"Immediately to the east of Mission San Jose, entirely within the hills, another crack opened with a strike of N. 18o to 20o W., which, converging upon the crack thus far traced, extended south as far as the County line".

 
Radbruch (1974) attempted to tie this description to a NW-trending lineament cutting across the subject landslide around elevation 800 feet.  There is little confidence in this position, which appears approximate at best.  The Warm Springs fault is the closest structure which approximates the trend and described area, but was not recognized until the early 1960s.   JHA Consultants (1987) felt they located this feature cutting across the Vineyard Heights subdivision (depicted in Plate 1).  An alternative possibility for the "1868 ground fissure" would have been dynamic consolidation of loose graben infill, up within the landslide graben-controlled valley, near the current headscarp.  But, this feature trends approximately N 40o  W, and is situated at considerable altitude above the old Mission District.  Co-seismic movement (strain energy release) along the Warm Springs fault or the toe thrusts associated with the large ancient slides are the most likely culprits for the 1868 ground break.

Wahrhaftig (1978)  opined that the prominent ground cracks noted in 1868 could be the tensile scarps  noted in vicinity of the landslide headscarp, between elevations 1700 and 1830, which are noticeable  in all historic aerial photos, predating the earliest in 1939.  Extension of these scarps had not occurred this Century, prior to this year"s slide.   Given the remoteness and inaccessibility of these cracks, this may be unlikely.  The road serving the old McClure Ranch below Mission Peak was accessed from upper Mill Creek, along the opposite side of Mission Ridge.  This is the same road that still provides  access to Mission Peak Regional Preserve from the north.

We conclude, therefore, that it is unlikely any reliable correlation can be drawn between the 1868 ground fissures and the subject slide, absent further information.  Based on the related description, we would suspect that these cracks were associated with co-seismic stress relief along the Warm Springs fault, or dynamic consolidation of the linear headscarp graben infill materials.

Perturbed Channel Profile

The most powerful suggestion for geologically-recent landslippage is the perturbed profile of Aliso Creek, presented in Fig.7. This profile suggests Aliso Creek is out-of-equilibrium with respect to its hydraulic grade, between elevations 500 about 810 feet, due to massive choking of the channel by landslide debris.  More debris has been produced than the channel has adequate stream power to remove, resulting in a profile with upward concavity.  A channel with normal balance between runoff and sediment yield (in "equilibrium") will tend to develop a decreasing profile with increasing watershed area, accumulated downstream (Schuster, 1997; Rogers, 1997).

Field evidence for hydraulic choking is also abundant, between elevation 500 and 520, where the channel gradient increases sharply, in a series of small drops.  Part of this change may be associated with vertical offset of the Warm Springs fault, which supposedly crosses Aliso Creek between elevation 500 and 505.  But, faulting cannot, of itself, account for the convex form of the channel profile between elevations 500 and 650, which is suggestive of a large sediment "plug".

Recent Channel Offsets

JHA Associates (1987) noted that Aliso Creek and the unnamed creek channels bordering the south side of the Vineyard Heights subdivision exhibited sharp drops in their channel profiles, which ranged between 8 and 10 vertical feet.   These drops appear coincident with the projected trace of the left-lateral Warm Springs fault, discussed previously.

During the week of March 22-29, 1998,  the channel of Aliso Creek appears to have been pushed about 9 feet southerly, between elevations 520 and 550, where the "Hammertime" block has been forced against the opposing (south) creek bank.

Near elevation 550  the landslide toe translated approximately 16-1/2 feet southwesterly, into the channel of Aliso Creek, creating a hydraulic choke.  This block was over 100 feet high, and  translated approximately 6-1/2 feet upward and 11-1/2 feet laterally, with a upward rake of approximately 30^o (rake dipping northeast)   The recurrence of this blockage through recent geologic time (past several hundred to several thousand years) also suggests geologically-recent activity of  the 1998 landslide complex.  Given the sharpness of channel perturbations, it would appear that the slide has only been dormant 200 to 600 years since its last episode of movement, which was likely of similar magnitude to that observed this year.

