conchita area 

and its landslide history, a comparison of the 1995 and 2005 

landslides, and a discussion of continuing landslide hazards in the 

La Conchita area.

Setting of La Conchita

La Conchita is located on the southern California coastline 

midway between Ventura and Santa Barbara (fig. 1). The 

11-ha (28-acre) community was first established in 1924 when 

subdivision created about 200 lots that mostly contain singlefamily residences. La Conchita lies on a narrow coastal strip 

about 250 m (800 ft) wide between the shoreline and a 180-m 

(600-ft) high bluff having a slope of about 35°; above the top of 

the bluff is a gently rising terrace surface covered by avocado and 

citrus orchards (fig. 2). 

The bluff above La Conchita consists of poorly indurated 

marine sediment of the Monterey and Pico Formations. The upper 

part of the slope consists of interlayered siliceous shale, siltstone, 

and sandstone of the Middle to Upper Miocene Monterey 

Formation. The lower part of the slope is siltstone, sandstone, 

and mudstone of the Pliocene Pico Formation (O’Tousa, 1995). 

Rock of both formations is very weakly cemented and has been 

regionally associated with extensive landslide activity (Morton, 

1971; Harp and Jibson, 1995, 1996; Parise and Jibson, 2000). The 

two formations are in fault contact along the active Red Mountain 

Fault, which extends across the slope face.

Landslide History

The bluff above La Conchita has produced a variety of 

landslides over an extended period of time. Figure 3 shows 

LIDAR and false-color infrared images of the bluff above La 

Conchita and the surrounding area, and several sizes, types, 

and ages of landslides are visible. The arcuate bench at the top 

of the bluff is the head of a very large prehistoric landslide that 

affected the entire bluff. Several smaller, more recent slumps 

and earth flows also are visible, as is the 1995 slump–earth flow 

Figure 1. Location map showing the La Conchita area (after 

O’Tousa, 1995).

Figure 2. Topographic map showing the La Conchita area. 

Approximate outlines of 1995 (blue) and 2005 (yellow) 

landslides are shown. Base from U.S. Geological Survey Pitas 

Point 7.5' quadrangle, contour interval 20 ft (6.1 m), datum mean 

sea level; map center is at UTM 11 275256E 3805431N.
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Figure 3a. Oblique false-color 

infrared photograph of La 

Conchita taken in 2002. The 

arcuate bench near the top of 

the bluff in the center of the 

photograph is the main scarp of 

an ancient landslide that involved 

the entire bluff. The 1995 

landslide is visible in the right 

center of the photograph. Other 

landslides of various ages and 

sizes are visible on the slopes. 

Photograph courtesy of Robert 

Larson, GeoArchives Photography.

.

AB Figure 3b. Oblique LIDAR 

image of La Conchita after 

the 2005 landslide. Outline of 

1995 (blue) and 2005 (yellow) 

landslides shown; arrows show 

examples of other landslides in 

the area; red line outlines main 

scarp of an ancient landslide that 

involved the entire bluff. The 

deep canyon on the left produced 

major debris flows in both 1995 

and 2005. Image courtesy of 

Airborne 1 Corporation, El 

Segundo, Calif.
5

(terminology after Varnes, 1978). Similar combinations of large 

ancient landslides and smaller, recently active landslides also 

are present in areas southwest of the 1995 and 2005 landslides. 

In addition, large and small ravines that incise the bluff have 

produced debris flows recently and in the past.

Historical accounts dating back to 1865 have reported 

landslides in the area around La Conchita as being a regular 

occurrence (Hemphill, 2001). The Southern Pacific rail line 

that extends along the coastal strip was inundated by landslide 

debris in 1889 and again in 1909, when a train also was buried 

(Hemphill, 2001). Since that time, landslides frequently have 

inundated roads, railroads, cultivated land, and more recently, the 

La Conchita community (O’Tousa, 1995).

1995 La Conchita Landslide

The following summary of the 1995 La Conchita landslide is 

extracted from O’Tousa (1995) and Anderson (Robert Anderson, 

RJR Engineering, 2005, personal commun.). On March 4, 1995 

at 2:03 p.m. PST, the La Conchita landslide failed and moved 

tens of meters in only a few minutes. The landslide, a complex 

slump–earth flow, destroyed or severely damaged nine houses. 

