An efficient and robust cross‐correlation technique for estimation of surface wave Green's function between two locations is applied to a set of ambient noise data recorded by accelerograph networks in the southern Korea. We have estimated 1843 group velocities of Rayleigh waves in the period band of 2–4 seconds from correlation pairs of 91 accelerograph stations. Relatively dense and uniform coverage of the pairs provides us with high‐resolution tomographic image of surface‐wave group velocities in the region. The resulting group‐velocity distribution correlates well with the characteristic features of geological map of the region, displaying low velocities for the sedimentary basins and regions of volcanic and metasedimentary rocks, and high velocities for the regions where granitoid basement rocks outcrop. The short‐period information from the accelerograph network can enhance resolution of the subsurface structures at depths within a few kilometers, which may be predominantly responsible for simulation results of strong ground motions.
S U M M A R YThe suspicious seismic event that occurred in the northern Korean Peninsula on 2009 May 25 was declared to be the second underground nuclear test (NK2ND) by North Korea. We investigated the characteristics of NK2ND using seismic signals recorded at regional-distance stations in South Korea and China. The Pn/Lg ratios of NK2ND definitely discriminate this event from two nearby natural earthquakes at frequencies above 4 Hz. Full moment tensor inversion of full waveform data shows that NK2ND had a very large isotropic component. Pure isotropic moment tensor inversion also resulted in good recovery of observed waveforms, with clear indication that NK2ND was explosive in origin. The moment magnitude (M w ) from the full moment tensor inversion was estimated to be 4.5 and network-averaged values of 4.6 and 3.6 were calculated for rms m b (Lg) and M s (VMAX), respectively. Although m b − M s signature has been considered one of the most reliable discriminants for separating explosions and earthquakes, this signature showed poor discrimination in the case of NK2ND. The Pn/Lg ratios and moment tensor inversion give more reliable evidence than does the m b − M s for classifying the suspicious event in the northern Korean Peninsula as a possible explosion. The characteristics of NK2ND are also quite similar to those of the first North Korean nuclear test on 2006 October 9.
A radial collector well is used for the extraction of a large amount of groundwater without causing a deep drawdown at the well's center, and it is appropriate for the supply of municipal water through riverbank filtration (RBF). Flow path changes caused by water extraction through a radial collector well were simulated to estimate the amount of river water induction at a RBF site associated with Nakdong River in South Korea. The structure of the screened horizontal arms of a radial collector well was examined with respect to effective riverbank filtration. The relative ratio of the river water induced to the radial collector well compared to the total groundwater extraction was estimated to be 27-52%. The amount of induced river water varies with the distance of a horizontal arm from the river, indicating that the location and structure of the collector well is significant for RBF. In all simulation cases, the maximum drawdown of the groundwater level near the collector well was 2.1m, which is not significant considering the substantial pumping rate at the study site. It was concluded that RBF radial collector wells can be used at the study site for a sustainable water supply.
Strong ground motions induce large dynamic stress changes that may disturb the magma chamber of a volcano, thus accelerating the volcanic activity. An underground nuclear explosion test near an active volcano constitutes a direct treat to the volcano. This study examined the dynamic stress changes of the magma chamber of Baekdusan (Changbaishan) that can be induced by hypothetical North Korean nuclear explosions. Seismic waveforms for hypothetical underground nuclear explosions at North Korean test site were calculated by using an empirical Green’s function approach based on a source-spectral model of a nuclear explosion; such a technique is efficient for regions containing poorly constrained velocity structures. The peak ground motions around the volcano were estimated from empirical strong-motion attenuation curves. A hypothetical M7.0 North Korean underground nuclear explosion may produce peak ground accelerations of 0.1684 m/s2 in the horizontal direction and 0.0917 m/s2 in the vertical direction around the volcano, inducing peak dynamic stress change of 67 kPa on the volcano surface and ~120 kPa in the spherical magma chamber. North Korean underground nuclear explosions with magnitudes of 5.0–7.6 may induce overpressure in the magma chamber of several tens to hundreds of kilopascals.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.