Geometry of the 1998 Landslide Complex

In order to estimate the likely depths of landslippage and volume of slide mass, 16 longitudinal and four transverse geologic cross sections were constructed across the 1998 landslide area and adjacent ground.  The locations of these cross sections were shown in Fig. 22 of Vol. 2 of the reports submitted to the City of Fremont.  These cross sections utilize subsurface data, surface measurements and published physical relationships reported by others who have studied similar landslides, and principles of block kinematics.  The purpose of reconstructing balanced cross sections was to estimate the likely depths of sliding, and delineate those  blocks of ground involved in the recent movements.  A representative sample of these cross sections is presented in Fig. 8.

Three principal cross sections taken through the headscarp area (Section H-H is presented in Fig. 8)  present our estimate of the underlying conditions in the headscarp area of the 1998 slide, where the block movements appear deepest.  These cross sections suggest an initial headscarp graben width of approximately 400 feet, which extends downward between 110 and 180 feet.  As the upper translated downslope, an extensive graben, or pull-apart structure, was thereby formed.  Grabens  collect surface runoff and talus from the newly-evacuated headscarp.  As this headscarp depression infills with eroded material and water,  the graben periodically extends itself, allowing the graben block to back-rotate and break into smaller pieces.  The pull-apart graben would typically support sag ponds like those observed this past year near elevation 1280. Within a few years the ponds begin to fill with sediment and coarse talus shed from the eroding headscarp.  In this manner, grabens gradually become filled with fine grained pond sediment and coarse talus, often including house-size boulders.  This mixture was observed across the headscarp depression area following this year"s movements.

About 60,000 cubic yards of coarse sandstone talus appears to have fallen onto the headscarp graben this past summer, due to retrogression of the headscarp within Mission Ridge.  The reasons for this regression are not fully understood, but appear related to the geometry of the overturned Briones beds, their joint patterns, and the close proximity of low-strength Orinda beds, which exhibit evidence of shearing inclined clay seams, in "toe kick-out".  In this manner, enormous wedges of Briones sandstone are "brought down", which increases the load on the landslide, which leads to more movement, and so on.  Geomorphic evidence of rapid slippage, landslide movement and headscarp erosion are abundant within the 1998 slide area. This situation does not appear so acute at other locations along Mission Ridge, suggesting this as the most likely location for periodic landslide activity.

As the landslide becomes increasingly loaded with rock talus and absorbs runoff, the slide mass should  periodically reactivate above elevation 1300.  It would appear that this block has moved about 400 to 650 feet or more from its original position, as the entire exposed face (between elevations 1125 and 1275) appears comprised of Orinda formation materials.  Additional movements are also fostered by percolation of water into the headscarp sag ponds (near  elevation 1280 at the present time).  The entirety of the "Titanic" block, between elevations 925 and 1215, appears to be comprised of displaced Orinda beds, devoid of recognizable structure.  This is indicative of a much older sequence of  translational slippage, lying at some unknown depth.

Below elevation 1125 on the north side of the Titanic block, and elevation 1150 on the south, are a series of deep-seated earth flows which mobilized this past March.  Those on the north side extend down to elevations 700 to 775, while those on the south side are much smaller, and only extend to elevation 855.  Cross Sections H-H" (Fig. 8)  and L-L" (Fig. 25 in Vol 2 of the City report) suggest that the northern earthflows could extend to depths of 70 to 100 feet, but this has not been confirmed.  However, based on the 35 feet height of the lateral scarp along the north side of the "Titanic" block, these estimates appear reasonable. The massive earthflows along the north side of the middle slope area begin to thrust upward around elevation 875, and continue in a series of toe thrust "steps", down to elevation 700.  Some of these toe thrusts appear to have lifted as much as 110 feet, as suggested in Cross Sections A-A" and B-B" (Fig. 27 of Vol. 2 of the City report).

Below elevation 775 feet, a series of ancient landslide blocks appear to have been loaded by these recent toe thrusts, and have reacted in passive compression, along pre-existing slip surfaces (associated with past landslippage).  Representative samples of these blocks are presented in Fig. 8 and Figs. 27 and 28 of Vol. 2 of the City report.   In this lower slope of the 1998 slide, we observed  considerable evidence of compression, including multiple toe thrusts and back-thrusts, as shown on Plate 1 and Fig. 8.

