Frequency-dependent Lg attenuation in Northeast China and its implications

Lg attenuation and site responses in Northeast (NE) China are estimated by implementing the Reverse Two Station Method (RTSM) at narrow band central frequencies of 0.5, 1.0, 2.0 and 3.0 Hz using 453 earthquakes recorded by 201 seismic stations deployed in the region from 1995 to 2013. The RTSM has the advantage of removing source and site effects without requiring a priori models. Tomographic images produced at a resolution of 2° × 2° at all frequencies exhibit a high degree of lateral variation of Lg attenuation in NE China. The Great Xing'an, Lesser Xing'an and Songen-Zhangguangcai Ranges show Q values above 400 at all frequencies. At central frequencies of 0.5 and 1 Hz, Sanjiang Basin Songliao Basin, Erlian Basin and Hailar Basin consistently show Q values lower than 400. Holocene and Pleistocene volcanoes, including the Wudalianchi and Jingpuhi volcanic fields, also appear as regions of low Q (< 400) at 0.5 and 1 Hz and at higher frequencies the effects of volcanoes diminish. At high frequencies (≥2 Hz), the sedimentary basins show Q values higher than 400 and overall Q values increase with frequency in NE China, thus obeying a power-law frequency dependence. A linear regression of frequencies in the range of 0.25-4.5 Hz results in the parameters describing the power-law frequency dependence of the region, with an average Q₀ value of 428 and a frequency-dependent factor (°), describing the strength of dependence, of 0.68. Overall, Lg attenuation in NE China appears to be due to thick late Cretaceous sediments, Holocene and Pleistocene volcanism, moderate to high heat flow, partial melts and variation in the thickness of the crustal wave guide. The site responses calculated at 0.5, 1.0, 2.0 and 3 Hz show a high degree of lateral variation as well as variation with frequency in NE China. At 0.5 and 1.0 Hz, the Great Xing'an and Lesser Xing'an Ranges show deamplification while the Hailar, Erlian and southern Songliao basins and the Songen-Zhangguangcai Range show amplification. At higher frequencies (≥2 Hz), the pattern nearly reverses, with deamplification east of the Songliao Basin and amplification to the west of the Basin. The deamplification observed east of the Songliao Basin could be caused by basaltic lava flows resulting from Cenozoic volcanism in the region.