Airy Phase Research Articles

A measurement system for shear speed using interface wave dispersion.

Our recent work has highlighted the effect of shear on the dispersion of acoustic normal modes. Recently, Pierce et al. showed the effect of shear on compressional wave attenuation and its frequency dependence. Potty and Miller [(2010)] showed that sediment shear speed can significantly impact compressional modal arrival times near the Airy phase. We have observed that neglecting shear in compressional wave speed inversions results in a bias in our long range sediment tomography inversions. All of these factors emphasize the importance of estimating shear wave speeds in semiconsolidated shallow water sediments. One of the most promising approaches to estimate shear speed is to invert the shear speed profile using the dispersion of seismoacoustic interface waves (Scholte waves) that travel along the sediment-water boundary. The propagation speed and attenuation of the Scholte wave are closely related to shear-wave speed and attenuation over a depth of one to two wavelengths into the seabed. A shear measurement system being developed at the University of Rhode Island based on this concept will be presented. Test data collected on land will be shown and preliminary analysis techniques will be discussed. [Work supported by the Office of Naval Research.]

The origin of resonance in the vertical component of earthquakes recorded on soft soil at Wainuiomata, New Zealand

Occasionally-observed resonances in the vertical components of earthquakes recorded at the Wainuiomata, New Zealand, soft site, are likely to be manifestations of the Airy phase of fundamental-mode Rayleigh waves which traverse the site. These packets of waves exist only when a soft, water-saturated layer of soil overlies a substrate with a much higher velocity. Other soft sites in Wellington also show the phenomenon, which may have implications for hazard estimates.

Deep rock damage in the San Andreas Fault revealed by P- and S-type fault-zone-guided waves

Abstract Damage to fault-zone rocks during fault slip results in the formation of a channel of low seismic-wave velocities. Within such channels guided seismic waves, denoted by F g , can propagate. Here we show with core samples, well logs and F g -waves that such a channel is crossed by the SAFOD (San Andreas Fault Observatory at Depth) borehole at a depth of 2.7 km near Parkfield, California, USA. This laterally extensive channel extends downwards to at least half way through the seismogenic crust, more than about 7 km. The channel supports not only the previously recognized Love-type- (F L ) and Rayleigh-type- (F R ) guided waves, but also a new fault-guided wave, which we name F Φ . As recorded 2.7 km underground, F Φ is normally dispersed, ends in an Airy phase, and arrives between the P - and S - waves. Modelling shows that F Φ travels as a leaky mode within the core of the fault zone. Combined with the drill core samples, well logs and the two other types of guided waves, F Φ at SAFOD reveals a zone of profound, deep, rock damage. Originating from damage accumulated over the recent history of fault movement, we suggest it is maintained either by fracturing near the slip surface of earthquakes, such as the 1857 Fort Tejon M 7.9, or is an unexplained part of the fault-creep process known to be active at this site.

Studies on the effect of shear on compressional wave dispersion.

The results of a study to examine the effect of shear on compressional wave dispersion are presented. Modal arrival time data from different shallow water sites will be analyzed to examine the influence of shear. The modal travel times corresponding to the Airy Phase regions are extremely sensitive to shear. Simple inversion schemes will be developed to estimate the shear speed in the sediment by comparing theoretical predictions with experimental data. The estimated shear speeds will also be compared with shear speeds calculated from core data. The effect of shear on compressional wave attenuation will also be investigated. Synthetic data will be generated for elastic bottom, with different shear speeds, and these data will be inverted for compressional wave attenuation. One of the most promising approaches to estimate shear speed is to invert the relation between seismo‐acoustic interface waves (Scholte waves) that travel along boundaries between media and shear wave speed. The propagation speed and attenuation of the Scholte wave are closely related to shear‐wave speed and attenuation over a depth of 1‐2 wavelengths into the seabed. A shear measurement system being developed at the University of Rhode Island based on this concept will be presented. [Work supported by the Office of Naval Research.]

