The city of Los Angeles sits near the San Andreas Fault and all faults associated with the North American and Pacific plate boundary. Therefore, the city has a relatively high likelihood of experiencing a major earthquake within the next century. As such, Los Angeles needs to be prepared for this event, and smaller seismic occurrences. Preparation includes hazard analysis, detailed mitigation strategies, and foreknowledge on what effects earthquakes will have on the city.
Los Angeles is a city of over four million people living within 503 square miles (World Population Review, 2018). This highly dense city faces many issues when it comes to how to mitigate seismic hazards. The climate of Los Angeles is mild to hot, which contributes to how arid it is. Although classified as a Mediterranean climate, temperature can vary by as much as 36 degrees due to the differences between the valley and coastline throughout the Greater Los Angeles area. Los Angeles plays a role in American economics as a financial hub, the entertainment capital of the world, and Hollywood’s home. This is extremely important as the United States entertainment industry generates over 497 billion dollars every year (Hollywood Reporter, 2018). Therefore, Los Angeles creates an environment that draws more people to move there, as shown by a growing population and a high demand for housing within the area. When preparing mitigation strategies for such a dense city, it is important to use different methods of analysis to predict how many earthquakes will occur, and of what magnitude, within a given period of time.
To simulate this, we used a Gutenberg-Richter Analysis to quantify the seismic hazard over the next 10, 30, and 100 years. For our interest, we decided to study an area that did not include the San Andreas Fault, but rather included a fault system made up of the Hollywood and Raymond Faults. Based on a 50-year assessment by the USGS (USGS Hazard Map, 2018), a calculation to find the “a” value within the Gutenberg-Richter Analysis equation produced a=6.1999. As noted in many geological classes of this level, we assumed the slope, or “b” value, was equal to one. This value could actually vary between about 0.91 and 1.20, yet 1 was a good estimate for our predictions. After finding these values, we plugged in different magnitudes to see how many earthquakes would occur within the given period of time (Table 1). We then put this quantifiable data in graph form to further show how many earthquakes would occur. Unfortunately, we cannot accurately assess how many earthquakes can truly occur below about a magnitude of 2.5. This is because some earthquakes release energy that cannot be detected or could be confused for other events, yet the data is still important to understand how many earthquakes to expect.
Table 1:
Magnitude
10 Years
30 Years
100 Years
Assumed Stats
1
15848.9
47546.7
158489
a=6.1999
2
1584.89
4754.67
15848.9
b=1
3
158.489
475.467
1584.89
4
15.8489
47.5467
158.489
5
1.58489
4.75467
15.8489
6
0.158489
0.475467
1.58489
7
0.0158489
0.0475467
0.158489
To calculate our largest earthquake possible, which is our scenario earthquake, we had to find the length of our fault system, which totaled to 29.6 miles. Then, by using an empirical graph provided in lecture (Lekic, 2018), we determined that a 7.3 magnitude earthquake could occur just from the Hollywood and Raymond Faults within our limits.
In comparing the estimates from the ShakeMap to statistics from the United States Geological Survey, we noticed that our estimates were slightly lower. According to a hazard map released by the USGS, there is a 2% chance that the peak ground acceleration will exceed 0.8 on the USGS scale in 50 years for Los Angeles (USGS Hazard Map, 2018). This peak ground acceleration of 0.8 on the USGS scale correlates to an intensity of 9 on the Mercalli scale. However, according to our shakemap, which is based on a 7.3 magnitude earthquake, the intensity in Los Angeles would only be 8.5 on the Mercalli scale. This correlates to a peak ground acceleration of about 0.7 on the USGS scale (USGS Hazard Map, 2018). Our underestimate could be attributed to the geography of Los Angeles and that we did not reliably factor in how this changes the perceived intensity of the earthquake as well as the overall peak ground acceleration. This underestimate could also be due to which fault system we used within our boundaries. While we used the Raymond and Hollywood Faults for our earthquake, in a realistic scenario, other faults could have slipped to cause a larger earthquake. This could in turn lead to a domino effect of earthquakes releasing energy through the Los Angeles area, which in turn would lead.
Upon creating a scenario 7.3 magnitude earthquake through the ShakeMap application, its impact on the city of Los Angeles cannot be underestimated. While analyzing certain aspects of the earthquake, such as shake intensities, rupture length, liquefaction potential, and building retrofitting, we can paint a clear picture of how this earthquake would look if it actually occurred. With a rupture length of about 29.6 miles, the earthquake’s shaking would last for roughly 30 seconds. Because of high shaking intensities, various buildings would need repairs and many would even collapse past the point of possible repair. Thankfully, this earthquake would likely not occur within our lifetimes. According to our Gutenberg-Richter analysis, not even one magnitude 7.3 earthquake will hit Los Angeles within 100 years. According to data expressed by the USGS ShakeMap application, the earthquake would produce, a minimum of category 8 intensity shaking in downtown Los Angeles. This would cause considerable damage to well-built structures. Even cities as far as Tijuana, Tecate and Mexicali would experience category 3 intensity shaking, which may even cause parked cars to rock slightly (USGS ShakeMap, 2018). Liquefaction, a very dangerous hazard, may result from a large earthquake where the shaking causes loosely packed, water saturated silt to behave like a liquid. Multiple areas within Los Angeles have this type of sediment within their geology, and therefore are categorized as hazard zones for liquefaction. (Liquefaction Zones USGS, 2018). Unfortunately, this particular area of Los Angeles is filled with heavy traffic roads, businesses, and residencies that, if affected, may cause devastating loss of property and life.
