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Home , Earthquake, Effective stress, Landslide, Sand

kChapter 4

GERALD F. WIECZOREK TRIGGERING MECHANISMS

1. INTRODUCTION 2.INTENSE RAINFALL

andslides can have several causes, including that produce intense rainfall for periods as L geological, morphological, physical, and hu- short as several hours or have a more moderate in- man (Alexander 1992; Cruden and Vames, Chap. tensity lasting several days have triggered abun- 3 in this report, p. 70), but only one trigger (Varnes dant in many , for example, 1978, 26). By definition a trigger is an external (Figures 4-1, 4-2, and 4-3). - stimulus such as intense rainfall, shak- documented studies that have revealed a close ing, volcanic eruption, , or rapid stream relationship between rainfall intensity and acti- that causes a near-immediate response in vation of landslides include those from California the form of a landslide by rapidly increasing the (Campbell 1975; Ellen et al. 1988), North stresses or by reducing the strength of slope mate- Carolina (Gryta and Bartholomew 1983; Neary rials. In some cases landslides may occur without an and Swift 1987), (Kochel 1987; Gryta apparent attributable trigger because of a variety or and Bartholomew 1989; Jacobson et al. 1989), combination of causes, such as chemical or physi- (Jibson 1989; Simon et al. 1990; cal of materials, that gradually bring the Larsen and Torres Sanchez 1992)., and slope to failure. The requisite short time frame of (Wilson et al. 1992; Ellen et al. 1993). cause and effect is the critical element in the iden- These studies show that shallow landslides in tification of a landslide trigger. and weathered often are generated on The most common natural landslide triggers are steep slopes during the more intense parts of a described in this chapter, including intense rainfall, storm, and thresholds of combined intensity and rapid snowmelt, water-level change, volcanic erup- duration may be necessary to trigger them. In the tion, and earthquake shaking, and examples are pro- Santa Monica of , vided in which observations or measurements have Campbell (1975) found that rainfall exceeding a documented the relationship between triggers and threshold of 6.35 mm/hr triggered shallow landslides landslides. Some geologic conditions that lead to that led to damaging flows (Figure 4-4). susceptibility to landsliding caused by these triggers During 1982 intense rainfall lasting for about 32 are identified. Human activities that trigger land- hr in the San Francisco of California slides, such as excavation for cuts and irriga- triggered more than 18,000 predominantly shallow tion, are not discussed in this chapter. To the extent landslides involving and weathered rock, possible, examples have been selected that illustrate which blocked many primary and secondary landslide damage to transportation systems. (Ellen et al. 1988). Those landslides whose times

76 FIGURE 4-1 Landslide blocking State Highway 1 near Julia Pfeiffer-Burns State Park, California: debris slides and flows from toe and flanks of reactivated landslide. Intense storm Februdry 28—March 1, 1983, triggered many debris flows that blocked primary and secondary roads along coastline. G.F. WIF(70REK, MARCh 125, 933

FIGURE 4-3 Landslide blocking State Highway 1: excavation of of estimated 6.1 million .. RV m3 in 6-m-wide ' 7 W!i1; •3.: ,,'. '' D ? ; 4t benches extending about 300 m above roadbed made this the largest highway repair job ';: undertaken in California history. Highwayropennd in April 1984 (Works 1984). CALIFORNIA 1)EPARTMENT OF TRANSPORTAThON

FIGURE 4-2 (above left) Landslide blocking State Highway 1: May 1, 1983, massive rock slide of 1.2 million m3 incorporated entire hillside. Exceptionally heavy rainfall during winters of 1981-1982 and 1982-1983 was responsible for raising levels and triggering slide. During excavation, groundwater flow of approximately 378,000 Llday was collected and drained from cut (Works 1984). CALIFORNIA DEPARTMENTOFTP,ANSF'ORTATION 78 Lands/ides: /nvestigation and Mitigation

