Junkee Rhie

Excitation of Earth's continuous free oscillations by atmosphere–ocean–seafloor coupling

The Earth undergoes continuous oscillations, and free oscillation peaks have been consistently identified in seismic records in the frequency range 2–7 mHz (refs 1, 2), on days without significant earthquakes. The level of daily excitation of this ‘hum’ is equivalent to that of magnitude 5.75 to 6.0 earthquakes, which cannot be explained by summing the contributions of small earthquakes. As slow or silent earthquakes have been ruled out as a source for the hum (except in a few isolated cases), turbulent motions in the atmosphere or processes in the oceans have been invoked as the excitation mechanism. We have developed an array-based method to detect and locate sources of the excitation of the hum. Our results demonstrate that the Earths hum originates mainly in the northern Pacific Ocean during Northern Hemisphere winter, and in the Southern oceans during Southern Hemisphere winter. We conclude that the Earths hum is generated by the interaction between atmosphere, ocean and sea floor, probably through the conversion of storm energy to oceanic infragravity waves that interact with seafloor topography.

A study of the relation between ocean storms and the Earth's hum

We previously showed that the Earths “hum” is generated primarily in the northern oceans during the northern hemisphere winter and in the southern oceans during the summer. To gain further insight into the process that converts ocean storm energy into elastic energy through coupling of ocean waves with the seafloor, we here investigate a 4-day-long time window in the year 2000 that is free of large earthquakes but contains two large “hum” events. From a comparison of the time functions of two events and their relative arrival times at the two arrays in California and Japan, we infer that the generation of the “hum” events occurs close to shore and comprises three elements: (1) short-period ocean waves interact nonlinearly to produce infragravity waves as the storm reaches the coast of North America; (2) infragravity waves interact with the seafloor locally to generate long-period Rayleigh waves; and (3) some free infragravity wave energy radiates out into the open ocean, propagates across the north Pacific basin, and couples to the seafloor when it reaches distant coasts northeast of Japan. We also compare the yearly fluctuations in the amplitudes observed on the two arrays in the low-frequency “hum” band (specifically at 240 s) and in the microseismic band (2–25 s). During the winter, strong correlation between the amplitude fluctuations in the “hum” and microseismic bands at BDSN is consistent with a common generation mechanism of both types of seismic noise from nonlinear interaction of ocean waves near the west coast of North America.

Slip of the 2004 Sumatra-Andaman Earthquake from Joint Inversion of Long-Period Global Seismic Waveforms and GPS Static Offsets

The 26 December 2004 Great Sumatra-Andaman earthquake opened a new era for seismologists to understand the complex source process of a great earth- quake. This is the first event with moment magnitude greater than 9 since the de- ployment of high-dynamic-range broadband seismic and Global Positioning System (GPS) sensors around the globe. This study presents an analysis of the ruptured fault- plane geometry and slip distribution using long-period teleseismic data and GPS- measured static surface displacements near the fault plane. We employ a rupture geometry with six along-strike segments with and without a steeper down-dip ex- tension. The fault segments are further subdivided into a total of 201 � 30 30 km fault patches. Sensitivity tests of fault-plane geometry and the variation in rupture velocity indicate that the dip and curvature of the fault plane are not well resolved from the given data set and the rupture velocity is constrained to be between 1.8 and 2.6 km/sec. Error estimations of the slip distribution using a random selection of seismic and GPS station subsets (50% of all stations) illustrate that slip is well re- solved along the whole rupture and the mean slip uncertainty is less than 1.5 m (about 11%). Although it is possible that near-field GPS data include contributions from additional postseismic transient deformation, our preferred model suggests that the Sumatra-Andaman earthquake had a magnitude of Mw 9.20 0.05/0.06. Online material: Comparison of slip models, GPS modeling, waveform fit, fault geometry, and inversion parameters.

