FAR-FIELD AFTERSHOCK FORECAST FROM CENTRAL CHILE QUAKE (MW~6.5) 01 APRIL 2002

A large earthquake and aftershocks hit the region of central
Chile this afternoon (April 01, 2002). This earthquake probably
had Mw around 6.5 (NEIS gives initially Mb 6.4, GSSC gives MS 6.1).
Normally a shock of this size would not be considered likely to
generate much of a far-field aftershock sequence, however in combination
with other recent large shocks in the Ryukyu Islands, Northern Chile,
and Taiwan.

The initial parameters (NEIS and GSR) for this event and aftershock are:

O: 1APR2002 19:59:35 29.1S 71.2W MS=6.0 GSR NEAR COAST OF CENTRAL CHI
O: 1APR2002 19:59:32 29.5S 71.1W MW=6.4 NEI NEAR COAST OF CENTRAL CHI

PRECURSORY PHENOMENA:

ANTIPODAL TRIGGERING:

This is the second shock of Mw 6.4-6.5 in the region in the past
week. The area had been expected to see additional activity on the
basis of the earthquakes both in Chile and in the Ryukyu Islands/Taiwan
region. This type of antipodal triggering is not uncommon since
seismic energy is concentrated in antipodal regions by geometric
focussing. Studies have shown that seismic amplitudes at or near
the antipode can be as high as they are within 500 km of the epicenter
of the earthquake, so this is a prime area to look for far-field
triggering.
It is uncommon for this type of ping-pong effect where an earthquake
occurs, then one near the antipode, then another near the original
and then a further near the antipode, etc. but it is not unheard of.

GEOMAGNETIC EFFECTS:

A moderate geomagnetic storm began this morning (UT) at about
03:00 UT reaching it's maximum about 06:00 UT and continuing until
about 09:00 UT. This region of Chile was antipodat at the time, and
would have experienced near-maximum effects from this geomagnetic
storm. The earthquake continues a series of events which began
after the "strongest geomagnetic storm in five months (March 24).
Generally larger events associated with geomagnetic storms tend to occur
about 4-6 days following those storms and occur in regions which were
near sub-solar or anti-solar at the time of the storm. The storm
commenced at about 1:50 UT on 24 March and was most intense
around 05:00 UT while Chile was anti-solar. The geomagnetic
storm could have influenced the timing of this event." (see Far-Field
Aftershock forecast for Northern Chile, 28 March, 2002 for source
of quotation).

FAR-FIELD EFFECTS:

The region of this earthquake is nearly antipodal to the epicenter
of the Ryukyu Islands earthquake of 25 March and the Taiwan earthquake
of March 31, 2002 (Ms 7.3) and is 9 degrees from the Chile earthquake
of March 28, 2002 (Ms 6.5). The far-field forecast from northern
Chile had specifically noted this area as a site of likely future
activity of Ms>=6 as follows:

"9-10 degrees: ... Santiago, Chile areas"

and

"The most likely events of Mw>=6 in the next 10 days appear to be:

and from the Taiwan Far-field (March 31, 2002) we read:

"The most likely events of Mw>=6 in the next 10 days appear to be:
(Number in paranthesis indicated number of times and event of M>=6
has followed a Taiwan mainshock within 2-3 weeks).

... Central Chile"

REGIONAL EARTHQUAKE HISTORY

Activity within 200 km of this epicenter has been relatively
moderate at the Mw>=7.5 level with 6 such events recorded in
the past 200 years. The parameters of these events follow:

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

12 17 10 0 0 1849 -29.950 -71.370 0 7.5
5 20 17 55 10 1918 -28.500 -71.500 80 7.9
11 11 4 32 36 1922 -28.500 -70.000 60 8.4
4 6 16 7 15 1943 -30.750 -72.000 55 8.3
2 8 0 0 0 1954 -29.000 -70.500 0 7.7
10 15 1 3 33 1997 -30.933 -71.220 58 7.6

All have occurred at depths between 0 and 100 km. All of these
events have generated tsunamis including a 9 meter wave which
did considerable damage near Chanaral, Chile in November, 1922.
All have done considerable damage and events in 1849, 1918,
1922, 1943 and 1997 were deadly, killing a total of about
500 people.

