Download 1/29/17 1 Lecture 9: Earthquakes

1/29/17
EARTHQUAKES AND TSUNAMI IN HAWAI‘I (GG103)
~50 years of
Hawai‘i seismicity
2006
1975
Lecture 9: Earthquakes
• Assignment for Friday
• Finish last class
• Earthquakes – how they
work
• Relation to Hawai`i
Notable eruption: 1950 SW rift eruption
Largest Mauna Loa historical eruption
Line of fire fountaining as source (20 km long)
Split into several “lobes”
Fast flowing large volume made ocean entry in ~4 hours
USGS
Hawaii Air National Guard
http://hvo.wr.usgs.gov/maunaloa/history/50_06_01/
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Last eruption: 1984 eruption
-Started at summit Moku‘āweoweo
-Quickly turned into an eruption along
a line (fissure)
-Eruption shifted to various
lines/fissures, progressively further down the rift zone
-Did not get to
Hilo, but the
next one might
http://hvo.wr.usgs.gov/maunalo
a/history/50_06_01/
Broken down in pulses
-Effectively 3 pulses
EFFECTIVE EFFUSION RATES
Effusion: flowing out (so
how fast lava appears)
1984 flows, ~2900 m vent
-As time progressed,
eruption rate dropped,
increase in flow length
became slower and
slower
Flow front positions from Lockwood et al. (1987)
Effusion rates from Lipman et al. (1985), Lipman & Banks (1987)
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Did we learn anything from this eruption?
Slow progress due to levee
breakdown & creation
of subparallel flows
This happened due to:
(1) relatively gentle slopes
(2) dense vegetation through
which the flows moved
(3) the relatively low temperature of the erupted lava, which
made it relatively viscous
(4) the gradual decline of eruption rates.
(From USGS website)
Model for activity and collapse
-Similar to Kilauea, idea: alternate eruptions between crater
and flank
-Crater collapse
occurs when
magma supply
wanes
-Renewed
activity may
partly fill crater
again
Diagram by Robin Holcomb,
from:
http://hilo.hawaii.edu/~kenhon/
GEOL205/kilauea/default.htm
Hazards at Mauna Loa
Similar to
Kilauea: crater +
rift zones are
most significant
http://pubs.usgs.gov/gip/hazards/mauna-loa.html
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Current activity at Mauna Loa
Seismicity: minor earthquakes in the past month on the
southwest flank of Mauna Loa. All recent earthquakes small
relative to earthquake sequences before eruptions in 1975, 1984.
Deformation: GPS data has been showing variable inflation rate
since the start of the current, renewed inflationary period.
Earthquakes & ground motion
What happens during an earthquake?
Kobe, Japan 1995
When an earthquake happens…
epicenter
hypocenter
Rocks break @ fault,
energy released as
seismic waves
Volcanoes also get
earthquakes from
moving magma
Hypo/epicenter:
Where the break
started
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Seismic Waves: 1st type: P-waves
•
P (Primary) or compressional waves
•
Direction of wave is same direction of force
•
Fastest waves, travel through Earth’s interior
Seismic Waves: S-waves
•
S (Secondary) or shear waves
•
Direction of wave is perpendicular to force
•
Slower than P, also travel through Earth’s interior
Seismic Wave Observations
Seismic waves give us the best info on the Earth’s interior (from crust through core)
1) P & S waves “bend” as they travel through Earth layers
2) P & S waves can “bounce” off different density layers
liquid outer core
3) S waves can’t travel through liquid
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Body Waves vs. Surface Waves
Body Waves
Seismograph
Seismogram
reading:
P wave
ground shaking
S wave
Surface
wave
ground REALLY shaking
Locations: finding the action
Relative arrivals of seismic waves at different stations gives:
Distance earthquake – to – seismic station
This uses time difference, assumed/modeled velocity of the waves
If you record given
earthquake on >=3 seismic
stations, there is 1
unique epicenter
solution
This is actually done
more rigorously by
USGS
Earthquake hazard zones
Used mainly for building codes
http://hvo.wr.usgs.gov/earthquakes/hazards/hazardmap.gif
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3 types of Earthquakes in Hawaii
1) Directly related to volcanic activity
2) Related to gravity and volcanic structure
3) Related to underlying plate
Type 1 earthquakes
Most (small)
Hawai‘i
earthquakes from
propagating dikes
in rift zones
Other types occur
that are related to
actual magma
Type 2 earthquakes
Bigger Hawai‘i earthquakes occur when the south flank of Kīlauea
and the southeast or southwest flanks of Mauna Loa slide
seaward (gravity, magmatic activity)
Examples:
1868 Ka‘ū (M > 8?)
1951 Kona (M = 6.9)
1975 Kalapana (M = 7.2)
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Slow landslides
The south flank
of Kīlauea,
showing
numerous
down-dropped
blocks
3D image by Frank Trusdell and John Sinton
Displacements associated with the 1975 M 7.2 Kalapana earthquake
http://hvo.wr.usgs.gov/earthquakes/destruct/
1975Nov29/75eqHoriz.gif
Halapē before the 1975 earthquake
http://hvo.wr.usgs.gov
/earthquakes/destruct
/1975Nov29/3042430
3_005_L.jpg
Halapē after the 1975 earthquake
Arrows for horizontal
motion, contoured
vertical motion
http://hvo.wr.usgs.gov/earthquakes/destruct/1975
Nov29/30424303_006_L.jpg
Small, short-lived eruption in Kīlauea caldera following the Kalapana earthquake
http://hvo.wr.usgs.gov/earthquakes/destruct/1975Nov29/75erup_l.jpg
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[Watts and Ten Brink, 1989]
3rd type of earthquake
Plate bending
under the
weight of the
islands
-2006 Kīholo
Bay (M 6.9)
-1973
Honomu (6.2)
-1938 Maui
-1929
Hualalai
-1871 Lanai
Slide by Carolina Anchieta and Cecily Wolfe
The October 15, 2006 Kīholo Bay and
Hāwī earthquakes
From: Reconnaissance Following the October 15th, 2006 Earthquakes on the Island of Hawai`i
By Ian Robertson, Peter Nicholson, Horst Brandes (UH Mānoa School of Engineering)
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