A discussion on Subduction Zones:A discussion on Subduction Zones:Links between Structure and SeismogenesisLinks between Structure and SeismogenesisA discussion on Subduction Zones:A discussion on Subduction Zones:Links between Structure and SeismogenesisLinks between Structure and Seismogenesis

Aron MeltznerAron Meltzner

TO Brownbag Discussion TO Brownbag Discussion

GroupGroup

29 November 200529 November 2005

Aron MeltznerAron Meltzner

TO Brownbag Discussion TO Brownbag Discussion

GroupGroup

29 November 200529 November 2005

Image of southwestern Japan. Figure from Sugiyama (1994) overlying image from Google Earth ©MDA EarthSat.

BackgroundBackground

• coseismic slip in great subduction coseismic slip in great subduction earthquakes is usually nonuniform earthquakes is usually nonuniform and contains and contains asperitiesasperities: regions : regions of higher slip or moment releaseof higher slip or moment release

• debate lingers as to whether debate lingers as to whether asperities represent permanent asperities represent permanent geologic structures that control geologic structures that control the rupture process or rather the rupture process or rather represent the filling of seismic represent the filling of seismic gapsgaps

• there is evidence to support both there is evidence to support both argumentsarguments

BackgroundBackground

• in some cases, in some cases, successive large successive large earthquakes within a given fault earthquakes within a given fault segmentsegment have not ruptured the same have not ruptured the same source regionsource region (e.g., the 1957–1986–1996 (e.g., the 1957–1986–1996 sequence along the Andreanof segment of the sequence along the Andreanof segment of the Aleutian arc)Aleutian arc); stress transfer and ; stress transfer and other dynamic processes may be other dynamic processes may be responsible for the sequenceresponsible for the sequence

• some asperities appear to persist some asperities appear to persist from one seismic cycle to the nextfrom one seismic cycle to the next: : GPS data from Japan and southern GPS data from Japan and southern Alaska indicate that centers of slip Alaska indicate that centers of slip in previous great earthquakes are in previous great earthquakes are now preferentially accumulating now preferentially accumulating strainstrain

BackgroundBackground

• in great earthquakes, rupture may in great earthquakes, rupture may extend over 1000+ km in length and the extend over 1000+ km in length and the entire width of the seismogenic zone, entire width of the seismogenic zone, producing huge asperitiesproducing huge asperities

• structures potentially correlative with structures potentially correlative with these asperities should be recognizable these asperities should be recognizable in the in the large-scale architecture of the forearclarge-scale architecture of the forearc

• along the Nankai Trough, some locked along the Nankai Trough, some locked patches and high-slip regions appear to patches and high-slip regions appear to correlate with offshore basins; history correlate with offshore basins; history suggests that the source regions have suggests that the source regions have persisted over many seismic cyclespersisted over many seismic cycles

Wells et al. (2003)

19441944

19461946

Basic Structural FeaturesBasic Structural FeaturesBasic Structural FeaturesBasic Structural Features• inverted inverted LL-shaped high -shaped high or composite anticline or composite anticline fringing a forearc fringing a forearc basinbasin

• reverse faults along reverse faults along unit boundary showing unit boundary showing relative uplift of left relative uplift of left (west) block(west) block

• nose structure protruding landwards nose structure protruding landwards along unit boundaryalong unit boundary

• folds with axes perpendicular to folds with axes perpendicular to trench on the right (east) side of the trench on the right (east) side of the unit boundaryunit boundary

• dextral and sinistral megakink folds dextral and sinistral megakink folds on the right and left sides of the on the right and left sides of the unit boundary, respectivelyunit boundary, respectively

Coseismic Coseismic DeformationDeformationat the Earth’s at the Earth’s SurfaceSurfacefor a N-dipping right-for a N-dipping right-oblique thrustoblique thrust

