Course:EOSC311/2020/ Cascadia Subduction Zone Hazard Mitigation

The study of geography entails a multitude of disciplines and uses these different areas of study to analyze how the earth systems work as well as how humans are impacting or are impacted by these systems. This multidisciplinary area of study incorporates many aspects of geology, and can use geology to better understand physical processes that affect humans. One of these physical processes that adversely affect human life are earthquakes. Earthquakes can cause major disruption to the human use system especially if the location of one is near a densely populated area. However, when a geological physical process occurs away from a human population, it is not considered a hazard. Through geology, one can better understand the mechanisms of an earthquake. This can provide a lot of useful information to geographers about how to incorporate mitigation efforts in an attempt to lessen the damage of this hazard. The world's population is growing exponentially and our climate is changing, so understanding both the geological physical processes as well as how humans are impacted by these are extremely important and relevant. These changes are placing more people at risk as well as causing more extreme and unpredictable natural hazards. Within this article, information will be taken from a geology and a geography perspective. This includes understanding the mechanisms of the geological physical process of earthquakes, a case study of the Cascadia subduction zone, and what humans can do in terms of mitigation efforts in preparing for an earthquake.

The Geological Process of Earthquakes

The Formation of Earthquakes

Figure 1. Tectonic plate boundaries

As shown in Figure 1, the earth's crust is made up of different tectonic plates that are constantly moving, and when these plates interact with one another a variety of geological processes can occur. One of these processes is an earthquake. Earthquakes are not always hazardous, despite media representation and general public knowledge. This is because, if an earthquake was to occur far from a population or deep underground where it did not adversely affect humans, there is little disruption caused and most people would likely not hear about it. A natural hazard only becomes hazardous when human life is disrupted because of it. However, when the natural system interacts with the human system, like an earthquake near a populated area, the natural event turns into a hazard. The plate interactions that are creating these geological processes occur at the plate boundaries. Earthquakes can occur at convergent, divergent and transform plate boundaries. A convergent plate boundary is when two plates are moving toward each other and collide. A divergent plate boundary is when two plates are moving away from each other. Lastly, a transform plate boundary is when two plates slide past each other horizontally. An earthquake occurs at a fault line at a plate boundary. The movement of tectonic plates can cause friction which releases energy that can cause some form of disturbance. This energy can be in the form of seismic activity as seismic waves that travel underground as Primary (P) waves and then Secondary (S) waves. If this seismic activity is significant, it is what humans would feel as an earthquake. The hypocenter is the area underground where the earthquake started, and the epicenter is the area on the earth's surface above the hypocenter.

What are the Precursors

Figure 2. The recording of seismic waves onto a seismograph

Earthquakes have a very fast speed of onset, providing minimal warning and preparation time to local inhabitants. This creates a level of uncertainty that can only be lessened through mitigation preparation and responses. One way geologists have attempted to predict earthquakes is through recording previous events using seismograms that create a seismograph displaying seismic activity. A seismograph is shown in figure 2. This information is interpreted by seismologists and then communicated to the general public. Scientists are unable to predict the exact timing and magnitude of an earthquake, however using the recurrence interval and past records scientists can make predictions about the likelihood of one happening. The Geological Survey of Canada (GSC) is one of the many organizations that interprets the recurrence interval of hazards, including earthquakes. During the period of an earthquake there are often foreshocks which are smaller earthquakes that precede a large one. Then the mainshock occurs which is usually the largest earthquake that causes the most damage. The aftershocks will occur after the large earthquake and can last for weeks, to months to years. Often these can be just as damaging to the local area.

How Earthquakes are Measured and Recorded

Figure 3. Seismic Waves showing Primary (P) and Secondary (S) waves on a seismograph

