Rotational slip. National Oceanic and Atmospheric Administration, 1994a, Climatological data annual summaryCalifornia 1994: National Climatic Data Center, v. 98, no. Expansion of man-made developments into fire-prone wildlands has created situations where wildfires can destroy lives and property, as can the flooding and debris flows that are common in the aftermath of the fires. 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 dominant mode of movement is lateral extension accompanied by shear or tensile fractures. The latter landslide killed 10 people, and destroyed or damaged dozens of houses. Steep, recently burned areas in southern California are especially susceptible to debris flows (seeSouthern CaliforniaWildfires and Debris Flows- Fact Sheet 2001-3106). Figure 3 shows LIDAR and falsecolor infrared images of the bluff above La Conchita and the surrounding area, and several sizes, types, and ages of landslides are visible. Lesson All About Landslides: Land on the Run - TeachEngineering.org There are five basic categories of flows that differ from one another in fundamental ways. This cookie is set by GDPR Cookie Consent plugin. Mudflow: A mudflow is an earthflow consisting of material that is wet enough to flow rapidly and that contains at least 50 percent sand-, silt-, and clay-sized particles. During the winter season in California, beaches typically erode thereby allowing waves to reach further inland and to inundate the bottoms of coastal cliffs. Analytical cookies are used to understand how visitors interact with the website. These include oversteepening of the base of the slope, such as by natural erosion or excavation, and loading of the slope, such as by an inflow of water, a rise in the groundwater table, or the accumulation of debris on the slopes surface. Types of landslides Factors contribute to landslides Landslide in Utah. This website uses cookies to improve your experience while you navigate through the website. 4 What type of mass wasting took place in La Conchita? LaConchita Community Landslide | FEMA.gov Although many types of mass movements are included in the general term landslide, the more restrictive use of the term refers only to mass movements, where there is a distinct zone of weakness that separates the slide material from more stable underlying material. The bench at the top of the bluff is the head of a very large prehistoric landslide that affected the entire bluff. A .gov website belongs to an official government organization in the United States. They move very quickly. U.S. Department of the Interior | U.S. Geological Survey Saturated soil slumps down a curved surface. Bits of rock fall off the cliff face, usually due to freeze-thaw weathering. 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. How many people died in the landslide on Rincon Mountain? Examine Figure 1 (an oblique air photo of the slide features on the coast at La Conchita), Figure 2 (a close-up view of the 1995 landslide), and Figure 3 (some common types oflandslides). A summary of recent and past landslides and debris flows caused by rainfall in Southern California. People were evacuated and the houses nearest the slide were completely destroyed. Recent Landslides in La Conchita, California Belong to a Much Larger Several smaller, more recent slumps and earth flows also are visible, as is the 1995 slumpearth flow (terminology after Varnes, 1978). 1B, p. S319S332. Since that time, landslides frequently have inundated roads, railroads, cultivated land, and more recently, the La Conchita community (O'Tousa, 1995). Of primary interest to the general public and various Governmental entities is the current state of hazard at La Conchita. [2] It is likely that the new rainfall did not enter the deposits left behind after the 1995 landslide, but slid off it taking surface debris and few new materials with it.[2]. Figure 3: Some major types of mass movements. People were evacuated and the houses nearest the slide were completely destroyed. Landslides can also be activated by processes that weaken the shear strength of a slopes material. What are various methods available for deploying a Windows application? 3I). Analysis of the 1995 and 2005 La Conchita, CA Landslides - NASA/ADS Even modest rain storms during normal, non-El Nio years can trigger post-wildfire debris flows. Our editors will review what youve submitted and determine whether to revise the article. This information provides tools to help make PDF files accessible. Heavy rains of January 1995 led to a March 4, 1995, landslide in the La Conchita Community of Ventura County (County), California. 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. landslide, also called landslip, the movement downslope of a mass of rock, debris, earth, or soil (soil being a mixture of earth and debris). The cookie is used to store the user consent for the cookies in the category "Other. In a rotational slide the axis of rotation is roughly parallel to the contours of the slope. Eyewitness accounts indicate, however, that similar to 2005, dust was in the air and much of the deposit included relatively dry material. [2] The top of the bluff was covered by avocado and citrus orchards. 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. Falls of large volume can trap enough air to facilitate the very rapid flow of rock or debris, forming rock avalanches and debris avalanches, respectively. Landslides occur when gravitational and other types of shear stresses within a slope exceed the shear strength (resistance to shearing) of the materials that form the slope. Some, but not all, of the major winter storms that have caused landslide fatalities and property damage in southern California have occurred during El Nio (1997-98 info) conditions. This was not the first destructive landslide to damage this community, nor is it likely to be the last. Corrections? The 2005 landslide mobilized the south flank of a larger landslide that occurred in March 1995, and destroyed 9 homes. Or will it remain metastable? Photo by Pam Irvine, CGS. 8 What year did the landslide happen in Ventura CA? 