FIGURE 4-4 700 Cumulative rainfall at selected recording 25 gauges in Santa 600 Monica and San z E Gabriel Mountains, ' 20 500 -J —j southern California. —j —I 4 4 Known times zLI. U. of debris flows SAN DIMAS 400 Z 15 F— TAN BARK FLAT 4 indicated by heavy cc dots. Steepness of F- LECHUZA PT Ui — I- STAFC3528 300 > cumulative rainfall ------ -- -- I- 10 --- 4 line indicates BELAIRFC 108 intensity of rainfall SEPULVEDA 200 (modified from F BURBANK VALLEY \ ' CIVIC CTR PMPPLT Campbell 1975). 100 FLINTRIDGE FC 2808

il 1''11 11 111 11 111 1 I 111111111111111, 11111111.1111 E BEEEE EE E EEE EEEE BEEBE EEEE EE E EEEE E EE EEEE BE EE E E 0.00 aa.o d.a ,i 0 d.0.o 0.0.000.0. d d.d.o , d.Q.0 QQ 000.0.00 ä.ä. a d. j coc.awcqWCJ wc.Jwccoc.JwrJ C0CgC0NWOJWCWCIW1WC €004 ,00JCOCSJ tgWCvWOJw'cucOOJ w JANUARY 1969

FIGURE 4-5 Rainfall thresholds that triggered abundant landslides in San Francisco Bay region, California. Thresholds for high E and low mean annual L Threshold for high MAP 1 20 precipitation (MAP) . I- Threshold for I 0wMAP areas are indicated as C curves representing got combination of /1 I C 0.4 rainfall intensity and S duration (modified _110 from Cannon and 02 S. Ellen 1985).

10 15 20 25 30 35 40 45 Duration (hours)

could be well documented were closely associated sures, is generally believed to be the mechanism by with periods of most intense precipitation; which most shallow landslides are generated dur- this documentation permitted identification of ing storms. With the advent of improved instru- landslide-triggering rainfall thresholds based on mentation and electronic monitoring devices, both rainfall intensity and duration (Figure 4-5) transient elevated pore pressures have been mea- (Cannon and Ellen 1985). Such thresholds are sured in hillside soils and shallow during regional, depending on local geologic, geomorphic, rainstorms associated with abundant shallow land- and climatologic conditions. sliding (Figures 4-6 and 4-7) (Sidle 1984; Wilson The rapid of rainfall, causing soil and Dietrich 1987; Reid et al. 1988; Wilson 1989; saturation and a temporary rise in pore-water pres- Johnson and Sitar 1990; Simon et al. 1990).

Landslide Triggering Mechanisms

Loose or weak soils are especially prone to land- 1.50 30 slides triggered by intense rainfall. may — INTENSITY — ------1.00 -- produce a water-repellent (hydrophobic) soil layer 20 below and parallel to the burned surface that, to- gether with loss of vegetative cover, promotes rav- eling of loose coarse soil grains and fragments at UZ :::: the surface. Increased overland flow and nIl for- mation then lead to small debris flows ( 1987). On the lower parts of hill slopes and in NEST 2 stream channels, major storms generate high sedi- ment content in streams (hyperconcentrated flows) or large debris flows (Scott 1971; Wells et al. 1987; Weirich 1989; Florsheim et al. 1991). 75 cm DEPTH Shortly after midnight on January 1, 1934, an ----150cm DEPTH intense downpour after more than 12 hr of rainfall -100 resulted in debris flows from several recently 100 burned canyons into the La Canada Valley of NEST 3 southern California and caused significant prop- erty damage and loss of life (Troxell and Peterson 1937). Following an August 1972 wildfire north of Big Sur in central coastal California, storms -- - IOcmDEPTH with intensities of 19 to 22 mmfhr triggered two 75 cm DEPTH episodes of debris flows. During the second, more 120 cm DEPTH -100 devastating storm on November 15, 1972, large debris flows reached Big Sur about 15 min after in- 100 NEST 4 — 75 cm DEPTH tense (22-mmfhr) (Johnson 1984). Debris ----125 cm DEPTH flows blocked California State Highway 1 with , , and vegetative debris; the flows 1r\r\ -I, , '1 ,' - •'—. '\ partly buried, heavily damaged, or leveled struc- I I

