CALIFORNIA SEISMICITY REBOUNDS AFTER STRONGEST GEOMAGNETIC STORM IN 5 MONTHS

SUMMARY

The California region appeared to be subjected to regional activation
today after a week of quiet. In the past week which only one event of Ml>=3
was recorded in the California region and in the last several days the region has been extremely quiet at all magnitudes. A series of earthquakes occurred
today suggesting that the area may be rebounding from this strong quiescence.
Northern and Central California saw 11 earthquakes of Ml>=2 today compared
with one such event in the previous 4 days. In southern California 4 events
of Ml>=2 were observed compared with 5 in the previous 4 days (3 of which
occurred as a swarm near Brawley yesterday morning. This is a total of
15 events of Ml>=2 today compared with 6 in the previous four days.
Today's earthquakes appeared to be scattered throughout the state.
Nine different epicentral areas were active today - Petrolia, Coalinga,
the Geysers, Quincy, Ft. Ross, Tres Pinos, Borrego Springs, Port
Huenene and Calexico indicating some kind of regional activation.

The activity in Central California began shortly after a strong, long-
lasting M2.4 occurred while California was exactly anti-solar and after
the strongest geomagnetic storm since November 2001 began at about
09:00 UT. Areas which experience the strongest effects from geomagnetic
and solar events are anti- and sub-solar. The first events in central or
Northern California occurred in the Geysers at 09:20 UT, followed by the
additional events over the next 13 hours. Even for skeptics, it must be
hard NOT to see a correlation here between the geomagnetic storm/solar flare
and the sudden increased seismicity in Central and Northern California today.

As is usual for strong geomagnetic storms, a seismic watch is being
issued for a possible moderate or strong earthquake within 30 degrees
longitude of 120W or 60E or at high latitudes (>45 N or S) for the
next three days. Small events in these regions may be foreshocks.

GLOBAL

There was also a strong rebound in global seismicity today at the
ML>=4 level. Fifteen earthquakes of Ml>=4 have been listed thus far
by NEIS since the last summary. The largest of these was a Mb 5.4
in Southern Iran. One other event of Mb>=5 occurred in the Fiji
Islands, another active area at this time also experiencing quakes
of 4.0 and 4.1 last night.
The most unusual earthquakes occurred as a Ml 4.5 in northern
Australia and as a Ml 4.8 and 4.1 in southern Italy. The mid-ocean
ridge systems continued their current high levels of seismicity with
a Mb 4.7 on the Mid-Indian Ridge.
Other events were recorded in Luzon, Philippines (Mb 4.6); the Rat
and Andreanoff Islands in the Central Aleutians (Mb 4.2 and 4.6);
off the Coast of Peru (Mb 4.6) and in northern and Central Chile.
Earthquakes in Asia included a Ml 4.7 southeast of Honshu, a
Mb 4.0 in Hindu Kush and an event of undetermined magnitude in
Irian Jaya.

U.S./CANADA

The largest earthquakes in the U.S./Canada today occurred in the
Central Aleutians of Mb 4.6 (Andreanoff Islands) and Mb 4.2 (Rat Islands).
Two other earthquake of ML>=3 also occurred in central Alaska today.
No event of Ml>=3 occurred within the Hawaiian Islands today.
The strong activation in the California region today (see Summary
section above), was not completely unexpected, as this is a common
occurrence following a strong quiescence. Today's events occurred
as a Ml 2.8 north of Coalinga, preceded by a Ml 2.5 foreshock; a series
at Tres Pinos (Ml 2.4, 2.2, 2.1); a Ml 2.0 near Fort Ross on the
coast north of San Francisco; a Ml 2.2 north of Petrolia; Ml 2.2
west of Quincy preceded by a Ml 2.0 foreshock;; earthquakes of Ml 2.4 and
2.1 near the Geysers. In southern California shocks were observed
of Ml>=2 at Port Huenene (Ml 2.5); Borrego Springs (Ml 2.3 and 2.1)
and in the Calexico region (Ml 2.2).
No other events of Ml>=2 were reported from other areas of the
U.S. or Canada today.