On March 10, a subsequent debris flow from a canyon to the 

northwest (see canyon on left part of figs. 2 and 3) damaged five 

additional houses in the northwestern part of La Conchita. The 

main mass that failed is on the southeastern margin of the larger 

ancient landslide that encompasses the entire front of the bluff 

(see fig. 3). The 1995 slide was 120 m (400 ft) wide, 330 m (1100 

ft) long, and covered approximately 4 ha (10 acres). The depth 

was estimated at greater than 30 m (100 ft), and the volume was 

estimated at 1.3 million m3 (1.7 million yd3). 

Incipient movement of the upper part of the slide was 

reported as early as the summer of 1994, when surface cracks 

were observed in the upper part of the slope (O’Tousa, 1995). 

Deformation continued as the rainy season began, and by 

December 1994, several open cracks on the hillside were 

channeling surface runoff into the subsurface. Several smaller 

landslides occurred between the summer of 1994 and March 

1995, when the large slide occurred.

The 1995 landslide apparently occurred as a result of an 

extraordinarily wet year. Mean seasonal rainfall at Ojai (20 km 

[12 mi] northeast of La Conchita) from October 1 through March 

3 (the day before the landslide occurred) is 390 mm (15.37 in) 

(National Oceanic and Atmospheric Administration, 1994a, 

1995a). In 1994–95, about twice as much rain—761 mm (29.96 

in)—fell during that period (National Oceanic and Atmospheric 

Administration, 1994b, 1995b). Figure 4 shows the rainfall 

Figure 4. Daily rainfall at Ojai (20 km [12 mi] northeast of La Conchita) from October 1, 1994, through March 

31, 1995 (data from National Oceanic and Atmospheric Administration, 1994b, 1995b). The 1995 landslide 

occurred more than 1 month after the heaviest rainfall of the season.
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distribution during the 1994–1995 rainy season. Most of the 

excess rain fell in January, which had 623 mm (24.53 in) as 

compared to a normal rainfall of 108 mm (4.26 in). February 

1995 produced only about one-third of the normal rainfall, but a 

modest storm on March 2–3 produced 21 mm (0.81 in) of rain. 

The La Conchita landslide then occurred on March 4.

As discussed above, the 1995 landslide was relatively deep and 

primarily moved as a coherent slump–earth flow. This mode of 

movement suggests that the landslide formed as a result of rising 

ground-water levels in response to deep infiltration of antecedent 

seasonal rainfall. In particular, the extraordinary rainfall of 

January 1995 probably was the principal contributing factor 

to the elevated ground-water levels and, hence the landslide 

movement. The storm of March 2–3 also may have played a 

role in triggering initial movement of the hillside, which already 

was approaching instability due to longer-term seasonal rises 

in ground-water levels. Eyewitness accounts indicate, however, 

that similar to 2005, dust was in the air and much of the deposit 

included relatively dry material. Eyewitnesses also reported 

seeing material failing from the main scarp and lateral margins of 

the landslide as a result of removal of lateral support.

2005 La Conchita Landslide

The 2005 La Conchita landslide occurred at about 12:30 p.m. 

on January 10. Little or no newly failed material was involved 

in the landslide; rather, it consisted of a remobilization of the 

southeastern portion of the 1995 landslide deposit, involving 

about 200,000 m3 (250,000 yd3) (James O’Tousa, RJR 

Engineering, personal commun., 2005). The landslide area was 

approximately 350 m (1,150 ft) long and 80–100 m (260–330 ft) 

wide. The landslide entered the La Conchita neighborhood 

destroying 13 houses and severely damaging 23 others (figs. 5 

and 6). There were 10 confirmed fatalities.

Earlier that morning, debris flows from canyons northwest of 

La Conchita reached Highway 101. Law enforcement officers 

and media representatives were in the area, which facilitated 

capturing the moving landslide on video. The KCAL-TV video 

indicates that the landslide material mobilized simultaneously and 

nearly instantaneously into a highly fluid, rapidly moving debris 

flow. I estimate from viewing the video that high on the slope, the 

landslide was moving perhaps 10 m/s (30 ft/s). The developed 

part of the slope where the houses were impacted has a flatter 

slope, and so the flow probably slowed to no more than 5 m/s 

(15 ft/s) in the neighborhood. This slower rate also is suggested 

by eyewitnesses who stated that some residents were able to 

outrun the advancing flow, which would not have been possible at 

the higher upslope velocity.

