FAR-FIELD AFTERSHOCKS ABOUND FROM CHILE, MEXICO

SUMMARY

The far-field aftershock sequence from the Mw 6.7 in northern Chile
and the 6.3 in Guerrero, Mexico on April 18, 2002 was in full bloom today.
Moderate earthquakes have now occurred near all the latitude/longitude
pairs listed in the far-field aftershock forecast and for most of the
other locations indicated as well. More details are included below in
the "earthquake occurrence" section of this report.
FFA have included events in the Solomon Island/New Britain region
(Mb 5.0, 5.1), Hokkaido/Kurils (Mb 4.7, 4.4); Colombia (Mb 4.1, 3.9, 3.9);
New York (Mb 5.1, 4.0); New Madrid (Mb 3.0, 2.5); Guatemala (Mb 4.4);
Calexico, CA (Ml 2.5); Vanuatu Islands (Mb 5.5); Jalisco, Mexico (Ml 5.0)
among others.

GLOBAL

The largest earthquakes of the day occurred in the Solomon Islands/
New Britain region and were probably FFA from northern Chile (Mb 5.1, 5.0).
Other earthquakes of Mb>=4 were listed as occurring in Taiwan (Mb 4.6/
Ml 5.1); northern (Mb 5.2, 4.5) and central (Ml 4.7) Peru; the Fiji
Islands (Mb 4.4) and the Alaska Peninsula (Mb 3.8) by NEIS.
Local networks also recorded events in the western Caroline Islands
(Mb 4.5); western Turkey (Ml 3.8) and in Belgium/Germany (Ml 3.5)
today.

U.S./CANADA

The largest earthquake listed in the U.S./Canada region on April 22
was a Mb 3.8 in the Alaska Peninsula region of western Alaska. A Ml 3.5
was also listed as having occurred in the Kilauea region of Hawaii,
however, these events are usually withdrawn upon manual inspection.
No other events of Ml>=3 were located in the U.S. or Canada today.
The largest earthquake located in the U.S/Canada outside Alaska and
Hawaii was a Ml 2.5 in the New Madrid Missouri seismic zone. This is
the second event in this area in the past two days (A Ml 3.0 occurred
on April 20 following the New York state earthquake).
In the California/Nevada region, seismicity remained at very low levels.
The proton storm may be inhibiting the elements of the geomagnetic
field needed to trigger earthquakes in this region. A similar phenomena
was observed following the X-flare of November 25, 2001, the last such
flare. During the three days of the strong proton storm which followed
that flare, only 3 events of Ml>=2 were located in southern California.
Since the current proton storm began three days ago no earthquakes
of Ml>=2 had been located in southern California until this evening
when two events in the Calexico region (Ml 2.1 and 2.5 occurred). It
is often the Calexico region which is the first to respond with Ml>=2
events after such a lull. Quiet periods in southern California are
often followed within 3-4 days by significant regional events or
swarms. In the past 4 years, only after the November 2002 quiet period
did more than 5 days pass before a significant regional even occurred.
Three successive days of no events of Ml>=2 in southern California
was unprecedented in recent history. Only two days in 2002 had previously
seen no events of Ml>=2 in southern California - in mid February, which were
followed by a Ml 5.7 in Calexico; the Nov. quiet in 2001 was followed by a
Ml 4.4 in Calexico. A quiet day on Nov. 11 by a swarm of 38 events of Ml>=2
at Bombay Beach, CA on Nov 13 including a Ml 4.1. The previous quiet
day on Feb 8, 2001 had been followed on Feb. 10 by a Mb 5.1 at Big Bear.
1999 saw no quiet days in the region and 2000 saw one - on March 6,
followed the next day by a Ml 3.7 south of Los Angeles.
In northern and Central California, things weren't much more active
than in southern California. One earthquake of Ml>=2 (Ml 2.3) was
seen at the Markleeville, CA region this afternoon in this area.
The Pacific northwest also saw one earthquake in this magnitude
range today - a Ml 2.2 east of Tacoma.

NEW ZEALAND

No new earthquakes were located in the New Zealand region today by
IGNS. A previously unlisted Mb 4.2 was listed by NEIS in the Kermadec
Islands as having occurred on the 20th of April.

