MODERATE QUAKES HIT NORTHERN CALIFORNIA COAST; NELSON AREA, NEW ZEALAND

SUMMARY

A strong swarm of earthquakes occurred today in the region off the
Coast of Northern California. These included at least 16 of Ml>=2 and
12 events of Ml>=3 the largest of which was a Ml 4.3 which occurred mid-way
through the swarm. The last swarm similar to this occurred on August 1, 1987
when 20 event of Ml>=3 occurred without a mainshock. The difference between
a swarm and an aftershock sequence is that a swarm has many events of
approximately the same size without a single strong event, while an
aftershocks sequence has one or more strong events followed by a series
of weaker earthquakes. Strong aftershock sequences occurred in this
area from April 25-27, 1992 with 23 events of Ml>=3 on the 26th of
April, 11 on the 27th and 10 on the 25th of April, 1992. This was the
strongest aftershock sequence recorded in the area. The other strong
aftershock sequence occurred on June 26, 1968 when 14 aftershocks occurred
of Ml>=3 after a Ms 5.9 mainshock.
Swarms such as today's often occur in volcanic regions or in regions
where a new fault is trying to rupture. Because neither of these
scenarios appera to apply to today's swarm off the northern Coast of
California, the cause of this swarm is a mystery at this time. Smaller
but similar swarms in the region have occurred on July 12, 1964 (6 events
of Ml>=3); and August 1 - August 2, 1993 (8 events of Ml>=3). Neither
of these two swarms lasted more than a day nor did they culminate in a
strong regional shock, so it seems likely that no strong shock will follow o
today's swarm.

In New Zealand, a Ml 5.5 earthquake occurred on northern South Island,
in the Vicinity of Nelson. This was probably an FFA from the Guam
earthquake of April 26 as it lies at 60.5 degrees from that event in
a region where strong FFA events were expected from Guam. The FFA
forecast had stated in this regard:

"59-61 degrees: ... South Island, southern North Island, New Zealand .."

A sequence of moderately strong earthquakes has been occurring
in the South Island to southern North Island since the Guam earthquake
including a Ml 4.9 near Te Anau (South Island, 61 deg. from Guam);
Ml 4.5 near Porirua (Wellington, North Island, 61 degrees from Guam),
and today's Ml 5.5. The FFA forecast had emphasized the likelihood
of strong FFA from Guam in this area.

GLOBAL

Strong to moderate earthquakes also occurred today in the Fiji/Tonga
region. A Mb 5.8 was recorded in Fiji, and a Mb 5.0 in Tonga.
The Guam FFA forecast had also expected this area to see increased
activity at this time and had stated:

"Secondary location where event tend to occur but are not
as large or as likely . MB>=4.8 considered likely in next
10 days.

Fiji/Tonga Islands"

The strongest earthquake yesterday was a Mb 5.2 in the Loyalty
Islands region, at 44 degrees from Guam, another apparent FFA:

"Secondary location where event tend to occur but are not
as large or as likely . MB>=4.8 considered likely in next
10 days.

...Loyalty Islands"

While NEIS was concentrating on the swarm off the coast of Northern
California other teleseismic networks recorded an active day in
global seismicity.

GSSC (Russia Geological Survey) recorded events of Mb 4.3 in
Pakistan (71 degrees from Guam, in an area where FFA were expected);
a Mb 4.5 in the Solomon Islands; a Mb 4.9 on Mindanao, Philippines;
a Ms 4.3 off the East Coast of Honshu; a Mb 5.3 on the northern
Chile/Argentine border; a Mb 4.8 in Micronesia near the southern
Caroline Islands (this event lies 9 degrees from Guam, and is
likely a FFA) - the Guam FFA forecast had noted:

"On two occasions, a large M>=7 regional shock in the Caroline
Islands has followed Guam events within a week. This should
also be considered a possibility in this case."

Preliminary locations have also placed two moderate (Mb 5.8 and 5.2)
earthquakes along the southern Mid-Atlantic Ridge, continuing the
strong mid-Ocean Ridge activity reported on in yesterday's
Daily Summary.

