Magnitude 9: The Largest Earthquakes on Earth Happen in BC
Simon Fraser University・79 minutes read
Professor John Cay discusses the history of earthquakes on the west coast of Canada, highlighting the seismic activity and the impact of plate tectonics theory on geology and understanding seismic events. The presentation emphasizes the importance of scientific progress in studying seismic events and the need for collective action to ensure safety against earthquakes and related risks in vulnerable areas like Vancouver.
Insights
- Earthquakes on the west coast of Canada are influenced by fault area and displacement, leading to significant seismic activity and historic earthquakes causing substantial damage, emphasizing the region's seismic risk.
- Basaltic rocks on ocean floors record Earth's magnetic field shifts every 5,000-10,000 years, showing alternating normal and reverse polarity bands that help understand Earth's history and geology.
- Geological records indicate an average recurrence of magnitude nine earthquakes every 5 to 600 years in the region, necessitating improved dating methods, mapping of fault interfaces, and understanding precursor signals for impending earthquakes to enhance preparedness and safety measures.
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Recent questions
What is the focus of Professor John Cay's talk?
The history of seismic events in the region.
How do plate tectonics revolutionize geology?
By providing a better understanding of earthquakes.
How do basaltic rocks record Earth's magnetic field?
By cooling and crystallizing on ocean floors.
What geological evidence indicates past earthquakes?
Sharp contacts between peat and mud layers.
How do tidal marshes provide clues about earthquakes?
By exhibiting peat-mud couplets as a geological barcode.
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Summary
00:00
"Earthquake History and Seismic Risk Analysis"
- Week five of the Deep Time Global Change series features Professor John Cay, a renowned expert in natural hazards and geology.
- Professor Cay has held prestigious positions at SFU and the Geological Survey of Canada, with a background in geology and archaeology.
- His talk focuses on the history of the region over the past two million years, particularly on the largest earthquakes occurring on the coast.
- The magnitude of earthquakes depends on fault area and displacement, with the west coast of Canada experiencing significant seismic activity.
- Historic earthquakes in British Columbia, like the 1946 Vancouver Island earthquake, have caused substantial damage and are a concern for seismic risk.
- The plate tectonic paradigm, established 50 years ago, revolutionized geology and led to a better understanding of earthquakes.
- Earthquakes occur due to the slippage of Earth's crust along faults, with seismic activity concentrated on the west coast and a linear trend of earthquakes to the north.
- Notable earthquakes in the region include the 1949 Haida Gwaii earthquake and the 1946 Vancouver Island earthquake, with varying levels of damage and energy release.
- The presentation highlights the shift in understanding seismic events, from smaller historic earthquakes to the realization of rare but massive magnitude 9 earthquakes.
- The talk emphasizes the importance of scientific progress and building upon predecessors' knowledge, exemplified by the groundbreaking work of Henry Hess and subsequent researchers in plate tectonics.
17:20
Earth's Magnetic Field Shifts Every 5,000 Years
- Earth's magnetic field undergoes a 180-degree switch in history, documented previously.
- Basaltic rocks on ocean floors record Earth's magnetic field when they cool and crystallize.
- Magnetometers are used to measure the magnetic polarity of basaltic rocks on ocean floors.
- Earth's magnetic field undergoes a 180° shift every 5,000-10,000 years due to metal convection in the core.
- Basalts on ocean floors show alternating normal and reverse polarity bands symmetrically about ridges.
- Seafloor spreading away from ridges records normal and reverse polarity bands like a tape recorder.
- The spreading and creation of crust at mid-ocean ridges is recorded by the polarity of basalts.
- Subduction zones consume new crust created at ridges, preventing Earth's expansion.
- Subduction zones, like off the coast of British Columbia, lead to the biggest earthquakes on Earth.
- Plate tectonics theory includes spreading centers, subduction zones, and transform faults like the San Andreas Fault.
35:18
Tidal muds reveal earthquake history in Washington.
- Tidal muds deposited in lower tidal zones resemble a couplet seen in earthquakes, with a marsh surface from 1700 AD dropping 2 meters and being covered by tidal muds.
- Brian discovered this phenomenon in the Nisqually River Estuary in Washington, then searched the coast for similar evidence to understand the spatial pattern of subsidence.
- Before an earthquake, a tidal marsh backed by a coastal forest experiences subsidence, while the land behind rises, causing the marsh to be covered by tidal waters and muds post-earthquake.
- The presence of sharp contacts between peat and mud layers, along with rooted marsh plants buried by tidal muds, indicates sudden burial events linked to earthquakes.
