Landslide Hazards at La Conchita, CaliforniaBy Randall W. JibsonOpen-File Report 2005-1067U.S. Department of the InteriorU.S. Geological SurveyThis report has not been reviewed for stratigraphic nomenclature.Any use of trade, firm, or product names is for descriptive purposes only and does not implyendorsement by the U.S. Government.IntroductionOn January 10, 2005, a landslide struck the community of LaConchita in Ventura County, California, destroying or seriouslydamaging 36 houses and killing 10 people. This was not thefirst destructive landslide to damage this community, nor is itlikely to be the last. On January 14, 2005, at the invitation ofthe California Geological Survey, I visited La Conchita withJames O’Tousa, contract geologist for Ventura County, andPamela Irvine, Janis Hernandez, and Terry Jones, all from theCalifornia Geological Survey. This report describes my fieldobservations and provides a description of the La Conchita areaand its landslide history, a comparison of the 1995 and 2005landslides, and a discussion of continuing landslide hazards in theLa Conchita area.Setting of La ConchitaLa Conchita is located on the southern California coastlinemidway between Ventura and Santa Barbara (fig. 1). The11-ha (28-acre) community was first established in 1924 whensubdivision created about 200 lots that mostly contain singlefamily residences. La Conchita lies on a narrow coastal stripabout 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 ofthe bluff is a gently rising terrace surface covered by avocado andcitrus orchards (fig. 2).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 PitasPoint 7.5' quadrangle, contour interval 20 ft (6.1 m), datum meansea level; map center is at UTM 11 275256E 3805431N.The bluff above La Conchita consists of poorly induratedmarine sediment of the Monterey and Pico Formations. The upperpart of the slope consists of interlayered siliceous shale, siltstone,and sandstone of the Middle to Upper Miocene MontereyFormation. 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 beenregionally associated with extensive landslide activity (Morton,1971; Harp and Jibson, 1995, 1996; Parise and Jibson, 2000). Thetwo formations are in fault contact along the active Red MountainFault, which extends across the slope face.Landslide HistoryFigure 1. Location map showing the La Conchita area (afterO’Tousa, 1995).3The bluff above La Conchita has produced a variety oflandslides over an extended period of time. Figure 3 showsLIDAR and false-color infrared images of the bluff above LaConchita and the surrounding area, and several sizes, types,and ages of landslides are visible. The arcuate bench at the topof the bluff is the head of a very large prehistoric landslide thataffected the entire bluff. Several smaller, more recent slumpsand earth flows also are visible, as is the 1995 slump–earth flowAFigure 3a. Oblique false-colorinfrared photograph of LaConchita taken in 2002. Thearcuate bench near the top ofthe bluff in the center of thephotograph is the main scarp ofan ancient landslide that involvedthe entire bluff. The 1995landslide is visible in the rightcenter of the photograph. Otherlandslides of various ages andsizes are visible on the slopes.Photograph courtesy of RobertLarson, GeoArchives Photography..BFigure 3b. Oblique LIDARimage of La Conchita afterthe 2005 landslide. Outline of1995 (blue) and 2005 (yellow)landslides shown; arrows showexamples of other landslides inthe area; red line outlines mainscarp of an ancient landslide thatinvolved the entire bluff. Thedeep canyon on the left producedmajor debris flows in both 1995and 2005. Image courtesy ofAirborne 1 Corporation, ElSegundo, Calif.4(terminology after Varnes, 1978). Similar combinations of largeancient landslides and smaller, recently active landslides alsoare present in areas southwest of the 1995 and 2005 landslides.In addition, large and small ravines that incise the bluff haveproduced debris flows recently and in the past.Historical accounts dating back to 1865 have reportedlandslides in the area around La Conchita as being a regularoccurrence (Hemphill, 2001). The Southern Pacific rail linethat extends along the coastal strip was inundated by landslidedebris in 1889 and again in 1909, when a train also was buried(Hemphill, 2001). Since that time, landslides frequently haveinundated roads, railroads, cultivated land, and more recently, theLa Conchita community (O’Tousa, 1995).1995 La Conchita LandslideThe following summary of the 1995 La Conchita landslide isextracted from O’Tousa (1995) and Anderson (Robert Anderson,RJR Engineering, 2005, personal commun.). On March 4, 1995at 2:03 p.m. PST, the La Conchita landslide failed and movedtens of meters in only a few minutes. The landslide, a complexslump–earth flow, destroyed or severely damaged nine houses.On March 10, a subsequent debris flow from a canyon to thenorthwest (see canyon on left part of figs. 2 and 3) damaged fiveadditional houses in the northwestern part of La Conchita. Themain mass that failed is on the southeastern margin of the largerancient landslide that encompasses the entire front of the bluff(see fig. 3). The 1995 slide was 120 m (400 