EARTHQUAKE MW 6.2 in TONGA ISLANDS PROBABLY TRIGGERED BY GEOMAGNETIC EFFECTS

SUMMARY

A strong earthquake in the northern Tonga Islands occurred this morning
with the following parameters:
02Oct2001 00:48:19 16.1S 173.8W MW=6.2 NEIS TONGA ISLANDS
This event followed a strong sudden commencement of a geomagnetic
storm which began at 22:30 UT while the area was exactly sub-solar.
A M1.1 class solar flare then followed at 23:40 UT also as the area
was sub-solar. The combined effect of these two sudden geomagnetic
effects appears to have been enough to trigger this earthquake.
This is the third earthquake of M>6 in the two days since the end of the
proton storm on 29 Septembe. The other events occurred as a Mw 6.3
in Vanuatu, a Ms 6.1 aftershock yesterday in Vanuatu and today's
earthquake in the Tonga Islands (Ms 6.2).

GLOBAL

Seismic activity has been moderately high throughout the world since
29 September. In addition to the three events of Ms>6, there have been
earthquakes of Mb 5.3 in the South Sandwich Islands, a swarm of events
(Maximum Mb 5.2) in the Easter Island region, and a Mb 5.4 in the Kuril
Islands. All of these events have been at high latitudes or near the
geomagnetic equator and it is expected that the geomagnetic storms
increased their eventual magnitudes considerably.
Other earthquakes of Mb>=4.5 have occurred in Alaska in the Fox
Islands (Ml 5.1) and in the Andreanoff Islands (Mb 4.6) and in the
Kuril Islands (Mb 4.8) (at high latitudes); and in the Mariana Islands
(Mb 4.7) and Bali (Mb 4.8) at the geomagnetic equator; and in
Loyalty Islands, Vanuatu, Tonga and the Fiji Islands (under seismic watch).
No earthquakes of Mb>=4.5 have been listed by NEIS which were not either
along the geomagnetic equator, at high latitudes or under seismic watch
conditions since the end of the proton storm.

U.S./CANADA

Activity in the U.S. and Canada outside Alaska has been fairly
quiet since the last report.
The most unusual earthquake in the California/Nevada region today was
an Ml 2.5 in the Saratoga area this evening. Only 7 events have occurred
within 40 km of this events since Jan 1, 1999, so the event is fairly
rare. The last such event occurred as a Ml 2.6 on July 15, 2001.
An Ml 2.9 in the Mammoth Lakes area and a Ml 3.1 in
the Baja California region occurred yesterday. Both Baja and Mammoth Lakes
often becomes activated during and following strong geomagnetic storms.
Other events of Ml>=2 in the western U.S. occurred near Napa (Ml 2.2);
Big Bear City (Ml 2.6) and Running Springs CA (Ml 2.3). An event of Ml 2.3
occurred in Utah near Wallsburg today.
The largest earthquake in the U.S./Canada outside Alaska this weekend occurred
off the coast of Oregon/California. PNSN gives the following parameters:

29SEP2001 9:13: 9 41.9N 128.1W ML3.0 PNSN OFF THE COAST OF CALIFORNIA

PNSN has difficulty locating and assigning magnitudes to events off
the coast and this distant from the network. The magnitude is probably
3.5-4.5 Ml.

An unusual earthquake was also recorded today near Newport, Oregon.
PNSN gives the following parameters for this event:

01OCT2001 20:53:32 44.4N 124.6W ML=1.8 PNSN WSW of Newport, OR

Only 15 earthquakes of all sizes have ever been recorded within
50 km of this epicenter, of which 5 have been larger (Ml 2-3). The
last event of this size in this region occurred on July 17, 2000 (Ml 2.6)
and Sept. 1, 1996 (Ml 2.7).

NEW ZEALAND

The on-line seismometer was busy again today. Two regional events
(probably in Tonga) were recorded along with many light events. One
event near the seismograph was registered this afternoon (Ml 3-4).

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 very active today. Starting last
night (UT) a moderate storm continued for 12 hours. This morning
at about 04:00 UT a strong storm developed out of the moderate
storm and continued for about 12 hours before dropping back
to quiet conditions, probably due to the added effects of a
strong M9.1 solar flare at 04:41 UT. This was followed at about 23:00 UT by
a strong resurgent sudden commencement storm which was probably
enhanced by a M1.2 flare at 23:40 UT. Areas affected
by these storms will be within 20 degrees longitude of 60W and
120E and 175W and 5E in addition to the regions affected
by last night's storm. This covers a good portion of the globe,
so it is reasonable to issue a seismic watch for the world -
a seismic watch means that maximum magnitudes would be increased by up
to 1.0 units for the next three days.
A strong M-class solar flare occurred this afternoon and a weak
M-class flare occurred this evening (UT) with the following parameters:

Flare # START MAX END CLASS
390 0441 0515 0523 M9.1
530 + 2340 2345 2351 M1.2

The second of these was followed about an hour later by a Ms 6.2
earthquake in the region directly sub-solar and near the geomagnetic
equator - in the Tonga/Samoa Islands. This event was probably
triggered by geomagnetic effects.

For general space weather conditions see:
http://sec.noaa.gov/today.html
For Solar flare data see:
http://sec.noaa.gov/ftpdir/indices/events/20011001events.txt

TROPICAL STORMS

Juliette is now at the northern end of the Gulf of California and
is a weak tropical depression. It may affect seismicity in the
area of 30N 113W for the next day and then die a natural death.
For tracking details see:
http://www.wunderground.com/auto/HurricaneSite/tropical/tracking/ep200116.html