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At 11:30 on September 3, 2017 (Beijing time), a strong earthquake occurred in Democratic People′s Republic of Korea (DPRK). International seismic monitoring agencies, e.g., the China Earthquake Network Center (CENC) and the United States Geological Survey, suspected that it is an explosion. Based on seismic data from the China National Digital Seismic Network (CNDSN) and Global Seismic Network (GSN), we investigated characteristics of this event and an aftershock 8 min after the main event. The P- and S-wave excitation functions of explosion and earthquake sources are scaled differently. Therefore, the P/S-type spectral ratios can be an effective discriminant for separating explosions from earthquakes. Using the P/S spectral ratios Pg/Lg, Pn/Lg and Pn/Sn as discriminants, we confirmed the 3 September 2017 event was an explosion. For the aftershock occurred after the main event, we identified it is a collapse, likely caused by the failure of the explosion generated cavity. Using a pre-calibrated regional seismic network in Northeast China and the Korean peninsula, and the regional Lg-wave attenuation model developed previously, we obtained the Lg wave body wave magnitudes for the 3 September 2017 main event and its aftershock to be m b(Lg)=5.6±0.2, and 3.95±0.04. We used a group of historical events to calibrate the regional network for calculating Rayleigh wave magnitude. After correcting for the site responses, the network averaged surface wave magnitudes for the main shock and the aftershock were obtained to be M s=5.1±0.2 and 3.95±0.08. To test body-wave versus surface-wave magnitude as a potential discriminant, we compared the M s (Rayleigh) and m b (Lg) for all 6 DPRK nuclear explosions and a group of earthquakes in Northeast China and the Korean peninsula. The explosion and earthquake populations were largely overlapped with each other. The above results show that the P/S ratio method is a more effective discriminant than the m b(Lg)- M s criterion in Northeast China and the Korean Peninsula. The seismic yield of an underground nuclear explosion can be estimated from its magnitude using a calibrated empirical magnitude-yield relation. However, the DPRK test site (DPRKTS) is an uncalibrated test site. Considering that the DPRKTS is located at a granite site in a stable geology platform, we adopted the fully-coupled hard-rock site equation used at the Novaya Zemlya test site to calculate the yield at the DPRKTS. The estimated yield for the 3 September 2017 explosion was 56 kt using this relationship and assuming a normally scaled burial depth. Transferring the measurement error of ±0.2 magnitude unit to the yield calculation introduced uncertainties between 30 and 100 kt. However, if the explosion was over buried at depths between 1000 and 2400 m, the yield could be increased to 100–200 kt.

We estimate the Lg-wave quality factor (Q) across the Australian continent from vertical-component Lg waveforms. A tomographic inversion is performed to construct an Lg attenuation model for 58 frequencies between 0.05 and 10.0 Hz. The available spatial resolution is approximately 1.5° × 1.5° for the 0.5–2.0 Hz band. At 1.0 Hz, the Lg-wave Q over the whole island continent varies from 50 to 1250 with an average value of 850. Significant regional variations in the Lg-wave Q images tie well with many geological features and boundaries in Australia. The cratons in western, northern and southern parts of Australia usually have higher Q values (700–1250), while the volcanic regions, sedimentary basins and orogenic areas in eastern Australia are characterized by increased attenuation (lower Q values, 50–650). We determine the frequency-dependent Q of Lg waves for different blocks across Australia, and find the frequency dependence of Q is much more complex than the traditional single power law representation. When combined with the assumed geometrical spreading relation, the Lg Q maps provide a new way of assessing potential ground motion across the continent for any event location.

We investigate the geometric spreading and attenuation of seismic Pn waves in Northeast China and the Korean Peninsula. A high-quality broadband Pn wave data set generated by North Korean nuclear tests is used to constrain the parameters of a frequency-dependent log-quadratic geometric spreading function and a power law Pn Q model. The geometric spreading function and apparent Pn wave Q are obtained for Northeast China and the Korean Peninsula between 2.0 and 10.0 Hz. Using the two-station amplitude ratios of the Pn spectra and correcting them with the known spreading function, we remove the contributions of the source and crust from the apparent Pn Q and retrieve the P wave attenuation information along the pure upper mantle path. We then use both Pn amplitudes and amplitude ratios in a tomographic approach to obtain the upper mantle P wave attenuation in the studied area. The Pn wave spectra observed in China are compared with those recorded in Japan, and the result reveals that the high-frequency Pn signal across the oceanic path attenuated faster compared with those through the continental path.