ft) wide, 330 m (1100ft) long, and covered approximately 4 ha (10 acres). The depthwas estimated at greater than 30 m (100 ft), and the volume wasestimated at 1.3 million m3 (1.7 million yd3).Incipient movement of the upper part of the slide wasreported as early as the summer of 1994, when surface crackswere observed in the upper part of the slope (O’Tousa, 1995).Deformation continued as the rainy season began, and byDecember 1994, several open cracks on the hillside werechanneling surface runoff into the subsurface. Several smallerlandslides occurred between the summer of 1994 and March1995, when the large slide occurred.The 1995 landslide apparently occurred as a result of anextraordinarily wet year. Mean seasonal rainfall at Ojai (20 km[12 mi] northeast of La Conchita) from October 1 through March3 (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 (29.96in)—fell during that period (National Oceanic and AtmosphericAdministration, 1994b, 1995b). Figure 4 shows the rainfallFigure 4. Daily rainfall at Ojai (20 km [12 mi] northeast of La Conchita) from October 1, 1994, through March31, 1995 (data from National Oceanic and Atmospheric Administration, 1994b, 1995b). The 1995 landslideoccurred more than 1 month after the heaviest rainfall of the season.5distribution during the 1994–1995 rainy season. Most of theexcess rain fell in January, which had 623 mm (24.53 in) ascompared to a normal rainfall of 108 mm (4.26 in). February1995 produced only about one-third of the normal rainfall, but amodest 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 andprimarily moved as a coherent slump–earth flow. This mode ofmovement suggests that the landslide formed as a result of risingground-water levels in response to deep infiltration of antecedentseasonal rainfall. In particular, the extraordinary rainfall ofJanuary 1995 probably was the principal contributing factorto the elevated ground-water levels and, hence the landslidemovement. The storm of March 2–3 also may have played arole in triggering initial movement of the hillside, which alreadywas approaching instability due to longer-term seasonal risesin ground-water levels. Eyewitness accounts indicate, however,that similar to 2005, dust was in the air and much of the depositincluded relatively dry material. Eyewitnesses also reportedseeing material failing from the main scarp and lateral margins ofthe landslide as a result of removal of lateral support.wide. The landslide entered the La Conchita neighborhooddestroying 13 houses and severely damaging 23 others (figs. 5and 6). There were 10 confirmed fatalities.Earlier that morning, debris flows from canyons northwest ofLa Conchita reached Highway 101. Law enforcement officersand media representatives were in the area, which facilitatedcapturing the moving landslide on video. The KCAL-TV videoindicates that the landslide material mobilized simultaneously andnearly instantaneously into a highly fluid, rapidly moving debrisflow. I estimate from viewing the video that high on the slope, thelandslide was moving perhaps 10 m/s (30 ft/s). The developedpart of the slope where the houses were impacted has a flatterslope, and so the flow probably slowed to no more than 5 m/s(15 ft/s) in the neighborhood. This slower rate also is suggestedby eyewitnesses who stated that some residents were able tooutrun the advancing flow, which would not have been possible atthe higher upslope velocity.The 2005 landslide occurred at the end of a 15-day period thatproduced record and near-record amounts of rainfall in manyareas of southern California. At Ventura (20 km [12 mi] southeastof La Conchita) seasonal antecedent rainfall from October 1,2004 through January 10, 2005 totaled 493 mm (19.4 in) ascompared to the mean value of 122 mm (4.8 in). From December27, 2004 through January 10, 2005, Ventura received 378 mm(14.9 in) of rainfall, only slightly less than its mean annual totalof 390 mm (15.4 in) (Wofford, 2005; National Oceanic andAtmospheric Administration, 1994a, 1995a). Although rainfallintensities were not extreme, moderate- to high-intensity rainfallpersisted for more than 2 weeks, and the landslide occurred at theculmination of this 15-day high-rainfall period (fig. 7).2005 La Conchita LandslideThe 2005 La Conchita landslide occurred at about 12:30 p.m.on January 10. Little or no newly failed material was involvedin the landslide; rather, it consisted of a remobilization of thesoutheastern portion of the 1995 landslide deposit, involvingabout 200,000 m3 (250,000 yd3) (James O’Tousa, RJREngineering, personal commun., 2005). The landslide area wasapproximately 350 m (1,150 ft) long and 80–100 m (260–330 ft)Figure 5. View of the LaConchita landslide takenJanuary 14, 2005. The lightcolored, exposed rock in theupper part of the photographis the main scarp of the 1995slide. The southeast part ofthe 1995 deposit (right sideof photograph) remobilizedin 2005. At the bottom centerof photograph, a wall builtafter the 1995 slide is visible;the 2005 slide overtoppedand tilted parts of the wallforward.6Figure 6. View from the main scarpdown the length of the 2005 LaConchita landslide. Water visiblein the center of the photographwas