ML20151P842
| ML20151P842 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 05/06/1988 |
| From: | Melosh H ARIZONA, UNIV. OF, TUCSON, AZ |
| To: | |
| Shared Package | |
| ML20151H012 | List:
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| References | |
| NUDOCS 8808100204 | |
| Download: ML20151P842 (1) | |
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16 A Finite Element Study of Strain and Uplift in the Pacific Northwest H. J. MELOSH Lunar and Planetary lab and Department of Geosciences, University of Arizona, Tucson, AZ 85721 Considerable controversy has surrounded the interpretation of geodetic measurements in the Pacific Northwest.
On the one hand Ando and Balazs (1979) argue that uplift in western Washington indicates that the Juan de Fuca plate is subducting aseismically. On the other hand Lisowski et al. (1987) have measured a compressive strain rates ranging between 0.03 and -0.11 pstrain/yr in the Puget trough and vicinity that they argue indicates accumulating stress on a locked fault. I constmeted a finite element model of the Juan de Fuca plate and western Washington to study both uplift and strain in a consistent manner. The two dimensional model incorporates the known geophysical structure of the region, including the bend in the slab at about 40 km depth. Earthquake dislocations are introduced via an efficient split node technique; steady slip is simulated by either internal slide lines or prescribed split node displacement rates. A large number of models was investigated to determine the slip configuration that best fits both the uplift and horizontal strain data.
None of the runs with steady slip were able to reproduce both the observed uplift I
and compressive strain. Steady slip models that fit the uplift data predict a compressive strain more than 20 times smaller than observed. Models in which the fault is presen locked fit the data best, although they predict a rather complex pattern of uplift and strain.
The evidence for coseismic coastal downwarping presented by Atwater (1987) requir the coseismic slip drops to zero in the vicinity of the coast. The fault raay be slippin aseismically beneath the Puget Trough and fartherinland. Recurrence times on the order of 300 to 500 years seem to fit the data best. In the models investigated so.far, the best fits are obtained in the last 95% of the earthquake cycle. Such models must be tested by furt measurements of contemporary strain and uplift.
REFERENCES Ando, M. and Balazs, E. I. (1979). /. Geophys. Res. 84, 3023 3028.
Atwater, B. (1987). Science 236, 942 944.
Lisowski, M., Savage, J. C., Prescott, W. H., and Dragert, H. (1987). EOS 68, 1240.
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