I tures and caused one fatality (Jackson 1977). .s, '. I • — ..I In and regions, intense storms can trigger debris - flows in thin loose soils on hillsides or in 0 24 48 72 96 120 144 168 in stream channels (Woolley 1946; Jahns 1949; TIME(h) Johnson 1984). On September 14, 1974, an in- tense passed over Eldorado Canyon, FIGURE 4-6 Nevada, and although the duration of the rainfall Response of pore was short (generally less than an hour), the inten- August 1981 a wildfire had removed all , pressure to rainfall in sities were very high—from 76 to 152 mm/hr for exposed bare soil, and converted 15 percent of the shallow hillside soils 30 min. The intense rain eroded shallow soils, burned area to a hydrophobic condition; by June of northern California. leaving nills on some of the sparsely vegetated hill- 1982 reseeded grasses were establishing themselves Positive peaks of pore pressure correspond sides, and the high runoff scoured unconsolidated because of the wet winter of 1981-1982. In a recording gauge 1.2 km away, rainfall of 46 mm in to periods of high alluviuin from the larger stream channels. The rainfall intensity; initial debris-flow surge, heavily laden with sedi- 6 mm, 76 mm in 18 mm, and 101 mm in 27 mm negative pore ment and with the consistency of fresh , was measured during the height of the storm. This pressures indicate emerged from the canyon with a high steep front, intense rain triggered a by sheet and nIl soil tension in partly damaging a marina and killing at least nine people erosion from shallow soils and from erosion of al- saturated soil at beginning of storm (Glancy and Harmsen 1975). luvium within tributary gullies as well as the main gully. The resulting debris flow and closed or during periods On June 18, 1982, a very intense thunderstorm between rainfall occurred over a recently burned steep drainage of California State Highway 50 for 5 hr while main- (modified from the South Fork American in California be- tenance crews removed rocky debris from the Johnson and Sitar tween the towns of Kyburz and Strawberry. In (Kuehn 1987). 1990). 80 Landslides: Investigation and Mitigation

FIGURE 4-7 (a) Acceleration time (a) histories and (b) response of pore pressure in liquefied Horizontal-360 silty layer from November 1987 Superstition Hills (California) ('I earthquake. 0 Acceleration time 200 Horizontal-090 histories were C recorded at ground .2 0 surface and beneath . -200 liquefied layer. a) Horizontal-360 0 P1, P2, 0 P3, and PS are in liquefied silty sand Up 0 - layer; P6 is in 0 layer that did not a liquefy (modified Horizontal-090 from Holzer et al. 1989). J I I I I I I 0 20 40 60 80 100 Time (s) (b) 11 1 111 1 1 1 1 1 1 1 1 1 'I P6 (Depth = 12.Om) Ca a- 0 P5 (Depth = 29m) 3) 0 Cd, 3) 100 P3 (Depth = 6.6m) a)0. P2 (Depth = 3.0m)

P1 (Depth = 5.0m)

0 20 40 60 80 100 Time (s)