NEW ZEALAND

A strong swarm of earthquake began yesterday south of Lake
Taupo, New Zealand. These include events of Ml 4.8, 4.3, and 4.8
among smaller associated shocks. The Taupo region has not experienced
strong shocks in the last several years although subject to many light
and micro earthquakes. The two Ml 4.8 events are the strongest we have on
record within 50 km of 38.8S 176E since a Mb 6.2 in the area on
Oct 25, 1999. The Ml 4.8 is apparently the largest shallow focus earthquake
in this region since March 5, 1984 when a Ml 5.5 occurred, although many
slightly smaller event have occurred in the intervening years.

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

GEOMAGNETIC/SOLAR FIELDS

A noted in the summary, the increased activity in California today
coincided with a strong geomagnetic storm, apparently precipitated by a
strong, long-lasting M2.6 solar flare which occurred while California
was exactly anti-solar. Parameters are listed below for this flare:

Flare # START MAX END CLASS

1590 0746 0824 0957 M2.6

The strong geomagnetic storm Kp 7 (AA 73 at mid-latitudes) was the
strongest recorded at mid-latitudes since November, 6, 2001.
Data on Geomagnetic indices for 2002 is available at:

http://sec.noaa.gov/ftpdir/indices/DGD.txt

and

http://sec.noaa.gov/ftpdir/indices/old_indices/2002Q1_DGD.txt

Data on Geomagnetic indices for 2001 is available at:

http://sec.noaa.gov/ftpdir/indices/old_indices/2001_DGD.txt

For general space weather conditions see:
http://sec.noaa.gov/today.html
For Solar flare data see:
http://sec.noaa.gov/ftpdir/indices/events/20020417events.txt

SEC is expecting another strong geomagnetic storm tomorrow.

TIDAL EFFECTS
April 19 is the sixth day after the beginning of the 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

Indian Ocean MB>=4.0 17 2.1 0.05
Kamchatka MB>=4.0 35 2.6 0.03

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

Global MB>=6.0 -17 -2.8 0.02
West Pacific MB>=4.0 -20 -1.9 0.05
Yellowstone MB>=0.0 -19 -2.0 0.05
Puerto Rico MB>=4.0 -17 -1.9 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 18 0 0 0 1352 35.600 105.300 0 7.0 So. China
Most damage was done in Kansu and Shensi. At Chuanglang, Tinghsi in
Liangchou Fu. At Kansu many houses were destroyed, a mountain
was moved and a valley was filled up.

4 18 0 0 0 1902 14.000 -91.000 0 7.5 Guatemala
2000 killed at Quetzaltenango and San Marcos, and
Amatitlan, Guatemala. Felt from Tabasco, Jalisco, Guerro, Chiapas,
Oaxaca, Hidalgo and Mexico City, Mexico. Destroyed the city of
Quetzaltenango. Extensive loss of life. Activity continued until
September 23 when an earthquake was strongly felt and an eruption of
the Volcano Santa Maria began.

4 18 13 12 0 1906 37.670 -122.480 0 8.3 Central California
The Great San Francisco earthquake of 1906 was felt over
1 million square km. 700 were killed and many more were injured.
The zone of damage extended from Eureka to Salinas.
Although the earthquake did some damage to buildings in general, by
far the worst damage occurred, as in 1868 to construction on made land
and along the bay shore. The fire that followed the earthquake caused
most of the destruction. As in other great earthquakes, the water system
was destroyed and the fire raged for 3 days before a favorable change
of wind and concentration of efforts along parks and wide street confined
it. Dynamiting of structures did not appear to be very successful. The
Fire Chief had been killed by a fallen wall at th time of the earthquake.
The problems of caring for the disposessed and re-establishing city
life and services was great, about half a million people temporarily
left the city.
The concentration of shaking was quite complicated. The highest values
were aligned along the San Andreas, but areas of higher intensity along
the Berkeley Hills and in the San Joaquin Valley near Los Banos suggest
movement on other faults as well. Sympathetic movement probably took
place along the Hayward, and faults in the San Joaquin valley.
Dextral strike slip movement took place on the San Andreas fault from
San Juan Bautista to at least Punta Arenas. Another fault trace was
found to extend to Petrolia. The largest displacement was in Marin
County on the Point Reyes Peninsula where displacement of up to
7 meters were observed in soft alluvium and up to 5 meters on solid
ground.
Vertical displacements of up to half a meter were noted in many
places. These tended to be small and disappear with the passage of
time. Similar effects have been noted in other large earthquakes (notably
in Chile in 1835 and 1960).
Aftershocks continued for mor than a year. A far-field aftershock
on the same day as the San Francisco quake but in Brawley was felt
over all of the Imperial Valley and into San Diego.
As a result of the California earthquake of 1906, the Seismological
Society of America was formed and the elastic rebound theory of earthquakes
was enunciated by Ried (1910). If it were not for this earthquake
the true nature of the San Andreas Fault might not yet be recognized.

