Teleseismic body and surface wave studies reveal a broad (200-km-wide) lowvelocity region in the upper mantle beneath the East Pacific Rise (EPR), indicating a
broad thermal anomaly and melt-containing region in the mantle upwelling zone. In
contrast active-source seismic experiments reveal a narrow (6-km-wide) low velocity
region in the crust, indicating a narrowly focused crustal magmatic system. We use
short-period Love wave data to investigate the transition between the deep and shallow
portions of the magmatic system beneath the southern EPR. The data consist of
waveforms generated by regional earthquakes and recorded on the MELT seismic array
of ocean bottom seismometers located near 17S. The data provide information on the
structure of both the crust and uppermost mantle. We filter the waveforms at a range of
center frequencies between 3-20 s to obtain a suite of wave packets and then model the
group time, phase, and amplitude of each packet. Our method accounts for lateral
refraction of surface wave energy. We invert for the phase velocity distribution as a
function of frequency and position from the spreading center, and constrain the width
and magnitude of the low velocity region beneath the EPR. Love wave energy traveling
parallel to the EPR experiences relative travel time delays and amplitude variations as a
function of distance from the spreading center. The data show clear evidence for lateral
focusing of seismic energy due to the low velocity region beneath the rise. At longer
periods, energy is refracted towards the rise such that stations near the rise record
relatively larger amplitude arrivals. The data are consistent with a broad (100-km-wide)
low-velocity region in the uppermost mantle. At shorter periods a shallow and narrow
(10-km-wide) low-velocity region also focuses energy and acts as a lateral wave guide
for the surface wave energy. Energy trapped in this guide exhibits significant
multipathing and the character of the wave packets is more complex. Inversions for
shear wave structure show an abrupt transition from the deep and broad low-velocity
region to the shallow and narrow region. While the shallow, narrow low-velocity
region is roughly symmetrical beneath the rise, the deeper, wider low-velocity region is
strongly asymmetric across the rise. In addition the lithosphere thickens more slowly
west of the rise as compared to the lithosphere east of the rise.