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1.
Abstract
The San Andres Fault (SAF) has a history of M7 – M8 earthquakes approximately every 100 years. However,
the southernmost segment has not had an earthquake in over 320 years, and is long overdue for a great earthquake.
The SAF is a strike slip plate boundary between the Pacific Plate and the North American Plate. One area of interest
along the SAF is the traspreessional region where the Garlock Fault collides with the SAF. This region is thus
littered with thrust faults, and is also the area where LA is located. Through trenching and carbon dating, scientists
have determined the recurrence interval to be 100 years. Two of the most recent earthquakes were the M 7.6 1906
earthquake and the 1989 M6.9 Loma Prieta earthquake. If an earthquake starts in the southern most segment and rips
northward, LA will be hit with seismic waves that will be extremely damaging. California has tried to mitigate the
damage from such an event by creating The Great California Shakeout, a state-wide earthquake drill based on a
1460 SAF earthquake. Based on an extremely through computer earthquake simulator program called UCERF3, the
likelihood of an earthquake on this scale in the next 30 years is 36 percent. LA will be hit with intense shaking for a
long duration, so having mitigation strategies is important. In addition to The Great California Shakeout, CA needs
to implement an early warning system that will alert citizens when an earthquake is on its way. To address the threat
an earthquake on the southernmost segment of the SAF, the history of the SAF will be reviewed, the magnitude and
recurrence interval explained, and other mitigation options investigated. s
2.
Introduction
The San Andres Fault (SAF) is the strike slip boundary between the Pacific Plate and the North American Plate
that is overdue for a major earthquake. While the SAF generally runs from south to north, there is a segment called
the Big Bend where another fault, the Garlock Fault, is offsetting the movement of the SAF. Because the Pacific
Plate is continuously moving northward, rocks are broken and compressed, resulting in an area rich in thrust faults.
Such an area is called a restraining bend, and the transpression of the rock creates new faults that add to the danger.
There have been a few important earthquakes on the SAF in the past. One of the most important ones was the M 7.6
1906 earthquake that hit San Francisco. This earthquake was devastating for San Francisco. Following the quake,
Reid developed the elastic rebound theory. Another important earthquake was the 1989 M6.9 Loma Prieta
earthquake, which revealed weaknesses in infrastructure. To study the recurrence interval of the SAF, researchers
have to dig trenches, search for faults, and use carbon dating to date the layers above and below the fault. One of the
downsides of this method is that it may not give an exact date for a fault, but instead a range. Because the recurrence
interval is based on historical events, any variation in the date of a past event lead to uncertainty in the recurrence
interval. Despite this, the recurrence interval of the SAF in approximately 100 years. Looking at earthquake records,
the southern segment and the southernmost segment are overdue for an earthquake, with the southernmost segment
much more so. If a rip starts in the south and moves north, LA will be hit with seismic waves that could cause
movement of the thrust faults because of the interconnectivity of the fault system. In order to prepare, California has
created a drill called The Great California Shakeout which is modelled after an earthquake on the southernmost
segment of the SAF in 1460. Through this drill, California hopes to educate the public for what to do when an
earthquake hits, and to prepare companies that deal with lifelines for disaster. In addition, a sophisticated SAF
earthquake simulator called UCERF3 has evaluated the interconnectivity between different faults and had
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determined a likely scenario for the progression of shaking from a M7.5 – M8 earthquake in the southernmost
segment of the SAF. UCERF3 is predicting that seismic shaking will travel to LA by the Sierra Madre fault and that
once the shaking starts in LA it will take a few minutes to stop. This information is crucial for LA so that they can
plan for shaking on that scale.
The main concern is that a M7.5 – M8 earthquake will start in the Salton Sea and rip northward. In
particular, even though not directly on the SAF, LA will experience significant shaking and a suspension of lifelines
such as water and power for as long as 6 months. It is crucial that LA have other mitigation strategies besides The
Great California Shakeout, such as an early alert system. This paper will explore the background and history of the
SAF and discuss the effects that an earthquake of M7 or M8 will have. The general idea is to explore the risks so
that a strong mitigation approach can be developed. The science behind strike slip faults and historical earthquakes
are reviewed, and then the recurrence interval and risk of an earthquake in the southernmost segment of the SAF on
LA is explained before wrapping up with some mitigation strategies.
3.
