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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/ 1 1/29/17 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) 2 1/29/17 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 3 1/29/17 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 4 1/29/17 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 5 1/29/17 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 6 1/29/17 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) 7 1/29/17 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 8 1/29/17 [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) 9