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James 0. BerKland County of Santa Clara Environmental Management Agency
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Land [\.ove1oDment En9lneerin9 arid Survevir1:~
C::JL1nt·, ·~~vernnetit Ccnr~r-, East' \-/ins 70 We•t Heading Street San Jose, California 95110
Dear Jim;
This is to ackno·,;ledge rece.iot of the 11 reoort,. cc·sign<iteC: '1n vour it:~ter of May 4j 1977, Eig!:t cf -::--::-:~.e rcDor-ts (fa'"" C2-·-: Pet"rota. \.-.:eeksf C~o~e~ Levln, Sci~, Kend2 1 ~, ~:;r-,d ~;,i~Jcr;::1 a:--~.~ i7 c~ sites ~-:it~~i~1 the Soecia'J Studie.s Zones an·: hnve been aporoved bv Santa Cl~r~ CoLl~ty in com~liance ~lth tho ~lquist-Driolo So~~~~: Studies Zo 11es Ac.t. They \\ri 11 be placed or: open fl ~E'. a~ rcc!J'. !'"C·.-·'
The t1·:01 report" (Trovao, Bumoarner) for sites outs id" cf th" zones wi li be Placed in our informal consu't1ng file.
For you: informatior1, have m.f'.)de ;J qLi::::,: check (1 f cur filf:S and determined thal 69 percent nf the repcrts rcceiv~d from Santa Clara County during the 1975-76 period indic<it<· the Uo;C
of seisriic refraction in f~urlt investi9ations. Compar~tivc.: d?.t;:; for northern and southern Cvlifornia for the sarne period o,-c C'!Vf'1 in the enclosed summary. I do not hove sto:f~.tic:s fo: 1 ?7(~"'Tl't bt!t 7 of rhr~ 8 r·enort~:; _iu:,t filed b~, yo._: f":- sites t·.rirhin Spr·.:::i, .. : Studies Zones also indicate use of seisl""".iC: refr.:iction. I cannot help but wonder ahout the popul~rity nf thl• method. Mosr geoloqist~ and geophysicists with whom I have discuss~'.d this rn;;ittC'r con.:.::-ide;- seisrdc rt·:rr,·:jction to be fnt:'.")j"'lclusive. tit tlcst vJith res:;.:.ird to ~::i:valuating the h~z~rd nf faul.~ ru~ture. ti.t worst it i~; ~islf'~.:.iding and ni~y nc~t. iclcritifv ex!s~·:r·,~: faults.
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EARL W. HART
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~:~!·c i·•~·-:-.o::!·.' ic ~!~ate h·1 genphysic~l
Office of the Stcte Gcolo;~s~ C~E.G. 93~:
t=.nvlrDnmen)•.·; f"."'nr.?~l"~erit Ag(·ni:::y Lend Devl:!''orme!l'~ En~1'."l~~t'lt'!1:1 t<'!lrJ .Sl1lll'l7:~'.1l'":r:
County of Santa Clara
C<·i1.r~1\·1 Govcrnn~~:n\ (::··, .. ,'.'.·\ t-·~::.,:1 v.:~~:,.:
7(1 WP.:-.~ H~·')dr>11 0t~P.~)\ San JoSf!, Cri.l1lnrn1a 9~:;110 299-287~ Area Code 41)8
California
Report No. -.,--. -·
2.
3. " ,,
4. ·.'L
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6. "':* '
7.
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9. #
10.
State Geologist C/0 Earl Hart California Division of Mines
& Geology Ferry Building
May 4, 1977
San Francisco, California 94111
Dear Sir:
Attached are ten copies of geological r•~orts submitted to James Berkland, Santa Clara County Geologist, under the provisions of the Alquist-Priolo Act. The reports are as listed below:
P.erort Street Owner Fi 1 e No. Geoloqist Date
Aborn !load Perry Carr 18576.ll J.C. Prendergast l /7 / 77 (CEG 955) Addendum
Aborn Road Frank Perrota l8S76. 5 ,. J. c. Prenderqast 1/10/77 (C:O:G 955) Addendum
Aborn Road S.G. Weeks 77Bl8.3648 R. Burton Rose 4/7 /77 (CEG 760)
Call of the Wild Chope 76Rl2.602 R. Burton Rose 12/8/76 (CEG 760)
Chula Vista Dr. Pichard Levin 17574.9 J.C. Prenderqast 9/H/76 (CEG 955)
Clayton r.oacl r~anuel Trovao 77r117. l :19~ Ed1-1a rd Danehy l/l~/77
76Rl7.602 (C<:G 8)
Cochran/r-1a in Weis 21575. l PT Surton Rose l 0/23/?E 75G21.208 (CEG 760)
Hawkins Lane !lumgarner 21574.20 .Joseph Gabriel 9/tl/75 (CEG 256)
Hebard Road Bruce Kenda 11 74Ml2.864 R. Burton Rose 2/14/77 (CEG 760) Addendum
Higuera Road Stan finberg 18S7S.2 1 r (_J. - • Prendcr0ast 11/16/76 (CE:S 95S)
An Equal Opportunity Employer
Approval Date , ..... -~·---·-
3/~/77 Rec'd :l/ l /77
3/ ~I 77 Rec'C: J/l/77
4/25/77
l /2!:/77
ll/l~/76
1/l!J/77
2/25/77
4/~0/77
3/4/77 Rec'd v~1n
ll/2~·'1C·
Mr. Earl Hart Page 2 May 4, l 977
* These reports concern areas not in Alquist-Priolo Zones but are included for your files.