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). The KCALTV video indicates that the landslide material mobilized simultaneously and nearly instantaneously into a highly fluid, rapidly moving debris flow. This cookie is set by GDPR Cookie Consent plugin. Creep: Creep is the imperceptibly slow, steady, downward movement of slope-forming soil or rock. Experts are tested by Chegg as specialists in their subject area. Landslides are a type of "mass wasting" which denotes any down slope movement of soil and rock under the direct influence of gravity. In the past two decades major landslides devastated the town of La Conchita, California. 1 it is The landslide area was approximately 350 m (1,150 ft) long and 80100 m (260330 ft) wide. YUARA SLUMP EARTHFLOW DEBRIS SLIDE ROCKFALL SCARP ORIGINAL LAND SURFACE -SLIP PLANE. A lock ( [1] The USGS report stated that "no part of the community can be considered safe from landslides. 12). Figure 4 shows the rainfall distribution during the 19941995 rainy season. There are generally three types of creep: (1) seasonal, where movement is within the depth of soil affected by seasonal changes in soil moisture and soil temperature; (2) continuous, where shear stress continuously exceeds the strength of the material; and (3) progressive, where slopes are reaching the point of failure as other types of mass movements. Often referred to as "mudflows," debris flows are a type of landslide made up of a rapidly moving mixture of dirt, rocks, trees, and water (and sometimes ash) that start on a hillside and travel downvalley. There are four different types of mass movement: The landslide, which occurred about 130 km northwest of Los Angeles, California, mobilized over 40,000 cubic yards of wet debris into a large scale debris flow (commonly referred to in the media as a mudslide) that flowed into a residential community at the foot of the slope, killing 10 persons and damaging or . A translational slide typically takes place along structural features, such as a bedding plane or the interface between resistant bedrock and weaker overlying material. Examine Figure 1 (an oblique air photo of the slide features on the coast at La Conchita), Figure 2 (a close-up view of the. A classification system based on these parameters is shown in table 1. Reported landslides began in 1865. 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). It is often triggered by heavy rainfall. Debris avalanche: This is a variety of very rapid to extremely rapid debris flow (fig. 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. 251270. These schematics illustrate the major types of landslide movement. La Conchita (1995) La Conchita landslide, Ventura County. 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. Over time, upslope ponding of water by such back-tilted blocks can enlarge the area of instability, so that a stable condition is reached only when the slope is reduced to a very low gradient. The landslide, a complex slumpearth flow, destroyed or severely damaged nine houses. Sometimes more than one type of movement occurs within a single landslide, and analysis of each requires detailed interpretation of landforms and geological sections. { "10.01:_Why_It_Matters" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "10.02:_Classifying_Mass_Movements" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "10.03:_Erosion_and_Deposition_by_Gravity" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "10.04:_Driving_and_Resisting_Forces" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "10.05:_Landslide_Types_and_Processes" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "10.06:_Putting_It_Together" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, { "00:_Front_Matter" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "01:_Science_of_Geology" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "02:_Rock_Forming_Minerals" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "03:_Rocks_and_the_Rock_Cycle" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "04:_Soil" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "05:_Plate_Tectonics" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "06:_Earth\'s_Interior" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "07:_Crustal_Deformation" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "08:_Earthquakes" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "09:_Volcanic_Activity" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "10:_Mass_Movement" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "11:_Hydrology" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "12:_Geological_Implications" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()", "zz:_Back_Matter" : "property get [Map MindTouch.Deki.Logic.ExtensionProcessorQueryProvider+<>c__DisplayClass228_0.b__1]()" }, [ "article:topic", "showtoc:no", "license:ccby", "licenseversion:40" ], https://geo.