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A stochastic method called the random vibration theory (Boore, 1983) has been used to estimate the peak ground motions caused by shallow moderate-to-large earthquakes in the Taiwan area. Adopting Brune’s ω-square source spectrum, attenuation models for PGA and PGV were derived from path-dependent parameters which were empirically modeled from about one thousand accelerograms recorded at reference sites mostly located in a mountain area and which have been recognized as rock sites without soil amplification. Consequently, the predicted horizontal peak ground motions at the reference sites, are generally comparable to these observed. A total number of 11,915 accelerograms recorded from 735 free-field stations of the Taiwan Strong Motion Network (TSMN) were used to estimate the site factors by taking the motions from the predictive models as references. Results from soil sites reveal site amplification factors of approximately 2.0 ~ 3.5 for PGA and about 1.3 ~ 2.6 for PGV. Finally, as a result of amplitude corrections with those empirical site factors, about 75% of analyzed earthquakes are well constrained in ground motion predictions, having average misfits ranging from 0.30 to 0.50. In addition, two simple indices, R0.57 and R0.38, are proposed in this study to evaluate the validity of intensity map prediction for public information reports. The average percentages of qualified stations for peak acceleration residuals less than R0.57 and R0.38 can reach 75% and 54%, respectively, for most earthquakes. Such a performance would be good enough to produce a faithful intensity map for a moderate scenario event in the Taiwan region.

Attenuation of seismic wave energy and its spatial distributions in Taiwan area have been investigated using a large amount of short-period seismograms recorded from the local earthquakes with magnitude ranging from 2.5 to 4.5. The codaQvalues were estimated in the frequency range 1.5∼18 Hz, applying the single backscattering model proposed by Aki and Chouet in 1975. The estimated average frequency dependent codaQvalues within the crust give the relationship,QC= 93f0.75, while varying from 143±33 at 1.5 Hz to 904±164 at 18 Hz central frequencies. However, using the data from deeper earthquakes, the average codaQrelationship is given asQC= 122f0.71in related to the material property of the crust and upper lithosphere, in which the average codaQvalues vary from 180±67 at 1.5 Hz to 1000±144 at 18 Hz. The comparison of different lapse times starting at double the time of the primaryS-wave from the origin time and corresponding to the datasets with different focal depths, reveals that codaQvalues increase with the coda generating volume. As a whole, the regressive relationships of frequency dependent codaQfor Taiwan region approximately agree with those by previous investigators. In addition, the average codaQvalues for three subregions of Taiwan were calculated and correlated with the geology and tectonic features, respectively. The lowestQvalues were obtained in the region consisting of the Coastal Range and the Longitudinal Valley of eastern Taiwan, whilst the highestQfor the region underlain by the Peikang High beneath the western Coastal Plain. Such distinguishable difference has definitely explained that the seismic waves should exhibit lower decay rate in a stable region than within a seismoactive plate boundary.

Strong ground motions in the period range of 1 to 5 sec observed in Taiwan in a dense network during the Tapu earthquake (ML = 5.8) of 15 December 1993 were dominated by fundamental-mode Rayleigh waves. The shallow crustal structure beneath this network was determined from the group velocity dispersion data using standard inversion techniques. Results show that an alluvial layer with a thickness of only about 160 m exists over the sedimentary structure in the western coastal plain. A clear lateral variation in the shear-wave velocity along a cross section perpendicular to the western structural grain (Ho, 1982) was resolved. For the purpose of retrieving the ground motions to confirm the proposed model, forward modeling with a two-dimensional finite-element method was used and agreed well with the observed seismograms when slightly lower velocities than those of the inversions were used. It is concluded that the velocity model estimated using group velocity data cannot definitively correspond to the real one at such short distances and in such a complex structure. On the basis of quantitative simulations, a very soft surface layer must be crucial in the interpretation of the slow wave trains with long duration and the rare prograde particle motions. The results suggest that these surface waves may be generated by the conversion of body waves at the boundary of the western coastal plain and foothills.

In engineering applications, the development of attenuation relationships in a seismic hazard analysis is a useful way to plan for earthquake hazard mitigation. However, finding an optimal solution is difficult using traditional mathematical methods because of the nonlinearity of many relationships. Furthermore, using unweighted regression analysis in which each recording carries an equal weight is often problematic because of the non-uniform distribution of the data with respect to distance. In this study, the least squares method (LSM) and a genetic algorithm (GA) were employed as optimization methods for an attenuation model to compare the robustness and prediction accuracy of the two methods. Different (equal and unequal) weights of each recording were used to compare the adaptability of the weighting for practical application. The unequal weights of each recording were defined as functions of the hypocentral distance or the shortest distance from a station to the fault on the earth’s surface. Finally, regression analysis of horizontal peak ground acceleration (PGA) attenuation model in southwest Taiwan was shown.

