ESYS 150 LECTURE 9 TRANSFORM FAULT EARTHQUAKES IN SOUTHERN CALIFORNIA

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ESYS 150LECTURE 9TRANSFORM FAULT EARTHQUAKES IN
SOUTHERN CALIFORNIA

• Transform Fault Earthquakes
• Earthquake Prediction
• Thrust Earthquakes around Transforms
• San Diego County Earthquakes

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NORTH AMERICA 38MA - PRESENTDEVELOPMENT OF SAN
ANDREAS FAULT
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TRANSFORM FAULT EARTHQUAKESSAN ANDREAS
FAULTCape Mendocino to Gulf of California

• Coast line in blue. San Andreas fault heavy dark
  line.Very complex system of faults especially in
  southern California.

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TRANSFORM EARTHQUAKESLOCKED SECTIONS1857 Fort
Tejon and 1906 San Francisco

• Two sections of the fault locked for long periods
  of time.
• Then gave way very quickly creating two major
  ruptures
• Fort Tejon 360 km long
• San Francisco 430 km long.
• Two very large earthquakes

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TRANSFORM EARTHQUAKES1906 SAN FRANCISCOLocked
section moved

• Moved 6m along 430 km of fault.
• Had 60 secs of shaking.
• Magnitude 7.8 earthquake
• Most damage due to fire and disease.
• Important for developing elastic rebound theory
  for earthquakes

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TRANSFORM EARTHQUAKES1989 LOMA PRIETADuring
World Series

• Bend of San Andreas, where two faults split off
  from main fault, ruptured.
• Earthquake did not rupture the ground.
• Moved 1.9 m horizontally and 1.3 m vertically.
  Magnitude 6.9
• Caused a lot of damage in San Francisco.

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TRANSFORM EARTHQUAKESLOMA PRIETA 1989Damage in
Marina District San Francisco

• Water saturated sediments were shaken became a
  slurry and flowed. Foundations collapsed
  especially soft buildings.
• (b) Bottom floors used for garaging cars
  collapsed.

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TRANSFORM EARTHQUAKES1989 LOMA PRIETAInterstate
880 collapse

• Portion of the elevated 880 built on soft mud.
  The soft mud resonated with the earthquake waves
  and the mud liquified. The hard rock had much
  smaller accelerations

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TRANSFORM EARTHQUAKES1989 LOMA PRIETAInterstate
880 collapse

• The bridge had roughly the same frequency as the
  amplified surface waves and shook severely.
• The support columns failed at the joints. There
  were 20 18 steel bars in the columns but they
  were discontinuous at the joints.
• With repeated shaking the joints failed and the
  bridge collapsed.

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EARTHQUAKE PREDICTIONHOW FAULTS WORKElastic
rebound theory

• Active fault with road as reference.
• Deformation occurs either side of the fault but
  because of friction at the fault, no movement
• Deformation so great fault ruptures and two side
  snap away from each other.
• Current models add propagation of fault break.
  Like a ripple on a carpet.

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EARTHQUAKE PREDICTIONPALEOSEISMOLOGYSag ponds

• Topography along the central San Andreas
• Sag ponds - down dropped areas along the fault.
• Sediments deposited allow the prehistoric record
  to be read.

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EARTHQUAKE PREDICTIONPALEOSEISMOLOGYResults
from trenching

• Diagram of section across a trench wall. A fault
  has offset the sediment layers. No offset since
  upper layer of organic material formed. The 500
  year old layer is offset and 745 year old layer
  is offset twice as much.

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EARTHQUAKE PREDICTIONSEISMIC GAPSCross section
of seismicity, San Andreas 1969 to 1989

• Dense cluster in central creeping section of the
  fault.
• 1989 Loma Prieto earthquake and after shocks
  filled in a gap
• Gap exists south of San Francisco.

