Variations in Thickness of the Upper Mantle Transition Zone

We analyze recordings from more than 3,000 shallow earthquakes and measure the S410S S660S di erential travel times on the global scale. These di erential times are obtained using a cross-correlation technique that minimizes e ects associated with a priori assumptions of crustal thickness and velocity variations above the transition zone. Our measurements show absolute perturbations up to 8 seconds from the global average, which imply lateral variations of 20 km in transition zone thickness, on the length scale over 2,000 km. This range of variations is consistent with those reported by earlier studies of SS precursors (e.g., Flanagan and Shearer, 1998; Gu et al., 1998), but is signi cantly larger than a recent result from P -to-S converted waves (Chevrot et al., 1999). The transition zone under major subduction zones, e.g., western Paci c and South America, is signi cantly thicker than global average, which indicates the strong thermal in uence of subducted slabs. Anomalously thin transition zone is observed in the central Paci c, northern Atlantic and South Africa. We also observe a slight negative correlation between the S410S S660S times and the predicted S-travel time perturbations in the transition zone; this implies that largescale topographic variations of the transition zone discontinuities are considerably a ected by variations in temperature. A regional analysis shows a continent{ocean di erence, i.e., the transition zone is generally thicker under continents than under oceans, though we do not see a clear correlation of thickness with age. 1. The issue of a debate A number of studies of secondary phases have provided regional and global constraints on the thickness of the upper mantle transition zone. For example, Dueker and Sheehan (1997) observed a local thickness of 241 km across the Yellowstone hotspot track with absolute variations greater than 20 km. Gu et al. (1998) analyzed a large collection of SS precursors and obtained an average transition zone thickness of 240 km, with perturbations in excess of 25 km on the global scale; these estimates were supported by the results of Flanagan and Shearer (1998) from a similar but slightly smaller data set. Gossler and Kind (1996) found a correlation between thickness of the transition zone and surface tectonics, thus suggesting that deep roots of continents may extend into the upper mantle transition zone. The accuracy of these results were questioned, however, by a recent study using P -to-S converted waves (Chevrot et al. 1999). This study combined new and some existing measurements of Pds, and concludes that the transition zone is \ at" (with absolute lateral variations < 10 km) on the global scale. In addition, in areas where the Pds observations overlap with the former SS precursor observations, discrepancies of up to 15 km in predicted transition zone thickness are observed. The global average thickness of the transition zone, as inferred from Pds, is more than 8 kilometers thicker than that obtained by the SS precursors. Such large disagreements from using two di erent data sets require further explanation. We do recognize that there is a signi cant di erence in the resolution of the SdS and Pds phases in terms of imaging transition zone thickness. Our study, like the others that use SS precursors, is not aimed at the short wavelength structure associated, for example, with the topography of the 660 km discontinuity in the immediate vicinity of a subducted slab (e.g., Wicks and Richards, 1993). The size of the half-period Fresnel zone is on the order of 1; 500|2; 000 km, and hence such smallscale perturbations would be averaged out or strongly attenuated. Our resolution will also be limited at short wavelength because the size of the averaging cap (10 ) implies an e ective low-pass ltering with a corners at about ` = 10, or the half-wavelength of about 2,000 km. The half-period Fresnel zone of a 6-second P -to-S converted wave, on the other hand, has an oval shape and a diameter of 200{300 km. This means that most of the long-wavelength topography on the transition zone discontinuities, imaged by SS precursors, has little e ect on the travel times of Pds. Di erence in the lateral resolution of these two types of phases could, and should in theory, account for part of the discrepancy between the results of P -to-S converted waves and that of SS precursors. The rapid increase in the number of good quality stations from the global seismographic network in recent years, as well as the need to improve the modeling techniques, provide motivations for us to undertake a new global study of transition zone thickness.