zurich nuclear proliferation watchdog ctbto say saturday detect two seismic events north korea saturday probably deliberate explosions isolate country two seismic events 0829utc much small 0443utc unlikely man-made similar collapse event 8.5 mins dprk6 analysis ongoing ctbto executive secretary lassina zerbo say twitter post china earthquake administration say detect magnitude 3.4 earthquake north korea suspect explosion official south korea meteorological agency say initial view natural quake two seismic events north korea unlikely man-made ctbto