Seismic events along a geophysical fault-line
are recorded in (space, time) pairs, where the time component
represents the distance of the point along the fault. A small
fraction of the pairs result from bursts of energy which propagate
along the fault at approximately constant speeds, causing
seismic events as they go. These bursts generate roughly linear
clusters of points in the (space, time) plane, the gradient
of the line being proportional to the speed at which energy
travels along the fault. Identifying these clusters is of
intrinsic geophysical interest. The fault line in question
is a section of the San Andreas fault near Parkfield, California.
We discuss statistical methodology for identifying approximately
linear clusters of points. This problem has connections to
that of ley-line analysis, addressed in the UK in the 1980s.