  We report an analysis of the archival Rossi X-ray Timing Explorer (RXTE) data
from the December 2004 hyperflare from SGR 1806-20. In addition to the 90 Hz
QPO first discovered by Israel et al., we report the detection of higher
frequency oscillations at 150, 625, and 1,840 Hz. In addition, we also find
indications of oscillations at 720, and 2,384 Hz, but with lower significances.
The 150 Hz QPO has a width (FWHM) of about 17 Hz, an average amplitude (rms) of
6.8 %, and is detected in average power spectra centered on the rotational
phase of the strongest peak in the pulse profile. This is approximately half a
cycle from the phase at which the 90 Hz QPO is detected. The 625 Hz oscillation
was detected in an average power spectrum from nine successive cycles beginning
approximately 180 s after the initial hard spike. It has a width (FWHM) of 2 Hz
and an average amplitude (rms) during this interval of 8.5 %. We find a strong
detection of the 625 Hz oscillation in a pair of successive rotation cycles
begining about 230 s after the start of the flare. In these cycles we also
detect the 1,840 Hz QPO. When the 625 Hz QPO is detected we also confirm the
simultaneous presence of 30 and 92 Hz QPOs. The centroid frequency of the 625
Hz QPO detected with RXTE is within 1 Hz of the 626 Hz oscillation recently
found in RHESSI data from this hyperflare by Watts & Strohmayer. We argue that
these new findings provide further evidence for a connection of these
oscillations with global oscillation modes of neutron stars, in particular, the
high frequency signals may represent toroidal modes with at least one radial
node in the crust. We discuss their implications in the context of this model,
in particular for the depth of neutron star crusts
