The Great San Francisco Earthquake of 1906 was a huge wake up call to geologists worldwide. It was the first time that a large tremor could do serious damage to a modernized city. Over 80% of all buildings in San Francisco were destroyed. Some very old structures were destroyed by the earthquake themselves, but the predominant factor in the destruction was fire that followed. Gas lines ruptured and with live electrical wires and falling debris causing sparks, it was only a matter of time. In this earthquake you will learn about the history and geology of the San Francisco Earthquake and how geology as we knew it was changed forever.
Preluding the great quake, earthquakes were thought to be military experiments gone wrong. Large explosions underground by weapon testing was the most excepted cause of earthquakes. By 1906, however, things were about to change. On April 18, 1906 at about 5:12 am the now infamous San Andreas Fault unzipped, rupturing the span of about 200 miles. Within these 200 miles, the land moved by up to 24 feet as seen in Point Reyes. The earthquake measured a whopping 7.8 on the moment magnitude scale. It was the second largest in California in recorded history. The epicenter being only 2 miles was the worse case scenario. Over 3,000 people died and it still remains the deadliest natural disaster in California's history. Damages from the earthquake costed over $400 million in 1906. In todays modern economy, with inflation taken into account, that number would be nearly $6.2 billion.
The earthquake was a wake up call to the senate in Sacramento, so they hired the best geologists in the world to finally solve the cause of these odd explosions. They organized a team led by University of Berkley professor Andrew Lawson. He set out to map out the damages to see if they could have any clues. First, the went to Point Reyes where a perfectly strait fence line was made before the earthquake. After the quake, it was offset by nearly 30 feet. His team moved down the coast, searching for answers. A little more to the south in Muscle Rock Park, Lawson noticed that part of the land was missing, leaving behind cliffs that were not there before the earthquake. How could large pieces of land be missing if a bomb went off? He moved further south to San Andreas Lake (image below) where he noticed too that there was more offsets within local farms, moving in the same direction as what was witnessed in Point Reyes.
Lawson concluded his findings to the senate and to UC Berkley in the "Lawson Report" stating that when these three points were plotted on a map, they formed a near perfect strait line. What ever caused that explosion, must have been somewhere along this line. A crack must have been made within this line and physically moved these two pieces of land past each other. Lawson didn't know it yet, but he had made one of the biggest discoveries in geology. He discovered faults and ultimately plate tectonics, which were first thought of during Charles Darwin's expeditions to the Galapagos and South America in the 1850s.
Charles Darwin came to the conclusion that the earth was made of tectonic plates that are capable of moving entire continents when he discovered marine fossils in the Andes Mountains. The only way they could get above snow level was for the land around them to be uplifted due to some type of movement. Darwin never found out how they got up there. It wasn't until the aftermath of the 1906 earthquake that proved Charles Darwin was correct on his hypothesis.
When it became time to name this crack or line, Lawson chose the name San Andreas because of the lake it goes under (see image above). San Andreas Lake was named well before the discovery of the San Andreas Fault, not the other way around like most people believe. Further evidence of the fault were mapped out across the rest of California up to the Salton Sea. The 1906 Earthquake made one of the most important discoveries in geology possible.
Now that you've had your history lesson, it's time for a geology lesson. Lets first discus the main types of plates. There are oceanic plates and there are continental plates. Oceanic plates are made up of mafic rocks, meaning they are low in silica (SiO2) and appear dark in color. Oceanic plates are mostly made up of basalt due to the divergent plate boundary creating new land. I'll get to what a divergent plate boundary is in the next paragraph. Continental plates are made up of mostly granitic type rocks and are mostly felsic, or appear light in color due to their high silica content.
