Low-frequency submarine volcanic swarms at the southwestern end of the Okinawa Trough

A low-frequency (LF) submarine volcanic swarm has been first observed at the southwestern end of the Okinawa Trough where some submarine volcanoes have recently been identified. A swarm of more than 24 earthquakes was recorded at land seismic stations in Taiwan between September 1st and September 4th, 2006. These earthquakes without clear S-arrivals were substantially different from other abundant tectonic earthquakes in the area. The dominant frequency of around 1 or 2 Hz recorded at all stations shows that the LF seismograms were generated by neither path nor site effects, but directly from the source. These LF earthquakes can be characterized as B-type volcanic earthquakes, which have been observed in some active volcanoes before significant eruptions. The observations of these LF volcanic earthquakes beneath submarine volcanic cones and seamounts are strong indicator that some submarine volcanoes are still active at the southwestern end of the Okinawa Trough.