Very strong aftershock at a depth of 42 km close to Tokyo, Japan

Last update: April 22, 2011 at 9:47 am by By

UPDATE 15:45 UTC : Earthquake-Report is reporting the data as it comes in.
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Tokyo aftershock April 21 2011

UPDATE 15:43 UTC : USGS pager data is as good as the data itself. The difference in depth alone makes the difference in between a light shaking and a strong shaking. Luckily USGS was wrong this time.

UPDATE 15:40 UTC : Almost all USGS felt reports are mentioning a III to IV MMI shaking (light shaking)

UPDATE 14:59 UTC : We will keep an eye on the local earthquake emergency services and will report if we notice any damage or injuries.

UPDATE 14:58 UTC : We assume that the JMA data were correct (JMA data are in-line with the experienced shaking)

UPDATE 14:55 UTC : The Japanese press is rather silent on this aftershock (they are mentioning it as a serious shock). No additional footage.

UPDATE 14:30 UTC : At earthquake-Report we fear these close to Tokyo aftershocks. until now Tokyo got at most indirect aftershocks, but a shallow 6.0 hit could make a lot of damage in the City.

UPDATE 14:28 UTC : The greater Tokyo business area got an intensity 3 shaking.

UPDATE 14:23 UTC : All will depend on the exact earthquake numbers. USGS and JMA have different values for a) epicenter location b) magnitude and c) depth. Especially the depth of the hypocenter is very important as it will be the main factor of damage or no damage.

UPDATE 14:20 UTC : So far the local Japanese press is only mentioning the occurrence of the intensity 5 lower earthquake in Chiba prefecture, no word on any damage or injuries

UPDATE 14:17 UTC : Our iPhone application QuakeSOS has received a number of I AM SAFE alerts from the Tokyo area = people have been seriously shaken by the earthquake

UPDATE 14:12 UTC : As the epicenter is below or near land (depending on the data source),  and the magnitude of the earthquake is limited, there is NEVER been a tsunami risk

UPDATE 14:10 UTC : JMA, the local Japanese seismological service is still maintaining a depth of  70 km, which is a VERY BIG difference (less dangerous) than the 42 km from USGS.

UPDATE 14:09 UTC : Following the GDACS calculations 23 million people are living within a radius of 100 km from the epicenter. GDACS makes his theoretical calculations on the USGS data, which are more dangerous than the JMA data

UPDATE 14:05 UTC : GDACS published an ORANGE earthquake alert for this earthquake = moderate chance on damage and/or injuries

UPDATE 14:01 UTC : Following the USGS data, the epicenter is below land in the direct area of Chiba

UPDATE 14:00 UTC : JMA  Intensity 5- for Chiba-ken Hokutobu, 4 Ibaraki-ken Hokubu, Ibaraki-ken Nambu, Chiba-ken Hokuseibu

This aftershock was at an intermediate depth but very close to Tokyo.
USGS has now reported a magnitude of 6.1 at a depth of 42.9 km.
Only 64 km from Tokyo ! The earthquake occurred at late night.

Most important Earthquake Data:
Magnitude : 6.1
UTC Time : Thursday, April 21, 2011 at 13:37:03 UTC
Local time at epicenter : Thursday, April 21, 2011 at 10:37:03 PM at epicenter
Depth (Hypocenter) :  42.9 km
Geo-location(s) :
64 km (40 miles) E (95°) from TOKYO, Japan
83 km (52 miles) S (182°) from Mito, Honshu, Japan

Links to important maps
JMA (Japan) Did You Feel It Map
Google satellite map of the area showing the kind of terrain the earthquake took place
USGS historic earthquakes map
USGS Seismic hazard map
USGS Impact on Population map
USGS Shaking map
GDACS ORANGE earthquake report

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