1. uplift above the S and E margins of the fault plane

2. subsidence above the NW corner of the fault plane

3. E–W shortening along E margin of the fault plane

4. both (1) and (3) decrease northward

• (1) correlates with inverted L-shaped structural high

• (2) correlates with forearc basin

• (3) correlates with N–S-trending reverse faults and folds

• northward decrease of coseismic uplift agrees with northward tilting of marine terraces at promontories

• northward decrease of E–W shortening consistent with nose structure and megakink folds

BackgroundBackground

• the forearc basins may simply indicate the forearc basins may simply indicate strong forearc crust at depth, with strong forearc crust at depth, with passive basin fill trapped behind the passive basin fill trapped behind the growing accretionary prism growing accretionary prism [Byrne et al., 1988][Byrne et al., 1988]

• alternatively, the downward motion of alternatively, the downward motion of the lower plate during earthquakes may the lower plate during earthquakes may have created the depression overlying have created the depression overlying the rupture zone the rupture zone [Mogi, 1969][Mogi, 1969]

• Sugiyama [1994]Sugiyama [1994] considered the basins considered the basins in part to be the product of in part to be the product of cumulative interseismic subsidence not cumulative interseismic subsidence not recovered during earthquakesrecovered during earthquakes

Wells et al. (2003)

BackgroundBackground

• structural segmentation results structural segmentation results from from along-strike changes in plate along-strike changes in plate geometry geometry and strain partitioning in the and strain partitioning in the upper plateupper plate

• common causes include:common causes include:

• incoming fracture zonesincoming fracture zones• oblique subduction and margin parallel oblique subduction and margin parallel deformationdeformation

• inherited transverse structuresinherited transverse structures• ridge subduction and seamount ridge subduction and seamount “tunneling”“tunneling”

Kodaira et al. (2002)

Briggs et al.(submitted)

Sumatra:Uplift & Subsidence

Sumatra:Uplift & Subsidence

Sumatra: Bathymetry & March 2005 Coseismic SlipSumatra: Bathymetry & March 2005 Coseismic Slip

Briggs et al. (submitted); slip model by Y. Hsu

after Sieh & Natawidjaja (2000)

Wells et al. (2003)

Wells et al. (2003)

Wells et al. (2003)

Wells et al. (2003)

From Wells et al. (2003):

The lack The lack of of correlatiocorrelation between n between forearc forearc lows and lows and slip in slip in the great the great Alaska Alaska earthquake earthquake is in is in large part large part due to the due to the anomalous anomalous outer arc outer arc gravity gravity high.high.

The high The high may be may be caused by caused by the the unusually unusually flat-flat-lying, lying, dense dense lower lower plate, a plate, a local local doubling doubling of oceanic of oceanic crustal crustal thickness, thickness, and and resultant resultant uplift of uplift of the upper the upper plate due plate due to the to the collision collision of the of the Yakutat Yakutat terrane.terrane.

Sumatra: Bathymetry & December 2004 Coseismic SlipSumatra: Bathymetry & December 2004 Coseismic Slip

after Sieh & Natawidjaja (2000)

Subarya et al. (submitted); slip model by R. McCaffrey

Subarya et al. (submitted); slip model by M. Chlieh

Points for Discussion from Wells et al. (2003)

Points for Discussion from Wells et al. (2003)

• the commonly observed trenchward decrease the commonly observed trenchward decrease in seismic slip could reflect landward in seismic slip could reflect landward bias in geodetic observations, but tsunami bias in geodetic observations, but tsunami and seismic inversions give similar and seismic inversions give similar results and suggest the pattern of deeper, results and suggest the pattern of deeper, basin-centered asperities is realbasin-centered asperities is real

• although a few tsunami inversions document although a few tsunami inversions document coseismic slip beneath the prism, in coseismic slip beneath the prism, in general, general, the highest slip region tends to be deeper the highest slip region tends to be deeper

beneath the forearc basins and upper slopebeneath the forearc basins and upper slope

• how does the Nias-Simeulue earthquake fit how does the Nias-Simeulue earthquake fit it?it?

