SEISMCITY CONCENTRATES IN REGION OF SOUTHERN PHILIPPINES

SUMMARY

The Mw 7.2 Mindanao earthquake on March 5 has been succeeded by a series
of moderate regional earthquakes in the Mb>4.0 range since that
mainshock. Earthquakes of Mb 4.8 have occurred about 400 km NE
of the Mindanao mainshock (March 6); within the classical aftershock zone
and up to 200 km to the SW this evening in the Celebes.
The regional triggering of moderate events was not recognized
as reality by most seismologists until after the California Landers
earthquake in June, 1992 which clearly triggered small shocks
immediately as far away as 1000 km - in Yellowstone, Mt. St. Helens, WA;
the Geysers, CA; and Mammoth Lakes, CA, among others.

GLOBAL

In addition to the local and regional aftershocks, three earthquakes
have occurred on mid-ocean ridges since the Mindanao mainshock. All of
these appear to have been FFA as they lie at distances from the Mindanao
and Hindu Kush mainshocks where FFA are normally expected to be seen.
A Mb 5.1 in the Central Mid-Atlantic was the largest earthquake of the
day in the world and lies 147 degrees from Mindanao; a second moderate
Mb 4.7 earthquake struck about the same time on the East Pacific Rise
west of the Galapagos Islands. This was 147 degrees from the Hindu
Kush mainshock. An earthquake of M 4.8 on the southern mid-Atlantic
ridge yesterday was also a probable FFA from Hindu Kush.
Of the 8 non-local aftershock events of Mb>=4.5 in the past three days,
7 have occurred either at 102-107 or 142-147 degrees from either the Mindanao
or the Hindu Kush mainshocks. Events which are 142-147 degrees from a
mainshock see a strong increase in seismic energy arriving due to reflection
and refraction off the inner-outer core boundary, likewise locations between
102 and 107 degrees distant see strong energy boost from the reflections
and refractions off the core-mantle boundary. Stronger earthquakes often
occur at these distances from each other, probably due this additional
incoming seismic energy. Two earthquakes also occurred yesterday in Northern
Chile (Mb 4.7 and 4.3) which were located at 144 degrees from the
Hindu Kush earthquake. Moderately strong earthquakes also occurred
in Central America (Mb 4.7) and Guatemala (Mb 4.7) this afternoon -
locations which lie at 142-144 degrees from the Mindanao mainshock.
Two earthquakes have also occurred in the Central Vanuatu Islands
at 102-107 degrees from the Hindu Kush mainshock.
All other events of the past three days listed by NEIS
have also occurred at FFA distances from either Hindu Kush or Mindanao. These
include: The Fox Islands, Aleutians (Mb 4.2, distance to Mindanao 72 deg);
The NE Kuril Islands (Mb 5.2, HK distance ~60 deg); Off NE
Honshu (Mb 4.4, Mj 4.8; HK distance ~60 deg); Tonga (Mb 4.6; Mindanao
Distance ~72 deg); Turkey to Iran (Mb 3.6, HK distance 19 deg); Banda
Sea (Mb 5.2, ~72 degrees from HK).

U.S./CANADA

After a very quiet week, activity began to pick up in Central
and Northern California today. Two earthquake of Ml 2.5 and 2.6
occurred in the Parkfield area and a third slightly to the northwest
(Mb 2.0). In addition there were earthquakes of Ml 2.0 at Pleasant
Hill east of San Francisco and at Patterson east of San Jose CA. this
afternoon and evening. An earthquake of Ml 2.3 also occurred off the
northern Coast near Eureka.
Activity in most of southern California was at quiet levels again
today. The Calexico/Baja sequence saw only 3 aftershocks of Ml>=2
on this day, the largest of which was a Ml 3.1. Light earthquakes
occurred in the Simi Valley near Los Angeles and along the coast
further south.
The largest earthquake located in the U.S./Canada region today
was a Ml 3.5 near Jaspar, Alberta north of Idaho this afternoon.
An unusual earthquake was registered in Michigan on March 3 of Ml
2.2 in the Michigan area north of Grand Rapids. This earthquake
occurred several hours after the Hindu Kush event and may be FFA.
The area within 50 km of this epicenter has no record of any
previous earthquake. An event listed as occurring on March 6, 1965
in some catalogs appears to be a mistaken reading for a Ml 5.3
that occurred several minutes earlier near Fletcher, Missouri. This
error was first made in the Weston catalog, then carried into the
Decade of North American Geology (DNAG) catalog and from there
into urban legend. It is surprising how many events are listed
in catalogs for which the parameters are grossly in error. These
errors often propagate from one catalog to another and become
more and more difficult to overcome with fact. In this case we
know the event occurred in Missouri, because that is where it was
felt at the time of occurrence. Anyway, if the GSC (Geological
Survey of Canada) location is correct this is indeed a unique
event.
No events of M>=3 were located in Alaska today.

NEW ZEALAND

The IGNS on-line seismometer remained noisy today. In the
past week IGNS has located and QED has listed 5 events of M>=3
off or on the northern end of North Island, one in the Cook
Strait and 2 on the northern end of South Island.

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

GEOMAGNETIC/SOLAR FIELDS

Except for a minor geomagnetic storm about 12:00 UT today
the geomagnetic field remained at quiet to unsettled conditions.
There were no X- or M- class flares observed, however sun
did get more active towards the end of the day with two high
C-class flares.

For general space weather conditions see:
http://sec.noaa.gov/today.html
For Solar flare data see:
http://sec.noaa.gov/ftpdir/indices/events/20020307events.txt

TIDAL EFFECTS

March 08 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 SEISMIC HISTORY

The largest earthquake on this date occurred on March 8, 1812 in
a remote region of Central China (M 8.0). No damage or deaths
were recorded, however it is quite likely that they occurred.
The most destructive earthquake on this date occurred in Eastern
Greece in 1957. This event and a foreshock caused moderate
property damage in Larisa and Magnesia provinces while killing
two and leaving many injured. The event registed Mw 7.0.
Other earthquakes of Mw>=7 that have been recorded on March 8
include a Mw 7.0 in Northern Japan (property damage and causualties
were likely but not recorded Mw 7.0); in the Vanuatu Isl. (Mw 7.2, 1960);
and again in the Vanuatu Isl. in 1980 (Mw 7.1); and in the Komandorsky
Islands (Mw 7.0, 1991) and east in Kamchatka in 1999 (Mw 7.0).

U.S./CANADA

The largest earthquake to occur in the U.S. or Canada on March
8 was a Mb 6.2 in the Kodiak Island area of Alaska in 1990. This
event was felt (IV) at Kodiak, Larsen Bay and Port Lions and with
lesser effects at Homer, Pedro Bay and Seldovia. The second largest
earthquake in the U.S. on this date occurred in the same Kodiak
location exactly 10 years later with Ms 5.6.
No notable earthquakes have occurred in the western U.S. on
this date in recorded history.
A most interesting event on this date in the U.S./Canada
occurred on March 8, 1975 near the North Geomagnetic Pole, Canada
(Mb 5.2). The strongest geomagnetic storm of the year followed
two days later on March 10 (AP = 90; AA=110). It is not uncommon to see
earthquakes near the geomagnetic poles at times of strong geomagnetic
storms. In this case a solar flare was the likely cause of the
geomagnetic storm. The flare would have occurred 2-4 days earlier
about the time of the earthquake, but the flare catalog for 1975
in incomplete for that portion of the year, so this cannot be confirmed.