Sung-Joon Chang

Joint Analysis of Teleseismic Receiver Functions and Surface Wave Dispersion using the Genetic Algorithm

Teleseismic P-wave receiver function data and Rayleigh-wave phase velocity measurements are combined using the genetic algorithm, a global optimi- zation technique, to model crustal structure in southern Korea. The two datasets complement each other because receiver functions are sensitive to shear-wave ve- locity contrasts in layered structures, while surface wave dispersion is sensitive to averages of shear-wave velocities. The genetic algorithm is more useful than line- arized inversion in regions where there is little a priori information about local velocity structure because it is not sensitive to the initial model. The stability and variability of resulting crustal model parameters are quantified by using a Monte Carlo technique in specifying a suite of initial models. Depths to the Moho discon- tinuity in southern Korea were estimated to be 29-30 km for stations near the western coast and 33-36 km for inland stations. A well-resolved crustal low-velocity zone was inferred for some stations.

Crustal Structure in Southern Korea from Joint Analysis of Teleseismic Receiver Functions and Surface-Wave Dispersion

We estimated crustal structures under 18 broadband stations in southern Korea by combining receiver functions and surface-wave dispersion with the genetic algorithm (ga). Estimated crustal structures were analyzed together with previously determined structures under four stations (GKP, INCN, SNU, and TJN) in Chang et al. (2004). The trend of Moho depths estimated from the ga inversion generally coincides with the surface topography, ranging from 26 km to 36 km in the inland. However, the Moho depth distribution does not agree with the topography in the region around the Chugaryeong fault, which extends approximately north-northeast–south-southwest in the central Korean Peninsula. The shallow Moho depth under this region may be related to consequential crustal thinning processes along the fault caused by extensional tectonic movements. Another discrepancy is found in the Gyeongsang basin formed in a retroarc setting by the subduction of the Izanagi Plate in the early Cretaceous. A thick crust observed in the basin may be caused by two factors—maturity of the basin and underplating of magma materials. Average crustal velocities vary from 6.02 km/sec to 6.51 km/sec in southern Korea. This variation indicates that crustal structures in southern Korea involve diverse velocity profiles that change rapidly with distance. Remarkably, a clear velocity discontinuity is observed at the depth range of 8–10 km under several stations.

Crustal Structure in Southern Korea from Joint Analysis of Regional Broadband Waveforms and Travel Times

Regional broadband waveforms and travel times of seismic phases are combined to model crustal structure in regions where the signal-to-noise ratio (snr) of waveforms is low. The utilization of travel-time data enables us to overcome the problem occurring in waveform inversion with low snr data: the distortion of deep velocity structure. The genetic algorithm (ga) adopted as a search algorithm is useful in regions where there is little a priori information about crustal structure. After verifying the robustness of the joint inversion technique by numerical tests, we applied the technique to southern Korea in order to get a simple one-dimensional average crustal model, using broadband waveforms bandpassed between 0.03 and 0.3 Hz and travel times of Pg , PmP , and Pn waves recorded at ten broadband and five short-period stations from the 2 June 1999 Gyeongju earthquake ( M L 3.4), Korea. The P -wave velocities in the resulting three-layered crustal model are 5.67 ± 0.06, 6.05 ± 0.05, 6.67 ± 0.02, and 7.88 ± 0.02 km/sec from the top layer to the half- space, respectively, and the depths of layers are 5.1 ± 0.8, 16.7 ± 1.0, and 31.9 ± 1.0 km. Synthetic waveforms and travel-time curves corresponding to the resulting velocity model fit observed seismograms generally well. The synthetics are preceded by the observed waveforms in the southwestern part of Korean Peninsula, inferring the presence of a high-velocity zone in that area.