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fgeo.libretexts.org%2FBookshelves%2FGeology%2FFundamentals_of_Geology_(Schulte)%2F10%253A_Mass_Movement%2F10.05%253A_Landslide_Types_and_Processes, \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}}}\) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\), Original content from Kimberly Schulte (Columbia Basin College) and supplemented by, Creative Commons Attribution 4.0 International, Combination or two or more principal types of movement. [3] Historical accounts support this. Below is a summary of reports and maps to identify hazards associated with different types of landslides in this part of the state. ) or https:// means youve safely connected to the .gov website. Do you know what type of landslide occurred in La Conchita? A translational slide is sometimes called a mud slide when it occurs along gently sloping, discrete shear planes in fine-grained rocks (such as fissured clays) and the displaced mass is fluidized by an increase in pore water pressure. The U.S. Geological Survey, Multi Hazards Demonstration Project (MHDP) uses hazards science to improve resiliency of communities to natural disasters including earthquakes, tsunamis, wildfires, landslides, floods and coastal erosion. Other classification systems incorporate additional variables, such as the rate of movement and the water, air, or ice content of the landslide material. 2 and 3) damaged five additional houses in the northwestern part of La Conchita. In 2005, an additional landslide occurred at the end of a 15day period of near-record rainfall levels. Wofford, Michael, 2005, Ventura, California weather conditions. On March 4, 1995, the hill behind La Conchita failed, moving tens of meters in minutes, and buried nine homes with no loss of life. Similar combinations of large ancient landslides and smaller, recently active landslides also are present in areas southwest of the 1995 and 2005 landslides. Many readers will be familiar with the La Conchita landslide in California. You also have the option to opt-out of these cookies. Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. The catastrophic movement of the 2005 landslide damaged or destroyed 36 houses and killed 10 people. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. What was the type of landslide in recent 2005 La Conchita landslide? Debris avalanche: This is a variety of very rapid to extremely rapid debris flow (fig. They write new content and verify and edit content received from contributors. La Conchita Beach in Ventura, CA - California Beaches 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. Separation occurs along discontinuities such as fractures, joints, and bedding planes, and movement occurs by free-fall, bouncing, and rolling. You'll get a detailed solution from a subject matter expert that helps you learn core concepts. Maps displaying where shallow landslides are most likely to occur are based on observation of previous landslide activity, topographic slope, and information on the bedrock material (seePreliminary soil-slip susceptibility maps, southwestern California-Open-File Report 2003-17). La Conchita Landslide: Case History and Remedial Measures 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. In southern California, at least 25 cm (10 in) of rainfall during the winter is needed to nearly saturate the ground. 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. British Geological Survey - What is a landslide? Types of mass movement Flows are a mixture of water, rock and sediment. [2] The County of Ventura immediately declared the whole community a Geological Hazard Area, imposing building restrictions on the community to restrict new construction. Some, but not all, of the major winter storms that have caused landslide fatalities and property damage in southern California have occurred duringEl Nio(1997-98 info) conditions. Landslides in the United States cause approximately $3.5 billion (year 2001 dollars) in damage, and kill between 25 and 50 people annually. Landslides are described and classified in various ways depending upon the material (s) involved, mode of movement, rate of movement, and the water, air or ice content of the landslide material (see Figure 4). Debris flows are commonly caused by intense surface-water flow, due to heavy precipitation or rapid snowmelt, that erodes and mobilizes loose soil or rock on steep slopes. Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet. Types of Landslides The term "landslide" describes a wide variety of processes that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill, or a combination of these. https://www.britannica.com/science/landslide, Geology.com - Landslide Hazard Information. The material moves as a single mass along a slippery zone. Landslide Types and Processes - USGS Colorado, and northern New Mexico) are complex in rock type and climate, and so the landslides there are also complex. The 1995 landslide was a deep, coherent slumpearth flow that deformed plastically and moved slowly enough that people could get out of its way. These tools convert Adobe PDF documents into HTML or ASCII text, which then can be read by a number of common screenreading programs that synthesize text as audible speech. While shallow, that. 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. Large blocks of rock slide downhill. Agenda What is a landslide? Figure 2. Nearly all kinds of slope-failure processes have been or are currently active in the region. Mudflow. 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.2955. The 1995 landslide had a horizontal length, vertical length, and depth of 120 x 330 x 30 m and covered approximately 10 acres with a volume of 1.3x106 m3. There are generally three types of creep: (1)seasonal, where movement is within the depth of soil affected by seasonal changes in soil moisture and soil temperature; (2) continuous, where shear stress continuously exceeds the strength of the material; and (3) progressive, where slopes are reaching the point of failure as other types of mass movements. On January 10, 2005, the southeastern portion of the 1995 landslide deposit failed, resulting in shallow, rapid fluid flow, unlike the 1995 landslide. La Conchita, California - Wikipedia Southern California lies astride a major tectonic plate boundary defined by the San Andreas Fault and numerous related faults that are spread across a broad region. The 2005 landslide occurred on part of a previous landslide that occurred in 1995. A week later, debris flow in the same location damaged five more houses. Saturated soil (soil filled with water) flows down a slope. In 2005, an additional landslide occurred at the end of a 15day period of near-record rainfall levels. 