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Subduction Zone Earthquakes Earthquakes along the Cascadia subduction zone.

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Presentation on theme: "Subduction Zone Earthquakes Earthquakes along the Cascadia subduction zone."— Presentation transcript:

1 Subduction Zone Earthquakes Earthquakes along the Cascadia subduction zone

2 NORTH AMERICA 85 MA TO PRESENT PACIFIC PLATE/NORTH AMERICA PLATE

3 NORTH AMERICA 38MA - PRESENT DEVELOPMENT OF SAN ANDREAS FAULT

4 Location dateMw 1Chile22 May North Sumatra, Indonesia26 December Prince William Sound, Alaska28 March Andreanof Island, Aleutians9 March Kamchatka4 November Ecuador31 January Rat Island, Aleutians4 February Sumatra, Indonesia 28 March Assam, India-China border15 August Kamchatka3 February largest earthquakes since 1900 Pacific-NA along Aleutian trench Middle America - not as large

5 SUBDUCTION ZONE EARTHQUAKES ALASKA, MIDDLE AMERICA, CASCADIA Aleutian trench, middle America trench, Cascadian subduction Zone Ocean/Ocean, Ocean/Continent, Ocean/Continent collisions

6 SUBDUCTION ZONE EARTHQUAKES 1964 ALASKA EARTHQUAKE Pacific plate subducted beneath North American Plate km by 400 km slab moved 19.5 m with 3-4 min shaking. Produced uplift and subsidence Damage mostly due to tsunami Critical earthquake for under standing thrusting in subduction zones

7 SUBDUCTION ZONE EARTHQUAKES 1964 ALASKA EARTHQUAKE Pacific plate subducted beneath North American Plate. Ghost forest of trees died from salt water intrusion after the land sank some six feet during the 1964 Alaskan earthquake. Girdwood Flat, Turnagain Arm of Alaska's Cook Inlet.

8 SUBDUCTION ZONE EARTHQUAKE 1985 MEXICO CITY Earthquake in Middle America Trench  Earthquake in middle America trench but damage mostly in Mexico City.  Cocos plate moved 2.3 m at 18 o over 200 by 200 km area  Damage because soft lake sediments in Mexico City resonated with sec frequency of surface waves.

9 SUBDUCTION ZONE EARTHQUAKES 1985 MEXICO CITY East-West Accelerations at stations around Mexico City. UNAM on hard ground - small accelerations. SCT on soft lake sediments - high of acceleration due to amplification effect of resonance of soft lake sediments

10 SUBDUCTION ZONE EARTHQUAKES 1985 MEXICO CITY Building response problems during the earthquake storey high building worst hit. Buildings of different heights banged into each other. Buildings with different heights tore apart.

11 SUBDUCTION ZONE EARTHQUAKE GAP  West coast of Mexico showing dates of earthquakes and the fault areas.  Note that the Michoacan gap was filled by 1985 earthquake.  Geurrero gap is overdue for a major rupture. Important concept in predicting where next major EQ might occur

12 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST  Volcanoes  Youthful ocean floor and strong  Coupling with overriding plate  1872 M6.8, Lake Chelan  1949, M 7.1 earthquake, Puget Sound  1965, M6.5 earthquake, Puget Sound  2001 M 6.8 earthquake, Puget Sound  1700 M 9.0 earthquake, Cascadia

13 SUBDUCTION ZONE EARTHQUAKES CROSS-SECTION

14 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST 2/28/2001 M6.8 Nisqually earthquake

15 SUBDUCTION ZONE EARTHQUAKE PACIFIC NORTHWEST 2001 Nisqually earthquake Seismicity in the Pacific Northwest M6.8 Normal fault Earthquake hypocenter km depth under Tacoma Because it was so deep caused remarkably little damage at the surface.

16 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST 2001 Nisqually earthquake Normal fault solution at the bend in the Juan de Fuca plate as it descends into the mantle Below the zone where the most devastating thrust earthquakes occur. Normal fault and depth of rupture = damage was small.

17 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST

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19 SUBDUCTION ZONE EARTHQUAKES Stratigraphic evidence of past earthquakes  Interseismic: Uplift above the locked zone and oceanward submergence CONCEPT OF PIVOT LINE  Coseismic: Rapid submergence above the locked zone and oceanward uplift.  Two features demonstrate the subsidence typical of subduction zone earthquakes  Buried lowlands and sequences of soil horizons.