Joint inversion for three-dimensional S velocity mantle structure along the Tethyan margin

Abstract : For purposes of studying the lateral heterogeneity as well as for ultimately predicting seismograms for this region, we construct a new 3-D S-velocity model by jointly inverting a variety of different seismic data. We jointly invert regional waveforms, surface wave group velocity measurements, teleseismic S arrival times, and crustal thickness estimates from receiver functions, refraction lines, and gravity surveys. These data types have complementary resolving power for crust and mantle structures, vertical and lateral variations, shallow and deep mantle features, local and global structure. Therefore, a joint inversion of these data sets might help unravel the complexity of this tectonically diverse area. These measurements are made from a combination of mantle investigation of the deep suture between Europe and Africa (MIDSEA), Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL), GeoScope, Geofon, Global Seismographic Network (GSN), International Deployment of Accelerometeres (IDA), MedNet, national networks, and local deployments throughout the study region which extends from the western Mediterranean region to the Hindu Kush and encompasses northeastern Africa, the Arabian peninsula, the Middle East, and part of the Atlantic Ocean for reference. We have fitted the waveforms of regional S and Rayleigh waves from over 3800 seismograms using Partitioned Waveform Inversion. We include over 3000 crustal thickness estimates from receiver functions, gravity measurements, and refraction profiles. We include Rayleigh wave group velocities for hundred thousands of paths transecting the region. We have over 3000 teleseismic S arrival times measured through cross correlation and over 170000 from picks originally reported to the International Seismological Centre (ISC).

Joint inversion for global isotropic and radially anisotropic mantle structure including crustal thickness perturbations

We present a new global whole-mantle model of isotropic and radially anisotropic S velocity structure (SGLOBE-rani) based on ~43,000,000 surface wave and ~420,000 body wave travel time measurements, which is expanded in spherical harmonic basis functions up to degree 35. We incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure. This is possible since we utilize short-period group-velocity data with a period range down to 16 s, which are strongly sensitive to the crust. The isotropic S velocity model shares common features with previous global S velocity models and shows excellent consistency with several high-resolution upper mantle models. Our anisotropic model also agrees well with previous regional studies. Anomalous features in our anisotropic model are faster SV velocity anomalies along subduction zones at transition zone depths and faster SH velocity beneath slabs in the lower mantle. The derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts, which has been difficult to constrain due to poor access compared with continental crusts.

Moho Depth and Crustal VP/VS Variation in Southern Korea from Teleseismic Receiver Functions: Implication for Tectonic Affinity between the Korean Peninsula and China

We estimated Moho depths and V P / V S ratios of the crust under 21 broadband stations in southern Korea by using a grid search in the crustal thickness– V P / V S ratio ( H-κ ) domain. The Moho depth varies from 25.9 km to 32.5 km, and the V P / V S ratio ranges from 1.71 to 1.82 inland. Moho depths in the southernmost area of the Korean Peninsula were estimated shallower than those of the previous results obtained assuming a Poisson solid in the joint analysis of receiver functions and surface-wave dispersion. This southernmost area is roughly in accord with the Yeongnam massif, where relatively high V P / V S ratios of 1.78–1.82 are estimated. On the contrary, comparatively low V P / V S ratio measurements (1.71–1.76) are generally distributed in the Gyeonggi massif, which is located in the central area of the Korean Peninsula. The major factor for the high V P / V S ratios in the Yeongnam massif is thought to be the plagioclase-rich mafic composition of the lower crust rather than partial melting or crustal fluids, because high crustal S -wave velocities are reported in the Yeongnam massif. The mafic composition might have been supplied by the magmatic underplating. From the clearly divided feature of V P / V S ratios in southern Korea and the V P / V S ratio similarities between southern Korea and China, it seems that the Yeongnam massif might be related to the Sino-Korea craton, whereas the Gyeonggi massif is related to the Yangtze craton.

Upper- and mid-mantle interaction between the Samoan plume and the Tonga–Kermadec slabs

Mantle plumes are thought to play a key role in transferring heat from the core–mantle boundary to the lithosphere, where it can significantly influence plate tectonics. On impinging on the lithosphere at spreading ridges or in intra-plate settings, mantle plumes may generate hotspots, large igneous provinces and hence considerable dynamic topography. However, the active role of mantle plumes on subducting slabs remains poorly understood. Here we show that the stagnation at 660 km and fastest trench retreat of the Tonga slab in Southwestern Pacific are consistent with an interaction with the Samoan plume and the Hikurangi plateau. Our findings are based on comparisons between 3D anisotropic tomography images and 3D petrological-thermo-mechanical models, which self-consistently explain several unique features of the Fiji–Tonga region. We identify four possible slip systems of bridgmanite in the lower mantle that reconcile the observed seismic anisotropy beneath the Tonga slab (VSH>VSV) with thermo-mechanical calculations.

