  A double-correlation method is introduced to locate tremor sources based on
stacks of complex, doubly-correlated tremor records of multiple triplets of
seismographs back projected to hypothetical source locations in a geographic
grid. Peaks in the resulting stack of moduli are inferred source locations. The
stack of the moduli is a robust measure of energy radiated from a point source
or point sources even when the velocity information is imprecise. Application
to real data shows how double correlation focuses the source mapping compared
to the common single correlation approach. Synthetic tests demonstrate the
robustness of the method and its resolution limitations which are controlled by
the station geometry, the finite frequency of the signal, the quality of the
used velocity information and noise level. Both random noise and signal or
noise correlated at time shifts that are inconsistent with the assumed velocity
structure can be effectively suppressed. Assuming a surface-wave velocity, we
can constrain the source location even if the surface-wave component does not
dominate. The method can also in principle be used with body waves in three
dimensions, although this requires more data and seismographs placed near the
source for depth resolution.
