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A series of moderately strong to strong earthquakes hit in the short interval of several hours this afternoon. This sequence began in Guerrero, Mexico with an M 5.7 and moved northwestward along the Pacific Plate rim first to the Alaska Peninusla (M 5.8-6.0) fifty minutes later, then to Japan (M 6.0) after another 45 minutes. The migration then returned first with an earthquake in the Andreanof Islands of M 5.6-5.7 seven minutes later and a second event in the Alaska Peninsula of M 5.9-6.2 followed by an aftershock in Japan seven minutes later. The seven minute interval is about transit time from Alaska to Japan of the P-wave. It is likely that surface waves from one epicenter to the next were generally responsible for this strong series of earthquakes. These were not the only moderate to moderately large earthquakes today as such events also hit Sulawesi, Indonesia (M 5.8); the Ryukyu Islands of Japan near Okinawa (M 5.6) and the Fiji Islands. More on these is discussed further in this summary. Readers may recall that the region of Alaska is currently under a seismic watch as emphasized in our summary yesterday. On May 2, we emphasized this pattern as a result of the geomagnetic storm. As we have noted in the past geomagnetic triggering is often strongest at 4-5 days following the major storm, which would be May 5-6. Our watch issued in this summary on May 2 is reproduced in part below:
"The geomagnetic storm which began yesterday continued today and reached high AP values of 56 at high latitudes today. This is the strongest high latitude storm since a similar storm on March 11 when the high latitude AP also reached 56. Most readers will recall that March 11 is also the day of the great M 9.0 earthquake in Honshu, Japan, an earthquake and tsunami which killed more than 20,000 people. The current storm is also likely to increase global seismicity with the potential for a major earthquake in the next four days either as an aftershock in Japan, in Alaska or along the geomagnetic equator. ... the time of the tidal alignment include regions of central Mexico .. seismicity could be seen in these areas in the next several days" (May 2, 2011)
The earthquakes of M 5.9-6.2 in the Alaska Peninsula were felt with intensity V in Sand Point only 20 km from the epicenter and II-III in False PAss, Perryville, Cold Bay and King Cove, Alaska. Earthquakes in this region are not often felt even at this magnitude level. This is the strongest earthquake in this region of Alaska since an M 6.2 on October 2, 2007 and an M 7.0 on May 30, 1991. It is the largest earthquake in Alaska outside the Aleutian Islands in two years, since an M 6.0 on March 30, 2009 in the Kodiak area.
O: 05MAY2011 20:08:51 54.9N 160.3W ML=3.7 NEIC ALASKA PENINSULA
O: 05MAY2011 22:22:24 54.9N 160.3W ML=3.7 NEIC ALASKA PENINSULA
O: 05MAY2011 14:13:01 55.1N 160.5W ML=5.8 NEIC ALASKA PENINSULA
O: 05MAY2011 14:13:01 55.2N 160.6W mb=6.0 EMSC ALASKA PENINSULA
O: 5MAY2011 16:57:38 55.1N 160.6W ML=5.9 NEIC ALASKA PENINSULA
O: 05MAY2011 16:57:37 55.1N 160.6W MB=6.2 GFZ Alaska Peninsula
O: 05MAY2011 16:57:35 55.1N 160.7W ML=6.4 GSR Alaska Peninsula
O: 05MAY2011 14:12:58 55.1N 160.7W ML=6.3 GSR Alaska Peninsula
P: 10MAY2011 60372 54.7N 160.7W 3.5-5.9 AAAAA Unimak Islands, Aleutians
In our summary yesterday (see below) we discussed in detail the recent seismicity of the Andreanof Islands following an M 5.4-5.6 in the eastern Andreanof Islands yesterday. An M 5.4-5.6 followed today in the western Andreanof Islands. This earthquake was not reported felt in this remote epicentral area. It, like yesterday's earthquake of similar magnitude to the east is the strongest in the Andreanof Islands since an M 6.4 on October 8, 2010.