A seismometer is used to record seismic activity. These are set up at different earthquake seismic stations and record movements in the ground and interpret this information into a seismograph. As shown in figure 3, there are different lengths in the lines on a seismograph, the longer the line the more intense the seismic activity is. Seismographs also show the P waves and the S waves and these are important in providing insight to where the epicenter is. P waves occur first and travel faster than S waves. The interval of time between these two waves can determine how close the seismic station is to the earthquake. For example, if the P wave occurs and there is a long duration of time before the S wave is recorded, then it can be assumed that this station is far away. For an estimation of where the epicenter is, a method called triangulation is used. Three seismic stations are needed for this method, in which their seismograms have recorded seismic activity. Each station estimates how far they are to the earthquake drawing a circle around their station with the correct radius. When this is done, the intersection of all three of the stations is where the epicenter is. In determining the magnitude of an earthquake, the Richter scale which was developed in the 1930’s is commonly used. It measures the amount of energy released by a single earthquake in the form of magnitude. This rhetoric is used to communicate the intensity of an earthquake to the general public. It determines the different magnitude by the length of a fault line, so the longer the fault line the bigger the magnitude. The magnitude of an earthquake can start at a 0 and go up to a 9. Each number on the magnitude scale goes up in intensity by 10 fold, having a logarithmic pattern. So for example a magnitude 4 earthquake is 10 times more intense than a magnitude 3, and a magnitude 5 earthquake is 100 times more intense than a magnitude 3 and so on.

Cascadia Subduction Zone

Location of this Subduction Zone

Figure 4. Diagram of Subduction Zone

The Cascadia Subduction zone is a large oceanic continental convergent tectonic plate boundary that extends for 1,200km where there are two plates interacting causing tectonic activity. Figure 4 shows a simplified diagram of a subduction zone. The Juan De Fuca plate is subducting and moving eastward under the North American plate. As shown in figure 5, this subduction zone starts off at the coast of Northern California and extends upward to Central Vancouver. This places all of the cities along this coastline at risk. Geologists have found geological evidence to prove that this area is a subduction zone. This includes earthquakes and volcanoes along the plate boundary, and tertiary and quarterly sediments that have been deformed at the continental plate boundary. These are just a few of the geological processes that provide evidence for a subduction zone. Previous earthquake records have also provided evidence that there is an accumulation of elastic energy and strain happening at this plate boundary. Once this energy is released, a large magnitude earthquake of 8 or higher is predicted to occur.

What is the Likelihood of a Megathrust Earthquake

Figure 5. The tectonic plates involved at the Cascadia Subduction Zone

In the past 130 years there have been around 10 earthquakes recorded at the Cascadia Subduction zone ranging from magnitudes of 6-7 all within 250km of Vancouver and Victoria. There has been 12 megathrust earthquakes recorded at this subduction zone in the past 7,000 years. The last megathrust earthquake recorded at this subduction zone happened around 300 years ago. Since there has been large earthquakes recorded here in the past, it is almost guaranteed that large earthquakes will occur in the future. Previous earthquakes and Tsunamis that were a result from the movement of tectonic plates at the Cascadia subduction zone are shown in plant fossil records and tsunami deposits. From this information geologists are able to provide a recurrence interval which predicts how often an earthquake will occur. The GSC stated, “We use the recurrence rate of Cascadia subduction events to calculate time-dependent occurrence probabilities for various time frames.” From this information seismologists have predicted a recurrence interval for megathrust earthquakes at this Subduction zone to be every 500 years. The magnitude of this earthquake is also disputed as although many geologists and seismologists suggest the magnitude of an interface earthquake at this subduction zone is not to exceed 7.0, some have estimated it to be between 8.5 and 9.1. There are three types of earthquakes that can occur at the Cascadia subduction zone. The first is called a shallow crustal earthquake where an earthquake would occur in the North America plate. The second is a deeper sub-crustal earthquake where an earthquake would occur on the Juan de Fuca plate. The last one that has the most potential for destruction is a subduction interface earthquake that would occur at the plate boundary of these two plates interacting. Globally, earthquakes that are subduction interface earthquakes have been the largest in magnitude. This is why geologists are able to predict that with the amounting stress at this plate boundary, a megathrust earthquake will likely occur.

Implications of Earthquakes for the Vancouver Area

How Vulnerable is Vancouver

Figure 6. Map of the Vancouver area

In British Columbia, each year there are over 200 earthquakes recorded. Most of these earthquakes were small in magnitude, but historically there have been some large magnitude earthquakes that have caused disruption. For example, the last megathrust earthquake in British Columbia was a Magnitude 7.3 in 1946. This earthquake was not at the Cascadia subduction zone and it's epicenter was located in an area that had a small population density and therefore the hazard risk was low. However, towns nearby reported damages. The potential for megathrust earthquakes in Vancouver provide insight into how destructive one would be if it occurred at the Cascadia subduction zone. Figure 6, shows a map of the Vancouver area, which would be affected when a megathrust earthquake occurs at the Cascadia subduction zone. The areas near this plate boundary are densely populated and many are metropolitan areas putting not only infrastructure at risk but also human lives. In relation to a megathrust earthquake, “were a comparable earthquake to occur today near Vancouver, damage would likely be in the tens of billions of dollars.” It is also important to note that since the last megathrust earthquake, Vancouver has grown exponentially in population numbers and in terms of infrastructure development. This means that the damages and losses will be much more extreme than those experienced in 1946.