# These reports are for single building sites which are exempted from the Alquist-Priolo Act.
JOB:plk Enclosures
Very truly yours, ~ c/-, ,,. ~ ' :; '"' . . ,::,,.;;i. '\fl.t.,i+.X_
James 0. Berkland Sr. Engineering Geologist
.. ,;:
•
R. Bu.Ion Ro .. , M.A. GEOLQGllSiT
E:N~INE:~f:tlNG GEOLOG,_9T
ie ... u .. oP'!NIA RE:Gl~TRATl.N)
ALUtD GEOPHYSICS
P.O. eox $S3 SAN ,.IOSE, GA. 91!-106
STATE GE"oWGrsT
Copy (!_o ('_H RA I\/ I ()1 A I N
ZI 3 ?<;;,I 7S & z.1.20~
A-P4.31
re(e i~tJ 111\~1.. Mts ~J::is ll-3.·-fi:., ?08
ENGINEERING GEOLOGY "\ .. ,. County of Santa Cl~ra .....__ _ _ _ Land Development Engineering --- . _ County Government Center, East Wing 70 Wc;st Hedding Street ')en J0'"- CA 95110
Weis Minor Land Division
APN 727-02-009 59.82 ac.
East of NE end Main Street
Morgan Hill, California
T. 9 S. , R. J E. , MDBM
In-depth Geologic/Seismic Report
File 75G21.208
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R. B .. ..i.,,, R,,,., !VI.A.
E'.NC~IN~"f.~•NG G~Ol..0(;1~T
IGALll""O ... N!,._. Rr".Gl!ITrlATION)
ALLll'D Gl:OPHYSICS
P.O_ nox 503
SAN ..:.t051t, C:.a.. 9':•'0G
Mr. and. Mrs. We is 2210 East Main Street Morgan Hill, CA 95037
Dear Mr. and Mrs. Weis:
October 23, 1976
Re: Minor Land Division APN 727-02-009 59.82 acres East of Main Street Morgan Hill, CA
In-depth Geologic and Seismic Feasibility Study
Thank you for the opportunity of stud.ying your south west sloping land parcel for potential division to 4 parcels.
This study conclusion is favorable from the geologic and seismic viewpoints, with certain outlined recommendations.
' Consideration was given to additional possible alternative sites along the upper ridge lir1e. Several potential sites appear favorable, subject to site specific examination.
The detail back pocket maps together with the photographs represent an exceptional perspective of your land show the basic suitability of your proposed land division.
If you have any further questions, please call at your convenience.
ex-:;; # '-&=tl:_r'(U.!J ~ . .,,() UokC:-
ame s B. Baker Graduate Geologist
6 copies
Respeetfully yours,
R. Burton !lose C.E.G. 760 G.P. J97
• 'rABLE OF CONTEN'rs
Letter of Transmittal
He port
Bibliography
Appendix Data
Illustrations:
• Preface Plates and Aerial
Photographs
Seismic Refraction Lines and Index
(In pocket)
Geologic Plate
Geologic Gross Sections
Relative Geologic Stability Map
•
Alqu1st-P:riolo Special Studies
Zoning Act 1974
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ENt";lr-.IEEnlNQ GEOL0
0GIST
IGALt•o11r~IA l=/~"(';IATl~ATION)
ALL I Lil Gt:OPI IVSICS
P,!'.). Box 583
SA~ .JO~~;. CA. 9~106 (40Bl 256-2609
Weis Minor Land Division APN 727-02-009 59.82 acres East of NE end of Main Street. Morgan Hill, California
In-depth Geologic/Seismic Study
~: October 24, 1976
Introduction: This feasibility study and report addresses
the 1974 Alquist-Priolo Special Studies 2oning Act criteria
and supplementary requirements of Santa Clara County. The
acreage is evaluated for a specific building site on each
of the proposed parcels, with general consideration of
additional sites along the eastern ridgeline. Wood framed
home construct i.on is considered as basic. Both geologic
and seismic factors and hazards are reviewed and correlated.
Four parcels are partitioned; with an existing dwelling on
one. Seismic refraction survey lines were run across the
approved building sites on the additional three parcels as
shown on the index map and the detail geologic plate. Plus
one week of field time was spent at the site with separate
flight time for aerial photographs. Office preparation and
aerial analysis time was not logged in report completion •
1o,,,1000 I
INDEX: Weis Land Parcel
APN 727-02-009 59.82 ac.
NE end Main Avenue Morgan Hill CA
T,9 S., R.J E., MDBM Contour interval "' 40 ft~
Mt, Sizer U.S.G.S. 7t' Morgan Hill 11 "
quadrangle II
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727-02-009 2 •
Location; This sloping recorded land parcel extends from
the valley floor to over the ridge separating Santa Clara
valley and the Anderson Reservo1r on the eastern side of
the valley. The south west corner of the parcel is at the
north east end of Main Street, Morgan Hill, in Township 9
South., Range 3 East, MDBM. The. composite vicinity preface
plate shows the area parcel relationship, the orthophoto
and elevation contour map present the slope aspects of the
property, and the detailed 1• "' 100 1 pocket maps summarize
the specific field study results, l.n spatial relationship.
Data: The basic geologic, geopl1ysical, structural, seismic,
fault, topographic and slope essential information must be
dispersed. onto a series of revelaut maps presented in this
report. Tl'\e conclusions and mitigating or limiting list of
recommendations are a specific portion or content of the
minor land division approval. A reverse recorded seismic
refract ion survey line was run across the most suita.ble and
practical building site on each of the three proposed_ parcel
separations f1'om the existing home unit. A Bison signal
enhancement se.i.smograpl1 was used in the field. Stereo aerial
photos were analysed for structural and fault lineations.
