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09/28/2004 Aftershock Probabilities
Tue 28 Sep 2004 11:23 AM PDT
- This forecast is based on the statistics of aftershocks typical for California. This is not an exact prediction, but only a rough guide to expected aftershock activity. This forecast may be revised as more information becomes available.
MAINSHOCK: Tue 28 Sep 2004 10:15:24 AM PDT MAGNITUDE 6.0
11 km ( 7 miles) SSE (151 degrees) of Parkfield, CA
STRONG AFTERSHOCKS (Magnitude 5 and larger)
- At this time (one hour after the mainshock) the probability of a strong and possibly damaging aftershock IN THE NEXT 7 DAYS is approximately 50 PERCENT.
EARTHQUAKES LARGER THAN THE MAINSHOCK
- Most likely, the recent mainshock will be the largest in the sequence. However, there is a small chance (APPROXIMATELY 5 TO 10 PERCENT) of an earthquake equal to or larger than this mainshock in the next 7 days.
WEAK AFTERSHOCKS (Magnitude 3 to 5)
- In addition, approximately 30 to 70 SMALL AFTERSHOCKS are expected in the same 7-DAY PERIOD and may be felt locally.
Background Information About Aftershocks
Like most earthquakes, the recent earthquake is expected to be followed by numerous aftershocks. Aftershocks are additional earthquakes that occur after the mainshock and in the same geographic area. Usually, aftershocks are smaller than the mainshock, but occasionally an aftershock may be strong enough to be felt widely throughout the area and may cause additional damage, particularly to structures already weakened in the mainshock. As a rule of thumb, aftershocks of magnitude 5 and larger are considered potentially damaging.
Aftershocks are most common immediately after the mainshock; their average number per day decreases rapidly as time passes. Aftershocks are most likely to be felt in the first few days after the mainshock, but may be felt weeks, months, or even years afterwards. In general, the larger the mainshock, the longer its aftershocks will be felt.
Aftershocks tend to occur near the mainshock, but the exact geographic pattern of the aftershocks varies from earthquake to earthquake and is not predictable. The larger the mainshock, the larger the area of aftershocks. While there is no "hard" cutoff distance beyond which an earthquake is totally incapable of triggering an aftershock, the vast majority of aftershocks are located close to the mainshock. As a rule of thumb, a magnitude 6 mainshock may have aftershocks up to 10 to 20 miles away, while a magnitude 7 mainshock may have aftershocks as far as 30 to 50 miles away.
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Northern California Management Center |
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Last modified: Wed Jan 17 18:17:55 PST 2007