On September 7, 1991, a debris flow triggered 3. RAPID SNOWMELT by heavy rainfall (63 to 213 mm) within a 24-hr period damaged several in a subdivision of Rapid melting of a snowpack caused by sudden North Ogden, . Concentration of runoff from warming spells or by rain falling on snow can add the storm mobilized talus and other debris in trib- water to hillside soils. Horton (1938) examined utary channels and scoured material from the main the infiltration and runoff of melting snow into into a debris flow, which emerged from soil, including the special case of the effects of rain the canyon and traveled about 400 m down an al- on snow cover. He found that the process of melt- luvial fan before reaching the subdivision (Mulvey ing may provide a more continuous supply of mois- and Lowe 1992). ture over a longer time period compared with the Landslide Triggering Mechanisms 81 usual duration of infiltration from rain. Snowmelt down can occur when a river drops following a may also recharge shallow fractured bedrock and , the water level in a or canal raise pore-water pressures beneath shallow soils, is dropped suddenly, or the level drops follow- thus triggering landslides (Mathewson et al. 1990). ing a storm . Unless pore pressures within the Near Wrightwood, California, a steady thaw of slope adjacent to the falling water level can dissi- a heavy snowpack over a 40-day period in the pate quickly, the slope is subjected to higher spring of 1969 triggered mud flows in Heath Creek stresses and potential instability (Figure 4-8) from saturated debris in steep channels and from (Terzaghi 1943; Lambe and Whitman 1969). In steep faces in the toe area of the Wright terms of effective , Bishop (1954, 1955) in- landslide (Morton et at. 1979). In Utah during an troduced a method to estimate the pore-water unusually warm 10-day period from late May to pressure in terms of reduction of the principal early June 1983, a heavy winter snowpack along stresses and to analyze slope stability due to the re- the Wasatch Front began to melt rapidly and trig- moval of the water load during rapid drawdown. gered approximately 150 debris flows and other types of landslides (Pack 1984; Wieczorek et al. 1989). In the Wasatch Front above Farmington, Utah, during the height of this activity (May 28-30, 1983), snowmelt provided the equivalent of approximately 2.1 to 2.6 mm/hr of precipitation; yore pressure on May 30, 1983, a large debris flow emerged from hydrostatic under high water level the canyon of Rudd Creek into the community of .' Farmington (Vandre 1985). Rain-on-snow events commonly reduce the of the snowpack and add sufficient water to soils to be significant in triggering land- slides. In coastal , Sidle (1984) found that snowmelt before rainfall augmented the piezo- metric response. In a small watershed of western

Oregon, Harr (1981) found that 85 percent of —in water load landslides that could be dated accurately were as- sociated with snowmelt during rainfall. of slope A majority of the documented landslides in the central Sierra Nevada of California in mid-April 1982 and in early and mid-March 1983 occurred during rain-on-snow events (Bergman 1987). Landslides along Stump Springs Road, a major tim- ber-haul route in Sierra National Forest, Cali- fornia, were triggered by a rain-on-snow event that Pore pressure FIGURE 4-8 from transient included peak rainfall intensities of 14 to 18 mm/hr Response of slope supplemented by snowpack losses equivalent to LJ to rapid drawdown: 130 mm of water. Landslide repairs of Stump (C) (a) initial equilibrium Springs Road required an estimated $1.3 million condition, (b) after along a 23-km section during 1982 and 1983 drawdown but (DeGraffet at. 194). before consolidation adjustment, (c) after Pore pressure consolidation 4. WATER-LEVEL CHANGE hYdrostac under adjustment, and / ow water level ( final equilibrium The sudden lowering of the water level (rapid condition (Lambe drawdown) against a slope can trigger landslides in and Whitman 1969). REPP.INTED WITH , along coastlines, and on the banks of (d) PERMISSION OF , , canals, and . Rapid draw- JOHN WILEY & SONS, INC 82 Landslides: Investigation and Mitigation

