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How much is recent seismicity affected by DCFF of previous large earthquakes ?

How much is recent seismicity affected by DCFF of previous large earthquakes ?. Takeo ISHIBE and Kunihiko SHIMAZAKI (Earthquake Research Institute, Univ. of Tokyo). Motivation.

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How much is recent seismicity affected by DCFF of previous large earthquakes ?

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  1. How much is recent seismicity affected by DCFF of previous large earthquakes ? Takeo ISHIBE and Kunihiko SHIMAZAKI (Earthquake Research Institute, Univ. of Tokyo)

  2. Motivation Can we obtain fault mechanism or slip of historical large earthquakes from recent instrumental observation record ? 1ST STEP How much recent seismicity affected by previous large earthquakes that the fault mechanism and slip have been estimated ?

  3. Occurrence rate of felt earthquakes at Gifu Utsu et al. (1995) indicates that the number of felt earthquakes obeys in accordance with the Omori formula during 100 years from the occurrence of the 1891 Nobi earthquake. Elapsed time after the 1891 Nobi earthquake (days) It suggests that an aftershock activity can continue during long time period after a mainshock

  4. Previous Research • Muller et al. (2004) estimates locations and fault mechanisms of four large historical earthquakes occurred in the New Madrid from 1811 to1812 by using the dCFF and recent instrumental observation record. Although some aftershock activities are thought to continue during long period, there were few researches which examine aftershock distribution of historical earthquakes in detail.

  5. Table. Large earthquakes (M6.5+) occurred after 1900. (Hypocentral location and magnitude are based on the unified JMA catalog)

  6. CatalogData • unified Japan Metrology Agency (JMA) catalog (Oct . 1997 to Feb. 2008) All magnitude scale is JMA magnitude • M ≧ 0.0 hypocentral depth ≦ 20km (neglecting spatial heterogeneity of minimum magnitude of completeness in a narrow region )

  7. Positive dCFF region Recent seismicity We compare the static changes of the Coulomb Failure Function (CFF) caused by historical large earthquakes and epicentral distribution during the unified JMA catalog and discuss the availability to obtain some information about historical large earthquakes from recent seismicity. We supposed that mechanism of aftershocks are same as mainshock. Calculated depth is intermediate depth of fault plane. We treated earthquakes (magnitude 6.5+) which have strike-slip fault mechanism occurred from 1900.

  8. 2000 Western Tottori (M7.3) 　　＋　　　　1943 Tottori (M7.2) Most earthquakes (80%+) occur in positive dCFF (more than 0.1bar) region

  9. 1948 Fukui （M7.1)〔Kikuchi et al.,1999〕 0.1 0.4 0.7 • Relatively good agreement between epicentral distribution and positive dCFF region though southwestern part is activated by the occurrence of the 1995 Kobe earthquake (ex. Toda et al., 1998) • The degree of agreement becomes high as the apparent friction coefficient is high. 図6．1948年福井地震(M7.1)によるクーロン応力変化と一元化後の気象庁震源カタログによる震央分布（1997年10月～2008年5月、震源の深さ20km以浅）。

  10. 1927Tango earthquake （M7.3)〔Kasahara;1957,1958〕 Seismic activity thought to be aftershocks accompanied by the mainshock is slightly recognized. Earthquakes frequently occurs in +dCFF region. Large earthquakes such as the 1925 Tajima, 1943 Tottori, 1995 Kobe earthquake occurred in surrounding region. -> it is necessary to include the dCFF caused by these large earthquakes..

  11. 1931 Saitama earthquake（M6.9)〔Abe;1974a〕 1969 Gifu-ken-chubu earthquake (6.6)〔Mikumo,1973〕 Significant agreement between positive dCFF region and recent seismicity is not recognized.

  12. 1963Wakasa-bay （M7.1)〔Abe,1974b〕 Seismicity is quiet -> aftershock activity completely decays to background seismicity level? 1984 Nagano earthquake (6.8)〔Mikumo et al.,1985〕 Earthquake swarm is observed and seismicity rate might be affected by other stress change like a pore pressure changes accompanied by flux of liquid.

  13. Reason of disagreement 1 • Simplicity of supposed model • (1-1) Uniform slip model (-> heterogeneous fault slip distribution) • (1-2) Fixed receiver fault mechanism (-> considering to spatial heterogeneity of receiver fault mechanism ) • (1-3) Contamination of large earthquakes occurred prior to or after the target mainshock • (1-4) just comparing with epicentral distribution of recent seismicity (-> dCFF should be compared with seismicity rate change from background seismicity. (ex. Dietrich, 1994; Toda et al, 1998)…. though background seismicity for historical large earthquakes can not be estimated easily.

  14. Reason of disagreement 2 (2) Essential differences (2-1) aftershock activity completely decayed to background seismicity, that is to say, stress disturbance accompanied with historical earthquake disappears. (2-2) Dominant stress changes such as a pore pressure changes caused by liquid flux may be major reason of seismicity changes in the region.

  15. Conclusion • Positive dCFF region caused by some historical large earthquakes well agree with epicentral distribution from recent catalog. • It may suggest that some information such as fault mechanism or slip can be estimated from recent seismicity although the correspondence strongly depend on individual aftershock sequence.

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