Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays

Abstract

[1] Recordings of deep Tonga earthquakes from two arrays of 12 broadband seismographs each in the Fiji and Tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the Tonga subduction zone. The arrays operated as part of the Seismic Arrays in Fiji and Tonga (SAFT) experiment from July 2001 to August 2002. 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 topography and sharpness. 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-km discontinuity are observed. These anomalies are interpreted as being induced by the active, steeply subducting Tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil Vityaz trench, respectively. Interpreting the deflections of the 660-km discontinuity in terms of local temperatures implies a thermal anomaly of À800°K to À1200°K at 660 km depth. Except for the southern region where it may thicken, the width of the depressed 660-km discontinuity region implies that the Tonga slab seems to penetrate the 660-km discontinuity with little deformation. Waveform modeling suggests that both the 410-and 660-km 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-km discontinuity thickness shows somewhat more variability and ranges from 2 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.Citation: Tibi, R., and D. A. Wiens (2005), Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays,

Planetary Seismology

“…Moreover, the discontinuity almost disappears for the high 40% iron content. These transitions contrast with those on Earth, for which both the reduced iron content and the higher pressure gradient result in sharp discontinuities, with a typical width of < 10 km (e.g., Tibi and Wiens, 2005). Reflections from mantle discontinuities on Mars might be less impulsive than reflections on Earth, and as a result, they might only be observable only at LPs, when the seismic wavelength is much greater than the width of the discontinuity, or below frequencies of 0.05-0.1 Hz.…”

Deep Earth Structure - Transition Zone and Mantle Discontinuities

“…Xu et al (1998) and Xu et al (2003) found globally a sharp 660 (2 km or less) and a less sharp 410 from P 0 P 0 precursors. Tibi and Wiens (2005) concluded from short-period observations of local earthquakes a sharpness of 0-2 km for the 660 and 2-10 km for the 410 in the Tonga subduction zone. Castle and Creager (2000) identified that the 660 is locally < 10 km wide at the Izu-Bonin subduction zone using source side S-to-P conversions recorded in the western United States.…”

“…Gilbert et al (2001) confirmed such observations with ocean-bottom seismic data. Tibi and Wiens (2005) studied the 410 and 660 with local high-frequency seismic phases recorded by an array of broadband island stations. Tibi and Wiens (2005) studied the 410 and 660 with local high-frequency seismic phases recorded by an array of broadband island stations.…”

Deep Earth Structure – Transition Zone and Mantle Discontinuities

“…Castle and Creager (2000) found that the 660 is locally less than 10 km wide at the Izu-Bonin subduction zone using source side S-to-P conversions recorded in the western United States. Tibi and Wiens (2005) concluded from shortperiod observations of local earthquakes a sharpness of 0-2 km for the 660 and 2-10 km for the 410 in the Tonga subduction zone. Estabrook and Kind (1996) concluded from the missing precursors of PP from the 660 that the velocity and density jump here is globally significantly smaller than predicted by the IASP91 model.…”

“…Gilbert et al (2001): Ocean-bottom receiver functions; 410 uplifted by 30 km and 660 depressed by 20-30 km. Tibi and Wiens (2005): Reflections and conversions at the 410 and 660 of local high-frequency seismic phases recorded by an array of broadband island stations; practically, no change in the 410 depth; 660 depressed to 714 km (see Figure 9). Tibi and Wiens (2005): Reflections and conversions at the 410 and 660 of local high-frequency seismic phases recorded by an array of broadband island stations; practically, no change in the 410 depth; 660 depressed to 714 km (see Figure 9).…”