On Possible Plume-Guided Seismic Waves

Abstract Hypothetical thermal plumes in the Earth’s mantle are expected to have low seismic-wave speeds and thus would support the propagation of guided elastic waves analogous to fault-zone guided seismic waves, fiber-optic waves, and acoustic waves in the oceanic SOund Fixing And Ranging channel. Plume-guided waves would be insensitive to geometric complexities in the wave guide, and their dispersion would make them distinctive on seismograms and would provide information about wave-guide structure that would complement seismic tomography. Detecting such waves would constitute strong evidence of a new kind for the existence of plumes. A cylindrical channel embedded in an infinite medium supports two classes of axially symmetric elastic-wave modes, torsional and longitudinal-radial. Torsional modes have rectilinear particle motion tangent to the cylinder surface. Longitudinal-radial modes have elliptical particle motion in planes that include the cylinder axis, with retrograde motion near the axis. The direction of elliptical particle motion reverses with distance from the axis: once for the fundamental mode, twice for the first overtone, and so on. Each mode exists only above its cut-off frequency, where the phase and group speeds equal the shear-wave speed in the infinite medium. At high frequencies, both speeds approach the shear-wave speed in the channel. All modes have minima in their group speeds, which produce Airy phases on seismograms. For shear wave–speed contrasts of a few percent, thought to be realistic for thermal plumes in the Earth, the largest signals are inversely dispersed and have dominant frequencies of about 0.1–1 Hz and durations of 15–30 sec. There are at least two possible sources of observable plume waves: (1) the intersection of mantle plumes with high-amplitude core-phase caustics in the deep mantle; and (2) ScS -like reflection at the core-mantle boundary of downward-propagating guided waves. The widespread recent deployment of broadband seismometers makes searching for these waves possible.

Effects of Love Waves on Microtremor H/V Ratio

Abstract The horizontal-to-vertical (H/V) method has the potential to significantly contribute to site effects evaluation, in particular in urban areas. Within the European project, site effects assessment using ambient excitations (SESAME), we investigated the nature of ambient seismic noise in order to assess the reliability of this method. Through 1D seismic noise modeling, we simulated ambient noise for a set of various horizontally stratified structures by computing efficiently the displacement and stress of dynamic Green’s functions for a viscoelastic-layered half-space. We performed array analysis using the conventional semblance-based frequence-wavenumber method and the three-component modified spatial autocorrelation method on both vertical and horizontal components and estimated the contribution of different seismic waves (body/surface waves, Rayleigh/Love waves) at the H/V peak frequency. We show that the very common assumption that almost all the ambient noise energy would be carried by fundamental-mode Rayleigh waves is not justified. The relative proportion of different wave types depends on site conditions, and especially on the impedance contrast. For the 1D horizontally layered structures presented here, the H/V peak frequency always provides a good estimate of the fundamental resonance frequency whatever the H/V peak origin (Rayleigh wave ellipticity, Airy phase of Love waves, S -wave resonance). We also infer that the relative proportion of Love waves in ambient noise controls the amplitude of the H/V peak.

Calibrating infrasonic to seismic coupling using the Stardust sample return capsule shockwave: Implications for seismic observations of meteors

