Surface rupture of the great Mw 8.6 Assam earthquake in the eastern Himalayas

Transcription

1 Surface rupture of the great Mw 8.6 Assam earthquake in the eastern Himalayas Jérôme Van der Woerd Aurélie Coudurier Curveur (EOS, Singapore) Elise Kali (IPGS, Strasbourg) Paul Tapponnier (EOS, Singapore) Emile Okal (Northwestern University, Evanston, USA) Swapnamita Choudhury (Wadia Inst., Dehra Dun, India) Saurabh Baruah (NEIST, Jorhat, India) This work has been submiued to Science Advances with the Vtle «Dual surface breaks of the great 1950 Assam earthquake across the Mishmi/Abor Hills cusp», Coudurier Curveur et al.

2 Mw 7.5 for the last yrs Seismic slip deficit : where are the earthquakes? Stevens & Avouac,, 2015, 2016

3 Frontal scarp of MFT and surface breaks of the 1934 Bihar Nepal earthquake Sapkota et al., 2012 ; Bollinger et al., 2014

4 Great Himalayan Earthquakes of the 19 th 20 th century may not be blind, afer all Frontal scarp of MFT and surface breaks of teh 1934 Bihar Nepal earthquake Sapkota et al., 2012 ; Bollinger et al., 2014

5 Extension of 1934 Bihar Nepal surface rupture Sapkota et al., 2012 ; Bollinger et al., 2014

6 25 April 2015 Mw 7.8 Gorkha earthquake extension

7 Sub surface complexity of the thrust fault Incomplete historical and geological records Hubbard et al., Geology, 2016

8

9 Crossing rivers in Assam, the far east India

10

11 Tebang river cut

12 Tebang river cut

13

14 Mishmi thrust outcrop

15 10 m high scarp across terrace of Dibang tributary at Roing

16 Decametric scarp of the Mishmi thrust at Roing

17 14 m high scarp across terrace at Wakro

18 Topographic profile across scarp reveals complexity and occurrence of 2 events

19 Scarp across lower terrace along Kamlang river at Wakro

20 Scarp across lower terrace along Kamlang river at Wakro

21 7 m high scarp

22 Rapids in the present day Kamlang river bed Similar to Muzafarabad Kashmir 2005 Mw7.6 or Wenquan 2008 Mw7.9 thrust events

23 Emergence of the Mishmi thrust along a conznuous seismic scarp

24 High resoluzon Pleiades images DEM

25

26 Several meters high scarp in the city of Pasighat

27 Several meters high scarp in the city of Pasighat

28 Several meters high scarp in the city of Pasighat Evidence of step wise growth of the MFT scarp with terrace height

29 Landslides and rock falls, szll aczve in the Abor and Mishmi Hills

30 Remnants of large debris flow loaded with tree trunks

31 Remnants of large debris flow loaded with tree trunks

32 Beheaded trunks, remnants of a forest covered by a massif debris flow, presently eroding

33 Beheaded trunks, remnants of a forest covered by a massif debris flow

34 Landslides and debris flow extension across the Abor and Mishmi hills

35

36

37 Herrin et al., 1962 BenMenahem et al., 1974 Chen and Molnar 1977

38 Chen and Molnar 1977 ; BenMenahem et al., 1974; Herrin et al., 1962 this study

39

40

41

42 Roing Scarp / Depth profile 10Be Measurements of in situ produced cosmogenic 10Be concentrazons in exposed quartz rich boulders and cobbles Surface and sub surface

43 Metric quartz rich block at terrace surface

44 Surface Sub surface Upper terrace south (1700yrs), north (2000yrs) and sub surface (2300yrs) are consistent with abandonment at 2000±200yrs Youngest strath is less than 170 yrs old Consistent with last event in 1950 and penulzmate event between yrs

45 Conclusion - Evidence of recent surface faulting along the Abor and Mishmi hills front associated to the 1950 Assam earthquake - Field observations (continuous scarp up to 330 km, geometry of the scarp, amounts of offsets, landslides extension, ) are in agreement with magnitude and mechanism : - Vertical co-seismic offsets and dip of the MT and MFT imply up to 15 to 30 m of slip, in agreement with magnitude Mw 8.6 derived from long period surface waves - Resulting mechanism combines slip on both the MFT and MT with a major reverse faulting component compatible with first motion polarities - Given present-day geodetic convergence in Assam, about 17 mm/yr, and the maximum amount of slip estimated along the MT implies a recurrence time of about 1700 yrs, in agreement with the return time deduced from cosmogenic isotope dating ( yrs)

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63