THE BIG ONE: UNDERSTANDING WHY THE BIG EARTHQUAKE IS PREDICTED FOR VANCOUVER
This morning, the front page of the national newspaper reads “BC put on alert for huge quake.” Yet again. So do I hop on a plane to Calgary or continue obliviously sipping my coffee? The reality of an eventual massive earthquake on the British Columbian coast is a given. As my mom, discontent with raging snowstorms and -30oC temperatures, declares, “there are consequences to living in paradise.” Earth-shattering consequences. But do we understand the nature and the true extent of these consequences?
To understand why Vancouver is at such a high risk for earthquakes relative to Calgary or Toronto, one must consider the geology of where it is on the Earth’s surface and of what lies near and beneath it. At a quick glance at a world map, Vancouver is a coastal city located on the western margin of the North American continent. Stare at the map a little longer and you might see how Alfred Wegener came up with the Theory of Plate Tectonics, in 1915. He proposed that all the continents once fit together like puzzle pieces creating a supercontinent which he named Pangaea (Figure 1; 1).
Wegener argued that Europe and North America were juxtaposed so that the Caledonian Mountains in Scotland and Ireland were continuous with a similar mountain range in Newfoundland. He also showed that the Amorican mountain range of Europe also matched up with the Appalachian range of New England, USA (2). A third geologic belt composed of similar rocks of comparable ages was later shown to have been continuous throughout South America, South Africa, India and Australia (3). In addition to similar mountain ranges and rock types, fossil deposits derived from plant and animal material were also similar between regions Wegener proposed to have once been side by side in the Pangaean supercontinent. The splitting of Pangaea into fragments resembling current continental masses occurred in several steps, between 250 to 35 million years ago (4). To understand the breakup of Pangaea and the movement of its fragments over geologic time one must understand the composition of the layers of the earth.
The Earth is composed of a core, a mantle, and a crust. Each of these layers can be further subdivided: the core into the inner and outer cores, the inner and outer (upper) mantles, and the oceanic and continental crust (Figure 2; 1).
The core and inner mantle are molten. The outer mantle is solid, as is the crust , which sits atop of the mantle. Together the crust and mantle form the lithosphere, an assembly of fifteen fragments, or plates, of variable size (Figure 3; 5).
A particular plate may have a crustal layer composed of only a few kilometers of dense basaltic oceanic crust (6). Alternately, it may consist of continental crust with an underlying thin layer of oceanic crust. Continental crust is made of less dense rocks of granitic composition and is much thicker, averaging 35 km in thickness (6). Beneath the lithosphere is the hotter part of the mantle, known as the asthenosphere. Overlying solid lithospheric plates move on the molten asthenosphere. This motion can be compared to the flow of solid ice cubes in liquid water. The relative motion of the lithospheric plates is known as plate tectonics.
Interactions between tectonic plates cause stress to build up and the development of earthquakes. The friction generated from interaction between adjacent tectonic plate margins creates stress, a form of potential energy, which can ultimately lead to strain or deformation of the crust the principal cause of earthquakes. Three conditions may generate stress at plate boundaries (6). Plates may simply generate friction at a common parallel boundary as they slide past each other (Figure 4).
This is known as motion along a transform fault. Stress may also be generated as new crust is formed on the ocean floor at a divergent zone (Figure 5).
A divergent zone occurs where a volcanic ridge occurs at a boundary between two plates. There is high volcanic activity along these ridges, resulting in flow of basaltic lava from the mantle to the crust (7). New oceanic crust is created as lava solidifies at the surface, forcing plates apart. Stress results from the heat and kinetic energy of the molten rock rising from the asthenosphere and from the addition of new crustal material which forces plates to diverge. A third way in which stress may be generated is at the convergent boundary between a dense oceanic plate and a less dense continental plate (Figure 6; 4).
Interaction between these two plates forces the denser oceanic plate to subduct below the continental plate. The tip of the dense oceanic plate forced beneath the continental plate is slowly forced downward into the hot asthenosphere, where it becomes plastic and eventually melts. This compensates for the formation of new crust at ocean ridges and keeps the surface of the earth at a constant area. When one plate subducts below another at a convergent plate boundary, stress builds up in the brittle upper 10 km of crust (8). The stress that builds up in any of these three plate boundary interactions accumulates over a long time interval, over hundreds or millions of years. The hot lower crust is too weak to store stress (7). When stress in the upper crust reaches a critical threshold, the crust snaps and strain is converted into vibrating shock waves (5). This translates into what we feel as the earth shaking (6).