Shock waves produced by meteoroids are detectable by seismograph networks, but a lack of calibration has limited quantitative analysis of signal amplitudes. We report colocated seismic and infrasound observations from reentry of NASA's Stardust sample return capsule (SSRC) on 15 January 2006. The velocity of the SSRC (initially 12.5 km/s) was the highest ever for an artificial object, lying near the low end of the 11.2–72 km/s range typical of meteoroids. Our infrasonic/seismic recordings contain an initial N wave produced by the hypersonic shock front, followed ∼10 s later by an enigmatic series of weak, secondary pulses. The seismic signals also include an intervening dispersed wave train with the characteristics of an air‐coupled Rayleigh wave. We determine an acoustic‐seismic coupling coefficient of 7.3 ± 0.2 μm s−1/Pa. This represents an energy admittance of 2.13 ± 0.15%, several orders of magnitude larger than previous estimates derived from earthquake or explosive analogs. Theoretical seismic response was computed using in situ VP and VS measurements, together with laboratory density measurements from samples of the clay‐rich playa. Treatment of the air‐ground interface as an idealized air‐solid contact correctly predicts the initial pulse shape but underestimates its seismic amplitude by a factor of ∼2. Full‐wave synthetic seismograms simulate the air‐coupled Rayleigh wave and suggest that the secondary arrivals are higher‐order Airy phases. Part of the infrasound signal appears to arise from coupling of ground motion into the air, much like earthquake‐induced sounds.

Modelling of Rayleigh-type seam waves in disturbed coal seams and around a coal mine roadway

SUMMARY Wave propagation in coal seams is numerically modelled in order to identify approaches towards the reconnaissance beyond the heading face of an advancing coal mine roadway. Complete synthetic wavefields including P‐SV body waves and Rayleigh-type seam waves are calculated using a Green’s function approach for simple, laterally homogeneous models and a parallel elastic 2-D/3-D finite difference modelling code for more realistic geometries. For a simple three-layer model the wavefield within the seam is dominated by a fundamental Rayleigh seam mode symmetrical with respect to the centre of the seam on the vertical component and antisymmetrical on the horizontal component. If the seam contains an interleaved dirt band with higher velocities and density, higher modes dominate the wave propagation, depending on the thickness of the dirt band. Wave propagation in laterally inhomogeneous coal seam models with disturbances like seam ends, faults, thinning, washouts and seam splitting is strongly influenced by the type of disturbance. Amplitudes of seam waves reflected from these disturbances strongly depend on the fault throw and the degree of thinning or washout. In some cases, conversion to higher modes can occur. In all investigated models, those Rayleigh seam wave phases are preferably reflected, which have frequencies above the fundamental mode Airy phase. Lower frequency phases are preferably transmitted. However, seam waves are not reflected from a seam splitting disturbance. Thus a detection of seam splitting with reflected seam waves appears to be impossible. FD computations for 3-D models containing an ending tunnel parallel to the seam and a source beyond the heading face of the tunnel show that seam waves are converted into Rayleigh waves at the tunnel face. They propagate along the surface of the tunnel and interfere with the seam waves propagating beside the tunnel. This effect has to be taken into account for subsequent treatment of experimental data, where the locations of sources and receivers are restricted to a small seismic layout in the vicinity of the tunnel. As tunnel surface waves have slightly lower frequencies than seam waves, it may be feasible to separate tunnel waves from the seam wave reflections, particularly because higher frequency phases of the seam wave are preferably reflected at seam disturbances. Polarization analysis showed, that the elliptically polarized Rayleigh-type seam waves in the vertical‐radial plane can be distinguished from Rayleigh tunnel waves propagating on the sidewall of the tunnel adjoining the coal layer with elliptical polarization in the radial‐transversal plane.

Geoacoustic inversion using combustive sound source signals

In the summer of 2006, the combustive sound source (CSS) was deployed off the coast of New Jersey during the Shallow Water 2006 experiment (SW-06). CSS generates an oscillating bubble through the combustion of a fuel/oxidizer mixture, which in turn yields a low frequency acoustic pulse [Wilson, Ellzey, and Muir, IEEE J. Ocean. Eng. 20 (1995)]. The depth of these shots was 26 m in water depths of the order of 100 m. Source levels were monitored using a hydrophone placed close to the source. CSS data collected downrange on single hydrophone receiving units (SHRU) are presented. Five SHRUs were deployed by Woods Hole Oceanographic Institution (WHOI) in water depths ranging from 65 to 110 m. The CSS data collected indicate modal dispersion and are used for geoacoustic inversions. These inversions are based on matching the observed and modeled group speed dispersions including Airy phase. Historic sediment data from the location are utilized to constrain the inversions. [Work supported by the Office of Naval Research.]

