Vancouver’s Seismic SceneEven though not everyone in the city may look up every day, many of the mor

Vancouver’s Seismic SceneEven though not everyone in the city may look up every day, many of the more than 2 million inhabitants of the Vancouver, Canada area get daily reminders of the geology of the area. The city sits in a basin surrounded by rivers and the Strait of Georgia. Towering to the city’s east sits the Pacific Coast Mountain Ranges and the northern extent of the Cascade Mountains, while to the city’s west the mountains of Vancouver Island and the Olympic Mountains in Washington State can be spotted.The presence of so many mountain ranges surrounding the city definitively shows that Vancouver sits in a seismic hot spot and makes it a good place to learn more about the seismic threats facing major cities.Vancouver owes its origin to the long history of subduction along the western coast of North America. From the rocks high in the Cascades and the Interior Plateau to the Pacific Coastline, basically none of North America was present a few hundred million years ago. The entire western coast of Canada and the Pacific Northwest is made of accreted terranes – pieces of crust, such as island arcs or pieces of the ocean floor, that rode into the continent on the subducting oceanic plate and got stuck.The process of accreting crust is a violent one. It literally involves breaking off a piece of one plate and locking it to another, so it tends to generate faults and push up mountains. On top of that, normal subduction generates faults and volcanoes like those of the Cascade arc, generating further complexity.Vancouver itself sits on a small piece of one of these terranes wedged between several larger ones. Much of Vancouver Island is composed of a terrane called Wrangellia; it is composed of basaltic igneous rocks, erupted in the middle of an oceanic plate. Those rocks erupted on the ocean floor during the Jurassic and rode along on the subducting Farallon plate until about 100 million years ago when it slammed into the continent.After the arrival of Wrangellia, subduction of the Farallon plate resumed, causing continued building of faults and mountains near the coast as well as the growth of the Cascade volcanoes farther inland. Vancouver, therefore, sits in a basin, in-between mountains grown to the west from faulting and mountains grown to the east from a longer history of faulting combined with volcanism.Vancouver therefore has a lot of potential seismic threats. The most powerful fault in the area is the one that dominated its formation – the Cascadia Megathrust. Off the coastline of Vancouver Island, a small piece of the old Farallon plate named the Juan de Fuca plate still subducts, sinking beneath North America and driving the growth of the Cascade volcanoes.Evidence gathered over recent decades shows that the Cascadia Megathrust undergoes rare, massive earthquakes. It works much the same way as the faults in Indonesia and Japan that in the past 10+ years unleashed some of the most powerful quakes in recorded history; as one plate is pushed beneath another the fault plane becomes locked, storing and then releasing a huge amount of energy.Evidence from near the coastline shows that this region sees these large earthquakes. Submerged forests have been found along the coastline, from Oregon to British Columbia, implying up and down motion just like that seen during the tsunami-triggering quakes of the past decade.These quakes occur roughly every 500 years and the last major one was in the year 1700; its date is known because it triggered a tsunami recorded in Japan. Evidence suggests that the time between major Cascadia earthquakes can vary a lot; some are only a few hundred years apart and sometimes the fault goes over a millennium with little activity. Consequently, there’s estimated to be a 15% chance of a major earthquake on that fault over the next 50 years, but with a substantial uncertaintly.When the Cascadia fault ruptures, the first thing that will happen is the shaking. Downtown Vancouver is about 300 kilometers from the fault and the strongest shaking is always felt closest to the rupture, so the distance will protect the city somewhat. However, Vancouver has one thing going against it; the local geology.Vancouver is built in a sediment filled lowland called the Georgia Basin. Sediments have come down from the mountains on both sides, but mostly from the east, carried by the Fraser River. Sediment-filled basins are extremely vulnerable to earthquake waves. When seismic energy is released, it tends to pass through hard igneous or metamorphic rocks but when it enters a sedimentary basin it bounces around and reverberates. On top of that, buildings on sedimentary rocks are