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).