Geology: Lecture Series 6: Transform Plate Boundaries (Transform Faults, San Andreas Fault, Earthquakes)

Parking Garage at California State: Northridge

The parking garage at Cal State in Northridge collapsed during the earthquake. The outside of the garage was well-supported, but the inside wasn't, because space had to be accommodated for cars. As the inside of the garage pancaked, it brought the sidewalls in and down as well.

Fault Creep

The phenomenon when faults "creep" or move along slowly.

Earthquake Epicenter

The point on the Earth's surface directly above the focus of the earthquake. The vibrations are felt most strongly here.

What is the Order of Waves that Result from Earthquakes?

1) P or Primary Waves 2) S or Secondary Waves 3) Surface Waves (most felt)

Potash

A mineral produced by the evaporation process; it's important for fertilizer.

Particle Motion vs. Wave Propagation

For P waves, the particle motion is back and forth, while the wave propagation is outward. For S waves, the particle motion is up and down, while the wave propagation is also outward.

Fill and Liquefaction

Fill is a soft material that liquifies, or turns to a soupy substance, when shaken by earthquakes.

Seismic Gaps

Gaps that exceed the recurrence interval of earthquakes. Seismic gaps indicate a high risk of earthquakes.

Elastic Rebound

After strain forces a move, things rebound to original shape (slinky and brick analogy). Elastic rebound in ocean trenches produces tsunamis.

Transverse Ridges

Areas of local compression or extension*

San Francisco Bay Area

At highest risk- higher population and property values. In the San Francisco Bay Area, it's not just the San Andreas Fault that poses a problem, but the Hayward Fault as well. The Hayward Fault is moving, but not enough to relieve its tension.

Great Glen-Cabot Fault

Both faults are left-lateral, active at same time, probably same fault, just close up the Atlantic to be sure.

Earthquake-Resistant Buildings

Buildings cannot necessarily be earthquake-proof, only earthquake-resistant. The key is to put the center of gravity lower in the building, so the weight is at the bottom. Earthquake-resistant buildings should also have sturdy foundations that simulate bedrock. These buildings should also have concrete or frame reinforcements. Buildings can also be put on springs, so the shock is reduced. Though there are stringent earthquake codes for buildings, these codes only affect public buildings and buildings built after the codes are established.

Dip-Slip Fault

Continent falls parallel to dip (like East African Rift). NOT transform faults. Dip-Slip Down= Normal, Dip-Slip Up= Reversal.

Creepmeter

Creepmeters measure how fast a fault is moving. If the fault is not moving fast enough to relieve stress, it is likely a big earthquake will occur to relieve the stress.

San Francisco Earthquake 1906

During the San Francisco Earthquake of 1906, the fault was offset by 18 feet. The magnitude of the earthquake was 8.3. The damage was mostly from the fires that the earthquake caused. The earthquake damaged many wooden structures that could not withstand the constant sideways motion. The total damage caused by the earthquake was half a billion dollars.

Earthquake Hazard Maps

Earthquake hazard and risk assessment upgrade and proof vulnerable areas. Areas of highest risk are done first, and earthquake hazard maps and seismic gaps are used to indicate which areas are at risk.

Stick-Slip Fault

Fault sticks, then slips. All transform faults are strike-slip faults. Strike-slip faults are also plate boundaries.

Fossil Transforms

Faults like the Minas Fault that are ancient, but still lines of weakness that produce occasional earthquakes.

Mid-Ocean Ridges and Faults

Faults link one mid-ocean ridge to another. Mid-ocean ridges aren't smooth, they are often offset. Each offset is a transform fault. Why? Plate motion has to occur 90 degrees to the ridge crest. Continents break up irregularly, but ridges can't be curved, so they're segmented. Transform faults are geometric requirements for straight ridges. Transform faults at mid-ocean ridges generate small earthquakes, not tsunamis- they don't lift up the seabed.

