FAR-FIELD AFTERSHOCK FORECAST FROM INDIAN ANTARCTIC RIDGE (MS 6.3)
08 APRIL 2002

A moderately large earthquake hit the region of the Indian-Antarctic
Ridge south of Australia this morning (UT, April 08, 2002). This earthquake
probably had Mw around 6.3 (NEIS gives initially Ms 6.2).
Normally a shock of this size would not be considered likely to
generate much of a far-field aftershock sequence, however because of
it's unusual location, it may have effects which are slightly
different than earthquakes with more common epicenters, so a
preliminary far-field forecast is being produced in the eventuality
that unusual effects occur. Also, in combination
with other recent large shocks in the Ryukyu Islands and Hindu
Kush, this event could have some effect on distant regions.

The initial parameters (NEIS) for this event are:

O: 8APR2002 03:48:59 51.0S 138.7E Mb=5.7 NEIA WESTERN INDIAN-ANTARCTIC RIDGE

FAR-FIELD EFFECTS:

The epicenter of this earthquake lies at 103 degrees from the
mainshock in northern Chile 10 days ago. This is at the beginning
of the shadow zone boundary from this event. The shadow zone
boundary is an area where FFA effects are expected to be strong.

REGIONAL EARTHQUAKE HISTORY

Surprisingly, although there have been about 30 earthquakes of
Mw>=6 within 200 km of this epicenter, there has been little significant
earthquake activity in this same distance range. No events of Mw>=7 have
ever been recorded in this region. Only 4 events of Mw>=6.5 have occurred
in this area as follows:

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

6 14 17 16 55 1927 -50.000 140.000 0 6.5
4 23 4 55 56 1946 -50.000 140.000 0 6.8
5 11 15 50 42 1976 -51.603 139.683 33 6.6
6 11 11 55 12 2000 -50.579 139.550 10 6.6

FAR-FIELD FORECAST:

The following areas are expected to see increased seismicity
in this period. Maximum increased magnitude might be 0.2-0.4
units from April 8-15 (slightly more at shadow zone boundaries).

0-5 degrees: Local aftershocks
On 6 different occasions, events of Mw>=6 have been followed
within a day by a second such event in this region, so the chance
of strong immediate aftershocks (or a double mainshock) is moderate.
9-10 degrees: Macquarie Island, Ridge
17-19 degrees: New South Wales, Australia, Milford Sound/Te Anau, New Zealand
34-36 degrees: South of the Kermadec Islands
43-45 degrees: Fiji, Santa Cruz, Solomon, Papua New Guinea, Banda,
Flores, Sumbawa Islands
59-61 degrees: No. Sumatera, Borneo, Mindanao, Atlantic-Indian Ridge
71-73 degrees: Luzon, No. Marianas
103-106 degrees***: Central Aleutians, No. Chile, Iran, Afghanistan,
Hindu Kush, Rat Island, Near Island, Kamchatka (wavefront parallel to
arc in this area - larger event possible)
142-146 degrees: Illinois/Indiana, New Madrid, Sicily, Adriatic Sea, Poland,
Svalbard, Puerto Rico/Virgin Islands (wavefront parallel to arc in
this area - larger event possible)
176-180 degrees: Near Flemish Cap (of Perfect Storm fame), Grand
Banks, Newfoundland, northern Mid-Atlantic ridge only local seismic
zones.

***: 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>=6.5) EARTHQUAKES IN
INDIAN-ANTARCTIC RIDGE AREA:

JUN 14 17 16 55 1927 -50.000 140.000 0 6.5

Jun 16 35N 86W Ml 4.2 Kentucky
Jun 18 44N 21E MW 4.9 Balkans
Jun 26 44N 34E MW 6.0 Black Sea/Caucasus
Jul 01 38N 23E MW 7.3 Greece

APR 23 4 55 56 1946 -50.000 140.000 0 6.8

Apr 23 36N 118W MW 4.0 Coso Junction, Calif
Apr 25 38N 122W ML 3.8 Central California
Apr 27 38N 118W MW 4.7 California/Nevada border

MAY 11 15 50 42 1976 -51.603 139.683 33 6.6

May 11 37N 20E ML 6.4 Ionian Sea, Greece
May 11 46N 13E MW 5.2 Northern Italy
May 15 11S 74W MW 6.6 Peru
May 17 40N 63E MW 7.1 Uzbekistan
May 22 36N 90W MW 3.2 New Madrid, MO
May 29 24N 99E MW 6.8 Yunnan, Burma

JUN 11 11 55 12 2000 -50.579 139.550 10 6.6

Jun 13 35N 27E ML 5.2 Dodecanese Islands, Greece
Jun 14 17N 120E MW 5.5 Luzon, P.I.
Jun 14 25S 178E ML 6.4 Fiji Islands
Jun 14 24S 67W ML 5.9 Northern Chile
Jun 14 5N 128E ML 6.1 Talaud Is. Mindanao/ P.I.
Jun 14 33N 115W MW 4.2 Southern California
Jun 14 17S 176E ML 6.0 Fiji Islands
Jun 15 34N 20E MW 5.2 Central Mediterranean
Jun 16 1N 118E MS 5.4 Boreno
Jun 16 42N 72W MW 3.1 New England
Jun 16 42N 72W MW 3.1 New England
Jun 16 33S 70W MW 6.4 Central Chile
Jun 16 33S 70W MW 6.4 Central Chile
Jun 17 64N 21W MW 6.8 Iceland
Jun 21 64N 21W MW 6.6 Iceland
Jun 20 13S 98E MW 7.8 South Indian OCean
Jun 19 16N 120E MW 5.7 Luzon Philippines

Summary of far-field events following Indian-Antarctic Ridge (Mw>=6.5):

It is difficult to see patterns with so few events (which are
small compared to the usual mainshocks considered for FFA).
The most prominent locations appear to be Sumatera and the
Indonesian archepelago; Luzon and Talaud/Halmahera Philippines;
North mid-Atlantic regional events - New England, New Brunswick,
Iceland; the Northern Greece/Italy region; Borneo; Fiji and
northern to Central Chile. In the U.S. events in the Illinois/MO/KY
regions have occurred in at least half the cases. Southern and
Central California also appears to be slightly affected.