Earth 375 - Midterm #2

Juan de Fuca Plate System (JdF)

Comprised of:

-Juan de Fuca plate

-Explorer Plate

-Gorda Plate

*includes all types of margins: spreading, subduction & transform

JdF Location

Located between Queen Charlotte Fault & San Andreas Fault

*right lateral transform faults

Crustal EQs

-occur in both oceanic & continental plates caused by deformation due to convergence

-smaller EQs in ocean crust

Intra-Slab EQs

-AKA Wadati-Beneioff EQs

-Occur within the subducting plate

Inter-plate EQs

-occur on the shear interface between subducting and over-riding plate

Megathrust EQs Evidence

-Coastal marsh soil profiles

-Drowned coastal forests

-Seabed turbite layers

-Japanese tsunami records

-First Nations records

Coastal Marshes

-Soil profiles show repeating sequence of layers:

1. Peat

2. Mud

3. Sand

Drowned Forests

-areas of dead cedar trees found along the west coast of Washington and Oregon

Seabed Turbite Layers

-sediment cores collected at base of the continental shelf showing alternating layers of fine mud and coarse sand.

Megathrust EQ Cycle

What happens?

1. Inter-seismic period: elastic deformation builds in over-riding plate between EQs.

   *”toe” of plate dragged down

   *uplift at coast, decreasing inland

   *crustal shortening

2. Co-seismic period: motion during EQ

   *”toe” of plate jumps up, rupturing seabed initiating tsunami

   *coastal subsidence

   *crustal extension

Geodetic Measurements

-accurate elevation and position measurements over time (GPS, surveying) indicate uplift and shortening that agree with deformation phase of EQ cycle.

Megathrust Hazard

-Paleoseismic evidence of 13 past megathrust EQs at CSZ

Seismic Waves

-seismometer: instrument that detects vibrations in the earth (ex. seismic waves initiated by EQ fault rupture)

-seismograph: instrument that records vibrations detected by seismometer

*modern seismographs provide digital measurements of 3D motion: N-S, E-W and vertical

Wave Properties

1) Function of Position (time-fixed snapshot):

-amplitude: maximum value

-wave length: distance of one wave cycle

-velocity: speed a point on the wave moves

2) Function of Time (watch wave at a fixed position):

-Period (T): time of one cycle

-Frequency (1/T): # of cycles per second

-long wavelength = long T and short frequency

-short wavelength = short T and high frequency

Wave Types

1. Body Waves: propagate through interior of the earth (compressional and shear waves)

2. Surface Waves: propagate along surface of earth (Rayleigh waves, love waves)

   -generally larger amplitude

Body Waves - Compressional Waves (P)

-direction of oscillation (movement of particles) is parallel to propagate direction

-propagate through solid, liquid & gas

-fastest seismic wave

-velocity increases with density and resistance to compression

Body Waves - Shear Waves (S)

-particle motion perpendicular to propagate direction

-velocity increases with density and resistance to shearing

Seismic Velocity Structure

Velocity of P & S waves increase through mantle, except for low-velocity zone in asthenosphere

Surface Waves - Rayleigh Waves (Lr)

-Particle motion is retrograde elliptic (opposite to propagation at top of ellipse) *boat on waves

-long wavelength wave

-Slow

-Only propagate through solids

-N→S

Surface Waves - Love Waves (Lq)

-Horizontal particle motion perpendicular to propagation direction

-Damaging to buildings which are built to withstand vertical stress, not horizontal

-only propagate through solids

-faster than Rayleigh waves

-long wavelength wave

-E→W

EQ Intensity (Modified Mercalli Intensity -MMI)

-Intensity: assess local EQ effects (what ppl feel, what damage is done)

-Intensity depends on: Magnitude, distance to hypocentre, local soil/rock type)

Logarithmic Scale

-Magnitude increases by 1 leads to 10x increase in shaking.

-Energy increases even more quickly with M

Magnitude Scales

-Attempts to measure EQ energy released

-4 magnitude scales:

1) Richter Magnitude

2) Surface Wave

3) Body Wave

4) Moment M (based on measurements of properties at EQ source)

Richter M

used only for small EQs

Surface Waves

not good for EQs bigger than 50km depth

Moment Magnitude

-large EQs

-doesn’t saturate (good for all EQs) but more difficult to measure than other scales

Saturation

-Problem: for very large EQs with large fracture areas, an increase fraction of seismic energy is radiated at lower frequencies and over a longer time.