Physical Geological Processes that Would Follow an Earthquake

The geological processes that are affected during and after an earthquake are specific from place to place. In the case of Vancouver, there are a number of processes that would occur. Firstly, the shaking of the ground would be affected by the type of bedrock that the seismic waves pass through. The differing velocities of seismic waves are impacted by what unconsolidated materials they travel through. “Shear-wave velocities in sediments overridden by Pleistocene glaciers, for example, are much higher than those in postglacial alluvial and deltaic sediments.” The damage from an earthquake can also be influenced by the local topography and soil conditions, so this is something that needs to be taken into consideration. Another effect from ground shaking would be liquefaction. This would mainly be a disruption in metropolitan areas as it could damage property when parts of the ground become liquid during seismic activity. This would also put a strain on public transport systems and cause severe damage. Landslides are another concern as this is common in areas that have mountains accompanied by seismic activity. For example over 300 landslides occurred after the last megathrust earthquake of a magnitude 7.3 that happened on Vancouver island in 1946. The last potential threat would be Tsunamis. Since the coast of British Columbia is a densely populated area, this would be extremely catastrophic. “A Tsunami generated at the Cascadia subduction zone will reach the British Columbia coast soon after the shaking stops. The first wave will strike western Vancouver Island in minutes to, at most, one hour. Travel times to Victoria and Vancouver might be as little as 1.5 and 3 hours, respectively.” All of these processes provide another layer of threat to the pre existing hazard of an earthquake occurring here.

Appropriate Responses and Mitigation Efforts to Earthquake Hazards

With the prediction of a megathrust earthquake to hit the Vancouver area, there has been mitigation response efforts put in place in order to protect the people and infrastructure within Vancouver and the Greater Vancouver Regional District (GVRD). The preparedness of Vancouver for an earthquake of this size is highly dependent on their budget and mitigation strategies they have in place. Vancouver has invested 20 million dollars as part of a 10 year budget for earthquake projects. Despite these efforts however, the hazard that an earthquake presents prevents the ability for humans to modify the natural event. Therefore, there are certain protective measures that can be put in place to lessen the severity of the damages caused, but these cannot entirely mitigate overall destruction.

Risk Factors

Figure 7. The catastrophic damage of an Earthquake in San Fransisco

Mitigation responses to an earthquake hazard are important and essential for the protection of the inhabitants of the Vancouver area, however individual perception is another factor that affects overall risk. “Although the seismic hazard has existed for a long time, it is only recently that measures have been taken to prepare for an earthquake. This is due to a lack of a major seismic event in the GVRD which diminishes the perceived risk.” Public perception and knowledge is an important component of perceived risk. Individual behaviour may be influenced by their perceived risk and this would therefore influence whether they undertake proper mitigation efforts at a personal level. This would include educating themselves on the geophysical process of an earthquake and ensuring their environment is earthquake resistant. Often if perceived risk is low, mitigation efforts are not implemented in their everyday behaviours and activities which therefore increases overall risk to an earthquake hazard. Vulnerability is another factor that either increases or decreases overall risk. This can be influenced by an individuals socio-economic status. For example, if an individual has a lower income and lives in an older building that is not up to date on seismic code, they may be unable to move and or invest in earthquake resistant materials to implement within their house. In order to reduce overall risk there needs to be an understanding of the geophysical process, changes in individual perception, modifications to the natural and human use system, and emergency responses put in place by the local government. Figure 7 details the catastrophic damage that an earthquake can have in a metropolitan area. Although this image is from San Fransisco, there are similarities in building structure and the density of the metropolitan areas in Vancouver, which puts into perspective what kind of damage would occur when an earthquake hits.

