GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 7, PAGES 1065-1066, APRIL 1, 2000 Comment on "Subduction in the Indo-Burma region' Is it still active?" by S. P. Satyabala Marco Guzmfin-Speziale Unidad de Ciencias de la Tierra, Instituto de Geofisica, UNAM, M6xico James F. Ni Department of Physics, New MexicoState University The nature of subductionin the Western Sunda Arc has been, indeed, a puzzle, as Satyabala[1998] (hereafter referred to as SA98) points out. That is why manypapers havebeenwritten about this arc [e.g.,Fitch, 1970;Currayet al., 1979;Lawyerand Curray, 1981; LeDam et al., 1984; Chen and Molnar, 1990]. We also undertook the problem andpublished two papers onthe subject: Ni et al. [1989] (hereafter NI89) and Gumnfin-Speziale andNi [1996] (hereafter GU96). A third [Gumnfin-Speziale and Ni, 1993]also discusses seismotectonic aspects of the area. In this commentary, we would like to refer to critical differ- ences betweenSA98 and our work, particularly in view that SA98 is probably not awareof the existence of GU96, much less of the conclusion we reached in thatpaper: Available evidence suggests thereis no activesubduction along the Burmese Arc at present. Shape of the Wadati-Benioff Zone Bothin NI89 andin GU96, we obtained the geometry of the subducted slab by fitting Hypocentral Trend Surfaces [Bevis and Isacks, 1984] to good-quality hypocentral data (from the International Seismological Centre- ISC). We found that the isodepths of the Wadati-Benioff zone (WBZ) are arc-shaped, closely following the trendof the Indo-Burman ranges. SA98 reproduced ourresults in his Figure 1. It is important to notethat the surface we fitted continues to the south to aboutlatitude 19 øN, while in SA98 it is truncated at 21 øN. It also looks as if the 100km and 140km (andpar- ticularly thelatter)isodepths areNS trending while ours arear- cuate features parallel to the Burmese Arc (Figure1 in NI89 and Figures 4.3, 4.4, and 4.5 in GU96). Thedifference is significant because the geometry of the WBZ at depth is key to the understanding of subduction and the state of stress within the subducted slab. We regret the reading Satyabala makes of NI89. For exam- ple, on page3190 he states that the "...dip angleof the non- planar WBZ surface determined by Ni et al. [1989]ranges from 20o-40 TM, while we clearly indicated in NI89 that it has a dip which "... varies from about 50 øin thenorth near theHimalayan syntaxis to about 30 ø in the Bay of Bengal area."(p. 68). He doesnot mention, either, that the WBZ doesnot have a uniform dip angle, aswe stress in GU96 (p. 67). The depth to crystalline basement beneath the Bengal Basin mentioned in Chen and Molnar[1990] (p. 12528) (incidentally, Copyright 2000 by theAmerican Geophysical Union. Paper number1999GL005428. 0094-8276/00/1999GL005428505.00 proposed by Evans [1964], andnot by these authors) can hardly be considered part of the subducted slab. The isodepths go from one to 13 km (not from 30 to 45 km, as stated in p. 3192 of SA98), they are in a localized area southwest of the Shillong Plateau, and the closest isodepth is at least 300 km from the Indoburman Ranges. Seismicity The depthof the earthquakes is another crucial parameter in a subduction environment. For thrust-faulting earthquakes, for instance, it determines whether the events are inter- or intra- plate which, in turn, solves the question of the title in SA98: is subduction still active? In our analysis we found strike-slipeventsalong the arc at depths fromm25 to 75 km. The pressure axes are consistently aligned along the arc whereas T axes pointtowards the steepest slopeof the subducted slab. Deeper earthquakes (from 90 to 150 km) show a similartrendfor P axes but T axes plunge more steeply, resulting in thrust-faulting earthquakes. In GU96 we interpreted the two groups of events as follows: shallow stresses are dominated by arc-parallel bendingwhereas deeperwithin the subducted plate, slabpull playsa majorrole. This analysis is not possible if bothgroups of earthquakes are taken together and/orif some of the isodepths of the WBZ are assumed to be oriented N-S, as is done in SA98. In any case, we found no interplate earthquakes along theBurmese Arc. Contrary to what is suggested by SA98, historic seismicity (although relatively abundant and with largemagnitudes in the period 1897 to 1952) is not directly relatedto subduction. The 1912, 1931, and 1946 events are mostprobably relatedto the right-lateral Sagaing Fault. Their shallow depths [Pacheco and Sykes, 1992], together with the descriptions that LeDain et al. [1984]makeof these earthquakes, agree with activity along this fault (phrases suchas ".zone of intensity VII elongated in a north-south direction.." or "...bent raikoad tracks..." are very illustrative). The great1897 and 1950Assam earthquakes are related to the shallow-dipping, east-west trending Himalayan detachment, while the 1947 event is a foreshock of the 1950 event [Seeber and Annbruster, 1981]. Further,isoseismals for bothearthquakes are E-W [Seeber andArmbruster, 1981 ], the fault-plane solution for the 1950event shows a shallow-dipping E-W trending plane, andsixmid-sized earthquakes (1963-1988) in the same areahaveP axes trending N-S [Chen andMolnar, 1990]. Thus, none of the large historic events is indicative of active subduction along theIndoburman Arc. The Sagaing Fault Another key elementin in the tectonics of Burma is the Sa- gaing Fault. This right-lateral fault extends for about1,100 km 1065