Foreshock - Main shock - Aftershock Sequence

The three earthquakes shown below occurred at virtually the same location (8 km ENE of Watsonville) and within 7 minutes of each other on May 9, 2000, as detailed in the table below.

Time, PDT Magnitude Latitude Longitude Depth Designation
00:59:06 M=1.7 36.939 -121.679 8 Foreshock
01:00:55 M=3.3 36.246 -120.821 8 Main shock
01:06:02 M=2.9 36.244 -120.829 8 Aftershock

The record from station HFP (Fremont Peak; 9 miles SW of Hollister, 18 miles SSE of Gilroy; 36.754, -121.492) is shown below. The closeness in time and location of these earthquakes to each other is no coincidence. Earthquakes "interact", meaning that the stress changes produced by one earthquake can encourage others to occur nearby, thereby producing clusters of events. The most common form of cluster is the main shock and its aftershocks, the main shock being first and having the largest magnitude. When an earthquake (like the M=1.7 event here) triggers a larger earthquake (the M=3.3 event here), the first is called a "foreshock".

Helicorder display