The use of closely spaced receiver arrays to delineate the structure of the upper mantle under Northern Australia

Abstract

Data from three seismograph arrays in the Northern Territory are used to investigate the velocity structure of the upper mantle above the 400 km discontinuity. These arrays are the permanent 26 km aperture Warramunga (WRA) array southeast of Tennant Creek, a 90 km aperture temporary digital array northeast of Tennant Creek, which includes a closely spaced sub-array (RDA), and a 500 km aperture analog-tape array straddling the two medium aperture arrays. Together, these arrays have 66 stations. The WRA and RDA arrays have receiver spacing of 2 km and 5 km, respectively, and are separated by 100 km. Both arrays can be phased to enhance arrivals with a given slowness and azimuth. A composite record section is made with seismograms from 17 shallow Indonesian earthquake sources 1250 km to 2150 km distant. Receiver spacing of less than 5 km is achieved over most of the record section as a result of the medium aperture arrays, while phases from individual events can be tracked at 40 km intervals across the 500 km analog array. This record section overcomes shortcomings of some previous studies which relied on either sparse, very-large-aperture arrays where phase correlation across the array was difficult, or on WRA data alone where phase correlation among different earth-quakes was uncertain. Beam forming is used to examine the character of the reflection from the 400 km discontinuity for two earthquakes 1600?1700 km away. Differences in beam-formed seismograms at the WRA and RDA arrays for one event suggest lateral heterogeneity near the 400 km discontinuity.