PACIFIC-ANTARCTIC RIDGE FAR-FIELD AFTERSHOCK FORECAST AUGUST 6, 2001

An earthquake of Mw 6.5+ occurred on the morning (UT) of August
6 along the Pacific-Antarctic Ridge. It was the largest event within
500 km of the epicenter since March 1971 when a Ms 6.6 occurred, and the
third largest in this area in recorded history. The largest event occurred
as a Ms 7.0 on September 3, 1944.

Current parameters for this event are (times are UT)

06Aug2001 03:52:59 55.6S 123.2W MW=6.5 NEIS SOUTHERN EAST PACIFIC RISE

The event is likely to have had a strike-slip mechanism. This
type of event seldom produces tsunamis, so a tsunami is unlikely
except as a landslide tsunami in local areas.

Although a Mw 6.5 earthquake is normally too small to produce
wide-spread far-field triggering, regional triggering is possible
especially when the event occurs in an unusual position with
resulting horizontal motions of the earth as did this one.

Following is a preliminary far-field forecast from this event.
Areas within 36 degrees are expected to see maximum magnitudes increase
by 0.4-0.7 units, at distances farther than this, increases in
maximum magnitude are expected to be about 0.5 units or less.
Seismicity is expected to increase in these areas for the next
7 days (i.e. through 13 August, 2001):

9-10 degrees: Pacific-Antarctic Ridge events and aftershocks
17-19 degrees: East Pacific Rise near 37S
34-36 degrees: East Pacific Rise near 20S
43-45 degrees: Central Chile (about 30 S), southern So. Sandwich Islands,
Alpine fault zone, New Zealand (strikes this area with
wave front parallel to the fault, this is a situation where
a moderate event can be triggered if strain conditions in this
region are otherwise favorable).
59-61 degrees: Loyalty Isl., Fiji, Samoa Ecuador, Galapagos
71-73 degrees: SE Solomon Islands, Santa Cruz Isl. Central
American subduction zone. The wave front is parallel to
the Mexican Subduction zone, a moderate to large event
is possible in this region especially given the effect
of landfall of Hurricane Barry tonight. This event could
be Ms>6 in the next 3 days along the Mexican Subduction zone.
103-106 degrees***: South of Sumatera (100-106E), Molucca Sea, Sulawesi,
Central Marianas, Vancouver B.C, Seattle, Spokane, WA, Central
Mid-Atlantic Ocean, Indian Ocean near Mauritius (triple junction)
142-146 degrees: Nepal, No. of Iceland, Greenland area,
France, Sicily, Arctic Ocean
176-180 degrees: Aseismic area 56N 58E east of Moscow, Russia

Most likely areas to be affected are 103-106 degrees distance and
at 142-146 degrees away from the epicenter.

Previous history

Sept 3, 1944 57S 122W Mw 7.0?

==> Sept 4, 1944 Kermadec Isl. 33S 176W Ms 6.4
==> Sept 5, 1944 Massena, New York 45N 75W Ms 5.9
This is the largest earthquake ever recorded in the
state of New York. It was followed by many aftershocks.
The area is currently active, so a repeat is possible.
==> Sept 6, 1944 Sudan/Ethiopia 7.2N 38.5E Ms 6.0
One of the 5 largest events ever recorded in this region
of Ethiopia
==> Sept 6, 1944 Loyalty Islands 22S 172E Ms 6.5
==> Sept 9, 1944 Colorado 39N 107W Mb 5.0
This is one of the 3 largest earthquakes ever recorded
in Colorado.
==> Sept 11, 1944 Moluccas 1.5N 125E Ms 7.2
==> Sept 14, 1944 So. of Java 8S 108E Ms 6.8

April 4, 1971 56S 123W Mw 6.6

==> April 5, 1971 Honshu 38N 142E Ms 5.8
==> April 5, 1971 Fox Islands 53N 171W Ms 6.1
==> April 6, 1971 Fiji Islands 22S 179W Ms 6.0
==> April 7, 1971 Moluccas 2N 129E Ms 6.6
==> April 7, 1971 Central Chile 32S 69W Mb 5.7
==> April 8, 1971 So. of Sumatera 4S 102E Ms 6.3
also events in Northern Montana (Ml 4.4) and Galapagos (Ms 4.5, 5.4, ,,)
==> April 12, 1971 So. Iran 28N 56E Ms 6.0
==> April 18, 1971 Galapagos Isl. 0N 91W Ms 6.0

