Wen-Tzong Liang

Astronomically calibrated ages for geomagnetic reversals within the Matuyama chron

We present a magnetostratigraphic record from the western Philippine Sea that is tied to a marine δ18O record for the past 2.14 million years. The ages of geomagnetic reversals were astronomically calibrated by tuning the oxygen isotopic stratigraphy, yielding a chronology for the following subchrons: Matuyama/Brunhes boundary, 781 ± 3 ka (slightly above δ18O Stage 19.3); top of the Santa Rosa polarity interval, 920 ± 2 ka (Stage 23/24); base of the Santa Rosa polarity interval, 925 ± 1 ka (Stage 24); top of the Jaramillo subchron, 988 ± 3 ka (Stage 27); base of the Jaramillo subchron, 1072 ± 2 ka (Stage 31); top of the Cobb Mountain subchron, 1173 ± 4 ka (Stage 35/36); base of the Cobb Mountain subchron, 1185 ± 5 ka (Stage 36); top of the Olduvai subchron, 1778 ± 3 ka (Stage 63/64); base of the Olduvai subchron, 1945 ± 4 ka (Stage 71/72); top of the Réunion II subchron, 2118 ± 3 ka (Stage 80/81); and base of the Réunion II subchron, 2133 ± 5 ka (Stage 81). This astronomically calibrated chronology independently confirms the ages of major reversals in recently published astronomically calibrated polarity timescales for the Matuyama chron. It also provides the first astronomically calibrated dates for the lower and upper reversals associated with the Cobb Mountain and Santa Rosa polarity intervals, respectively.

Multiresolution parameterization for geophysical inverse problems

A model parameterization based on a multiresolution wavelet representation is proposed for geophysical inverse problems in this study. The matrix equation for the wavelet representation of the model is constructed by dual wavelet transforms of a readily built matrix based on the pixel parameterization. We demonstrate the merits of the proposed parameterization using the simple examples of geophysical data gridding and 3D seismic tomography, and comparing the results with those obtained by the conventional pixel parameterization. It is shown that for the conventional damped least-squares solutions of pixel-parameterized models, regularization by norm damping deteriorates the underlying model correlation due to sparse and heterogeneous sampling, whereas roughness damping schemes impose a priori arbitrary correlation length irrespective of the heterogeneous sampling. The scale hierarchy embedded in the proposed multiresolution parameterization facilitates the compromise between the dual spatial and scale resolution. Depending on the local richness of the data constraints across different scales at a site, model variation of various scale spectra can be robustly resolved through the scale hierarchy. There is thus no need to invoke additional smoothness regularization.

Linkages between turbidites in the southern Okinawa Trough and submarine earthquakes

[1] Turbidite layers in surficial ( 6.8) submarine earthquakes recorded in the region since the 20th century. The repeating pattern of turbidite layers reported here on decadal to centennial time scales suggests what may be the long-term rhythm of seismic activities at this tectonically active plate boundary. INDEX TERMS: 0932 Exploration Geophysics: Radioactivity methods; 1035 Geochemistry: Geochronology; 3022 Marine Geology and Geophysics: Marine sediments—processes and transport; 3025 Marine Geology and Geophysics: Marine seismics (0935); 7221 Seismology: Paleoseismology. Citation: Huh, C.-A., C.-C. Su, W.-T. Liang, and C.-Y. Ling (2004), Linkages between turbidites in the southern Okinawa Trough and submarine earthquakes, Geophys. Res. Lett., 31, L12304, doi:10.1029/2004GL019731.

Testing the hypothesis of orbital (eccentricity) influence on Earth's magnetic field

Abstract We test a recent suggestion that orbital eccentricity has influenced the geomagnetic field by performing a range of spectral analyses, including estimation of statistical confidence limits, on a 2.2-million-year paleomagnetic inclination record from the western Caroline Basin (WCB). Our analyses indicate that the claimed ∼100-kyr signal, while present over discrete portions of the WCB paleomagnetic record, is not statistically significant for the entire record and is not modulated by the 404-kyr eccentricity component as would be expected if the signal was influenced by orbital eccentricity. This signal also has highly variable phase compared to orbital eccentricity variations and the paleomagnetic record shows no statistically significant coherency with the orbital signal. Our results from a nearby coeval western Philippine Sea record support the present analysis. Together, these data suggest that the hypothesis of orbital energization of the geomagnetic field has yet to be convincingly demonstrated.