FAR-FIELD FORECAST:

The following areas are expected to see increased seismicity
from April 01 through April 06, 2002. Maximum event size for
these regions is expected to increase by 0.3 to 0.5 magnitude units
in this period.

0-5 degrees: North-central Chilei/ Argentina
9-10 degrees: Northern Chile, Bolivia (20-22S, 65-70W),
southern Chile (38-39S, 70-75W)
17-19 degrees: Central Peru (10S), West Chile Rise
34-36 degrees: Colombia (3-6N), southern East Chile Rise, South Sandwich
Islands
43-45 degrees: Nicaragua, Costa Rica, Northern Venezuela, Trinidad
59-61 degrees: Central Mid-Atlantic (22-25N), Jalisco, 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***: Southern, Gulf of Alaska, Central, No. Vanuatu Is.
Lake Victoria, Sicily, No. Italy, Italy, Germany, U.K. North Sea, Iceland
142-146 degrees: Afghanistan, Hindu Kush, So. Pakistan, Uzbekistan,
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, but with the amount of energy in this area from recent
events, could see an event despite the non-seismic nature of the area.

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

OBSERVED EVENTS FOLLOWING PREVIOUS LARGE (Mw>=7.5) EARTHQUAKES IN
NORTHCENTRAL CHILE:

DEC 17 10 0 0 1849 -29.950 -71.370 0 7.5

Dec 28 34N 107W Ml>4.0 Arizona/New Mexico (aftershocks through Jan 1850)
Jan 18 59N 121E MW 6.1 Siberia

MAY 20 17 55 10 1918 -28.500 -71.500 80 7.9

May 22 17S 177W MW 7.0 Tonga/Samoa Islands
May 22 1S 153E ML 6.7 New Britain/Ireland
May 23 27N 111W ML 6.8 Gulf of California
May 25 31S 93W MW 7.0 West Chile Rise
May 25 44N 141E MW 5.8 Kuril Islands
May 28 35N 106W MW 6.3 New Mexico/Colorado border
Jun 07 24N 121E MW 6.0 Taiwan
Jun 07 19N 103W MW 6.6 Jalisco, Mexico
Jun 11 19N 62W MW 5.8 Puerto Rico/Virgin Islands

NOV 11 4 32 36 1922 -28.500 -70.000 60 8.4

Nov 18 36N 1E ML 5.6 Spain
Nov 24 45N 19E MW 5.3 Northern Italy
Nov 27 38N 88W MW 4.5 Illinois
Dec 02 24N 122E MW 6.3 Taiwan
Dec 06 37N 71E MW 7.5 Hindu Kush
Dec 06 37N 71E MW 7.5 Hindu Kush
Dec 07 41N 21E MW 5.7 No. Greece
Dec 08 31N 130E MW 6.5 Kyushu, Japan
Dec 08 41N 143W MW 6.8 Kuril Islands

APR 6 16 7 15 1943 -30.750 -72.000 55 8.3

Apr 05 39N 73E ML 6.5 Hindu Kush
Apr 05 8S 76W MW 6.5 Northern Peru
Apr 09 19N 146E ML 7.0 No. Mariana Islands
Apr 28 44N 147E ML 6.6 Kuril Islands
Apr 28 24S 180E ML 6.5 Tonga Islands
May 03 6S 80W MW 7.1 Northern Peru
May 03 12N 125E MW 7.4 Central Philippines

FEB 8 0 0 0 1954 -29.000 -70.500 0 7.7

Feb 11 39N 101E MW 7.5 N-Eastern China
Feb 15 5N 82W MW 6.4 South of Panama
Feb 17 52N 160E MW 6.3 Kamchatka
Feb 17 45N 151E MW 6.4 Northern Kurils
Feb 19 11N 86W MW 6.8 Nicaragua/Costa Rica (plus large aftershocks)
Feb 19 45N 92E MW 6.1 Northern Xinjiang, China
Feb 19 30S 177W MW 7.0 Kermadec Islands
Feb 19 19N 102W MW 6.0 Michoacan, Mexico
Feb 20 7S 124E MW 7.0 Banda Sea
Feb 21 41N 76W MW 5.7 Pennyslvania/Lake Erie
Feb 21 40N 107W MW 4.7 Central Colorado
Feb 22 34N 141E MW 6.0 Off E. Coast Honshu
Feb 22 57S 26W MW 7.0 South Sandwich Islands
Feb 23 28N 91E MW 6.4 Nepal/No. India
Feb 26 37N 71E MW 6.0 Hindu Kush
Mar 03 5S 142E MW 7.0 Papua, New Guinea
Mar 03 61N 147W MW 6.2 Southern Alaska
Mar 06 23S 179W MW 6.5 Tonga Islands
Mar 09 0S 30W MW 6.5 Central Mid-Atlantic Ridge
Mar 09 49N 157E MW 6.5 Kamchatka/No. Kuriles