U.S./CANADA

The Summary detailed the strong swarm which occurred today off the
coast of Northern California. While the Guam forecast had not specifically
noted this as a possiblity a minor swarm in this region did follow
the Guam event in May 1950. This swarm included event sof Ml 3.9 and
4.2, similar to those seen today. The Guam FFA forecast had also noted
incrased activity (Ml>=3) likely in Northern California.
In addition the Guam FFA forecast had noted for the U.S:

Areas of U.S. and Canada where triggering may occuR (Ml>=3)

New York/Pennsylvannia/So. Quebec, Canada
---> Events of Ml 2.7 and 1.9 occurred in New York/Quebec yesterday

Los Angeles region, CA
---> nothing significant at this time

Coso Junction region, CA
---> Swarm of events including Ml 3.8, 3.0, 3.0

Northern and Central California (scattered)
---> A series of light shocks at Gilroy (3.1, 3.0); Willits (Ml 3.4)
and Ukiah (Ml 3.5)

Unusual events may occur in

Iceland
---> Two unusual events occurred north of Iceland on Apr. 29
and 30 (Ml 4.3, 4.0) along the Reykjanes Ridge (Mb 4.2 on April 29)
and in the Svalbard area near the North Pole yesterday (Mb 5.4, 5.1)

New Mexico/Arizona
---> On April 29 a Ml 4.1 occurred south of Yuma, Arizona, the
largest in that region in more than 4 months.

Besides the events off the Northern Coast, the region near Redding (Ml 2.3)
and Viola/Lassen Peak was also active today (Ml 2.3, 2.0). Also in
northern California two earthquake occurred near Markleeville (Ml 2.4, 2.1).
The California southland was quiet at the Ml>=2 level today.
A Ml 2.5 near Paradise, UT was the largest event in the contiguous
U.S. outside of California. This area has seen a minor swarm over the
past 3 days, but this appears to be a different epicenter from those
of April 30 - May 1. A Ml 2.2 was again registered in the Jackson Lake
area.
In the eastern U.S./Canada, two events of Ml>=2 were recorded yesterday
along the Quebec/New York borderland (Ml 2.7 and 1.9).
Alaska also saw two events of Ml>=3 since last report - a Ml 3.7
in the Gulf of Alaska and a Ml 3.5 in Central Alaska. Other areas
either were not reporting today or had nothing significant to
report.

NEW ZEALAND

The strongest event of the day in New Zealand was the Ml 5.5
in the region of Nelson, South Island. This was discussed in the
summary section above. In addition a series of small events appear
recorded on the IGNS on-line seismometer.
IGNS did record and parameterized two smaller felt events north of
Lake Taupo (Ml 3.0) and south of Hawke's Bay (Ml 3.9) today.
All of these events (with the exception of the Taupo earthquake
appear to be FFA from Guam at 60-62 degrees from that event).

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 remained at quiet conditions today. There
were no M- or X-class solar flares observed.

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/20020504events.txt

TIDAL EFFECTS

May 06 is the twenty-third 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

Caribbean MB>=4.0 22 2.4 0.04
Central California MB>=0.0 32 2.1 0.05
Central California MB>=2.0 11 2.9 0.02
Central California MB>=3.0 23 2.9 0.02
Mammoth Lakes MB>=0.0 56 2.5 0.04
San Andreas MB>=0.0 14 2.4 0.04
San Andreas MB>=3.0 38 2.7 0.02
So Asia MB>=4.0 50 3.6 0.01
Canada MB>=2.0 26 2.2 0.05

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

No Regions

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

5 5 0 0 0 1875 38.100 30.200 0 7.0 Turkey
Maximum intensity IX, 1300 killed, moderate damage in Turkey.
71
5 5 6 16 0 1908 3.000 123.000 60 7.5 Mindanao, Philippines
5 5 13 45 57 1930 17.000 96.580 60 7.3 Andaman Islands
550 killed.

5 5 6 31 38 1958 -9.640 27.830 0 7.2 Lake Victoria area
5 5 4 52 51 1976 -29.900 -177.800 35 7.2 No. Kermadec Islands
5 5 3 53 45 1995 12.626 125.297 16 7.1 Luzon, Philippines
Felt on Cataduanes, Leyte and Masbate. Also felt in southern Luzon.

U.S./CANADA

Following are events of Mb>=5 which have occurred in this region on

MO DA HR MN SC YEAR LATIT LONGIT DEP MW LOCATION

5 5 4 14 9 1933 48.850 -129.650 0 5.5 Vancouver Island area
5 5 11 1 14 1954 27.500 -112.500 0 5.8 Gulf of California
5 5 13 9 49 1954 27.800 -112.000 0 6.6 Gulf of California
5 5 16 13 44 1964 58.200 -149.700 35 5.0 Southern Alaska
5 5 15 7 50 1967 59.340 -151.310 53 5.0 Southern Alaska
5 5 17 6 15 1967 63.720 -148.370 100 5.0 Central Alaska
5 5 5 32 47 1978 63.318 -150.971 134 5.1 Central Alaska
5 5 21 3 16 1978 32.211 -115.304 6 5.2 So. California
Press reported shock caused chandeliers to sway.

