FAR-FIELD AFTERSHOCK FORECAST FROM CENTRAL CHILE QUAKE (MW 6.7) 18 APRIL 2002

A large earthquake and aftershocks hit the region of northern
Chile this afternoon (April 18, 2002). This earthquake probably
had Mw around 6.7. This is large enough, especially in combination
with other events of Ms 6.3 in Guerrero, Mexico and of Ms 5.8 in
the South Sandwich Islands to have some minor Far-field triggering
effects on global seismicity.

The initial parameters (NEIS) for this event are:

O: 18APR2002 16:08:36 27.4S 70.0W MW=6.7 NEI NEAR COAST OF NORTHERN CHILE

PRECURSORY PHENOMENA:

FAR-FIELD TRIGGERING

The Chile earthquake occurred about two hours after an earthquake
at the southern end of the South Sandwich Islands and is at 45 degrees
from that event. This is a distance at which Far-field triggering
is expected, so the Chile earthquake may well be a Far field aftershock
from the South Sandwich Islands.
It also lies in the general area of the Mw 6.4 on April 1, 2002
which hit about 230 km to the southeast. It is not, therefore a
classical aftershock, but more like a regionally triggered event.

GEOMAGNETIC EFFECTS

A strong geomagnetic storm, the strongest in more than 5 months
occurred last night and this morning. The Chile earthquake occurred
very near to local noon, Chile time. The earthquake may have been
immediately triggered by a class III solar flare (#2030) which
began at 16:08 UT while Chile was directly sub-solar. The flare,
while minor would have had maximum effects on Chile at the time,
and with the strong geomagnetic storm may have been able to trigger
this earthquake at this time. Unfortunately, the ACE satellite was
off-line at the time so details on the variation in the geomagnetic
field are not available for this event.

REGIONAL EARTHQUAKE HISTORY

Activity within 100 km of this epicenter has been relatively
moderate at the Mw>=6.5 level with only 11 such events recorded
in the past 300 years.
The parameters of these events follow:

MO DA HR MN SC YEAR S LAT W LONG DEP MW

3 30 10 45 0 1796 -27.350 -70.350 0 7.7
4 11 15 0 0 1819 -27.350 -70.350 0 8.3
10 5 12 0 0 1859 -27.350 -70.350 0 7.6
12 18 12 44 0 1918 -27.300 -70.500 0 8.2
8 3 19 57 12 1920 -27.500 -70.000 0 6.8
11 11 0 0 0 1922 -27.300 -70.000 0 8.3
4 18 6 22 45 1939 -27.000 -70.500 100 7.4
9 15 7 39 7 1956 -26.600 -69.400 100 6.6
11 18 12 46 48 1987 -26.700 -70.700 0 6.7
3 15 16 9 1 1993 -26.930 -70.480 45 6.7

All have occurred at depths between 0 and 100 km. Tsunami
accompanied events in 1819, and 1922. Severe damage occurred
in 1796, 1819, 1859, 1918, and 1922. Moderate damage occurred
with the smaller magnitude events.

FAR-FIELD FORECAST:

The following areas are expected to see increased seismicity
from April 19 through April 29, 2002. Maximum event size for
these regions is expected to increase by 0.5 to 0.7 magnitude units
in this period.

0-5 degrees: North-central Chile/ Argentina
9-10 degrees: Southern Peru (About 17-18S), southern Chile (about 36-37S)
17-19 degrees: Central Peru (11S), West Chile Rise, Southern Chile (45S)
34-36 degrees: Colombia (6-8N), Colombia, No. Venezuela
43-45 degrees: Virgin Islands, El Salvador, Pacific-Antarctic Ridge (120W)
59-61 degrees: Coast of Jaslisco, Mexico Central Mid-Atlantic Ridge, Bermuda
71-73 degrees: Northern Gulf of California, Texas Panhandle, Oklahoma,
Missouri/Arkansas, New Madrid, southern New England, N.Y.
103-106 degrees***: SE Alaska, Iceland, Central, Vanuatu Is.
Lake Victoria, Sicily, No. Italy, Italy, Germany, U.K. North Sea
142-146 degrees: Afghanistan, Hindu Kush, So. Pakistan, Uzbekistan,
So. Kuril Isl. Central Mariana Islands, Guam, Moluccas, Sulawesi, South of
Sumatera, Java.
176-180 degrees: East-Central China near the Yangtse River. Relatively
a-seismic

***: The strongest triggering is always expected to be along the
shadow zone boundary between 103-107 degrees from the epicenter.

Far Field effects from the other two events today should be minimal
but are most likely in combination with Chile. Far field aftershocks
are most like from these as follows:

Mexico Ms 6.3

0-5 degrees: Mexico from Oaxaca to Jalsico
9-10 degrees: Gulf of California (23N 110W), El Salvador/Guatemala
17-19 degrees: Mexicalli/Calexico CA, Panama,
34-36 degrees: Virgin Islands, NE Venezuela, Central Peru, Vancouver Isl.
New England
43-45 degrees: SE Alaska, Northern Chile
59-61 degrees: Santiago Chile area, Unimak Island, Alaska Peninsula
71-73 degrees: Near Island, Rat Isl. Aleutians
103-106 degrees***: SE of Honshu, Greece
142-146 degrees: Andaman Islands, Bali
176-180 degrees: South Indian Ocean, mostly a-seismic (17S 79E)

And from the South Sandwich Islands:

0-5 degrees: South Sandwich Islands
9-10 degrees: Southern Atlantic Ocean
17-19 degrees: Falkland Islands
34-36 degrees: Southern Chile south of Santiago
43-45 degrees: Rep of So. Africa, Northern Chile (27-28 S)
59-61 degrees: Northern Peru (11-13S)
71-73 degrees: Ecuador, central Venezuela
103-106 degrees***: off Coast of Jalisco, Mexico, Sicily, So. Carolina,
Southern Spain, So. of Sumatera, So. of Java, No Vanuatu, Samoa
142-146 degrees: Northern Ryukyu Isl, Queen Charlotte, Svalbard
176-180 degrees: North of Kamchatka (60N 155E) some regional activity
in the Kamchatka/Komandorsky Islands is possible.

COMBINATION:

From the combination effects of these events several areas seem
likely to see events of Ms>=5.5 in the next 10 days. These include

Colombia around 7N 73W
Southern Kuril Islands 43N 147E
New Britain/New Ireland 5S 153E
northern Vanuatu Is. 14S 167E

Other areas where an earthquake of Mb>=4.5 may be and FFA.

Off the Coast of Jalsico Mexico (about 21N 106W)
Virgin Islands
Peru 11-14S
Sicily, Italy
Santiago, Chile region
El Salvador/Guatemala
Hindu Kush area

U.S./CANADA

Areas of southern California near the Mexican border are the most
likely (Calexico/Imperial Valley) to see increased seismicity
from this pattern. Seismic activity may increase at:

SE Alaska
Southern New England
continued micro-earthquakes in So. Carolina, New Madrid MO.