Large Effects of Moho Reflections (SmS) on Peak Ground ...€¦ · Large Effects of Moho Reflections (SmS) on Peak Ground Motion in Northwestern Taiwan by Kun-Sung Liu and Yi-Ben

Large Effects of Moho Reflections (SmS) on Peak Ground Motion

in Northwestern Taiwan

by Kun-Sung Liu and Yi-Ben Tsai

Abstract A total of 336 three-component strong-motion recordings from theMw 6.35 Niu Dou earthquake of 25 June 1995 at a focal depth of 39.9 km in northernTaiwan are used to study the effects on strong ground motion due to Moho reflectionof S waves. The residuals of both horizontal peak ground acceleration (PGA) and peakground velocity (PGV) recorded from the earthquake are analyzed. The results confirmthat many Class E soft soil stations in the Taipei Basin and the Ilan Plain had theexpected large amplification of about 1.7 and 1.5 times, respectively, the predictedmedian PGA values. Surprisingly, a large group of Class C or D dense and stiffsoil sites in Taoyuan (TCU007), Lungtan (TCU013), Guanshi (TCU021), Hsinchu(TCU095), and Miaoli (TCU047) areas in northwestern Taiwan had unusually largeamplification of about 3.4–8.1 and 1.7–3.3 times the predicted median PGA and PGVvalues, respectively. They are interpreted in terms of focusing and interference be-tween SmS waves reflected from the horizontal and inclined portions of an east-dipping Moho discontinuity in this area. This interpretation is supported by the closeagreement between the expected amplitudes and arrival times of the largest shearwaves with the observed data. Our results suggest that when a damaging earthquakeoccurs near an inclined Moho boundary, the reflected SmS waves can result in sig-nificantly amplified ground motions at distances beginning about 50 km. The exactdistance range will depend on the thickness of the crust and the dip angle of the Mohoboundary.

Introduction

Ground-motion characteristics are known to depend onthe properties of seismic source, wave propagation, and siteresponse. The most commonly used method for characteriz-ing ground motion is through attenuation relations, in whichthe effects of earthquake source, wave propagation, and siteresponse are typically parameterized by the earthquake mag-nitude, fault type, source-to-site distance, and site condi-tion. Conversely, we can study the contribution of source,path, and site effects by examining the residuals of observedground-motion data at individual sites with reference to thevalues predicted by the empirical attenuation relations.

The path effects on strong ground motion due to crustalstructures have been known for some time. Burger et al.(1987) investigated the attenuation relations of eastern NorthAmerica, which show amplitudes in the distance range of60–150 km to be higher than that at smaller and greater dis-tances. They showed that the observed interval of relativelyhigh amplitudes can be attributed to postcritically reflectedS waves from the Moho discontinuity. The presence and lo-cation of the interval of relatively high amplitudes is highlydependent on the crustal velocity structures and may there-fore be expected to show regional variations.

Somerville et al. (1990) found significant influence ofcritical reflections from the lower crust on ground-motionattenuation from the large set of strong-motion recordingsof the Saguenay, Quebec, earthquake of 25 October 1988.At distances beyond 64 km, the peak ground motions oc-curred at times corresponding not to the direct S wave butto strong critical reflections from the lower crust. The ampli-tude of recorded ground motions did not significantly de-crease between 50 and 120 km, but it abruptly decreasedbeyond 120 km.

In central California, Bakun and Joyner (1984) sug-gested that the large positive residuals inML at distances be-tween 75 and 125 km could be due to Moho reflections.Large-amplitude reflections from the Moho (PmP) were alsoobserved in seismic refraction data near the source region ofthe Loma Prieta earthquake (Walter and Mooney, 1982).Somerville and Yoshimura (1990) presented evidence of en-hanced amplitudes of strong ground motion from the LomaPrieta earthquake recorded in the San Francisco and Oaklandareas, which were found to exceed the levels predicted bystandard empirical attenuation relations. Their analysis of ac-celerograms with known trigger times strongly suggests that

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Bulletin of the Seismological Society of America, Vol. 99, No. 1, pp. 255–267, February 2009, doi: 10.1785/0120080258

enhancement of ground-motion amplitudes in the distancerange of approximately 40–100 km was due to critical reflec-tions from the base of the crust. The effect of critical reflec-tions in amplifying peak accelerations of the Loma Prietaearthquake in the San Francisco and Oakland regions wasas large as the amplification effect of soft soil site conditions.