The IGNS on-line Seismometer can be found at:
http://www.gns.cri.nz/news/earthquakes/latest_drum.html

GEOMAGNETIC/SOLAR FIELDS

The geomagnetic field was quiet today except for a brief disturbance
at about 03:15 UT. Electron flux was at low levels due to the strong
proton storm which is expected to end at the 100 MeV proton levels
early on April 23 and at the 10 MeV level on April 24. SEC also
expects a moderate to strong geomagnetic storm today from the CME
of the X1.5 flare. The loss of the buffering protons and the new
geomagnetic storm may bring back moderate to strong seismicity in
many areas.
No X- or M- class solar flares were observed today.

Flare # START MAX END CLASS

For general space weather conditions see:
http://sec.noaa.gov/today.html
For Solar flare data see:
http://sec.noaa.gov/ftpdir/indices/events/20020422events.txt

TIDAL EFFECTS

April 24 is the eleventh day after the beginning of the new lunar month.
Regions most likely to experience tidal triggering
on this day (With associated magnitude ranges; the percent change in
seismicity rate over background seismicity rate on this
day of the lunar month; statistical z-value and
significance level) are:

Region Magnitude range %change z-value Significance level

So. Alaska MB>=3.0 9 2.7 0.02
Australia MB>=4.0 33 2.6 0.03
New Madrid MB>=0.0 34 3.4 0.01
So Indonesia MB>=4.0 42 3.1 0.01

Regions expected to experience reverse triggering (i.e. they are
unlikely to experience a significant event at this period of the
lunar cycle) are:

Region Magnitude range %change z-value Significance level

Geysers MB>=0.0 -6 -2.2 0.05
NE Europe MB>=4.0 -10 -2.3 0.04
Tonga MB>=4.0 -29 -2.0 0.05

TOMORROW IN EARTHQUAKE HISTORY

GLOBAL:

The following events of Mw>=7 have occurred on this date in
recorded history:

MO DA HR MN SC YEAR LATIT LONGIT DEP MW LOCATION

4 23 15 0 0 1771 24.000 124.030 0 7.4 Taiwan
Quake was felt throughout the Ryukyu Islands. The tsunami arrived
about 1 hour after the quake. The first of three was the largest
killing 11,941, destroying 3137 houses. The heights of the tsunami
ranged from 12-115 meters. Seven villages were totally washed away.

4 23 15 29 18 1915 -8.000 -68.000 650 7.2 Brazil
4 23 3 16 0 1923 27.000 126.000 10 7.2 Ryukyu Islands
4 23 11 15 39 1949 -8.000 121.000 80 7.1 Flores Island area
4 23 16 24 17 1953 -4.000 154.000 0 7.5 New Britain
$13,000 Damage at Rabaul. Accompanied by very serious landslides.

4 23 5 58 12 1962 42.233 143.917 60 7.0 Hokkaido, Japan
Felt earthquake.

4 23 3 32 50 1964 -5.300 134.000 33 7.2 West Irian, PNG
Felt at Darwin, Australia.

4 23 16 15 23 1985 17.700 119.700 189 7.3 Luzon, Philippines
Felt (IV) at Manila and Santa and III at Calloo Caves. Pasuquin
and Quezan City. Felt by personnel on four naval vessels in
Subic Bay.

4 23 9 27 23 2000 -28.307 -62.990 609 7.0 Argentina
Felt at La Rioja province, too deep to effect damage.

U.S./CANADA

The following events of Mw>=5 have occurred on this date in
recorded history:

MO DA HR MN SC YEAR LATIT LONGIT DEP MW LOCATION

4 23 14 56 31 1964 57.300 -151.810 21 5.4 Southern Alaska
4 23 13 22 35 1968 31.966 -116.683 10 5.0 So. California
Series of shocks felt IV in the San Diego area. Also felt
lightly at Leucadia. Many were frightened at Mexicali, Mexico.

4 23 20 29 14 1968 58.700 -150.000 23 6.3 Southern Alaska
4 23 13 17 52 1976 57.773 -155.907 104 5.1 Alaska Peninsula
4 23 14 43 17 1982 60.154 -141.092 3 5.0 Central Alaska
4 23 23 18 23 1982 51.142 -179.880 39 5.6 Andreanoff Islands, Aleutians
4 23 19 21 5 1989 66.950 -156.160 9 5.9 Northern Alaska
4 23 2 59 44 1990 52.840 -167.850 30 5.8 Fox Islands, Aleutians
4 23 0 7 12 1992 50.540 -132.030 3 5.1 British Colombia
4 23 4 50 23 1992 33.961 -116.318 12 6.3 So. California
Thitry-two people were treated for minor injuries. Light to moderate damage
(VII) at Joshua Tree, Banning, and Yucca Valley. Felt strongly
in many parts of So. California. Felt from San Diego, CA
to Los Vegas, NV and from Santa Barbara to Yuma, AZ.

