Characteristics and generation mechanism of low-frequency earthquakes in the continental plate

Abstract

Low-frequency earthquakes (LFEs) are anomalous earthquakes with a lower predominant frequency than that expected from the earthquake magnitude. LFEs are also unique with respect to their anomalously deep focal depths, where ordinary earthquakes do not occur. Due to these characteristics, the generation mechanisms of LFEs have attracted seismological, lithological, and geochemical attention. Here, we review the observation and their interpretation of LFEs. Most LFEs worldwide occur near active volcanoes, although some LFEs in Japan are detected far from such volcanoes on account of the high sensitivity of the seismic network. The focal mechanisms of LFEs include double-couple that represent fault slip, isotropic and compensated linear vector dipole (CLVD) that suggest a volume change in the source. Tomographic studies have shown that the source areas of LFEs are characterized by low velocity and high V_P/V_S ratios, suggesting the contribution of geofluids to LFE occurrence. Recent observations in Japan have revealed the occurrence of LFEs at shallow depths, even in the upper crust, sometimes located close to ordinary earthquakes. These observations and the variation in focal mechanisms support an LFEs’ source model of tensile-shear crack. LFEs can occur in the upper crust if the pore fluids are close to H₂O rather than magma. The focal mechanism is double-couple if shear motion is dominant but is characterized by isotropic or CLVD components if crack opening is dominant, with the different focal mechanisms probably reflecting the pressure of pore fluids.