issuing from the base of themain scarp when the photographwas taken (January 14, 2005). The2005 landslide remobilized about15 percent of the 1995 deposit andfollowed the left margin (lookingdownslope) of the 1995 slide. Thevegetated ridge in the left part ofthe photograph is intact materialthat did not fail in 1995 or 2005;the material in the right part of thephotograph is 1995 deposit that isstill in place.Figure 7. Daily rainfall atVentura (20 km [12 mi]southeast of La Conchita)from October 1, 2004through January 20, 2005(data from Wofford,2005). The 2005 landslideoccurred at the culminationof the heaviest rainfall ofthe season.7Inspection of the site within a few hours of the landslideindicated 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 floweddownslope. Thus, it appears that the landslide mobilized on asaturated layer deep in the 1995 deposit but that much of thematerial above this saturated zone was dry or nearly so. Thevideo shows relatively intact vegetation being rafted on thesurface of the rapidly flowing mass, which indicates that much ofthe landslide mass simply was being carried on the fluidized layerat depth, which presumably was much more saturated.Such a failure scenario, involving a significant amount of drymaterial that fully mobilized on a saturated layer, indicates thatmost of the rain that fell on the surface of the 1995 deposit didnot infiltrate but drained off the surface. The rising ground-waterlevel within the 1995 deposit would thus have resulted fromdeeper recharge from rainfall infiltration upslope. Figure 8 showsan aerial photograph of La Conchita taken in September 2004.The lush, green vegetation visible in the southeastern (lower rightin photograph) part of the 1995 deposit clearly indicates thatdrainage on and within the 1995 landslide deposit concentratedwater in the part of the mass that failed in 2005. At the time ofour visit (January 14, 2005) water was still issuing from the baseof the main landslide scarp and was ponding at several locationson the 2005 deposit (fig. 9).The 2005 landslide pushed many of the houses off theirfoundations and into each other at the toe of the landslide(figs. 10 and 11). A wall built after the 1995 landslide to keepminor 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 flowedrapidly, was quite viscous and pushed structures in front of itrather than flowing around them or filling them with mud, assometimes occurs with fully saturated debris and mud flows. Thisapparently resulted from a highly hazardous situation involvinga two-phased landslide mechanism: (1) a saturated, highly fluidlayer at depth on which the landslide mobilized that (2) carried athick layer of drier, much more viscous material that effectivelyacted as a battering ram.be the object of future research, and it is much too complex toanalyze in detail at this time. A few things, however, can be said.The timing of the two landslides with respect to the triggeringstorms 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 offailure in 1995 is consistent with this delay: deeper landslidescommonly are triggered by deep infiltration of rainfall, whichcan take weeks or months to occur (for example, Morton andCampbell, 1989). The 2005 landslide occurred at the culminationof an extremely wet 2-week period (fig. 7). This also is consistentwith the shallower, fluid mode of failure: shallow, rapid debrisflows most commonly occur during periods of prolonged, intenserainfall with little or no lag time (Campbell, 1975; Keefer andothers, 1987; Jibson, 1989).However, this still leaves some troubling questionsunanswered. Why did the landslide material not mobilize intoa rapid debris flow in 1995? What about the remaining 1995deposit? Since only about 15 percent of the 1995 depositremobilized in 2005, could the remainder also mobilize into arapid debris flow, or is it more likely to remobilize as a deepslump? Or will it remain metastable? Currently, we haveinsufficient data and understanding of the failure mechanismsof this landslide to adequately answer these questions, but itis clear that the hazard from renewed landslide movement isconsiderable.Continuing Hazards at La ConchitaComparison of 1995 and 2005 LandslidesThe movement of the same landslide mass in 1995 and 2005by two very different mechanisms, and with markedly differentresults, is difficult to explain. The 1995 landslide was a deep,coherent slump–earth flow that deformed plastically and movedslowly enough that people could get out of its way. The 2005landslide was a shallower remobilization of the very samematerial into a rapid, highly fluid debris flow that buried 10people. Although it is not uncommon for subsidiary debris flowsto occur from the toes or scarps of existing landslides (Mortonand Campbell, 1989), that is not what happened in 2005. Thiswas a wholesale remobilization of a significant portion of the1995 deposit. How and why the same material failed twice in10 years by fundamentally different mechanisms certainly will8Of primary interest to the general public and variousGovernmental entities is the current state of hazard at LaConchita. While this preliminary report does not representa detailed