4:30 PM - 4:45 PM
[S13-4-01] Earthquake Source Spectral Studies beyond the Standard Omega-Square Model
A scientific methodology is to understand natural phenomena as general properties and variation around them. As for earthquakes, source spectral studies have simplified the information from seismic data and helped us enhance our knowledge of earthquake rupture process. The standard model in spectral studies is the omega-square model, where the source spectrum is flat equivalent to the seismic moment at lower frequencies and inversely proportional to the square of frequency at higher frequencies. The corner frequency borders the lower and higher frequencies and is related to the inverse of the source duration, therefore employed for estimating representative stress drop under an assumption of circular crack model. The omega-square model and the corner frequency represent the general property and the variation, respectively.
Recent studies in earthquake source spectra have been examining the validity of the omega-square model as a general model. Denolle and Shearer (2016) claimed the existence of another corner frequency which is scaled by Mo^(-1/5), for M > 5.5 earthquakes. However Uchide and Imanishi (2016) claimed that small earthquakes also have two corner frequencies, whereas some of small earthquakes are fitted well by the single-corner-frequency model, as revealed by a spectral ratio analysis using many empirical Green's function events for each target event.
We show the universality of the double-corner-frequency source spectra by analyzing small inland earthquakes in Japan by the method of Uchide and Imanishi (2016). In addition, as a byproduct of the study, the same analysis indicated the underestimation of the size of microearthquakes by the local magnitude scale, which significantly affects the b-value estimation.
Then what is the physical meaning of the second (higher) corner frequency? It is still difficult to answer this question, and we need case studies and scaling studies on the second corner frequency.
Recent studies in earthquake source spectra have been examining the validity of the omega-square model as a general model. Denolle and Shearer (2016) claimed the existence of another corner frequency which is scaled by Mo^(-1/5), for M > 5.5 earthquakes. However Uchide and Imanishi (2016) claimed that small earthquakes also have two corner frequencies, whereas some of small earthquakes are fitted well by the single-corner-frequency model, as revealed by a spectral ratio analysis using many empirical Green's function events for each target event.
We show the universality of the double-corner-frequency source spectra by analyzing small inland earthquakes in Japan by the method of Uchide and Imanishi (2016). In addition, as a byproduct of the study, the same analysis indicated the underestimation of the size of microearthquakes by the local magnitude scale, which significantly affects the b-value estimation.
Then what is the physical meaning of the second (higher) corner frequency? It is still difficult to answer this question, and we need case studies and scaling studies on the second corner frequency.