Our 7.3 Magnitude ShakeMap:
Construction in downtown Los Angeles is made up of moderately tall buildings, all of which are constructed out of a concrete facade. While it is hard to know what they are constructed out of internally, there were little to no brick facades or plain wood. Based on what we have learned in class about California and its mitigation strategies, it is safe to assume that most buildings downtown are created with reinforced concrete or a wooden frame. Outside of downtown, the buildings tend to be shorter, with some brick facades. In the suburbs, there are many smaller houses, none of which have brick on the outside. A few are made with wooden exterior walls, yet most seem to be comprised of concrete walls. While the houses were short, they were also extremely close together, with only a few feet between each.
With the epicenter not quite in downtown LA, very little damage would occur in the downtown area during the magnitude three earthquake scenario. Only light shaking should affect most of downtown, with little shaking felt in the surrounding areas, and little damage actually done to the buildings. However, in a magnitude four, shaking would increase to light and moderate shaking downtown, which means that buildings might start to exhibit signs of shaking. Therefore, it is possible that the buildings might accumulate small amounts of damage. Once the earthquake scenario becomes a magnitude five or six, however, the earthquake generates heavy shaking, meaning that the buildings will sustain fairly large amounts of damage. The actual damage will vary depending on the building itself. Luckily, there are very few brick buildings, so there will not be a lot of masonry shattering. Fires will also not be too prevalent, as there are not many wooden buildings which could keep a fire fueled. The most likely damage will result from liquefaction near concrete buildings and increased pressure on steel supports, causing damage to either the rebars or structures held together by concrete. Liquefaction also brings about the possibility that buildings will sink into the ground, fall over, or become unstable.
When it comes to mitigation strategies, the city of Los Angeles is fairly well prepared. On a hazard assessment document of the city, earthquakes rank highest on hazard likelihood (L.A. 2018). The city also ran risk assessments for four different likely scenario earthquakes, of magnitudes ranging from 6.8 to 7.8. According to this assessment, one of the biggest problems is how critical facilities located within LA would react. This is especially important because these facilities include utility buildings, hazardous waste storage sites, and administrative buildings (L.A.). There are also hundreds of thousands of buildings built before both 1933, when there were no building codes in regards to earthquakes, and 1975, when the city began to enforce building codes. This means that 74% of buildings standing in Los Angeles today do not meet the standards of any current building codes pertaining to earthquakes (L.A.). To mitigate the possible damages to these buildings for magnitude three or four earthquakes, basic seismic retrofitting should be observed, such as securing loose items in homes, helping people figure out the best way to evacuate a tall building, and keeping building codes enforced. However, mitigation for a magnitude five earthquake should start with looking at older buildings and seeing their potential weaknesses. While damage could still be minimal for this type of earthquake, adding in metal bracing to walls and floors and securing wooden structures would help reduce damages even further. For magnitude six and larger earthquakes, mitigation would include slowly updating the city infrastructure and buildings. While this would be very expensive, it is worth it, considering the high hazard zone for earthquakes. Replacing buildings that were built before 1975 and that also do not have historical significance would cut down on how many buildings could collapse. This not only decreases costs of damage overall but also decreases how many people an earthquake could injure. For buildings that do have historical significance, seismic retrofitting could include energy dissipation devices, which are effective and still can be fairly inexpensive to install. However, if expenses are not a concern, base isolation techniques would decrease damage very effectively, while still preserving the exterior structure.
Los Angeles, a highly seismically active area that lies near many faults, has a large seismic hazard. According to our Gutenberg-Richter analysis, Los Angeles will have at least 1 magnitude 6 earthquake within the next 100 years. This is in addition to a multitude of smaller magnitude earthquakes that will occur more frequently. In comparison, with the hazard map published by the United States Geological Survey, agreed with our estimates for smaller earthquakes. However, given that the hazard for larger earthquakes was higher in our calculations, their results deviated from ours for magnitude 5 earthquakes and larger. Discontinuity in results and subsequent misinformation may have effects on how the area prepares for these events. Although Los Angeles is generally well prepared for earthquakes, it can still implement more mitigation strategies, such as retrofitting, to reduce risk during earthquakes. Since earthquakes of category 8 intensity on the MMI scale are likely, it would benefit Los Angeles to strengthen more of its buildings to protect its infrastructure and people.