extremely weak – sediments tend to undergo liquefaction during seismic shaking, taking away all their strength. Buildings built on sediments can lose their support and suffer heavy damage or collapse (http://tmblr.co/Zyv2Js1WTUw1o).Vancouver has plenty of old buildings, including brick buildings that are extremely weak against seismic forces and will likely collapse in any strong earthquake. Vancouver’s city leadership notes that it has the strongest seismic building codes in Canada and modern buildings constructed since 2000 under these codes are likely to survive any earthquake, potentially with some damage. However, older buildings were constructed before those codes and thus they retain substantial risk. Early surveys suggest possibly 10,000 buildings in the city could be at risk of major damage or collapse; full surveys of at-risk buildings are underway but in the U.S. these have taken decades. Refurbishing or replacing that many old buildings is an expensive, multi-decade job; an old “dark joke” in seismically prone areas is that the upgrades will be fully completed after the demolition is done for free. On top of the direct damage, after a major quake getting aid to millions of people will be difficult as electricity will be out and major transportation methods, including most roads in and out of the basin, are likely to be damaged and impassible. For these reasons, during a major earthquake, significant casualties will happen, and preparation for residents along the lines spelled out in these articles or in emergency plan documents is highly recommended (http://tmblr.co/Zyv2Js1pz9oUR, http://tmblr.co/Zyv2Js1puIWDj).The other threat during a Cascadia quake is tsunami. The Cascadia Megathrust will generate another tsunami when it ruptures and that wave will be comparable to the one that struck Japan. The city of Vancouver is mostly shielded from the direct effects of the wave, but anything on the Pacific coastline of Vancouver Island will be directly exposed. Any construction in low-lying areas should be done with tsunami evacuation in mind. The wave will be blocked from the city by Vancouver Island, but the waves energy will still be able to propagate somewhat into the coastal straits, causing stronger than normal currents which have the ability to damage shipping.Finally, the Cascadia Megathrust is not the only seismic threat to Vancouver. Many smaller earthquakes have been recorded throughout British Columbia in recent decades, including a magnitude 7.3 quake on Vancouver Island in 1946. The mountains throughout the area are laced with faults; the mountains grew on faults and forces shift from one fault to another as the plates move. There are major faults within 50 kilometers of Vancouver that could be even more direct threats to the city; although they could not produce a quake as large as the Cascadia Megathrust, these smaller faults could dump energy directly into the Georgia Basin, causing even more damage to buildings than the megathrust could.The lesson from seismic regions around the world is that geologists are working to map and understand these faults, but without written records it is difficult to tell how active they are. Furthermore, there are often buried faults beneath the Earth’s surface that aren’t recognized until they actually rupture.Vancouver, Canada is in a seismic hot zone. Like many cities of the Pacific Northwest it is built on sedimentary rocks next to rivers, making its structures vulnerable to quakes that the mountains and megathrust will bring at some point in the future. Building codes and seismic upgrades are happening, but that work takes time. Preparation at a personal level, including stored water, emergency kits, and participation in events like the Shakeout drill (http://shakeoutbc.ca/) are absolutely essential for anyone living in this city, or in any other similar city in a fault-prone region.This piece written at the request of a Vancouver Resident who messaged our page, hope I did it justice. If you have a topic request, please send it to the page, we take requests and photographer submissions as well!-JBBImage credit: Tim Shieldshttps://flic.kr/p/9wRJYfReferences:http://bit.ly/16WgbpWhttp://bit.ly/1fUHEv2http://bit.ly/1wohsAMhttp://bit.ly/1OO826Ihttp://bit.ly/1IilWfFhttp://link.springer.com/article/10.1007/s11069-011-9958-6http://bit.ly/1IWgjHthttp://fas.org/irp/imint/docs/rst/Sect2/Sect2_1b.htmlhttp://bit.ly/1OO8xNUhttp://bit.ly/1eQZGi9http://bit.ly/1TJyIJhhttp://www.aema.alberta.ca/catastrophic_earthquake_in_british_columbiahttp://bit.ly/1g5QV4Mhttp://corelab-www.oce.orst.edu/earthquakes.htmhttp://www.gac-cs.ca/publications/VancouverGeologyMap.pdfhttp://bit.ly/1Hr35Lv -- source link
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