4 18 23 52 26 1907 13.500 123.000 60 7.2 Luzon, Philippines
These earthquakes were felt in the greater part of Luzon, Philippines.
through an area of 800 km in diameter. Two violent earthquakes occurred
in slightly less than an hour. Damage to buildings was done by the
second as they had been weakened by the first. The customs house
in Manila was badly damaged along with many other building in Manila.

4 18 4 1 48 1916 53.300 -170.000 170 7.5 Fox Islands, Aleutians
4 18 19 22 46 1928 42.000 24.700 33 7.0 Bulgaria
An MMI X earthquake hit Chirpan, Bulgaria.

4 18 6 22 45 1939 -27.000 -70.500 100 7.4 Coast Central Chile
4 18 21 34 49 1949 -15.500 -173.500 100 7.0 Tonga Islands
4 18 13 39 19 1990 1.186 122.857 26 7.4 Celebes Sea
At least 3 people killed and 25 people injured. More than 1140 houses
were damaged in the Balaan area. Felt strongly throughout the
Minahassa area. Followed by a possibly p-wave triggered shock
of Ml 5.4 in Central California which did extensive damage in the
Santa Cruz-Watsonville area.

U.S./CANADA

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

4 18 12 0 0 1872 36.000 -118.000 0 5.0 California/Nevada area
Very severe shock at Inyo County.

4 18 13 12 0 1906 37.670 -122.480 0 8.3 Central California
See description above for the great San Francisco earthquake.

4 18 4 1 48 1916 53.250 -170.000 170 7.5 Fox Islands, Aleutians
4 18 3 55 0 1931 48.700 -122.200 0 5.0 Washington state, U.S.
At Bellingham, Washington. At Acme, a few bricks fell; at Saxon
a severe twisting motion was observed. Intensity V also was
assigned to Bellingham and other towns in the area. Felt slightly
at Vancouver and Victoria, British Colombia.

4 18 22 15 28 1935 70.500 -73.000 0 5.6 Baffin/Davis Strait, Canada
4 18 11 0 22 1956 51.800 -177.700 33 6.5 Andreanoff Islands, Aleutians
4 18 1 32 18 1964 56.400 -152.800 20 5.0 Southern Alaska
4 18 7 47 0 1964 57.360 -149.950 10 5.0 Southern Alaska
4 18 20 16 16 1964 56.100 -153.700 30 5.9 Southern Alaska
4 18 23 38 3 1964 59.300 -147.500 10 5.1 Southern Alaska
4 18 6 33 58 1965 41.440 -127.310 17 5.4 Off Coast of No. California
4 18 8 50 40 1970 59.900 -152.800 94 5.7 Southern Alaska
4 18 10 32 46 1976 59.797 -153.283 156 5.4 Southern Alaska
4 18 13 21 7 1979 51.393 -170.652 47 5.0 Andreanoff Islands, Aleutians
4 18 21 16 2 1980 46.208 -122.183 0 5.0 Washington state, U.S.
Volcanic Activity at Mt. St. Helens

4 18 6 55 46 1984 52.046 -169.753 33 5.0 Fox Islands, Aleutians
4 18 19 31 31 1984 60.706 -151.840 81 5.1 Central Alaska
4 18 2 1 41 1987 61.820 -151.200 77 5.9 Central Alaska
4 18 2 41 53 1989 51.700 -168.600 0 5.1 Fox Islands, Aleutians
4 18 13 41 39 1990 36.918 -121.670 6 5.0 Central California
4 18 15 46 4 1990 36.932 -121.695 9 5.2 Central California
Slight damage in the Santa Aptos-Watsonville-San Juan area. This was
the strongest aftershock of the Loma Prieta earthquake.

