Comprehensive Study Notes on Earthquakes and Tectonic Hazards

Causes and Mechanisms of Earthquakes

• Tectonic Movements and Faulting:

  • Earthquakes are driven by mechanical stresses resulting from tectonic movement along fracture zones in the Earth's crust known as faults.

  • Fault displacements are classified based on movement direction:

    • Dip-Slip Faults: Feature vertical displacement along the inclination of the fault plane (e.g., normal or reverse faults).

    • Strike-Slip Faults: Feature horizontal, lateral displacement parallel to the strike of the fault plane.

    • Oblique-Slip Faults: Feature a combined movement involving both horizontal (strike-slip) and vertical (dip-slip) displacement components.

  • Faulting is predominantly concentrated along tectonic plate boundaries.

• Anthropogenic and Magmatic Causes:

  • Hydraulic Fracturing (Fracking) and Fluid Injection: Earthquakes can be induced by human activity through high-pressure fluid injection into deep subsurface rock layers during oil and natural gas extraction.

  • Subsurface Fracture Dynamics: In regions like Texas and Oklahoma where active faulting was historically rare, continuous pumping of high-pressure fluids increases pore pressure, fracturing the subsurface rock. This process has generated unprecedented regional earthquakes ranging from magnitude $2.0$ and $3.0$ up to magnitude $4.0$

  • Magma Movement: The upward force and subsurface migration of magma induce stress shifts and rock fracturing, generating localized seismic swarms.

Anatomy of an Earthquake and Seismic Wave Properties

• Structural Anatomy:

  • Focus (Hypocenter): The subterranean point of origin where the initial physical rupture and rock fracture occur.

  • Epicenter: The position on the Earth's surface situated directly perpendicular to (above) the subterranean focus.

  • Fault Line: The surface or subsurface line depicting the trace of the fault plane along which displacement takes place.

  • Fault Scarp: The exposed vertical or step-like offset on the Earth's surface created by surface displacement along the fault line.

• Seismic Wave Characteristics:

  • Primary Waves ($P$-Waves):

    • Velocity: The fastest seismic waves generated by a rupture.

    • Medium Propagation: Capable of propagating through both solid rock and liquid media.

    • Motion Dynamics: Compressional waves characterized by alternating compression and expansion along the wave's directional axis (similar to the cone movement of an audio subwoofer speaker).

    • Early Warning Applications: Because $P$-waves travel faster than other seismic waves, real-time warning applications detect them to provide a lead time of $5\text{--}30$\,seconds before the arrival of destructive shear waves.

    • Automated Safety Systems: In California, automated natural gas shutoff valves connected to $P$-wave sensors automatically close residential gas valves before destructive waves hit, preventing gas line ruptures, fires, and explosions.

  • Secondary Waves ($S$-Waves):

    • Velocity: Slower than $P$-waves, arriving second at recording stations.

    • Medium Propagation: Propagate strictly through solid media; they cannot travel through liquids.

    • Motion Dynamics: Shear waves that displace ground material side-to-side and up-and-down perpendicular to the direction of propagation.

    • Impact: Responsible for the majority of structural damage during an earthquake.

  • Surface Waves:

    • Velocity and Motion: Travel along the Earth's surface and typically exhibit high wave periods.

    • Impact: Produce complex rolling surface motion that creates significant ground deformation.

• Triangulation of Epicenters:

  • Methodology: The origin point of an earthquake epicenter is located by measuring the time lag between the arrival of initial $P$-waves and subsequent $S$-waves at different recording locations.

  • Triangulation Requirements: Requires data from a minimum of three independent seismograph stations (e.g., stations located in Minneapolis, Detroit, and Charleston).

  • Mechanism: The time interval between $P$-wave and $S$-wave arrivals at each station calculates the exact distance from that station to the epicenter. Drawing a circle corresponding to that distance around each station reveals the exact epicenter where all three circles intersect.

Major Historical Intraplate Earthquakes

• The New Madrid Earthquake Sequence ($1811\text{--}1812$):

  • Overview: Represents one of the largest series of earthquakes recorded in the contiguous United States.

  • Timeline and Magnitude: Consisted of three primary quakes in December $1811$, January $1812$, and February $1812$. The February $1812$ event was the largest, approaching a magnitude of $8.0$

  • Tectonic Context: Occurred within the interior of the North American tectonic plate along the New Madrid Fault Line, a region devoid of well-known active boundary faults.

  • Geomorphic and Physical Effects:

    • Low populated settlement in the early $19^{\text{th}}$ century resulted in minimal structural property damage.

    • Extensive shaking caused the Mississippi River to reverse its flow direction for a duration lasting from a few hours to a full day.

    • Triggered widespread liquefaction along the low-lying terrain of the Mississippi River basin.