4 23 5 40 8 1992 50.490 -132.020 3 5.5 British Colombia
4 23 19 48 53 1992 62.140 -150.410 82 5.0 Central Alaska
4 23 2 55 51 1995 50.000 -179.000 0 6.3 Andreanoff Islands, Aleutians
4 23 8 41 37 1995 36.603 -121.201 7 5.0 Central California
Felt IV at Aptos, Hollister and POzicines. Also felt in Monterey,
San Luis Obispo and to San Francisco to Fresno.

4 23 20 3 29 1996 51.490 -178.410 48 5.1 Andreanoff Islands, Aleutians

EARTHQUAKE LISTINGS AND FFA NOTES

ASIA

O: 22APR2002 12:15:21 10.7N 140.8E MS=4.5 GSR WESTERN CAROLINE ISLANDS
Note: This is probably a FFA from the Ms 6.7 in No. Chile on 18 April
as it lies at the IOC shadow zone boundary from that event - 146 degrees.
The forecast had expected FFA in the general vicinity of this event:

"142-146 degrees: ... Central Mariana Islands, Guam"

O: 21APR2002 19:24:03 5.4S 108.3E MB=5.0 NEI JAVA SEA
Note: This is probably a FFA from the South Sandwich Islands event
of 18 April (at 104 degrees at the CMB shadow zone boundary); and the
Ms 6.7 in No. Chile and Ms 6.3 in Mexico on 18 April (it lies ear the IOC
shadow zone boundary from those events - 147 degrees.)
The forecast had expected FFA in the general vicinity of this event:

"142-146 degrees: ... South of Sumatera, Java. "

O: 22APR2002 15:14:28 24.3N 122.3E MB=4.6 NEI TAIWAN REGION
O: 22APR2002 15:14:30 24.4N 122.0E ML=5.1 TAI Ilan, Taiwan

O: 19APR2002 01:42 32.7N 141.0E Mj=4.0 NIED E_OFF_HACHIJOJIMA_ISLAND, Japa

O: 20APR2002 21:30:33 41.7N 142.1E MB=4.7 NEI HOKKAIDO, JAPAN REGION

O: 20APR2002 07:44:49 2.8S 139.4E MB=4.8 NEI NEAR N COAST OF IRIAN JAY

O: 20APR2002 09:35:14 27.4N 56.7E MB=4.5 NEI SOUTHERN IRAN

O: 19APR2002 19:58 42.2N 144.8E Mj=4.4 NIED SE_OFF_TOKACHI, Japan

O: 19APR2002 11:35 45.0N 149.6E Mj=3.7 NIED SE_OFF_ETOROFU, Japan

O: 17APR2002 00:14 32.1N 140.8E Mj=5.0 NIED E_OFF_HACHIJOJIMA_ISLAND, Japa

SOUTH PACIFIC ISLANDS

O: 20APR2002 08:27:08 6.2S 149.3E MB=4.4 NEI NEW BRITAIN REGION, P.N.G

O: 22APR2002 19:48:52 7.0S 150.4E MB=5.1 NEI NEW BRITAIN REGION, P.N.G

O: 22APR2002 01:56:25 6.5S 154.9E MS=4.5 GSR SOLOMON ISLANDS
Note: This event lies at 105 degrees from the Mexico mainshock of
18 April. The area had been expected to see FFA in the forecast we read:

"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

New Britain/New Ireland 5S 153E"

O: 21APR2002 17:58:46 20.6S 162.2E MS=5.2 NEI NEW CALEDONIA
Note: This is probably a FFA from the Ms 6.3 in Mexico on 18 April
as it lies at the CMB shadow zone boundary from that even - 102 degrees.
The FFA forecast had anticipated this event:

"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

northern Vanuatu Is. 14S 167E"

O: 22APR2002 04:07:58 25.0S 179.6E MB=4.3 NEI SOUTH OF FIJI ISLANDS

CARIBBEAN/CENTRAL/SOUTH AMERICA

O: 21APR2002 04:54:17 6.8N 73.2W Mc=4.1 ULA Dpto. Santander, Colombia.
O: 21APR2002 11:46:50 6.5N 73.6W Mc=3.9 ULA Dpto. Santander, Colombia.
O: 21APR2002 09:35:23 6.8N 73.2W Mc=3.9 ULA Dpto. Santander, Colombia.
Note: These events lie at 35.5 degrees from the northern Chile Mw 6.7
earthquake of April 18 at an epicenter which had been expected to
see FFA at this time. The FFA forecast had stated:

