Presentation on theme: "Is Hawaii a deep plume… Or a wide slow deep fracture zone? Don L. Anderson December 2009."— Presentation transcript:
Is Hawaii a deep plume… Or a wide slow deep fracture zone? Don L. Anderson December 2009
Enormous delays compared to Hawaii All in crust & shallow mantle
>3 second delays; much greater than Hawaii
Science, Dec. 2009
TOMOGRAPHY IS THE STUDY OF SHADOWS As Plato showed, shadows can be deceiving
… things are not always what they seem. In Plato's cave people see only shadows and these are the only reality there is … they kill the messenger from the outside …
Steeply arriving teleseismic rays give a fuzzy view and can easily be misinterpreted (parallax, smoothing, smearing, streaking, bleeding, rabbit- earing, shallow structures…) SKS waves have the additional complication of Ultra Low Velocity Zones at the CMB & other complications in the complex D” region… The controversy between Zhang-Tanimoto and Dziewonski et al. about the depth of midocean ridges was traced by Masters to the effect of D” heterogeneity…which was misinterpreted as being due to deep slow ridges…
A cylinder and a pillbox both give a tent or cone
A suture or fracture zone (FZ) can be 50 km wide, 200 km deep and have S-wave deficits of 8 % (fluids, mineralization, cracks) to 35 % (high pore pressure, open dikes, wet rifts…). This will affect teleseismic arrivals, but in a different way than a shallow pillbox or a vertical cylinder or than most resolution tests. A finite fault zone can give rabbit ear artifacts…or fuzzy drips off the edge of the structure. If 8 % anomalies are ruled out (Wolfe) then the shallow fault zone will be smeared into the deep mantle…
SKS delays are right above the FZ; FZ area and delay are correctly determined S delays are smaller, shifted; far side; average delay (station term) is very much smaller, and effect is over a larger area. A wide FZ under the Hawaiian chain? farside source S SKS
If there are only a few stations to the SE of the Hawaiian FZ anomaly, the FZ slowness will be projected to great depth and will not be cancelled out by crossing rays. Elementary optics…
If there are only a few stations to the SE of the Hawaiian FZ anomaly, the FZ slowness will be projected to great depth and will not be cancelled out by crossing rays. Elementary optics… This is not a plume…it is Sherlock…
SKS S & SKS A possible artifact of station and source sparcity to the SE There are few rays or crossing rays at depth of proposed plume “Science” Dec.2009
Laske, Phipps Morgan & Orcutt NORMAL Cracks through thick lid (70 km) to fertile blob from 120 km to 200 km DEPTHDEPTH ULVZ (E.Pacific) SKS Wolfe, Solomon
~1000-km arrays; ~3-sec delays Shallow rift Deep plume Jimez Lineament Hawaii 2 sec Similar data sets led to quite different conclusions A simple approach to the joint inversion of seismic body and surface waves applied to the southwest U.S. Michael West, New Mexico State, Gao &S.Grand
360 km RISTRA 3 sec total variation HAWAII source farside A simple approach to the joint inversion of seismic body and surface waves applied to the southwest U.S. Michael West, New Mexico State, Gao &S.Grand
West et al.
Body wave only inversion Rabbit ear artifact West et al.
Rabbit ears artifacts Wolfe et al Yellowstone Hawaii Rabbit ear artifacts?
Iceland Hawaii SKS ray 00
These are among the latest arrivals to the Hawaiian array from any earthquake (which is to the west); slow trend is along chain; there are also more slow stations (>4) for this event than for others Typical S-delays in western N.America are much larger & are due to upper mantle
University of Texas
These good men of HartO ’ Stan To dogma much inclined, Much feared to see an Elephant (Though all of them were blind), Since any observation Might stultify the mind
The Noble Men, one fore two aft Inhaled the pachyderms exhalations "Tis from the deep bowels" offered one "A signature of intestinal fermentations". "Tis primordial gas", gasped another We'll get a Nobel prize for our exertations Or at least a Science paper, with no deliberation! It’s a PLUME! I did resolution tests!
THE TAIL OF THE PLUME These good souls of Academe To dogma much inclined, Much feared to see an Elephant (Though all of them were blind), Since any observation Might stultify the mind The First, a physicist, approached, Leaving Guyot Hall in fear Fell against his sturdy side, And spoke so we could hear “God bless me! but the Elephant Is exactly like a sphere!" It was six souls in research strong To learning much inclined, Who went to study mantle plumes Which they no doubt would find. Lacking an Alternative, Nothing else could be divin'd. With bulbous Head, and skinny Tail A pachyderm paced by, undetect’d. Emitting fluids and various gases, Enormous lips and trails of ejecta This might be the plume Which they all had expect'd.
The Second, feeling around below, Cried, “Ho! we have here a mushroom Very round and smooth and straight! No further need to fume. This wonder of an Elephant Is certainly a plume!” The Third, demurred, he had his own theory, But he tripped and fell (there was little room), A squirming trunk brushed his face, He spake with a sense of doom: “I fear,” quoth he, “the Elephant Really is a plume!” The Fourth, who chanced to touch its brow, Said: “Not an animal at all. I, instead Can tell what this resembles most; Deny not the fact" she said "This marvel of an Elephant Is just a plume head!” The Fifth no sooner had begun About the beast to groom, Than, seizing on the swinging tail That tangled with his broom, “I agree,” quoth he, “the Elephant Is very like a plume!”
ex cathedra …let’s look [only] at…features where a plume…makes…geological sense and investigate those. Hofmann & Hart, Science 2007
…let’s look [only] at…features where a plume…makes…geological sense and investigate those. H & H, Science 2007 “Seek and ye shall find…”