The lithospheric geography of the British Columbian coast is complex. While along the Californian coast, the North American continental plate has a single boundary with the Pacific plate (Figure 7), further north three small plates lie between these two major plates (Figure 8a; 7). These plates originate from a single oceanic plate, the Farallon plate, which also lay between the North American continental plate and the Pacific plate (9). Of the three smaller plates that now exist, the Juan de Fuca plate is the largest. It is roughly the size of Oregon and Washington states combined (7). The other plates are the Explorer Plate, to the north, and the Gorda Plate, to the south (Figure 8.). Each of these plates is subducted below the less dense, North American plate and is descending below the continental plate at a rate of 45 mm/year (5, 10). Subduction occurs along the Cascadia fault, also named the Cascadia subduction zone (Figure 8a, b). This is the second largest, tectonically active fault system in North America, extending 1200 km from northern California to Vancouver Island (10).
The Pacific Northwest is predisposed to earthquakes because of plate boundaries in the region. It is more complicated and even more predisposed to earthquakes as it has transform, divergent, and convergent zones. Most coastal regions predisposed to earthquakes have only a type of zone, such as along the southern Californian coast (Figure 7). The simplest plate boundary in the Pacific Northwest is a convergence zone called the Cascadia subduction zone (Figure 8.). This zone has ruptured several times over history, causing great earthquakes similar to the Japanese Nankai Trough and the southern Chilean subduction zone earthquakes that have upward of 8 on the Richter scale (13, 14). Much of the force that leads oceanic plates to subduct beneath the North American plate is generated by a ridge system, or divergent zone. The largest ridge in the Vancouver region is the Juan de Fuca ridge, located between the Mid-Pacific Rise and Juan de Fuca fault (Figure 9). The other two ridges are the Gorda and Explorer ridges (Figure 9; 11).
These volcanic ridges add new crust to the Pacific plate to the west and to the triplet of Juan de Fuca plates to the east. Consequently, these plates are spreading apart at a rate of 60 mm/year (7). As new crust is created, old crust is pushed along the Cascadia subduction zone to return to the asthenosphere (Figure 8b). In addition to convergent and divergent zones, the Pacific Northwest also has many transform faults between the Pacific plate and the triplet of Juan de Fuca plates. These are the Sovanco, Blanco, Nootka, and Mendocino faults (Figure 10; 7).
Along these faults, the Pacific plate grinds against the Explorer, Juan de Fuca and Gorda plates as it is pushed northwest. The greatest force pushing the Pacific plate in this direction is crustal formation at the Mid-Pacific Rise in the Pacific Ocean. The movement of the Pacific plate has been confirmed by mapping the two-dimensional pattern of changes in sea-level at tidal stations over years (9). Grinding of this plate as it moves along the aforementioned faults generates stress. Plate interaction along any of these convergent, divergent, or transform faults may cause the buildup of stress which may potentially lead to an earthquake.
A major earthquake affecting Vancouver Island and Vancouver would most likely be produced by stress and strain generated at a transform fault or along a subduction zone. The magnitude and intensity of earthquakes produced by diverging plates are low. Tremors would not be felt unless one were on the ocean floor it is only known that they occur by measuring seismic activity (15). Both transform faults and subduction zones can cause very violent earthquakes measuring up to 9 on the Richter scale (7, 13). Most of these earthquakes have a shallow epicenter with a focal depth of less than 70 km (1). Earthquakes occurring close to the earth’s surface are typically the most destructive. Many historical earthquakes in the region have been along the Cascadia subduction zone (16).The Alaskan earthquake of March 27, 1964, the second largest of the twentieth century, was one such earthquake and triggered tsunamis that killed 130 people, some as far away as California and Hawaii. High magnitude earthquakes historically occurring in the vicinity of Vancouver and Vancouver island are listed in Table 1. While any major earthquake predicted to occur in southwestern British Columbia would most likely result from convergent activity along the Cascadia subduction zone, activity along transform faults could also trigger violent, damaging earthquakes as activity generated from tectonic movements along fault lines can cause high magnitude initial shock waves and many aftershocks (12).
