zurich nuclear proliferation watchdog ctbto examine unusual seismic activity north korea take place around 50 km 31 miles previous nuclear test isolate country say saturday analysts look unusual seismic activity much small magnitude dprk ctbto executive secretary lassina zerbo say twitter post korean peninsula unusual seismic activity lat=41.36 lon=129.76 mb=3.5 50km prior test ctbt analysts investigate say subsequent post china earthquake administration say detect magnitude 3.4 quake north korea suspect explosion raise fear pyongyang might conduct another nuclear bomb test ctbto spokeswoman say zerbo remark small magnitude refer monitor event north korea sept. agency describe consistent man-made explosion stop short call nuclear blast pending test airborne radioactivity north korea describe sept. incident test advance hydrogen bomb long-range missile mark dramatic escalation regime stand-off unite state ally ctbto preparatory commission comprehensive nuclear test ban treaty organization found 1996 monitor compliance accord negotiate 1990s ban nuclear explosions treaty come effect eight countries nuclear technology yet sign ctbto look `` unusual seismic activity `` north korea