The purpose of this paper is to document the climate characteristics ofthe Global Forecast System(GFS),which is an atmospheric general circulationmodel developed at the Central Weather Bureau(CWB),Taiwan.This paper documents the winter(December-February)and summer(June-August)climate characteristics of simulated hydrological processes andatmospheric circulation from a 2-year control simulation conducted withthe GFS using an annually repeating prescribed sea surface temperatureclimatology.In most regards,the climate characteristics of hydrological processesand atmospheric circulation are reproduced reasonably well by the GFSwhen compared to observations and analyses of the atmosphere.As for theclimate characteristics of hydrological processes,the major features of observedprecipitation,such as the Intertropical Convergence Zone(ITCZ),the Asian monsoon regimes,and the extratropical storm tracks,are wellcaptured in the GFS simulation.Similarly,other climate features of observedprecipitation,namely the regions of low precipitation rates over thesubtropical subsidence zones and polar areas,are also well defined by theGFS.The simulated precipitation pattern,however,exhibits some obviousdiscrepancies from the observed in the tropics.Excessive precipitation issimulated by the GFS over some tropical regions where there are complextopographic variations among oceans and lands.Otherwise,the GFS precipitationin the remaining tropical regions is generally underestimated.Inparticular,the underestimate of model precipitation over the tropical easternPacific results in a local ITCZ that is less organized in spatial structurethan the observed.This model precipitation deficiency is linked to underestimatesof precipitable water content and water vapor convergence overthe tropical eastern Pacific in the GFS simulation.Regarding the climate characteristics of the zonal mean state,the zonalmean climatologies of temperature and zonal wind are adequately simulatedby the GFS when compared to analyses.The major difference betweenthe simulated and analyzed zonal mean temperatures is a systematiccold bias in the model troposphere.This cold bias is generally within 4°K ofthe analyses for most of the tropospheric domain bounded by 40°S and40°N.The modal cold bias becomes significant at the polar tropopause,wherethe simulated zonal mean temperature can be from 8°K to 18°K colderthan the analyzed.Also noteworthy is the spatial relationship between thezonal mean temperature bias and zonal mean zonal wind bias.This is foundto be consistent with the spatial relationship between the real temperatureand the real zonal wind fields known as the thermal wind relationship.Thisfinding suggests that interactions between the thermal and dynamic fieldsin the GFS simulation must be to a great extent consistent with analyseswith regard to the thermal wind relationship.Regarding the climate characteristics of atmospheric circulation,theprimary circulation features associated with the summer monsoon systemand winter teleconnection pattern are well represented in the GFS simulationwhen compared with analyses.Nevertheless,in winter,major differencesbetween the analyzed and simulated circulation fields include theunderestimate of the East Asia subtropical jet features and the overestimateof the North America subtropical jet features in the GFS simulation.In the summer simulation,the major circulation bias is that the zonalwavenumber-2 component of the Northern-Hemisphere stationary eddy issimulated with larger amplitude than analyses.This circulation bias is accompaniedby excessive precipitation biases over the subtropical centralNorth Pacific west of the date line and the Central America/Caribbean Searegion.

The IPEI payload onboard ROCSAT-1 has observed many well-knownlow-latitude ionospheric phenomena at 600 km altitude,such as the diurnalvariations of ion density,composition and temperature.The equatorialspread F events,such as bubbles and blobs,together with the flow variationsare also recorded with an unprecedented high time resolution at asampling rate of 1024 Hz.The satellite-borne data on power spectral densityof the equatorial spread F event on the transitional scale(10-100m)ispresented here for the first time.The local-time and longitudinal variationsof the vertical drift velocity in the dawn sector indicated strong hemi-spheric asymmetry.

A new spectral moist convection model that employs both the least assumptionsin moist physics and a very accurate solution method is presented.The temperature and pressure in the model are diagnostically determinedfrom thermodynamics.There is no need to predict water vaporand condensate separately;rather,they are diagnostically separated fromthe predicted total airborne water.The model allows a modular separationof dynamics and thermodynamics;the link between dynamics and thermodynamicsis through the pressure gradient force.The modular separationallows the possibility of having a detailed,fine resolution,nonhydrostaticcloud model and a coarse resolution,hydrostatic model which can be runside by side with the identical moist thermodynamics.The height coordinateof the nonhydrostatic model can also extend into the hydrostatic regime.The only differences between the hydrostatic and nonhydrostaticmodels are spatial resolution and the way vertical motion is computed.Wehave performed numerical experiments in the nonhydrostatic model foracoustic adjustment and moist convection.The discontinuity in thermodynamicsdue to phase change is modified in the model by the”gradual saturation”technique.

A three-dimensional,time-dependent,MHD model of solar-disturbance-caused storms(Wu,1993,Wu et al.,1996)is used to predict the turningdirection of the interplanetary magnetic field(IMF)at Earth.More explicitly,the authors examine the polarity of B(subscript z)caused by solar disturbances on theSun.A specific solar disturbance,the solar flare,is used in this paper.Thedata set used is a subset of that used by Smith and Dryer(1995)and is basedon observations of coronal shocks as seen in metric type Ⅱ observations.These observations are associated with both source flares and geomagneticstorms which serve as markers of shock arrival at Earth.The Omni IMFdata are used for comparison with the generalized simulation results of the3D model.Eight events are studied in this paper.The results of six of themare consistent with the prediction model.