Physical Consequences of an earthquake
In the event that the predicted great earthquake did occur, its after-effects might cause greater and more widespread damage than the earthquake itself. Aftershocks commonly occur, 36% within the first month following an earthquake. An aftershock is a smaller wave than the initial shaking in an earthquake; aftershocks further destabilize buildings, pipelines, and other structures damaged by the first wave. Damage to underground fuel lines, gas connection points, and facilities in which toxic and combustible materials are stored can start fires. Historically, many earthquakes have caused fires, such as the 1906 San Francisco earthquake (5). Tsunamis and flooding are other important consequences of earthquakes which occur along subduction zones between continental and oceanic plates. Tsunami is a Japanese word meaning “harbor wave.” These waves can be up to 10 metres high, traveling at velocities upward of 1000 km/h and may occur in a series of repetitive waves that may last up to 12 hours (5). To understand how tsunamis are generated, it is important to understand that strain accumulating along a fault causes the coastline to rise. When an earthquake results, the uplifted coastline falls and the offshore region is lifted up (Figure 11; 17).
This motion displaces the overlying water, leading to the creation of tsunamis. It can be assumed the Cascadia subduction zone has been responsible for significant earthquakes in the past because of dating of rapidly buried soils and the sand sheets left by tsunamis on top of these soils (18). Low-lying land masses are at highest risk for extensive flooding in the event of a tsunami. Damage to coastal British Columbia would be great. A final concerning consequence of a major earthquake is that of landslides. Although surface soil is mainly made of compact layers of silt or clay, the less compact layer below is composed of sand and highly susceptible to liquefaction. Sandy soils that are water-saturated can behave more like a liquid than a solid during an earthquake (7). Movement of the more solid surface layer over liquefied sand can destabilize building foundations and damage bridges, roads, and pipelines. In the Greater Vancouver Area, areas made of lowlands sediment such as Richmond and Delta, are most susceptible to landslides and liquefaction (Figure 12;19).
Medical and Human consequences of an earthquake
Beyond physical damage, earthquakes wreak havoc on human health. Crush injuries, resulting from compression of the trunk and the extremities, affect between 2 and 15% of the population in urban centers affected by earthquakes (5). Muscle trauma from the crush injury, defined medically as rhabdomyolysis, causes the release of muscle cell components and electrolytes into the bloodstream. As a result, blood pH and potassium concentrations rise. This leads to organ damage, particularly to the heart and kidneys, in addition to skin and soft tissue injury and physical trauma to the chest and extremities resulting from the impact (5). Fires induced by earthquakes substantially increase human morbidity and mortality beyond that attributed to the earthquake itself (5). There may be many burn victims from the aftereffects of an earthquake. In the event of a tsunami, patients may need to be treated for asphyxiation from near-drowning, for acute respiratory distress syndrome, aspiration pneumonia, and for extremity fractures, lacerations, and sprains (20, 21). A final worry in medical treatment following an earthquake is that of infection, particularly of soft tissue injuries. A soil fungus called coccidiodomycosis is a common infective agent and contaminated water following a tsunami may spread leptospirosis and Vibrio vulnificus (21).
Vancouver and Vancouver island lie within an area of complex geology that predisposes this coastal region to high magnitude earthquakes. The last great earthquake that caused massive destruction occurred in this area in 1700, reaching 9 on the Richter scale, and was caused by movement along the Cascadia subduction zone. Urban legend has it that major earthquakes occur in the region every few hundred years and that we are long-overdue for a quake. The Geological Survey of Canada has stations continually measuring seismic activity, with the intent of detecting the precise location and magnitude of occurring earthquakes and of predicting when and where in the near future an earthquake is likely to occur. Unfortunately, this proves about as accurate as predicting the weather in Vancouver and people quickly become immune to earthquake predictions. It is important to remember that while earthquake forecasts are merely predictions and while it would become rather costly to fly out of town at every predicted threat, the Pacific Northwest is among the regions of the world with the highest susceptibility to large magnitude earthquakes. While we cannot prevent an earthquake from happening and cannot predict where we will be when it occurs, the best strategies are to push for buildings engineered to withstand moderate earthquakes, to be aware of earthquake preparedness and disaster protocols, and to have adequate supplies and nourishment in the event of a natural disaster striking.
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