A peculiar aspect of the slide"s toe is the thrusting which occurred adjacent to the Mei Residence at 2978 Grapevine Terrace. In this area the landslide made a sudden 70 degree right-hand turn, about 50 feet northeast of their home, along a pre-existing plane of weakness, akin to a fault (see annotation "abrupt toe thrust" on Fig. 8).  Past landslippage appears to have occurred along this boundary, sufficient to have elevated a toe-thrusted hillock over 85 feet high6  overlooking Aliso Creek.  This low strength boundary may be the lateral scarp of a much older landslide block, or a transverse fault.

Recent movement at the slide toe appears to have been close to 5 meters, the majority of which occurred between March 22-29, 1998.  Components of this movement were both vertical (6.6 feet, or 2 meters) and horizontal (11.5 feet, or 3.5 meters), for an upward rake (incline) of approximately 30 degrees (dipping northeast).  This movement has shifted the afore-mentioned hillock 11.5 feet (3.5 m) towards Aliso Creek, killing 3 mature trees astride the channel, and constricting channel flow by creating a landslide dam.  The channel of Aliso Creek above elevation 550 feet appears to have been elevated between 6.6 feet (2 m) and 8.2 feet (2.5 m) in the toe area (see footnote ⁶ ).

Apparent Offset of Landslide by the Underlying Mission Fault

The prominent bench developed along the landslide"s headscarp graben and the observed back-rotation of this block suggests the basal landslide slip surface flattens in vicinity of the headscarp, as shown in Figs. 6 and 8.   This change  appears to occur just upslope of the prominent ridge that pervades the area, between elevations 1000 and 1300 across the slide area  (increasing in elevation preceding southeasterly).   This prominent ridge appears to form the western side of the landslide headscarp graben-controlled valley (Plate 1).  At first glance one assumes that this secondary  ridge has simply pulled away from Mission Ridge, but close inspection reveals it to be comprised of Orinda beds that are pervasively sheared and exhibits sporadic structure.  The resistant nature of the promontory is ascribable in part to conglomerate lenses and overlying blocks of Briones sandstone, but some of the elevation may be tectonically-induced, by ongoing uplift of the underlying Mission fault.

Our evaluation of exposed outcrops at the base of Mission Ridge (described earlier under "Mission fault studies") suggested that the Mission fault does not lie at the base of the ridge, as suggested by Hall (1956, 1958) and others who succeeded him.  The sharp break in slope appears to be stratigraphically controlled, at the gradational contact between overturned sequences of the Orinda/Cierbo formation and older Briones sandstone, which is much more resistant.  Graymer, Jones, Brabb and Helley (1994) suggested that the Mission fault is located about 500 to 600 feet  downslope of the contact (Fig. 2), which appears to be supported by our more recent work.

In order to evaluate the potential impact of uplift along the Mission fault on the subject landslide complex, a series of 14 cross sections were drawn through the headscarp graben area (Fig. 6), and small increments of downslope translation were modeled along with even smaller increments of vertical uplift along the Mission fault, assuming its underlying position to be about 500 feet downslope of Mission Ridge.  One of the interesting aspects of this model was the compressional flower structure that is suggested to develop above the fault, within the landslide.  A similar "step" in the prototype landslide"s basal slip surface appears to exist, as evidenced by the marked backward rotation of the headscarp with simple translation for several hundred feet downslope, wherein the basal slip surface(s) steepen markedly, downslope of the prominent "secondary ridge" forming the southwest side of the landslide headscarp graben.

In addition, recent uplift along the Mission fault could also account for the prominent "secondary ridge", which parallels Mission Ridge, about 500 feet southwest, along the strike of the Mission fault.  Although this is conjecture, based upon geometric relationships and assumed rates of offset and uplift, such a model does appear to replicate the physical aspects of the field situation.  If such a model were true, the ongoing movement and uplift along the Mission fault is one of the primary mechanisms reactivating the landslide, probably every few hundred or few thousand years.