Now that you are familiar with plates, we can discuss how they interact with each other. There are three main plate boundaries called convergent, divergent, and transverse (transform). A convergent plate boundary is when two plates collide. Think of it as a head on collision in a car crash. There are two types of convergent plate boundaries. One type is called subduction, where one plate slides under another which almost always occur off the coast of a continent. The plate that subducts is always an oceanic plate because it's more dense due to basalt being more dense than granite. There are more atoms per square inch in basalt than in granite, making it more dense and more heavy. Hence objects tend to sink towards the bottom like a penny in a glass of water. Lighter objects will float like an ice cube. Ice cubes can move around in the surface of the water, like how tectonic plates move and interact.
Divergent plate boundary are when two plates move away from each other. These happen almost always in oceanic plates because they are much thiner than continental plates, more easily to tear apart. It's easier to rip a tortilla than a pizza. In the ocean, we call these mid-ocean ridges. They create mountains in the ocean. As lava pours out and hardens, new lava flows on top and so on. It's like a scap. Its height keeps growing. Sometimes it even reaches the surface of the ocean. Iceland is a great example where you can literally stand in the mid-ocean ridge.
Transverse, transform, or strike-strip plate boundaries whose names are synonymous, slide passed each other. The San Andreas Fault is a transform fault. Most transform faults occur in the ocean because it's hard to rip continental crust from side to side due to it's thickness. The San Andreas Fault and it's associated faults is a rare and unique exception. There are two types of transform faults and it depends on how you view it. One is called a left-latteral strike-strip fault and the other a right-lateral strike-strip fault. To know the difference, you can observe the direction of relative movement. If you are on one side of the plate, looking towards the other and the land moves to your left then it's a left-lateral strike-strip fault. Likewise, if the land moves to your right, it's a right-lateral strike-strip fault. Seems easy enough, right?
Now that you are familiar with the different types of faults, we can discus as to why they move and the forces behind it. The answer can be found in your kitchen, literally. When boiling pasta on the stove, the hotter pasta floats to the surface then when it cools it sinks. This effect has a name. Convection currents in your pot at home and in the earth are nearly identical. There are a few difference though. One is with water, rather than boiling rocks and the other is that one is less a few inches while one is miles deep. Convection currents are continues cycles in the earth's mantel that make hotter rock rise, and cooler rock fall and the cycle begins over and over again. When this happens, it causes fractures in the earth's crust. When fractures occur, a fault line is created.
Now at this point I'm sure you are asking, "What is a fault line?" Well it's more of a fault zone since it's a collection of cracks rather than one single crack. Simplicity, it is place where two tectonic plates meet and create friction. When this friction is released, an earthquake is formed. This friction is formed by these convection currents. When they move in a circular pattern in one direction, they can push rock with them. When tension is released you get an earthquake that moves in waves on the X, Y, and Z axises. X is up or down, Y is right or left, and Z is pivoting side to side. There are a few types of earthquake waves which I'll go into detail.
When an earthquake occurs, the all the types of waves are produced at once. But these different waves move at different velocities in different fluids. Fluids such as hard rock can pass energy more efficiently than softer rock. Because the west coast of California is generally made up of soft, sedimentary rock, it absorbs more energy from these waves then if it was a hard rock like granite. It's the reason why a 5.0 earthquake in Virgina in 2011 was felt across the entire east coast. Most rocks there are hard granitic types which transfer more energy efficently. When a 5.0 happens in California, it is only felt no more than 150 miles from the epicenter. Because earthquake waves move at different velocities, they arrive at different times at any specific point on earth. This time differences helps geologists determine the exact epicenter within a few meters of accuracy.
There are two types of seismic waves which are called body and surface waves. Body waves travel in the interior of the earth and bounce back to the surface and so on. Depending on different types of rocks and their densities, these raves can be refracted like a rainbow. Body waves can bend downward and upward as they travel through the earth. Surfaces waves, on the other hand, can only travel on the surface. Because of this, they take less travel time and are the first to arrive from the epicenter.
There are two types of surface waves which are called primary (P-wave) and secondary (S-wave). Primary waves are the first seismic waves to arrive during an earthquake, hence the name. They are compressional waves, meaning the particles expand and contract on the horizontal axis. An example would be stretching a slinky and pushing it back to gather. Secondary waves move in the opposite direction. After the P-wave has passed, the S-wave will move particles up and down vertically similar to jumping up and down where your only motion is vertical.
