EARTHQUAKE ML 6.1 HITS CENTRAL MEXICO COASTLINE IN MICHOACAN/GUERRERO REGION

08OCT2001 02:59:57 16.9N 100.2W ML=4.5 SSN COSTA DE GUERRERO , MX
08OCT2001 02:39:20 17.0N 100.2W ML=6.1 SSN COSTA DE GUERRERO , MX

(UT time above - source give Local time)
Source:
http://www.ssn.unam.mx/~yi/SSN/ssn-ultimos.html

Note: This event occurred after several region of Mexico were
subjected to hurricane landfalls or forces - Hurricane Loreno hit
on Oct 5 in the area to the west of this epicenter and Hurricane
Iris was approaching from the east at the time of this event.
This is the largest event within 100 km of this epicenter since
a Ms 6.4 on July 15, 1996. Before that, the last event of Ml>6.1 had
occurred as a Ms 6.1 on July 25, 1991.
Earthquakes in this area have also tended to cluster during periods
of high geomagnetic activity, with 3 between 1989 and 1991 (Ms 7.3,
6.4 and 6.1); two between 1978 and 1979 (Ms 6.6 and 6.9);
7 between 1956 aand 1962 (Ms 6.5, 6.2, 6.2, 6.2, 7.0 and 7.5); 4 during
1907-1909 (Ms 8.3, 7.5, 7.8, 7.8, 7.6). Since 1966 (the development of
good recording of seismic events), with the one exception of the
event in 1996, EVERY earthquake in this region of Ms>=6 has occurred
during or slightly after a period of strong geomagnetic storminess.

The events follow:

Date MS Geomagnetic Storm AA Index (Maximum for storm)

Oct 08, 2001 6.1 ST 23-OCT 4 2001 83
Jul 15, 1996 6.4 No geomagnetic storm
Jul 25, 1991 6.1 Jul 14-21, 1991 61
May 31 1990 6.4 May 26-30, 1990 58
Apr 25, 1990 7.3 Apr 23-26, 1990 79
Jul 14, 1984 6.4 Jul 13-14, 1984 82
Jan 26, 1979 6.1 Jan 25 1979 52
Mar 19, 1978 6.6 Mar 16-18, 1978 45
Jun 07, 1976 6.7 My 30- JN 5, 1976 40
Jul 16, 1973 6.2 Jul 15-16, 1973 46

It is also interesting to note that all but one of these events occurred
in the 5-month summer period from March through October, with only
one occurring in the winter months.

This event had been considered a possibility due to the effects
of Hurricanes Iris and Lorena. Speculation posted on September 3
had suggested an event in the Ml 5-6 range was possible in the
Michoacan region. Guerrero is on the eastern border of Michaoacan.

Report of October 7:

"Iris was upgraded to Category 1 Hurricane status today. Hurricane
Iris is taking a path slightly south of that originally anticipated and
is expected to hit the eastern coastline of Guatemala and Belize
tomorrow (8-9 October) as a category 2 hurricane. Two earthquakes were
recorded in Guatemala today with Ml 4.6 and 4.1. The low atmospheric pressure
associated with the hurricane system may have allowed enough uplift of the
continental plate near Guatemala/El Salvador for these earthquakes to have been
triggered due to reduced normal stresses across the the subduction zone."

Report of October 3:

" A new tropical storm - Tropical Storm Lorena formed off the coast
of southwestern Mexico today. Lorena is not expected be be long-lasating
or powerful and forecasters expect it to hit the coast of Jalisco,
Mexico tomorrow. Seismicity along the Mexican subduction zone, especially
in Michoacan and Jalisco may be affected as the winds push Mexico
north reducing normal stress on the subduction zone and the tidal
surge adds about 1 million tons per sq. mile to the subduction zone off
the coast, pushing it down. These two effects tend to decouple the
subduction zone from the North American plate. Larger earthquakes
can occur under these circumstances (Mb 5-6). The force of Lorena
is relatively minor, so this effect should not increase the maximum
magnitude in the region by more than 0.5-0.75 units over previous
estimates. This watch is in effect until 6 October, 2001. "

• Earthquakes in Mexico