Spatial and temporal characteristics of low-magnitude seismicity from a dense array in western Nagano Prefecture, Japan

We analyze an earthquake catalog and waveform data from a dense seismic array in western Nagano Prefecture in order to investigate seismic properties and source parameters. The catalog used here consists of about 30,000 smaller magnitude earthquakes that were located within a source volume of order (10 km)(3). Moving window analysis shows that the seismic rate is steady over time and the temporal change in the size distribution of earthquakes shows only statistical fluctuations. Spatial analyses, however, reveals that certain features of seismicity are depth-dependent. At shallow depths (<3 km), we find an increase in average stress drop with depth that may be related to the characteristic depth of seismicity (the depth where the frequency of earthquakes is maximum). The b-value is also found to be elevated throughout depths where the seismicity is most intense but then the b-value slowly decreases as depth increases. Another result from our analysis of waveform data is a clear relation between earthquake magnitude and stress-drop in the magnitude range -0.5 to 2.5.