The 2005 landslide occurred at the end of a 15-day period that 

produced record and near-record amounts of rainfall in many 

areas of southern California. At Ventura (20 km [12 mi] southeast 

of La Conchita) seasonal antecedent rainfall from October 1, 

2004 through January 10, 2005 totaled 493 mm (19.4 in) as 

compared to the mean value of 122 mm (4.8 in). From December 

27, 2004 through January 10, 2005, Ventura received 378 mm 

(14.9 in) of rainfall, only slightly less than its mean annual total 

of 390 mm (15.4 in) (Wofford, 2005; National Oceanic and 

Atmospheric Administration, 1994a, 1995a). Although rainfall 

intensities were not extreme, moderate- to high-intensity rainfall 

persisted for more than 2 weeks, and the landslide occurred at the 

culmination of this 15-day high-rainfall period (fig. 7). 

Figure 5. View of the La 

Conchita landslide taken 

January 14, 2005. The lightcolored, exposed rock in the 

upper part of the photograph 

is the main scarp of the 1995 

slide. The southeast part of 

the 1995 deposit (right side 

of photograph) remobilized 

in 2005. At the bottom center 

of photograph, a wall built 

after the 1995 slide is visible; 

the 2005 slide overtopped 

and tilted parts of the wall 

forward.
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Figure 6. View from the main scarp 

down the length of the 2005 La 

Conchita landslide. Water visible 

in the center of the photograph 

was issuing from the base of the 

main scarp when the photograph 

was taken (January 14, 2005). The 

2005 landslide remobilized about 

15 percent of the 1995 deposit and 

followed the left margin (looking 

downslope) of the 1995 slide. The 

vegetated ridge in the left part of 

the photograph is intact material 

that did not fail in 1995 or 2005; 

the material in the right part of the 

photograph is 1995 deposit that is 

still in place.

Figure 7. Daily rainfall at 

Ventura (20 km [12 mi] 

southeast of La Conchita) 

from October 1, 2004 

through January 20, 2005 

(data from Wofford, 

2005). The 2005 landslide 

occurred at the culmination 

of the heaviest rainfall of 

the season.
8 

Inspection of the site within a few hours of the landslide 

indicated that much of the deposit consisted of fairly dry material 

(James O’Tousa, RJR Engineering, personal commun., 2005). 

Also, the video shows dust in the air as the landslide flowed 

downslope. Thus, it appears that the landslide mobilized on a 

saturated layer deep in the 1995 deposit but that much of the 

material above this saturated zone was dry or nearly so. The 

video shows relatively intact vegetation being rafted on the 

surface of the rapidly flowing mass, which indicates that much of 

the landslide mass simply was being carried on the fluidized layer 

at depth, which presumably was much more saturated. 

Such a failure scenario, involving a significant amount of dry 

material that fully mobilized on a saturated layer, indicates that 

most of the rain that fell on the surface of the 1995 deposit did 

not infiltrate but drained off the surface. The rising ground-water 

level within the 1995 deposit would thus have resulted from 

deeper recharge from rainfall infiltration upslope. Figure 8 shows 

an aerial photograph of La Conchita taken in September 2004. 

The lush, green vegetation visible in the southeastern (lower right 

in photograph) part of the 1995 deposit clearly indicates that 

drainage on and within the 1995 landslide deposit concentrated 

water in the part of the mass that failed in 2005. At the time of 

our visit (January 14, 2005) water was still issuing from the base 

of the main landslide scarp and was ponding at several locations 

on the 2005 deposit (fig. 9).

The 2005 landslide pushed many of the houses off their 

foundations and into each other at the toe of the landslide 

(figs. 10 and 11). A wall built after the 1995 landslide to keep 

minor landslide debris off the road was tilted forward and(or) 

overtopped in places by debris from the 2005 landslide (fig. 12). 

This indicates that the landslide material, although it flowed 

rapidly, was quite viscous and pushed structures in front of it 

rather than flowing around them or filling them with mud, as 

sometimes occurs with fully saturated debris and mud flows. This 

apparently resulted from a highly hazardous situation involving 

a two-phased landslide mechanism: (1) a saturated, highly fluid 

layer at depth on which the landslide mobilized that (2) carried a 

thick layer of drier, much more viscous material that effectively 

acted as a battering ram.