McGarr et al. (1991) presented observations in the epi-central distance range of 59–95 km, including the San Fran-cisco International Airport, of the Moho-reflected phasesPmP and SmS from the aftershocks of the Loma Prietaearthquake to support the hypothesis that the phase SmS ac-counted much of the enhanced peak ground motion experi-enced from the mainshock throughout most of the SanFrancisco Bay area.

Mori and Helmberger (1996) used closely spaced after-shock data from the 28 June 1992 Landers earthquake to cre-ate event record sections that showed clear examples ofvarying amplitudes of SmS. Some of the data showed strongSmS phases to be two to five times larger than the direct Swaves. They interpreted the amplitude variations in terms ofthe local crustal and Moho structures in southern California.

Yeh et al. (1988) demonstrated, on the basis of an as-sumed velocity structure, that focusing of seismic-wave en-ergy in the Taipei Basin can occur for some earthquakesunder favorable conditions. The wave focusing phenomenawere also supported by other strong-motion data collected bythe accelerograph network in Taiwan.

Several later studies have shown additional evidencethat reflected seismic energy can significantly intensify theground shaking at an appreciable distance from the epicenterof an earthquake (Atkinson and Boore, 1995; Catchingsand Kohler, 1996; Atkinson and Boore, 1997; Somervilleet al., 1997; Toro et al., 1997; Chen, 2003). Thus, focusingof seismic-wave energy can play an important role in enhanc-ing seismic intensity even in regions some distance awayfrom the epicentral zone. It follows that it may be importantto consider such path effects in engineering practice. The ob-jective of this article is to analyze large effects of the Mohoreflection on the amplification of peak ground motion (peakground acceleration [PGA] and peak ground velocity [PGV])observed at several sites located in northwestern Taiwan dur-ing the 1995 Niu Dou earthquake.

Strong-Motion Data

The Central Weather Bureau has undertaken the Tai-wan Strong-Motion Instrumentation Program (TSMIP) sinceJuly 1991 to collect high-quality instrumental recordings ofstrong earthquake shaking. Thus far, more than 640 free-fieldaccelerograph stations have been deployed in populatedareas of Taiwan. This network has provided large numbersof recordings to form an excellent database for studyingstrong-motion characteristics and for developing attenuationrelations. Each operating free-field station includes triaxialaccelerometers, a digital recording subunit, a power supply,and a timing system. The transducers for the accelerograph

must respond accurately in the frequency range from d.c. to50 Hz in order to faithfully record the near-source groundmotion caused by large earthquakes. In order to record awide range of earthquakes on scale, it is required that thecomplete system be digital and have at least a 16-bit resolu-tion and a 96 dB dynamic range (Liu et al., 1993). The fullscale of the recording system is �2g at a sample rate of200 samples=sec. Each recording system is operated in trig-ger mode with a 20 sec preevent memory and is set to recordan additional 10 sec of data after the signal dropped below apreset threshold (Liu et al., 1999).

In this study, we use the TSMIP strong-motion data ofthe Mw 6.35 Niu Dou earthquake of 25 June 1995 at a fo-cal depth of 39.9 km in northern Taiwan (Central WeatherBureau, 1995). A total of 336 three-component accelero-grams were recorded at epicentral distances ranging from4.9 to 244.7 km. We have analyzed these data to study thedependence of PGA on the seismic-wave propagation path,especially to examine the effects of Moho reflection. Figure 1shows a contour map of the mean PGA of two horizontalcomponents, together with the epicenter location (24.606° N,121.669° E) of the Niu Dou earthquake. The largest PGA ofall of the records is 259 gal (gal � cm=sec2). The heavy linesin the map represent the PGA value of 80 gal. Similarly, Fig-ure 2 plots a contour map of the mean PGV of two horizon-

Figure 1. Horizontal PGA contour map with the epicenter of the25 June 1995 Niu Dou earthquake. Nine stations with large ampli-tude located at Class C orD dense and stiff soil sites are also shown.

