Mass Movements in Southern California and Hazard Reduction

POSTED: 10 June 2020

UPDATED: 23 October 2023: Case studies updated; reference to Pineapple Express clarified

• General Triggers of Mass Movements

• preexisting instability
• rain
• earthquakes
• thawing of frozen ground
• human construction (roads, houses, grading)
• irrigation
• failing infrastructure (water mains, etc.)

• Plate Tectonics

• causes uplift, therefore relief/topography
• causes faulting which weakens the crust
• causes volcanism, weathering and erosion
• causes earthquakes that trigger landslides/slumps

• The Case Southern California

• southern CA has been an active plate boundary for at least the last 50 Mio yrs (see lectures 4 and 10)
• weak substrate in general as part of mesozoic (250 - 67 Mio years ago) accretionary prism that was later uplifted and now forms much of southern California
  (recall that whole North American west coast used to be subduction zone of old and now gone Farallon plate!)
• faulting during past 15 Mio years (fractures are planes of weakness; steep cliffs)
• compression across SAF (oblique strike-slip!) causes uplift (slopes!)
• earthquakes shake the ground loose/reduce friction of potentially sliding masses

• Environmental Effects in Southern California

• chemical weathering in faults (e.g. through salt water) weaken them
• physical weathering in faults (e.g. through heating and freezing in the high desert) weaken them
• wave attack near bottom of cliffs leads to undercuts; upper portions of cliffs lose support from below
• semiarid climate causes sparse vegetation
• brush fires destroy vegetation, leading to increased mudflow risk in the next rainy season
• west coast climate brings occasional but heavy rains

• Human Effects in Southern California

• irrigation on unstable cliffs
• planting of non-native vegetation that requires increase in irrigation
• development on unstable ground, e.g. on old landslides (Mt. Soledad) and too close to cliff edge (some La Jolla Farms homes; western edge of Palos Verdes, CA)
• landscape alteration by aggressive development (oversteepening, overloading)
• changes to groundwater table (see Lecture 11)
  • excess withdrawal can lead to subsidence.
    Some extreme examples outside of Southern California:
    • groundwater withdrawal for farming and cities in San Joaquin valley resulted in drop of groundwater table by 120m since 1925, surface subsided by 8.5m
    • groundwater withdrawal for city of Las Vegas has resulted in subsidence by up to 3m since 1935
  • irrigation and leaky water/sewer systems can raise the ground water table above glide horizon, which lowers friction and increases risk of mass movements (reduce friction/resistance force) (Point Fermin, CA; see below)
  • OK: ground water table below glide horizon
  • NOT OK: ground water table above glide horizon
• failing infrastructure (e.g. leaky pools, leaky irrigation, leaky water pipes, leaky septic tanks, defective storm drains)

• CASE STUDY 1: Slumping in La Conchita

1995 slide (source: USGS)

La Conchita: a small town along SoCal coastline between Ventura and Santa Barbara area with steep, unstable hillsides is prone to mudslides 1909: devastating mudslide just north of village 1995: on March 4, a mudslide buried or damaged 7 homes, after rainfall; a debris flow followed on March 10 that damaged another 4-5 homes and a banana plantation; 2005: on January 10, a massive mudslide buried houses under some 9 m (30 ft) of dirt; 10 fatalities. 13 homes destroyed, 23 damaged; home insurance often do not cover mudslides; NB: mudslides may actually not be covered at all in La Conchita because of the known recurrence of slides

Possible natural trigger: in both 1995 and 2005, heavy rain was the likely trigger of mass movements though the timing in both cases was different. The slide in 2005 was preceded by heavy rain from a Pineapple Express that lasted from January 7 to 11 (see topic 16). A Pineapple Express is an atmospheric river that brings particularly moisture-laden are from a subtropical area near Hawaii to the U.S. West Coast. It brings relatively warm but extensive and severe rainfall, often causing flooding where the atmospheric river makes landfall. The name Pineapple Express was once created by news media and comes from the fact that Hawaii used to be a major producer of pineapples.
Possible trigger by human activity: Farming-related irrigation. La Conchita Ranch Co., a 700-acre avocado and citrus ranch that is situated on the cliffs above the town, was sued by homeowners after the slides for causing the slides as a result of excess irrigation. In a lawsuit in 2008, the ranch was found "50% negligent" because it did not provide adequate drainage of its orchards. In a settlement, the ranch agreed to turn over its ranch to the victims and their families to settle the suit.⁽¹⁾

• CASE STUDY 2: Portuguese Bend, Palos Verdes Peninsula

• one of the geologically most unstable areas in the world
• luxury cliff homes on unstable grounds; many built on ancient slides
• geologic setting: steep cliff underlain by seaward dipping weak volcanic tuff resting on shale


• slow slump: 1956-1985 of the entire Portuguese Bend area
• 0.3-2.5 cm/day moved a total of 200m destroying 150 homes


• possible cause for failure 1): during construction, developers dumped sediments near the top of the slope, along with hundreds of thousands of gallons of water
• possible cause for failure 2): already existing homes did not have modern sewer systems. Instead, each home used septic tanks that eventually leaked into the ground, thereby raising the groundwater table
• subsequently elevating groundwater table rose above glide horizon
• ultimately reducing friction in glide horizon

Reality after the slump:
• now stabilized by installing an above-ground drainage system
• however, surface road still hazardous to driving, with speed limits at 10 mph at some places.
• to date, the ground is still moving, and road is under constant repair.