Background
The SAF is the dextral strike slip plate boundary between the Pacific Plate and the North American Plate, and it
has the potential to cause a major earthquake. The fault runs from the Gulf of California north past San Francisco
and eventually into the Pacific Ocean, and it is getting longer with time. Over 25 million years, there has been about
300-400 km of total offset. The Pacific Plate is moving northward at about the rate of 35 cm per year. There are
three main segments of the SAF: the northern segment, the southern segment, and the southern-most segment. The
amount of accumulated strain on each segment depends on the time since the most recent earthquake to hit that
segment and the plate rate. Currently, the segment that has the most accumulated strain is the southern-most
segment, which will shift approximately 11-12 meters when a major earthquake occurs. The last major earthquake in
this area was between 1680-1690, and is thus overdue for a big earthquake. However, the southern segment is also
due for an earthquake. The last major one was in 1857, and this segment has built up strain of approximately 5.7
meters. Historically, earthquakes on the SAF are usually M7.5 – M8. However, earthquakes on the SAF are also
shallow, leading to higher intensity of shaking on the ground that can be highly damaging. The SAF plate boundary
between the Pacific Plate and the North American Plate is overdue for an extremely large earthquake.
The SAF is not straight, and the continuous northward movement causes crust at the Big Bend to break. The
main reason for the Big Bend is a sinistral strike slip fault called the Garlock Fault, which is 250 km long and has a
slip rate of 2-11 mm per year. The Garlock fault appears to have irregular recurrence intervals that are tied directly
to SAF slip. The resulting bend on the SAF causes the rocks to smash into each other and experience a lot of
compressional stress. Contraction results in uplift and thrust faults. LA is built at the base of the Big Bend, directly
on the crust that is being compressed and broken in an area called the foreland basin. There are many faults that go
right under the city. Many of these faults are blind thrust faults, a contractual fault that is buried by young, recent
sediments. However, many of these faults are unmapped and therefore unknown. The hypothesis is that when the
SAF experiences a major earthquake in the southernmost segment, shaking may also propagate and cause the faults
under LA to also shift and intensify the shaking. LA is built at the base of the Garlock Fault and directly on top of
blind thrusts, so the city is very susceptible to experiencing earthquakes.
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In the last 200 years there have been a few major earthquakes, notably the 1906 San Francisco earthquake
and the 1989 Loma Prieta earthquake. The 1906 SF earthquake was a M7.9, had a rupture length of 450 km, and
caused almost 80 percent of SF to be destroyed, either through the shaking or from fires afterwards. However, this
earthquake initiated lots of new science, and there were many important outcomes from this event. First of all, strike
slip faults in California were first recognized and mapped by G.K. Gilbert. His mapping showed surface rupture
offsets (Gilbert et al 1907). The second major effect of the 1906 SF earthquake was the spawning of ideas related to
elastic rebound theory by Harry Reid in 1910. He theorized that as the crust deforms and the plates continue moving,
elastic strain begins to accumulate. As soon as the force from the strain is great enough to overcome the friction
from the plates being stuck, and earthquake occurs, and the plate spring back into their original shape (Reid 1910).
Elastic rebound results in a nearly regular recurrence interval that is related to plate rate and resistance due to
friction. This theory is the primary hypothesis used for repeated ruptures, and therefore recurrence intervals. The
1906 SF earthquake was important in kickstarting science that has led to theories such as the elastic rebound theory.
Another important earthquake on the SAF was the 1989 Loma Prieta earthquake. This earthquake had a
rupture length of 40 km and was a M6.9. One of the major outcomes from this earthquake was that it revealed major
issues with critical infrastructure, including the Nimitz freeway collapse and the Bay Bridge. Although tragic,
scientists have examined what went wrong in that event and have tried to mitigate by reinforcing or redesigning
existing structures and reducing the number of hazards, such as double decker bridges. It also ushered in a new era
of fault hazard mapping. The 1989 Loma Prieta earthquake was crucial in revealing existing weaknesses in
infrastructure so that cities in CA can be more prepared in the future.
Although not directly on the SAF, the Ridgecrest faults are a series of dextral slip faults that connect with
the Garlock Fault. Despite not being on the SAF, this area is important because of how it connects with the Garlock
Fault at approximately a 90-degree angle. Because scientists now understand that CA has a system of interconnected
faults instead of separate, disjointed faults, slippage in the Ridgecrest area results in greater strain on the Garlock
Fault. If the Garlock Fault were to slip, there is a chance that this would increase stress on the SAF and trigger an
earthquake there. However, the chance of this happening is slim. Due to the 2019 Ridgecrest earthquakes, the tips of
the rip are now stressed, meaning the Garlock Fault is not 100 times more likely to rupture in a large scale. If the
rupture came within 45 km of the SAF, there is a 50 percent chance that the Mojave section of the SAF would
follow. Because of this, the interconnectedness of the fault system is not recognized, which is a relatively new idea.
