Regional seismic observations of the 9 October 2006 underground nuclear explosion in North Korea and the influence of crustal structure on regional phases

Abstract

[1] The crustally guided shear wave, Lg, is typically the most prominent phase of a nuclear explosion at regional distance. This Lg phase is analyzed often to discriminate a nuclear explosion from a natural earthquake. In addition, the Lg phase allows us to determine the size of the detonation. A nuclear explosion test in North Korea was conducted on 9 October 2006. The epicenter was located close to the eastern shore of the Korean Peninsula, resulting in raypaths that vary significantly according to the azimuths. In particular, rays radiated in the southern direction experience lateral variation of crustal structures at the continental margin. We examine the influence of raypaths on regional seismic phases by comparing the spectra and waveforms from different raypaths. Three natural earthquakes in North Korea are also examined to determine the raypath effect. We find that the Lg from the nuclear explosion dissipated significantly as result of energy leakage into the mantle resulting from variations in crustal thickness along the portion of the raypath traversing the western tip of the Sea of Japan (East Sea). Some of the leaked energy develops into mantle lid waves (Sn), causing a large energy increase to Sn. A similar feature is observed in the records of natural earthquakes. This feature is confirmed by seismic waveform modeling. The raypath effect also causes underestimation of magnitude. The Lg body wave magnitude, m b (Lg), is estimated to be 3.8-4.2 for records from pure continental paths and 2.6-3.4 for records from paths crossing continental margins. This result illustrates the need to consider raypath effects for the correct estimation of magnitudes of regional events, including a nuclear explosion.

Lg Body-Wave Magnitude Scaling for the Continental Margin around Korea and Japan

“…On the other hand, the seismograms from raypaths across the East Sea show weak Lg waves due to abrupt lateral variation of crustal structures (HONG, 2010). We also find that the Lg waves from raypaths across the East Sea display longer wavetrains than those from pure continental raypaths (HONG et al, 2008).…”

“…Regional Lg is dominant at frequencies around 1 Hz (NUTTLI, 1973;HONG et al, 2008). Thus, quality factors for 1 Hz Lg waves (i.e., Q 0 ) were implemented as reference quality factors in this study.…”

“…The Lg wavetrains, however, vary significantly with crustal structures (e.g., KENNETT and FURUMURA, 2001;HONG et al, 2008). In particular, the Lg waves are dissipated strongly in thin or undulated crusts due to energy leakage into the mantle (e.g., KNOPOFF et al, 1979;KENNETT, 1986;ZHANG and LAY, 1995).…”

“…The m b ðLgÞ scaling has been limitedly applied to regional events around the Korean Peninsula (HONG et al, 2008;ZHAO et al, 2008). However, the application conditions for m b ðLgÞ scaling in the continental margins were rarely investigated.…”

Lg wave propagation in the area around Japan: observations and simulations

“…The phase is dominated by relatively high frequencies between 0.2 and 5 Hz. Therefore, Lg phases have been used to magnitude estimates of shallow events (m b (Lg), see, e.g., Nuttli 1973;Herrmann and Nuttli 1982) and to map crustal heterogeneities by estimating Lg attenuation variations (Q Lg ; e.g., Phillips et al 2000;Xie et al 2006;Chung et al 2007;Hong et al 2008). The Lg phase is also utilized for discriminating manmade sources from natural earthquakes, such as by using the Lg/Pg and Lg/Pn amplitude ratio (e.g., Kennett 1993;Walter et al 1995;Taylor 1996;Fisk 2006;Kim and Richards 2007).…”