Thick uniform deposits of low-permeability clays produced high water pressures in tension cracks and are particularly susceptible to landsliding behind the free face and were not associated with triggered by rapid drawdown. Morgenstern (1963) rapid drawdown. listed 16 cases in which rapid drawdown triggered During and following of Grand landslides in the upstream face of earth dams. Coulee Dam in Washington State, some 500 land- Rapid drawdown triggered four landslides in slides were noted between 1941 and 1953 along the very low-permeability in the Fort of Franklin D. Roosevelt . Accurately Henry and Ardclooney embankments, Ireland. dated landslides among this sample were most fre- The best documented of these slides occurred after quent during the filling stage of the reservoir and a drawdown of 1.1 m in 10 days; during the last subsequent to filling during two periods of rapid 24 hr the average drawdown rate was 0.35 rn/day drawdown (Jones et at. 1961). Even larger draw- (Massarsch et at. 1987). In the coastal area of downs during the period from 1969 to 1975 were re- Zeeland, Netherlands, Koppejan et al. (1948) ob- sponsible for additional earth slumps, earth spreads, served that excessive tidal differences of 2.8 to 4.6 earth flows, and debris flows (Schuster 1979). m during spring or coinciding with gales triggered Increases in groundwater levels on hill slopes wet sand flows. From a few observations, they con- following periods of prolonged above-normal pre- cluded that movement started during drawdown of cipitation or during the raising of water levels in the ebb tide between half tide and low water. rivers, lakes, reservoirs, and canals build up pore- Springer et at. (1985) inspected more than water pressure and reduce effective strength of sat- 6500 km of the Ohio River system and examined urated slope materials and can trigger landslides 120 landslide sites in detail. They observed several (Figures 4-2 and 4-9). The initial filling of characteristic types of instabilities, including mas- Yellowtail Reservoir, , and also of the sive slumps evidently triggered by rapid drops in Panama Canal were cited by Lane (1967) as exam- river level following . Other landslides, co- ples in which large landslides were triggered by ini- hesive wedges of material sliding on thin sand tial raising of the water levels on natural or cut FIGURE 4-9 strata, were triggered by recent precipitation that slopes. Rising groundwater levels can also acceler- Blucher Valley landslide, approximately 6 km south of Sebastopol, California, began moving after series of 1983 winter storms. Deep-seated (>18 m deep) translational earth block slide on nose of spur ridge moved along bedding planes inclined at only 5 to 8 degrees. Cumulative seasonal rainfall during 1981-1 982 and 1982-1983 preceding landslide had been highest recorded historically. Triggering of landslide attributed to high groundwater levels in cracks and grabens (Spittler 1983). Landslide Triggering Mechanisms 83 ate landslide movement, as observed at Vaiont nels, and inundating agricultural land, debris flows Dam, , where a slowly moving landslide rapidly have destroyed most major highway bridges near accelerated during the weeks following the initial the (Pierson 1992). filling of the reservoir (Lane 1967). Volcanic eruptions have triggered some of the The Mayunmarca landslide of April 25, 1974, largest and most catastrophic historic landslides. blocked the Mantaro River in , and the rising As a result of the May 18, 1980, eruption of Mount water level behind the dam caused by the landslide St. Helens, Washington, a massive 2.8-km3 rock resulted in more landslides along the shores of the slide—debris rapidly descended from the lake, which destroyed a regional highway (Lee and north slope of Mount St. Helens and traveled Duncan 1975). Sudden breaching of the landslide about 22 km down the valley of the North Fork dam and rapid drawdown of the lake level trig- Toutle River (Voight et al. 1983). The avalanche gered still more landslides along the banks of the destroyed nine bridges and many kilometers of lake (R.L. Schuster, personal communication, highways and roads. As a result of rapid melting of 1992, U.S. Geological Survey, Denver, Colorado). snow and ice from the eruption, mud flows surged There are other examples in which gradually ris- down several of the valleys that radiated from the ing groundwater levels caused by irrigation and pro- mountain. The largest and most destructive of longed or intermittent low- to moderate-intensity these mud flows entered the valleys of the North rainfall have resulted in landslides. These cases are Fork and South Fork Toutle River and destroyed not cited because the relation of trigger and land- or heavily damaged about 200 homes, buried half sliding is not as closely documented with respect to of the 27-km portion of State Highway 504 and time as it is for those cases described here, which other highways and roads, destroyed 27 km of rail- involve more rapid changes in water levels. way, and destroyed or badly damaged 27 highway and railroad bridges (Figure 4-10) (Schuster 1981). On November 13, 1985, pyroclastic flows and 5. VOLCANIC ERUPTION surges from a relatively small eruption melted snow Deposition of loose on hillsides com- and ice on the summit of volcano monly is followed by accelerated erosion and fre- in and produced large volumes of melt- quent mud or debris flows triggered by intense water, initiating debris flows in steep channels that FIGURE 4-10 rainfall (Kadornura et at. 1983). Irazu, a volcano in swept down and killed more than 23,000 inhabi- St. Helens Bridge, central Costa Rica, erupted ash almost continu- tants of Armero and other areas at or beyond the 75-rn steel bridge ously from March 1963 through February 1965. base of the volcano (Pierson et al. 1990). on State Highway 504 carried about Intense rain and high runoff accompanied by sheet 1/2 km downstream and nIl erosion of ash-covered slopes triggered 6. EARTHQUAKE SHAKING and partially buried more than 90 debris flows in valleys on slopes of by mud flow in this volcano. A large debris flow in the Rio Strong ground shaking during has 1980. Reventado valley destroyed more than 300 homes triggered landslides in many different topographic ROBERT L SCHUSTER and killed more than 20 persons. High runoff and debris flows incised deep channels, resulting in slumping and of valley walls and reactiva- tion of landslides, which in turn supplied addi- tional material for debris flows (Waldron 1967). Following the June 1991 eruption of Mt. Pina- tubo in the , monsoon and triggered many debris flows that originated in thick volcanic-ash deposits (Pierson 1992). Debris flows as deep as 5 m traveled down major channels; during the most rainy periods, three to five debris flows a day were common. Most debris flows were triggered by monsoonal rainstorms with intensities that seldom exceeded 80 to 100 mm over several hours. In addition to disrupting natural drainage patterns, causing lateral migration of river chan 0"M 84 Landslides: Investigation and Mitigation