Pull-Apart Basins

Holes that form at divergent bends when the transform fault pulls apart. The holes will fill up with water, or sediment, or volcanic activity, if it fills with basaltic magma.

North Anatolian Fault: Hot Springs

Hot springs are common on transform faults. Surface water trickles down and gets heated. The material being deposited is silica, also known as sinter or "geyserite."

Tangshan 1976

In 1975, geologists predicted an earthquake would occur at Haicheng, and evacuated people. The geologists based their predictions off of many things: observing animal behavior, and foreshocks (small earthquakes before big ones). Sure enough, a huge earthquake struck shortly thereafter. A year later, in 1976, a huge earthquake struck without warning in Tangshan, and leveled the city, with astronomic fatalities.

North Anatolian Fault

In 1999, the North Anatolian Fault was responsible for a devastating 7.6 earthquake in Turkey. As the Arabian plate pushes up, the Anatolian plate is pushed left. The fault is right-lateral.

Transverse Ridges

In California, most mountain ranges are parallel to the fault, except around Los Angeles, where they're East-West oriented. Hence, there are transverse ridges. The reason is that they converged in a bend in the fault.

Northridge Earthquake 1994

In February 1994, the Northridge Earthquake struck, with a magnitude 6.8/6.9. The epicenter seemed to be at the Hypothetical Elysian Park Fault in the San Fernando Valley. The quake was close to LA, but there was little damage there.

Nimitz Freeway (I-880) Collapse

In the Loma Prieta earthquake most deaths occurred here. The Nimitz Freeway was a double-decker freeway, and the top part collapsed on the bottom part. The purpose for double-decker freeways was minimizing cost and maximizing space usage.

Seismometer

Instrument we use to measure earthquake motion. Usually 3 seismometers are set up to measure forward & back, side to side, and up & down motion. A real seismometer never produces a straight line.

Mesada

Mesada is a sort of rock fortress, which has a pull-apart basin behind it. At Mesada, the Jews held out against the Romans, though the fort shows signs of earthquake damage.

Earthquake Recurrence Intervals

Most active faults have a recurrence interval, or a period of time in between major earthquakes, but they are only average, and hence can be inaccurate. The San Andreas Fault has a recurrence interval of once about every 120 years.

Loma Prieta (World Series) Earthquake 1989

On October 17th, 1989, the World Series was taking place, when the 7.1 magnitude Loma Prieta earthquake struck, and TVs lost contact with the game. As a result of the earthquake, 47 people died.

Primary (P) Waves

P waves can travel through any medium. They are the first to arrive after an earthquake occurs, as they are faster. P (push-pull) analogy= pushing or pulling a slinky spring (condensed coils). Wave propagation is where the condensed coils move together. Sound molecules are much the same- they have a spring-back pattern.

How Can One Determine How Far Away an Earthquake Is?

P-S time interval, or how long it takes to transition from P to S waves. The longer the time, the further away the earthquake is. If you know how fast each is traveling, you can figure out when they started. Problem: what direction? We can narrow it down with other seismometers by drawing circles and seeing where they intersect.

Pancaking and Houses

Pancaking in structures occurs when the ground floor, which in the Bay Area was typically the garage, is inherently weak. Garages need to accommodate cars, and so the structures are generally weak and widespread. When shaking occurs, the garage collapses. When it hits, the first floor support collapses, and so on, with the upper floors all collapsing onto the lower floors.

Secondary (S) Waves

S waves travel down rope and back with decreasing force. The rope goes up and down, while the loop goes back and forth. S waves only work in a solid/rigid medium.

Scintillation Counter

Scintillation counters measure radioactivity; as cracks in the earth start, water fills them, and gas is released occasionally. The gas is radon, which is produced by the breakdown of uranium.

Foreshocks

Small earthquakes that occur before bigger ones. The problem with foreshocks is that it's hard to tell if a foreshock is really a foreshock or an aftershock of an earthquake that occurred elsewhere.

