Tohoku earthquake: A surprise? Yan Y. Kagan and David D. Jackson Dept. Earth and Space Sciences, UCLA, Los Angeles, CA 90095-1567, ykagan@ucla.edu, http://eq.ess.ucla.edu/~kagan.html Tohoku earthquake: A surprise? http://moho.ess.ucla.edu/~kagan/AGU11.ppt Manuscript is available at: http://eq.ess.ucla.edu/~kagan/tohoku_index.html

Outline of the Talk Maximum size estimates for subduction zones relevant for Tohoku (Japan) M9 earthquake: Historical method. Statistical method. Moment-conservation method (tectonic versus seismic moment rates). Long- and short-term seismicity rate forecasts in Tohoku region.

Flinn-Engdahl seismic regions: Why select them? Regions were defined before GCMT catalog started (no selection bias), and it is easier to replicate our results (programs and tables available). Kagan (JGR, 1997)

A log-likelihood map for the distribution of the scalar seismic moment of earthquakes in the Flinn-Engdahl zone #19 (Japan--Kurile-Kamchatka)

Kagan’s [1999] hypothesis of uniform b still stands. Review of results on spectral slope, b – Bird & Kagan, 2004 Although there are variations, none is significant with 95%-confidence. Kagan’s [1999] hypothesis of uniform b still stands.

DETERMINATION OF MAXIMUM MOMENT CONCERVATION PRINCIPLE (CORNER) MAGNITUDE: MOMENT CONCERVATION PRINCIPLE Seismic moment rate depends on 3 variables -- The number of earthquakes in a region (N), The beta-value (b-value) of G-R relation, The value of maximum (corner) magnitude. Tectonic moment rate depends on 3 variables -- 1. Width of seismogenic zone (W - 30 -- 104 km), 2. Seismic efficiency coefficient (chi - 50 -- 100%), 3. Value of shear modulus (mu - 30GPa -- 49GPa).

Tectonic rate for 1977-1995/6/30 period is calculated by using Kagan (JGR, 1997) parameters: W=30 km, mu=30 GPa, chi=1.0. Tectonic rate for 1977-2010/12/31 period is calculated by using Bird & Kagan (BSSA, 2004) parameters: W=104 km, mu=49 GPa, chi=0.5.

Repeat time for the m9 and larger events in the Tohoku rectangle (5 by 6 degrees), depending on the assumed b-value, is between 300 and 370 years.

Repeat time for the m9 and larger events in the Tohoku rectangle (5 by 6 degrees), depending on the assumed b-value, is between 300 and 370 years.

Geller, R. J., 2011. Nature, 472(7344), 407-409. Stein, Geller, and Liu, 2011. Seismol. Res. Lett., 82(5), 623-626.

Maximum ratio of short-term to long-term rates is of the order of few percents.

Maximum ratio of short-term to long-term rates is greater than one hundred.

Maximum ratio of short-term to long-term rates is of the order of one hundred.

Maximum ratio of short-term to long-term rates is of the order of one thousand.

Maximum ratio of short-term to long-term rates is of the order of one hundred.

Maximum ratio of short-term to long-term rates is of the order of few tens.

END Thank you

Abstract We consider three issues related to the 2011 Tohoku mega-earthquake: (1) Why was the magnitude limit for the Tohoku region so badly underestimated, and how can we estimate realistic limits for subduction zones in general? (2) How frequently can such large events occur off Tohoku? (3) Could short-term forecasts have offered effective guidance for emergency preparation? Two methods can be applied to estimate the maximum earthquake size in any region: statistical analysis of available earthquake records, and the moment conservation principle -- how earthquakes release tectonic deformation. We have developed both methods since 1991. For subduction zones, the seismic record is usually insufficient, and failed badly for Tohoku, because the largest earthquakes are so rare. However, the moment conservation principle yields consistent estimates for all subduction zones. Various measurements imply maximum moment magnitudes of the order 9.0--9.7.

Abstract (cont.) Comparison of inter-earthquake secular strain accumulation and its release by coseismic slip implies a similar maximum earthquake size estimate. Since 1977 we have developed statistical short- and long-term earthquake forecasts (earthquake rate per unit area, time, and magnitude). Beginning in 1999 we have made such forecasts for the northwest Pacific, including Japan, based on the GCMT catalog. We have posted them on the web and included expected focal mechanisms as well. Long-term forecasts indicate that the average frequency for magnitude 9 earthquakes in the Tohoku area is about 1/400 years. This rate is consistent with that of moderate earthquakes recorded in the GCMT catalog. We have archived several forecasts made before and after the Tohoku earthquake. As expected, the Tohoku mega-earthquake changed the forecasted long-term rate by just a few percent. However, the magnitude 7.5 foreshock increased the short term rate to more than 100 times the long-term rate, and the magnitude 9 event increased it briefly to more than 1000 times the long-term rate. These results could well justify development of an operational earthquake forecasting plan.

GCMT catalog of shallow earthquakes 1976-2005

Tectonic rate for 1977-2010/12/31 period is calculated by using Bird & Kagan (2004) parameters: W=104 km, mu=49 GPa, chi=0.5.