5 5 19 49 55 1982 61.198 -149.536 50 5.3 Central Alaska
5 5 23 45 35 1990 60.020 -141.010 39 5.2 Central Alaska
5 5 10 30 4 1999 59.234 -151.343 73 5.6 Southern Alaska
5 5 5 48 48 2001 56.448 -156.609 62 5.1 Alaska Peninsula

EARTHQUAKE LISTINGS

ASIA

O: 4MAY2002 06:07:46 28.9N 69.2E Ms=4.3 GSSC PAKISTAN

O: 4MAY2002 10:52:57 5.2N 124.6E Ms=4.9 GSSC MINDANAO, PHILIPPINE ISLANDS

O: 4MAY2002 11:35:08 35.7N 140.2E Ms=4.3 GSSC NEAR E.CST EASTERN HONSHU

O: 4MAY2002 14:29 35.9N 140.1E Mj=4.0 NIED SOUTHERN_IBARAKI_PREF, Japan
O: 4MAY2002 11:35 35.5N 140.5E Mj=4.8 NIED KUJUKURI_COAST_BOSO_PEN, Japan

SOUTH PACIFIC ISLANDS

O: 4MAY2002 16:07:14 20.0S 173.8W MS=5.0 NEI TONGA ISLANDS
O: 4MAY2002 16:07:32 19.1S 174.2W Ms=4.8 GSSC TONGA ISLANDS

O: 4MAY2002 07:00:48 17.9S 178.8W MW=5.8 NEI FIJI ISLANDS REGION

O: 4MAY2002 08:08:23 7.7S 156.1E Ms=4.5 GSSC SOLOMON ISLANDS REGION

CARIBBEAN/CENTRAL/SOUTH AMERICA

O: 4MAY2002 12:51:38 23.1S 64.5W MB=5.3 NEI JUJUY PROVINCE, ARGENTINA

O: 3MAY2002 06:00:18 17.9N 68.4W ML=3.8 PRSN Eastern Dominican Republic

O: 1MAY2002 16:01:28 19.2N 68.4W ML=3.7 PRSN Eastern Dominican Republic

O: 1MAY2002 03:55:58 18.9N 64.3W ML=3.8 PRSN Virgin Islands

O: 4MAY2002 06:39:10 12.3N 87.4W MC=4.1 SIS Nicaragua, Central America

O: 4MAY2002 14:02:39 13.1N 89.1W MC=4.1 SIS El Salvador, Central America

O: 5MAY2002 00:09:16 9.5N 85.4W ML=3.1 SIS Costa Rica, Central America

NEW ZEALAND/AUSTRALIA

O: 4MAY2002 12:59:00 41.4S 172.3E ML=5.5 IGNS SE of Karamea, N.Z.

O: 4MAY2002 04:22:00 38.6S 176.0E ML=3.0 IGNS NW of Taupo, N.Z.

O: 4MAY2002 05:05:00 40.5S 176.2E ML=3.9 IGNS East of Pahiatua, N.Z.

