Part I: Variation of Seismic Source Parameters and Stress Drop within a Descending Slab as Revealed from Body-Wave Pulse-Width and Amplitude Analysis. Part II: A Seismological Investigation of the Subduction Mechanism of Aseismic Ridges

Citation

Chung, Wai-Ying (1979) Part I: Variation of Seismic Source Parameters and Stress Drop within a Descending Slab as Revealed from Body-Wave Pulse-Width and Amplitude Analysis. Part II: A Seismological Investigation of the Subduction Mechanism of Aseismic Ridges. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/79jx-8334. https://resolver.caltech.edu/CaltechTHESIS:07072024-191031375

Abstract

In Part I, a least squares searching technique has been developed to estimate the source dimensions of intermediate and deep focus earthquakes from the azimuthal variation of body-wave pulse-width. Modes of rupture propagation, seismic moment, stress drop and orientation of the slip plane can also be determined if both pulse-width and amplitude data are used, With this method, 17 intermediate and deep earthquakes in the Tonga-Kermadec arc system have been investigated in order to determine changes in the state of stress and source properties within a subducting slab. Three different modes of rupture, unilateral, bilateral, and circular faults, are compared and tested against observations, Results indicate that the unilateral fault is the best model for most of the earthquakes studied, the bilateral fault is the best model for some shocks, but the circular fault, in no cases, is better than the other two fault models. Stress drops of intermediate and deep focus earthquakes vary within a very large range, from 20 bars to 4617 bars, and change significantly with focal depth. Two high stress drop regions at depths about 360 km and 640 km seem to correlate with the depth ranges in which phase transitions occur. A relative minimum of stress drop is found at about 450 to 560 km where the annual number of earthquakes is particularly high, Earthquakes which occurred at the northern end of Tonga arc, where the Benioff zone is laterally bent, show systematically higher stress drops than other events at comparable depths but away from the bend. Also events in low seismicity regions appear to have higher stress drops than those in high seismicity regions. Apparent stress is found to be smaller than half of the stress drop, and the upper bound of seismic efficiency appears to decrease as depth increases. A comparison of deep and shallow earthquake source parameters is also made.

In Chapter l of Part II, the subduction of the D'Entrecasteaux fracture zone-aseismic ridge system in the New Hebrides island arc is investigated on the basis of the focal process of the New Hebrides earthquake of January 19, 1969 (m_b = 6.4, h = 107 km), mechanisms of some related events, seismicity, and regional tectonics. A notable feature of this island arc is the discontinuity of the New Hebrides Trench in the central New Hebrides where the ridge­ fracture zone is subducting and intersecting the arc. The 1969 New Hebrides earthquake occurred along the subducted portion of the fracture zone and is characterized by unusual wave forms with remarkably large excitation of long period waves. Body-wave and surface wave analyses reveal the earthquake was a complex event with a change of fault strike during the rupture process. The location and mechanism of this earthquake suggest that the D'Entrecasteaux fracture zone structurally extends to the east of the Trench. This structural boundary at depth seems to be reflected in the spatial distribution of two earthquake swarms which are bounded sharply at the latitude of 15.2°S. At the extension of the ridge-fracture zone, the activity of intermediate-depth earthquakes, which are characterized by a very consistent pattern of down-dip extensional mechanism, is much higher and their depths are systematically shallower than in the adjacent regions. These features can be interpreted as a consequence of subduction of a buoyant ridge and the resultant increase in the extensional stress at the intermediate depths of the sinking slab. Fault-plane solutions of 22 earthquakes suggest that the subduction of aseismic ridges in the New Hebrides is characterized by high angle thrusts. The lithospheres on the two sides of the D'Entrecasteaux fracture zone under the arc subduct more or less independently and generate alternating left-lateral and right-lateral earthquakes along the subducted portion of the fracture zone.

In Chapter 2, tectonic features associated with a subducting fracture zone-aseismic ridge system in the New Hebrides island arc are investigated. Several notable features including a discontinuity of the trench, peculiar locations of two major islands (Santo and Malekula), regional uplift, and the formation of a basin are interpreted as a result of the subduction of a buoyant ridge system. The islands of Santo and Malekula are probably formed from uplifted mid-slope basement high while the interarc basin of this particular arc is probably a subsiding basin instead of a basin formed by backarc opening. The situation can be modeled by using a thin elastic half plate overlying a quarter fluid space with a vertical upward loading applied at the plate edge. This model is consistent with topographic and geophysical data. This study suggests that subduction of aseismic ridges can have significant effects on tectonic features at consuming plate boundaries.

In Chapter 3, seismicity near locations of aseismic ridge sub­ duction has been investigated for five seismic regions: New Hebrides, Bonin-Mariana, Tonga-Kermadec, Peru and Northern Chile. The maximum focal depth of intermediate-depth earthquakes beneath areas of aseismic ridge subduction is generally shallower than that on either side of the subducting ridge. This variation of focal depth in some cases forms a well-defined gap or quiet zone for intermediate-depth events. This phenomenon may be a consequence of differential subduction and difference in material properties between an aseismic ridge and the rest of the oceanic plate. Although at locations of aseismic ridge subduction large shallow earthquakes occur less frequently compared with the adjacent regions, small shallow earthquake activity is not always reduced. In addition, there is a high correlation between trench discontinuities and aseismic ridge subduction suggesting that a trench can be interrupted and divided into two sections by ridge subduction if the buoyant force associated with the ridge is strong enough.

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