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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.
Typically, the chance of an earthquake comparable to or larger than today's earthquake is 5-10% in the next 7 days. Of the six historical earthquakes of comparable size that have occurred in the Parkfield region, one, in 1857, was followed 9 hours later by a larger earthquake that ruptured the San Andreas fault to the south, and caused widespread damage in southern California. Since that time our measurements indicate that insufficient slip has accumulated to allow that event to repeat and so we judge the probability of that event recurring soon as unlikely and we note that the other five historical Parkfield earthquakes were not followed by a larger earthquake.
In this unlikely event that a comparable or larger event occurs, such an earthquake would likely rupture the San Andreas fault toward the south, and be felt most strongly in southern California. It is unlikely that a larger earthquake will rupture the San Andreas fault toward the north (toward the SF Bay region) because this portion of the San Andreas fault is slowly creeping and not thought to have accumulated sufficient stress for a larger earthquake.
The likelihood of all aftershocks, including the relatively unlikely larger earthquakes described above, is greatest during the first day of aftershocks, and diminishes rapidly with time.