Regional historical seismicity outlines the Mid-Atlantic Ridge System in the North Atlantic that forms the plate boundary between the North American Plate and the Eurasian Plate. While this spreading ocean ridge is offset by many transform faults, the Charlie-Gibbs Fracture Zone is one of the largest.

40 years of regional seismicity – most earthquakes plotted here were smaller than M 6.0

M 7.1 EarthquakeNorth American Plate

Eurasian Plate

Map created using the IRIS Earthquake Browser: www.iris.edu/ieb

Charlie-GibbsFracture Zone

Mid-

Atla

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Ridg

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Mid-Atlantic R

idge

Magnitude 7.1 NORTHERN MID-ATLANTIC RIDGEFriday, February 13, 2015 at 18:59:12 UTC

Following the earthquake, it took 9 minutes and 39 seconds for the compressional P waves to travel a curved path through the mantle from the epicenter to the station.

PP waves are compressional waves that bounce off the Earth’s surface halfway between the earthquake and the station. PP energy arrived 11 minutes and 43 seconds after the earthquake.

Surface waves, both Love and Rayleigh, travel the 6194 km (3849 miles) along the perimeter of the Earth from the earthquake to the recording station.

The record of the earthquake on the University of Portland seismometer (UPOR) is illustrated below. Portland is about 6194 km (3849 miles, 55.8°) from the location of this earthquake.

S and SS are shear waves that follow the same path through the mantle as P and PP waves, respectively.

P PP S SS

Magnitude 7.1 NORTHERN MID-ATLANTIC RIDGEFriday, February 13, 2015 at 18:59:12 UTC

Teachable Moments are a service of

IRIS Education & Public Outreach and

The University of Portland

Magnitude 7.1 NORTHERN MID-ATLANTIC RIDGEFriday, February 13, 2015 at 18:59:12 UTC

AS-1 Seismometer

Seismic Event Recording

Interdisciplinary: Earth Science, Physics and Mathematics

• Seismic waves behave in a similar manner to light waves; they reflect, refract, and diffract.

– how well the reflect and refract tell us about the material differences across a boundary since good reflections require good acoustic impedance (density x velocity) contrasts, and good refractions require good velocity contrasts between materials

Reflection and refraction of seismic waves in the Earth (paths and sample seismogram)

Refractions across North America from explosions in Lake Superior (1966)

Reflections from within a sedimentary basin

Diffractions from corners of buried fault blocks

• They provide direct information... • about material properties, because their velocities are

dependent on density and elastic properties of the Earth

•e.g., vP = √ [K + (4/3) μ ] / ρ for those of you who like equations, where ρ is density, μ is the shear modulus, and K is incompressibility.

• about Earth structure and processes, through quantitative analysis and modeling of waves (frequency, magnitude, triangulation, synthetics, tomography)

... both shallow and deep

A seismogram is the sum of an infinite series of periodic functions

Triangulation was used to locate earthquake epicenters in the past

Tomography allows us to find variations in velocity as a function of location

Time for the Activity!

2012-04-11OFF W COAST OF SUMATRAO = 08 38 36; Δ ~ 135°

0 20 40 60 80 100 120 140 160 1800.0

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14.0Average Chord Velocities

Distance (Geocentric Degrees)

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