• CASE STUDY 3: Point Fermin, Palos Verdes Peninsula, 1929/1930

• seaward dipping layers (sandstone and clay-rich mudstone)
• a vertical fault cut across the layers and cut into a weak zone
• block slide broke loose 30 m cliff
• Jan 1929; 800 m long block slides slowly (2m in one year)
• no one killed but houses on top of cliff twisted and had to be removed
• today, much of slide is cordoned off and still looks like it happened recently
• possible cause: irrigation

• CASE STUDY 4: Rancho Palos Verdes Golf Course/Ocean Trails

this is an interesting example to emphasize just how expensive a fix can get after a cliff collapse/slumping has occurred
• Jun 2, 1999: a cliff collapse destroyed the 18th hole of the cliff-top golf course
• "wet bentonite behaves like liquid soap"
• cost of repair: $20 Mio; owner goes bankrupt in process
• Donald Trump buys golf course for $27 Mio
• $260 Mio extreme makeover to open Trump National Golf Club Los Angeles
• spent $87 Mio to fix 18th hole: 115 shear pins (each 42 in in diameter, 106 ft deep) to anchor sliding mass to ground below
• possible cause: leaky sewer pipes; irrigation; rain (??)

• CASE STUDIES IN OUR OWN BACKYARD

• 1920: landslide closed tunnel in Carrizo Gorge
• 1949: Torrey Pines slump (ca. 500 m long)
• 1993: flooding in Tijuana triggers international disaster relief
• 1997: remnants of Hurricane Pauline triggers landslide in Tierra Blanca, burying school with 10 children
• Aug 07, 02: Houses of Cerro Maravilla bulldozed by city because the area was allegedly "unsafe due to landslide threat" (issue controversial)
• Jan 02, 03: Evacuation of Cumbres del Rubi (23 houses) "overbuilt hillside could collapse after heavy rain"
• Feb ??, 03: small landslide due to inadequate drainage
• Since October 03: mudslides in county after fire
• Winter season 04/05: numerous slides and sinkholes
  (e.g. large sinkhole in Fashion Valley, took many months to repair)

• CASE STUDY 5: Mt. Soledad Slump on October 3, 2007

this slump (dubbed erroneously by some as sinkhole) occurred on the morning the San Diego Union Tribune published a piece on ongoing tell-tale observations that the hillside was in motion. The newspaper article reported on Oct 3:
• hillside slid a few inches in past months, causing cracked streets, water-line breaks, buckling of alley
• street started cracking in July
• minor damage to 2 homes; a photo showed a half-foot gap in a homes foundation
• potential threat to more than 7 homes
• cause and mitigation unclear at this point
• engineers: "still gathering data"; more soil analysis planned for next 2 days; S.D. senior engineer: "until you know the exact geometry (of the slide plane), you can't fix it"
• city: "we are moving quickly (to prevent catastrophe)"
• water main disconnected; speed limit imposed on Soledad Mt. Road (main thoroughfare; heavy traffic); but no evacuation
• a letter warning residents about imminent danger was sent 2 months ago; officials have been knocking on residents' doors for last 2 days
during the morning hours, 200 ft of Soledad Mt. Road gave way, leaving behind a head scarp and a toe that buried Desert View Drive below. The slump destroyed 2 homes and left at least 7 uninhabitable. The cause of the slump is uncertain though it was not caused by rain (there was no rain in a long time!). Officials: "these types of failures are not the same as landslides caused by heavy rains, such as in 2005 (see La Conchita).
Possible causes include:
• breaking water mains (there is a chicken-and-egg question as whether to whether the breaking water mains caused the slide or the slide caused the pipes to break
• irrigation
• leaky pools on the top-side of Soledad Mt. Road though these were never discussed
• vibrations from heavy traffic
• vibrations from an unusual off-shore earthquake cluster earlier in the summer (as suggested by Pat Abbott)
For more info on this event, see appendix below.

