DETAILED STRUCTURE OF THE TOP OF THE MELT BODY BENEATH THE EAST PACIFIC RISE AT 9-DEGREES-40'N FROM WAVE-FORM INVERSION OF SEISMIC-REFLECTION DATA

We have applied waveform inversion to multichannel seismic reflection data collected at the East Pacific Rise at 9 degrees 40'N in order to determine the precise velocity structure of the magma body causing the axial magma chamber reflection. Our analysis supports the idea of a m olten sill as previously suggested from forward modeling of seismic da ta from this location. Our inverted solution has a 30-m-thick sill wit h a P wave seismic velocity of 2.6 km s(-1). Although not well constra ined by the data we believe that the S wave velocity in the sill is no t significantly different from 0.0 km s(-1). The low P-and S wave velo cities in the sill imply that it contains less than 30% crystals. The molten sill is underlain by a velocity gradient in which the P wave ve locity increases from 2.6 to 3.5 km s(-1) over a vertical distance of 50-m. The shape of our velocity-depth profile implies that accretion o f material to the roof of the sill is minor compared to accretion to t he floor. The underlying velocity gradient zone may represent crystal settling under gravity. We suggest that only material from the 30-m-th ick layer can erupt.