There are two main types of body waves which are called Love and Rayleigh waves. Love waves travel faster than Rayleigh waves and they do the most damage to structures. Body waves are what you feel when an earthquake is occurring. Rarely do you feel surface waves. Love waves move side to side, but in a perpendicular angle of the direction of the wave. The push and pull objects from side to side. It's the most destructive because they shake violently. Rayleigh waves roll the land like a wave at the beach. They pick up an object pull it forward and pull it back before setting it right where it started. When you feel a rolling motion in an earthquake, it's a Rayleigh wave. An easy way to remember the difference between them is that the "L" in love waves makes a 90 degree angle in the direction of the wave. Rayleigh waves roll, so just think what type of motion an "R" makes, especially if you are a Spanish speaker who rolls their "R"s a lot.
Earthquakes have another, even more destructive force called liquefaction.Liquefaction is literally turning the ground into quick sand. Buildings are not being supported by anything so they ultimately fall and sink in this water. To understand liquefaction, think of it like a water bottle with water at the bottom and dry sand at the top. When you shake it, water travels up to the surface as the sand settles. What really happens is that when you shake and bottle full of sand, the elevation gets lower due to small particles of sand go under the larger ones. In reality, this squeezes up groundwater to the surface which makes puddles and can deform the land up to a few feet.
Knowing the types of earthquake waves will help you determine the magnitude. There are three ways in which that can be done. The first type of modern measuring of earthquakes was developed by Giuseppe Mercalli, an Italian volcanologist, in the late 1800s. The Mercalli scale, or today called the Modified Mercalli scale uses eye witness accounts of the shaking and damages to draw a visual map of damage and relative shaking from a general point. Given many interviews and surveys of the damage, a relative map could be made of the intensity of the shaking. It was meant to measure intensity, not angry realized so for deep earthquakes it was highly inaccurate since depth wasn't a factor in determining a number. The number scale is written in roman numerals to differentiate it from other methods of measurement. Numbers go from I (not felt) to XII (extreme) for a total of 12 numerical values.
Before the invention of the modern seismograph in the 1900s, we could only guesstimate the magnitude of an earthquake with no mathematical measurements. So only a relative number based off of interviews and surveys wouldn't be accurate, hence why there is a disagreement with large earthquakes before the Richter Scale, the next magnitude measurement scale I'll talk about. There isn't a definitive answer as to how big the 1906 earthquake was due to this lack of information. It wasn't until Charles Richter, a seismologist professor at California Institute of Technology in Pasadena invented his own system in 1939.
The Richter scale was intended to measure the magnitude of earthquakes based on energy released by measuring the largest wave produced on a seismometer and drawing a chart based on fixed proportions. I'll explain how a seismometer works in a few paragraphs. It was intended to measure earthquakes in California, since all the rocks are nearly the same. The scale was logarithmic (see above), meaning that each increasing number would be about 32 times greater, in terms of energy released, than the previous one. Theoretically, there is no number that an earthquake can be measured at. But, looking at passed measurements the scale has only measured earthquakes no more than a 10. The Richter scale was the first accurate way to give a magnitude to an earthquake, but it too had it's draw backs. For one, it didn't take into account the type of rocks, depths, pressures, or type of faulting. The Richter Scale would usually cap out around a 7 or 8 meaning that for very large earthquakes, this scale was not accurate. It became time to develop a new method to measure earthquakes, that took every little variable into account.
The Moment Magnitude Scale (below) measures earthquakes in terms of energy released based on a number of factors. It took into account the area of the fault plane that had shifted as well as the rigidity of the rocks, or the hardness. It also took into account the type of movement of the fault plane. It took a long time to calculate and retrieve all the data, but in the end it is way more accurate. The Ricther Scale, in contrast, had not enough variables and is easy to calculate based on actual recordable data. The Moment Magnitude Scale has a rating from 1 to 10, but like it's predecessor there is no end limit as to how energetic an earthquake can get. Think of it like this: Richter is a fixed number scale, while Moment Magnitude is an infinite scale.
