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.
Mw 7.5 for the last 200-500yrs Seismic slip deficit : where are the earthquakes? Stevens & Avouac,, 2015, 2016
Frontal scarp of MFT and surface breaks of the 1934 Bihar Nepal earthquake Sapkota et al., 2012 ; Bollinger et al., 2014
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
Extension of 1934 Bihar Nepal surface rupture Sapkota et al., 2012 ; Bollinger et al., 2014
25 April 2015 Mw 7.8 Gorkha earthquake extension
Sub surface complexity of the thrust fault Incomplete historical and geological records Hubbard et al., Geology, 2016
Crossing rivers in Assam, the far east India
Tebang river cut
Tebang river cut
Mishmi thrust outcrop
10 m high scarp across terrace of Dibang tributary at Roing
Decametric scarp of the Mishmi thrust at Roing
14 m high scarp across terrace at Wakro
Topographic profile across scarp reveals complexity and occurrence of 2 events
Scarp across lower terrace along Kamlang river at Wakro
Scarp across lower terrace along Kamlang river at Wakro
7 m high scarp
Rapids in the present day Kamlang river bed Similar to Muzafarabad Kashmir 2005 Mw7.6 or Wenquan 2008 Mw7.9 thrust events
Emergence of the Mishmi thrust along a conznuous seismic scarp
High resoluzon Pleiades images DEM
Several meters high scarp in the city of Pasighat
Several meters high scarp in the city of Pasighat
Several meters high scarp in the city of Pasighat Evidence of step wise growth of the MFT scarp with terrace height
Landslides and rock falls, szll aczve in the Abor and Mishmi Hills
Remnants of large debris flow loaded with tree trunks
Remnants of large debris flow loaded with tree trunks
Beheaded trunks, remnants of a forest covered by a massif debris flow, presently eroding
Beheaded trunks, remnants of a forest covered by a massif debris flow
Landslides and debris flow extension across the Abor and Mishmi hills
Herrin et al., 1962 BenMenahem et al., 1974 Chen and Molnar 1977
Chen and Molnar 1977 ; BenMenahem et al., 1974; Herrin et al., 1962 this study
Roing Scarp / Depth profile 10Be Measurements of in situ produced cosmogenic 10Be concentrazons in exposed quartz rich boulders and cobbles Surface and sub surface
Metric quartz rich block at terrace surface
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 1200 2600 yrs
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 (1200-2600 yrs)