Frequently Asked Questions about the Magnitude 5.2 near Redding
Rick McKenzie, David Oppenheimer, Lind Gee
Is this an unsual earthquake for Redding?
Seismicity in the Redding area is not new.
Searching our catalog for
magnitude 3 and larger earthquakes in the area reveals many events dating
back to 1950. Prior to 1950, we don't have magnitude information on
events (the Richter Magnitude scale wasn't used systematically in
northern California until the 1940s or so).
Granted, the magnitude 5.2 earthquake which hit on Thanksgiving Day just
as we all sat down to our luncheon feasts is the largest event with
magnitude given in the file. But that does not mean a lot considering
how brief the catalog is in the geologic scheme of things.
As of 12/03/98,
35 aftershocks have been recorded, the largest of
which was magnitude 4.4. This earthquake occurred in the same area
as a magnitude 4.5 event 0n 07/21/98. The depth of most of these events ranges
between 23-25 km (~ 15 mi). Most earthquakes in California are relatively
shallow, that is, they occur at depths less than 12-15 km. However, in
the northernmost part of the state, the
Gorda plate (which is part of the
Juan de Fuca plate) is subducting under North America
and we see earthquakes at depths up to 90 kms. The location
of the Gorda plate is marked by diffuse deep seismicity between 40 and 42
degrees latitude.
The Redding sequence is located in North American
plate, above the depth where seismologists believe the Gorda plate is
subducting.
Will there be larger earthquakes?
We have no indication that we should expect a larger event beyond the
statistics for California as a whole. These give a probability of between
1-5% that something significantly larger will follow based on past
sequences throughout the state. It is much more likely that this
sequence will continue as it has been with fewer and fewer events as
time passes. This does not say anything about the size of those events.
It is entirely likely that there will one, or more, events in the
magnitude 4+ range weeks, or even months from now. You will find
more information in the aftershock
warning which was issued after the earthquake.
Can you tell me the name of the fault Redding lies on?
No. There is a small fault trace in the generally vicinity of the recent
epicenter on the California Division of Mines and Geology Fault Map of
California (1994), but they do not show a name for this fault. As the
plot thickens, these hypocenters are 23-25 km (around 15 miles) deep. As
of yet there is no evidence to link them with this or any other surface
fault trace. Our analysis goes on and perhaps will eventually reach some
conclusion on this.
For more information on fault and seismic hazard maps, please see our
FAQ on this subject
which lists resources for maps.
Do these earthquakes have any relation to the nearby volcanoes?
For more information on Mt. Shasta and Medicine Lake, we recommend the
USGS Cascade Volcano Observatory
Web page. These earthquakes do not appear to be related to any
current unrest at Mt. Shasta and/or Mt. Lassen and there is not forecast statement for these
volcanoes. The USGS operates a dense network of stations to monitor
these volcanoes and there is no anomalous activity at the present.
There have stories about the release of methane gas in the
Redding vicinity, but this appears to be biogenic in origin and is
appearing now because of the ground shaking during the earthquake.
Magnitude 3-4 seismicity in this area during the period 1944-1963
has been associated with the filling of Lake Shasta (Perman et al.,
1981, 1983). According to Tousson Toppozada of the California
Division of Mines & Geology, events since 1963 "occurred mainly
during periods in reservoir storage associated with droughts"
(1998 AGU abstract). The relationshop between the recent earthquakes
and Lake Shasta is an active area of research. Stay tuned!
Where can I learn more about earthquake preparedness
Return to information on the Redding earthquake
Return to UC Berkeley Seismological Laboratory