Today, the most common ways to measure earthquakes are the Richter Scale and the Moment Magnitude Scale. In order to differentiated these from each other, they were given a small call sign in front of the number they represent. All earthquakes measured using the Richter Scale have an (ML) where the L is a sub letter. All earthquakes measured using the Moment Magnitude Scale have an (Mw) where the W is a sub letter. Seismographs help geologist determine the intensity as well as the epicenter and focus of an earthquake.
Seismographs work by creating electricity. Inside each modern seismometer, are three small gyroscopes. One is placed in the vertical direction, one is placed in the horizontal direction and one in the front and back direction. They are placed like this so that they cover the three main planes of motion the X, Y, and Z axises. When an earthquake happens, a seismometer will move with the ground it is buried in. Because the gyroscopes had a coper wire in the middle, it sends electrical signals to a transmitter as the seismometer moves and the gyroscopes stay in their resting positions. The signal is transmitter to a receiver where a seismograph can record the data. When an earthquake happens and you see a graph with a pen and moving paper, that's what has been recored from a seismometer. The P-waves come first, followed by a small break before the S-waves arrive. Rayleigh waves are next followed by Love waves. Because the gyroscopes are set in each axis, we can easily determine the type of wave that is being recorded.
The difference between the arrival of the P and S (seen below) waves will tell us the epicenter and focus of the earthquake. We know the velocities of the P and S wave are fixed, so if we know the time difference between them we can calculate a distance. You need at least three seismometers placed in just the right spots ignored to know where the epicenter is. Let's say that you calculated the distance from the epicenter to three different seismometers. Station A has a distance of 50 miles. Station B, 100. And Station C, 150. From each station, you'd draw a three-dimensional sphere of those distances at each station. Where they all intersect in the ground is the focus, the exact point where the earthquake began. Where that point lies directly above the ground is the epicenter of the earthquake.
On April 18, 1906 at about 5:12, the citizens of San Francisco didn't have access to such technologies to know how bad the damage was or what really caused the earth to move. It wasn't until that infamous earthquake that we had a full understanding of how the earth works, including the San Andreas Fault. The San Andreas Fault is roughly 20-25 million years old. Before then, the west coast of California was an active subduction zone where the Farallon Plate subjected under the North American Plate, creating volcanoes in the present day Sierra Nevada. What's left of those ancient volcanoes are the granite monoliths. Half Dome and El Capitan are the most notable.
When subduction ceased here it created two knew plates that can still be seen today as the Juan de Fuca plate off the coasts of Northern California, Oregon, Washington, and British Columbia. It is this plate that still is being subjected under the North American Plate that creates the Cascade Volcanoes. The other plate is known as the Cocos Plate in Central America. Each plate were technically the same at one point, but they were discovered before this connection was made, hence why the different names of these plates. Back in California, the tension began to tear the boundary between the Pacific Plate and the North American Plate. The end result was a new type of plate boundary, a transform fault called the San Andreas. Over millions of years, the fault grew, eventually becoming nearly 800 miles long from just south of Eureka, California to the Salton Sea, just east of San Diego. This new fault would become the world's most famous, especially since it was the responsible for the 1906 earthquake here in San Francisco.