‘And Somewhere the Dove Rose’: Seamus Heaney y su Fase Aérea en la Crítica Literaria

This article concentrates on the different analyses made by critics regarding the beginning of the so-called airy phase in Seamus Heaney’s oeuvre. In contrast with the generalized critical acceptance of a new airy, visionary, escapist phase, which follows a first earthy, sensorial, more politically committed phase, the main objective of this article is to show the outstanding critical disagreement over the collection/s which introduce/s this poetic shift and, consequently, the fragility of such critical hypothesis. After providing evidence on this discrepancy among critics and claiming our own disagreement with this airy phase, we offer a re-examination of Heaney’s poetic evolution which detaches itself from the binary opposition earth vs. air in favour of emphasizing tension and balance between both of them –as in many other Heaneyian dualities– all throughout his literary career.

‘And Somewhere the Dove Rose’: Seamus Heaney y su Fase Aérea en la Crítica Literaria

This article concentrates on the different analyses made by critics regarding the beginning of the so-called airy phase in Seamus Heaney’s oeuvre. In contrast with the generalized critical acceptance of a new airy, visionary, escapist phase, which follows a first earthy, sensorial, more politically committed phase, the main objective of this article is to show the outstanding critical disagreement over the collection/s which introduce/s this poetic shift and, consequently, the fragility of such critical hypothesis. After providing evidence on this discrepancy among critics and claiming our own disagreement with this airy phase, we offer a re-examination of Heaney’s poetic evolution which detaches itself from the binary opposition earth vs. air in favour of emphasizing tension and balance between both of them –as in many other Heaneyian dualities– all throughout his literary career.

Numerical study of seismic scattering and waveguide excitation in faulted coal seams

ABSTRACTFinite‐difference P‐SV simulations of seismic scattering characteristics of faulted coal‐seam models have been undertaken for near‐surface P‐ and S‐wave sources in an attempt to understand the efficiency of body‐wave to channel‐wave mode conversion and how it depends on the elastic parameters of the structure. The synthetic seismograms clearly show the groups of channel waves generated at the fault: one by the downgoing P‐wave and the other by the downgoing S‐wave. These modes travel horizontally in the seam at velocities less than the S‐wavespeed of the rock. A strong Airy phase is generated for the fundamental mode.The velocity contrast between the coal and the host rock is a more important parameter than the density contrast in controlling the amplitude of the channel waves. The optimal coupling from body‐wave energy to channel‐wave energy occurs at a velocity contrast of 1.5. Strong guided waves are produced by the incident S‐sources for source angles of 75° to 90° (close to the near‐side face of the fault). As the fault throw increases, the amplitude of the channel wave also increases. The presence of a lower‐velocity clay layer within the coal‐seam sequence affects the waveguiding characteristics. The displacement amplitude distribution is shifted more towards the lower‐wavespeed layer. The presence of a ‘washout’ zone or a brecciated zone surrounding the fault also results in greater forward scattering and channel‐wave capture by the coal seam.

Development of a Time-Domain, Variable-Period Surface-Wave Magnitude Measurement Procedure for Application at Regional and Teleseismic Distances, Part I: Theory

