Earthquake Data and Probability

(Graph 6 is the important part for people to read.)

The original Japanese Press Release can be found here:
http://www.jma.go.jp/jma/press/1103/13b/kaisetsu201103131255.pdf

Press Release
13 March 2011, 12:55 PM
Japan Meteorological Agency

Regarding the 2011 Tohoku Pacific Coast Earthquake (15th Release)

Earthquake Magnitude
After detailed analysis, the magnitude of the earthquake has been judged to be 9.0 rather than 8.8.

As a result of using data from international earthquake monitors and examining the progress of ruptures in the epicenter area (the rupture process), it was found that three giant seismic ruptures occurred in succession, in an unusually complicated pattern. Based on this data, the magnitude of the earthquake was found to be 9.0 after reanalysis.

As the tremor pattern revealed by the first earthquake was extremely rare, when unusual waveforms were found that didn’t match the first giant rupture, a further survey revealed that these abnormalities were caused by the occurrence of second and third giant ruptures that followed the first.

[note omitted]

Aftershocks and Future Developments

Aftershocks are occurring from the Iwate coast to the Ibaraki coast, spanning an area that is approximately 500km long and 200km wide. This is highly likely to be the area where the epicenter of the initial earthquake was. Aftershock activity is also especially frequent, given the wide area of occurrence and past case studies of earthquake activity in such circumstances. In addition, because the aftershocks are taking place over such a wide area, the local effects of any aftershock will vary greatly depending on where tremors originate, even if the magnitude is the same.

Please be warned that with the large amount of aftershock activity, earthquakes at a lower 5 rating on the Japanese seismic intensity scale will take place from time to time, and that there is also a high possibility of earthquakes on the lower 6 to upper 6 rating.

[Added Note: Wikipedia link]

There is also the risk of a tsunami occurring due to large aftershocks; please heed any tsunami warnings and follow instructions. [Added Note: The tsunami keihō (警報) warning is more serious than the tsunami chūihō (注意報) warning.]

The pressure axis of the convergent (reverse) fault runs in a west by northwest – east by southeasterly direction. The region of pressure is to the east (oceanic plate) and the region of tension is to the west (continental plate). The more dense oceanic plate converges with the continental plate and is subducted.

The origin (centroid) is measured at:
37 degrees 49 minutes north latitude
143 degrees 3 minutes east longitude
Depth: about 10 kilometers
*The centroid is the location in the layer that produced the earthquake, which experienced the greatest seismic activity.

Examples that focus on regions of pressure
1. Convergent (reverse) faults: Plates converge. One plate is pushed up while one plate is subducted in either direction.
*Force is exerted laterally and tension occurs as the plates are pushed together at the fault.

2. Transform faults: Plates slide by each other in either direction.
*Force is exerted laterally and tension occurs perpendicularly as the plates are pushed perpendicularly at the fault.

Examples that focus on regions of tension
1. Divergent faults: Plates diverge in either direction
*Force is exerted outward from the point of the fault and tension occurs laterally as the plates are pushed apart.

2. Transform faults: Plates slide by each other in either direction.
*Force is exerted perpendicularly and tension occurs laterally as the plates are pushed laterally at the fault

CTAO vertical seismographic readings (Australia)
The seismic waves that registered 9.0 reached their highest amplitude at 30 minutes and subsided at 50 minutes.

KEV vertical seismographic readings (Finland)
The seismic waves that registered 9.0 reached their highest amplitude at 30 minutes and subsided at 50 minutes.

Epicenter Distribution Map

(From 11 March 2011 12:00PM, to 13 March 2011, 7:00AM, to a depth of 90km or less, where magnitude (M) is greater or equal to 5.0)

The size of the circles represents how great the magnitude was. Tremors with a magnitude of greater than 7.0 have the details written in boxes.

The above chart shows time and space distribution within the rectangle.

The greatest seismic activity in that region was registered as follows:
March 11, 2011 (14:46 M 8.8)
March 11, 2011 (15:08 M 7.5)
March 11, 2011 (15:15 M 7.3)
March 11, 2011 (15:25 M 7.4)


Aftershock Distribution

The x-axis shows time, and the y-axis shows the area from A to B on the map above. The circles show how great the magnitude of the tremors were, and when they occurred.

The greatest seismic activity was registered on March 11 during the times of 14:46 and 15:25. Aftershocks ranging between M 5 and 6 continued thereafter through March 12.

(PLEASE READ)
Further seismic activity is predicted as follows:

Greater than M 7
Within 3 days of March 13 70%
Within 3 days of March 16 50

*An M7 earthquake that occurs on the land plate or near the coast (within 100 km of land) is expected to cause tremors of Japanese Seismic Intensity 6 weak through 6 strong.

*An M7 earthquake that occurs more than 100 km off the coast is predicted to cause tremors of Japanese Seismic Intensity 4 to 5 weak, and in some places 5 strong.

*Areas where the bedrock/land has been greatly weakened by previous earthquakes may experience tremors of greater intensity.

Insert by PA – What does this mean?

[It means that even if an M7 earthquake does occur, depending on where it takes place, the intensity that you actually feel will vary. So, for example, if an M7 occurs 100 km off the coast of Miyagi, the actual tremors in a place like Chiba may register as anywhere from 4-6, which we’ve already been experiencing on a regular basis over the last few days. If it were to take place closer to land, then yes, the tremors might be larger, but in most areas, M6 intensity quakes are not new.

The reason why there have been so many aftershocks, as explained above, is that the original epicenter area was so large that there is a lot of pressure being released along the subduction zone. The bigger the shift at the fault, the more energy to release, the more energy to release, the longer the aftershocks take to peter out. So, in reality, what you’re waiting for is not the “next big one,” rather, you’re waiting for the plates to settle down.

Panicking about it accomplishes nothing at all, whereas planning for an emergency evacuation does.]

[GRAPH 7]

Comparison of the Number of Aftershocks (Greater than 5.0 Magnitude) Following Major Earthquakes at Plate Boundaries

2011 Tohoku Pacific Coast Earthquake (M9.0)
1994 Hokkaido Toho Offshore Earthquake (M8.2)
1952 Tokachi Offshore Earthquake (M8.2)
1933 Sanriku Earthquake and Tsunami (M8.1)
2003 Tokachi Offshore Earthquake (M8.0)
1994 Sanriku Haruka Offshore Earthquake (M7.6)
1963 Iturup Offshore Earthquake (M8.1)
1958 Iturup Offshore Earthquake (M8.1)
2005 Miyagi Offshore Earthquake (M7.2)

(y) Total number of aftershocks

(x) Number of days after initial earthquake

*Includes initial earthquake

*This chart is based on current information that may be corrected with further investigation in the following days.

The March 11, 2011 (M 9.0) has experienced over 150 aftershocks as of March 12th. (1 day after initial earthquake).
1994 (M 8.2) experienced about 75 aftershocks after 3 days.
1952 (M 8.2) and 1933 (M 8.1) experienced over 25 aftershocks after 3 days.
2003 (8.0), 1994 (7.6), 1963 (8.1), 1958 (8.1) experienced about 5 to 25 aftershocks after 3 days.
2005 (7.2) experienced less than 5 aftershocks after 3 days.