On October 17, 1989 at 5:08 PM Pacific Time the Loma Prieta Quake struck. The magnitude 6.9 quake caused 63 deaths and extensive damage along the Peninsula and up into San Francisco and Oakland.
Extensive damage occurred to bridge and freeway structures in San Francisco and Oakland. Bridges and overpasses on the Peninsula generally fared better and were reopened after inspections. The exception is the Mora Drive overpass crossing Interstate 280. During the 1989 Loma Prieta quake, compression buckling of the road surface near here was witnessed by a USGS geologist. The buckling was fixed by CalTrans within 24 hours but not recorded in repairs made by the agency. The single support pillar was cracked and the bridge was closed to auto traffic for some time.
As it happens, I had to cross this bridge daily and was forced to walk across rather than drive during this closure. Walking gave me an opportunity to see things one might not notice from a car. In particular, where the West abutment meets the bridge, a horizontal and vertical offset had appeared following the quake. Clearly, the bridge and the abutment had moved in different ways as a result of the quake. You can see this in this picture I took shortly after the quake. Since the bridge remained vertical, although the support column was damaged, I surmise that the embankment is what moved.
Across the road, the sidewalk suffered substantial cracking as this picture shows. Asphalt has been used to patch the crack in the sidewalk. Later the entire sidewalk was given a repaving with a cap of ashphalt.
Fast forward to the present and I paid the area another visit only to discover things had changed since 1989. For example, the bridge abutment had moved a lot further. In this Earthcache, you will consider possible causes of the changes based on background information presented here and onsite observations.
Background Information
Adjacent faults
There is a complex network of smaller faults nearby including the Berrocal fault and the Monte Vista-Shannon fault. The initial abutment movement was due to the Loma Prieta quake or by coseismic activity on the Monte Vista fault. Perhaps, aftershocks or movement from these nearby faults could account for the change in offset. See the map for locations of adjacent faults to Mora Drive [Pampeyan]. The nearest fault is the Monte Vista fault which runs quite close to this location. Geological reports give a .4 mm per year slip rate for this fault [AEG San Francisco Section Newsletter]
Liquefaction
Another significant factor in earthquake damage is the nature of the soil at a location. Certain soils are subject to liquefaction - a process where saturated soils lose strength and behave like a liquid under stresses like an earthquake. Severe damage in the Marina area of San Francisco during the Loma Prieta quake is an example of such behavior. The Mora Drive area is not subject to liquefaction. However, more knowledge about the soils is worth considering since it may be relevant.
Soil Classifcation
Soil classification is a way of characterizing soils based on particle sizes and relative percentages of the particles. Three particle sizes are used: sand, silt and clay. Of these, sand particles are the largest, silt the second largest and clay particles are the smallest. If most particles are large, the soil will feel like sand. If most are very small, it will feel slippery when wet, think of a potter's clay. In the middle is silt which may feel like flour.
The Soil Triangle (image at right) defines names for the 12 classes of soil. Assuming our soil was 30% sand, draw a line diagonally up and to the left from the sand axis starting at 30% (green in this diagram). Then if we have 60% clay, draw a horizontal line starting at 60% (purple in this diagram). The point of intersection defines the name for the soil type -- in this example "Clay". Note that you only need two percentage values to determine the name.
The relative percentages of sand, silt and clay is a factor in how soil behaves. However particle size is also a factor. In general, the smaller the particles forming the soil, the less well they drain. As the liquid content increases, the soil can flow and slide more easily. The Unified Soil Classification System assigns a two letter code to a soil that characterizes it. Some examples are, GW - Gravel, Well Graded and CH - Clay, High Plasticity. The CH type is also known as fat clay. It can hold a lot of water and expands in doing so. Under dry conditions, it can shrink considerably leading to cracking.
Soil in this area
Now for some background on the area you are investigating. Interstate 280 is in a cut in this area and soil was removed to adjacent areas to create the roadbed which is cut into Monterey Shale bedrock. Soil cores taken in 1963 before building the Mora Drive overcrossing in 1967 show the following composition [Yashinsky, 1998].
Elevation below local ground level
- 340 ft: Very soft organic silty clay
- 325 ft: Very stiff to compact tan and gray fine sandy silt with thin brown clay layer
- 320 ft: Dense brown and gray partly compacted fine sandy silt with gray clayey sand streaks to compact mottled brown and gray silt
- Olive brown stratified claystone and tan silty fine grain sandstone.
- 310 ft: olive brown siltstone
Recent test borings along nearby Eastbrook drive found 30-40 feet of fill overlaying the bedrock. The fill typically consisted of medium dense to dense clayey/silty gravel and stiff to very stiff high plasticity silt/clay, while the alluvium generally consisted of stiff to very stiff high plasticity clay. A reasonable surmise is that this fill material came from the Interstate 280 cut that runs nearby as Caltrans did not haul the material from the cut very far and tried to use it for fill where necessary.
Logging Requirements
Observe the area around ground zero and send me the answers to the following questions using my email address in my geocaching.com profile. If you fail to provide answers or omit some answers, I will delete your Found Log.
- List the name "GC76J1W Loma Prieta Quake - A Quarter Century Later" in the first line of your email. Also, list the number of people in your group.
- Using the soil triangle, what would be the name for the soil if you had 30% sand, 30% clay and 40% silt?
- Is the area at the same level as the surrounding terrain? If not, why might it be different?
- Consider the above photo of the displacement of the abutment to the bridge taken in 1989. Measure the horizontal and vertical displacement now and report it. How much would you estimate it has changed?
- The cracks in the sidewalk shown in the 1989 photo were patched and later a new layer of asphalt was laid over them. How is that pavement holding up? Report what you see.
- Based on supplied information and your answers to 3-5, hypothesize as to the cause for what happened here post Loma Prieta earthquake that would account for what you observe.
- (Optional) There are other things you can observe in the area to support the hypothesis. Feel free to mention them since corroborating data is always good to strengthen belief in a hypothesis.
Sources
- AEG San Francisco Section Newsletter. "August Section Meeting Notes, Slip rate of Monte Vista-Shannon fault", 9/2004
- Yashinsky, Mark. "The Loma Prieta California Earthquake of October 17, 1989-Highway Systems", 1998/
- Pampeyan, Earl. "Map Showing Type and Distribution of in Northern Santa Clara and Southern San Mateo Counties", 1995
- Using the Soil Triangle
- Wikipedia, "Unified Soil Classification System"
- Groffie, Frank. "Damage in Los Altos Hills from the 1989 Loma Prieta Earthquake
- Groffie, Frank. Geologic Map
Congratulations to KCSearcher for FTF.