1999 Annual Meeting Visiting Trenchss, San Bernardino and San Gorgonio Pass/Burro

SCEC ANNUAL MEETING OPTIONAL "EN ROUTE" FIELD STOPS, Sunday, September 26

WHAT WE THINK YOU CAN SEE, LOGISTICS, AND DIRECTIONS are given below for
each site.

Sally McGill will be at the Plunge Creek trench site from 11:00 a.m. to
1:00 p.m. Doug Yule will meet interested persons at 2:00 p.m. at the
Morongo Indian Reservation (directions below) to escort the group to the
Burro Flats site.

We hope to see many of you on Sunday, September 26!

Plunge Creek Site: Science
At this site, you can view evidence that suggests that the two most
recent large earthquakes (AD 1812 and AD ~1700) at Wrightwood and Pitman
Canyon in the Cajon Pass area may not have extended southeastward along the
entire length of the San Bernardino segment of the San Andreas fault, as
has been assumed in the past. The Plunge Creek site is located on the south branch of the San
Andreas fault in Highland, California, just a few miles east of San
Bernardino. This site was originally investigated in 1991 by Stephen C.
Suitt, who dug five trenches as part of a fault hazard study for a
potential development project. Sally McGill began work at the site in
1996. Her work at trench 7 revealed strong evidence for a faulting event
that occured between A.D. 1440 and A.D. 1640. Supplemental work at trench
7 by SCEC intern Safaa Dergham revealed evidence suggestive of an older
event as well. The older event would post-date a date within the range AD
1235-1410. Trench 7 was available for viewing before and after last year's
SCEC meeting. It is still open, and will be available for viewing by
visitors on their way to this year's SCEC meeting, but the condition of the
exposure has detiorated somewhat. No evidence for earthquakes younger than the AD 1440-1640 event was
found in trench 7. If the most recent surface-faulting event on this part
of the fault did indeed occur between AD 1440-1640, then neither of the two
most recent ruptures (A.D. 1812 and A.D. ~1700) at Wrightwood and Pitman
Canyon extended as far southeast as Plunge Creek. Unfortunately, a zone
lacking clear stratigraphy in trench 7 made it impossible to rule out the
possibility of a younger event. Trench 8 was excavated on the edge of a very young alluvial fan in
attempt to unambiguously determine the age of the most recent large
earthquake at this site. Safaa Dergham is logging this trench as part of
her master's thesis. The trench was excavated in three stages. In each
stage the trench was widened and was deepened by 1.5 meters, reaching a
total depth of 4.5 meters. The first stage of the trench (0 to 1.5-m deep)
exposed well-stratified sand and gravel that spanned the entire width of
the fault zone and yet was clearly unfaulted. Similar unfaulted layers
are currently exposed in the uppermost tier of the present stage of the
excavation.

The 1.5- to 3-m deep portion of the second stage of the excavation
exposed discontinuous sedimentary layers in which no distinct faults were
visible, but neither could their presence be ruled out. This stage was
visible to those who visited our site on their way to last year's SCEC
meeting. The deepest tier (3- to 4.5-m deep) of the third and present
stage of the excavation reveals several fault strands in the southern half
of the trench. Some of these faults strands may continue upward into the
middle tier of the trench (1.5- to 3-m deep), but all are capped by the
unfaulted strata in the uppermost tier. We have not yet determined with
certainty whether or not multiple faulting events may be exposed. By the
time of the meeting and field trip we expect to receive the results of
radiocarbon dating that should bracket the age of at least the most recent
event. This may allow us to to place constraints on the southeastern
extent of ruptures previously dated at Wrightwood and Pitman Canyon.

Plunge Creek Site: Directions and Logistics
The Plunge Creek site will be available for viewing from 11:00 am
to 1:00 pm on Sunday, September 26. From Los Angeles: Go east on Interstate 10 past San Bernardino to
Redlands, then go north on Hwy 30. After a few miles on Hwy 30, exit onto
5th street and turn right (east). Continue east on 5th street, going
straight through the stoplights at Boulder and Church streets. At one of
these lights, 5th street will turn into Greenspot road. Continue east on
Greenspot road until you have passed the new housing developments. Then
you will cross a concrete bridge over a creek (Plunge Creek). About 0.2 to
0.3 miles past the creek (about 3 miles from Hwy 30) you will turn left
onto a dirt road with a small, black mailbox at the corner. The dirt road
you want is just before you get to a left turn lane for a second dirt road
into a construction site. If you get to Cone Camp Road on the right you
have gone about 100 yards to far. If you get to the iron bridge across the
Santa Ana River, you have gone several miles too far. Note: As you drive east on Greenspot Road, you can recognize the
south branch of the San Andreas fault as a linear break in slope at the
base of the mountain front. The north branch of the San Andreas fault is
behind the first ridge. Ahead of you (beyond Plunge Creek), you may see a
notch in the hillside where a canyon (Morton Canyon) has eroded along the
south branch of the San Andreas fault, which lies parallel to but
mountain-ward from the actual mountain front. Once you turn left onto the dirt road, there will be a locked gate.
I will wait at the locked gate until the first party arrives. Those who
arrive after that can let themselves in using a key that will be hanging
from a paper clip near chest height on the left side of the gate. If this
key is missing, there should be another key beneath an orange-painted rock
to the left of the gate. Please leave the gate in the condition that you
found it, and return the key to the place from which you took it.
Once you go through the gate, the dirt road heads north toward the
mountain front. To get to Trench 7, take this dirt road for about 1/2 mile
until it ends at the mountain front. To get to Trench 8, take the dirt
road toward the mountain front and then turn right just after crossing a
flood control channel. After paralleling the flood control channel for
about 100 m turn left into the orchard, and then right to the trench.