and geologic settings. Rock falls, soil slides, and rock slides from steep slopes, involving relatively thin or shallow disaggregated soils or rock, or both, have been the most abundant types of landslides triggered by historical earthquakes (Figures 4-11 and 4-12). Earth spreads, earth slumps, earth block slides, and earth on gentler slopes have also been very abundant in earthquakes (Keefer 1984). For 40 historic earthquakes, Keefer (1984) de- termined the maximum distance from to landslides as a function of magnitude for three general landslide types (Figure 4-13). Using the expected farthest limits of landsliding during

FIGURE 4-1 1 Rock slide—avalanche onto Sherman triggered by Alaska earthquake: Sherman Glacier on August 26, 1963, showing conditions before earthquake (Post 1%/).

FIGURE 4-12 (bottom left) Rock slide—avalanche onto Sherman Glacier triggered by March : collapse of Shal.lered Peak in rriiiJdle dki-itu formed avalanche (Post 1967).

E 1 0

..u..,,.u,.au.uu, , .0 I .0 0.0 0.3 W., .3 Magnitude (M)

FIGURE 4-13 (above) Maximum distance to landslides from epicenter for earthquakes of different magnitudes. - - - -, disi upied IdlIS did slides, — - -, buui id Iui t..uI ci ciii slides; .....hound for spreads and flows (Keefer 1984). Landslide Triggering Mechanisms 85 earthquakes of specific magnitude and location, an basis of records of strong ground shaking (Wilson outer distance limit for landsliding was prepared and Keefer 1983; Jibson 1993). for a hypothetical earthquake in the Los Angeles Landslides involving loose, saturated, cohe- region (Figure 4-14) (Harp and Keefer 1985; sionless soils on low to moderate slopes commonly Wilson and Keefer 1985). The amount of landslide occur as a result of earthquake- induced liquefac- displacement during an earthquake is a critical fac- tion, a process in which shaking temporarily raises tor in assessment; a seismic analysis of earth pore-water pressures and reduces the strength of dams (Newmark 1965) was modified to calculate the soil (Figure 4-15). Sedimentary environment, the displacement of individual landslides on the age of deposition, geologic history, depth of water

- . E .R N

ANTELOpF- - - ••--•;-•••. -- V -- MJAVE IDES1ER!