Ground and a.i.r photos arc incli:_d,od to emph~size pertinent
geologic and structural features •
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Contour interval ~ 40 ft .
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727-02-009 ) •
Geologic Structure:
Two main structure types are present:
Faults-: Three distinctive faults have been mapped
through the property; all trending subparallel in a NNW to
SSE direction.
The Coyote Creek fault cuts through the property at
the highest elevation and is marked by linear outcrops of
silica carbonate at the serpentine - Santa Clara contact.
Another fault marks the juxtaposition of Franciscan
greenstone with the Santa Clara gravels. This contact was
crossed by one of the seismic refraction survey lines and
interpretive profile (A-B) and is clearly disce:rnable •
The third fault appears to mark the alluvial contact
with the Franciscan melange and may be marked by the break
in slope where not concealed by landslide debris.
All three of these faults are considered probably active
by reason of their juxtaposing recently deposited materials
and_ the older rock uni.ts.
Regional active faults include the San Andreas, the
Hayward, and the Calaveras; all shown on an appendix map.
Foldin~: The Plio-Pleistocene Santa Clara gravels
were mapped by Dibblee (197)) as being in a gentle synclinal
t'old striking NNW - SSE. This geometry is depicted on the
cross sect.ions in the back pocket, but were not verified in
this study_ since attitudes were not e:x:posed within the Weis
property limlts.
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Vert.ical aerial photo and overlay diagram of parcel.
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727-02-009 4 •
Geologic Rook Types:
Several distinctive rook types (units), as well as
s1gnifice.nt surf1cial materials, were identified on the
Weis property,
Field mapping, air-photo interpretation, and seismic
refraction survey profiling were used to delineate their
geographic extent (see geologic map, back pocket). The
following sections describe these units in detail. One
type group requires additional perspective:
Franciscan melange (KJfm) Cretaceous age
The term "melange" is now commonly used to designate
masses of rocks cf diverse origins which chaotic mixing
has arranged in a sheared matrix (Hsi.i, 19'/1 ), Current
hypotheses suggest that sometime during l,ate Triassic
time (Tithonian) a submarine trench .opened near the western
coast of the North American continent as the Paciflc plate
was subduoted beneath it. Terriginous, biogenio, and volcanic
layers intermittently accumulated in this linear trough
(eugeosyncline){Blake and Jones, 1974). As these materials
were forced down beneath the continent, extr•eme stress
sheared and mixed them while producing low grade metamorphism.
Innumerable dislocation surfaces have disrupted the
stratal continuity to such degrees that mapping of' different
rock units in the m~lange yields little systematic under
standing of their distribution.
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727-02-009 5 •
GEOLOGY - Rock Units:
This following summary of rock types exposed in the
Weis land parcel is a composite of site field observations
and literature reference descriptions.
Cretaceous age metamorphic isolated "knocker• (Tectonic block)
This isolated boulder ("knocker•) of mica schi3t,
approximately 20 feet across, is located behind the residence
at the south west corner of the property, just above the
valley floor. It has a striated chlorite-actinolite
·peripheral shell. Where cracked in half, the metamorphosed
interior shows veinlets of lawsonite (?). The shell coating
of chlorite minerals is slickensided and striated parallel
with the rounded surface of the block.
This "knocker" 1.s a classic indication Of the melange
nature of the Franciscan unit as exposed here. A mass of
elastic rock, carried to a great depth (10-20 km) by plate
subduction, must have been altered to schist and then worked
its way back up the sabduction zone to the shallower depths
where it eventually became mixed witl'i. the graywackes and
cherts which also outcrop here (Hsu, 19'71). Its presence also
suggests. that a fault intersects the surface near this break
in slope •
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Single ellip3oidal knocker of schist
(cracked in half) located .i.mmediatt0•ly
behind the a xi_ sting rcsi<honc e
(photo courto<;y of Tony Nisich)
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727-02-009 6 •
Franciscan graywacke with shale (KJfmss>
Exposures of these cJ_ast ic rocks are dark, foliated,
and discontinuous. Isolated outcrops, separated by wid,e
expanses of nondescript light-colored sandy soil, exhibit
poorly defined and irregular bedding which is usually too
foliated to yield reliable attitudes. Mild metamorphism
has produced flakes of mica which create a distinctive
glisten when reflecting sunlight. Overall, st1•ucture is
discontinuous and chaotic. Veins of quartz and calcite lace
most exposures and indicate the great extent of tectonic
disruption these materials have undergone. The rock is
hard and dense, yet so foliated and shattered that outcrops
are usually surrounded by talus accumulations of platey
fragments,
Franciscan chert (KJfmch)
Isolated blocks of chert outcrop in a broad band
along the first ridge line. Bedding is apparent in most of
the masses which are fine grained (probably Badiolaria) and
veined with quartz (see photo). A notable exception is a
large outcrop of rhythmically banded chert in which
brilliant blue and white bands of chert are shattered and
dislocated with fine veins of lighter colored quartz that
runs transverse to bedding planes; Conspicuous cavities
extend along the bedding planes indicating the altei•at ions
and recrystalization which these rocks have undergone •
Foliate<i and minorly recrystalized graywacke anci_ mudstone with fist-siz.e cobbles of basic igneous rock are typical in isolated outcrops in the area of Fran6iscan m~lange.
Blocks of reddish radiolarian chert with veinlets of quartz are exposed in several isolated outcrops and are indicative of the intimate mixing of the different rock types which has occured in the melange.
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727-02-009 7.
Franciscan greenstone (KJfmg)
Actual outcrops of these altered extrusive basalts
are non-existant on the property. Rather, the surface is
littered with small pebbles (i"-1") of aphan1t1c greenstone
with characteristic vesicles and dark pseudomorphs after
olivine. Some cobbles of tuffaceous greenstone are also
present.
