“The  Kumamoto,  Japan,  Earthquake  Series  of  April  2016”  Kenji  SATAKE,  Earthquake  Research  Institute,  the  University  of  Tokyo  The	   April	   2016	   Kumamoto	   earthquakes	   caused	   devastating	   damage	   including	   50	  casualties.	   The	   epicentral	   region	   is	   located	   in	   the	   Beppu-­‐Shimabara	   graben	   belt,	   running	  east-­‐west	   direction	   through	   central	   Kyushu	   with	   north-­‐south	   tension.	   Mt.	   Aso,	   an	   active	  volcano	   in	   the	   large	   caldera	   (diameter	   of	   ~	   20	   km),	   is	   located	   in	   the	   graben,	   east	   of	   the	  earthquake	  source.	  The	  graben	  structure	  parallels	  the	  Median	  Tectonic	  Line,	  which	  continues	  to	  east	  in	  Shokoku	  as	  an	  active	  fault.	  It	  continues	  to	  the	  southwest	  toward	  Okinawa	  trough,	  a	  backarc	  basin	  of	  Ryukyu	  trench.	   	   	  The	   earthquake	   series	   started	   with	   an	   M	   6.5	   event,	   called	   foreshock,	   on	   April	   14,	  followed	   by	   M	   6.4	   event	   ~2.5	   hours	   later.	   On	   April	   16,	   ~	   28	   hours	   after	   the	   first	   event,	   the	  largest	   event	   with	   M	   7.3	   occurred.	   This	   earthquake,	   called	   mainshock	   of	   the	   Kumamoto	  earthquake	   series,	   triggered	   earthquakes	   with	   M~6	   in	   Aso	   (~	   50	   km	   from	   the	   epicenter)	   and	  Oita	   (~	   80	   km)	   regions.	   The	   aftershock	   activity	   as	   well	   as	   triggered	   seismicity	   in	   Aso	   and	   Beppu	  regions	   was	   very	   high,	   nearly	   250	   earthquakes	   with	   M	   >	   3.5	   occurred	   within	   30	   days	   of	   the	  foreshock.	   	  Both	   foreshock	   and	   mainshock	   have	   predominantly	   strike-­‐slip	   mechanism	   with	  north-­‐south	   tensional	   axis.	   The	   focal	   mechanism	   solution	   and	   aftershock	   distributions	   indicate	  left-­‐lateral	   strike	   slip	   faultings	   with	   slightly	   different	   strikes.	   The	   maximum	   peak	   acceleration	  from	   the	   foreshock	   and	   mainshock	   were	   1580	   gal	   and	   1791	   gal,	   respectively.	   The	   maximum	  horizontal	   displacements	   recorded	   on	   GNSS	   network	   during	   the	   foreshock	   and	   mainshock	  were	   20	   cm	   and	   98	   cm,	   respectively.	   The	   satellite	   (InSAR)	   data	   indicate	   the	   mainshock	   fault	  extended	  about	  35	  km	  from	  the	  epicenter.	   	  The	   foreshock	   occurred	   on	   the	   ~	   6	   km	   long	   northern	   segment	   of	   Hinagu	   fault,	  whereas	   the	   mainshock	   occurred	   on	   the	   ~	   28	   km	   long	   eastern	   segment	   of	   Futagawa	   fault.	  
Postseismic	   field	   surveys	   indicate	   that	   the	   surface	   rupture	   with	   maximum	   offset	   of	   2	   m	  appeared	   along	   these	   active	   faults.	   The	   Earthquake	   Research	   Committee	   evaluated	  possibilities	   of	   an	   M	   6.8	   earthquake	   on	   the	   northern	   segment	   of	   Hinagu	   fault	   and	   an	   M	   7.0	  earthquake	  on	  the	  eastern	  segment	  of	  Futagawa	  fault.	   	   Because	  the	  size	  and	  fault	  length	  of	  foreshock	   and	   mainshock	   were	   similar	   to	   the	   forecast,	   these	   events	   may	   be	   considered	   as	   two	  main	  shocks	  rather	  than	  foreshock-­‐main	  shock	  series.