256 K.-S. Liu and Y.-B. Tsai

tal components. The largest PGV of all of the records is10:8 cm=sec. The heavy lines in the map represent thePGV value of 6 cm=sec. From these PGA and PGV contourmaps, a surprising feature is noticed: unusually large PGAand PGV anomalies were observed at several soft-rock anddense soil sites (Lee et al., 2001) in the Taoyuan, Lungtan,Guanshi, Hsinchu, and Miaoli areas in northwestern Taiwan(see Fig. 1 for the location of the cities).

It is well known that ground-motion characteristics canbe affected by local site response. According to the site clas-sification criteria of Table 1 (Lee et al., 2001), each record-ing station in Taiwan can be classified as a Class B, C, D, orE site. Examination of the records with different site classi-fications confirmed that many Class E soft soil stations in theTaipei Basin and the Ilan Plain had large PGA values. How-ever, many Class C and D stiff soil sites, especially thoselocated in the Taoyuan, Lungtan, Guanshi, Hsinchu, andMiaoli areas, also had unusually large PGA and PGV valuesduring the 1995 Niu Dou earthquake. As shown in the fol-lowing, this unusual PGA and PGV amplification can be ex-plained by large effects of Moho reflection.

Attenuation Model and Residual Analysis

A strong-motion attenuation relationship expresses anearthquake ground-motion parameter as a function of simpleparameters characterizing the earthquake source, the propa-gation path between the earthquake source and the site, andthe geologic conditions beneath the site. The following equa-tion form is used in this study:

lnY � a ln�X � h� � bX� cMw � d� σ; (1)

Figure 2. Horizontal PGV contour map with the epicenter ofthe 25 June 1995 Niu Dou earthquake. Nine stations with large am-plitude located at Class C or D dense and stiff soil sites are alsoshown.

Table 1Comparison between the 1997 UBC Provisions and the Simplified Site Classification Used in This Study (after Lee et al. [2001])

Site Class Site Class Description of 1997 UBC Provisions Site Class Description of Lee et al. (2001)

A Hard rock, eastern United States sites only, �VS > 1500 �m=sec�. (not used)B Rock, �VS is 760–1500 (m=sec) Miocene and older strata, along with limestone, igneous rocks,

metamorphic rocks, etc.C Very dense soil and soft rock, �VS is 360–760 (m=sec), undrained

shear strength us≧2000 psf (us≧100 kPa) or N≧50 blows=ftPliocene and Pleistocene strata, along with conglomerates,

pyroclastic rocks, etc. and geomorphologic lateritic terracesD Stiff soils, �VS is 180–360 (m=sec), stiff soil with undrained shear

strength 1000 psf≦us≦2000 psf (50 kpa≦us≦100 kPa) or15≦N≦50 blows=ft.

Late Pleistocene and Holocene strata, geomorphologic fluvialterrace, along with stiff clays and sandy soils with averageSPTN≧15 in the upper 30 m

E Soft soils, profile with more than 10 ft (3 m) of soft clay defined assoil with plasticity index PI > 20, moisture content w > 40%,and undrained shear strength us < 1000 psf(50 kPa) orN < 15 blows=ft

Holocene deposits and fills, etc. with average SPT N < 15 in theupper 30 m

F Soils requiring site specific evaluations: (1) soil is vulnerable topotential failure or collapses under seismic loading: for example,liquefiable soils, quick and highly sensitive clays, collapsibleweakly cemented soils; (2) peats and/or highly organic clays(10 ft [3 m] or thicker layer); (3) very high plasticity clays: (25 ft[8 m] or thicker layer with plasticity index > 75); (4) very thicksoft/medium stiff clays: (120 ft [36 m] or thicker layer)

(not classified in the present study and will be studied in the future)

Note: the provisions of 1997 NEHRP and 1997 UBC are similar.

Large Effects of Moho Reflections (SmS) on Peak Ground Motion in Northwestern Taiwan 257

where Y is the ground-motion parameter, X is the hypocen-tral distance, Mw is the moment magnitude, a is the geo-metrical spreading coefficient, b is the anelastic attenuationcoefficient, c is the magnitude coefficient, d is a constant, his the close-in distance saturation coefficient, and σ is thestandard deviation. The coefficients a, b, c, d, and h areto be determined by regression from the data (Liu and Tsai,2005). This equation form is similar to the one used by Joy-ner and Boore (1981), except for the difference in distancedefinitions.