-scales can miss this increase and saturate.

-M(L) saturates at M6

-M(b) saturates at M6.5

-M(S) saturates at M8.5

Site Response

-EQ fatalities from fault rupture are rare → most occur due to ground shaking

-3 factors that depend on the local geology increase seismic hazard due to shaking:

1) soil liquefaction

2) amplification

3) resonance

Soil Liquefaction

-intense/prolonged shaking of water saturated sandy soil can increase water pressure, cause grains to lose contact and float in water

-can affect: building sink, tilt or collapse, people sink, landslide

-structure built on potential liquefaction sires require deep pilings as support

Amplification

-low velocity , low density (soft) rock/soil layers increase amplitude of seismic waves

-seismic wave velocity depends on material (faster through hard rock, slower through soft rock and soil)

-waves passing from hard to soft materials slow down and amplitude increases for wave to carry energy

-soft soil = bad

Resonance

-fraction of seismic energy “trapped” in layer by successive reflections

-energy increases as later waves enter

-wavelengths that are trapped and later entering waves are in phase and are amplified (resonance)

building resonance should not match soil resonance

Building design - safety levels

• no major damage in minor EQ

• no structural damage in moderate EQ

• no collapse in largest EQ

Building considerations

-avoid slopes, and soft soil

-don’t have different materials under 1 foundation

-foundation: bolt structure to foundation to avoid slip in horizontal shaking

-wood is a good material

-base isolation: devices on ground or within structure to absorb EQ energy

-box shape is best

San Andreas Fault

Sections of the fault have different:

-locked sections that produce large EQs

-creeping sections that release slowly over time by small EQs or constant motion

1964 Alaska EQ

-megathrust cycle

Magma & Lava

Magma: melted rock within earth

Lava: melted rock on earth’s surface

Plutonic Rock: magma solidified below surface

Volcanic Rock: lava solidified above surface

What does magma contain?

Dissolved gases (volatiles) with gas solubility increasing with pressure and decreasing with temperature

-water (steam) is the most abundant dissolved gas

Viscosity

Magma viscosity depends on:

-Temperature

-Mineral crystal content (increased viscosity)

-Silica content (increases viscosity)

What does eruption explosiveness depend on?

1. Magma Viscosity

2. Amount and ease of release of dissolved gas in magma

Eruption Types (Viscosity)

-Low viscosity, easy gas escape: peaceful eruptions

-High viscosity, difficult gas escape: explosive eruptions

Magma Types

1. Basalt: fist crystallizing minerals (highest melt point) more peaceful

2. Andesite: intermediate (middle melting point)

3. Rhyolite: last crystallizing minerals (lowest melting point)

Volcanic Eruption

-begins with heat at depth (heated rock rises, pressure reduces causing decompression melting)

-reduced pressure allows dissolved gas to form bubbles propelling magma upwards

-bubble volume may overwhelm magma, fragmenting it into pieces which explode out as gas jet

Volcanoes & Plate Tectonics

-Volcano type depends on tectonic setting

-90% of volcanism at plate tectonics

-10% at hotspots

-difference is primarily due to magma type

• subduction zones: explosive eruptions, rhyolitic magma

• transform: little to no volcanoes

• spreading: peaceful eruptions, basaltic magma

MOR Volcanism

-80% of all magma produced at MORs

-peaceful eruptions/no hazard

-as hot plates diverge:

• hot plastic (solid) astheno rises to fill gap (basaltic magma)

• pressure reduced

• melt increases

Continental Subduction Zones

-10% of all magma

-subducting oceanic plate carries water saturated sediments into hot astheno

-by approx 100km, heated plate releases water which lowers melting point of mantle material, straps steam.

-hot material rises through continental crust, melting low melt point components → changes magma composition

-produces andesitic-rhyolitic volcanism with viscous magma, explosive eruptions, high silica ash, stream

Other Tectonic Boundaries

-oceanic subduction zones: somewhat less explosive than continental since magma rises through oceanic crust (thinner and silicate content than continental)

-collision zones: no volcanism (crust is not subducted)

-transform: no volcanism

Hot Spots

-account for 10 % of all magmas

-location of mantle plumes of rising hot, partially molten material

Oceanic Hot Spots

-similar to MORs

-hot basaltic magma upwells through oceanic crust

-results in smooth, low viscosity magma flow and peaceful eruptions

-Iceland & Hawaii

Continental Hotspots

-similar to continental subduction zones

-partially melts crust, high SiO2 content, cooling

-explosive volcanism

Eruption Types: Icelandic

creates low, wide volcanic plateaus (ex. Iceland hotspots)

