Motivation and Research Questions

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6) displacements at the surface (from the CES) are less than the cumulative displacement of ... Canterbury Earthquake Sequence (CES) and led to damage of.
Geology of liquefaction-induced lateral spreading, new results from Christchurch, New Zealand 1

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Gregory De Pascale Jeff Bachhuber Ellen Rathje Jing Hu Peter Almond Christian Ruegg 5 1 Mike Finnemore Department of Geology and Geothermal Centre of Excellence (CEGA), University of Chile 3

Earthquake Engineering, U. Texas at Austin Geosciences, Pacific Gas and Electric (PG&E), San Francisco 5 4 Department of Soil and Physical Sciences, Lincoln University, NZ Southern Geophysical Limited (SGL), NZ 2

Motivation and Research Questions

Note westward displacement & vertical step / headscarp under house & fence

Earthquake triggered liquefaction and lateral spreading was widespread in Canterbury, New Zealand during the 2010-2012 Canterbury Earthquake Sequence (CES) and led to damage of ~$20 Billion NZD (~$15 Billion USD). Geological controls on lateral spread are poorly understood, thus: 1) What are the geological controls on lateral spreading? 2) How do lateral spreads appear in the subsurface? 3) What can we learn from the CES that we can apply towards global lateral spreading and liquefaction hazard?

Trench Site Post-CES

Faulted silt lenses Note Trench Blah Blah Blah

East

West

Christchurch Avonside Drive Trench

4.0

4.0

Top of trench Top of trench

West

3.5

Post EQ (2011) Fill

N80W

3.5

Plastic

Post-EQ (2011) Fill Base of Fill is Older Fill gravelly, dense, with old historical artifacts

ELEVATION (meters)

2.5

Top of trench

Post-EQ (2011) Fill

Post-EQ (2011) Fill 2.0

L

Blue pottery

Black Soil

Coal

Old Fill

Coal

Loose sand

Sand dikes

Black Soil

Silt

Coal

Black Soil below thin Old Fill. Fill debris pushed down into Soil

Post EQ Fill Silt Bedded Sand

Sand blow?

J

Nail

RC-2

Sand dike

Bioturbated

Bioturbated Soil

Bioturbated zone

Massive Sand

Gravel

Sand

Plastic bag

Fill

Sand seam

Paleosol?

Tile

Sand Clay

Old Fill

Square rusty nail

D

Sand

Gradational

B1 B2

(D) Highly mottled plastic

Sand dike

Mottled Clay rip-up?

Sand

RC-GDP-ASI (D) 2555 ± 24 ybp

B Sand, slightly darker (with bioturbation)

Upper Sand

Sand

Sand

Mottled Clay

C

Old excavated outline

Sand seam

(E) Sand, subrounded 1/16 - 1/8 mm gray

Clay

Clay

H

Index silt lens Massive Sand

Paleosol

Ib

Ia E

AS OSL-3

“Faulted” index silt lens

Crack

Bottle

String line

1.5

Paleosol

ch

Clay

Bioturbated zone Highly bioturbated

Sand blow?

B Stringline

(B) Sand with Silt (blocky ped structure)

Sand

Gradational

A

String line

A Sand (subangular, 1/8 - 1/4 mm, well sorted) oxidized (possibly old dune sand?)

Bottom of trench

AS OSL-2

Lower Mottled Clay

RC-AS4-GDP-2016 1452 - 1622 A.D.

1.0

2.0

Mean high tide water level

Green-gray Clay

Massive Sand

2.5

1.5

Key

Mottled Clay

Bottom of trench

3.0

Paleosol Gradational

OLD FILL

Possible down-dropped Paleosol/Fill?

M

AS OSL-3

Massive Sand

Clay pipe

Bone K

F

Sand dikes

Sand dike

Pottery

±10 cm step

Yellowish Old Fill (thick glass)

Coal

Down-dropped old soil wedge (Soil G?)

Roots, organic seams/lifts

ELEVATION (meters)

3.0

A AS OSL-1 RC-GDP-ASI (D) 2555 ± 24 ybp

Blue-gray Clay

Soil description sampling location Soil sample for possible age dating

1.0

Radiocarbon sample location

0.5

0.5

Zone of extension and vertical deformation recorded by “faulted” and down-dropped silty lenses . -10

-9.5

- 9.0

-8.5

-8.0

-7.5

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5. 0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

9.5

HORIZONTAL DISTANCE (meters)

Preliminary results from our 1st lateral spread trench (1 of 2) demonstrates:

1) displacements observed from satellite-image modeling correspond well with on-the-ground and subsurface displacement measurements & headscarp margins; 2) Holocene sediments along headscarp margins show ground cracks & vertical steps at the surface correspond with liquefaction-ejecta filled cracks in the subsurface & extension indicated by normal faulting (see photo) and horst and graben structures 3)sediments are laminar, sub-horizontal bedded sands with occasional silt interbeds; 4) discrete displacements are well exhibited by progressive vertical-lateral displacements of silt marker beds and can be used to measure total movement of the lateral spread mass; 5) radiocarbon dating indicates that the translated sands are late-Holocene (