Lian-Feng Zhao

Regional Seismic Characteristics of the 9 October 2006 North Korean Nuclear Test

We investigate the regional seismic signature of the 9 October 2006 North Korean nuclear test. Broadband regional data for the nuclear test and a group of earthquakes close to the test site were obtained between December 2000 and No- vember 2006. Epicentral distances from the stations to the test site are between 371 and 1153 km. We first use these regional events to calibrate the Lg-wave magnitude in the network. Then the network is used to calculate mbLg �� 3:93 for the North Ko- rean nuclear explosion. Using a modified fully coupled magnitude-yield relation, the yield of the North Korean nuclear test is estimated to be 0.48 kt. Because of large uncertainties in the source depth, the estimate is preliminary. The P=S-type spectral ratios Pg=Lg, Pn=Lg, and Pn=Sn are calculated for the nuclear explosion and a group of earthquakes close to the test site. At frequencies above 2 Hz, the network-averaged P=S spectral ratios clearly separate the 9 October 2006 explosion from the regional earthquakes. Our result indicates that a single-blast explosion in the North Korea re- gion shows different seismic characteristics from an earthquake. Any well-coupled single-blast explosion detonated in this region with yield similar to that for the North Korean nuclear test has a large probability of being identified by a regional seismic network such as the one adopted in this study.

Lateral Variation in the Sedimentary Structure of West Bohai Bay Basin Inferred from P-Multiple Receiver Functions

The widely distributed Cenozoic sediments in the Bohai Bay Basin give rise to noticeable modification of broadband teleseismic P waveforms. At one station in the west Bohai Bay Basin, the observed amplitude of tangential P-receiver func- tions is significantly above the noise level, and sedimentary reverberations (e.g., the P-type wave PpPp) remain one of the most prominent features in both the radial and tangential components. To investigate the lateral heterogeneity structure under this site, a 3D raytracing technique is used to compute the teleseismic P-wave response, and a fast simulated annealing algorithm is applied to the simultaneous inversion of radial receiver functions for different backazimuths. An upper crustal structure con- sisting of shallow dipping sedimentary layers with low seismic velocities and large Poissons ratios is proposed to interpret the observed seismic data. The west-dipping interfaces we obtained are consistent with the north-northeast-south-southwest sur- face geology in North China, and Tertiary extensional fault structures may be re- sponsible for the formation of dipping sedimentary layers.

Pn wave geometrical spreading and attenuation in Northeast China and the Korean Peninsula constrained by observations from North Korean nuclear explosions

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.

The 9 September 2016 North Korean Underground Nuclear Test

We characterize the seismic events that occurred in North Korea on 9 September 2016 and South Korea on 12 September 2016. The 9 September 2016 event was identified as an explosion, and the two 12 September 2016 events were identified as natural earthquakes using the P/S (P-and S-wave) spectral ratios, Pg/Lg, Pn/Lg, and Pn/Sn as discriminants. The explosive event was relocated within the North Korean nuclear test site using a relative location method and the 2006 North Korea underground nuclear test as the master event, and the epicenter was identified at 41.2976 degrees N latitude and 129.0804 degrees E longitude. From the regional Lg and Rayleigh waves, the body-and surface-wave magnitudes for the 9 September 2016 event were calculated as m(b) (Lg) = 4: 8 +/- 0: 2 and M-s = 4: 2 +/- 0: 1. By adopting an empirical magnitude-yield relation for the body-wave magnitude, and assuming that the explosion was fully coupled and detonated at a normally scaled depth, we estimated that the seismic yield was similar to 6 kt, and the uncertainty range was between 3 and 11 kt. If an overburied depth range between 780 and 1200 m was applied, then the yield would be increased to 16-22 kt.

Restoration of clipped seismic waveforms using projection onto convex sets method

The seismic waveforms would be clipped when the amplitude exceeds the upper-limit dynamic range of seismometer. Clipped waveforms are typically assumed not useful and seldom used in waveform-based research. Here, we assume the clipped components of the waveform share the same frequency content with the un-clipped components. We leverage this similarity to convert clipped waveforms to true waveforms by iteratively reconstructing the frequency spectrum using the projection onto convex sets method. Using artificially clipped data we find that statistically the restoration error is ~1% and ~5% when clipped at 70% and 40% peak amplitude, respectively. We verify our method using real data recorded at co-located seismometers that have different gain controls, one set to record large amplitudes on scale and the other set to record low amplitudes on scale. Using our restoration method we recover 87 out of 93 clipped broadband records from the 2013 Mw6.6 Lushan earthquake. Estimating that we recover 20 clipped waveforms for each M5.0+ earthquake, so for the ~1,500 M5.0+ events that occur each year we could restore ~30,000 clipped waveforms each year, which would greatly enhance useable waveform data archives. These restored waveform data would also improve the azimuthal station coverage and spatial footprint.

High‐Precision Relocation and Event Discrimination for the 3 September 2017 Underground Nuclear Explosion and Subsequent Seismic Events at the North Korean Test Site

On 3 September 2017, a strong seismic event occurred at the North Korean nuclear test site near the border between China and North Korea. Using P/S-type spectral ratios calculated from regional seismic data, we identified this event as an explosion. Using a high-precision relative location method, with the first North Korean nuclear test as the master event, we obtained that the epicenter of this explosion was at 41.3018° N and 129.0696° E with a relative location precision of ∼87 m based on the error ellipse. This explosion was followed by three moderate seismic events, which occurred 8 min after the explosion, on 23 September and 12 October 2017, respectively. The relocation suggests that the first postexplosion event occurred under the same mountain as the nuclear tests, but the other two events were located about 8 km to the northeast of the explosion. Based on their spectral ratios, these postexplosion events are clearly distinguished from the explosions. Electronic Supplement: Table of Pn differential travel times at individual stations and figures of Pn waveforms recorded at YNB station and compared among North Korean test site (NKTS) events and spectral ratios from individual stations and events.

Lateral Variation of Crustal Lg Attenuation in Eastern North America

We perform QLg 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 QLg 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 QLg 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.

Seismic characteristics of the 15 February 2013 bolide explosion in Chelyabinsk, Russia

The seismological characteristics of the 15 February 2013 Chelyabinsk bolide explosion are investigated based on seismograms recorded at 50 stations with epicentral distances ranging from 229 to 4324 km. By using 8–25 s vertical‐component Rayleigh waveforms, we obtain a surface‐wave magnitude of 4.17±0.31 for this event. According to the relationship among the Rayleigh‐wave magnitude, burst height and explosive yield, the explosion yield is estimated to be 686 kt. Using a single‐force source to fit the observed Rayleigh waveforms, we obtain a single force of 1.03×1012 N, which is equivalent to the impact from the shock wave generated by the bolide explosion.

Low temperature hydrogen embrittlement behavior of Zr-4 alloy

The mechanical properties and hydrogen embrittlement behaviors (HE) of Zr-4 alloy in the temperature range of 40-4.2 K have been investigated. The hydrogen free Zr-4 alloy sample exhibits a very good tensile strength and superior ductility as high as a 21% increase at liquid hydrogen temperature (20 K) over that at room temperature. It also shows the typical features that appeared in austenitic stainless steel, such as unstable plastic flow, strengthening and multi-necking behaviors. The difference is that the temperature of these behaviors occurred much lower than those in austenitic stainless steel. The experiment on hydrogen charged sample shows a heavy hydrogen embrittlement behavior at low temperatures. The hydride cracking mechanism of HE has been discussed in the paper.