USGS Updates Magnitude of Japan’s 2011 Tohoku Earthquake to 9.0

The USGS has updated the magnitude of the March 11, 2011, Tohoku earthquake in northern Honshu, Japan, to 9.0 from the previous estimate of 8.9. Independently, Japanese seismologists have also updated their estimate of the earthquake’s magnitude to 9.0. This magnitude places the earthquake as the fourth largest in the world since 1900 and the largest in Japan since modern instrumental recordings began 130 years ago.

The USGS often updates an earthquake’s magnitude following the event. Updates occur as more data become available and more time-intensive analysis is performed. There are many methods of calculating the energy release and magnitude of an earthquake. Some methods give approximate values within minutes of the earthquake, and others require more complete data sets and extensive analysis. Due to inherent uncertainties in the modeling of energy and magnitude, the results from different agencies often vary slightly. These magnitude discrepancies arise from the use of different data and techniques. For more information on why magnitudes change, see the Earthquake Hazards Program FAQ website.

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Parameter	Value	Description
Magnitude	mb	The magnitude for the event.
Longitude	° East	Decimal degrees longitude. Negative values for western longitudes.
Latitude	° North	Decimal degrees latitude. Negative values for southern latitudes.
Depth	km	Depth of the event in kilometers.
Place		Textual description of named geographic region near to the event. This may be a city name, or a Flinn-Engdahl Region name.
Time	1970-01-01 00:00:00	Time when the event occurred. UTC/GMT
Updated	1970-01-01 00:00:00	Time when the event was most recently updated. UTC/GMT
Timezone offset		Timezone offset from UTC in minutes at the event epicenter.
Felt		The total number of felt reports
CDI		The maximum reported intensity for the event.
MMI		The maximum estimated instrumental intensity for the event.
Alert Level		The alert level from the PAGER earthquake impact scale. Green, Yellow, Orange or Red.
Review Status		Indicates whether the event has been reviewed by a human.
Tsunami		This flag is set to "1" for large events in oceanic regions and "0" otherwise. The existence or value of this flag does not indicate if a tsunami actually did or will exist.
SIG		A number describing how significant the event is. Larger numbers indicate a more significant event.
Network		The ID of a data contributor. Identifies the network considered to be the preferred source of information for this event.
Sources		A comma-separated list of network contributors.
Number of Stations Used		The total number of Number of seismic stations which reported P- and S-arrival times for this earthquake.
Horizontal Distance		Horizontal distance from the epicenter to the nearest station (in degrees).
Root Mean Square	sec	The root-mean-square (RMS) travel time residual, in sec, using all weights.
Azimuthal Gap		The largest azimuthal gap between azimuthally adjacent stations (in degrees).
Magnitude Type		The method or algorithm used to calculate the preferred magnitude for the event.
Event Type		Type of seismic event.
Event ID		Id of event.
Event Code		An identifying code assigned by, and unique from, the corresponding source for the event.
Event IDS		A comma-separated list of event ids that are associated to an event.

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USGS Updates Magnitude of Japan’s 2011 Tohoku Earthquake to 9.0

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