These rocks were probably deposited as extrusive
flows on the sea floor and subsequently subd.ucted to depths
where mi.nerals were altered and original structures sheared.
A band of this rock ls in apparent fault contact with the
Pllo-Pleistocene Santa Clara gravels about half way up the
property slope •
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727-02-009 a.
Cretaceous and Later serpentinite •serpentine• (sp)
This ultramafic rock is in apparent fault contact
with the Plio-Pleistocene Santa Clara gravels (Coyote Creek
fault). The mass, which extends eastward at least as far
as Anderson Lake, is most likely in intrusive relation to
the basement rocks.
Slopes underlain by these rocks are characteristically
boulder-strewn and. covered wlth distinctively different and
less dense vegetation {see photo) owing to the peculiar
chemistry of weather products. Oxidation of abundant iron
minerals gives a distinctive reddish-green .hue to the soil.
Digger pines and manzanita thrive with these conditions.
A five foot cut into "fresh" rock at a building site 20
feet beyond the north eastern property line in Holiday Lake
Estates revealed the 1 blocks in a sheared matrix' character
of the serpentine mass. The blue-green blocks consist of
completely serpentinized peridotite or dunite with the
original textures preserved. Olivine and pyroxene crystals
have been replaced by serpentine minerals (ant1gor1te,
crysot1lc and bowl1nglte) and outcrops are typically bumpy
owing to differential weathering of these pseudomorphs.
Blocks are rectangular to rounded and set in a sheared
matrix of soap-like pulverized rock with many foliations
and slickensldes •
i !
Look i.ng north along the strike of the Coyote Creek fault. (Note the topographic expression and BOK pond at left.) Blocky outcrops in foreground are typic~l serpentine areas •
.... ,
Outcrop of silica carbonate rock follovJs the trend of the fault, undulatinr, foliation dips steeply ( 75°) to east.
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727-02-009 9 •
Cretaceous to Pliocene silica-carbonate rock (scr)
Along the fault contact between the serpentine and
the Santa Clara gravels is a relatively thin (1-3 feet)
layer of dense lenticular rock that crops out along the
strike of the contact as rock-wall type exposures. The mix
of silica minerals (chalcedony, opal and quartz) associated
with carbonate (chiefly ferroanmagnesite) ls apparently the
result of hydrothermal alteration of serpentine along the
fault plane. Shiney black crystals of chromite remain from
the original serpentine nature of the rock.
Because of the resistance to erosion, these relatively
thin lenses stick out of the ground in a very characteristic
fashion (see photo), as well as forming the "backbone• of
the r-idge. Weathering of the. carbonates has reduced the
rock to a porous network of silica veinlets stained with
residual iron oxides (Bailey, 1964, p. 87) which produces
a typical "thunk" when struck with a hammer.
The sheared and undulating structure of the rock
suggests that it ls the alteration product of equally
sheared and foliated serpentine masses and further confirms
the fault related nature of the material. The hydrothermal
solutions which originally produced the rock commonly carry
metals (like mercury) into the rock; however, no cinnabar
was found on the Weis property •
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727-02-009 10.
Plio-Pleistocene Santa Clara gravels {Qtso)
As used herein, the term applies to a poorly
consolidated unit of coarse, angular gravels, sand and
clay, which is poorly exposed and underlies more than
half of the subject property. The charaoteristio reddish
brown soil that it produces, and the small rounded chips
of lithic fragments which are brought up by burrowing
animals, clearly indicate the extent of the rook material.
However, no good indication of the total thickness of the
unit as exposed here could be found. The syt-iollnal nature
of a broad fold indicated in the cross sections is
primarily based upon that interpretation presented by
Dibblee (197)), Generally, however, the geometry of the
several slope failures suggest that this synclinal relation
is probably correct.
Quaternary to Recent alluvium (Qal)
The flat area at the south western end of the property
is the beginning of the broad valley. Unconsolidated sands,
gravels, and clays have been carried out of the hills
by streams and laid down as a broad continuous apron. 'rhe
fault mapped along the ridge front could be responsible
for the abrupt nature of the alluvial contact with the more
resistant Franciscan rocks.
This is the only clear exposure of ~anta Clara Formation on the parcel, owing to the ease with which it ero,les and developes a soil proL.le. 'J'he vravel appears to be predominately locally derived Freenstone from the Fr•anciscan. 1rhe clays make a very poor cement anci contribute to th8 unstable naturH of slopes underlain by th_i.s l.lnit.
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•
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Stereo triplet vertical aerial allows for three dimensional view of parcel. Slope failures show by their char•acteristlc bowl-t>haped head scarps and bulbous toes. Also, two of the three faults are clearly shown by the abrupt discontinuities in soil tone.
0
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727-02-009 11.
Quaternary to Recent landslides (Qls)
Several large rotational - flow type slope failures
are indicated on the accompanying geologic map and cross
sections. The forms of these landslides are somewhat
modified by erosion, obscuring their geometry and extent.
Analysis of stereo-paired aerial photographs of various
scales and dates provide the different lighting conditions
and soil moisture distributions·necessary for interpretive
delineation of these masses.
A complex of several generations of failure is
evident at the ridge front in the Franciscan. The other
slides, several of which have well preserved, bowl-shaped
head scarps, occur in the Santa Clara whe1•e deep inc 1.s ion
has undercut the stability of the slopes. A very large
landslide (Spencer slide) extends from the ridge line out
onto the valley floor. The mass of this slide is mostly
sheared serpentine which weathers out as blocks on the
surface. This distribution led to an erroneous designation
on the adjoining parcel as serpentine bedrock. Actually,
the blockaof serpentinite observed on the adjacent knoll
are coming from a cap of landslide debris resting on Santa
Clara gravels (see photos) as the cap erodes away.
