S44A-02
Complexities in earthquake source spectra revealed by spectral ratios to multiple empirical Green’s function events
Abstract:
Earthquake source spectra are useful for characterizing source process. In order to eliminate the path and site effects and extract the source effect, we often take the ratio of the observed spectrum of target event to that of collocated small event ("EGF"). The spectral ratio is fit by the omega-square model to estimate the corner frequency of the target event.In general, events two units of magnitude smaller than target event are preferable for EGFs to constrain the corner frequency of the target event. However in case to estimate the entire shape of source spectrum, smaller events are preferable, since small events have higher corner frequencies, below which the source effect of EGF must be negligible. Different purpose prefers different magnitude of EGFs.
We propose a spectral ratio analysis to fit the spectral ratios of single target event to multiple EGFs simultaneously to estimate the corner frequency of the target event and those of the EGFs by grid search, and calculate the moment ratios. Larger EGFs are useful to constrain the corner frequency of the target event, and smaller ones work to investigate the shape of the source spectrum of the target event up to high frequencies.
We focus on Fukushima Hamadori and northern Ibaraki areas where shallow normal-faulting seismicity was significantly activated after the 2011 Tohoku-oki earthquake. We apply this method to earthquakes whose moment magnitudes were estimated by NIED F-net, employing 10-50 EGFs within 0.5 km from each target event.
One example is shown in Figure. We observe bumps on the spectral ratios to different EGFs in common. This common feature must be from the source effect of the target event and indicate incoherence in the rupture process. Some of other events have smooth spectra.
We investigate the spatial variation in characters of the source spectra. It is difficult to find any spatial tendencies in the amplitude of the spectral bumps. We will compare the results in this region and other region in future. The stress drops inferred from the corner frequencies have some spatial dependence. This fact will be a clue to understand fault properties in the source region.
[Figure] Spectral ratios normalized by moment ratios.
