Discussion of the September 2004 magnitude 6.0 Parkfield Earthquake

Author: Andrew Michael, United States Geological Survey (USGS)

Tele-conference attendees

These are talking points for the September 28, 2004, M6 earthquake at Parkfield, California. They were developed with a lot of collaboration including Mary Lou, William Ellsworth, Mike Reichle (State Geologist), Dave Oppenheimer, Lind Gee, OES, Paul Reasenberg, Andrew Michael and many others on a conference call at 11:15 AM. The probabilities are based on both the generic model and Agnew and Jones' characteristic model.

  1. This is the anticipated Parkfield earthquake, Mw=6.0 on the San Andreas fault. It ruptured rougly the same segment of the fault that broke in 1966.
  2. This earthquake occurred at 10:15 AM PDT on September 28, 2004 had a hypocenter of 35 degrees, 49 minutes north, 120 degrees 22 minutes west, and a depth of 8 km or 5 miles. From this point along the San Andreas fault, about 7 miles SW of the town of Parkfield, it ruptured primarily northwest along the fault.
  3. Strong shaking during this event lasted for about 10 seconds.
  4. 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.
    1. 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.
    2. The chance of a magnitude 7 or larger earthquake occurring in the next 7 days is a few percent. In this unlikely event, 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.
    3. The likelihood of all aftershock, including the relatively unlikely larger earthquakes described above, is greatest during the first day of aftershocks, and diminishes rapidly with time.
  5. The previous two earthquakes ruptured the opposite direction from NW to SE along this section.
  6. The Parkfield area has been the focus of intensive research since the mid-1980s. Todays earthquake will provide us with a very detailed view of a large earthquake on the San Andreas fault and will provide scientists with new data on how such earthquakes occur and create damage.
  7. It is the seventh in a series of repeating earthquakes on this stretch of the fault. The previous events were in 1857, 1881, 1901, 1922, 1934, and 1966.
  8. Scientists from the USGS are carefully monitoring the instruments in this region for any hints of changes in deformation that might help us update these probabilities.
  9. Just to the north of the rupture, the National Science Foundations EarthScope Program, in partnership with USGS, is drilling a scientific borehole that will ultimately intersect the San Andreas fault at depth. Drilling operations ended a week ago at a depth of 8500 feet and a distance of 1500 feet from the fault.

URL: http://eqinfo.ucsd.edu/faq/parkfield04.php [Last updated: 2015-10-22 (295) 23:07:44 UTC]