Panoramic mosiac looking NE and East. Both property lines are marked by fences at left and right. Features in this area indicate relatively active rotational landslides with'exLensive erosional incision.
Closeup looking upslope to the northeast of the most active landslide.
Low sun angle view looking north at higher elevations of the slide area accentuates the hummocky surface and active head scarps.
Looking south at isolated knoll just beyond limits of the parcel. Note the horizontal discontinuity in soil color and feature. Blocks of sheared serpentine litter the surface.
Another view looking SI< shows trie knoll and an area of similar material in the foreground. Both appear• to be the remnants of landslide debris. 'rhe material on the knoll is now an isolated cap cut off from the main body of the Spencer landslide.
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727-02-009 12.
Recent expansive colluvium (exc)
Accumulations of slope wash and creep ( colluvium) soi[
with high clay content exhibit a strong tendency to
shrink and swell with the change of dry and wet season
cycles. Winter rains saturate the material and the water
absorbing clay minerals increase in volume. The following
summer, this material drys, shrinks and cracks. Cyclic
cracking mechanics indicates that the cracks open towards
the down slope side and close from the upslope side. The
bulk behavior of these solls on slopes is to slowly move
the broken material down hill, termed •creep•. This cyclic
movement constitutes a significant hazard to development
and these areas have been mapped on the basis of extensive
cracking observed during the dry su111111er •
' .
Aerial view looking north. surf'ace in midct.le elevatior1s, slope failure.
Note hum~ocky indicative of
•
•
•
727-02-009 13 •
Seismic Refraction Survey:
Three reversed recorded seismic refraction survey
lines were run, one at each of the proposed building sites
on the undeveloped division parcels. A Bison, Model 1550,
signal enhancement seismograph was used. Time of the shock
wave travel was measured in milliseconds, between hammer
blow stations and the end point recorder. Velocities of
sub surface horizons were calculated in feet per second.
Depths were computed in feet below the surface at the line
end points, offset in the direction of measurement. These
depttsprovide the data for the geoseismic profile in context
with the geologic and other facets Of this report. Three
distinctive velocity ranges were recognized:
Vl 770-1)00 ft,/sec.
v2 2000-4000 • n
VJ 5600-12000 • ,,
Soll mantle
Weathered bedrock
Fresher bedrock
Depth
31-18 1
33'-38'
A setback of 20 feet is recommended from the possible fault
contact on line A--B at station 80 •
•
•
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C 1, INSTRUMENTS ~ 5708 - 36th Street West
St. Lou is Park, Minn. 55416 Tr-i~'"honP 61 ~ 1fJ?6-l"M6
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TRAVERSE C-£)
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FIELD NO TES _::!2 /a:># SE CoR.Pc/. 4
13
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F!ELO NOTES---~--·----------------------------"1't=-o..,.-n.
•
•
•
727-02-009 14 •
Relative Geologic Sta~liti:
A map of the relative geologio stability has been
generated as an overlay on the geologic map and is included
in the back pocket. The following table explains the
meaning of the stepped categories. The condition ls
described, the hazard outlined, and the potential for
residential development given. Three building sites were
spec1f ically investigated and approved. These areas are
labled with "A" and colored green.
'l'he several broad areas labeled "B" are also suited
for building, but require additional site specific study
to verify the general analysis of the stability of these
at~eas .
The other categories are increasingly more restrictive
in t<?rms of information needed and possible problems that
confront development.
The areas labeled "F" and coloreci red are deemed
unsuitable for residential structures due to the definite
indication of active hazards; such as active fault rupture
potential or la.nd surface failure.
It is noted that all of the parcel, surrounding area,
all of central and southern California are subject to
future severe earthquake snaking; a pervasive hazard that
requires engineering design attention •
• SYMBOL
A
E
c
D
E
F
Questionably stable
Least stable
• RELATIVE GEOLOGIC STABILITY CAJ'EGOB.IES
CONDITION
Site investigatiori specific, including seismic refraction line (see profiles}
Relatively flat areas apparently free poor geologic factors (no site specific data)
Excessively steep slopes: may become unstable when graded and/or loaded
Expansive soils are indicated by some contraction crac~ing in summer
Setback areas adjacent to inferred faults or accumulation expansive colluvium (extensive)
Identified potentially active area {faults & landslides a problem)
HAZARD
Minimal hazard_ *
Suggested minimal geologic hazards by general analysis *
Structures subject to distortion from differential slope failure action
Shrink-swell behavior of soil may affect foundations and structures
Likely major ground distortion or downslope creeplid.versely affect foundations
.
Surface rupture due to faulting or slope failure could damage any type structure
.
• DEV ELO p ViEN'r POT Eci'T" IAL
Suitable for single family wood fra:ned structure with approved soils engineering
Probably suitable with normal geotechnical site specific study
Special engineered :ype foundation design may be required
Subsurface data on soil properties necessary for specific evaluation
In-depth study required for possible placement of any structures
Unsuitable risk associated with any development: open space category
* ~round shaking is a nervasive hazard in all areas a~d re~ion
• 727-02-009 15 •
Conclusions:
This proposed minor land division is approved on the
basis of the plot plan lines used in the index reduction for
the seismic refraction survey positions; with the following
requisite recommendations: from a geologic and seismic basis,
Recommendations:
One of the recommendation made in the field has been
incorporated into the road design; that or increasing the
culvert size for maximized runoff capacity under the major
fill section. Other mitigating requisites include:
1.) Wood framed house construction.