Comparison of 1995 and 2005 Landslides

The movement of the same landslide mass in 1995 and 2005 

by two very different mechanisms, and with markedly different 

results, is difficult to explain. The 1995 landslide was a deep, 

coherent slump–earth flow that deformed plastically and moved 

slowly enough that people could get out of its way. The 2005 

landslide was a shallower remobilization of the very same 

material into a rapid, highly fluid debris flow that buried 10 

people. Although it is not uncommon for subsidiary debris flows 

to occur from the toes or scarps of existing landslides (Morton 

and Campbell, 1989), that is not what happened in 2005. This 

was a wholesale remobilization of a significant portion of the 

1995 deposit. How and why the same material failed twice in 

10 years by fundamentally different mechanisms certainly will 

be the object of future research, and it is much too complex to 

analyze in detail at this time. A few things, however, can be said.

The timing of the two landslides with respect to the triggering 

storms is of primary interest. In 1995, after a very wet January, 

the landslide did not move until more than a month later, 

during which time very little rain fell (fig. 4). The deep mode of 

failure in 1995 is consistent with this delay: deeper landslides 

commonly are triggered by deep infiltration of rainfall, which 

can take weeks or months to occur (for example, Morton and 

Campbell, 1989). The 2005 landslide occurred at the culmination 

of an extremely wet 2-week period (fig. 7). This also is consistent 

with the shallower, fluid mode of failure: shallow, rapid debris 

flows most commonly occur during periods of prolonged, intense 

rainfall with little or no lag time (Campbell, 1975; Keefer and 

others, 1987; Jibson, 1989).

However, this still leaves some troubling questions 

unanswered. Why did the landslide material not mobilize into 

a rapid debris flow in 1995? What about the remaining 1995 

deposit? Since only about 15 percent of the 1995 deposit 

remobilized in 2005, could the remainder also mobilize into a 

rapid debris flow, or is it more likely to remobilize as a deep 

slump? Or will it remain metastable? Currently, we have 

insufficient data and understanding of the failure mechanisms 

of this landslide to adequately answer these questions, but it 

is clear that the hazard from renewed landslide movement is 

considerable.

Continuing Hazards at La Conchita

Of primary interest to the general public and various 

Governmental entities is the current state of hazard at La 

Conchita. While this preliminary report does not represent 

a detailed evaluation of those hazards, a few reasonable 

observations can be made.

1. Historical accounts and geologic evidence show that 

landsliding of a variety of types and scales has been occurring 

at and near La Conchita for many thousands of years, and on 

a relatively frequent basis, up until the present. There is no 

reason to believe this pattern of landsliding will stop.

2. Even in the absence of additional significant rainfall this 

year (2005), the remainder of the 1995 landslide could still 

remobilize, most likely as a deep slump–earth flow similar 

to that in 1995. This mode of movement would most likely 

be relatively slow (compared to 2005) but still could pose 

serious hazards to property and, perhaps, life.

3. If significant additional rainfall occurs, either this year or 

in future years, several landslide scenarios are possible: 

(a) deep movement of the 1995 deposit, as described above, 

(b) mobilization of the 1995 (and possibly the 2005) deposit 

into a rapid debris flow such as occurred on January 10, 2005, 

(c) triggering of subsidiary landslides from parts of the 1995 

and 2005 deposits or scarps, (d) triggering of slumps and 

(or) earth flows on adjacent hillsides, and (e) triggering of 

rapid debris flows from various nearby slopes, particularly in 

ravines.
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Figure 8. Othorectified 

airphotograph of La 

Conchita taken in 

September 2004. The 

1995 landslide is outlined 

in blue; the area that 

remobilized in 2005 is 

outlined in yellow. Note 

the concentration of lush, 

green vegetation in the 

area that remobilized in 

2005, which indicates 

concentration of surface 

and subsurface drainage 

into this area. Image from 

orthophotography provided 

courtesy of AirPhoto USA 

and County of Ventura. 

Figure 9. Water was 

issuing from the base of 

the main scarp at the time 

of our visit (January 14, 

2005). The water ponded 

at several locations on the 

2005 deposit.
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Figure 10. The 2005 

landslide moved as a 

rapid debris flow, but it 

was quite viscous and 

pushed houses in its 

path rather than flowing 

around or through them. 

The left part of the house 

was detached from the 

right part and was pulled 

by the landslide several 

decimeters toward 

the upper left of the 

photograph.