4 18 15 5 58 1999 60.387 -151.852 73 5.3 Central Alaska

EARTHQUAKE LISTINGS

ASIA

O: 17APR2002 22:46:48 24.4N 122.0E ML=4.5 TAI Ilan, Taiwan

O: 17APR2002 00:15 32.3N 140.9E Mj=5.2 NIED E_OFF_HACHIJOJIMA_ISLAND, Japa

O: 17APR2002 00:10 36.4N 140.6E Mj=4.0 NIED NORTHERN_IBARAKI_PREF, Japan

O: 15APR2002 06:10 37.4N 141.7E Mj=4.5 NIED E_OFF_FUKUSHIMA_PREF, Japan

O: 17APR2002 00:14:57 31.7N 140.4E MB=4.7 NEI SOUTHEAST OF HONSHU, JAPA

O: 16APR2002 23:55:18 14.4N 123.0E MB=4.6 NEI LUZON, PHILIPPINES

O: 16APR2002 23:13:58 36.0N 69.4E MB=4.0 NEI HINDU KUSH, AFGHANISTAN

O: 17APR2002 04:46:58 4.8S 138.8E MB>4.0 NEI IRIAN JAYA, INDONESIA

O: 17APR2002 08:47:22 27.6N 56.8E MB=5.4 NEI SOUTHERN IRAN

SOUTH PACIFIC ISLANDS

O: 16APR2002 14:31:03 5.9S 149.7E MB=4.2 NEI NEW BRITAIN REGION, P.N.G

O: 16APR2002 21:21:54 18.2S 178.5W MB=4.0 NEI FIJI ISLANDS REGION

O: 16APR2002 18:26:03 21.1S 179.0W MB=4.1 NEI FIJI ISLANDS REGION

O: 17APR2002 16:42:23 24.7S 176.4W MB=5.1 NEI SOUTH OF FIJI ISLANDS

CARIBBEAN/CENTRAL/SOUTH AMERICA

O: 17APR2002 05:28:56 17.0N 100.2W ML=4.3 SSN COSTA DE GUERRERO, MEXICO
O: 17APR2002 03:03:51 17.0N 100.1W ML=3.4 SSN COSTA DE GUERRERO, MEXICO
O: 16APR2002 20:07:54 16.9N 100.1W ML=4.0 SSN COSTA DE GUERRERO, MEXICO

O: 16APR2002 16:16:50 15.8N 98.9W ML=4.2 SSN COSTA DE OAXACA, MEXICO

O: 16APR2002 18:33:07 6.7N 73.2W Mc=3.8 ULA Dpto. Santander, Colombia.

O: 16APR2002 04:54:28 19.8S 69.6W MB=4.3 NEI NORTHERN CHILE

O: 17APR2002 02:11:02 31.7S 70.4W HN>4.0 NEI CHILE-ARGENTINA BORDER RE

O: 17APR2002 11:35:39 14.5S 76.6W MB=4.6 NEI NEAR COAST OF PERU

NEW ZEALAND/AUSTRALIA

O: 17APR2002 21:15 38.9S 176.2E ML=4.8 IGNS SE of Taupo, N.Z.

O: 16APR2002 10:13 38.8S 175.8E ML=4.8 IGNS SW of Taupo, N.Z.