Eruption Types: Hawaiian

-peaceful lava outpouring (slightly more viscous than Icelandic)

-energy gas release, forms lava fountains

-forms high shield volcanoes

-ex.: Hawaii located on oceanic hot spot

Eruption Types: Strombolian

-increase magma viscosity, explosive eruptions but not powerful enough to break volcanic cone

-ex.: Stromboli

Eruption Types: Vulcanian

-alternates between viscous magma flows and explosions throwing rock and ash

-often first phase for more violent eruptions

-often characterized by strato or composite volacanoes

-ex. Vulcano

Eruption Types: Plinian

-powerful eruption of gas, ashes and rock to great heights

-most violent eruption type →common final phase of major eruptions

-typically 2 or 3 per century

Volcanic Explosivity Index (VEI)

-eruptions range from icelandic and hawaiian to plinian

-eruption intentisity measured (0-8) based on:

• volume of material erupted

• height of erupted colum

• duration of eruption

Volcanic Hazards

-ash falls

-pyroclastic flows

-lahars

-volcanic landslides

-volcanic tsunamis

-lava flows

-volcanic gases

Ash Falls

-dust-sized ash forms cloud above volcano, drifts with wing and falls to ground

-can blanket wide areas

-not usually immediately life threatening but can cause difficulty breathing, make agricultural land and water unusable, etc.

Pyroclastic Flows

-also called hot ash flows

-high speed avalanche of volcanic fragments suspended in gases (acts like fluid)

-most destructive volcanic process →destroys everything in its path

Lahars

-also called mud flows

-volcanic fragments (mud, ash, sand and rock) suspended in water

-can be hot or cold, very fast

-water from melting ice or snow on volcanoes

Volcanic Landslides

-many volcanoes are built on successive layers of lava and ash (composite or strato volcanoes)

-structurally weak and prone to landslides

-explosive eruptions and EQs can trigger landslides

Volcanic Tsunami

Caused by:

-violent underwater eruptions (not MOR)

-volcanic landslides into bodies of water

-caldera collapse

Lava Flows

-not immediately life threatening (unless very low viscosity) but can destroy buildings and roads

Volcanic Gases

-most common→H20

-high concentrations of C02 can displace 02 at low elevations, leading to asphyxiations

-can include fluorine and Sulphur → bad for agricultural lands

Cascade Volcanism

-Cascade volcanoes extend length of CSZ

-more than a dozen potentially active

-recent major eruption: Mt. St. Helens

-typically tall strato-volcanoes with large ice/snow cover

-explosive eruptions with andesitic-rhyolitic magma

-hazards: landslide, lahar, pyroclastic flow, ash fall

Mt Rainer

-greatest threat in Cascades

-great height

-ice/snow cover

-frequent EQs

-active hot springs weaken mountain

Mt Shasta

-2nd tallest cascade volcano

Mt St Helens

-most active volcano

-1980 eruption:

• most deadly and $$ in US history

• hazards involved: landslide, pyroclastic flow, lahar, tsunami, ash fall

Mt St Helens (Pre Eruption 1880s)

-volcano developed conical shape

-Si02 rich lava dome grew at peak

-andesitic lava flows on high slopes

*these events causes weaknesses in volcanic core

Mt St Helens (Pre Eruption 1980s)

-rising magma within mountain caused bulge to grow 1.5m daily

-many small EQs and steam eruptions due to magma movements

Mt St Helens Eruption

-Landslide

-Lateral Blast (unexpected)

-Pyroclastic Flow

-Lahars

-Ash Fall

*Plinian Eruption

BC Volcanoes

1) Subduction (Garibaldi Volcano)

2) Hot Spot (Nazko Cone)

3) Continental Rifting (Tseax Cone)

Garibaldi (cascade range)

-strato-volcano

-lava pooled behind ice dam, which melted leaving steep face of volcanic rocks (barrier)

Nazko Cone

-youngest volcano in the Anahim hot spot chain

Monitoring & Warning

-define normal vs significantly changed behaviour

-scientists monitor: seismicity, gas emissions, ground deformation & gorund temp

-magma chamber indications: increase activity, heating/inflation of ground, gas escape increase