Receiver function imaging of the mantle transition zone beneath the South China Block

Upper mantle discontinuities are influenced by convection-related thermal heterogeneities arising in complex geodynamic settings. Slab rollback of the Pacific plate and mantle upwelling in the Meso-Cenozoic caused the extension and spreading of continental segments in the South China Block leading to profound variations of the local temperature conditions. We processed 201 teleseismic events beneath 87 stations in the Hainan, Guangdong, and Fujian provinces in the South China Block, and extracted 4172 high-quality receiver functions. We imaged the topography of the local mantle discontinuities by using phase-weighted common conversion point stacking of the receiver functions, which effectively improves the P-to-S-converted phases. We found that the average depths of the discontinuities at 410 and 660 km depth are 414 and 657 km, respectively, while no clearly defined discontinuity at 520 km depth was detected. We mapped the thickness of the mantle transition zone (MTZ), which can reflect temperature and/or compositional heterogeneities as well as the presence of water, and discussed possible geodynamic implications. In particular, we found that the MTZ beneath the Leizhou Peninsula in the Hainan province is 42 km thinner than average. This scenario suggests that the Hainan plume is responsible for positive temperature anomalies between ∼270 and 380 K and between ∼200 and 240 K at the 660 and 410 km discontinuities, respectively. We also observed a prominent uplifting of the 660 km boundary beneath the coast regions that may be indicative of lateral flow of the Hainan plume.

3D SH-wave Velocity Structure of East Asia using Love-Wave Tomography and Implication on Radial Anisotropy

We present a 3D SH-wave velocity model of the crust and uppermost mantle and seismic radial anisotropy beneath East Asia. The SH-wave velocity structure model was built using Love-wave group-velocity dispersion data from earthquake data recorded at broadband seismic networks of Korea, Japan, and China. Love-wave group-velocity dispersion curves were obtained by using the multiple filtering technique in the period range of 3 to 150 s for 3,369 event-station pairs. The inverted model using these data sets provides a crust and upper mantle SH-wave velocity structure down to 100 km depth. At 10 ~ 40 km depths SH-wave velocity beneath the East Sea is higher than beneath the Japanese island region. We estimated the Moho beneath the East Sea to be between 10 ~ 20 km depth, while Moho beneath the Korean Peninsula at around 35 km based on the depth where high-velocity anomalies are detected. We estimated the lithosphereasthenosphere boundary beneath the East Sea to be at around 50 km based on the depth where strong low-velocity anomalies are observed. Widespread low-velocity anomalies are found between 50 ~ 100 km depth in the study region. Positive radial anisotropy (VSV > VSH) is observed down to 35 km depth, while negative radial anisotropy (VSV > VSH) is observed for deeper depth.

S-wave Relative Travel Time Tomography for East Asia

We performed seismic imaging based on relative S-wave travel times to examine S-wave velocity of upper mantle structure beneath East Asia. We used teleseismic events recorded at 129 broadband stations of the Korea Institute of Geoscience and Mineral Resources (KIGAM), Korea Meteorological Administration (KMA), and National Research Institute for Earth Science and Disaster Prevention (NIED). Relative travel time residuals were obtained by a multi-channel cross-correlation method designed to automatically determine accurate relative phase arrival times. The resulting images show high-velocity anomalies along plate boundaries around the Japanese islands region. These anomalies may indicate subducting Pacific and Philippine Sea plates. On the other hand, a low-velocity anomaly is revealed beneath east of the Korean peninsula down to around 300 km depth, which is thought to be related to the formation of the Ulleung basin and the Ulleung island. Low-velocity anomalies revealed beneath the Jeju island may imply that the formation and volcanism of the Jeju island have been caused by magmatic sources from the deep mantle.