San Andreas Fault: Offset Streams

Streams cutting across the San Andreas bend right because this is a right lateral fault. Streams are actively being offset, a sign that this fault is active.

Transform Fault: Stress and Strain

Stress is an outside force- the motion of plate tectonics. Because the rocks are being pushed, they break (strain) as a result.

Sinistral Fault

Strike-slip fault, opposite side moves to the left. Greek for "Left."

Dextral Fault

Strike-slip fault, opposite side moves to the right. Greek for "Right."

Earthquake Waves: Surface Waves

Surface waves only affect surfaces. They can only travel on a surface- Earth's surface. The best examples of these are water ripples or waves. They can move up and down, back and forth. They are slow, and have farther distances to go.

Dead Sea Transform

The Arabian Plate is moving faster than the African Plate. Above the Dead Sea Transform, the two plates are converging at the top, while there is a transform on one side of the plates, and a divergent on another side.

San Andreas Fault: Carizzo Plain

The Carizzo Plain is Northwest of Los Angeles. There is uplift on either side; it seems to be slightly convergent. Streams that try to run across the plain are defaulted right by the San Andreas Fault.

Dead Sea Rift

The Dead Sea Rift is part of the Levant Fault. The Dead Sea Rift is in fact not a rift, but a pull-apart basin. The Dead Sea Rift is the lowest point anywhere on land- it's 1200 feet below sea level.

Dead Sea

The Dead Sea is salty, as the Jordan River drains into it. The Dead Sea is also in an arid climate, with enormous amounts of evaporation, which makes the Dead Sea even saltier. The water here is 10 times saltier than ocean water, and as a result, is heavy.

Great Glen Fault

The Great Glen Fault is in Scotland, and runs down a great valley. This transform fault was active 300 million years ago, but has been inactive since, much like the Minas. The fault has a lake at the bottom- Loch Ness.

Levant Fault

The Levant Fault is a left-lateral fault. It is the fault between the African and Arabian plates, which are moving the same direction, but at different speeds.

Bay Bridge Collapse

The Loma Prieta 1989 earthquake caused an upper segment of the Bay Bridge to collapse onto the lower. The collapse had occurred right on top of the support beam. The reason why the bridge collapsed is because metals expand and contract when heated and cooled during the day, and these movements must be accommodated by parts that slide. One part slid so far from the shaking that it collapsed.

Motagua Fault (Guatemala, February 1976)

The Motagua is a left-lateral or sinistral fault. The earthquake it caused in February 1976 caused 20,000 fatalities as the fault moved 11 feet. Prior to 1976, the fault hadn't moved for so long, that geologists didn't know which fault was the plate boundary, but it was this one. Buildings in Guatemala were cheaply built brick walls with heavy roofs, which weren't designed for sideways motion.

Blind Faults

The Northridge Earthquake occurred on a fault that no one knew existed. Blind faults exist underground, but never come up to the surface. The problem arises when it is unknown whether these faults are earthquake-producing, thus producing a serious risk. Fortunately, the Northridge fault was one where the top of the fault rose up, so most of the shock was diverted northward, away from Los Angeles, and hit the Santa Susanna Mountains.

Salton Sea

The Salton Sea is a pull-apart basin in the San Andreas Fault.

San Andreas Lake

The San Andreas Lake is a lake along the San Andreas Fault by the San Francisco Bay Area. There is a dam (built 1868) across the lake and the fault which survived the quake of 1906. The dam was constructed when no one knew about faults or plates.

San Andreas Fault: Earthquakes and Fault Movement

The San Andreas is the most important of a bunch of faults. The Pacific Plate is moving Northwest, the American plate Southeast; this movement is accommodated by a lot of faults, mainly the San Andreas.

