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I. Great earthquakes and seismic coupling at subduction zones. II. The structure of the lowermost mantle determined by short period p-wave amplitudes

Citation

Ruff, Larry J. (1982) I. Great earthquakes and seismic coupling at subduction zones. II. The structure of the lowermost mantle determined by short period p-wave amplitudes. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-09062006-105439

Abstract

Part I:

Seismic coupling has been used as a qualitative measure of the "interaction" between the two plates at subduction zones. Kanamori (1971) introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern Pacific subduction zones. Great earthquakes (MW>8.5) occur in only a few subduction zones: notably the northern Pacific and South American subduction zones. A quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other subduction zone variables: age of the subducting lithosphere and convergence rate. The largest earthquakes occur in zones with young lithosphere and fast convergence rates, while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes. Two other correlations are of interest; maximum depth of the continuous Benioff zone is correlated to lithosphere age, and horizontal length of the Benioff zone is correlated to convergence rate. The simplest explanation of these correlations is "preferred trajectory": the subducting slab descends into the mantle with the vertical and horizontal rates determined by the plate age and convergence rate respectively. The mechanism of preferred trajectory is also consistent with the obversation that back-arc spreading occurs behind subduction zones that are subducting old lithosphere at a slow rate.

The rupture process of a great earthquake indicates the distribution of weak and strong regions on the fault zone between the subducting and over-lying plates. The rupture process of three great earthquakes (1963 Kurile Islands, MW=8.5; 1965 Rat Islands, MW=8.7; 1964 Alaska, MW=9.2) are studied by using WWSSN stations in the core shadow zone. The main result is that maximum earthquake size is determined by the asperity distribution on the fault plane (asperities are the strong regions that resist the motion between the two plates). The subduction zones with the largest earthquakes have very large asperities (the Alaskan earthquake is characterized by a giant asperity of length scale 150-200 km), while the zones with smaller earthquakes have small scattered asperities. This observation can be translated into a simple model of seismic coupling, where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface.

If asperity size determines earthquake size, and earthquake size is correlated to plate age and rate; then plate age and rate must be related to the asperity distribution. Plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the preferred trajectory. Indirect influences are many, including: oceanic plate topography and the amount of subducted sediments.

All subduction zones are apparently uncoupled below a depth of about 40 km, and the basalt to eclogite phase change in the down-going oceanic crust may be largely responsible. This phase change shouldstart at a depth of 30-35 km, and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change.

Part II:

The seismic velocities in the D" region (lowermost 200 km of the mantle) are recognized to be anomalously low, though the details of the velocity structure are not known. The details of D" are important, in particular whether a smooth velocity model is appropriate or not. A smooth decrease in the seismic velocities would be consistent with a thermal boundary layer at the base of the mantle. We have used the amplitudes of short period (T = 1 sec) P waves to investigate the internal structure of D". A short period amplitude data set is obtained by using underground nuclear events as sources and applying receiver corrections to the amplitudes. Receiver effects are largely responsible for the factor of ~ 8 scatter in the amplitudes of the North American WWSSN stations. Applying receiver corrections reduces the scatter to a factor of ~ 2, thereby providing a quantitatively useful amplitude profile into the core shadow. Using Soviet events and North American WWSSN statios, the D" layer beneath the north polar regio is well sampled. The core shadow (at T = 1 sec) begins sharply at a distance of [delta] = 95.5 and the slope of the amplitude decay is well defined. Also, the amplitudes decrease slightly from [delta] ~ 87 to [delta] ~ 90, then increase to [delta] = 95.5. Synthetic seismograms are used to test various earth models, with the important conclusion that the amplitudes from smooth D" models with a nearly constant velocity in D" decay too slowly in the shadow. This mismatch cannot be satisfactorily explained by random forward scattering or a thin low-Q layer within D". Anelastic calculations show that a thin low-Q layer in D" decreases the amplitudes gradually before the shadow, with little effect on the decay slope in the shadow. All of the features of the observed amplitude profile can be explained as the interference effects of a model that has a low velocity zone in the upper part of D" followed by a normal velocity gradient in the lower part of D". This type of model (POLAR series) also explains the scatter often observed in dT/d[delta] beyond [delta] ~ 90. The interference effects and required velocity changes in D" are small, and long period amplitudes will respond only to the averaged velocity gradient in D". The POLAR models imply a compositional and/or phase change at the top of D". Thus, the preferred seismological model does not allow the D" region to be interpreted as a single thermal boundary layer between the mantle and core.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geological and Planetary Sciences
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Kanamori, Hiroo (advisor)
  • Anderson, Donald L. (advisor)
  • Helmberger, Donald V. (advisor)
Thesis Committee:
  • Unknown, Unknown
Defense Date:14 December 1981
Record Number:CaltechETD:etd-09062006-105439
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-09062006-105439
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:3357
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:22 Sep 2006
Last Modified:26 Dec 2012 02:59

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