Low-frequency Earthquakes in the Continental Plate and Their Seismological and Tectonic Implications

Low-frequency earthquakes(LFEs)occurring in the continental plate are reviewed. Most LFEs in the continental plate occur at depths of similar to 15-45 km in the uppermost mantle to the lower crust beneath volcanoes, but they also occur within the same depth range beneath non-volcanic areas. Because they occur at greater depths than the typical depth limit for shallow regular earthquakes, they are called "deep low-frequency earthquakes(deep LFE)." However, a recent study reveals that LFEs also occur at depths shallower than 15 km in the upper crust where many regular earthquakes occur. This indicates that LFEs occur over the entire depth range from the uppermost mantle to the upper crust. In the upper crust, LFEs and regular earthquakes coexist and occur in close proximity. Focal mechanisms and activity patterns of LFEs show that tensile-shear crack is the dominant mechanism generating LFEs. In addition, the long duration of waveforms is probably caused by resonance in the fluid-filled crack. Distributions of peak frequency(fp)and frequency index(FI)values of waveforms, both of which are expected to be significantly small for LFEs and large for regular earthquakes, show that there is no clear boundary for fp and FI values between LFEs and regular earthquakes; rather, they are distributed continuously. It is presumed that the distribution of high and low pore fluid pressures in source faults creates such distributions of small and large fp and FI values, respectively, and a LFE occurs when the pore pressure is extremely high. This indicates that pore pressure is directly related also to the genesis of regular earthquakes. In source areas of recent large inland earthquakes, LFEs are activated by the mainshock, and FI and fp values and stress drop synchronously decrease immediately after the mainshock, gradually recovering thereafter. The activation of LFEs by the mainshock and such temporal changes of FI, fp, and stress drop after the mainshock can be explained within the framework of fault-valve behavior. Furthermore, fp and FI values tend to be small along prominent tectonic lines, such as the Itoigawa-Shizuoka tectonic line. It is inferred that pore fluid pressure is locally high along those tectonic lines, thereby facilitating the current crustal deformation.