Using this approach we have integrated all of the tidal and leveling records from coastal Oregon to determine the 3d uplift pattern along the coast.

Topography

Benchmark uplift rates projected contour-perpendicular

• Interseismic deformation due to plate convergence

Updip Deformation Sources

Trench Central OR Coast

1975 1980 1985 1990 1995 2000 2005 2010 20153.05

3.1

3.15

3.2

3.25

3.3

3.35

3.4

3.45

f(x) = 0.00541644496829082 x − 7.48308026515348North Spit minus Crescent City

Both NOAA’s calculated absolute sea level trend estimates and the more precise difference between the two sites suggest that North Spit subsides relative to Crescent City at about ~5.4 mm/yr. Given eustatic sea level rise of ~2.3 mm/yr for 1977 to 2010, Crescent City rises, absolutely, at ~3 mm/yr and North Spit sinks at about ~2.5 mm/yr.

1980 1985 1990 1995 2000 20053.1

3.15

3.2

3.25

3.3

3.35

3.4

3.45

April 25, 1992

Black line is long term trend andred curve is 12 month smooth

meters

North Spit Minus Crescent City (monthly means)

• Interseismic deformation due to plate convergence

Updip Deformation Sources

Trench Central OR Coast TrenchCrescent CityNorth Spit

Possibly?

Tidal records from La Push (LP) and Point Grenville (PG) will confirm (or deny!) this hypothesis, and IF 2nd order leveling lines could be repeated between them and HW101 these tide gauges could stabilize the coast.

Preliminary reanalysis of the HW101 line from the Strait of Juan de Fuca (BM) to Aberdeen (Ab) supports the hypothesis that at least part of the westernmost coast is subsiding and there are very large gradients in uplift. However, this is based on a short repeat time in the releveling and is a very long distance unsupported by tide gauges to assess cumulative error in the leveling.

Earthquake geology indicates a megaquake occurs every ~700 years

Nanayama, et al., Nature (2003)

Sawai, et al., Nature (2003)

1857 Earthquake Surface Slip > 420 offset features identified (previous work identified ~150 offsets)

Courtesy ofO. Zielke

Courtesy ofO. Zielke

For the Carrizo Plain and Big Bend region we see repeated “characteristic” offsets of about five m.

Much of the deformation at the Wrightwood site is distributed across complicated small faults and folds that, in the few places we have deeply exposed them, root into low angle structures. To calculate the slip associated with these structures we have used a cross sectional area balancing approach often used to relate growth strata or erosional unconformities to detachments at depth.

Event Mean age(1σ range)

MeanInterval

Offset (m),(1σ range) 1

W1857 1857 (Historic) 44 1.0 (0.5-2.0)W1812 1812 (Historic) 130 3.5 (1.0-7.0)2

W3 1685 (1662-1700) 150 3.5 (1.0-7.0) 2

W4 1536 (1518-1542) 49 7.0 (3.0-9.0)W5 1487 (1463-1502) 127 0.7 (0.0-2.8)3

PC-T3 1360 (1343-1370) 97 0.7 (0.0-2.8) 3

W6 1263 (1230-1286) 148 3.7 (1.9-5.6)W7 1116 (1071-1152) 101 1.8 (1.1-3.4)W8 1016 (981-1039) 165 1.5 (0.7-3.1)W9 850 (825-864) 70 6.6 (3.0-9.9)W10 781 (758-794) 60 5.2 (2.5-7.5)W11 722 (706-729) 31 3.0 (1.2-6.3)W12 697 (676-708) 65 4.1 (1.4-8.2)W13 634 (602-658) 110 1.8 (1.0-5.1)W14 534 (464-594) 1.9 (1.0-3.8)

See Weldon et al., BSSA, 2002; GSA Today, 2004, for details

Summaryof Wrightwood Upper Section’s ages and offsets. Note the huge uncertainties in slip. It is difficult to measure slip per event because the deformation zone is broad and complex. But if the zone was simple we could document few earthquakes due to deformation overprinting.

WW compared to new AMS dates at Pallett Creek

70% (10 of 14) correlate

Last two quakes that don’t correlate have good evidence to be southern events.

Paleoseismology, if done well is repeatable and gives us long interpretable records.

Cluster at Wrightwood is made of multiple northern and multiple southern events, so can’t just be overlap.

The data Some interpretation

1857-like ?