In all, it takes about 100 years for the fault to recharge its potential energy and create yet again another large earthquake. In some parts of California, for example Los Angeles, that last earthquake was in 1857. A study was conducted by the U.S. Geological Survey in 2008 to predict probabilities of a large destructive earthquakes of known faults in the Bay Area from 2006 to 2036. Their findings are not surprising. Los Angeles is more likely to get a larger slip of the San Andreas than San Francisco is due to the 1906 earthquake. You may be thinking, do a lot of smaller earthquakes release the needed energy to save us from a catastrophic larger one? The answer is unfortunately, no. Remember, earthquakes and their energy are proportional to a logarithmic scale. So one hundred 3.0s don't release enough energy to prevent a 8.0 from occurring. The best thing to do is remember that that it's not "if" an earthquake happens, it's "when" an earthquake happens. The San Andreas Fault is locked and loaded, it's just waiting for someone to pull the trigger. When will the 1906 Earthquake happen again? Sometime between now and our lifetime. It will happen again, you just need to understand them to take precautionary measures so that we are prepared for them. That's why we have building codes and other laws to prevent damage and loss of life. Because the last thing we need it 80% of San Francisco to go up in flames again.
Works Cited
http://www.scienceinschool.org/2012/issue23/earthquakes
http://earthobservatory.nasa.gov/blogs/elegantfigures/2011/05/10/one-of-my-favorite-maps-the-1906-san-francisco-earthquake/
http://msemac.redwoods.edu/~dbazard/geography/tectonics/index.html
https://www.esgsolutions.com/technical-resources/microseismic-knowledgebase/what-is-moment-magnitude
http://www.bgs.ac.uk/discoveringGeology/hazards/earthquakes/MeasuringQuakes.html
http://www.businessinsider.com/san-francisco-1906-earthquake-and-now-2012-9
http://pubs.usgs.gov/fs/1996/fs094-96/
http://sepwww.stanford.edu/oldsep/joe/fault_images/BayAreaSanAndreasFault.html
https://en.wikipedia.org/wiki/1906_San_Francisco_earthquake
https://tools.wmflabs.org/geohack/geohack.php?pagename=1906_San_Francisco_earthquake¶ms=37.75_N_-122.55_E_
https://wiki.ubc.ca/Course:Math110/003/Teams/Ticino/The_Richter_Scale
https://study.com/academy/lesson/what-is-the-epicenter-of-an-earthquake-definition-location.html
TO LOG A FIND ON THIS CACHE YOU MUST GIVE ME THE CORECT ANSWERS. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER. ANY INCORRECT ANSWERS WILL RESULT IN A DELETED LOG
1. "1906 San Francisco Earthquake Epicenter" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. Note, this IS cheating as only the person who sent their answers has learned something, while the others get a "free ride". It's not fair to others. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.
2. If you were standing at (a) GZ, (b) Oakland, (c) San Jose, (d) Sacramento on April 18, 1906 at about 5:12 am, name the type of seismic waves that would reach you in order AND explain why this is always a constant variable.
3. Based on your knowledge of liquefaction, list at least two (2) areas from this vantage point that would be at the greatest risk AND explain why.
4. Would a tsunami happen if the San Andreas Fault ruptured offshore again like it did in 1906? Is there a current tsunami threat from the San Andreas Fault? Explain your reasoning.
5. If a tsunami did happen, Tell me (a) how has the city prepared the coastline, (b) what obstacles are here now that could slow down an impeding wave, (c) what is missing that the city could build or add, and (d) how these different measures would slow down or stop a tsunami if implanted correctly.
6. From this vantage point, what has the city done to prepare for another 1906 earthquake? Is there anything that would prevent a disaster or, at least, lower the risk? What precautionary effects have been taken into consideration? Don't google the answer, this is from your observations from GZ.
7. Look around the surround area of GZ. Where would you place a seismometer based on your geologic knowledge? Mark the location by either street locations or GPS coordinates and explain why it might be a good place to add one, considering hazards that could damage or impede on accurate data such as liquefaction.
8. Why are other faults in the region such as the Rogers Creek Fault more likely to have a large earthquake than the San Andreas in the next 30 years?
9. Would many thousands of 3.0 Mw earthquakes along this section of the San Andreas Fault release enough energy to stop an impeding 8.0 Mw? Explain why this can or cannot be the case.
10. What are the difference(s) between Richter Magnitudes and Moment Magnitudes? What are their similarities? Why don't geologists and seismologist use both?