One rupture may trigger others within the system. The Ridgecrest fault is not directly connected to the SAF;
however, it plays a role when looking at the entire system.
4.
Magnitude of events
The next rupture on the SAF is most likely to occur in either the southern or southern-most segment between
the Big Bend and the Salton Sea. Based on the history of the area, the earthquake is expected to be M7.5 – M8. The
worst case would be if the southern-most segment ripped, starting at the south near the Salton Sea and ripping
northward. In this case, the SAF would push a lot of energy directly into the LA basin. For LA, this scenario is
particularly dangerous because of the number of blind thrusts under the city. Many of these faults are especially
dangerous due to fault participation, which would increase the intensity and/or duration of the shaking. Three of the
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major faults are the Sierra Madre, Elysian Park, and Compton. In particular, intensity will be a major problem for
LA not only because of the thrust faults, but also because they are in a basin. The basin will trap seismic waves, both
increasing the duration of the shaking and very likely creating a magnification effect as waves collide. When the
next rupture on the southern part of the SAF occurs, LA is going to experience tremendous shaking.
When the southern SAF earthquake occurs, there will be major damage to infrastructure. In particular, lifelines
that cross the SAF are going to be completely compromised, including cutting water, roads, and power. With these
systems badly damaged, it is much harder to recover from the earthquake. For instance, there will be a lack of clean
drinking water, or water to fight fires with. According to Lucy Jones, it may take up to six months for enough of the
pipes to be repaired to restore water (Seismologist 2016). It will be hard to get more supplies until the major roads
are passable. Additionally, infrastructure within LA will experience over 3 minutes of shaking. Many older
buildings will collapse or be damaged. There is also the possibility of fires erupting that cannot be contained due to
the lack of water. Furthermore, Lucy Jones warns that there is the chance that the economy of LA will collapse.
Without these lifeline systems, many people will relocate to other areas of the country and many businesses will
close (Earthquake 2019). Following an earthquake on the southern or southernmost part of the SAF, lots of
infrastructure and lifelines will be greatly damaged and it will take months to repair.
5.
Expected recurrence interval
Each segment of the SAF has a recurrence interval of approximately 100 years. The southern-most segment of
the SAF is the most overdue because there has not been a major earthquake there since at least 1690. UCERF3 has
modelled more than 350 fault segments in the SAF system. UCERF3 is able to simulate how segments of rock will
transmit stress. Due to the concept of fault participation, the magnitudes of predicted earthquakes are larger and
because of the increasing number of faults, the probability of larger earthquakes increases. UCERF3 also measures
fault readiness, which is how long it has been since a recent event. Values greater than one are overdue in the
context of average recurrence interval. The probability of an M7 earthquake in southern California in the next 30
years according to UCERF3 is 36 percent. Meanwhile, the probability of a M7 earthquake occurring in northern
California in the next 30 years is 20 percent. The southern segments of the SAF are the most overdue for a M7 or
above earthquake.
Recurrence intervals rely on cross-cutting relationships of sedimentary deposits and carbon dating. Dating is done
almost exclusively by C14 dating. The process is that scientists dig a trench to gain access to the sedimentary layers
where faults are visible. Faults can be recognized by the cross-cutting line that is visible in the sediment. In order to
date a fault, organic material from the layer directly above and below where the fault ends must be taken and
analyzed. However, C14 dating methods can only go back approximately 50,000 years. Additionally, carbon dating
does not give an exact year that an event occurred, only the range between which it could have. Thus, scientists
cannot pinpoint exactly when some earthquakes that happened before written records were kept occurred. This lack
of precision is harmful because if scientists do not have reliable dates, it is difficult to determine an accurate
recurrence interval. Carbon dating and trenching are a way to get fairly accurate dates for past earthquakes.
6.
Risk to people and cities
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If the southern SAF earthquake scenario occurs, all cities along the rip, along with LA, are going to be greatly
affected. The cities directly on the fault will experience the strongest intensity of shaking, and the intensity will
decrease further from the fault. The amount of shaking that LA experiences depends on the directivity of the rip. If
the SAF rips from north to south, LA will experience moderate shaking. However, if instead the SAF rips from
south to north, this will push a large amount of seismic energy along the Sierra Madre fault and directly into LA.
