ML20151P847

Geologic Comparisons of Cascadia & Other 'Similar' Subduction Zones, Presented at 880506-08 Meeting in Seattle,Wa

ML20151P847
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Site:	Trojan File:Portland General Electric icon.png
Issue date:	05/06/1988
From:	Gina Davis SOUTHERN CALIFORNIA, UNIV. OF, LOS ANGELES, CA
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Text

17 '

GEOLOGIC COMPARISONS OF THE CASCADIA AND OTHER "SIMILAR" SUBDUCTION ZONES

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Davis, Gregory. A., Dept. Geol. Sci., UAiv. So. California, Los Angeles, CA 90089-0740

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Geologic comparisons have been made between the Cascadia subduction zone (CSZ) and six other rones deemed most similar to the CSZ in terms of youthful age (t) of subducting crust and rate of plate convergence (v), e.g. Heaton and Hartzell (1986, 1987). Such comparisons neither prove nor disprove the potentit! for great earthquakes along the JdF/NA plate interface, but do provide a scientific basis for concluding that subduction along the CSZ may be occurring in a manner unlike that of the seismogenic zones with which it has been compared. Specifically, geologic differences with the six zones support the 'conclusion that slow subduction of the hot, topographically smooth, and sediment-laden JdF plate may be occuring by stable sliding rather than stick slip behavior. The table below summarizes t and v parameters of the 7 subduction zones, and cites major geologic differences of each with the CSZ. Values for two independently derived measures of coupling between converging plates -

Jarrard's (1986) strain class numbers based largely on geologic characteristics of the upper plate, and alpha (the seismic slip rate / relative total plate convergence rate) --

are considerably lower for the CSZ than for any of the other zones. Such low values are consistent with geologic arguments that W, the downdip width of the seismogenic plate interface, may be very narrow or nonexistent for CS7-.

Zone (Jarrard, Plate ace Convercence Geolecie differences with Cascadia S. 2.

'S6 strain class) at trench rate (em!vr)

Cascadia 8 Ma 3.4.......... Alpha = 0.3 (Kanamori & Astiz, '85) for (3 4a) t = 1015 Ma; 0.0 to < 0.3 (this analysis).

SW Japan 17-24 2.9-3.7 ...

(5)

Major topographic asperities on subducting plate (abandoned spreading ridge); no active volcanic arc; upper and lower plates in possible subhorizontal contact beneath Shikoku and Honshu; alpha = 0.88 - 1.0.

S Chile 5 35 9 ... . . . . .

Very high convergence rate; 1960 M 9.5 (5) event initiated in subduction zone where oldest crust is being subducted; accretionary prism very narrow (<100 km); W may be an-SW Columbia omalously wide; wpha = 1 (Peterson/Seno '84) 8-17 8...

(6)

High convergence rate; zonc is highly seis-mogenic; major asperities (an abandoned spreading ridge and Carnegie Ridge ) on sub-ducting plate make this a collisional zone; upper plate under strong compression; adj.

Rivera Andean mts. very high (6 km); alpha = 0.8 5.

8-10 2.3 . .

Topographic asperities (seamounts) on

(?)

Rivera plate; subducting plate has only thin sediment cover; trench is well defined with only localized ponded sediments; narrow (?)

N Cocos accretionary prism = wide (?) W; alpha = ?.

3-20 5.9 . ..

Zone is exceedingly seismogenic (42 M 7+

<6) events since 1900); subducting plate has only thin sediment cover; trench is well defined wiin only localized ponded sediments; very narrow accretionary prism (35 km) may Tierra del Fuego 8 10 produce anomalously wide W: a lph a = 0.5.

2.1 . . No active volcanic arc opposite northem

(?)

part of zone where an active spreading ridge ridge was subducted during past 6 Ma; thermal characteristics of this aseismic ses-ment presumably anomalous; alp h a = ?.

N '

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