lecture 16 Flashcards
megathrust earthquake cycle
interseismic period: b/w earthquakes
- seafloor dragged downwards as plates r stuck and shortens as coastline bulges up
coseismic period: the earthquake
- fault slips, upper plate extends, seafloor uplifted, land subsides
after: tsunami waves spread out away from uplift which displaces and causes ocean-wide tsunami
subduction zone
all oceans contain at least 1 subduction zone but pacific ocean contains the most
- most dangerous
2004 indian ocean tsunami
boxy day - 9.2 megathrust in Sunda subduction zone
- third largest earthquake, fault length 1300km, 8 min rupture and slip 20m
worst affected countries of 2004 indian ocean tsunami
- indonisia: more than 2 hours after earthquake
- sri lanka: earthquake not felt, peak arrived first
- india ^
- thailand: earthquake felt, preceded by drawback
- myanmar
- somalia
total –> 228,000
why was no alert issued 2004 indian ocean etsunami
didnt have ability to share information at the time and didnt have a warning system in place
2011 tohoku tsunami
tectonic setting: westward subduction of pacific and phillippine sea oceanic plates beneat the japan arc on the continental eurasian plate
- 0 tohoku earthquake ruptured megathrust fault
- japan squeezed and elastic rebound
cost of tohoku tsunami
1,5000 killed, mostly burning
economic: 360 bill USD
run ups of tohoku
kuji: funnelled into bay, RU 13m
miyako: RU 39m
kesennuma: funnelled up river valley
sendai plain: RU 19m a distance of 8km
okuma: RU 15m, overtop 6m seawall and flood powerplant
Japan GPS measurement
pioneered GPS measurements of sea floor motions
- ships use triangulation
- satellitles pinpoint precise locations
- GPS picks up signs of ground deformation
why were the run ups larger than expected in tohoku tsunami
trench area was thought to be creeping and incapable of generating an earthquake, but slip was much larger than expected (mw 9.0) and ruptured up to trench, uplifting the sea-floor and creating large run up heights
1700 CE orphan tsunami
ghosts forests in oregon washington discovered in 1980s trees were killed as they were lowered into salt water tidal zone by earthquake substidence
- radiocarbon dating and tree ring studies indicated forests were killed in 1700 CE
- records from Japan confirmed 2m high tsunami on january 21 1700
average recurrence interval of orphan tsunami
similar events at 1310 CE, 810 CE, 400 CE, 170 BCE, 600BCE
average = 400-600 years
- suggested 15m run ups
pacific tsunami warning center
set up in 1949, worlds first tsunami warning system
- uses seismogram data from around and assess:
- epicenter: did earthquake occur beneath sea
the depth: was it shallow enough to generate tsunami
magnitude: was it large enough to generate tsunami
if answers are yes then regional tsunami watch is issued and ugraded to tsunami warning if generated
tsunami detection
deep ocean assesment and reporting of tsunami sensors added to pacific tsunami warning center in 200
- bottom pressure sensors
- communicate with floating buoy through accoustic telemetry, communicates with control centre thru satelites
- dart sensors
WERA
wave radar run by ocean networks canada: shore based remote sensing system in tofino
- transmitting and receiving radar antennas that monitor opean current speed in real time
- can detectl arge waves up to 80km which can be 20min of warning
tsunami evacuation
with knowledge of the shape of the coastline and the offshore bathmetry (seafloor topography) tsunami hazard maps can be used to predict wave heights at diff points along a
- raised earth parks can be built in cities where it is difficult to evacuate to high ground ex. padang
tsunami mitigation
structural countermeasures include tsunami walls to block incoming waves, breakwaters and underwater berms to increase friction and dissipate wave energy and angled walls and ditches to steer waves away from vulnerable infrastructure
1958 lituya bay tsunami
july 9, 7.8 earhtquake triggered huge rockfall
- giant slab of rock (30mill m) fell into the bay causing giant wave to run up hillside opposite, stripping trees of hillside
nuuugaatsiaq tsunami
june 17 tsunami located on fjord in greenland
- 11 houses swept away and 4 killed
- triggered by landslide
landslide was so large it confounded in seismic signals making it seem like a mw 4 earthquake
1929 grand banks tsunami
18 nov, Mw 7.2 earthquake occured in grand banks region, rich fishing grounds on eastern continental shelf
collapse of slope in huge submarine landslide
- deadliest earthquake in Canada 28 ppl
how do we know about the 1929 grand banks landslide
formed turbidity current –> severed several trnsatlantic telgraph cables
around 100km ffrom epicenter an hour after and 700km away 13 hours
cause of 1929 grand banks tsunami
abrupt increase in sea floor depth caused sea surface to warp downwards
- r apid failure of shelf causes changes in sea floor
- rapid response of sea level to changing depth generates tsunami
- waves propagates outwards
sieche
standing wave in enclosed or partially enclosed body of water, slosh back and forth
- can be set up by strong winds
how to survive a tsunami
abandon belongings, dont stay to watch, get elevated as quickly as possible, dont return to the coastline
