seoul series tremor landslides near north korea nuclear test base likely mean country sixth large blast destabilize region punggye-ri nuclear site may use much long test nuclear weapons experts say small quake detect early friday near north nuclear test site south korea weather agency say unlike quake associate nuclear test appear manmade tremor late string least three shock observe since pyongyang sept. nuclear test cause 6.3 magnitude earthquake friday quake magnitude 2.7 depth km north hamgyong province north korea korea meteorological administration say unite state geological survey usgs measure quake 2.9 magnitude depth km series quake prompt experts observers suspect last test north claim hydrogen bomb may damage mountainous location northwest tip country north korea six nuclear test conduct explosion sept. test power exist tunnel within underground test site might cave say kim so-gu head researcher korea seismological institute think punggye-ri region pretty saturate go ahead another test area could risk radioactive pollution accord 38 north washington-based project monitor north korea numerous landslides throughout nuclear test site detect via satellite image sixth test disturbances numerous widespread see north previous test 38 north say explosion sixth test large enough residents chinese border city yanji 200 km 125 miles north north korea nuclear test site feel grind shake beneath feet reason punggye-ri become north korea nuclear test field area consider stable rarely saw tremor past say hong tae-kyung professor earth system science yonsei university seoul recent small quake suggest test might trigger crust deformation south korea spy agency say recently north ready possibly two tunnel follow late test accord rule democratic party lawmakers brief issue tunnel use pyongyang first nuclear test shut test second tunnel use follow five national intelligence service cite say last month second tunnel may cave sixth test intelligence officials say north korea hint next test could grind foreign minister ri yong ho say last month north could test unprecedented scale hydrogen bomb pacific ocean response u.s. president donald trump threat totally destroy country arm experts say detonate nuclear-tipped missile pacific ocean see logical final step prove success weapons program would extremely provocative carry huge risk another issue could keep north korea use punggye-ri nuclear test nearby active volcano mt paektu yonsei university hong say 2,744 meter 9,003 ft mountain straddle northwestern border china north korea last erupt 1903. since north korea begin test nuclear capabilities experts debate whether explosions punggye-ri could trigger another volcanic eruption late north korea earthquake sign instability nuclear test site-experts