One magnitude unit reduction in detection threshold by cross correlation applied to Parkfield (California) and China seismicity

From a local high-resolution base catalog at Parkfield, California, 5076
earthquakes (M 0.2 to 6) are used to study the comparative performance of a correlation
detector and standard energy detector on the sparse regional network of continuously
operating stations. Eighty-six percent of the events detected by a standard
energy detector can also be detected by cross correlation. Correlation detection is able
to find additional events by lowering the detection threshold by about 1 unit beyond
what standard processing detects for Parkfield, a factor of 10 increase in number of
events such as those predicted by Gutenberg–Richter. Most event separation distances
for events that correlate at Parkfield are less than 1 km. The distribution of magnitude
differences for events that correlate at Parkfield is not distinguishable from the input
magnitude distribution. More robust measures to quantify reduction in detection
threshold are introduced. Detection magnitude threshold reduction of about 1 unit
holds for large-scale application to the 18,886 events in China and 5,076 events
in Parkfield with false-alarm rates of a few percent. Large and small events are seen
to correlate well enough for detection. Two examples are shown with magnitude
differences as large as 2.3 and 3.3 units. The correlation detector also finds two cases
of buried aftershocks in the coda of mainshocks that were previously unreported in the
Annual Bulletin of Chinese Earthquakes (ABCE).