RD

Earthquakes Lecture Review

Earthquakes: An Overview

  • Earthquakes are caused by the movement of parts of the Earth, often occurring when rocks break and slip along a fault, a crack in the Earth's surface.

    • It is important to note that parts of the Earth are always moving, usually at rates too slow for humans to perceive.

  • Aftershocks are the tremors felt for hours or days following the primary earthquake event.

The Structure of the Earth

  • The Earth consists of several layers with distinct properties:

    • The Crust: The outermost layer, composed of large rocks broken into sections known as tectonic plates.

    • The Mantle: Lies beneath the crust and is partially made up of molten rock.

  • Tectonic plates move at very slow rates, typically between 2 to 12 cm/year.

Tectonic Plates and Their Movement

Types of Plate Movements

  • Tectonic plates can:

    • Separate: Moving away from each other.

    • Collide: Bumping into each other, which can cause earthquakes.

    • Slide past: Moving adjacent to one another, also resulting in stress build-up.

Earthquake Generation

  • When tectonic plates meet, they can create significant pressure below the surface.

    • This pressure continues to build until it exceeds the friction holding the rocks together, leading to a sudden release.

    • The release of energy results in seismic waves, which are the vibrations felt as an earthquake.

Causes of Earthquakes

Natural and Human-Induced Sources

  • Natural Sources:

    • Volcanic Eruptions: Can release large amounts of energy, leading to earthquakes.

  • Human Activities:

    • Activities such as mining, dam creation, and nuclear explosions can trigger minor earthquakes, typically felt only near the activity site.

Effects of Earthquakes

Energy Release During an Earthquake

  • Energy released during an earthquake manifests as:

    • Movement along faults

    • Heat generated from friction

    • Seismic waves that radiate outwards, causing ground shaking

  • The term seismic comes from the Greek word seismos, meaning "a shaking".

Seismic Focus and Epicenter

Definitions

  • Hypocentre (Focus): The underground point where the earthquake originates, typically found between 1 to 100 km deep in Canada.

  • Epicentre: The point directly above the focus on the Earth's surface.

Factors Affecting Ground Motion

  • Ground motions caused by seismic waves depend on:

    • The depth of the focus

    • The type of local rock

    • The magnitude of the earthquake

Faults

  • Faults: Breaks in the Earth’s crust formed under pressure from moving tectonic plates.

  • Faults can be classified based on their sizes and lengths, ranging from a few centimeters to hundreds of kilometers.

Measuring Earthquake Magnitude

Definition of Magnitude

  • The magnitude measures the energy released during an earthquake, with the Moment Magnitude Scale being the current standard for measurement.

Historical Context

  • The largest recorded earthquake occurred in southern Chile in May 1960, with a magnitude of M_w = 9.5.

  • The second largest was in Anchorage, Alaska, in March 1964, with a magnitude of M_w = 9.2.

Scale Significance

  • Each full step on the Moment Magnitude Scale (e.g., from Mw = 8 to Mw = 9) represents a 32-fold increase in earthquake size.

Earthquake Monitoring in Canada

  • Canada began systematic earthquake recording in the late 1890s.

  • The Geological Survey of Canada currently operates around 220 seismograph stations for monitoring seismic activity.

Earthquakes in Canada

General Statistics

  • Canada experiences approximately 4000 earthquakes annually, with only about 50 being strong enough to be felt.

  • From 1900-2000, only around 20 recorded earthquakes caused significant damage.

Regions of Activity

  • The most earthquake-prone area in Canada is the Pacific Coast, followed by Eastern Canada. Saskatchewan and Manitoba are least affected.

  • The largest historic Canadian earthquake occurred off Vancouver Island on January 26, 1700, with an estimated magnitude of 9, noted in both First Nations oral histories and distant tsunami records in Japan.

  • The largest recorded earthquake by seismograph in Canada took place off Haida Gwaii on August 22, 1949, with a magnitude of 8.1, felt across BC, the Yukon, and into the United States.

  • Due to the presence of active faults, the West Coast of Canada experiences earthquakes predominantly in three major zones linked to plate movements.

Eastern Canada Earthquake Activity

Geographic Stability

  • Eastern Canada is situated in a stable region of the North American Plate, notably distant from major plate boundaries, which explains the relative infrequency of earthquakes.

  • Seismic activity can still occur due to stress built up within the plate or from postglacial rebound, which results from the land pushing back after being depressed by glaciers.

    • Postglacial rebound: Occurs when ice sheets melt, allowing the mantle to flow back and lift the crust to its original position.

Seismic Zones in Eastern Canada

  • Montreal is part of the Western Quebec Seismic Zone, with seismic activity concentrated along the Ottawa River and the Montreal-Maniwaki axis.

  • On average, about one earthquake every five days occurs in the Western Quebec Seismic Zone.

    • Notable earthquakes included:

    • Montreal in 1732 (magnitude 5.8)

    • Temiscaming in 1935 (magnitude 6.2)

    • Cornwall/Massena region in 1944 (magnitude 5.6)

    • Saguenay in 1988 (magnitude 5.8) causing over $25 million in damages.

Recent Earthquake Incidents

  • On June 23, 2010, a magnitude 5.0 earthquake impacted south of Echo Lake, QC, with effects felt across several northeastern US states.

  • Global instances of large earthquakes include:

    • The January 12, 2010, Haiti earthquake of magnitude 7.0, leading to 230,000 deaths and significant infrastructural damage.

    • The December 26, 2004, Indian Ocean earthquake, measuring 9.1, which resulted in a massive tsunami and devastation for millions.

Preparing for Earthquakes

Mitigation Strategies

  • Seismologists use geological data to predict potential earthquake-prone areas (not precise timing).

  • The National Building Code of Canada ensures that buildings are designed to withstand seismic forces.

Emergency Preparedness

  • Residents should check local emergency management organizations for earthquake risk and develop personal emergency plans.

  • Natural Resources Canada recommends preparing an emergency kit for survival over 72 hours in anticipation of potential earthquake events.

Acknowledgments and Sources

  • This material was developed under the guidance of Drs. Olivia Jensen and William Minarik, Earth and Planetary Sciences at McGill University, and with inputs from various resources including Natural Resources Canada and academic institutions.

  • Funding for the research and creation of these materials was provided through the PromoScience program of NSERC.