Detection Accuracy of Active Fault Based on Seismic Reflection Method Data

Transcription

1 Detection Accuracy of Active Fault Based on Seismic Reflection Method Data Katsumi Ebisawa, Toshio Kogure, Hideaki Tsutsumi, Shohei Motohashi and Masaharu Sakagami Japan Nuclear Energy Safety Organization (JNES) 1

2 Introduction The Japan Nuclear Energy Safety Organization (JNES) has has been been conducting the the development of of methodology for for evaluating the the exceedance probability spectra of of seismic motion occurred by by the the blind fault. As As a part part of of the the evaluation of of detection accuracy for for the the blind faults, in in order to to estimate the the detection accuracy of of active faults by by the the seismic reflection method, JNES examined the the accuracy using active faults data. JNES also also examined the the relationship between the the fault fault displacement and and the the fault fault length obtained by by trenching, boring and and outcrop investigation, etc. etc. 2

3 Overview of active fault data Overview of active fault data The active fault data of Headquarters for Earthquake Research Promotion (HERP), and National Institute of Advanced Industrial Science and Technology (AIST) were used to examine. HERP s data are the displacement data obtained by the seismic reflection method, and fault length and average slip rate data etc. obtained by other investigation methods that are the trench, boring and outcrop, etc. In the same way, AIST s data are also the displacement data obtained by the seismic reflection method, and fault length and average slip rate data etc. obtained by other investigation methods that are the trench, boring and outcrop, 3

4 Results of of seismic reflection method for for Kyoto-Nara basin fault zone Obitoke fault Tenri frexure Sanbyaku fault Takai fault Level (m) -400 basement -600 Profile location Profile location Result of seismic reflection method 4

5 Example of of seismic reflection method for for Tonami west west plain fault faultzone Tonami plain fault zone Depth (m) Mudstone Sandy mudstone Gravel Altitude (m) Sandy mudstone Mudstone Faults location Result of seismic reflection method with columnar by boring 5

6 Examples of of Active fault fault data data of of HERP Seismic reflection method Fault characteristics Active fault name Ishikari-teichi-toen fault zone Kitakami-teichi-seien fault zone Nagamachi-Rifusen fault zone Motoarakawa fault zone Displacement at basement (m) Displacement at sedimentary Layer (m) Slip direction Average velocity rate Fault length (m/10 3 years) (km) Confidence Confidence Confidence (5) Dip *** * 66 ** 200 Dip *** ** 62 ** Dip *** ** * 200! (0.05)! 25! Isehara fault Dip *** ** 21 ** Kurehayama fault zone Morimoto & Togashi fault zone 500 Dip *** * 22 ** Dip *** 1 * 26 **! : The type of active fault could not be decided. Blank: Not description Dip: Dip-slip fault, Strike: Strike-slip fault, Dip & strike: Dip & strike-slip fault *** :Confidence of decision for dip and strike- fault- High ** : the above confidence- Middle * : the above confidence- Low 6

7 Histograms of of displacement at at sedimentary layer layer or or basement detected by by seismic seismic reflection method method 10 Numbers of faults : National Institute of Advanced Industrial Science and Technology (AIST) : Headquarters for Earthquake Research Promotion (HERP) 2 Numbers of faults (m) Displacement at sedimentary layer (m) AIST HERP Displacement (m) at basement (m) 7

8 Detection accuracy of active fault by seismic reflection method (SRM) Detection accuracy of active fault by seismic reflection method (SRM) Classification of data Contents Number Case 1 Fault can be detected and its displacement is also estimated by SRM 34 Case 2 Fault can be detected but its displacement can not be estimated by SRM 13 Case 3 Fault can not be detected by SRM but that can be detected by other methods, trench investigation and outcrop etc. 5 Percentage of Detection by SRM The detection percentage of Case 1 & 2 is about 90 %. The active faults that could not be detected by SRM are about 10 %. Reason:- The application of SRM to the strike-slip faults is not suitable. - Since the decision of the fault segmentation is difficult, the profile location predicted by SRM is not always suitable. 8

9 Relationship between displacement by by seismic reflection method and fault length by by other geological investigation methods Displacement (m) at basement (m) 25 : AIST : HERP 6 Fault 25length (km) M6.1 M7.0 M8.0 M : Magnitude estimated by Matsuda Equation log L=0.6M-2.9 (M: ) Vertical resolution value (VRV) Length of wave=vp (2000m/s)/ Win (20Hz) =100 m Vp: Primary velocity at sedimentary layer Win: Frequency of input wave by seismic reflection method Low of 1/4wave length by Rayleigh VRV=100 m/4=25 m 9

10 Conclusion - When the seismic reflection method was applied suitably against the active faults under sedimentary layer, it was demonstrated that the possibility of their detection was quite large and the detection percentage was about 90 %. - It may well be that the active fault over about fault length of 6 km will be detected by seismic reflection method. 10

11 Future Plan Future Plan In order to to obtain high quality data concerning geophysical prospecting, the main issues are as as follows: - To To accumulate the related geophysical prospecting data with the deep boring data - To To examine the detection accuracy of of blind faults using the various geophysical prospecting data In order to to realize these plans, it it is is important that the deep boring data are accumulated. 11