We perform Q Lg tomography for the northeastern part of North America. Vertical broadband seismograms of 473 crustal earthquakes recorded by 302 stations are processed to extract the Lg amplitude spectra. Tomographic inversions are independently conducted at 58 discrete frequencies distributed evenly in log space between 0.1 and 20.0 Hz. This relatively large dataset with good ray coverage allows us to image lateral variation of the crustal attenuation over the region. Obtained Q Lg maps at broadband and individual frequencies provide new insights into the crustal attenuation of the region and its relationship to geological structures and past tectonic activity in the area. The Q Lg shows more uniform values over the older, colder, and drier Canadian Shield, in contrast to higher variations in the younger margins. Results confirm the correlation of large-scale variations with crustal geological features in the area. Existence of low-velocity anomalies, thick sediments, volcanic rocks, and thin oceanic crust are potential sources of observed anomalies. The mean Q values are inversely correlated with average heat flow/generation for main geological provinces.

As a prominent geophysical anomaly, unusually high seismic wave attenuation is observed in the crust and upper mantle of the Tibetan Plateau, particularly along its northern area. Theoretical and laboratory investigations show that the strong seismic attenuation can indicate high temperatures and partial melting, which may decrease the viscosity of the material and cause it to flow. Thus, seismic attenuation distribution may provide useful constraints to the crust flows if they exist. Using Lg-wave Q tomography, we construct a 0.05–10.0 Hz broadband high-resolution crust attenuation model for the Tibetan Plateau and its surrounding regions. The maximum spatial resolution is approximately 1.0 ° × 1.0 ° in well-covered areas and for frequencies between 0.05 and 1.5 Hz. This attenuation model reveals an apparent low-QLg belt stretching along the northern and eastern Tibetan plateau. Combining the Lg-wave Q model with other geophysical data, two possible crust flow channels are found in the Tibetan Plateau. The main flow channel is from north to east and then turns to southeastern Tibet along the western edge of the rigid Sichuan basin, while a second channel starts from southern Tibet and crosses the Eastern Himalayan syntaxis.

Lian-Feng Zhao,1,2 Xiao-Bi Xie,2 Wei-Min Wang,3 Jin-Hai Zhang1 and Zhen-Xing Yao1 1Key Laboratory of the Earth’s Deep Interior, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. E-mail: zhaolf@mail.iggcas.ac.cn 2Institute of Geophysics and Planetary Physics, University of California, Santa Cruz, CA 95064, USA 3Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China

The rupture process of the 1999 Chi-Chi, Taiwan earthquake is investigated by using co-seismic surface displacement GPS observations and far field P-wave records. In according to the tectonic analysis and distributions of aftershocks, we introduce a three-segment bending fault plane into our model. Both the elastic half-space model and the layered-earth model are employed to invert the distribution of co-seismic slip along the Chi-Chi earthquake rupture. The results indicate that the shear slip model can not fit horizontal and vertical co-seismic displacements simultaneously, unless we add the fault-normal motion (tensile component) to inversion. And then, the Chi-Chi earthquake rupture process is obtained by inversion using the seismograms and GPS observations. Antidilatational fault normal motions determined by inversion, concentrating on the shallow northern bending fault, where the surface earthquake ruptures, reveal more complexities and the developed flexural slip folding structures than the other portions of the rupture zone. For understanding the perturbation of surface displacements caused by nearsurface complex structures, we have taken a numerical test to synthesize and inverse the surface displacements for a pop-up structure that is composed of a main thrust and a back thrust. Our result indicates that the pop-up structure, the typical shallow complex rupture that occurred in the northern bending fault zone, can be modelled better by a thrust fault added a negative tensile component than by a simple thrust fault. We interpret the negative tensile distributions, that concentrate on the shallow northern bending fault, as a synthetic effect including the complexities of property and geometry of rupture. The earthquake rupture process also exhibits more spatial and temporal complexities at the northern bending fault. And the results clearly show an inverse correlation between the aftershock distribution and the mainshock slip.

Using Lg-wave Q tomography, we construct a broadband crustal attenuation model for the Middle East. The Q Lg images reveal a relationship between attenuation and geological structures. Strong attenuation is found in the continental collision orogenic belt that extends from the Turkish and Iranian plateau to the Pamir plateau. We investigate the frequency dependence of Q Lg in different geologic formations. The results illustrate that Q Lg values generally increase with increasing frequency but exhibit complex relationships both with frequency and between regions. An average Q Lg value between 0.2 and 2.0 Hz, Q Lg (0.2–2.0 Hz), may be a critical index for crustal attenuation and is used to infer the regional geology. Low-Q Lg anomalies are present in the eastern Turkish plateau and correlate well with low Pn-velocities and Cenozoic volcanic activity, thus indicating possible partial melting within the crust in this region. Very strong attenuation is also observed in central Iran, the Afghanistan block, and the southern Caspian Sea. This in line with the previously observed high crustal temperature, high-conductivity layers, and thick marine sediments in these areas, suggests the high Lg attenuation is caused by abnormally high tectonic and thermal activities.