Burro Flats paleoseismic site -- Science.
Burro Flats is an intermontane basin filled by south-dipping,
coalescing alluvial fans and bounded on the southwest by the San Andreas
fault. The fault shows north-facing scarps and a vegetation lineament at
the surface, and forms a barrier to ground water. The site occurs in a
marsh at a right step in the fault where modern alluvium has buried two
parallel scarps.

Two intersecting trenches, 40 and 60 meters long, expose an ~7.5
meter section of well-stratified distal fan, pond, peat, and peaty soil
deposits. With one exception, thirteen samples from peat and peaty soil
yield stratigraphically consistent radiocarbon ages that range in age from
A.D. 1900 near the top to BC 1600 1.0 meter above the base of the exposed
section. The trenches expose two fault zones. Faulting and associated
folding provides convincing evidence for four rupture events and less clear
evidence for three additional events. Calibrated calendar radiocarbon ages
from peat layers that loosely bracket each event constrain the timing of
the events to between: 1465-1225 A.D. (two events), 1300-695 A.D., 605-30
A.D., 225 AD-960 BC, 920-1685 BC, and pre-BC 1685. Describing the most
recent event was the focus of Ashley Streig, a 1999 SCEC summer intern.
All ages are from AMS analyses of residue after acid/alkali/acid pre
treatment. We expect to obtain a more precise chronology with further
dating of peats closer to each event horizon and by dating both solute and
residue fractions (see abstract of Seitz et.al., 1998 SCEC Annual Meeting).

Our work this summer suggests that the San Bernardino segment, San
Andreas fault has a recurrence interval that is longer than the recurrence
intervals determined for the Mojave segment to the northwest and the
Coachella Valley segment to the southeast. Less frequent events on the San
Bernardino segment requires larger slip per event or strain partitioning
onto other faults in the region.

Burro Flats paleoseismic site -- Logistics.
The site is located on property owned by the Morongo Band of
Mission Indians. They have asked that you adhere to the following
schedule. To visit the trench site you must sign a waiver form that
releases the Indians from any liability in the event that something
unforetold may happen. To facilitate this matter, it is important that
everyone meet at 2:00 p.m. at the Morongo Administrative Center (directions
below). Please send email to me so I know how many people to expect.

Burro Flats paleoseismic site -- Directions.
The trench site is located ~6.5 miles north of Interstate 10 near
Banning, CA. The last 4.5 miles are on a once-yearly graded dirt road that
most passenger cars can manage.
For eastbound traffic, exit Interstate 10 at Fields Road, ~3 miles
east of Banning and ~0.5 miles east of a truck weigh station. Turn left
(north) at the bottom of the offramp, drive underneath I-10, turn left
(west) at the stoplight, proceed a short distance past the Chevron gas
station, and turn right (north) at Fields Road. A cattle guard lies just
ahead at the entrance to the "Sovereign nation of the Morongo Band of
Mission Indians".
For westbound traffic, exit Interstate 10 at Fields Road, ~2 miles
west of Casino Morongo. Turn right (north) at the bottom of the offramp
and proceed to the cattle guard at the entrance to the "Sovereign nation of
the Morongo Band of Mission Indians".
As you temporarily leave the U.S., refer to the following set of
directions.

Mile 0.0 Cattle guard. Head north on Fields Road.

Mile 0.65 Note: A southwest facing fault scarp of the San Gorgonio
Pass fault system cuts obliquely across Fields Road and behind a small
house to the west of the road. The low hill in the foreground to the left
at 10:00 to 11:00 consists of Early Holocene(?) alluvium on the northeast
side of this fault strand.

Mile 1.0 Intersection with Morongo Road. Turn left (west).

Mile 1.35 Intersecion with Potrero Road. Turn right (north).

Mile 1.45 Note: Folded Holocene alluvium can be seen to the right at
3:00 at the top of a 10 to 15 meter incision by Potrero Creek.

Mile 1.75 Morongo Indians Administrative Center, at 11581 Potrero
Road, on left. Meet here at 2:00 p.m.

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