2.) Relative geologic stability chart and map be
• included in development planning.
•
J.) Minimum acceleration factor of 0.2 g be used in
design from a geologic and seismic viewpoint.
4.) Engineered foundations be used.
5.) Grading cuts be kept minimal to the extent possible.
6.) Runoff water be controlled around home sites and
6 copies
no ponding permitted adjacent to structures.
Respectfully submitted,
R. Burton Rose, M.A. C.E.G. 760 G.P. 397
<-? 'B,_"Lc--ames B. Baker
Graduate Geologist
•
•
•
BIBLIOGRAPHY
Bailey 1 Ed.gar 11. 1 William P, Irwin, and David L. Jones; "Franciscan and Related Rocks, and their Significance in the Geology of Western California"; Bulletin 183, California Division of Mines and Geology, 1964
Blake, M. c., Jr, and David L. Jones: "Origin of Franciscan Melanges in Northern California"r in Modern and Ancient Geosynclinal Sedimentation; R-:- H. Dott,Jr., and Robert H. Shaver, ed.; Soc. of Eco. Paleo. and Mineralogists, Special Publication No. 19, 1974
Berkland, J. 0., et. al.; "What is Franciscan?"; American Association of Petroleum Geologists Bulletin, vol. 56, no. 12, pp. 2295-23021 1972
California Division of Mines and Geology; Seismic Hazards in Santa Clara County; Special Report 107, 1974
Cooper, Clark & Associates; Preliminary Technical Report Geotechnical Investigation, City of San Jose's Sphere of Influence, April 24, 1974
Cotton, W. R. ; Preliminary Geologic Map of the Franciscan Rocks in the Central Part of the Diablo Range, Santa Clara and Alameda Counties, California; USGS and HUD, SFBRS, Basic Data Contribution 39, 1972
Cummings, ,T, C. ; "The Santa Clara Formation and Possible Post-Pliocene Slip on the San Andreas Fault in Central California"; in Proceedings of the Conference on Geologic Problems of the San Andreas Fault System, Stanford University Publications, Geological Sciences, vol. XI, pp. 191-207, 1-68
Dibblee, 'l'. W., JR.; "Preliminary Geologic Maps of the Mt. Sizer and Morgan Hill Quadrangles, Santa Clara County, California", USGS Open File Maps, 1973
Dickenson, William R.; "Table Mountain Serpentinite ~xtrusion in California Coast Ranges"; Geol. Soc. of Amer. Bulletin, vol. 77, no. 5 1 pp. 451-471, May, 1966
•
•
•
Dickenson, W. R., et al.; "Test of the New Global Tectonics; Discussion"; Amer, Assoc. of Petroleum Geologists Bulletin, vol. 56, no. 2, pp. 375-384, 1972
Helley, E. J., and E. E. Brabb; "Geologic Map of Late Cenozoic Deposits, Santa Clara County, California"; USGS and HUD, SFBRS, Basic Data Contribution 27, 1971
Hst\, K. Jinghwa; "Fransican Melanges as a Model for Eugeosynclinal Sedimentation and Underthrusting Tectonics"; in Jour. of Geophy. Research, vol. 76, no. 5, pp. ll62-IT70, Feb. 1971
lrwin, William P,; "Franciscan Groups in the Coast Ranges and its Equivalents in Sacramento Valley, California"; in Bull. of Amer. Assoc. Of Petrol. Gaol., vol. 41, no. 10, Oct. 1957
Leighton, F. B.; "Landslides and Hillside Development"; in R. Lung and R. Proctor, ed., Engineering Geology of Southern California; Association of Engineering Geologists, Los Angeles Sect~on, Special Publication, pp. 149-200, 1966
Mooney, Haral M.; Handbook of Engineering Seismology: (in progress edition) , Bison Instruments, Inc., 1973
Page,B.M.; "Time of Completion of Underthrusting of Franciscan Beneath Great Valley Rocks West of Salinian Block, California"; in Geol. Soc. of Amer, Bulletin, vol. 81, pp. 2825-2834-,-September 1970
Rogers, T, H. ; Environmental Geologic Analysis of the Santa Cruz Mountains Study Area, Santa Clara County, California; California Division of Mines and Geology, Open File Report 72-21, 1972
Rogers, T. H., and C. F. Armstrong; Environmental Geologic Analysis of the Monte Bello Ridge Mountain Study Area, Santa Clara County, California; California Division of Mines and Geology, Preliminary Report 17, 1973
•
•
•
Rose, R. Burton; "Portable Seismic Refraction Studies of Shallow Geologic Structures''; in XXth International Geologic congress, Mexico, publI'Shed 1958
Scott, James; Anderson Reservoir Landslide Investigation; Santa Clara Valley Water District, January 1976
Aerial Photograhs:
low altitude, oblique, color photo courtesy of James o. Berkland (35 mm taken in June 1976)
vertical stereo triplet - enlargements of 24: frames 25,26,27, original black-and-white 9" X 9" s scale 1:12850, flown 5/27/65 (available courtesy of USGS Library)
vertical frame (ac.companied by diagram) -enlargement of 9-19A, original color 9" X 9" scale 1:21300, flown 6/26/74 (available courtesy of USGS Library)
vertical stereo-pair (not included in this report) -AV-546-01-03,04, glossy black-and-white Q" X 9" s scale 1:12000, lown 6/26/63 (acquired from Pacific Aerial Surveys)
Map Base:
proposed parcel boundaries and road alignment from Preliminary PARCEL MAP, MH Engineering
topographic contours generalized from USGS Mt. Sizer Quadrangle and modified with detail from field observations
•
•
•
•
PM:5TJK!-'.D LPin1"'t~:R ot•
IJ>ll..IL l8, 190~ r.ARI'HQU~
t\AG. 8. J
l'P.ESUMi:.:0 !:PICE.l'l'rtR OF...__,~'"l JUIB1lB38 °E.Po.Kl'l-IQL'll'l.KE
MA~ITUOE 7.0iQ.~
~M-t (. ; P+.IJ P'RCSUMEO EPlCl-".Ni'ER or OC'l"Q?..RR 11. 1869, •'.J!.RT\!QUAAl'. MAG~HTUOE 7 .. o:to. 3
. . .. . . PRESUMED t;PICENTER OP' / JULY 1 1 l9ll I:ARI'llQ'JAKE •:.