IMPACT OF  PREVIOUS LANDSLIDE MAPPING ON LAND USE PLANNING  (To Top of Page)

The subject landslide was initially recognized in the published literature in the late 1950s (Hall, 1958).  Between 1970-75 the Federal Department of Housing and Urban Development (HUD) sponsored a joint project with the Western Regional Office of the U.S. Geological Survey termed the "San Francisco Bay Region Environment and Resources Planning Study".  Its purpose was to develop geologic hazard maps for planning purposes in the San Francisco Bay Region.  This was an unprecedented undertaking, which involved compiling maps on a wide range of features, including bedrock geology, landslides, groundwater resources, hydrology, water quality, and toxicity.  Of particular note, the program paid for the preparation of reconnaissance-level bedrock geology and landslide hazard maps, initially at 1:62,500 scale. These were followed upon by special bulletins dealing with landslide mapping and correlations between rainfall and historic activity, which culminated in the following documents: T.H. Nilsen, F.A. Taylor and E.E. Brabb, 1976, Recent Landslides in Alameda County, CA (1940-71): An Estimate of Economic Losses and Correlations with Slope, Rainfall, and Ancient Landslide Deposits: USGS Bulletin 1398, 21 p, 1 pl.; and T.H. Nilsen, F.A. Taylor and R.M. Dean, 1976, Natural Conditions That Control Landsliding in the San Francisco Bay Region - and Analysis Based on Data From the 1968-69 and 1972-73 Rainy Seasons: USGS Bulletin 1424, 35 p., 1 pl.  The landslide maps contained in these bulletins were also released at a scale of 1:62,500, or about 1 inch = 1 mile.

The key products of the HUD program were fifty-seven 7.5-minute USGS landslide quadrangle maps by Tor Nilsen, termed  "Preliminary Maps of Landslides and Surficial Soil Deposits", and released as open file reports, beginning in 1975.   Nilsen"s landslide map of the Niles 7.5-minute quadrangle was released as USGS Open File Report 75-277-40 in 1975.  The Nilsen map (Fig. 9) presented a more thorough appraisal of landslides in the Mission Peak and Mission Ridge area.  Few consultants took the Nilsen maps seriously until the severe winters of 1981-82 and 1982-83, when an unprecedented number of older, deep-seated bedrock slides reactivated.

The area also received attention from reconnaissance-level seismic risk assessments by Radbruch (1974) and Herd (1978), who suggested the entire slope area below Mission Ridge was a ancient landslide complex; and a somewhat less inclusive area, north of Aliso Creek was inferred as massive landslide deposits by Dibblee; Crane and Graymer, et al (1994).  An overlay of the landslide areas mapped by Hall, Nilsen, Herd, Dibblee and Graymer, et al is presented in Fig. 10.

Fremont was one of those cities who benefited from the HUD/USGS program, having been incorporated in 1956. Fremont experienced a building boom in the 1980s which paralleled development in the Silicon Valley, bringing more than 3,000 new business to the City.  As part of this boom, ranches located on the moderate slopes east of Mission Boulevard were allowed to develop.  Amendments to the City"s development policy were made in 1982, 1986,1987 and 1989 (latest revision).  These included limiting development on slopes steeper than 30%.  The Vineyard Heights-II Subdivision was created in the mid-1980s, but not actually developed until the early 1990s, because of a slow-down in construction which occurred between 1989-94.

During the building of the 1990s, residential construction was brought uphill to Aliso Creek and halted, the Mei parcels being the uppermost units of Vineyard Heights, with the former Hammertime mansion the highest parcel of the Vista del Sol subdivision, across Aliso Creek.  Given what we know at present, these subdivisions were carried just about as far upslope as appropriate, so as to avoid reactivated landslides.

Adjacent Slide Complex

Additional landslide mapping was undertaken of the area upslope of Alameda County Water District"s (ACWD) Mayhew Reservoir by Dr. Robert Wright of Harlan-Tait Associates in 1994.  Dr. Wright had previous experience in regional landslide mapping with the U.S. Geological Survey in the late 1970s⁷ .  This map was the most detailed assessment undertaken till that time, covering the slope area between Ohlone College and Aliso Creek (Wright"s 1994 map was reproduced as Fig. 13 in Vol. 2 of the City report). Like Herd (1978), he interpreted the entire slope area below Mission Ridge to be landslides.  Wright mapped the 1998 slide area as "Qls 2113"; a grading and identification scheme which equated with: "Dormant, definite landslide, slump-flow complex, more than 15 feet deep".    Nilsen"s 1975 map and that of Harlan-Tait (1994) were the only products focused entirely on landslides, and thereby, exhibit much greater detail with respect to individual slides within the larger complex.

