NSTA 2015 MIQuakes presentation and activity Part 2

1. 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 Earthquake
North American Plate
Eurasian Plate
Map created using the IRIS Earthquake Browser: www.iris.edu/ieb
Charlie-Gibbs
Fracture Zone
Magnitude 7.1 NORTHERN MID-ATLANTIC RIDGE
Friday, February 13, 2015 at 18:59:12 UTC

2. 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 RIDGE
Friday, February 13, 2015 at 18:59:12 UTC

3. Teachable Moments are a service of
IRIS Education & Public Outreach
and
The University of Portland
Magnitude 7.1 NORTHERN MID-ATLANTIC RIDGE
Friday, February 13, 2015 at 18:59:12 UTC

4. AS-1 Seismometer

5. Seismic Event Recording

8. 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

9. • 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

13. 2012-04-11
OFF W COAST OF
SUMATRA
O = 08 38 36; Δ ~ 135°

14. 0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
0 20 40 60 80 100 120 140 160 180
AverageVelocity(km/sec)
Distance (Geocentric Degrees)
Average Chord Velocities