Breathing of the seafloor: Tidal correlations of seismicity at Axial volcano

Figure 1. Locations and names of three ocean-bottom seismographs (OBS) used to locate events and NOAA-PMEL’s bottom-pressure recorder (BPR). All located earthquakes are shown as green dots. Purple star indicates location of water-column anomaly as recorded during OBS deployments. Lower right figure shows outline of 1998 flow (Embley et al., 1999) compared with location of 1994 seismicity. Lower-left figure shows location of Axial volcano (AS is Axial Seamount) on Juan de Fuca Ridge, off coast of northwestern United States. ABSTRACT Tidal effects on seafloor microearthquakes have been postulated, but the search has been hindered by a lack of continuous long-term data sets. Making this observation is further complicated by the need to distinguish between Earth and ocean tidal influences on the seafloor. In the summer of 1994, a small ocean-bottom seismograph array located 402 microseismic events, over a period of two months, on the summit caldera of Axial volcano on the Juan de Fuca Ridge. Harmonic tremor was also observed on all instruments, and Earth and ocean tides were recorded on tiltmeters installed within the seismometer packages. Microearthquakes show a strong correlation with tidal lows, suggesting that faulting is occurring preferentially when ocean loading is at a minimum. The harmonic tremor, interpreted as the movement of superheated fluid in cracks, also has a tidal periodicity.