The 12 September 2016 Gyeongju earthquakes: 1. Observation and remaining questions

Two earthquakes (ML 5.1 and 5.8) ruptured branches of the Yangsan Fault System in Gyeongju, S. Korea on September 12, 2016. After the ML 5.8 earthquake, aftershock earthquakes continued to occur, including two notable earthquakes (ML 4.3 and 4.5) on September 12 and 19, 2016. This paper details the early reports of the Yangsan Fault System in the Gyeongsang Basin from various geological and geophysical/seismological perspectives. Based on a review and an initial seismological analysis of the results of the three earthquakes (ML 5.1, 5.8, and 4.5), we present and discuss the following topics: (1) the tectonic setting and the geophysical/seismic structure of the Yangsan Fault System, (2) historical seismicity and inferred seismic hazard from historical (literature) data, and (3) source mechanisms of the three earthquakes. In the end, we highlight some of the outstanding issues with regard to earthquakes and future research topics.

Regionally heterogeneous uppermost inner core observed with Hi-net array

Studies of velocity structure in the Earths inner core and its volumetric variations illuminate our understanding of inner core dynamics and composition. Here we use an extensive number of seismograms recorded by the Hi-net array to construct complete empirical curves of PKPBC-PKPDF differential traveltimes. The nature of these curves implies that significant variations in traveltimes are accumulated during the passage of PKPDF waves through the uppermost inner core and rules out outer core structure effects. Uniform cylindrical anisotropy of a plausible strength in the uppermost inner core can also be ruled out as a cause of the observed traveltime variations because the range of sampled ray angles is too narrow. The configuration and strength of inhomogeneities from a recent tomographic model of the lowermost mantle cannot account for the observed traveltime variations. Therefore, we infer that either variations of P wave isotropic velocity on the scale of about hundred km and less are present in the uppermost inner core or the material may be organized in distinctive anisotropic domains, and both of these features may be superimposed on long-wavelength hemispherical structure. If the former holds true, the absolute magnitude of required P wave velocity perturbations from referent values is 0.60 ± 0.10% in the quasi-eastern and 1.55 ± 0.15% in the quasi-western hemisphere (0.85 ± 0.05% and 1.10 ± 0.10%, respectively, with the lowermost mantle correction). The existence of these variations is a plausible physical outcome given that vigorous compositional convection in the outer core and variations in heat exchange across the inner core boundary may control the process of solidification.

The 12 September 2016 Gyeongju earthquakes: 2. Temporary seismic network for monitoring aftershocks

The ML 5.8 earthquake in Gyeongju, southeastern Korea, on September 12, 2016 11:32:54 (UTC) was the largest earthquake on the Korean Peninsula since instrumental monitoring began in 1978. It was preceded by an ML 5.1 foreshock and is being followed by numerous aftershocks. Within an hour of the mainshock, the first temporary seismic station to monitor aftershocks was installed at about 1.5 km east of the announced epicenter. The current temporary seismic network consists of 27 stations equipped with broadband sensors covering an area of ∼38 × 32 km in the mainshock region. This is the first high-density aftershock monitoring array in the Korean Peninsula. Initial results, using data from both the regional seismic networks and the aftershock monitoring array, indicate that earthquakes during the first 10 days following the mainshock are related to the Yangsan Fault System. Establishment of an official rapid-response team to monitor aftershocks of major earthquakes is advised.

Topography of the 410 and 660 km discontinuities beneath the Korean Peninsula and southwestern Japan using teleseismic receiver functions

Topography of the 410 and 660 km seismic upper mantle discontinuities beneath the Korean Peninsula and southwestern Japan were determined using teleseismic receiver functions. P receiver functions were migrated from delayed times to corresponding piercing (conversion) points of P-to-S converted phases, using one-dimensional (1-D) and three-dimensional (3-D) models. Receiver functions were then stacked using Common Conversion Point (CCP) techniques, to enhance signal-to-noise ratios and thereby reduce uncertainty (noise). The 410 and 660 km discontinuities were clearly imaged, as positively valued amplitude peaks of CCP stacked receiver functions in the study area. Topographic variations were roughly consistent with the low temperature of the subducting Pacific Plate. However, the complex structure of the subducting Pacific Plate produced distinct changes of upper mantle discontinuities, which cannot be explained by temperature variations alone. Depression of the 410 km discontinuity, observed in a wide region extending from the Korean Peninsula to Kyushu Island, may be related to trench rollback history. Furthermore, the topography of the 660 km discontinuity varies significantly with latitude. At latitudes higher than 38°N, its depth remains unchanged, despite the presence of the stagnant slab, while significant depression has been observed at latitudes below 36°N. This may have been caused by differences in the angles of subduction of the Japan slab and the Izu-Bonin slab. However, heterogeneity of the water content of slabs may also have contributed to this topographical difference.

