Seismic moment of aftershocks during the August 23, 1995 deep earthquake sequence in the Mariana amounts up to 17% of that of the main shock (Mw 7.0). Such a high aftershock activity is extremely unusual. Based on the inversion of broadband P and SH waveforms, however, we found nothing anomalous in the source mechanisms. Among the 33 largest deep earthquakes (Mw≥7.0) since 1958, only events in the Mariana have high aftershock activity. We associate this observation with the unique configuration of the Mariana slab which seems to plunge sub-vertically into the lower mantle with little distortion. ¿ 1999 American Geophysical Union