Wentsouth

Wentsouth

Extra NorthBend events

HistoricPeriod

Event Mean age(1σ range)

MeanInterval

Offset (m),(1σ range) 1

W1857 1857 (Historic) 44 1.0 (0.5-2.0)W1812 1812 (Historic) 130 3.5 (1.0-7.0)2

W3 1685 (1662-1700) 150 3.5 (1.0-7.0) 2

W4 1536 (1518-1542) 49 7.0 (3.0-9.0)W5 1487 (1463-1502) 127 0.7 (0.0-2.8)3

PC-T3 1360 (1343-1370) 97 0.7 (0.0-2.8) 3

W6 1263 (1230-1286) 148 3.7 (1.9-5.6)W7 1116 (1071-1152) 101 1.8 (1.1-3.4)W8 1016 (981-1039) 165 1.5 (0.7-3.1)W9 850 (825-864) 70 6.6 (3.0-9.9)W10 781 (758-794) 60 5.2 (2.5-7.5)W11 722 (706-729) 31 3.0 (1.2-6.3)W12 697 (676-708) 65 4.1 (1.4-8.2)W13 634 (602-658) 110 1.8 (1.0-5.1)W14 534 (464-594) 1.9 (1.0-3.8)

See Weldon et al., BSSA, 2002; GSA Today, 2004, for details

Summaryof Wrightwood Upper Section’s ages and offsets. Note the huge uncertainties in slip. It is difficult to measure slip per event because the deformation zone is broad and complex. But if the zone was simple we could document few earthquakes due to deformation overprinting.

Completeness – Constant sedimentation?Are there gaps in the record?

Age Resolution – # of units per unit time?Number of dated units?

Unit resolution – Thickness and distinctiveness of units.Sharpness and planarity of contact surfaces?

Site characteristics – Narrow or broad zone of deformation?

Degree of Exposure – Number of fault crossing trenches?Frequency of exposure of key layers? Ray’s Rule!

Evidence quality – Some sort of paleoseismic index? Kate’s?

•Good sections include closed depressions that grow peat between frequent deposits of sediments, like this example from the Wrightwood paleoseismic site. Pieces of charcoal or other organic material can be used as well but detrital charcoal may be older than the unit that contains it.

Volume Time

With a sedimentation rate of ~1 m/100 yrs, and individual clastic units representing on average 5-10 years, we can distinguish events that are likely separated by decades.

Here are two earthquakes separated by just 20-30 cms, and thus likely 20-30 years (as the C-14 shows in the next slide).

Completeness – Constant sedimentation?Are there gaps in the record?

Age Resolution – # of units per unit time?Number of dated units?

Unit resolution – Thickness and distinctiveness of units.Sharpness and planarity of contact surfaces?

Site characteristics – Narrow or broad zone of deformation?

Degree of Exposure – Number of fault crossing trenches?Frequency of exposure of key layers? Ray’s Rule!

Evidence quality – Some sort of paleoseismic index? Kate’s?

Catalogue of deformation

EQ Trench Meter Type Upper Unit

Lower Unit

Q Comment

W610 T21SE 46-51 gs 620 610 4 Older units folded into syncline, units above debris flow undeformed. Significant coarsening and thickening of debris flow. Excellent example of single event growth strata.

W610 T37SE 14 - 20 gs 620 610 3 Moderate thickening, no coarsening.

W592 T41ANE 10 -16 gs 592.2 592 3 Unit thickens by 5x in fold, also coarsens, lower boundary more arcuate.

W590 T21SE 62 fis 592 590 4 Fissure has relative normal separation, nice clean break of 590 and excellent fill genetics.

W592 T31NE 4-5 fis 594 590 2.5 Very large fissure oriented sub parallel to trench. Edges of fissure have been subsequently faulted making correlation of thin upper units across feature difficult. Timing constraint is poor. Has very similar internal structure as fissure in T37, possibly continuation of same feature.

E3 T31SW 2, 3 fis 592/592.2 582 2 Large fissure with abstract shape. Filled with silt, not cg. Fis is very thin in cross-section (seen in T37SE) and has poorly defined upper horizon. Note bedding parallel shape under 520, suggests liquefaction feature.

W594 T35NE 14 fis 610 592 1 Possible hanging wall accommodation feature

E3 T31SW 2 ut 592/592.2 582 2 Normal separation, consistent offset below

W520 T35SW 6 ut 530 520 1  

Dep

ositi

onal

Ord

erEvent Quality vs. Stratigraphic Level

Sum

Sum of Quality

Sum of Observations

Event Quality vs. Stratigraphic Thickness

Peat Thickness

Debris Flow Thickness