  Tectonic tremor has been explained as a swarm of low-frequency earthquakes,
which are located on a narrow fault at the plate boundary. However, due to the
lack of clear impulsive phases in the tremor signal, it is difficult to
determine the depth of the tremor source with great precision. The thickness of
the tremor region is also not well constrained. The tremor may be located on a
narrow fault as the low-frequency earthquakes appear to be, or distributed over
a few kilometers wide low shear-wave velocity layer in the upper oceanic crust,
which is thought to be a region with high pore-fluid pressure. Lag times of
peaks in the cross-correlation of the horizontal and vertical components of
tremor seismograms, recorded by small-aperture arrays in the Olympic Peninsula,
Washington, are interpreted to be S minus P times. Tremor depths are estimated
from these S minus P times using epicenters from a previous study using a
multibeam backprojection method. The tremor is located close to the plate
boundary in a region no more than 2-3 kilometers thick and is very close to the
depths of low-frequency earthquakes. The tremor is distributed over a wider
depth range than the low-frequency earthquakes. However, due to the uncertainty
on the depth, it is difficult to conclude whether the source of the tremor is
located at the top of the subducting oceanic crust, in the lower continental
crust just above the plate boundary, or in a narrow zone at the plate boundary.