San Fernando Earthquake 1971

The San Fernando Earthquake of 1971 caused the land to move in waves. An overpass was being constructed when the earthquake struck, causing it to collapse in the middle of the road below. The magnitude of the earthquake was anywhere from a 6.7 to a 9. The damage caused was half a billion dollars.

Berkeley Football Stadium

The UC Berkeley Football Stadium sits astride the Hayward Fault- its movement is evident in the seats.

What Is A Combination of P&S Waves Called?

The combination of P&S waves is called body waves. Body waves both travel through the Earth's interior.

Earthquake Focus

The focus of the earthquake is the point in the Earth where the snap actually happened. Somewhat deep, sends sound vibrations all over.

Earthquake Risk by Geology of an Area

The geology of an area can also indicate how at risk the area is for earthquake damage. Areas with bedrock are safe, as bedrock is hard material, and durable during an earthquake. However, if the slope on which it rests is steep, it can prompt a landslide. Other parts of an area are fill, which are not durable for a quake, and will in fact liquify when shaken.

Earthquake Waves: Primary Waves

The initial shock wave that results from an earthquake is a P wave, or a primary wave. A P wave is exactly the same as a sound wave. A P wave can travel through air.

Earthquake Magnitude

The problem with earthquake magnitude, or the energy released by an earthquake, is that it is not measured on a linear scale. It is rather measured on a logarithmic scale. Each earthquake, according to the Richter scale, is about 32X the power of the preceding magnitude.

San Andreas Earthquake Danger

The problem with earthquakes is that it is hard to tell when and where they will happen. In the case of the San Andreas, earthquakes are more likely to happen where there haven't been any in a long time. The fault north of Los Angeles, for example, hasn't moved since 1857. The one by San Francisco hasn't moved since 1906. The San Francisco area, though it's been a short while, is more at risk, since it has a higher population and higher property values.

Earthquake Waves: Secondary Waves

The second shock that results from an earthquake is an S wave, or a secondary wave. A secondary wave is a vibration that is felt. An S wave can't travel through air- it needs a solid medium to travel through.

Minas Fault

The world's highest tide is in the Minas Basin. The Minas Fault is still a huge crack, lines of weakness which cause occasional earthquakes. Faults such as this are often known as "fossil transforms." The Minas Fault was active as Pangaea came together, but became inactive when it coalesced. The Minas Fault is right-lateral, or dextral.

Marina District

This area suffered heavily from liquefaction during the Loma Prieta earthquake.

Water/Gas Mains Rupture

Though earthquakes can do a lot of damage, it is often the rupturing of water and gas mains that does the most damage. When gas mains are severed, the gas is let out, and fires can be sustained for a long time, burning everything nearby, and the water is not sufficient to put out the blaze. When water mains are broken, the water becomes unusable, as there is no water pressure, and small floods may occur.

Tiltmeter

Tiltmeters measure a change in slope. As cracks open, rocks can only move up, causing tilting, indicating that an earthquake is imminent.

Convergent/Divergent Bends

Transform faults have sideways motions, which are easy if the fault is straight, but most faults are curved, which causes issues. What happens at these curves depends on the movement of the fault and the location of the bend.

California Isn't Associated With Volcanoes: Why?

Volcanoes aren't associated with California because cracks have to open, and transform faults don't do that. The only time volcanic activity can happen is in the pull-apart basins.

Earthquake Waves

When an earthquake happens, rocks that were locked let go and lurch, which releases shock waves. There are, however, two different types of shock waves that result from the one motion. The best analogy to explain this is snapping a twig: when it snaps, you both feel and hear it, two shock waves that result from two different vibrations.

Earthquake Prediction

While earthquakes are not completely predictable, all geologists can do is measure things as precisely as possible- detect risks, distinguish foreshocks, etc. Instruments such as creepmeters, seismometers, scintillation counters, and tilt meters help predict earthquakes. The problem with these instruments, however, is that they don't tell when an earthquake will occur or how big it will be. Predictions must be accurate, and evacuation is a huge issue.