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EARTHQUAKE PREDICTIONPREDICTION FOR SAN ANDREAS

• Prediction of magnitude and probability of
  occurring before 2030.
• Forecasts based on historic records, dated trench
  wall offsets and gps

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THRUST QUAKES SAN ANDREASBEND IN SAN
ANDREASComplex plate pattern south of bend
Left stepping right lateral fault creates
compression at the bend Leads to many little
plates south of the bend. Compression creates
the Transverse Ranges and the San Gabriel
Mts. Have thrust faults under LA.
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THRUST QUAKES SAN ANDREASBLIND THRUSTSSan
Fernando 1971, Northridge 1994

• Blind thrusts close together but dipping in
  opposite directions
• 1994 Northridge on Pico Fault dipping to the
  south.
• Overthrust plate moved up 3.5 m
• Intense ground shaking in cities on overriding
  plate.
• 1.8 g horizontal accel.
• 1.2 g vertical accel.

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THRUST QUAKES SAN ANDREASTHE BIG ONEComparison
with 1957 Mongolian quake

• Comparison at same scale of Gobi-Altay Mongolia
  and San Andreas
• Offsets on faults are similar and both have
  regions of thrust faults.
• Whole Mongolian fault system went in 1957. gt 8.
  Magnitude

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SAN DIEGO COUNTY EARTHQUAKESMAJOR FAULTS

• San Andreas 30 mm/yr
• San Jacinto 12 mm/yr
• Elsinore 5 mm/yr
• Newport/Inglewood
• /Rose Canyon 2 mm/yr

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SAN DIEGO COUNTY EARTHQUAKESHISTORICAL
EARTHQUAKES

• 1800 damaged San Diego Mission, 6.5 M
• 1862 damaged town, Rose Canyon Fault, 6.0 -7.0 M
• 1892 Laguna Salada Fault (extension of the
  Elsinore), gt 7.0 M
• 1933 Long Beach earthquake on extension of Rose
  Canyon Fault. Very damaging earthquake, affected
  schools

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SAN DIEGO COUNTY EARTHQUAKESROSE CANYON PLAN VIEW

• Freeways (red)
• Rose Canyon Fault (black)
• Separates La Jolla, Pacific Beach and Point Loma
  from the rest of San Diego.
• Portions of Interstate 5 run down the fault.
• It runs out to sea off Coronado

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SAN DIEGO COUNTY EARTHQUAKESROSE CANYON FAULT
MOUNT SOLEDAD

• Uplift of Mount Soledad created by left step in a
  right lateral fault.
• La Jolla/Cabrillo block is running into a bend in
  fault and being compressed creating uplift.

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SAN DIEGO EARTHQUAKESTRENCHING STUDY ROSE CANYON
FAULT

• Site was located in the SDGE parking lot.
• Right by Interstate 5
• Where active section of Rose Canyon fault is
  known to run.
• Used pre War II air photos
• to locate the fault.

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SAN DIEGO EARTHQUAKESTRENCHING IN ROSE CANYON
FAULT

• Southward view of hand-excavated trench.
• Fault strikes from upper left to lower right
  corner

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SAN DIEGO EARHQUAKESPLAN VIEW OF ACTUAL TRENCH

• Grey shows area dug out to a depth of 2 m. Dashed
  lines show strands of the
• fault. The redline shows buried channel offset
  across the Rose Canyon Fault

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SAN DIEGO EARTHQUAKESMAP AND RECONSTRUCTION OF
CHANNEL

• Total offset 8.7 m
• Age of channel 8.1 - 6.0 Kyr
• Slip rate 2.0 to 1.1 mm/yr
• Three major offsets suggests
• recurrence rate 2 - 3 Kyr for M 6.3
• events
• Last event 500 yrs BP

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SAN DIEGO COUNTY EARTHQUAKESCOASTAL FAULT ZONE

• Paleoseismic motions on the Coastal Fault Zone
  with dates of most recent rupture. Note the
  northward propagation of the events. All except
  1933 Long Beach earthquake were determined from
  trenching.