O: 16APR2002 10:59 38.8S 175.9E ML=4.3 IGNS SW of Taupo, N.Z

O: 16APR2002 09:08:36 15.8S 127.3E MB=4.5 NEI WESTERN AUSTRALIA

OCEANIA

O: 17APR2002 05:15:53 39.2S 78.5E MB=4.7 NEI MID-INDIAN RIDGE

EUROPE/MIDDLE EAST/AFRICA

O: 17APR2002 07:57:37 39.7N 16.9E MB=4.1 NEI SOUTHERN ITALY
O: 17APR2002 06:42:53 39.8N 16.8E MB=4.8 NEI SOUTHERN ITALY

U.S/CANADA

CALIFORNIA/NEVADA

O: 16APR2002 16:39:54 35.8N 119.7W ML=2.0 SCSN SSW of Corcoran, CA

O: 17APR2002 02:56:03 32.5N 115.2W ML=2.2 SCSN SE of Calexico, CA

O: 16APR2002 20:10:21 34.1N 119.2W ML=2.5 SCSN SW of Port Hueneme, CA

O: 17APR2002 04:20:36 33.3N 116.2W ML=2.1 SCSN ENE of Borrego Springs, CA
O: 17APR2002 03:16:54 33.3N 116.2W ML=2.3 SCSN ENE of Borrego Springs, CA

O: 17APR2002 04:31:28 36.7N 121.3W ML=2.4 NCSN SSE of Tres Pinos, CA
O: 17APR2002 05:43:47 36.7N 121.3W ML=2.2 NCSN SSE of Tres Pinos, CA
O: 17APR2002 05:46:00 36.7N 121.3W ML=2.1 NCSN SSE of Tres Pinos, CA

O: 17APR2002 06:50:03 38.3N 123.1W ML=2.0 NCSN W of Bodega Bay, CA

O: 17APR2002 07:22:17 38.8N 122.8W ML=2.4 NCSN E of The Geysers, CA
O: 17APR2002 02:20:36 38.8N 122.7W ML=2.1 NCSN ESE of The Geysers, CA

O: 17APR2002 03:37:11 40.0N 121.2W ML=2.0 NCSN WNW of Quincy, CA
O: 17APR2002 12:46:42 40.0N 121.3W ML=2.4 NCSN W of Quincy, CA

O: 17APR2002 15:31:56 36.5N 120.4W ML=2.8 NCSN SW of San Joaquin, CA
O: 17APR2002 13:50:17 36.5N 120.3W ML=2.5 NCSN SW of San Joaquin, CA

O: 17APR2002 15:47:38 40.4N 124.3W ML=2.2 NCSN NNW of Petrolia, CA

O: 17APR2002 17:57:05 35.8N 121.3W ML=2.0 SCSN NNW of San Simeon, CA

PACIFIC NORTHWEST/CANADA

O: 17APR2002 12:34:24 58.8N 142.8W ML=2.7 PGC WSW Yakutat AK

O: 15APR2002 15:12:34 53.2N 132.9W ML=2.2 PGC W Sandspit BC

O: 14APR2002 20:49:57 52.9N 119.5W ML=1.8 PGC W Jasper AB

O: 14APR2002 15:03:09 62.3N 124.3W ML=3.5 PGC WNW Ft. Simpson NT

O: 17APR2002 10:54:49 47.7N 120.1W ML=1.0 PNSN E of Entiat, WA

ROCKY MOUNTAIN AREA

EASTERN U.S./CANADA

ALASKA

O: 17APR2002 03:32:30 60.5N 151.5W ML=3.3 AEIC Homer, Alaska

O: 17APR2002 11:38:25 62.9N 148.6W ML=3.0 AEIC Cantwell, Alaska

O: 17APR2002 03:32:31 60.5N 151.4W MB>3.0 NEI KENAI PENINSULA, ALASKA

O: 16APR2002 14:59:16 60.4N 152.1W HN=0.0 NEI SOUTHERN ALASKA

O: 16APR2002 13:25:25 52.0N 170.1W MB=4.8 NEI FOX ISL, ALEUTIAN ISLANDS

O: 17APR2002 13:01:02 52.2N 179.4E MB=4.2 NEI RAT ISL, ALEUTIAN ISLANDS

O: 17APR2002 11:38:27 63.0N 148.5W MB>3.0 NEI CENTRAL ALASKA

HAWAII