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The Deep Earthquakes of 1997 in Western Brazil

Department of Geological Sciences
Northwestern University
Evanston, Illinois, 60208

The first 20% of the full text of this article appears below.

	Introduction

 
We report here on three 1997 deep earthquakes in Western Brazil, which extend the Northern end of the Peru–Brazil deep cluster and close the spatial gap with the deep shocks of 1921–1922, previously studied by Okal and Bina (1994) (hereafter Article I).

Deep seismicity in South America has been described by many authors, most recently Kirby et al. (1995). It is characterized by significant lateral heterogeneity in the along-strike direction (generally north–south), with the activity arranged in clusters (see Figure 1). In particular, prior to the great 1994 Bolivian earthquake, no deep earthquakes were known between the southern termination of the Peru–Bolivia cluster at 13.5°S, 69.3°W and the northern end of the central Bolivia cluster at 16.8°S, 64.4°W. The great event of 09 June 1994, its aftershocks, and the subsequent earthquakes on 08 August 1994, 14 March 1995, and 28 November 1997 mapped an essentially continuous line along the Bolivian jog in the Benioff Zone, suggesting that the slab in that region is warped, rather than torn and fragmented (Kirby et al., 1995). To the north, we described in Article I the three major shocks of 18 December 1921, 17 January 1922, and 31 July 1970 as isolated, with a gap of 265 km between the northern end of the Peru–Brazil cluster (then mapped at 6.67°S; 71.82°W) and the 1921–1922 hypocenters, and a further 238 km between the latter and the 1970 event to the North. The 1997 shocks essentially close the first of those two gaps.

View larger version (74K): [in this window] [in a new window]   Figure 1. Map of South American deep seismicity (h > 400 km). The open circles are NEIC epicenters (1963 to present), complemented by the relocations of the . . . [Full Text of this Article]

	The 1997 Events: Location

	Focal Mechanism

	Discussion