ESYS 150 LECTURE 9 TRANSFORM FAULT EARTHQUAKES IN SOUTHERN CALIFORNIA. Transform Fault Earthquakes Earthquake Prediction Thrust Earthquakes around Transforms San Diego County Earthquakes. NORTH AMERICA 38MA - PRESENT DEVELOPMENT OF SAN ANDREAS FAULT.
Transform Fault Earthquakes
Thrust Earthquakes around Transforms
San Diego County Earthquakes
Coast line in blue. San Andreas fault heavy dark line.Very complex system of faults especially in southern California.
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
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
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.
(b) Bottom floors used for garaging cars collapsed.
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
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.
Current models add propagation of fault break. Like a ripple on a carpet.
Topography along the central San Andreas
Sag ponds - down dropped areas along the fault.
Sediments deposited allow the prehistoric record to be read.
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.
Gap exists south of San Francisco.
Prediction of magnitude and probability of occurring before 2030.
Forecasts based on historic records, dated trench wall offsets and gps
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.
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.
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. > 8. Magnitude
San Andreas 30 mm/yr
San Jacinto 12 mm/yr
Elsinore 5 mm/yr
/Rose Canyon 2 mm/yr
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), > 7.0 M
1933 Long Beach earthquake on extension of Rose Canyon Fault. Very damaging earthquake, affected schools
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
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.
Site was located in the SDG&E parking lot.
Right by Interstate 5
Where active section of Rose Canyon fault is known to run.
Used pre War II air photo’s
to locate the fault.
Southward view of hand-excavated trench.
Fault strikes from upper left to lower right corner
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
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
Last event 500 yrs BP
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.