Other moderately large events in the area occurred as follows:

April 3, 1963 (Ms 6.5 at 55S 128W)
Was followed by an Mb 5.7 in Southern Alaska about an hour later.
==> April 7, 1963 Fox Islands, 53N 171W Ms 6.0
The wave front is parallel to the subduction zone at the
Fox Islands (128 degrees away), this may explain the common
occurrence of events in this region following large earthquakes
in this region of the Pacific-Antarctic ridge (see also 4/5/71)
==> April 7, 1963 So. of Sumatera, 4S 103E Ms 6.6
==> April 7, 1963 Colorado 39N 104W Ml 3.4+ many aftershocks

May 2, 1957 (Ms 6.3 at 56S 123W)

==> May 2, 1957 Northern Canada 72N 68W Ms 6.4
This is probably the largest event located in this area
==> May 2, 1957 Fox Islands 53N 169W Ms 6.3, 6.4, 6.5
==> May 2, 1957 Flores Islands 7S 120E Ms 6.6
==> May 3, 1957 Colorado 38N 107W Ml 4.3
==> May 4, 1957 Irian Jaya 3S 134E Ms 6.4
==> May 12, 1957 South Sandwich Islands 60S 25W Ms 6.0
==> May 12, 1957 South of Java 8S 107E Ms 6.7

Oct 19, 1956 (Ms 6.4 at 56S 123W)

==> Oct 19, 1956 Rat Islands 52N 177E Ms 6.8
==> Oct 23, 1956 Galapagos Islands, 2N 95W Ms 5.6
==> Oct 23, 1956 Philippines 15N 120E Ms 6.7
==> Oct 24, 1956 Nicaragua 11N 87W Ms 7.3
==> Oct 24, 1956 Nicaragua 11N 87W Ms 7.3, 6.4
==> Oct 26, 1956 Santa Cruz Isl. 13S 166E Ms 6.5
==> Oct 28, 1956 So. of Kermadec Isl. 33S 178W Ms 6.9

Nov. 13, 1943 (Ms 6.5 at 55S 129W)

==> Nov 13, 1943 Loyalty Islands 19S 172E Ms 7.2
==> Nov 17, 1943 So. of Japan 33N 137E Ms 7.0
==> Nov 24, 1943 Alpine Fault, N.Z. 45S 167E Ms 6.1
==> Nov 24, 1943 Taiwan 22N 122E Ms 6.9
==> Nov 26, 1943 So. of Sumatera 2S 100E Ms 7.1
A series of events M4.0-5.0 also occurred in northern
California on 11/14 and 11/15 at 39N 122W.

SUMMARY OF EXPECTED FAR-FIELD EVENTS

Areas where the wave-front is parallel to the local
faulting have experienced large events following earthquakes
in this region of the Pacific-Antarctic Ridge. These
areas include the Central American/Mexican subduction
zone, the subduction zone near the Fox Islands and
the Alpine fault in New Zealand. These areas should be
on seismic watch for the next 10 days. Earthquakes
of Ms>6.0 are possible in these areas. The area south
of the Kermadec Islands is also subject to strong
far-field triggering.
Regions at distances where seismicity is expected
to increase often do show moderate to large earthquakes
following events in this area of the Pacific-Antarctic
Ridge. Some of these which have a record of strong far-field
seismicity include the subduction zone south of Java and
Sumatera; the area south of the Loyalty Islands, the Moluccas
Tonga/Fiji and the Galapagos Islands.
In the United States seismic events in Colorado often
follow earthquake in this area of the Pacific. The largest
event ever in New York State followed by one day the largest
event ever recorded in this area of the south Pacific in
1944. One of the largest earthquakes in eastern Canada
was also associated with a strong event in the South
Pacific. The zone across the northern U.S. should be carefully
monitored for the next 10 days for far-field activity.