Shear wave anisotropy beneath the Aegean inferred from SKS splitting observations

[1] SKS splitting parameters are measured in the Aegean region using events recorded at a dense temporary network in the south Aegean and the operating permanent networks, especially focusing in the back‐arc and the near‐trench areas of the Hellenic arc. In general, fast anisotropy directions are trench perpendicular in the back‐arc area and trench parallel near the trench. Anisotropy measurements near the volcanic arc mark the transition between these two regions. In the back arc, a gradual increase is observed in delay times from south to north, with a prevailing NE‐SW direction. In Cyclades, this pattern is correlated with GPS velocities and stretching lineations of metamorphic core complexes. Our preferred source of anisotropy in the back‐arc region is the mantle wedge flow, induced by the retreating descending slab. The westernmost termination of the trench reveals directions parallel with the Kefalonia Transform Fault and perpendicular to the convergence boundary. Beneath Peloponnese, the trench‐parallel flow is probably located beneath the shallow‐dipping slab, although scattered measurements may also reflect fossil anisotropy from a past NW‐SE strike of the trench. In western Crete, which may be entering a stage of continental collision, the anisotropy pattern changes to trench perpendicular, with a possible subslab source. Good nulls in central east Crete indicate a change in the anisotropy origin toward the east. At the easternmost side of the trench, fast directions are trench parallel. This reflects a similar subslab flow that may become toroidal around the slab edge beneath western Turkey. This may also produce a trench‐parallel flow within the mantle wedge.

Anomalous Pn Waves Observed in Eastern Taiwan: Implications of a Thin Crust and Elevated Oceanic Upper Mantle beneath the Active Collision-Zone Suture

Normal Pn waves are commonly observed in Taiwan from shallow regional earthquakes at epicentral distances larger than 120 km, similar to the observations in many other continental regions. However, the critical distances to observe Pn waves for shallow eastern Taiwan earthquakes vary with azimuth corresponding to a significant variation of crustal thickness. In particular, anomalous Pn waves are commonly observed for shallow eastern Taiwan earthquakes recorded on seismic stations at epicentral distances as small as 60 km along the collision zone suture, the Longitudinal Valley. For the same event, normal Pn waves are observed at other seismic stations elsewhere on the island. The apparent velocity of the anomalous and normal Pn waves from the same event is 7.8 ± 0.15 km/sec, which is consistent with the average Pn velocity in the Taiwan area. Thus, the unusually short critical distance for Pn waves in eastern Taiwan suggests that the crust beneath the collision zone suture must be very thin and the upper mantle beneath the Longitudinal Valley must be relatively elevated compared with that beneath the other parts of Taiwan. Assuming a simple 1D layered velocity model, the Moho depth beneath the suture zone can thus be estimated at ∼23 ± 2 km. This observation is consistent with the recent report from a high-resolution 3D tomographic inversion that a narrowly confined, anomalously elevated, and north-northeast–south-southwest elongated oceanic upper mantle was imaged beneath the Longitudinal Valley from Hualien in the north to Taitung in the south (Kim et al. , 2005, 2006). Furthermore, the preceding observations may also support the interpretation that the conduction of excess heat supply from the elevated hot oceanic upper mantle into the adjacent mid- to-lower continental crust over a long period of geological time may play an important role in the crustal deformation beneath the continent, including metamorphism, thickening, and uplifting.

Characteristics of short period secondary microseisms (SPSM) in Taiwan: The influence of shallow ocean strait on SPSM

Taking advantage of a unique opportunity provided by a dense array of coastal short-period seismic stations and the diverse bathymetry around Taiwan, we examine how the long-range coherent ambient noises are influenced by surrounding ocean settings using the cross-correlation functions (CCFs) between pairs of stations. The effective energy of the CCFs derived from three components of short-period seismometer data falls within the frequency range of the short period secondary microseism (SPSM). The spatial variations mapped from the amplitude asymmetry of CCFs and source migration images evidently demonstrate that the SPSM strengths are closely linked to the drastic changes in offshore ocean characteristics and result in much stronger SPSM in the shallow and narrow Taiwan Strait than in deep open seas of eastern Taiwan. The temporal variations of the CCF strengths exhibit very good correlations with the wind speeds and wave heights, explicitly indicating the observed SPSM is dominated by local sources generated from wind-driven ocean waves around offshore Taiwan.