A major problem with time-domain measurements of seismic surface waves is the significant effect of nondispersed Rayleigh waves and Airy phases, which can occur at both regional and teleseismic distances. This article derives a time-domain method for measuring surface waves with minimum digital processing by using zero-phase Butterworth filters. The method can effectively measure surface- wave magnitudes at both regional and teleseismic distances, at variable periods between 8 and 25 sec, while ensuring that the magnitudes are corrected to accepted formulae at 20-sec reference periods, thus providing historical continuity. For applications over typical continental crusts, the proposed magnitude equation is, for zero- to-peak measurements in millimicrons: M s(b) = log( a b ) + 1/2 log(sin(Δ)) + 0.0031(20/ T ) 1.8 Δ − 0.66 log (20/ T ) − log( f c ) − 0.43, where: f c ≤ 0.6/ T √Δ. To calculate M s(b) , the following steps should be taken: Determine the epicentral distance in degrees to the event Δ and the period T . Calculate the corner filter frequency f c using the preceding inequality. Filter the time series using a zero-phase, third-order Butterworth bandpass filter with corner frequencies 1/ T − f c , 1/ T + f c . Calculate the maximum amplitude a b of the filtered signal and calculate M s(b) . At the reference period of 20 sec, the equation is equivalent to von Seggern’s formula (1977) scaled to Vanĕk (1962) at 50 degrees. For periods 8 ≤ T ≤ 25, the equation is corrected to T = 20 sec, accounting for source effects, attenuation, and dispersion. Online material: Design and realization of Butterworth filters.

Development of a Time-Domain, Variable-Period Surface-Wave Magnitude Measurement Procedure for Application at Regional and Teleseismic Distances, Part II: Application and Ms-mb Performance

The Russell surface-wave magnitude formula, developed in Part I of this two-part article, and the M_s(VMAX) measurement technique, discussed in this article, provide a new method for estimating variable-period surface-wave magnitudes at regional and teleseismic distances. The M_s(VMAX) measurement method consists of applying Butterworth bandpass filters to data at center periods between 8 and 25 sec. The filters are designed to help remove the effects of nondispersed Airy phases at regional and teleseismic distances. We search for the maximum amplitude in all of the variable-period bands and then use the Russell formula to calculate a surface-wave magnitude. In this companion article, we demonstrate the capabilities of the method by using applications to three different datasets. The first application utilizes a dataset that consists of large earthquakes in the Mediterranean region. The results indicate that the M_s(VMAX) technique provides regional and teleseismic surface-wave magnitude estimates that are in general agreement except for a small distance dependence of −0.002 magnitude units per degree. We also find that the M_s(VMAX) estimates are less than 0.1 magnitude unit different than those from other formulas applied at teleseismic distances such as Rezapour and Pearce (1998) and Vanĕk et al. (1962). In the second and third applications of the method, we demonstrate that measurements of M_s(VMAX) versus m_b provide adequate separation of the explosion and earthquake populations at the Nevada and Lop Nor Test Sites. At the Nevada Test Site, our technique resulted in the misclassification of two earthquakes in the explosion population. We also determined that the new technique reduces the scatter in the magnitude estimates by 25% when compared with our previous studies using a calibrated regional magnitude formula. For the Lop Nor Test Site, we had no misclassified explosions or earthquakes; however, the data were less comprehensive. A preliminary analysis of Eurasian earthquake and explosion data suggest that similar slopes are obtained for observed M_s(VMAX) versus m_b data with m_b <5. Thus the data are not converging at lower magnitudes. These results suggest that the discrimination of explosions from earthquakes can be achieved at lower magnitudes using the Russell (2006) formula and the M_s(VMAX) measurement technique.

An Estimate of the Bottom Compressional Wave Speed Profile in the Northeastern South China Sea Using “Sources of Opportunity”

The inversion of a broad-band "source of opportunity" signal for bottom geoacoustic parameters in the northeastern South China Sea (SCS) is presented, which supplements the towed source and chirp sonar bottom inversions that were performed as part of the Asian Seas International Acoustics Experiment (ASIAEX). This source of opportunity was most likely a "dynamite fishing" signal, which has sufficient low-frequency content (5-500 Hz) to make it complimentary to the somewhat higher frequency J-15-3 towed source (50-260 Hz) signals and the much higher frequency (1-10 kHz) chirp signals. This low frequency content will penetrate deeper into the bottom, thus extending the other inverse results. Localization of the source is discussed, using both a horizontal array for azimuthal steering and the "water wave" part of the pulse arrival for distance estimation. A linear broad-band inverse is performed, and three new variants of the broad-band inverse, based on: 1) the Airy phase; 2) the cutoff frequency; and 3) a range-dependent medium are presented. A multilayer model of the bottom compressional wave speed is obtained, and error estimates for this model are shown, both for the range-independent approximation to the waveguide and for the range-dependent waveguide. Directions for future research are discussed.