evaluation of those hazards, a few reasonableobservations can be made.1. Historical accounts and geologic evidence show thatlandsliding of a variety of types and scales has been occurringat and near La Conchita for many thousands of years, and ona relatively frequent basis, up until the present. There is noreason to believe this pattern of landsliding will stop.2. Even in the absence of additional significant rainfall thisyear (2005), the remainder of the 1995 landslide could stillremobilize, most likely as a deep slump–earth flow similarto that in 1995. This mode of movement would most likelybe relatively slow (compared to 2005) but still could poseserious hazards to property and, perhaps, life.3. If significant additional rainfall occurs, either this year orin 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) depositinto a rapid debris flow such as occurred on January 10, 2005,(c) triggering of subsidiary landslides from parts of the 1995and 2005 deposits or scarps, (d) triggering of slumps and(or) earth flows on adjacent hillsides, and (e) triggering ofrapid debris flows from various nearby slopes, particularly inravines.Figure 8. Othorectifiedairphotograph of LaConchita taken inSeptember 2004. The1995 landslide is outlinedin blue; the area thatremobilized in 2005 isoutlined in yellow. Notethe concentration of lush,green vegetation in thearea that remobilized in2005, which indicatesconcentration of surfaceand subsurface drainageinto this area. Image fromorthophotography providedcourtesy of AirPhoto USAand County of Ventura.Figure 9. Water wasissuing from the base ofthe main scarp at the timeof our visit (January 14,2005). The water pondedat several locations on the2005 deposit.9Figure 10. The 2005landslide moved as arapid debris flow, but itwas quite viscous andpushed houses in itspath rather than flowingaround or through them.The left part of the housewas detached from theright part and was pulledby the landslide severaldecimeters towardthe upper left of thephotograph.Figure 11. This housewas pushed about 3.5 m(12 ft) off its foundationtoward the right in thephotograph when the2005 landslide toppeda wall. The tops of theH-beams of the wall arevisible in the left part ofthe photograph.10Figure 12. Steel-andtimber wall built afterthe 1995 landslide wasovertopped and tiltedforward in places by the2005 landslide.4. The landslide scenarios sketched above potentially couldimpact any part of the La Conchita community. Futurelandslide activity could move into the same areas thatrecently have been damaged or could mobilize in otherdirections that could damage any or all of the developed area.ConclusionThe La Conchita area has experienced, and will likelycontinue to experience, a rather bewildering variety of landslidehazards. Different landslide scenarios are more or less likely tooccur as a result of different specific rainfall conditions, and nopart of the community can be considered safe from landslides.Unfortunately, we currently lack the understanding to accuratelyforecast what might happen in each possible rainfall scenario.Prudence would certainly dictate, however, that we anticipaterenewed landslide activity during or after future periods ofprolonged and(or) intense rainfall. Future earthquakes, of course,also could trigger landsliding in the area (Harp and Jibson, 1995,1996).ReferencesCampbell, R.H., 1975, Soil slips, debris flows, and rainstormsin the Santa Monica Mountains and vicinity, southernCalifornia: U.S. Geological Survey Professional Paper 851,51 p.Harp, E.L., and Jibson, R.W., 1995, Inventory of landslidestriggered by the 1994 Northridge, California earthquake:U.S. Geological Survey Open-File Report 95–213, 17 p.11Harp, E.L., and Jibson, R.W., 1996, Landslides triggered by the1994 Northridge, California, earthquake: Bulletin of theSeismological Society of America, v. 86, no. 1B, p. S319–S332.Hemphill, J.J., 2001, Assessing landslide hazard over a 130-yearperiod for La Conchita, California, in Association of PacificCoast Geographers Annual Meeting, Santa Barbara, Calif.,September 12–15 2001.Jibson, R.W., 1989, Debris flows in southern Puerto Rico, inSchultz, A.P., and Jibson, R.W., eds., Landslide Processes inEastern North America and Puerto Rico: Geological Societyof 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 duringheavy rainfall: Science, v. 238, p. 921–925.Morton, D.M., 1971, Seismically triggered landslides above SanFernando Valley: California Geology, v. 24, no. 4–5.Morton, D.M., and Campbell, R.H., 1989, Cyclic landslidingat Wrightwood, southern California, in Sadler, P.M., andMorton, D.M., eds., Landslides in a semiarid environmentwith 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.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 AEGNews, v. 38, no. 4, p. 22–24.Parise, Mario, and Jibson, R.W., 2000, A seismic landslidesusceptibility rating of geologic units based on analysisof characteristics of landslides triggered by the January17, 1994 Northridge, California, earthquake: EngineeringGeology, v. 58, p. 251–270.Wofford, Michael, 2005, Ventura, California weather conditions:http://www.venturaweather.com/daily.htm, last visitedJanuary 28, 2005.12