    • The New Madrid Fault Zone has exhibited no major seismic activity since $1812$

Primary and Secondary Earthquake Hazards

• Landslides, Avalanches, and Mass Wasting:

  • Seismic shaking destabilizes steep slopes, severing transportation routes like railroad tracks.

  • Glacial Seismic Events: A massive glacier collapse along the Nepal-China border sent ice falling $7{,}000\,\text{ft}$. The impact generated ground shaking equivalent to a magnitude $5.2$ earthquake on regional seismometers and produced a massive flood wave downstream.

• Soil Liquefaction:

  • Mechanism: Occurs in low-lying relief areas with water-saturated, uncompacted soils. Repeated seismic shaking breaks grain-to-grain contact among soil particles while water pressure surges, transforming solid ground into a liquid slurry.

  • San Francisco ($1989$ World Series Earthquake): Artificial landfills along San Francisco Bay liquefied, causing severe structural failure, including a four-story apartment building that sank two full stories into the ground.

  • Good Friday Earthquake (Early $1960\text{s}$): Triggered widespread liquefaction and ground slumping in Turnagain Arm near Anchorage, Alaska.

• Tsunamis and Subduction Zone Dynamics:

  • Generation Mechanism: Most major oceanic tsunamis originate at subduction zones. As an oceanic plate subducts beneath a continental plate, friction locks the boundary, bending the overriding continental plate downward. When the lock ruptures, the continental plate snaps back upward, displacing a massive volume of ocean water.

Case Study: The 2011 Tohoku Earthquake and Tsunami (Fukushima)

• Seismic Event Parameters:

  • Date and Location: Occurred on March 11, 2011 ($03/11/2011$), centered approximately $80\,\text{miles}$ off the coast of Northern Japan.

  • Magnitude: Magnitude $9.0$; the largest quake ever recorded in Japan and one of the five largest in global history.

  • Human Toll: Resulted in over $28{,}000$ individuals confirmed dead or missing.

• Tsunami Propagation Dynamics:

  • Deep Water Behavior: Waves expanded outward in all directions, traveling up to $500\,\text{mph}$ with heights of only a few feet, making them largely undetectable to ships in deep ocean water.

  • Shallow Water Behavior: As the waves entered shallow coastal waters, seafloor friction reduced the wave velocity while forcing the wave height to surge upward.

  • Inundation: Tsunami waves penetrated up to $3\,\text{miles}$ inland across Japanese coastal regions, forming a debris-laden liquid slurry ("tsunami soup") carrying vehicles, building debris, structural rubble, and human remains.

  • Surface Displacement Velocity: GPS data showed displacement waves moving at speeds up to $5\,\text{miles/second}$. Waves surged upstream into rivers, such as the Naka River in Hitachi-Naka City.

• Coastal Engineering and Historical Warnings:

  • Seawall Overtopping: In Kamaishi City, an $18\,\text{ft}$ seawall—the world's deepest and largest seawall—was completely overtopped and destroyed by the tsunami surge.

  • Monitoring Data: Geoscientists at the University of Alaska processed spatial data from Japan's network of over $1{,}200$ high-precision GPS stations to map the displacement waves.

  • Historical Warning Markers: Ancestral stone markers throughout the coastal hills carried ancient warning inscriptions: "High dwellings are the peace and harmony of our descendants. Remember the calamity of the great tsunamis. Do not build any homes below this point." Centuries later, modern communities built homes below these markers behind artificial seawalls.

Case Study: The 1992 Landers and Big Bear Earthquake Sequence

• Seismic Event Overview:

  • Date and Location: Occurred in June $1992$ in Southern California as a magnitude $7.2$ (or $7.0$) quake in Landers, followed approximately one hour later by a magnitude $6.0$ earthquake in Big Bear.

  • Tectonic Location: Located along the Eastern Mojave Rift Zone (Eastern Mojave Shear Zone) rather than the main San Andreas Fault plane. The secondary event occurred along the separate Big Bear Fault.

• Fault Geomorphology and Field Observations:

  • Displacement Dimensions: Surface rupture showed a vertical uplift of approximately $2\,\text{m}$ and a horizontal offset of approximately $6\,\text{m}$

  • Fault Classification: Identified as an oblique-slip fault, showing concurrent horizontal strike-slip movement ($\sim 6\,\text{m}$) and vertical dip-slip movement ($\sim 2\,\text{m}$).

  • Scarp Features: Exposed clay and soil faces exhibited distinct fault striations indicating the precise angle and direction of fault movement. Field sites displayed double fault scarps and overhanging reverse scarps.

  • Rupture Extent: Produced a surface rupture extending over $68\,\text{miles}$ from Joshua Tree National Monument north toward Barstow, California, making it the longest surface rupture recorded in North America up to that time.