The Basin-Edge Effect from Weak Ground Motions Across the Fault-Bounded Edge of the Lower Hutt Valley, New Zealand

Basin-edge effects are investigated in the Lower Hutt Valley, Wellington, New Zealand, using recordings of weak-motion earthquakes. Twelve seismographs were deployed in a closely spaced array along the fault-bounded edge of the valley. Peak ground motions appear to be locally amplified at 2.0-2.5 Hz in the edge-parallel component at around 116-172 m from the fault and in the edge-normal component more than 295 m from the fault. Group velocity observations made of edge-generated Love waves show the fundamental-mode Airy phase frequency at 2.0-2.5 Hz. This frequency corresponds to the fundamental resonance mode of the Holocene sediments as well as the third-mode resonance of the whole basin. Large differential particle motions are observed at neighboring stations less than 50 m apart. The most plausible explanation to account for these observations is constructive interference between edge-generated surface waves and the delayed direct arrival, commonly referred to as the basin-edge effect. Manuscript received 23 October 2001.

Propagation of a ground‐penetrating radar (GPR) pulse in a thin‐surface waveguide

Field observations are tested against modal propagation theory to find the practical limitations upon derivation of layer permittivities and signal attenuation rates from a radar moveout profile over two‐layer ground. A 65‐MHz GPR pulse was transmitted into a 30‐60‐cm‐thick surface waveguide of wet, organic silty to gravelly soil overlying a drier refracting layer of sand and gravel. Reflection profiles, trench stratigraphy, resistivity measurements, and sediment analysis were used to quantify the propagation medium and possible attenuation mechanisms. Highly dispersive modal propagation occurred within the waveguide through 35 m of observation. The fastest phase velocity occurred at the waveguide cutoff frequency of 30 MHz, which was well received by 100‐MHz antennas. This speed provides the refractive index of the lower layer, so the near‐cutoff frequencies must match a lower layer refraction. A slower, lower frequency phase of the dispersed pulse occurred at about 60–70 MHz, with an average attenuation rate of about 0.4 dB/m. Similar events appear to have reflected back and forth along the waveguide. Modal theory for the average layer thickness shows all primary events to be different aspects of a TE1 mode, predicts the correct 30–70‐MHz phase speeds and low‐frequency cutoff phenomenon, but also predicts that the 60–70‐MHz group speed should be slightly lower than observed. An Airy phase was apparently out of the bandwidth. Two‐dimensional finite‐difference time‐domain modeling qualitatively simulates the main field results. After accounting for an inverse dependency of amplitude on the square of the range, the high resistivity of the surface layer accounts for the 0.4‐dB/m attenuation rate for the 60–70‐MHz phase of the pulse. However, erratic amplitudes, interface roughness, and the reflected packets indicate scattering. We conclude that permittivities can be well estimated from dispersive moveout profiles given an average surface layer thickness, and the wide bandwidth of GPR antennas allows the full dispersion to be seen. Attenuation rates appear to be derivable from the higher frequency part of our dispersive event, for which attenuation might be least affected by the waveguide dispersion.

