A study of squeaky glass beads squeezed between powerful pistons offers one explanation for how these earthquake warning signals form. The findings were published online Nov. 4 in the journal Geophysical Research Letters.

The signals range from tiny shocks along the fault, which may be beyond the limit of detection by today's seismic monitors, to earthquakes large enough to rattle houses. The common thread is that the final rip — the main earthquake — strikes at or near the site of the smaller, earlier breaks.