Points for Discussion from Wells et al. (2003)

Points for Discussion from Wells et al. (2003)

• some transverse, intrabasin highs apparently some transverse, intrabasin highs apparently overlie areas of lower slip in great overlie areas of lower slip in great earthquakesearthquakes

• 1952 and 1968 earthquakes off Hokkaido 1952 and 1968 earthquakes off Hokkaido ruptured on either side of the gravity high ruptured on either side of the gravity high at Cape Erimo, but little or no slip occurred at Cape Erimo, but little or no slip occurred beneath the Cape in either earthquakebeneath the Cape in either earthquake

• the Cape is either a potential asperity the Cape is either a potential asperity storing up great slip, or it is relatively storing up great slip, or it is relatively weak, possibly because of trapped heat or weak, possibly because of trapped heat or fluids beneath the thicker crustfluids beneath the thicker crust

• GPS inversion at Cape Erimo is consistent GPS inversion at Cape Erimo is consistent with less than full locking of the underlying with less than full locking of the underlying plate boundaryplate boundary

Points for Discussion from Wells et al. (2003)

Points for Discussion from Wells et al. (2003)

• a weaker fault beneath the Cape could a weaker fault beneath the Cape could allow some aseismic slip there, causing allow some aseismic slip there, causing earthquakes earthquakes to nucleate along the Cape Erimo to nucleate along the Cape Erimo gradient and rupture primarily under the gradient and rupture primarily under the adjacent basinsadjacent basins

• analogies include:analogies include:

• the Kii Peninsula, bounding the 1944 & the Kii Peninsula, bounding the 1944 & 1946 earthquakes along the Nankai 1946 earthquakes along the Nankai TroughTrough

• the Shumagin Islands high off Alaskathe Shumagin Islands high off Alaska• the Batu Islands and the “Simeulue the Batu Islands and the “Simeulue Saddle” ??Saddle” ??

after Sieh & Natawidjaja (2000)

Points for Discussion from Wells et al. (2003)

Points for Discussion from Wells et al. (2003)

• not all forearc highs are areas of lower not all forearc highs are areas of lower seismic slipseismic slip

• in the 1952 Kamchatka earthquake, one of the in the 1952 Kamchatka earthquake, one of the three asperities was centered on a prominent three asperities was centered on a prominent transverse hightransverse high

• in the 1946 Nankaido event, significant slip in the 1946 Nankaido event, significant slip occurred beneath the southwestern Kii occurred beneath the southwestern Kii PeninsulaPeninsula

• along the Nankai margin, the transverse along the Nankai margin, the transverse anticlinal highs accommodate significant anticlinal highs accommodate significant oblique slip on splay faults and may be oblique slip on splay faults and may be important components of coseismic deformationimportant components of coseismic deformation

Points for Discussion from Wells et al. (2003)

Points for Discussion from Wells et al. (2003)

• the tendency for coseismic slip to be the tendency for coseismic slip to be focused beneath the terrace and its basins, focused beneath the terrace and its basins, rather than beneath the intervening highs, rather than beneath the intervening highs, could reflect along-strike variations in the could reflect along-strike variations in the temperature, fluid pressures, and stresses temperature, fluid pressures, and stresses on the megathrust caused by variations in on the megathrust caused by variations in overlying crustal thickness and densityoverlying crustal thickness and density

• large-scale segmentation of the source zone large-scale segmentation of the source zone would then result from oblique convergence, would then result from oblique convergence, subducting fracture zones, or some other subducting fracture zones, or some other second-order processsecond-order process

• the question remains whether there is a the question remains whether there is a relationship between forearc basin evolution relationship between forearc basin evolution and seismogenesisand seismogenesis

The EndThe End

19441944

19461946

Wells et al. (2003)

19441944

19461946

Wells et al. (2003)