Wu-Cheng Chi

Finite-Source Modeling of the 1999 Taiwan (Chi-Chi) Earthquake Derived from a Dense Strong-Motion Network

The 1999 Chi-Chi earthquake (M W 7.6) (20 September 1999, 17:47:15.9 UTC) (located at 23.853 N, 120.816 E, and depth of 7.5 km) inicted severe re- gional scale damage to Taiwan. The strong-motion waveeld was captured by a dense network of stations (with average station spacing of 5 km), which represents the most complete strong-motion dataset to date to use to study the kinematic source process of an earthquake. We inverted velocity waveforms recorded by 21 stations for the spatial variation in slip on a planar fault model composed of 416 subfaults, each with a dimension of 3.5 km by 3.5 km. The planar model has a strike of N5 E and a dip of 30 E, and the inversion solves for the direction and magnitude of the slip. To account for possible temporal source complexity we allowed each subfault to slip within 10 overlapping time windows, each with a duration of 3 sec. The results show that the source is composed primarily of three major asperities, the rst of which is mainly dip slip, extending from the hypocenter to the northern end of the surface rupture. In this asperity, slip occurred in two pulses separated in time by 5 sec. The dislocation rise time for each pulse is short (3n4 sec), yielding an approximate av- erage slip velocity of 80 cm/sec. The second asperity is located at shallow depth near the northern end of the rupture where very large ground velocities were observed. This asperity is on average oblique and shows a temporal rake rotation from pure dip-slip to strike-slip. The rotating rake suggests a low initial shear-stress on the northern end of the fault. Slip in this asperity is dominated by a large pulse with a dislocation rise time of 8 sec. A station near the northern end of the surface rupture recorded a peak velocity of 390 cm/sec, which we nd to be due to the constructive interference of energy radiated from the rst two asperities. The third asperity is located south of the epicenter. The total moment from the three asperities is 4.1 10 27 dyne cm, which was released over a period of 30n35 sec within an area of 900 km 2 . Synthetics calculated from the three-asperity model explain 85% of the data and represent 98% of the total variance reduction. Our results indicate that slip is conned to the shallow region of the fault, deep slip patches are less constrained, and that the slip distribution may be representative of fault segmentation along the Chelungpu fault system.

Evolution of shallow, crustal thermal structure from subduction to collision: An example from Taiwan

We study crustal thermal evolution by examining heat flow patterns along a convergent boundary from a young subduction zone to a more structurally mature collision zone. More than 8000 km of seismic profiles covering an offshore region of 45,000 km 2 in southern Taiwan show widespread bottom-simulating reflectors (BSRs). We derived 1107 BSR-based heat flows before combining 42 additional, published, offshore thermal probe data and 86 on-land heat flow data to document the shallow forearc thermal structures from the subduction zone to the collision zone. In the subduction zone, the geothermal gradient ranges mostly within 30–80 °C/km, and decreases toward the arc due to slab cooling, intensive dewatering at the toe, sediment blanketing, topographic effects, and other processes. The geothermal gradient ranges mostly from 30 to 90 °C/km in the collision zone, and increases, instead of decreases, toward the arc, possibly caused by exhumation, erosion, topographically induced ground-water circulation, and some upper mantle processes related to collision. Heat flow in the collision zone ranges from 80 to 250 mW/m 2 . The high heat flow in the collision zone correlates with a shallower seismicity zone and high seismic attenuation, while the lower heat flow in the subduction zone might allow the earthquakes to rupture to greater depth. The heat flow increases along the topographic high from subduction to collision zone due to increasing geothermal gradients and higher thermal conductivities of the exhumed basement rocks. This heat flow variation may generate an artificial exhumation rate pattern, if a conventional 30 °C/km geothermal gradient was used in fission-track studies.