O: 05MAY2011 15:06:08 51.5N 178.3W MB=5.6 GFZ Andreanof Islands, Aleutian
O: 5MAY2011 15:06:12 51.5N 178.3W ML=5.4 NEIC ANDREANOF ISLANDS, ALEUTIAN I
P: 10MAY2011 60361 50.6N 179.2W 3.5-5.9 BAAAA Andreanoff Isls, Aleutians
A strong earthquake in Honshu, Japan of M 6.0-6.2 also occurred in this series. It was felt in Honshu but there were no damages reported and no tsunami was expected or observed. This is the strongest aftershock in the region of the M 9.0 quake of March 11 in the three weeks since an M 7.1 on April 11.
O: 05MAY2011 14:58:21 38.4N 144.1E ML=6.0 GSR Off east coast of Honshu, Japan
O: 05MAY2011 14:58:19 38.3N 144.0E mb=6.2 EMSC OFF EAST COAST OF HONSHU, JAPAN
O: 5MAY2011 14:58:21 38.2N 144.0E ML=6.1 NEIC OFF THE EAST COAST OF HONSHU,
P: 7MAY2011 60323 38.0N 142.0E 4.0-6.4 AABAA Off East Coast Honshu
P: 29APR2011 60167 38.0N 144.0E 4.0-6.4 CAAAA So. of Honshu
NEIC reported the earthquake of M 5.7 in Guerrero, Mexico was widely felt within Mexico on this Cinco de Mayo, Mexican Independence Day. The strongest intensity was VI felt in San Marcos but intensity IV was felt within about 200 km of the epicenter in the regions of Tlalnepantla, Ixtapaluca, Huisquilucan, Mexico and Acapulca, Chilapa and Ometepec, Guerrero. Intensity II-III was felt in Guerrero at Acamixtla, amd Zihuatanejo; in Mexico at Toluca, Tenancingo, Mexico City, Cuautitlan Izcalli, and Atizpan; in Puebla at Atlixco, and Puebla; and in Oaxaca at Santa Lucia del Camino, Puerto Escondido, Oaxaca, Mitla and Huatulco, Huajuapan. The earthquake sent people fleeing into the streets of Acapulco and swayed buildings as far away as Mexico City but no significant damage was reported.
Earthquakes of similar magnitude hit Guerrero and Oaxaca on June 30 and Feb. 9 2010 and with M 5.9 on June 14, 2004. No major earthquakes other then these have occurred in the region since an M 7.5 about 100 km to the east of today's epicenter twelve years ago on September 30, 1999. In recent weeks we have explored in this summary the possibility of a strong earthquake in this region of Mexico. This was first discussed in our summary of March 24 and most recently repeated in the summary of April 27. This event was a foreshock to the earthquake today. Readers are invited to re-read our summaries from these days. In part we noted in the April 27 summary:
"At this time earthquakes of M>=5 are fairly uncommon in Mexico so having three within half a day in three different areas is quite unusual. We have argued over the past month that a strong earthquake is overdue in eastern Mexico and these events may preceded such an occurrence. The earthquake in Chiapas was at 103-104 degrees from the northern end of the great Honshu, Japan rupture of March 11 while the event in Guerrero was at this same distance from the southern end of the rupture. Seismic energy is concentrated at the surface at this distance following major earthquakes by reflection and refraction off the core-mantle boundary to this distance and may promote regional seismicity in this distance zone." (April 27, 2011)
The earthquake today was also at 102-104 degrees from the Japan rupture zone.
O: 05MAY2011 04:48:33 16.2N 98.1W ML=3.9 UNAM SUROESTE de PINOTEPA NACIONAL, OAX
O: 05MAY2011 11:35:58 16.2N 98.3W ML=3.8 UNAM SUROESTE de PINOTEPA NACIONAL, OAX
O: 05MAY2011 13:24:07 16.6N 98.9W ML=5.5 UNAM OESTE de OMETEPEC, GRO
O: 5MAY2011 13:24:06 16.8N 98.5W ML=5.7 NEIC GUERRERO, MEXICO
P: 9MAY2011 60291 17.0N 98.0W 3.5-6.1 BAAAA Oaxaca, Mexico
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