Modification to the Natural Use System

The main mitigation strategy when it comes to modifying the natural system for the case of Vancouver would be in relation to buildings in the metropolitan area. There are seismic and structural design codes are put in place to ensure the safety of buildings and houses. These are attempts to decrease the severity of the damage to a metropolitan area. One of these codes is providing enough separation between buildings. This ensures that buildings won’t collide with seismic activity that causes buildings to shake and move. However, often with older areas and older buildings, they are not up to seismic code which produces a major threat. Another issue with this is the high cost of land, especially in congested metropolitan areas like Vancouver. Ensuring that buildings are up to date in terms of abiding by seismic codes is another important mitigation effort. Currently the British Columbia Building Code (BCBC) that is based off of the National Building Code of Canada (NBCC) have placed specific requirements for buildings in relation to earthquake preparedness. These requirements include things like, using reinforced concrete, having wood frames, and electrical components of buildings being seismically braced. The flaw in this effort is that many of the older buildings that are out of date and that don’t meet the current seismic code do not have to update or rebuild. This creates a hazard for the people living or working within these buildings as they are placed at a higher risk when an earthquake happens. This especially becomes difficult for heritage buildings as the city wants to continue the preservation of these buildings but this does not diminish overall risk to an earthquake event. Vancouver currently has a system of determining which buildings are up to seismic code. They use Levels, where level 1 means the buildings are currently up to date, and level 2 and 3 are not. The last mitigation effort is the installation of soft material layers. These materials include things like rubber which act as a shock absorber at the base of buildings known as a shock absorber device (SAD). This is effective for buildings that are older and not up to seismic code as they can be installed without needing to rebuild.

Modification to the Human Use System

The response and approach to an earthquake hazard in Vancouver is a bottom up approach in which individuals are mostly responsible for their own protection. This doesn’t mean that there aren’t structures put in place by the government, but individuals would need to be responsible for their own safety. Depending on the severity of the earthquake it would determine whether local, regional, provincial, national or international support is needed after the event. The government however is responsible for warning residents of Vancouver to potential earthquake threats. Another mitigation strategy with the human use system is ensuring the public is aware and educated about earthquake hazards. This includes offering public presentations about earthquake hazards and keeping media outlets updated with information about tectonic activity. Ensuring that schools and hospitals are well informed is also a key part of this mitigation effort.

Emergency Adjustments

Emergency adjustments are put in place through government organizations that deal with earthquake mitigation responses. The associations that are involved with the mitigation and emergency responses are: Emergency operation centers, the provincial governments, and Emergency preparedness Canada. The efforts put forward by these organizations are mainly in the form of rapid mobilization responses. Firstly, ensuring there is adequate medical care for people who are affected. This would include ensuring that hospitals are well equipped and emergency first aid kits are available to responders. The second is providing temporary housing to people whose homes were destroyed during an earthquake event. This may prove difficult considering Vancouver's high population density and limited space. One solution would be to attempt to build shelters outside of the metro Vancouver area, perhaps utilizing the Fraser Valley. Another rapid mobilization response is ensuring there is proper food distribution planning in effect. This is an extremely important emergency response as an earthquake can be quick to shut down supermarkets and make access to food difficult. Lastly, providing financial aid and relief to the people living in the GVRD is important. This ensures that they have the means to survive if their livelihood is majorly disrupted. The financial aid may vary depending on insurance, so there may be some discrepancies in who receives aid and how much. This also relates back to increased or decreased risk based off of socio-economic status.

Adjusting to Losses After an Earthquake Occurs

Due to the fast speed of onset of an earthquake and the inability to predict the exact timing and exact magnitude of an earthquake, there is little time for local inhabitants to prepare. This is why having appropriate responses implemented in an earthquake relief plan is particularly important for the Vancouver area. This would include financial aid and relief on a long term scale. This is more extensive than emergency aid, as it may provide people will longer financial relief following an earthquake. The rebuilding of homes and buildings that were destroyed would be another adjustment Vancouver would have to do. The time frame and cost would be dependent on the severity of the event. With this, it would be important to ensure future buildings are up to seismic code. The potential for a megatrhust earthquake to hit the Vancouver area requires an understanding of the geophysical event as well as proper mitigation efforts implemented to ensure the safety of the locals of Vancouver.

References

This Earth Science resource was created by Course:EOSC311.