3J). La Conchita lies on a narrow coastal strip about 250 m (800 ft) wide between the shoreline and a 180m (600ft) 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. Since that time, other slides have occurred, covering at times cultivated land, roadways, and the community itself.[4]. Earthflow: Earthflows have a characteristic hourglass shape (fig. The La Conchita landslide then occurred on March 4. Movement is caused by shear stress sufficient to produce permanent deformation, but too small to produce shear failure. The costliest landslide in U. history occurred in the Thistle, Utah area in April 1983. 3D). [2], Geologic evidence indicates that landslides have been occurring at and near La Conchita for many thousands of years up to the present, on a geologically frequent basis. Some materials with a loose, open sediment fabric will weaken if they are mechanically disturbed or flooded with water. The latter landslide killed 10 people, and destroyed or damaged dozens of houses. A slide happens when a section of soil or rock suddenly gives way and moves down a slope. [6] Figure 2. Officials in Southern California say a coastal landslide closed a historic cultural center, shut down rail service in the area and forced the evacuation of nearby residences. A) La Conchita, California The community of La Conchita in Ventura County, California experiences frequent landslides. Chapter 12 Flashcards | Quizlet The already loosened sediment from the 1995 landslide made the 2005 event a lot more catastrophic. Landslides occur when gravitational and other types of shear stresses within a slope exceed the shear strength (resistance to shearing) of the materials that form the slope. 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 OpenFile Report 95213, 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. This dynamic tectonic environment has created a spectacular landscape of rugged mountains and steep-walled valleys that compose much of the regions scenic beauty. If people continue to live in La Conchita, more lives will . By clicking Accept All, you consent to the use of ALL the cookies. 45. National Oceanic and Atmospheric Administration, 1995a, Climatological data annual summaryCalifornia 1995: National Climatic Data Center, v. 99, no. The term "landslide" describes a wide variety of processes that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill, or a combination of these. 3G). What type of slide is the La Conchita slide? - Heimduo Some move slowly, while others can move rapidly with little notice. The two major types of slides are rotational slides and translational slides. The project engages emergency planners, businesses, universities, government agencies, and others in preparing for major natural disasters. People were evacuated and the houses nearest the slide were completely destroyed. TYPES OF LANDSLIDES The term "landslide" describes a wide variety of processes that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill, or a combination of these. Below are other science projects associated with this project. Creep: Creep is the imperceptibly slow, steady, downward movement of slope-forming soil or rock. This is a typical type of landslide. The USGS has conducted hazard assessments forpost-wildfire debris flowsfor four recent fires in southern CA, as well as numerous fires across the Western U.S. including central and northern California. The tragedy came 10 years after the hill collapsed in 1995 when nine homes were lost. Alaska and Hawaii also experience all types of landslides. The entrance to La Conchita is from the northbound lanes of Hwy 101. The plaintiffs also sued the County of Ventura for damages, claiming that a wall that the County built at the base of the landslide caused or contributed to the landslide. Wildland fires are inevitable in the western United States. An unstable hillside The rain caused debris from an older landslide to fall about 150 feet. October 17, 2022. Law enforcement officers and media representatives were in the area, which facilitated capturing the moving landslide on video. Landslides are most abundant in areas underlain by marine shale . Other hazards, such as flash floods and debris flows, now become the focus. Triggered by earthquake shock or torrential rain in mountainous relief with steep gradients, a huge volume of avalanching rock or debris (of up to millions of metric tons) can reach a velocity of more than 50 metres (160 feet) per second and leave a long trail of destruction. 10 and 11). Unfortunately, we currently lack the understanding to accurately forecast what might happen in each possible rainfall scenario. Landslides are generally classified by type of movement (slides, flows, spreads, topples, or falls) and type of material (rock, debris, or earth). Surface cracks in the upper part of the slope were noted as far back as the summer of 1994. Debris-flow source areas are often associated with steep gullies, and debris-flow deposits are usually indicated by the presence of debris fans at the mouths of gullies. 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).
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