Three‐dimensional crustal structure in central Taiwan from gravity inversion with a parallel genetic algorithm

The genetic algorithm method is combined with the finite-element method for the first time as an alternative method to invert gravity anomaly data for reconstructing the 3D density structure in the subsurface. The method provides a global search in the model space for all acceptable models. The computational efficiency is significantly improved by storing the coefficient matrix and using it in all forward calculations, then by dividing the region of interest into many subregions and applying parallel processing to the subregions. Central Taiwan, a geologically complex region, is used as an example to demonstrate the utility of the method. A crustal block 120£ 150 km 2 in area and 34 km in thickness is represented by a finiteelement model of 76 500 cubic elements, each 2£ 2£ 2 km 3 in size. An initial density model is reconstructed from the regional 3D tomographic seismic velocity using an empirical relation between velocity and density. The difference between the calculated and the observed gravity anomaly (i.e., the residual anomaly) shows an elongated minimum of large magnitude that extends along the axis of the Taiwan mountain belt. Among the interpretive models tested, the best model shows a crustal root extending to depths of 50 to 60 km beneath the axis of the Western Central and Eastern Central Ranges with a density contrast of 400 or 500 kg/m 3 across the Moho. Both predictions appear to be supported by independent seismological and laboratory evidence.

Rifting under steam—How rift magmatism triggers methane venting from sedimentary basins

During opening of a new ocean magma intrudes into the surrounding sedimentary basins. Heat provided by the intrusions matures the host rock creating metamorphic aureoles potentially releasing large amounts of hydrocarbons. These hydrocarbons may migrate to the seafloor in hydrothermal vent complexes in sufficient volumes to trigger global warming, e.g. during the Paleocene Eocene Thermal Maximum (PETM). Mound structures at the top of buried hydrothermal vent complexes observed in seismic data off Norway were previously interpreted as mud volcanoes and the amount of released hydrocarbon was estimated based on this interpretation. Here, we present new geophysical and geochemical data from the Gulf of California suggesting that such mound structures could in fact be edifices constructed by the growth of black-smoker type chimneys rather than mud volcanoes. We have evidence for two buried and one active hydrothermal vent system outside the rift axis. The vent releases several hundred degrees Celsius hot fluids containing abundant methane, mid-ocean-ridge-basalt (MORB)-type helium, and precipitating solids up to 300 m high into the water column. Our observations challenge the idea that methane is emitted slowly from rift-related vents. The association of large amounts of methane with hydrothermal fluids that enter the water column at high pressure and temperature provides an efficient mechanism to transport hydrocarbons into the water column and atmosphere, lending support to the hypothesis that rapid climate change such as during the PETM can be triggered by magmatic intrusions into organic-rich sedimentary basins.

Fault-perpendicular aftershock clusters following the 2003 Mw = 5.0 Big Bear, California, earthquake

To explore aftershock-triggering mechanisms for the 2003 Big Bear, California earthquake sequence, we determined differential travel-times and applied the double-difference technique to relocate these events, which formed three clusters. The main cluster coincides with the 3 km long northwest striking sub-vertical mainshock fault. The other two sub-vertical clusters, located at opposite ends of the mainshock rupture, are almost perpendicular to the mainshock fault, contradicting the 60° separation angle of conjugate faults as predicted from frictional laws. Allowing for a 30° uncertainty in the cataloged strike, dip and rake values about 75% of the aftershocks are strike-slip as determined from first motion and complete waveform moment tensor inversions. We use a mainshock conceptual slip model to derive Coulomb Failure Stress regions, and assess correlations between stress increases and aftershock locations. We conclude that the perpendicular aftershock clusters were triggered by the mainshock static stress perturbation.

Fluid venting and seepage at accretionary ridges: the Four Way Closure Ridge offshore SW Taiwan

Within the accretionary prism offshore SW Taiwan, widespread gas hydrate accumulations are postulated to occur based on the presence of a bottom simulating reflection. Methane seepage, however, is also widespread at accretionary ridges offshore SW Taiwan and may indicate a significant loss of methane bypassing the gas hydrate system. Four Way Closure Ridge, located in 1,500 m water depth, is an anticlinal ridge that would constitute an ideal trap for methane and consequently represents a site with good potential for gas hydrate accumulations. The analysis of high-resolution bathymetry, deep-towed sidescan sonar imagery, high-resolution seismic profiling and towed video observations of the seafloor shows that Four Way Closure Ridge is and has been a site of intensive methane seepage. Continuous seepage is mainly evidenced by large accumulations of authigenic carbonate precipitates, which appear to be controlled by the creation of fluid pathways through faulting. Consequently, Four Way Closure Ridge is not a closed system in terms of fluid migration and seepage. A conceptual model of the evolution of gas hydrates and seepage at accretionary ridges suggests that seepage is common and may be a standard feature during the geological development of ridges in accretionary prisms. The observation of seafloor seepage alone is therefore not a reliable indicator of exploitable gas hydrate accumulations at depth.