- The "Ghost Forest" of the Copalis River in Washington reveals trees rooted 2 meters below the surface, associated with the 1700 AD forest that dropped into the tidal zone during the earthquake.
- Tidal marshes in Washington exhibit a pattern of peat-mud couplets, acting as a geological barcode across British Columbia, Washington, Oregon, and California, indicating earthquake occurrences.
- Geological records show seven earthquake events in the last 3,500 years, with an average recurrence of magnitude nine earthquakes every 5 to 600 years, supported by deep-sea records spanning 10,000 years.
- The 1700 AD earthquake was pinpointed by Japanese scientists through tree rings and carbon dating, revealing a tsunami that struck Japan without ground shaking, originating from North America.
- Geophysicists use precise technology like satellite GPS to monitor ground movements, showing strain accumulation between Vancouver and Victoria due to the locked plate boundary, hinting at a future earthquake.
- Future research aims to map the locked fault interface accurately, improve dating methods for past earthquakes, understand the non-random clustering of earthquakes, and explore precursor signals for impending earthquakes.
52:43
Earthquake impacts on infrastructure and safety
- Geological traces like liquefaction and landslides give clues about earthquake impacts on infrastructure.
- Better estimates of damage from future earthquakes are sought.
- Canadian building codes are updated to consider seismic events on the West Coast.
- Risk of damage is higher from more frequent smaller earthquakes than rare large ones.
- Local seismic sources like sedimentary basins can amplify ground motions.
- No global correlation between large earthquakes in different parts of the world.
- Atwater's study on ground subsidence in Japan as a precursor to earthquakes is controversial.
- Animals' behavior changes before some earthquakes, but not all.
- Critical infrastructure in Vancouver needs better protection against earthquakes.
- Vulnerability to floods, sea level rise, and earthquakes in Vancouver necessitates collective action for safety.
01:10:35
Earthquake Risks in Vancouver Island Region
- Vancouver Island is located beneath the Strait of Georgia and north of a complex tectonic environment known as the North End of the Wuka Plate.
- Seismic waves from earthquakes can reverberate through the Basin beneath the Strait of Georgia, causing varying intensities of shaking due to geological factors like sediment thickness and topography.
- Differences in ground shaking during earthquakes can vary significantly within urban areas, with up to three to five times differences in ground acceleration based on subsurface geology and earthquake assumptions.
- Hydraulic fracturing, like in the Horn River area in northeastern BC, has induced small earthquakes, with ongoing research to assess potential risks to infrastructure and reservoirs.
- Gravitational forces from the Sun and Moon can potentially trigger earthquakes but cannot be the sole cause, requiring tectonic stresses for earthquake production.
- The expansion of tank farms, like Kinder Morgan's on Burnaby Mountain, should consider seismic stability, prompting public debate and questioning of provisions for earthquake risks.
- Liquefaction during earthquakes causes localized subsidence and cracking in the ground, affecting subsurface infrastructure like gas and water lines, with Richmond being susceptible to liquefaction damage.
- Liquefaction occurs below the water table in granular, water-saturated soils, causing sand venting and cracking, potentially impacting subsurface infrastructure.
- The determination of locked zones is based on surface deformation patterns and temperature thresholds, with rocks becoming less brittle at higher temperatures, affecting strain storage along fault interfaces.
- The southern end of the subduction zone, near the San Andreas fault, experiences more frequent and varied earthquakes due to additional strains from right lateral strike-slip movements along the fault.
01:26:38
Earthquake Frequency and Size: Myths Debunked
- Earthquakes may not necessarily increase in magnitude with longer time periods between them, as indicated by a chart showing earthquake size over time.
- The method of determining earthquake magnitude using turbidites in the deep sea is considered a weak proxy by the speaker.
- Crustal faults, distinct from subduction quakes, can also generate significant earthquakes, with examples in Washington and Vancouver.
- Evidence of paleo earthquakes, such as land level drop and liquefaction, has been found in areas like Boundary Bay and Anastasia Island.
- Episodic tremor and slip events occur every 12 to 14 months, leading to micro earthquakes and surface deformation, potentially increasing the likelihood of larger earthquakes.
- The impact of changes in mass balance due to glacial retreat on earthquake frequency and size has not been thoroughly studied.
- Christchurch's experience with earthquake insurance and rebuilding highlights challenges in compensating for damages and obtaining permits.
- The speaker personally opts out of earthquake insurance due to high deductibles and the belief that most likely scenarios involve less catastrophic events.
- The Christchurch earthquake, despite not being of high magnitude, caused significant damage due to its shallow depth and direct energy transfer into the city center.