OCT 15 1 3 33 1997 -30.933 -71.220 58 7.6

Oct 15 51N 177W MW 6.4 Central Andreanoff Isl.
Oct 17 57S 148E MW 6.2 Macquarie Islands
Oct 19 22S 175W MW 6.0 Tonga Islands
Oct 22 44N 146E MW 5.6 Kuril Islands
Oct 23 26N 100E MW 5.5 Yunnan, China
Oct 23 26N 100E MW 5.5 Yunnan, China
Oct 25 22N 45W MW 5.5 Northern Mid-Atlantic ridge
Oct 28 4S 77W MW 7.1 No. Peru
Oct 31 13S 167E MW 6.0 No. Vanuatu Isl.
Nov 02 38N 118W MS 5.4 California-Nevada Border
Nov 03 30S 71W MS 6.2 Central Chile
Nov 03 6S 129E MS 6.1 Banda Sea
Nov 03 20S 175W MS 6.0 Tonga
Nov 05 38N 22E MS 5.5 No. Greece
Nov 06 11N 86W MS 5.7 Nicaragua
Nov 08 35N 87E MS 7.9 Xizang, China
Nov 09 13N 88W MS 6.5 Costa Rica
Nov 10 0N 17W MS 5.6 Central Mid-Atlantic Ridge
Nov 10 12N 145E MS 5.5 Guam/Marianas

Summary of far-field events following North-Central Chile mainshocks (Mw>=7.5):

In general, the events of Mb>=5 tend to fall in the areas
described by the listing of far-field events expected above
from distance considerations. Effects from smaller mainshocks
such as this one may be lesser and vary.

The most likely events of Mw>=6 in the next 10 days appear to be:
(Number in paranthesis indicated number of times and event of M>=6
has followed a North-central Chile mainshock within 2-3 weeks).

Taiwan aftershocks
Hindu Kush aftershocks
North or Central Peru
No. India/Nepal into Xinjiang province, China
Nicaragua/Costa Rica - magnitude in low 6's or high 5's
East or east Central China - several occurrences in Yunan and
north east, good possibility at this time.

The most likely events of Mw>=5 in the next 10 days appear to be:

Guam/Central Mariana Islands
Banda Sea
Tonga
So. of Panama/Colombia to Trinidad
Northern Kurils/Kamchatka
Jalisco/Michoacan Mexico
Central Mid-Atlantic Ocean
South Sandwich Islands
Northern Greece (Ml 4.5-5.5)
Sicily/Italy (Ml 4.0-5.0)

Possible Events (Ml>=3) in the U.S. and Canada include:

Arizona/New Mexico to Colorado - Several large events in these
states have been associated with north-central Chile earthquakes.
These might reach Ml 3.5 with the effects of this and other
recent large global earthquakes.
A moderately strong earthquake also followed in the California-
Nevada region in 1987, but seems unlikely this year.

There is a chance of an event in the New Madrid north to Illinois
region with Ml 2.3-3.5.

The recent activity in the northern Gulf of California
swarm could be enhanced by this Chile earthquake.

SUMMARY:

For large events in the Chile/Bolivia border region (Mw>=7.5)
strong far-field aftershock sequences appear to be the norm.
It is expected that some far-field triggering will occur event
with this Mw 6.4+ event, however.
The most interesting events could occur in the Central Mid-
Atlantic Ocean. Regions which consistent are associated with
strong shocks in this area of Peru continue to be Taiwan and
Hindu Kush. The northern and Central Kurils may also see some
moderate or strong event in the next 3 weeks.
The major pattern in the U.S. is a grouping of succeeding
shocks in the northern New Mexico/Arizona/Colorado region. This
is a possiblity this time as well.