"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"

O: 22APR2002 04:57 12.5S 76.8W mb=4.7 IGP Chilca, Peru
Note: This epicenter is 60 degrees from the South Sandwich Is, 37
from the Mexico and 17 from the northern Chile mainshocks of 18 April.
It was an area where FFA were expected as noted in the forecast:

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

...Peru 11-14S"

O: 21APR2002 20:23:30 19.1N 105.1W MB=4.4 NEI NEAR COAST JALISCO, MEXIC
O: 21APR2002 15:23:32 19.2N 105.7W ML=5.0 SSN COSTA DE JALISCO, MEXICO
Note: This is probably an FFA from Chile, Mexico and South Sandwich Islands
events of 18 April. It lies at 58, 5 and 102 degrees from these respectively
and was in an area where FFA were expected as noted in the forecast:

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

Off the Coast of Jalsico Mexico (about 21N 106W)"

O: 21APR2002 14:50:52 13.8N 92.5W ML=4.4 SSN COSTA MEXICO-GUATEMALA, MEXICO

O: 21APR2002 15:12:00 19.5S 69.3W ML>4.0 RES Chiapa, Chile
O: 21APR2002 21:10:10 20.1S 69.6W ML>4.0 RES Cerro, Chile

O: 22APR2002 04:12:35 5.9S 81.0W MB=4.5 NEI NEAR COAST OF N PERU
O: 21APR2002 22:24:06 5.8S 80.9W MS=5.2 NEI NEAR COAST OF N PERU

O: 21APR2002 02:36:20 8.5N 71.3W Mc=2.3 ULA Municipio Campo Elías, Edo. Mé

O: 21APR2002 07:14:30 9.3N 83.9W MC=3.6 CASC Costa Rica, Central America

O: 20APR2002 11:38:13 7.6N 78.2W MB=4.2 NEI PANAMA

NEW ZEALAND/AUSTRALIA

O: 20APR2002 09:36:55 31.5S 179.6E MB=4.2 NEI KERMADEC ISLANDS REGION

OCEANIA

EUROPE/MIDDLE EAST/AFRICA

O: 22APR2002 20:54:43 39.3N 27.7E ML=3.8 KAN BALIKESIR, TURKEY

O: 22APR2002 21:17:05 50.6N 5.9E Ml=3.5 LDG BELGIUM/GERMANY
Note: This unusual event occurred at 103 degrees from the earthquake in
Central Chile Mw 6.7 on 18 April. FFA in this region were expected in the
aftershock forecast:

"103-106 degrees***: ...Germany, U.K. North Sea"

U.S/CANADA

CALIFORNIA/NEVADA

O: 22APR2002 13:02:16 38.6N 119.8W ML=2.3 NCSN S of Markleeville, CA

O: 22APR2002 14:06:55 39.1N 121.8W ML=1.7 NCSN S of Sutter, CA

O: 22APR2002 16:35:50 32.6N 115.6W ML=2.4 SCSN SW of Calexico, CA

O: 22APR2002 19:17:19 32.3N 115.3W ML=2.1 SCSN SSE of Calexico, CA

PACIFIC NORTHWEST/CANADA

O: 22APR2002 03:38:51 47.3N 122.3W ML=2.1 PNSN ENE of Tacoma, WA

O: 21APR2002 03:24:39 46.2N 122.2W ML=1.0 PNSN ENE of Mount St. Helens
O: 21APR2002 06:56:49 46.0N 122.3W ML=1.6 PNSN SSW of Mount St. Helens

O: 21APR2002 17:05:26 42.4N 122.1W ML=1.0 PNSN ESE of Mount McLoughlin

ROCKY MOUNTAIN AREA

O: 21APR2002 06:04:35 38.6N 112.3W ML=2.6 UUSN SSW of Sevier, UT

O: 22APR2002 12:18:00 43.3N 110.7W MD=2.2 USBR S of Hoback Jct, WY

EASTERN U.S./CANADA

O: 20APR2002 15:00:00 36.1N 89.4W ML=3.0 CERI N of Dyersburg, TN
O: 22APR2002 09:03:08 36.0N 89.0W ML=2.5 CERI NW of Trenton, TN

ALASKA

O: 22APR2002 19:40:28 56.2N 157.2W MB=3.8 NEI ALASKA PENINSULA

HAWAII

O: 21APR2002 14:25:17 19.2N 155.4W ML=3.5 HVO ENE of Pahala, Hawaii