A large natural depression occupies the head of this complex near elevation 900 feet, which may aid in recharging moisture to the slide complex.  Although this complex is not fed detritus or runoff directly from Mission Ridge, some manner of tensile scarp adjustment appears to have occurred along the base of Mission Ridge above this complex between early November 1980 and late February 1981, suggestive of a possible deep-seated connection.  Other portions of this complex appear relatively dormant, hence the variation in grading shown on the Relative Landslide Susceptibility Map prepared for the City.

LANDSLIDE HAZARD MAPPING UNDERTAKEN IN 1998  (To Top of Page)

A series of four landslide hazard maps were prepared for the City of Fremont in a format similar to that used by the California Division of Mines & Geology (CDMG) for implementation of the Landslide Hazard Identification Act (Chapter 7.7, Division 2 of the California Public Resources Code).  Similar products have been prepared in other parts of the S.F. Bay Region since 1976, when the State released "Geology for Planning: Central and Southeast Marin County, California", as CDMG Open File Report 76-2.

These maps included  "Geologic Features of the Mission Peak Landslide" (Map 1), presented at a scale of 1" = 200', similar to the enclosed Plate 1, but without the basal slip surface isopleths.  This map incorporated available published data and benefited from site visits and discussions with other geologists who've worked the area.  We focused our efforts on confirming work previously reported by others, and looking  for evidence of the Mission and Warm Springs faults.  Although we have not conclusively identified either fault, we believe we have localized their likely whereabouts, beneath the slide complex.  In this effort we were greatly aided by new exposures created by enlarging landslide headscarps, which have continued to expand since the initial failure.  Arrows indicate the relative direction of movement: single arrows for earthflows and twin parallel arrows for slumps and translational blocks, moving as semi-coherent blocks.  Open elongate triangles are used to describe old headscarp evacuation scarps, which may be developed upon older landslides.  Thrust features are shown with solid saw teeth on the upper plate.  This applies to backthrusts as well, which were common in the toe area (prior to the recent channelization work).  Lateral thrusting was also observed north of the headscarp graben, and along the western margins of the of the slide"s toe.  Uphill-facing tension scarps were also observed near the slide toe, and at isolated locations along the  southwest-facing slope of Mission Ridge, in the Briones sandstone.  Some of the more prominent cultural features, such as fire roads, stock ponds and the landslide graben-controlled valley, are also delineated on this map, in order to aid the reader in locations.

The other maps prepared for the City are not contained in this field guide because they are oversize sheets.  The maps have been posted on the City's website at http://www.ci.fremont.ca.us/citysvcs/development/engineering/missionpeakslide-a.html. Map 2 was titled "Landslides and Related Features Map of the Mission Ridge Area", and was prepared at a scale of 1" = 500' on an orthophoto-derived topographic base imaged in July 1998, with contour intervals of 5 and 10 feet.  It presents the results of our reconnaissance-level engineering geologic mapping of the entire Mission Peak landslide area, between Ohlone College, Mission Peak, Agua Caliente watershed and Mission Boulevard.  The map delineates recent and ancient slump-block slides, recent and dormant earthflows, displaced rock wedges, selected areas believed to be underlain by colluvium,  and prominent geologic discontinuities, such as faults. Hillslope areas previously subjected to mass grading for residential development were not mapped as part of this study.  This includes the areas below Hunter Lane, Cougar Circle and  Vineyard Avenue.

