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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2
4
3
5
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 (