N l"f t $·. J.,_.,,..,,. , .. 1 •,' p 1-S. \: ~
o~o '~·~ ~Rli:SUMEO E"S>ICENTER or ~($.I) z QCZ'. 12 1 1926 EA~HQU•\KE : 0 o
ff'l-tl 1.,_1'1 n-.; o • p 1') \": ~ 0 ••
0 0
FAULT JOAQUIN
FAULT ' _,.. ~.....,
.. /' ,/
/STANISLAUS
·• 0 ., '
A ) CALAVERAS
I , '..J''-".., /,
' <
/ , /
............ /!' 0
' "" 0
,
' 0
, I
\
PALO COLORADO - SAN GR~GOR!O ,?'f \ BtNITO '., •-.,
' " 0 0
FAULT '
0
'"' "' • A
1•.21
E'.<P1,,.ANAT_ION
M.(<"Jni.tudu 4.0 - 4.? M./l.:Jilit•J"e S.,O ~ ~.9
t~,,9ni'l:.1.de 6.0 ~ 6.9 1'1<.!1JHitu,iiJ 7,0 - 7.9 r..1gnitu,J~ ~ 0.0 Maqnitu<.le of selecte~ Q~rthq~.ti.k~s
Fil]Ut't'.!S b~oi<l~ t.hc lo<:~t.tQn~• iridit::.).te th!! nu.'l'i)ez- i;.11'.: (>•1rt:h'.lli.:a:A~~ a.t U1(: '0-.iz:·.e pla.::l!I,. all ~quc!l. tC! oc smallll'!~ t.hil.ll t.he a'~ p).otl'..@d.
~ N.
~ 0 " 16 ~
'S(;Al..E !N MU.ES
o.
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FAUL.T .J 0
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•'
0
Figure 12. Major faults ;ir1d earthquakes in the greater San Francisco Bay arc~ (modified after Rogers and Williams, 1974).
33
•
•
•
,_
, APPENDIX 8
General Comparison of Scales Used to Rate Earthquakes at the Earthquake Epicenter•
Jnt.4!!nsity
Ma'l!'l!t.ude Richter sea.14!1
Rali•i~FoU'l mivaler,t.
l l
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8.0
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Net. folt. •:icr;111111pt bJ' few \Q'lder f•v.)r•t.. c:it"et:m&t~
relt. noticeatily 1ndoOn
Pelt gene:r.ally i..n4oolW
:Palt by a.11, IMllJ' frtgh.t.aIW4
~~eyori• run.t CKAd.oon, f•lt:: i..D. 'ill()Vin.9 ca.~
h"1o
V•r')" de.!!1tr13C'il ve ~ 9ene;r~l c!.1m101.9~ t.o ~•o!lk 5t.t'tlet&r9~. Little dam.aoge t.o well-built .ut~e-t.una
'l'Qt.al i!est.rui;;t!on. w•tJc 11t.ri.u~tunis, eon.sidct'~h
d;w.~e "'•ll.-bui lt. atnet:.urU
?-'laaon.ry .ut.d !r~ .tr..ructl.U"U ~ly
destroy~. Cinly be•t build.in~• •\ll'Viv•
o.lte&t;zlr •U1J~ d:l!J•et.a i>W!l\f
ni.t:hi!'Sr v;i.ti.~. b~ pttndu.l\Qll <:locks atql
f'uni..i. t. UXll 'lflO'ill!;d._. d;iject.a \lf'S•t
,•
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t.uni.•d· S.anil and. ~~ t!jl!l.;:t.i!;d.. Ol&nl]9:• i.n WiSll-Wit.t.111.t' 111Rl..t
ro\tt!.dati01'1.!11 d~ve4~ \i,t'o~r·-9J:'Ound p:f.p.a bxuk<i:D.
Ct'Qund P.:.~ly er~, rails bent. We:t.eE' •l.Qpi;ed gVt!C bi~
'D'l'Qad. fi.$~~· ~ t•ul.i. •earpar Ul'J.dar'"""9't'l)'lln4 pipes out. of so~c.
a~c~le~&t.ion -..ee~
9r.s.vi t;:y. W;ave• i1111;•t1.
i.l'!. 9;t0urtd.. LiM:f. Of s.1.ght And. level d.i9.~t,.~, d;lj•~ __ ....,.u
NOT![ ~it.ude .&r14 l.ntenstty of ~ ~arthqiJ;U:.e arw 1SJ1.ti:1:elY dl!fe~t .measuremi!.b.t.9 ot .tt~.aJc.1111 ·~ o: ~tl"eCU,.. ~d. can oo.ly b1111· ~4.rej .ae th• •~qu~• ~_piccmt.er-.