1 S A N B E R N A D I b (

\

I'

N BERNARDINO- 1 - RNE I

I -V fg S I 0 E '\ Y '

FIGURE 4-14 Map of Los Angeles basin showing predicted limit for coherent landslides from hypothetical M 6.5 earthquake with epicenter on northern Newport- Inglewood zone ( straight \\\ Y line) (modified from

0 30 KUOMETERS " \ Wilson and Keefer 1985). 86 Landslides: Investigation and Mitigation

and the only highway from Quito to Ecuador's eastern rain forests and oil fields (Crespo et al. 1991; Schuster 1991). Economic losses, principally from landslide-induced damage to the oil pipeline and highway, were estimated to be U.S. $1.5 bil- lion (Nieto and Schuster 1988). On November 12, 1987, liquefaction of a silt and sand layer during an M 6.6 earthquake in Superstition Hills, California, caused sand boils to erupt and resulted in extensive ground cracking in- dicative of an earth spread. Nearby instrumenta- tion recorded excess pore pressures that began to develop when the peak horizontal acceleration reached 0.21 gabout 13.6 sec after the earthquake began (Figure 4-7) (Holzer et al. 1989). The pore- pressure buildup was high enough to be the main FIGURE 4-15 table, grain-size distribution, , and depth factor in reducing soil strength and causing the State Highway 1 determine whether a deposit will liquefy during an earth spread. bridge destroyed earthquake. Generally, cohesionless of The M 7.1 Loma Prieta, California, earthquake by strong shaking age or younger below the groundwater and liquefaction of October 17, 1989, triggered an estimated 2,000 of river deposits table are most susceptible to liquefaction (Youd to 4,000 rock, earth, and debris falls and slides that at Struve Slough and Perkins 1978). blocked a major highway and many secondary near Watsonville, The May 31, 1970, Richter magnitude (M) 7.7 roads in the San Francisco—Monterey Bay areas. A California, during Peru earthquake was the most catastrophic historic debris slide of about 6000 m3 closed the two north- 1989 Loma Prieta earthquake of the Western Hemisphere, causing bound lanes of California State Highway 17 for 33 earthquake (Plafker over 40,000 deaths. The earthquake triggered a days before repairs were completed (Plafker and and Galloway huge debris avalanche from the north peak of 1989). Galloway 1989; Keefer and Manson in press). Huascaran Mountain that buried the town of The Loma Prieta earthquake also caused lique- Yungay and part of the town of Ranrahirca with a faction and earth spreads between San Francisco loss of more than 18,000 lives. The earthquake and Monterey, including damage to the runways at also triggered many other landslides within a Oakland International Airport and the Alameda 30,000-km2 area that disrupted communities and Naval Air Station (Plafker and Galloway 1989; temporarily blocked roads; these slides seriously Seed et al. 1990). Numerous earth spreads (about hampered rescue and relief operations and kept the 46) destroyed or disrupted flood-control levees, full extent of the unknown until weeks pipelines, bridge abutments and piers, roads, after the earthquake (Plafker et at. 1971). houses and utilities, and irrigation works in the The M 7.5 earthquake of February Monterey Bay area (Plafker and Galloway 1989; 4, 1976, triggered more than 10,000 landslides, Tinsley and Dupre 1993). predominantly rock falls and debris slides from steep slopes of pumice deposits 7. SUMMARY ( and ash flows) or their residual soils (Harp et al. 1981). Pumice deposits, which stand in steep, Common landslide triggers, including intense near-vertical slopes, lose much of their strength rainfall, rapid snowmelt, water-level changes, vol- during seismic loading. Strong shaking increases canic eruptions, and strong ground shaking during stresses that may break down in ce- earthquakes, are probably directly responsible for mented soils or brittle rocks, such as , bess, the majority of landslides worldwide. As illustrated or (Sitar and Clough 1983). by the foregoing examples, these landslides are On March 5, 1987, two earthquakes (M 6.1 and responsible for much damage to transportation sys- M 6.9) 100 km east of Quito, Ecuador, triggered tems, utilities, and lifelines. These landslide trig- thousands of rock and earth slides, debris ava- gers have been well documented, and recent lanches, and debris and mud flows that destroyed monitoring has provided considerable into nearly 70 km of the Trans- Ecuadorian oil pipeline the mechanics of the triggering processes. Landslide Triggering Mechanisms 87

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