Ambient seismic noise tomography of the southern East Sea (Japan Sea) and the Korea Strait

Group velocity maps were derived for the southern East Sea (Japan Sea) and the Korea Strait (Tsushima Strait) for the 5–36 s period range, which is sensitive to shear wave velocities of the crust and the uppermost mantle. Images produced in our study enhance our understanding of the tectonic evolution of a continental margin affected by subducting oceanic slabs and a colliding continental plate. The seismic structure of the study area has not been described well because seismic data for the region are scarce. In this study, we applied the ambient noise tomography technique that does not rely on earthquake data. We calculated ambient noise cross-correlations recorded at station pairs of dense seismic networks located in the regions surrounding the study area, such as the southern Korean Peninsula and southwestern part of the Japanese Islands. We then measured the group velocity dispersion curves of the fundamental mode Rayleigh waves from cross-correlograms and constructed 2-D group velocity maps reflecting group velocity structure from the upper crust to uppermost mantle. The results show that three distinct anomalies with different characteristics exist. Anomalies are located under the Ulleung Basin (UB), the boundary of the Basin, and the area between Tsushima Island and the UB. 1-D velocity models were obtained by inversion of dispersion curves that represent vertical variations of shear wave velocity at locations of three different anomalies. The 1-D velocity models and 2-D group velocity maps of lateral variations in shear wave group velocities show that the high velocity anomaly beneath the UB originates from crustal thinning and mantle uplift. Confirming the exact causes of two low velocity anomalies observed under the UB boundary and between Tsushima Island and the UB is difficult because additional information is unavailable. However, complex fault systems, small basins formed by faulting, and deep mantle flow can be possible causes of the existence of low velocity anomalies in the region.

Intraplate volcanism controlled by back-arc and continental structures in NE Asia inferred from transdimensional Bayesian ambient noise tomography

Intraplate volcanism adjacent to active continental margins is not simply explained by plate tectonics or plume interaction. Recent volcanoes in northeast (NE) Asia, including NE China and the Korean Peninsula, are characterized by heterogeneous tectonic structures and geochemical compositions. Here we apply a transdimensional Bayesian tomography to estimate high-resolution images of group and phase velocity variations (with periods between 8 and 70 s). The method provides robust estimations of velocity maps, and the reliability of results is tested through carefully designed synthetic recovery experiments. Our maps reveal two sublithospheric low-velocity anomalies that connect back-arc regions (in Japan and Ryukyu Trench) with current margins of continental lithosphere where the volcanoes are distributed. Combined with evidences from previous geochemical and geophysical studies, we argue that the volcanoes are related to the low-velocity structures associated with back-arc processes and preexisting continental lithosphere.

Coda-Derived Source Parameters of Earthquakes and Their Scaling Relationships in the Korean Peninsula

We applied the coda-derived source spectrum method of Mayeda et al. (2003) to earthquakes in and around the Korean peninsula. After empirical calibra- tions, we derived source spectra of the earthquakes. From the coda-derived spectra, we estimated valuable source parameters such as the seismic moment, corner frequency, and radiated energy for small events with Mw <3:5. We derived simple linear relation- ships between the coda spectral amplitudes and local magnitudes reported from the Korea Meteorological Administration and Korea Institute of Geoscience and Mineral Resources. These relationships can be used to estimate stable local magnitudes for future earthquakes using a small number of stations. To investigate whether the earth- quakes occurring in this region obey self-similarity, we examined the scaling relation- ships between dynamic and static source parameters, such as the corner frequency, radiated energy, and scaled energy versus seismic moment. The scaling relationship between the corner frequency and seismic moment showed clear nonself-similarity with a scaling parameter of 0.54; this value is more or less consistent with previous results for different regions. Scaling relations of radiated energy and scaled energy versus the seismic moment also show size-dependent behavior that cannot be explained by self-similarity; this result implies that the rupture dynamics of small and large earthquakes are different in this region. Our observation provides further evidence supporting the nonself-similarity of earthquakes.