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The Padang quake and Seattle implications - J. Vidale, UW

M7.6 Padang Earthquake,30 September 2009, 05:16 PM88 km depth 1.2 M People Affected 1,120 Deaths (est.) 3,000 Serious Injuries 140,000 Damaged Structures 2,100 School Damaged or Destroyed No tsunami (too deep)

1,400 Large Hotels or Office Buildings Seriously Damaged

M7.6 Padang Earthquake,30 September 2009

OutlineRegional tectonicsRegional quake historyThis quakeSimilar quakes in the Pac NWFuture predictions for the Pac NW

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Sept. 30, 2009, first Samoa M8 (with tsunami)Sept. 30, 2009, first Samoa M8 (with tsunami)then,then, M7.6, PadangM7.6, Padang

LEMPENG PASIFIK

LEMPENG FILIPINA

LEMPENG EURASIA

LEMPENG INDO-AUSTRALIA

LEMPENG COCOS

LEMPENG AMERIKA UTARA

LEMPENG NAZCA

LEMPENG ANTARTIKA

LEMPENG AMERIKASELATAN

LEMPENG CARIBIA

LEMPENG SCOTIA

BMKG

INDONESIA BMGBMG

SOURCE : BPPT

PLATE TECTONIC MODELNORTH-SOUTH GEOLOGICAL CROSS SECTION THROUGH SOUTH OF JAWA

Background seismicity

Background seismicity w/ aftershocks

M7.6 mainshock

M6.6 aftershock on Sumatra fault

2004 Earthquake and Tsunami

1797

1833

18611907

1822

1926 (~7)

1984 (6.4(

1935

1987 (6.6)

19211916198419871892

1822

1943190919951952

1926

1942

18931900

193319941908

19901997

1936

19641967

1893

1892 (7.7)

1900

1908

1916

1933 (7.5)

1942 (7.3)

1936 (7.2)

1952 (6.8)

1979 (6.6)

1943 (7.3)

1990 (6.5)

1997 (6.5)

1964 (6.5)

1921 ( >7)

1994 (6.9)

1995 (7.0)

1909 (7.6)

1967 (6.1)

Historical Earthquakes

1797 (8.2)

1833 (8.7 – 9)

1861 (~8.5)

1907

1935 (7.7)

2000 (7.8)

2002 (7.4)

2004 (9.2)

2005 (8.7)

Subduction ZoneSumatran fault

2000

2005

2004

2002

???

Fig. 6 : Map of damaging earthquake in Sumatera (Hilman, 2004)

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PDSI*

PPI

SDSI

Acceleration

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Velocity

Landslides• Causes

– steep mountain slopes– triggered by heavy rain

combined with earthquake shaking

• Mud and debris flow• 5+ villages reported buried• 600+ casualties• Inaccessible

– damage to roads– lack of communication

AP News

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September Rainfall – Met. Station

Tabing, Pandang

Sept. 1 – 1.0 mmSept. 2 – 2.8 mmSept. 4 – 0.6 mmSept. 8 – 53.0 mmSept. 14 – 123.3 mmSept. 15 – 66.5 mmSept. 16 – 0.7 mmSept. 17 – 19.6 mmSept. 24 – 3.7 mmSept. 27 – 44.1 mmSept. 28 – 2.7 mmSept. 30 – 20.4 mm

Local Areas with Very High Levels of Damage (MMI 8-9).

Evidence of Strong Localized Site Effects.Near-surface geologyheight of the water tabledepth to bedrock

• Massive Triggered Liquefaction/Debris Flow Killed 100’s of People.

Conclusions I

Not the Anticipated M8+ Earthquake on the Sunda Megathrust - no tsunami.

Damage was greater than would be expected for a comparable magnitude earthquake on the subduction interface

• closer to land

• Strong need remains in region for earthquake preparedness.

• About 1 such earthquake of M>=7 per year

Conclusions II

M7.6 Intraplate event at 80 km depth.Unusual focal mechanism.Usually normal faulting - this was oblique thrustOnly 4 aftershocks that could be located - typical for large intermediate depth quakes

• Normal faulting would unload shallow thrusts, but this event may load thrust and render future big events more likely.

• Strong need remains in region for earthquake preparedness.

Conclusions III

Complicated controls on subduction earthquakes

23Audet et al., 2009

ETS & subduction cross-section

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Abers et al, 2009

Intraplate quakes in cross-section

Nisqually strong shaking

• Nothing is red• Radiation pattern is visible• Deep is milder than shallow

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Low Potential for Large Intraslab Earthquakes in the Central Cascadia

Subduction ZoneBulletin of the Seismological Society of America, Vol. 95, No. 5, pp. 1880–1902, October 2005

by Ivan G. Wong

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4 types of slab earthquakes

Three zones along

Cascadia

Hexagons are earthquakes

below megathrust

Note a-d profile latitudes

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• Source Investigation and Comparison of the 1939, 1946, 1949 and 1965 Earthquakes, Cascadia Subduction

Zone, Western Washington Pure appl. geophys. 164 (2007) 1905–1919

Katy Wiest, Diane Doser, Aaron Velasco, & James Zollweg

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Puget Sound intermediate depth earthquakes

11/13/1939 5.8 63 S. Puget Sound02/15/1946 6.3 26 Puget Sound04/13/1949 7.1 55 Olympia04/29/1965 6.6 63 Sea-Tac07/03/1999 5.7 40 Satsop02/28/2001 6.8 55 Nisqually

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Detailed conclusions

Our results suggest that the 1946 earthquake is not an intraslab eventthe 1939 event closely resembles the 1965 eventthe 1949 event is similar to the 2001 Nisqually earthquakealthough the 1949 event appears to have ruptured toward the south, causing significantly more damage than the Nisqually event.

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Less seismogenesissouth of Puget Sound

young plate age

slower convergence rate

contributes to heating

the insulating effect of the Siletz terrane above the plate

contributes to heat

less stress concentration

slab contortions, perhaps tears, are visible under Puget Sound

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Four profiles

Note paucity of events east

of 124° W, except in top

profile

1873 M7.3 earthquakeBrookings, ORNo reported aftershocks

Perhaps deep earthquake,Wong claims likely shallow

Plenty of lower plates earthquakes south of OR-CA border

Analogs:Japan

MexicoNankai

a few M6.5-7 events,

mainly at bends

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Implications

The Padang M7.6 earthquake in the same setting is a reminder that such activity is not exceedingly rare.

Usually we think in terms of Gutenberg-Richter distributions of earthquakes, but intermediate-depth events may be different, thus odds are difficult to judge.

Current estimate is 85% chance of M6.5+ intraplate event, similar to Nisqually, in next 50 years.

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