• Protection against Mass Movements

• some areas are subject to repeated hazards
• avoidance (don't build in hazardous areas)...yeah right, tell a southern Californian!
• geologists look for movements in the past (slump head scarps, swaths of flattened or tilted trees, piles of loose debris at base of hills, hummocky land surfaces) to assess potential hazard risk


* IDENTIFYING REGIONAL RISKS
• roads, buildings, pipes crack
• power lines too tight or loose
• cracks of ground (potential head of slumps)
• bulge of ground (toe of slump) ("pressure ridge")
• anomalies in vegetation (sudden surplus or lack of water)
• bent tree trunks
• instrumental surveys (tilts, distances)


* ITEMS FOR POTENTIAL HAZARD AND LANDSLIDE MAPS
• slope steepness
• strength of substrate
• degree of water saturation
• dip of bedding, joints, foliation relative to slope
• vegetation cover
• climate
• undercutting
• seismicity


* PREVENTING MASS MOVEMENTS
• re-vegetations
• re-grading
• reducing subsurface water (improve drainage, reduce irrigation; repair leaky pools)
• preventing undercutting (offshore breakwater or pile riprap)
• constructing safety structure (stabilize slopes; retaining walls; spray road cuts with shotcrete; protect highways with avalanche sheds)

• Submarine Mass Movements

• rotational slumps within river deltas;
• distorted rock masses at subduction zones
• submarine volcanoes
Hawaii: largest mass movement on Earth; slumps and debris avalanches; some debris avalanches over 200km (125mi) long; volumes > 5,000km³ (1120mi³) (10,000 large sub-arial flows! Blackhawk Event); catastrophic volcano flank collapses; 70 in last 20 Mio years; tremendous tsunami hazard; some corals and shells found 365m (1120ft) above sea level on Lanai and 60m (200ft) above water and 2km inland on Molokai; Molokai has no craters anymore (i.e. slid down into ocean);
next event quite likely to happen southeast of Hawaii where rift zone weakens structure of island; right now movement is 25cm/yr; early November 2000 as fast as 6cm/day (silent earthquake); 10,000 people live on it but more are threatens through tsunami; there is 1 such flank collapse per 10,000 years worldwide;

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1. active plate boundary weakens crust
2. occasional heavy rain causes erosion and reduces friction of slumping masses
3. earthquakes trigger mass movements
4. bedrock is ground up by frequent hurricanes
5. semiarid climate causes sparse vegetation that would anchor soil

1. steepening slopes of hillsides beyond angle of repose
2. development on ancient landslide
3. plant native, drought-resistant plants
4. plant tropical plants that need artificial irrigation
5. leaking pools and sewer systems

1. increases
2. decreases

1. build retaining walls
2. lower the slope of a hillside
3. install drainage ditches
4. remove dirt from the foot of a hillside
5. plant deep-rooting, drought-tolerant trees

1. excess farming
2. installing drainage systems
3. droughts
4. all of the above
5. only a) and b)

• Appendix: More Material on 2007 Mt. Soledad Slide

• On October 31, 2007, the fix of the slump began
• 37 shear pins "to shore up the landslide"; in reality, this only took secured the head scarp to make Soledad Mt. Road passable again
• shear pins are 42-48 in wide and 60-66 ft long (see also Trump Golf Course fix); shear pins to "anchor dislodged earth into solid bedrock 70 ft below"; "shear pins are like tacks through a deck of cards"
• the work was estimated to last 1 month and no $$$ amount was initially given
• "repair of slides with tie backs not yet discussed because it involves private property"; ~$10 Mio ??
• Soledad Mt. Road reopened on 16 October, 2008
• by October 17, 2008: "used $10 Mio of $26 Mio allocated for response to this slide"
Shortly after the slide, engineers were cited as saying "we mapped a small, uncharted fault 37 ft below the surface. The slide appears to have broken along the same track as the fault."
• Previous slides: the San Diego City Hazard Map shows that the whole hillside is unstable and has experienced similar slides in the last 50 years
• a landslide in 1961, about 10 homes south of the 2007 slide, destroyed 7 homes under construction
• in 1990, new home construction just south of this site started another landslide
• a landslide in 1994 affected a canyon further north, below Desert View Drive; it "was repaired by residents". The slide is still visible from I-5 below.
• homeowners' policies do not cover landslides
• state law: recent buyer should be informed of potential hazard
• since 1998: sellers have to present natural hazard disclosure report but do not have to disclose previous landslides
• in an interesting twist, the city attorney, Michael Aguirre, claimed that the city is at fault, before the mayor spoke to the press. The attorney's motive was that if the city was at fault, then the insurance would pay. But if the slide was of natural causes, then the city would have to pay.
• on 1 October 2009, (65) residents lost a lawsuit against the city. On 14 April 2010, the city settled and agreed to pay nearly $284,000 to homeowners, far less than the $100 Mio claimed. San Diego spent $6 Mio on private lawyers, who blamed the slide on unstable soil. (source: FOX 6 News)