4:15 PM - 4:30 PM

# [SSS03-10] Underestimate of the size of microearthquakes by the JMA magnitude scale and its infuence to earthquake statistics

Keywords:Earthquake statistics, JMA magnitude, Moment magnitude, Spectral analysis, Completeness magnitude, b-value

_{j}), based on amplitudes of seismograms are easy to estimate and therefore usually included in earthquake catalogs. The moment magnitude (M

_{w}) is based on the physical source parameter, seismic moment, however needs much effort for the estimation especially for microearthquakes. Though the consistency between M

_{j}and M

_{w}is guaranteed for the medium earthquakes, we need to check that for microearthquakes.

As for use of earthquake catalogs, we should know the completeness magnitude above which catalog is complete. A type of it is M

_{c}defined as a magnitude where magnitude-frequency distribution starts deviating from the Gutenberg-Richter’s (GR) law. Another one is based on earthquake detectability. Schorlemmer and Woessner [2008] proposed M

_{P}based on the detectability inferred from the pick information. They showed the Californian case that M

_{P}is smaller than M

_{c}, which indicates the breakdown of the GR law. It is important to confirm if the breakdown really occurs. Our study investigates if the discrepancies are also seen in case of M

_{w}.

__M__

_{w}Estimation for MicroearthquakesWe stably estimate seismic moment of microearthquakes based on moment ratios to nearby small earthquakes whose seismic moments are available in the NIED MT catalog, by a multiple spectral ratio analysis [Uchide and Imanishi, under review]. Applying this method to earthquakes in Fukushima Hamadori and northern Ibaraki prefecture areas, eventually we obtained the seismic moments of a total of 19140 earthquakes (M

_{j}0.4 - 3.8). The striking result of this study is that the change in slopes of the M

_{j}-M

_{w}curve: 1 and 0.5 at higher and lower magnitudes, respectively (see Figure). The discrepancies between M

_{j}and M

_{w}are significant for microearthquakes, suggesting that M

_{j}underestimates the sizes of microearthquakes.

__Completeness Magnitudes and b-values__

The result above must affect earthquake statistics. Here we study M

_{c}and b-value of the GR law. Following Ogata and Katsura [1993], we assume the earthquake detectability as the cumulative normal distribution with a mean, μ, and a standard deviation, σ, and estimate the GR parameters (a and b) together with μ and σ. We define M

_{c}= μ + 2.33 σ where the detection rate is 99 %. Applying this method to the monthly seismicity data in the study area, we found that the M

_{c}for M

_{w}is lower than that for M

_{j}converted into M

_{w}, however still larger than M

_{P}converted into M

_{w}. This may be due to the breakdown of the GR law for microearthquakes, though another possibility is that the incompleteness of earthquake catalog overestimates the detectability, resulting the underestimate of M

_{P}.

b-values for M

_{w}(b

_{w}) are systematically larger than those for M

_{j}(b

_{j}). The temporal trends for b

_{w}and b

_{j}are similar to each other. When b

_{j}increases, b

_{w}also increases. This does not affect discussions inferred from the qualitative temporal change in b-values [e.g., Nanjo et al., 2012]. b

_{w}is often larger than 1.5, indicating that the moment release is dominantly done by smaller earthquakes.

__Acknowledgement__

We used the JMA Unified Earthquake Catalog, seismograms from NIED Hi-net and the NIED moment tensor catalog.

__Figure__: Comparison between M

_{j}and M

_{w}inferred from the multiple spectral ratio analyses (color image for the distribution and circles for the median M

_{w}) and the NIED MT solutions.