g-Soo Jeon

Small-Aperture Seismo-Acoustic Arrays: Design, Implementation, and Utilization

A four-element, 1-km-aperture seismo-acoustic array has been designed and installed northeast of Seoul, Korea. Each element of the array consists of a GS-13 vertical seismometer (1 Hz) in a shallow borehole (∼10 m) and a low-frequency acoustic gauge connected to an 11-element hose array (7.6-m hoses) at the surface. The array is being used to assess the importance of colocated seismic and acoustic sensors for the purposes of (1) quantifying wind as a source of seismic and acoustic noise, (2) constraining propagation path effects in the atmosphere and solid earth, (3) locating the sources of the waves, and (4) characterizing the source type. Seismic noise estimates illustrate a level that is only slightly above the low-noise model on average. Acoustic noise levels resolve the microbaroms during low-noise times but document a nearly 50-dB increase in noise during the windiest periods. Infrasonic noise in the 0.01- to 5-Hz band increases rapidly with wind velocity. The seismic noise shows little or no dependence on wind velocity. Analysis of the data from a 2-month time period suggests that there are many more acoustic signals than seismic (4–10 times as many). Approximately 1/4 of all seismic signals are associated with an acoustic arrival. The vast majority of seismo-acoustic observations come from sources in the 30- to 200-km range and occur during working hours, local time. The 30- to 200-km observation distance is surprising in that average atmospheric velocity models predict no acoustic returns in this range. Average atmospheric models modified by meteorological data for the troposphere indicate the possibility of ducting in the troposphere as an explanation for these arrivals. Event location is based upon regional seismic phase identification ( P n, P g, P m P, L g, R g) using the array and backazimuth estimates from both the seismic and acoustic data. Many of the infrasound signals have good signal-to-noise ratios from 1 to beyond 4 Hz. Despite the small size of the array, event clusters are identified at regional distances. Events associated with acoustic signals are presumed to be from mining regions. The existence of Rg arrivals and dominance of P energy at high frequency are consistent with this interpretation. Ground truth in the form of in-mine observations has validated that two of the clusters come from construction and mine blasts.

An analysis of the infrasound signal from the Miyagi-Oki earthquake in Japan on 16 August 2005

Following the 16 August 2005 Miyagi-Oki earthquake in Japan, coherent atmospheric infrasonic waves were observed at regional distances (1200–1500 km) using three seismo-acoustic arrays on the Korean Peninsula. A source-location procedure was applied to the distinct long-duration infrasonic signals to construct earthquakegenerated infrasound source regions on the Japanese island arc. The results showed that the long-duration infrasonic signal was attributable to extensive seismic ground motions on land areas from the southwestern through to the northeastern part of the island arc as well as regions close to the earthquake epicenter. In many coherent infrasonic signals, an effect of seismic ground motions in sedimentary basins could be identified as a source of infrasound radiation from the large earthquake. These observations and interpretations were confirmed using predictions of possible infrasound arrival azimuth variation by converting real seismological data from the dense Japanese seismic network.

A compound linear discriminant method for small-magnitude seismic events and its application to the North Korea seismic event of October 9, 2006

On October 9, 2006, a seismic event (mb 3.9) occurred in North Korea, which was later announced as a nuclear test. We studied the seismic event from the viewpoint of seismological discrimination without a priori information of the source. This paper presents a compound discriminant method that enhances the ability to distinguish between earthquakes and explosions on the Korean Peninsula. The method includes four single discriminants based on seismic spectral amplitude ratios. These discriminants are used as single variables in a multivariate statistical analysis to derive a compound linear discriminant function that optimally separates the two populations. Short-period vertical component seismograms recorded by seismo-acoustic array (CHNAR) are used for the method. Combining the mutually complementary discrimination abilities of the single methods improves the discriminant power and lowers the probability of misclassification to 1.7% for the data sets. Using this compound method, we were able to identify the North Korea seismic event as explosion-induced. This discrimination technique can be applied automatically to new observations and extended to other seismic arrays or networks. The combination of these seismic discriminants with infrasound observations will increase our ability to detect surface explosions on the Korean Peninsula.