Detailed Structure and Sharpness of Upper Mantle Discontinuities in The Tonga Subduction Zone - fig. 2
Detailed Structure and Sharpness of Upper Mantle Discontinuities in The Tonga Subduction Zone - fig. 2
Credit:
Rigobert Tibi, Douglas A. Wiens • Washington University/IRIS Consortium
Description
Figure 2. (top) Vertical cross section along line A-A’ in Figure 1 showing the bounce and piercing point locations for s410p (hexagons), P660p (circles) and S660p (squares). Thin lines indicate the nominal depths of 410 and 660 km for the discontinuities. Seismicity (dots) is from Engdahl et al. (1998). Width of the cross section is 240 km. (bottom) Enlargement showing the detailed topography of the 660-km discontinuity.
The Seismic Arrays in Fiji and Tonga (SAFT) experiment, which consisted of two arrays of broadband seismic stations in the Fiji and Tonga islands operating from July, 2001, to August, 2002, provides an ideal opportunity to perform a detailed local investigation of upper mantle discontinuities near one of the Earth’s coldest subduction zones. Recordings of deep Tonga earthquakes from the arrays are stacked and searched for reflections and conversions from upper mantle discontinuities in the Tonga subduction zone. In comparison with the commonly used teleseismic approaches, the short path lengths for the local data provide smaller Fresnel zones and high frequency content for precise mapping of discontinuity structure. This is particularly important for a subduction zone, where variations in temperature and water content are expected to cause small-scale changes in topography and sharpness of the discontinuities. To enhance the low-amplitude discontinuity phases s410p, P660p and S660p, deconvolved seismograms from each event/array pair are aligned on the maximum amplitude of the direct P wave and subsequently slant-stacked. For the 410-km discontinuity, the results show no systematic variations in depth with distance to the cold slab. The 660-km discontinuity varies between 656 and 714 km in depth. For the southern and central parts of the subduction zone, the largest depths occur in the core of the Tonga slab. For the northern part, two separate depressions of the 660 are observed. These anomalies are interpreted as being induced by the active, steeply subducting Tonga deep zone and a subhorizontaly lying remnant of subducted lithosphere from the fossil Vityaz trench, respectively (Chen and Brudzinski, 2001). Interpreting the deflections of the 660 in terms of local temperatures implies a thermal anomaly of –800K to –1200K at 660 km depth. Except for the southern region where it may thicken, the width of the depressed 660 region implies that the Tonga slab seems to penetrate the 660 with little deformation. Waveform modeling suggests that both the 410 and 660 discontinuities are sharp. The 660 km discontinuity is at most 2 km thick in many parts of the region, and a first-order discontinuity cannot be precluded. The 410 thickness shows somewhat more variability and ranges from 2 km to 10 km outside the slab, and is at most 10 km thick within the slab. This suggests that the subduction process does not produce dramatic effects on the sharpness of the discontinuities.
Tibi, R., and Wiens, D.A., Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays, J. Geophys. Res. 110, B060313.
Photographer / Contributor: Rigobert Tibi, Douglas A. Wiens • Washington University
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