Nov 15: The Parkfield ShakeMap has been
recently recovered values of peak ground velocity
from the CGS stations.
Oct 01: A close review of the
Parkfield aftershock sequence has revealed
some overestimated magnitudes for events
occurring in the tail of larger earthquakes.
In particular, the event immediately after
the M6 mainshock is not a large as originally
determined. The revised magnitude is M4.2.
At this point in time, the M6 has two aftershocks
of M5 - one on 9/29 at 17:10 UTC (10:10 AM PDT) and one on 9/30
at 18:54 UTC (11:54 AM PDT).
Another magnitude 5.0 earthquake
shook the Parkfield area at 11:54 AM local time.
The Parkfield area experienced a sizeable aftershock today - an
at 10:10 AM local time. This earthquake occurred toward the northern end
the rupture zone in yesterday's M6. Also, an
occurred east of Bakersfield at 3:54 PM local time. The
earthquake near Bakersfield did not occur on the San Andreas
fault. Because of the timing and distance of this event with the 2004
Parkfield earthquake, there may be a correlation between the events.
This earthquake is the anticipated
Mw 6.0 on the San Andreas fault.
It ruptured roughly the same segment of the fault that broke in 1966.
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.
Strong shaking during this event lasted for about 10 seconds.
Today's earthquake 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.
The previous two earthquakes ruptured the opposite direction from NW to SE along this section.
Just to the north of the rupture, the National Science
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.
The Parkfield area has been the focus of intensive research since the mid-1980s. Today's
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.
The CISN Northern California Management Center produces
for events of M3.5 and higher. ShakeMaps are based on the observed ground motions from
seismic instruments combined with predicted motions in areas without sensors. In this sequence
ShakeMaps have been produced for several events.
The initial ShakeMaps did not have many direct observations. However, as data from
the many strong-motion sites associated with the Parkfield Prediction Experiment have
been processed, more values have been added. The
most recent update was
Nov 15th and included many recently recovered values of
peak ground velocity from the CGS stations.
The CISN Engineering Data Center
produces reports showing details of
ground motions for events of interest. Although data are still being
collected, the largest observed peak ground acceleration is 0.85G from
a site located about 9 km southeast of the epicenter.
Many earthquakes in this swarm have been widely felt. People with Internet access
can report their observations of shaking and damage through the
Did You Feel It?
Web site. In the links below, you can see the maps illustrating the reported
intensities for several events in this sequence. Unlike the ShakeMaps - which
are based on data from seismic instruments - these maps are based on human
observations. You can contribute to these data by filling out the form for each event.