Maanit:\.ld~ - it! .it nl,l"'".,ber de:-ive-d .f~ in.'l·t~n~a1 pr.8::9.urewnt.$. a?ld is th• W.4!1A".ln ot the •MJ!WJ" t-~1.euod. ~ u;;-;arthqullllte. 1'r.~e i$ ~ 1'A.';lniti.:.d~ <=41.l~l.a.U:d !or e;:r.i;h eart.hqt.!Aka.
~nsl~..t - i• ;a l.~al :i:-a.tin9 of e.u-tl".qt.tdkil!! effe~ oQsi!!;i=ved. m:'.ld :report~d by PeopU. at ari.y 9ivmt l.oeati~. Int~r.i:!Jity .ri.tin9'• for Ul'!.1:.- e.-.tth.JUa:.~e vlll"f c~ider~ly depend.i.n.g QI) "-1.st.an1;:t11 tram •pie:e:nt.911', th• r • .iit.w."e of the io.ol.l. &r1~ t'Qek beneatl, th• l'.:bse~i:, a.~ ~e ~•~.t"Ver'11 !.i'lt.e.eynut.i~n ot. evv:r:i.U .!leen ot' .C.lt.
36
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Table 4.1. Ior:AL1zro RrLA11os ar.rY.'ElN MAcr-:1ruoE. A!'-i'D Ll:.SGTH OF SLIPi":ED FAULT
Table 4.5. ExPEcr.a.t10~ oF EARTHQUAKES JN
CALIFOR!'>ilA (150,000 lnP-)•
Magnitude
4.n-5.25 5.25-5.75 5.75-6.25 6.25-6.75 6.75-7.25 7.25-7.75 7.75-8.25 8.25-8.75
Number per 100 years
250 140 78 40 19 7.6 2.1 0.6
•A~wci~1~d with large ~arthquakt!~ are .;h.i'Stc~ i;ir sm;:illtr ~arthq1,1l1kes that l'll"'Y prbdu~ repcli[i\'C ~h~king cif the- sarnit lo.;:11tion- To disCOu;J.t 1hi'S fQr pn;ibabililY 1;.11k11latio11s, some of Lhe eanb.Q\J.il1'C::lo ha\·ing .'IJ < 6.0 ha\C been Qomitled.
Table· 4.6. PERCENT P~OHAtHurr or A.CCE:LERATroN
AT A LOCATION IS CALifOR~IA
In period or years
.(\occ:leration 10 2S 50 JOO (%&)
~ s 65 92 99 99
~IO 37 70 88. 98
~ 15 19 41 64 87
~20 10 23 40 63 ..,, 25 s i2 22 37
~30 2.5 5.S 10 19 ..,, 35 1.0 2.5 4.4 8.7
of course, a po.sibility that the site is covered more than once by the >pccified ground acceleration and the probability of being covered 11 times is
( ) (a)"I -•'"' p n,a = A n!e (4.2)
It should be noted that the probabilities given in Table 4.6 are too high for some of the less seismic regions of California and too low for some of the highly seismic
regions.
1ables 4.1, !!:..:..2• 4.6, l!J. Earthquake Engineering
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BISON SICiNAL ENHANCEMENT SEISMOGRAPH MODEL 1550
Modal 1550 is a rugged. light·we1glH statc-uf-the-nrt signal enhanccrnent engineering seisn1ograph designed for rapid and effective subsurfaco cxplora,, .tiori. Its niost irnportant applications arc in determ1niri\:I depth to bedrock or other subsurface boundaries. rind for engineering classification of rnaterials at Uept·h. The field work is carried out cornplctely at thr: surf acc. If used ris a supplomer1t to an explorrJtory drilling progran1. custs C<In be greatly reduced while expanding the inforrr1ation obtained.
The 1550 is a snpt1isticntcd multifunction instrurnent with many of the same fealurP.s as H10 larqer, more expensive Bison seismographs. !"his incliides tt1e C(Jpabi\ity tor 5ign(JI enhanccn1ent, iritrod1iced by 81::;on lnstrun,ents which permits its use alrnost anywhere - even in congr.sted tr(Jffic and construction areas,
Its curnpact size and light weight, to-9ether with its brillicint cjisplay and func .. tional controls. m.o.ko it useful to civil engineers. geulO~Jists, contr(Jctors. well drillers. (.Ind geophysicists engaged in:
Highway co11strucllon Damsite Investigations Bidding and estimating Groundwater studies Gravel and quarry prospecting Foundation sludies Geological and mining surveys
Explanation of signal enhancement
Signal enl1anccrnent is a powerful signf11 processing technique which ainplifies the true seisrnic signal (.Ind reducc5 t11e noise interference produced by wind, traffic. and machinery. It accomplishes this by electronically summing and stonng the scisrnic waveforms produced by two or more hamrner impacts at the snrr1e locat!on. Tho noise signals arrive at random tirnes so they tond to cancel in surr1rnntiun. The true scisrnic signals are perfectly repetitive in arrival tirne and waveforrn so they are amplified and enhanced by summation.
The effect of :)igrial enhancement is tu • Increase the depth of investigation by
a factor of three or more, compared tc) a seismograph without signal enhancer11ent.
• Permit oper.J.tiun under adven=,1° cond.itions of wind, machinery, u1 traffic.
• ln"'lprove ease and accuracy Oi reading.
• f·~educe field tirne by reducing U11~ nu1nbcr of irnpacts necessary to get< reliable reading.
W<ivdorm without 1.1nhancem9ril. True seiS1Yiic sigr1;_:tl.~ ;1f~ difficult to (listingul$h from nOi$f':!
C::Qmpo111te (11umrr10d) wawefor11'1 generated by tl'!ree 11ucce11~IV~ Impact •lg:nala. SeisIT1ic $ignals arc a.mplifieO and clear\1-(listinguishilblo,