There are a series of further risks if this scenario occurs. First of all, LA is built on many thrust faults. Because
scientists now understand the interconnectivity of different faults, they guess that it is highly likely that a south to
north rip would cause thrust faults under LA to also rip, adding to the intensity of the shaking. Furthermore, LA is
built on lose sediment, which will jiggle and shake a lot in and earthquake. This could be very damaging to
buildings and other infrastructure. Finally, LA is also in a basin. So, seismic waves and shaking will reverberate
within the basin, possibly joining together to create powerful shaking. Because LA is such a big city with many
businesses and a strong economy, a high magnitude earthquake would devastate the city. An earthquake on the SAF
would affect many cities along the fault, however, the biggest city it would affect is LA.
7.
Mitigation Strategies
The Great California Shakeout is a major mitigation measure that attempts to address the next great earthquake
on the southern SAF. Every October, California performs a drill with the scenario of a M7.8 to raise awareness
about earthquake preparation. The 1460 earthquake on the southern SAF was used as a model for the earthquake,
and everyone, from school children to the companies who manage life lines such as water, participate. One of the
main goals is to teach school children what to do when and earthquake happens so that an entire generation grows
up with this knowledge. The Great California Shakeout has become the largest earthquake drill in the world.
California citizens understand that they will suffer major consequences if they don’t prepare, so this is one of the
major ways that they are ensuring that all citizens are educated.
Despite everything The Great California Shakeout does to prepare citizens, there is still more mitigation efforts
that need to be made, such as an early earthquake warning system. If an earthquake starts in the Salton Sea, LA will
have approximately 35 seconds before the shaking starts. This may not seem like much time; however, it is just
enough time to stop trains, surgeries, cars, etc. so that people can take cover. Currently, there is an app called
ShakeAlert App, but it has not been the success it needs to be. For instance, not everyone has downloaded it. For the
people who do have it downloaded, there is also a problem with how it should alert people: whether it be through a
notification through the app or through a more urgent alarm. Because this is such a large and important project, the
government should take the lead to get an alert system in working order. Another important mitigation strategy is
educating citizens on things to prepare in one’s own house before the earthquake. For instance, Lucy Jones
recommends having plenty of drinking water, buying earthquake insurance, purchasing fire extinguishers, and other
simple preparations. These preparations will determine one’s quality of life after an earthquake. There is still work
to be done so that LA has as much mitigation as possible before the earthquake occurs.
8.
Conclusion
The strike slip fault that runs the length of California, called the SAF, is long overdue for a great earthquake. In
particular, the southernmost segment is the most overdue, with over 320 years since the last event. Based on the
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recurrence interval of 100 years, this area could produce an earthquake at any time. One of the most vulnerable cities
is LA. Not only is LA a hub of economic activity, but it is also built on thrust faults and in a basin. Both of these
elements will lead to intensified shaking in LA, which will in turn make the damages more severe. In addition, many
of the lifelines that California depends on, such as water, roads, and electricity, cross the SAF. When the earthquake
ends, all of these will be broken, leaving California without the essentials to a quick recovery. Luckily, California
has been trying to mitigate with the implementation of The Great California Shakeout, which they hold every year.
The goal is to try and raise an entire generation of children who know what to do in an earthquake, and also to
prepare the companies that manage the lifelines. Still, more work needs to be done, in particular in the
implementation of an early alert system. Such a system could save lives by giving people enough time to stop what
they are doing and take cover. The SAF is overdue for a major earthquake in the southern most segment, and LA
needs to continue to prepare for the disaster this event will be.
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Works Cited
“Earthquake Safety Presentation - Dr. Lucy Jones.” YouTube, 8 July 2019,
www.youtube.com/watch?v=8NBAMYzkrN8.
Gilbert, G. K., et al. San Francisco Earthquake and Fire of April 18, 1906 and Their Effects on Structures and
Structural Materials. Washington, Govt. Print. Off., 1907.
Reid, H.F., The Mechanics of the Earthquake, The California Earthquake of April 18, 1906; Report of the State
Investigation Commission, Vol.2, Carnegie Institution of Washington, Washington, D.C. 1910.
“Seismologist Dr. Lucy Jones' Tips To Survive An Earthquake - CONAN on TBS.” Youtube, 11 May 2016,
www.youtube.com/watch?v=oqrYFJe2Es4.