Map 3 was titled "Relative Landslide Susceptibility Map for the Mission Ridge Area", and was prepared at a scale of 1" = 500',  on an orthophoto-derived topographic base imaged in July 1998, with contour intervals of 5 and 10 feet.  This map was prepared to aid in general land-use planning.   The mapped landslide features were divided into four categories, graded from 1 to 4 with increasing susceptibility to future landslippage.  Area 1 is the least susceptible area; 2 is marginally susceptible; 3 is generally susceptible; and 4  is graded as most susceptible.  This map was not intended for evaluation of individual sites, only to point out those areas worthy of further site-specific investigation should new construction be contemplated.  This map is a derivative map based upon information gathered for this study on: 1) the occurrence and distribution of landslides, other types of slope failures, and features indicative of slope instability; 2) the underlying geology of the area, including bedrock types and material properties relative to slope stability and distribution of the various earth materials, as well as the structural framework, such as folding and faulting of the strata; and 3) the relative behavior of slopes within the area as interpreted from analysis of aerial photographs, topographic expression and field observations.

Map 4 was titled "Potential Debris Flow and Water-Borne Debris Deposition Zones in the Mission Ridge Area", and covers a somewhat smaller area, at a scale of 1" = 300 feet, covering all the watercourses emanating from the Mission Ridge study area, between Ohlone College and Agua Caliente.  The base map is taken from the same July 1998 orthophoto imagery as the companion maps.  This map presents a radical departure from the traditional CDMG Map 4 format in that it sought to highlight areas where sediment deposited by debris flows and fluvial flood flows could be expected, as well as the likely material source areas.  Peer reviewers Bill Cotton, Bill Dietrich and Jonathan Stock provided significant impact on this map, which is something of a prototype hazard planning product. Three common depositional mechanisms were identified across the study area: 1) Active landslides supply  disaggregated particles, which can then be transported by fluvial flow within first and second-order ephemeral channels; 2) Debris emanating from landslide dams across channels, which propagate debris flows when the dams overtop and  breach; and 3) Debris emanating from rapidly-moving flow slides in steep upland slopes or active landslide faces comprised of predominately granular materials.

Abundant debris was noted to have collected within several of the ephemeral channels surveyed across the study area, usually within channels with more than 15% slope.  Much of this debris appears related to past episodes of debris being caught behind obstructions, such as trees, within these channels. In this fashion, debris is "stored" in ephemeral channels until such time when the debris is taken up in bulking and carried downstream in a debris flow.   Experience has suggested that the debris tends to be deposited along channels with slopes between 20% and 6%, depending on flow volume (which is related to tributary watershed area), channel shape and roughness. In a few locations, such as Agua Caliente (Hidden Valley Creek), old debris fan terraces were identified, which are incised by present-day channels.  Although debris flow deposits are relatively rare in this area, most of these materials appear to have been deposited on slopes greater than 10%.   Subsequent overland flows (runoff) could sieve these deposits of their fine-sized fraction, transporting it downstream through flood flow processes.

CONCLUSIONS REGARDING THE LIKELY FAILURE SEQUENCE  (To Top of Page)

Based on these evaluations, the pre-existing topography, and the aerial photo assessment, it would appear that the 1998 landslide sequence probably initiated in the steep slope that formerly existed below elevation 1275, where tension scarps associated with partial activation of large earthflow slides were noted in the 1984 and 1986 aerial photos.  These scarps were also visible in the 1995 and 1996 photos, though the slides were then dormant.

Given the available data and exposures in the slide headscarp, it appears that Mission fault lies 400 to 800 feet southwest of, and parallel to, the base of Mission Ridge.   We know from microseismic data collected since 1969 that the Mission fault is an active right-lateral fault with near-vertical inclination below a depth of 3 km.  Since the early 1990s there has been a consensus of opinion that the Mission fault likely represents a stress and seismicity transfer between the northern  Calaveras and south Hayward faults.  To date, no one has uncovered the surface trace of the Mission fault, because of massive landslides covering the interval between Mission Peak and Mission San Jose.   In 1997 the State of California and the International Conference of Building Officials jointly determined that the Mission fault would henceforth be zoned as the "Hayward fault south", as an A-type fault", subject to near-field loading evaluations within 2, 5 and 10 km, discussed previously under "Seismic Slope Stability" in Vol.2 of the City report.    The Hayward fault is recognized to have a slip rate of approximately 9 mm/year with a maximum moment magnitude, M_L , of 7.1.

