TOHOKU EARTHQUAKE: A SURPRISE?Yan Y. Kagan and David D. Jackson

Department of Earth and Space Sciences, University of California Los Angeles

Abstract

We consider three issues related to the 2011 Tohoku mega-earthquake:(1) Why was the magnitude limit for the Tohoku region so badlyunderestimated, and how can we estimate realistic limits for subductionzones in general?(2) How frequently can such large events occur off Tohoku?(3) Could short-term forecasts have offered effective guidance foremergency preparation?Two methods can be applied to estimate the maximum earthquake size inany region: statistical analysis of available earthquake records, andthe moment conservation principle -- how earthquakes release tectonicdeformation. We have developed both methods since 1991. For subductionzones, the seismic record is usually insufficient, and failed badly forTohoku, because the largest earthquakes are so rare. However, themoment conservation principle yields consistent estimates for allsubduction zones. Various measurements imply maximum moment magnitudes of the order 9.0--9.7. Comparison of inter-earthquake secular strain accumulation and its release by coseismic slip implies a similarmaximum earthquake size estimate. Since 1977 we have developedstatistical short- and long-term earthquake forecasts (earthquake rateper unit area, time, and magnitude). Beginning in 1999 we have madesuch forecasts for the northwest Pacific, including Japan, based on theGCMT catalog. We have posted them on the web and included expectedfocal mechanisms as well. Long-term forecasts indicate that the averagefrequency for magnitude 9 earthquakes in the Tohoku area is about 1/400years. This rate is consistent with that of moderate earthquakesrecorded in the GCMT catalog. We have archived several forecasts madebefore and after the Tohoku earthquake. As expected, the Tohokumega-earthquake changed the forecasted long-term rate by just a fewpercent. However, the magnitude 7.5 foreshock increased the short termrate to more than 100 times the long-term rate, and the magnitude 9event increased it briefly to more than 1000 times the long-term rate.These results could well justify development of an operationalearthquake forecasting plan.

URL: http://eq.ess.ucla.edu/~kagan/tohoku_index.html

Conclusions The maximum earthquake size offshore Japan should not be a surprise: the moment conservation principle as well as a comparison of inter-earthquake secular strain accumulation and its release by coseismic slip suggest the maximum magnitude of 9.0--9.7. The recurrence time of these earthquakes in the Tohoku area is of the order of 400 years. The short-term earthquake forecasts produced since 1999 for the western Pacific area can in principle be used for the development of an operational earthquake forecasting plan.

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Flinn-Engdahl Seismic RegionsWhy select them? Regions were defined before GCMT catalog started (no bias), and easier to replicate our results.

Fig. 1. Number of earthquakes in the Flinn-Engdahl zone 19 (Japan-Kamchatka) with moment magnitude (M) for the shallow earthquakes in the GCMT catalog during 1977--2010. Magnitude threshold M=5.8, the total number of events 425. The unrestricted Gutenberg-Richter law is shown by dotted line. Dashed lines show two tapered G-R distributions: the G-R law restricted at large magnitudes by an exponential taper with the corner magnitude. One corner magnitude Ms=8.7 is evaluated by the maximum likelihood method using earthquake statistical record (with no upper limit), another estimate Mm=9.4 is based on the moment conservation.

Figs. 2, 3. Parameter beta distribution in Flinn-Engdahl subduction zones. GCMT catalog is used. Average region's beta and +/-1.96 standard deviations are shown, the black line corresponds to the average in Fig, 2 (1977-1995/6/30) <beta>=0.633 and <beta>= 0.654 in Fig. 3 (1977-2010)

for all subduction zones.

Figs. 4-5. Corner moment magnitude M_c distribution in Flinn-Engdahl subduction zones, GCMT catalog (1977-1995/6/30 and 1977-2010, respectively). Figs. 6. Centennial catalog (1900-1976). Region's M_c and +/-1.96 standard deviations are shown, the black line -- average <M_c> for all subduction zones. Red circles show earthquakes with historical maximum magnitude M_x which occurred in the regions during the catalog time interval. The red line -- average <M_x> for all subduction zones. Magenta diamond shows the Tohoku earthquake magnitude. No statistically significant difference is found between different subduction zones in beta value and corner magnitude. Historical magnitudes underestimate corner magnitude but discrepancy shrinks with time.

Fig. 7. Ratio of seismic to tectonic rate in Flinn-Engdahl subduction zones for GCMT catalog 1977-1995/6/30 (blue) and 1977-2010 (red). Vertical lines show average ratio for all regions the black line -- ratio of 1.0.

Fig. 8. Long-term forecast rate for north-west NW Pacific calculated March 8, 2011, before M7.5 Tohoku foreshock.

Fig. 9. Long-term forecast rate for NW Pacific calculatedMarch 17, 2011, after Tohoku M9.1 mainshock. Little change compared to Fig. 8.

Earthquake forecast rates around the epicenter of the M7.5 foreshock.Long-term rates do not change significantly even after an occurrence of the M9.1 mainshock.Forecasted focal mechanisms are also essentially the same. The ratio of the short- to long-term rates (the last column) rises sharply both after the foreshock and after the mainshock.

GCMT catalog of shallow earthquakes 1976-2005