Yan Y. Kagan

Dept. Earth and Space Sciences, UCLA, Los Angeles,CA 90095-1567, ykagan@ucla.edu, http://eq.ess.ucla.edu/~kagan.html

Evaluation of Maximum Earthquake Size for Subduction

Zoneshttp://moho.ess.ucla.edu/~kagan/ESC12.ppt

Outline (EFP3: O8)Maximum earthquake size estimates for

subduction zones:

1. Historical method;

2. Statistical method;

3. Moment-conservation method: tectonic versus seismic moment rates – area- and site-specific.

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) used these regions to estimate Mmax for several tectonic categories.

A log-likelihood

map for the distribution of the scalar

seismicmoment of earthquakes

in the Flinn-

Engdahl zone #19(Japan--Kurile-

Kamchatka)

Review of results on spectral slope, – Bird & Kagan, 2004

Although there are variations, none is significant with 95%-confidence.Kagan’s [1999] hypothesis of uniform still stands.

DETERMINATION OF MAXIMUM (CORNER) MAGNITUDE:

MOMENT CONCERVATION PRINCIPLE

Seismic moment rate depends on 3 variables --1.The number of earthquakes in a region (N);2.The beta-value (b-value) of G-R relation;3.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 1977-2010/12/31 period is calculated by using Bird & Kagan (BSSA, 2004) parameters: W=104 km, mu=49 GPa, chi=0.5.

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.

DETERMINATION OF MAXIMUM (CORNER) MAGNITUDE: SITE-SPECIFICMOMENT CONCERVATION PRINCIPLE

1. General (area-specific) distribution of the earthquake size, for the simplicity of calculations we take it as the truncated Pareto distribution.2. Site-specific moment distribution – large earthquakes have a bigger chance to intersect a site, hence the moment distribution is different from area-specific.3. Geometric scaling of earthquake rupture. Length-width-slip are scale-invariant, proportional to the cube root of scalar moment.4. Earthquake depth distribution is different for small versus large shocks: at least for strike-slip earthquakes large events would penetrate below the seismogenic layer.5. Most of the small earthquakes do not reach the Earth surface and therefore do not contribute to the surface fault slip.

Calculation of Mmax for fault slip

Calculation of Mmax for fault slip (cont.)

For Tohoku area site-specific calculations yield Mmax estimates 8.5-9.5

END

Thank you

EVALUATION OF MAXIMUM EARTHQUAKE SIZE FOR SUBDUCTION ZONES

Y. Y. Kagan UCLA/ESS, Los Angeles, USA

Updating our previous work (Kagan, JGR, 1997), we analyze seismicity inthe Flinn-Engdahl seismic zones to infer the maximum earthquake sizefor major subduction zones. The maximum earthquake size is usuallyguessed based on the available history of earthquakes or on thesubdivision of a fault into separate segments. These methods are knownfor their significant downward bias. There are two quantitative methodswhich can be applied to estimate the maximum earthquake size in anyregion: (1) a statistical analysis of the available earthquake record,and (2) the moment conservation principle. The latter technique allowsus to study how much of the tectonic deformation rate in regions or inparticular fault sites is released by earthquakes. We demonstrate thatfor the subduction zones the seismic or historical seismicity record isnot sufficient to provide a reliable statistical measure of the maximumearthquake. However, the moment conservation principle producesconsistent estimates: for all the subduction zones the maximum momentmagnitude suggested by various measurements is of the order 9.0 to 9.7.

Abstract continuation

Moreover, the moment conservation method indicates that for all themajor subduction zones the maximum earthquake size is statistically thesame. Since such mega-earthquakes have occurred in several subductionzones, other zones would eventually be expected to have shocks ofsimilar magnitude. The 2004 Sumatra and the 2011 Tohoku earthquakesoccurring after 1997, demonstrated the validity of this prediction. Wealso consider another moment conservation method -- comparing thesite-specific deformation rate and its release by earthquakes rupturingthe site. Though depending on less reliable assumptions, this technique, applied to the Tohoku area, also suggests that the maximumearthquake size could be of the order M9. If the maximum earthquakesize is known, the magnitude-frequency relation yields a reliableestimate of the recurrence time of mega-events.