A detailed spatial analysis of the 1998 slide suggests the massive headscarp graben back-rotated as it dropped, translating southwestward on a slight incline.  A prominent "secondary ridge" exist parallel to Mission Ridge, between 600 and 1000 southwest.  At first glance this escarpment appears to be massive blocks of Briones sandstone, displaced by simple back-rotation from Mission Ridge, akin to giant Toreva blocks (Reiche, 1935; Strahler, 1940).  But, the topographic prominence of this lower ridge/bench may also be ascribable to tectonic uplift along the Mission fault, which underlies the ridge (as depicted in Fig. 6).  In either case, the secondary ridge creates an over-steepened slope between elevations 1050 and 1350 feet.    The initial sliding appears to have occurred along the southwest-facing slope of the secondary ridge, as a series of retrogressive slump blocks which converted into deep-seated earthflows as they translated downslope (Plate 1). The parent materials appear to be dominated by Orinda formation materials, seen from below as a mottled series of gray, red and olive colored sediments.  These earth flows extend between elevations 1275 and 700 feet.

When this mass of material moved downslope, the lateral restraint it provided for the next-adjacent blocks upslope was sufficiently diminished to allow for reactivation of these upper blocks as well, which appear to be upwards of 180 feet deep (Fig. 8).    This movement allowed a similar reduction in lateral restraint for the base of Mission Ridge, which began to crumble and fall into the headscarp, beginning on May 22, 1998.

The movement of the deep upper block appears to have involved the zone between surface elevations 1275 and 1425, while the breakup of the Mission Ridge block, above the original headscarp, extended from elevation 1375 up to 1830 feet.  The headscarp evacuation scar appears infilled with sandstone breccia and detritus, obscuring any underlying structure.  This testifies to past movements and separations, which were subsequently infilled with coarse slope wash, in the recent geologic past.

The lowermost blocks, between elevation 700 and 520, appear to have been caught in passive reaction to the more than 100 feet of surcharge brought down by the massive earthflows on March 22nd.  These lower blocks were pushed approximately 5 meters, extending beneath Aliso Creek to elevation 550, and down to Aliso Creek between elevations 550 to 520.    The likely sequence of sliding is sketched schematically in Fig. 11.

There exist ample geomorphic evidence to suggest that similar movements have occurred in the recent geologic past, causing the channel profile of Aliso Creek to have become choked.   A sobering aspect of the recent movement has been the observance of movement between Aliso Creek and elevation 700, within a zone of ancient landslide blocks that had appeared quite dormant, until close examination.  This suggests that the "no-build", or "buffer zone" between Holocene-age slides and the homes being deemed safe for development might be extended outward,  towards 1200 feet ⁸ , in order to allow for reactivation of older slide blocks situated below the more obvious zones

Reasons for 1998 Slope Movement

The reasons for reactivation of the Mission Peak landslide this past winter are listed below, in  relative descending importance.  In reviewing these reasons, the reader should be made aware that the prima facie reasons for reactivation of ancient landslides lies in the fact that they preexist.  Underlying geologic conditions control WHERE slides occur, while transient hydrologic conditions tend to control WHEN they occur.  Correspondingly, the reasons outlined below are therefore split into two categories: 1) geologic conditions; and 2) hydrologic conditions.

Geologic Conditions Promoting Failure:

1. This year's slide occurred at the location of a pre-existing deep-seated slide that has long obscured the underlying bedrock geology of the slope area below Mission Ridge north of Aliso Creek and south of Ohlone College.  This area was mapped by many different geologists, including: Hall (1956, 1958); Radbruch (1974); Nilsen (1975); Herd (1978); Dibblee (1980); Crane (1988), Graymer, Jones, Brabb and Helley (1994) and Harlan-Tait Associates (1994).

2. There exists considerable evidence for Holocene-age movement within the subject landslide complex over the past several hundred years.  The most recent movements likely occurred within the past 200 to 500 years, based on the geomorphic expression of the toe area, along Aliso Creek (Fig. 7).  In addition, there are a number of back-facing tensile scarps in the Briones sandstone outcrops on Mission Ridge, which also testify to recent incipient block movements, similar to the types of separations observed this year.