OCEANIA

EUROPE/MIDDLE EAST/AFRICA

O: 5MAY2002 02:26:25 40.7N 30.9E ML=3.1 KAN DUZCE, TURKEY

O: 4MAY2002 17:41:09 37.3N 36.3E ML=3.3 KAN OSMANIYE, TURKEY

O: 4MAY2002 16:09:49 40.8N 31.7E ML=3.3 KAN BOLU, TURKEY

O: 4MAY2002 14:02:43 39.2N 27.9E ML=3.5 KAN MANISA, TURKEY

U.S/CANADA

CALIFORNIA/NEVADA

O: 4MAY2002 19:33:00 40.3N 124.5W ML=3.0 NEI NEAR COAST OF N CALIF.
O: 04May2002 17:45:48 40.3N 124.5W ML=3.4 NEI NEAR COAST OF N CALIF.
O: 04May2002 14:52:47 40.3N 124.4W MD=3.0 NEI NEAR COAST OF N CALIF.
O: 04May2002 14:24:48 40.3N 124.5W ML=3.5 NEI NEAR COAST OF N CALIF.
O: 04May2002 13:56:31 40.3N 124.6W ML=4.3 NEI NEAR COAST OF N CALIF.
O: 04May2002 13:28:08 40.3N 124.5W ML=3.4 NEI NEAR COAST OF N CALIF.
O: 04May2002 13:05:36 40.3N 124.5W ML=3.6 NEI NEAR COAST OF N CALIF.
O: 04May2002 12:54:23 40.3N 124.6W ML=3.6 NEI NEAR COAST OF N CALIF.
O: 04May2002 12:17:00 40.3N 124.5W ML=3.9 NEI NEAR COAST OF N CALIF.
O: 04May2002 10:20:23 40.3N 124.5W ML=3.5 NEI NEAR COAST OF N CALIF.
O: 04May2002 09:39:53 40.3N 124.5W ML=3.0 NEI NEAR COAST OF N CALIF.
O: 04May2002 08:54:06 40.3N 124.4W ML=2.9 NEI NEAR COAST OF N CALIF.
O: 04May2002 04:58:52 40.3N 124.4W ML=3.1 NEI NEAR COAST OF N CALIF.
O: 04MAY2002 01:54:06 40.3N 124.4W ML=2.9 NCSN NNW of Punta Gorda, CA
O: 04MAY2002 02:39:53 40.3N 124.5W ML=3.0 NCSN NW of Punta Gorda, CA
O: 04MAY2002 03:20:23 40.3N 124.5W ML=3.6 NCSN WNW of Punta Gorda, CA
O: 04MAY2002 05:17:00 40.3N 124.5W ML=3.9 NCSN WNW of Punta Gorda, CA
O: 04MAY2002 05:54:23 40.3N 124.5W ML=3.7 NCSN WNW of Punta Gorda, CA
O: 04MAY2002 06:05:36 40.3N 124.5W ML=3.6 NCSN WNW of Punta Gorda, CA
O: 04MAY2002 06:28:08 40.3N 124.5W ML=3.4 NCSN WNW of Punta Gorda, CA
O: 04MAY2002 06:56:31 40.3N 124.6W ML=4.3 NCSN WNW of Punta Gorda, CA
O: 04MAY2002 07:24:48 40.3N 124.5W ML=3.5 NCSN WNW of Punta Gorda, CA
O: 04MAY2002 07:52:47 40.3N 124.4W ML=3.0 NCSN NW of Punta Gorda, CA
O: 04MAY2002 10:45:48 40.3N 124.5W ML=3.4 NCSN WNW of Punta Gorda, CA
O: 04MAY2002 10:52:58 40.3N 124.5W ML=2.7 NCSN NW of Punta Gorda, CA
O: 04MAY2002 11:06:56 40.3N 124.4W ML=2.9 NCSN N of Punta Gorda, CA
O: 04MAY2002 11:42:31 40.3N 124.5W ML=2.7 NCSN WNW of Punta Gorda, CA
O: 04MAY2002 12:24:30 40.3N 124.5W ML=2.8 NCSN NW of Punta Gorda, CA
O: 04MAY2002 12:33:00 40.3N 124.5W ML=3.1 NCSN WNW of Punta Gorda, CA

O: 4MAY2002 00:28:42 40.4N 122.0W ML=2.2 NCSN ENE of Cottonwood, CA
O: 4MAY2002 00:28:42 40.5N 122.0W Md=2.2 NCSN ESE of Redding, CA

O: 3MAY2002 22:53:13 38.6N 119.8W ML=2.1 NCSN S of Markleeville, CA
O: 3MAY2002 23:07:03 38.6N 119.8W ML=2.4 NCSN S of Markleeville, CA

O: 4MAY2002 22:54:06 40.6N 121.8W ML=2.3 NCSN NW of Viola, CA
O: 5MAY2002 01:58:23 40.6N 121.8W ML=2.0 NCSN NW of Viola, CA
O: 4MAY2002 22:54:06 40.6N 121.8W Md=2.3 NCSN WNW of Lassen Peak

O: 4MAY2002 00:13:12 39.7N 122.1W ML=2.0 NCSN WSW of Hamilton City, CA

O: 3MAY2002 23:26:49 31.8N 115.8W ML=2.3 SCSN SSW of Calexico, CA

PACIFIC NORTHWEST/CANADA

ROCKY MOUNTAIN AREA

O: 4MAY2002 13:12:03 41.5N 111.7W ML=2.5 UUSN ESE of Paradise, UT

O: 4MAY2002 11:19:35 43.6N 110.3W MD=2.2 USBR ESE of Kelly, WY

EASTERN U.S./CANADA

O: 3MAY2002 17:02:24 45.5N 75.6W MN=1.9 GSC SE FROM GATINEAU, QUE.

O: 3MAY2002 17:07:52 46.1N 75.0W MN=2.7 GSC W FROM STE-AGATHE-DES-MO

ALASKA

O: 3MAY2002 23:22:56 62.7N 151.2W ML>3.0 NEI CENTRAL ALASKA

O: 4MAY2002 07:11:36 62.9N 151.7W ML=3.5 AEIC Kantishna, Alaska

O: 4MAY2002 13:30:20 58.6N 150.8W ML=3.7 AEIC Seldovia, Alaska

HAWAII