1. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 VARIATION OF THE PROBABILITY AND SIGNIFICANCE OF FACTORS ON DEBRIS FLOWS OCCURRENCE AFTER THE 1999 CHI-CHI EARTHQUAKE Jia-Jyun Dong Applied Geology Institute, National Central Univeristy Jie-Ru Chen Chia-Nan Liu Tser-Ming Rou I-Tser Wu Civil Engineering Department, National Chi Nan University 2009/09/22

2. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 Backgrounds The annual rainfall of Taiwan is more than 2000 mm. The intense rainfall comes with typhoon usually induce landslides and debris flows to occur commonly. The occurrence of disastrous earthquake is not often, but it would have effect on the subsequent occurrence of landslides and debris flows. To study the effect of strong earthquake on debris flows.

3. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 Study Region Ground motion during Chi-Chi earthquake was strong, ground motion classes were 5, 6, and 7 + Abundant occurrence of debris flows in typhoon Herb (1996, before Chi-Chi earthquake) and in typhoon Toraji (2001, after Chi- Chi earthquake) To study the correlation between strong earthquake and debris flows.

4. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 Herb Event Mapped from aerial photos (1997) to identify the gullies of debris flows Add some gullies of non-debris flows into Herb database Ground motion class Gully of debris flows Gully of non-debris flows 51542 63458 71880 Debris Flows Inventory

5. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 Toraji Event Mapped from aerial photos (2001) to identify the gullies of debris flows Add some gullies of non- debris flows into Toraji database Ground motion class Gully of debris flows Gully of non-debris flows 51273 668106 711768 Debris Flows Inventory

6. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 OBJECTIVES (I) To investigate the variation of susceptibility of the debris- flow occurrence after strong earthquake (II) To investigate the variations of the performance of a statistical susceptibility model in predicting the debris- flow occurrence after strong earthquake (III) To Investigate the variation of contributions of causative and triggering factors on the debris-flow occurrence after strong earthquake

7. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 (I)Variation of Debris-flow Susceptibility → the susceptibility of debris flow is elevated after the strike of Chi-Chi earthquake. It is observed that the susceptibility of debris-flows are greater for the region of stronger ground motion. Ground motion Class Debris Flow Density (1/km 2 )Change (%) HerbToraji Study region0.01860.0547+194.1 50.02520.0202-19.8 60.03470.0693+99.7 70.01790.1163+549.7 Liu C. N., Huang H. F., and Dong J. J. (2008), Natural Hazards.

8. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 Retrieve the characteristics of each gully Causative Factors: (1) the length of stream, [log(L)] (2) the average slope of stream, [Ss] (3) the area of catchment, [log(A)] (4) the form factor of catchment, [F] (5) the slope of catchment, [Sc] (6) the geology of upslope region of catchment, [G] Triggering Factors: (7) the peak hourly rainfall intensity [I] (8) the accumulated rainfall [R]

9. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 Logistic Regression Analysis X i (i=1,8): causative and triggering factors Y : indicator for a debris flow occurred (Y = 1) or not (Y = 0) Coefficients  and  are calculated under the constraint of maximum likelihood EX: The Logistic Regression Model for Herb data (LRM_H) is

10. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 (II) Variations of the Performance → input the LRM_H into Toraji database to examine the performance of the Herb model to predict the occurrence of debris flows after Chi-Chi earthquake → the susceptibility model to predict the occurrence of debris flows for areas struck by a large earthquake should be re- verified or re-developed

11. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 (III) Variation of Contributions of Factors Jackknife technique: Picked-out one of the variables and rebuild the model, calculate the decreased total accuracy The signicance of the picked-out factor on the debris-flow occurrence is propotional to the decreased total accuracy Pick out X i accordingly to study the significance of Causative factors and Triggering factors

12. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 Significant causative and triggering Factors 1999 Chi-Chi earthquake After earthquake Before earthquake LRM_Tt (2001 Toraji) LRM_H (1996 Herb) Geology of upslope region of catchment G Slope of catchment Sc Accumulated rainfall R > Peak hourly rainfall intensity I Peak hourly rainfall intensity I > Accumulated rainfall R

13. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 Conclusions The susceptibility of the debris-flow occurrence after a large earthquake is higher than the one before the earthquake. In addition, it also is found that the susceptibility of debris-flow occurrence increases with the increasing ground motion intensity. LRM_H (developed using debris-flow inventory of 1996 typhoon Herb) cannot predict well the occurrence of debris flow during typhoon Toraji in 2001. The total accuracy of the Herb’s model is low (65.19%) in predicting the occurrence of debris flow in the region sustained a ground motion of intensity 7. A susceptibility model developed using a debris-flow inventory before a large earthquake could seriously underestimate the occurrence of debris flows in a subsequent typhoon after earthquake.

14. The Next Generation of Research on Earthquake-induced Landslides: An International Conference in Commemoration of 10th Anniversary of the Chi-Chi Earthquake, 2009 Conclusions Contribution of the factors on the debris-flow occurrence: Before earthquake - The geology of upslope region of catchment is the most important causative factor and the influence of the accumulated rainfall on the debris-flow occurrence is stronger than the peak hourly rainfall intensity. After earthquake - The most significant factor for predicting the occurrence of debris flow after the Chi-Chi earthquake is either the average slope of catchment or the average slope of stream. Furthermore, the peak hourly rainfall intensity has a similar or stronger effect on the debris flow occurrence than the accumulated rainfall. The significance of causative and triggering factors contributing to the variation of debris-flow susceptibility before and after a large earthquake are interesting issues deserved further study.

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