3. The site is tectonically active, with a high rate of uplift that serves to lift Briones sandstone along the west Mission fault, which serves as a sort of "loading ramp", providing a never-ending supply of sandstone blocks that are available to cascade down the slopes of Mission Ridge (Fig. 4).  Crustal compression between the Hayward and Calaveras faults has caused extreme folding and faulting of the intervening block of land.

4. This is an area of high seismicity.  Historic seismicity along this zone has only been monitored since 1969, but has exhibited one markedly active microseismicity.  To date, earthquake epicenters along the Mission fault have only been recorded and focal mechanisms confirmed at depths greater than 3 km, where the fault plane is essentially vertical.

Hydrologic Conditions Promoting Failure

5. The principal reason for this year's reactivation appears to be the unprecedented levels of moisture recorded in this area since January 1993.  Two, three, four, five and six-year running sums of cumulative precipitation recorded at the nearby Niles station suggest all-time highs in cumulative rainfall over that interim, having been preceded by a 6-year all-time low in cumulative precipitation between 1986-92.  This comparisons are based solely upon the Niles gage, in recordation since 1870 (presented in App. D of Vol. 3 of the City report).

REFERENCES CITED  (To Top of Page)

Andrews. D. J., Oppenheimer, D.H., and Lienkaemper, J.J., 1992, The Mission link between the Hayward and Calaveras faults: in Borcherdt and others, eds., Proceedings of the Second Conference on Earthquake Hazards in the Eastern San Francisco Bay Area: CDMG Special Publication 113, p. 217-223.

California Division of Mines & Geology, 1997, Guidelines for Evaluating and Mitigating Seismic Hazards in California: Special Publication 117, 74 p.

Crane, R.C., 1988, "Geologic Map of the Niles Quadrangle, Alameda County, California": in R.C. Crane, ed, Geology of the San Ramon Valley and Environs: Northern California Geological Society, San Ramon, 131 p., 17 pl.

Crittenden, M.D., 1951, Geology of the San Jose-Mount Hamilton Area, California: California Division of Mines Bulletin 157, 74 p.

Crozier, M.J., 1973, Techniques for the Morphometric Analysis of Landslips: Zeitschrift fur Geomorphologie, n. F, v. 17:1, p. 78-101;

Cruden, D.M.,  1991, Observation of Graben geometry in landslides: in R.J. Chandler, ed., Slope Stability Engineering: Institution of Civil Engineers, Thomas Telford, London, p. 33-35).

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¹ Although the "map distance" is only 4,550 feet, the actual traveled distance is computed accounting for curvature of flow and topographic relief. (Back)

² Max Crittenden (1951) had originally mapped the Tularcitos Syncline in the Milpitas area, and Hall (1956, 1958) extended it northward into the Niles Quadrangle, where he showed it to be truncated by the Mission Fault. (Back)

³ This act was signed into law on December 22, 1972 and codified in the State Public Resources Code as Division 2, Chapter 7.5. It has been amended 10 times through 1994. California Code of regulations Title 14, Div. 2 provides policies and criteria set forth by the Mining & Geology Board to guide cities and counties in their implementation of the law. (Back)

⁴ These beds contain abundant fragments of shells, gastropods and mollusks cemented together, deposited in a near-shore environment. The type sections of the Briones were originally described by Trask (1922). (Back)

⁵ The sharp truncation of vertically-inclined Briones beds at the base on Mission Peak is strongly suggestive of faulting, east of the presumed trace of the fault, within the linear valley. Sudden shifts in bedding attitudes also exist preceeding eastward from Mission Ridge. (Back)

⁶ The elevations of this area were substantially altered by emergency channelization of Aliso Creek in September 1998^.(Back)

⁷ Among his publications, in 1979 he co-authored "Relative Slope Stability for Land Use Planning - Selected Examples from the San Francisco Bay Region, CA", U.S. Geological Survey Professional Paper 944. (Back)

⁸ The appropriate offset would be a function of the parent slide's size. The larger the parent slide, the greater the requisite buffer zone. In some cases, substantially less than 1200 feet may be adequate. (Back)

 

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E-mail Dr. J David Rogers at rogersda@umr.edu.