20 SUBDUCTION ZONE EARTHQUAKES Stratigraphic evidence of past earthquakes  Rapid coseismic subsidence  Deposition of tsunami sand layers  Interseismic emergence

21 SUBDUCTION ZONE EARTHQUAKES Stratigraphic evidence of past earthquakes Coastal submergence, some fresh-water lowlands drop below sea-level. Vegetation in these areas is exposed to seawater and dies. Rapid coastal submergence during great earthquakes, large number of freshwater coastal plants die and leave evidence in the form of "peat".

22 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST 26 January Cascadia earthquake  Annual growth rings in drowned trees along coast from BC to Oregon show  Trees died at the same time  Rapid subsidence and sea water kills tree roots.  Large earthquakes cause extensive subsidence.

23 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST Japanese records show evidence of a tsunami January that was not from a local earthquake but probably from Pacific Northwest.

24 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST Kenji Sataki found Japanese records to tsunami occurrences along the country's eastern coastline between January 27 and 28, Careful analysis of these historic tsunami records indicated that several coastal villages were damaged. The following is a chronicle by the head of Miho village, 145 km southwest of Tokyo. This account tells of sea water covering land as if it were high tide. "The water also went into the pine trees of Ego. The receding water went out very fast, like a big river. It came in about seven times before 10 a.m. of that day and gradually lost its power…Because the way the tide came in was so unusual, and was in fact unheard of, I advised the villagers to escape to Miho Shrine…It is said that when an earthquake happens, something like large swells result, but there was no earthquake in either the village or nearby."

25 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST !700 TSUNAMI Tsunami Modeling: Size and source of distant tsunamis.

26 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST OFFSHORE TURBIDITE RECORD FOR PAST EARTHQUAKES A number of events can potentially trigger turbidity currents. Tsunamis Storm induced waves Slope failures Earthquakes The turbidite record strongly suggests that Cascadia experienced strong coast-wide shaking typical of a large subduction zone earthquake.

27 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST OFFSHORE TURBIDITE RECORD FOR PAST EARTHQUAKES 54 new cores in Cascadia, document the event record temporally and spatially. Thirteen post-Mazama and 18 Holocene events are found along~ 660 km of plate margin. The most recent event took place in 1700 AD (Satake et al., 1996; Nelson et al., 1995) An additional 12 turbidite events have occurred during the preceeding 7200 years, yielding a mean recurrence time of ~575 years.

28 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST OFFSHORE TURBIDITE RECORD FOR PAST EARTHQUAKES Triggering mechanisms : Are they Earthquakes? Are these events all triggered by earthquakes? Eight mechanisms for turbid flow triggering: 1) storm wave loading 2) great earthquakes 3) Tsunamis 4) sediment loading. 5) crustal earthquakes 6) slab earthquakes 7) hyperpycnal flow 8) gas hydrate destabilization.

29 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST OFFSHORE TURBIDITE RECORD FOR PAST EARTHQUAKES Triggering mechanisms : Are they Earthquakes? 13 turbidites had been deposited off coastal Washington and Oregon since the eruption of Mount Mazama Turbidity currents originating from different locations occurred simultaneously Simultaneous turbidity currents from different side channels merge, the main channel can be expected to show a single large turbidite. If the turbidites orignated at different times in the side channels, the main channel would record each separate turbidite event.

30 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST OFFSHORE TURBIDITE RECORD FOR PAST EARTHQUAKES Triggering mechanisms : Are they Earthquakes? The consistent number of turbidites in core samples from the side and main channels indicate that the turbidity currents were likely caused at the same time and by the same event. Large storms are an unlikely source of a coast- wide event because these storms produce waves not much larger than smaller, more common storms. If common and rare storms produce waves that are approximately the same magnitude, the turbidite record should reflect more than 13 events in the last 5,000 years.

31 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST OFFSHORE TURBIDITE RECORD FOR PAST EARTHQUAKES

32 Triggering mechanisms : Are they Earthquakes? The “confluence test” of Adams (1990) requires synchronous triggering within a few hours. 18 Holocene events, all of which pass this test of synchronous triggering.

33 SUBDUCTION ZONE EARTHQUAKES PACIFIC NORTHWEST Tsunami animations

34 PACIFIC NORTHWEST SUMMARY PALEOSEISMIC RECORD FOR PAST EARTHQUAKES


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