Coseismic indentor-related deformation during the termination of subduction and its associated geophysical characteristics: An example from Taiwan

Collision is among the important processes for the growth of continents, and the way in which subduction becomes a collision is still an active research topic. Here, I examine the seismogenic structures of southern and central Taiwan where the subduction along the Manila Trench has terminated and given way to an arc-continent collision on land. Based on focal mechanisms and seven finite-fault slip models, coseismic tectonic extrusion is active in this region, in which the basement highs on the incoming passive margin are acting as indentors and strongly modifying the seismic moment release patterns in the collision zone. At least three magnitude 7 earthquakes have ruptured both north and south of an indentor called the Peikang high in the past hundred years. After examination, the basement highs show little global positioning system (GPS)–recorded relative motion with respect to the incoming passive margin; high Bouguer gravity anomalies associated with denser materials of the basement; and low heat flow due to less dewatering and exhumation. With regard to seismogenic structures, faster GPS relative motions, lower Bouguer gravity anomalies, and higher heat flow characterize the regions surrounding the indentors. Similar processes might be operating in other arc-continent collision zones. For other regions where there are fewer seismic instruments to monitor earthquakes, it might be helpful to combine a geological survey with gravity and other geophysical data sets to help identify such potential seismogenic structures.

Electromagnetic and seismic investigation of methane hydrates offshore Taiwan — The Taiflux experiment

We report on an electromagnetic and seismic experiments carried out offshore Taiwan in April 2013 in the quest to quantify methane hydrate saturation (TAIFLUX project). The quantification of hydrates is essential to efficiently and cost effectively plan a drilling campaign and for a resource assessment of methane hydrates as an energy source to Taiwan. The work has been carried out in a collaboration of Taiwanese and German scientists and funding organizations. Preliminary and qualitative processing of seismic and electromagnetic data acquired on the Four-Way-Closure accretionary ridge situated on the active margin south-west of Taiwan shows the presence of hydrates and indication that hydrate concentrations are relatively high. We furthermore show that the derivation of quantitative hydrate concentration models from the data is difficult to achieve using either seismic or electromagnetic data alone. Best resolution and certainties in a hydrate concentration model may be achieved if both type of data sets are inverted together to a common model, since both types of data sets carry complementary information.

Geological controls on the gas hydrate system of Formosa Ridge, South China Sea

Formosa Ridge is one of many topographic ridges created by canyon incision into the eastern South China Sea margin. The northwestern termination of the ridge is caused by beheading of the ridge due to a westward shift of the canyon that originally formed to the eastern flank of Formosa Ridge. Below Formosa Ridge a bottom simulating reflector (BSR) exists. Its depth below sea floor coincides with the theoretical base of the gas hydrate stability zone and the reflection has reverse polarity suggesting that it is caused by free gas below gas hydrate accumulations. The BSR is ubiquitous but shows significant variations in depth below sea floor ranging from 150 ms TWT (or approximately 180 m) underneath the incised canyon in the north to up to 500 ms (or approximately 460 m) underneath the crest of Formosa Ridge. Predominantly this depth variation is the result of topography on subsurface temperature, but comparison with the average BSR depth underneath the surrounding canyons suggests that recent canyon incision in the north has perturbed the thermal state of the sediments. Formosa Ridge consists of a northern half that is dominated by refilled older canyons and a southern half that consists mainly of contourite deposits. However, judging by the reflection seismic data this difference in origin seems to have little effect on the distribution of gas hydrate.

Inversion of ground-motion data from a seismometer array for rotation using a modification of Jaeger's method

Abstract We develop a new way to invert 2D translational waveforms using Jaeger’s (1969) formula to derive rotational ground motions about one axis and estimate the errors in them using techniques from statistical multivariate analysis. This procedure can be used to derive rotational ground motions and strains using arrayed translational data, thus providing an efficient way to calibrate the performance of rotational sensors. This approach does not require a priori information about the noise level of the translational data and elastic properties of the media. This new procedure also provides estimates of the standard deviations of the derived rotations and strains. In this study, we validated this code using synthetic translational waveforms from a seismic array. The results after the inversion of the synthetics for rotations were almost identical with the results derived using a well-tested inversion procedure by Spudich and Fletcher (2009). This new 2D procedure can be applied three times to obtain the full, three-component rotations. Additional modifications can be implemented to the code in the future to study different features of the rotational ground motions and strains induced by the passage of seismic waves.