Figure 11. This house 

was pushed about 3.5 m 

(12 ft) off its foundation 

toward the right in the 

photograph when the 

2005 landslide topped 

a wall. The tops of the 

H-beams of the wall are 

visible in the left part of 

the photograph.
11

4. The landslide scenarios sketched above potentially could 

impact any part of the La Conchita community. Future 

landslide activity could move into the same areas that 

recently have been damaged or could mobilize in other 

directions that could damage any or all of the developed area.

Conclusion

The La Conchita area has experienced, and will likely 

continue to experience, a rather bewildering variety of landslide 

hazards. Different landslide scenarios are more or less likely to 

occur as a result of different specific rainfall conditions, and no 

part of the community can be considered safe from landslides. 

Unfortunately, we currently lack the understanding to accurately 

forecast what might happen in each possible rainfall scenario. 

Prudence would certainly dictate, however, that we anticipate 

renewed landslide activity during or after future periods of 

prolonged and(or) intense rainfall. Future earthquakes, of course, 

also could trigger landsliding in the area (Harp and Jibson, 1995, 

1996).

References

Campbell, R.H., 1975, Soil slips, debris flows, and rainstorms 

in the Santa Monica Mountains and vicinity, southern 

California: U.S. Geological Survey Professional Paper 851, 

51 p.

Harp, E.L., and Jibson, R.W., 1995, Inventory of landslides 

triggered by the 1994 Northridge, California earthquake: 

U.S. Geological Survey Open-File Report 95–213, 17 p.

Harp, E.L., and Jibson, R.W., 1996, Landslides triggered by the 

1994 Northridge, California, earthquake: Bulletin of the 

Seismological Society of America, v. 86, no. 1B, p. S319–

S332.

Hemphill, J.J., 2001, Assessing landslide hazard over a 130-year 

period for La Conchita, California, in Association of Pacific 

Coast Geographers Annual Meeting, Santa Barbara, Calif., 

September 12–15 2001.

Jibson, R.W., 1989, Debris flows in southern Puerto Rico, in 

Schultz, A.P., and Jibson, R.W., eds., Landslide Processes in 

Eastern North America and Puerto Rico: Geological Society 

of America Special Paper 236, p. 29–55.

Keefer, D.K., Wilson, R.C., Mark, R.K., Brabb, E.E., Brown, 

W.M., Ellen, S.D., Harp, E.L., Wieczorek, G.F., Alger, C.S., 

and Zatkin, R.S., 1987, Real-time landslide warning during 

heavy rainfall: Science, v. 238, p. 921–925.

Morton, D.M., 1971, Seismically triggered landslides above San 

Fernando Valley: California Geology, v. 24, no. 4–5.

Morton, D.M., and Campbell, R.H., 1989, Cyclic landsliding 

at Wrightwood, southern California, in Sadler, P.M., and 

Morton, D.M., eds., Landslides in a semiarid environment 

with emphasis on the inland valleys of southern California: 

Inland Geological Society, Riverside, Calif., v. 2, p. 174–

182.

National Oceanic and Atmospheric Administration, 1994a, 

Climatological data annual summary—California 1994: 

National Climatic Data Center, v. 98, no. 13, 60 p.

National Oceanic and Atmospheric Administration, 1994b, 

Climatological data—California, October–December 1994: 

National Climatic Data Center, v. 98, nos. 10–12.

Figure 12. Steel-andtimber wall built after 

the 1995 landslide was 

overtopped and tilted 

forward in places by the 

2005 landslide.
12

National Oceanic and Atmospheric Administration, 1995a, 

Climatological data annual summary—California 1995: 

National Climatic Data Center, v. 99, no. 13, 52 p.

National Oceanic and Atmospheric Administration, 1995b, 

Climatological data—California, January–March 1995: 

National Climatic Data Center, v. 99, no. 1–3.

O’Tousa, James, 1995, La Conchita landslide, Ventura County, 

California: Association of Engineering Geologists AEG 

News, v. 38, no. 4, p. 22–24.

Parise, Mario, and Jibson, R.W., 2000, A seismic landslide 

susceptibility rating of geologic units based on analysis 

of characteristics of landslides triggered by the January 

17, 1994 Northridge, California, earthquake: Engineering 

Geology, v. 58, p. 251–270.

Wofford, Michael, 2005, Ventura, California weather conditions: 

http://www.venturaweather.com/daily.htm, last visited 

January 28, 2005.