Joint Passive/Controlled Source Seismic Experiment Across Taiwan

Scientists who study Earth structures and interactions at tectonic plate boundaries illuminate the three-dimensional (3-D) subsurface primarily through passive or active source seismic studies. In passive source studies, natural earthquakes are observed through arrays of seismic stations, allowing seismologists to image structures at depth and infer physical properties of the Earth. In active (controlled) source studies, scientists provide an artificial seismic source to be recorded by seismic networks, allowing more detailed views of fine-scale structures buried within plate boundaries. The use of “continuous” modes on large-storage-capacity seismic recording systems enables seismologists to blur traditional lines between active and passive source seismology in regions of high earthquake activity. In addition to generating seismograms from controlled sources, a deployed dense array can record local and teleseismic earthquakes with exceptionally coherent signals across its aperture. As a result of such joint recording, the scientific value of the collected data increases. Moreover, whether or not the seismic sources are natural or engineered becomes less of a factor for data storage, sorting, and analysis methods. Local earthquakes can supplement controlled sources, multichannel processing and imaging methods traditionally used in active seismic studies become amenable to dense sets of passive source data, and active source data with known source origin times and locations are easily incorporated into regional seismic 3-D tomography and earthquake location studies.

Present‐day crustal stress field in Greece inferred from regional‐scale damped inversion of earthquake focal mechanisms

In this study we utilize regional and teleseismic earthquake moment tensor solutions in order to infer the contemporary crustal stress in the Greek region. We focus on crustal earthquakes and select only solutions with good waveform fits and well-resolved nodal planes. A data set of 1614 focal mechanisms is used as input to a regional-scale damped stress inversion algorithm over a grid whose node spacing is 0.35°. Several resolution and sensitivity tests are performed in order to ascertain the robustness of our results. Our findings show that for most of the Greek region the largest principal stress σ1 is vertically oriented and that the minimum principal stress axis σ3 are subhorizontal with a predominant N-S orientation. In the SW Peloponnese the orientation of σ3 axes rotates clockwise and in SE Aegean counterclockwise. These results are in agreement with the generally accepted model that slab rollback combined with gravitational spreading of the Aegean lithosphere are the main causes of the extension. Transitions between different faulting types in NW Greece or in the Aegean occur within narrow zones in the order of tens of kilometers. A visual comparison of the principal horizontal stress axes and the principal strain axes derived from GPS observations shows good agreement, suggesting that the crust in the Greek region behaves largely in an elastic manner.

Multiple Diving Waves and Steep Velocity Gradients in the Western Taiwan Coastal Plain: An Investigation Based on the TAIGER Experiment

Seismic data collected during explosion experiments performed as part of the TAiwan Integrated GEodynamics Research (TAIGER) project provide an excellent opportunity to obtain high‐resolution images of the structure of the crust and upper mantle beneath Taiwan. The most significant feature observed at near‐source stations located on the western coastal plain in Taiwan is high‐energy later arrivals. These high‐amplitude multiples almost completely mask the lower‐amplitude signals (seismic refraction and wide‐angle reflection) from the deep crust. The later arrivals are identified as free‐surface‐reflected multiples. The nature and generation of these high‐energy, multiple diving waves are demonstrated using synthetic examples. Their generation requires the presence of a steep velocity gradient in the shallow crust. A detailed analysis of the observation data provided information on the velocity gradients in this region. An accurate layer‐velocity model, including the boundary orientation and its depth, and velocity gradient, was constructed based on a 1D waveform simulation and 2D seismic raytracing modeling for travel times. The present results indicate that the thick sediment in the survey area dips shallowly to the east, has a surface P ‐wave velocity of ![Graphic][1] , and an average velocity gradient of about 0.72/s from the surface to 3.0‐km depth. The thick sediment of the 2D model shows lateral variations in velocity gradient, increasing from west to east. This velocity model may provide useful information for future data processing to reduce multiple diving waves with the aim of enhancing the deep‐surface refraction/reflection signal. The velocity gradient calculated for the thick sediment of the western coastal plain may require a revision of the regional seismic velocity model developed for southwestern Taiwan, to improve the accuracy of regional hypocenter determinations, and to predict the strong ground motions produced by large earthquakes beneath this region. [1]: /embed/inline-graphic-1.gif