A Numerical Investigation of Lg Geometrical Spreading

This study is a comprehensive numerical investigation of the L g-wave geometrical spreading for vertically inhomogeneous media. I modeled a suite of source and path parameters and measured different L g amplitudes including root-mean-square (rms), peak-to-peak, third-peak (Nuttli, 1980; Patton, 2001), envelope-peak, and spectral amplitudes for analysis. The main result of this investigation is that the estimation of L g spreading rates from the rms amplitudes and from the spectral amplitudes yielded the most reliable estimates that are basically independent of almost all the source and path variables within the parameter ranges that I simulated. I obtained a spreading rate of Δ-1.0 for the rms amplitude and a rate of Δ-0.5 for the spectral amplitude across the frequency band of the data. The relationship between the two spreading-rate estimates is consistent with Parseval's theorem. Spreading-rate estimates from other amplitude measurements show larger variations with the variation of source and path parameters. These variations suggest that the behavior L g peak amplitudes might be more complex than that of a single-mode Airy phase. The decay of the third-peak amplitudes appears to be less rapid than the decay of the peak-to-peak and envelope-peak amplitudes. There are no systematic changes in the spreading rates due to the variation of variables such as velocity model, source depth, and source mechanism. The introduction of a gradient zone at the crust-mantle boundary and the earth-flattening transformation did not change the spreading rates significantly, at least not beyond certain source-receiver distances. The most important variable affecting the L g spreading appears to be the closeness of the source to major velocity discontinuities in the velocity models. Manuscript received 24 January 2001.

Sediment tomography in the East China Sea: Compressional wave speed and attenuation inversions from Airy phase dispersion measurements and time series correlation

This paper discusses ongoing data analysis results from the acoustic bottom interaction experiment conducted in May–June 2001 in the East China Sea as part of the Asian Seas International Acoustics Experiment (ASIAEX-2001). Using time-frequency scalograms of broadband signals, the modal arrivals and group speed minimums (Airy Phase) of several modes are clearly observed. The structure of the Airy Phase signal is used to match the dispersion curves which forms the basis of this inversion technique. Utilizing the Airy Phase group speed minimums and corresponding pressure amplitudes of each observable mode, the sediment compressional wave speed and attenuation as a function of depth are derived. The group speed minimum for each mode provides additional information on the compressional wave speed in the modal sediment depth penetration interval. To refine the sediment parameters, synthetic and measured time series are correlated for goodness of fit and used in the inversion process. The synthetic time series is generated from the scalogram values corresponding to the times and frequencies of the calculated dispersion curves. Inverted speeds and estimated modal penetration depths are then used to develop the sediment profile. The estimated resolution is dependent on the number and frequency span of the observable modes. Estimated sediment properties from several areas are presented with verification from coring results. [Work supported by ONR.]

Surface wave excitation in the western coastal plain of Taiwan during the 1999 Chi‐Chi earthquake

During the 1999 Chi‐Chi Taiwan Earthquake, long‐period surface waves were excited in the western coastal plain of middle Taiwan. The seismic signals were well recorded by the Central Weather Bureau's strong motion seismometers. The surface wave period was between 2 and 10 seconds. Although there was no report on direct damage caused by surface waves, they were still very dangerous for structures like long‐period bridges and elevated railroad systems built for high‐speed bullet trains. In this study, we focus on simulating the factors that control the generation of long period surface waves in the western plain of middle Taiwan. The numerical method used is the pseudo‐spectral method. We use two criteria to justify the simulation results. The first criterion is the permanent displacement around the surface outcrop produced by a rupture fault. The second criterion is to match the Airy phase or the energy trains. We tested several parameters to see the effects on generating surface wave signals. The tested parameters are the rupture velocity, fault dip angle, and whether the fault rupture arrives at the ground surface or not. In this study, we reach three conclusions. First of all, if a medium‐sized earthquake occurs in the foothills of central Taiwan, long‐period surface waves can be generated in the western coastal plain only if the fault rupture has reached the top low velocity sediments or the ground surface. Second, rupture velocity will strongly control the duration of surface waves. Third, the excitation of surface waves is mostly controlled by the near surface fault rupture; the deeper part of the rupture will only affect body wave signals.