Regressions on the TSMIP data set without differentiat-ing site conditions have resulted in the coefficients of the at-tenuation relationships, as given in Table 2, for the horizontal(H) components of PGA and PGV for the whole of Taiwan

(Liu and Tsai, 2005). In Table 2, σ refers to the standard de-viation of ln�Y�. Assuming a log-normal distribution, thisvalue can be used to obtain the value of the parameter Y cor-responding to different probability levels. Specifically, ln�Y�at 84% probability (median plus one sigma) may be obtainedby multiplying the predicted median value by eσ.

Figures 3 and 4 compare the observed data from the1995 Niu Dou earthquake (Mw 6:35) with the PGA andPGV attenuation relations, respectively, for the Taiwan area.It can be seen in these figures that most observed data aredistributed within the range between �1standard deviationof the median attenuation curve. We can also find a clusterof very high-valued data at a hypocentral distance 66–84 kmfrom TCU007, TCU013, and TCU021 stations and so on,which are located in Taoyuan, Lungtan, and Guanshi areasin northwestern Taiwan, located in Figures 3 and 4.

Examination of the PGA residuals (i.e., the differencebetween logarithms of the observed and predicted PGA)for sites with different soil categories is a useful methodfor sets of records where site information is not completeand hence cannot be included explicitly within the equation(Abrahamson and Litehiser, 1989). Liu and Tsai (2005) ana-lyzed both the PGA and PGV residuals to study their varia-tions with respect to site conditions. The results showed that

Table 2Regression Coefficients for the PGA and PGVAttenuation Relation

(Liu and Tsai, 2005)

a b c d h σ

PGA �0:852 �0:0071 1.027 1.062 1.24 0.719PGV �0:857 �0:0023 1.486 �4:472 1.34 0.711

Note: ln�PGA; PGV� � a� ln�X � h� � b�X � c�Mw � d� σ.

Figure 3. Comparison of the horizontal PGA data for theMw 6.35 Niu Dou earthquake with the corresponding medianand median� σ values derived from the attenuation relationshipsof Liu and Tsai et al. (2005).

Figure 4. Comparison of the horizontal PGV data for theMw 6.35 Niu Dou earthquake with the corresponding medianand median� σ values derived from the attenuation relationshipsof Liu and Tsai et al. (2005).


the residual contour maps have high consistency with bothregional geology and topography of Taiwan. For example,positive residuals are expected in areas like the Taipei Ba-sin and the Ilan Plain that will experience amplification ofground motion due to soft soil site conditions.

In order to understand the relation between both PGAand PGV residuals and local sites as well as propagation patheffects, we plot the residual of horizontal PGAversus epicen-tral distance in Figure 5 and the residual of horizontal PGVversus station azimuth in Figure 6. The results confirm thatmany Class E soft soil stations in the Taipei Basin and theIlan Plain have, as expected, large positive PGA residuals ofabout 0.53 and 0.40, corresponding to an amplification fac-tor of 1.7 and 1.5, respectively, relative to the predicted me-dian values.

Surprisingly, residuals at a group of Class C or D denseand stiff soil sites, especially those in Taoyuan (TCU007),Lungtan (TCU013), Guanshi (TCU021), Hsinchu (TCU095),and Miaoli (TCU047) areas, have even larger PGA and PGVresiduals corresponding to an amplification factor of 3.36–8.13 and 1.73–3.30, respectively, relative to the predictedmedian values. As shown in Figures 5 and 6, this groupof unusually large PGA and PGV sites is distributed in a nar-row range of epicenral distances (i.e., 50–75 km) and azi-muths (i.e., 270°–331°). Figures 1 and 2 clearly show that

these sites form a northeast–southwest-trending narrow bandof anomalously large PGA and PGV in northwestern Taiwan.

Why did these stiff soil sites in northwestern Taiwan,which were previously shown to have normal local site re-sponse (Liu and Tsai, 2005), have such large amplificationsduring this earthquake? A plausible answer to this question ispresented in the following section.

Crustal Velocity Structures

We first analyze the path effects, especially regardingthe role of crustal structure in affecting strong ground mo-tion. In order to pick seismic phases and calculate the the-oretical travel times of direct and reflected waves from thehypocenter, an appropriate crustal model for the Taiwan areais needed. Some attempts to determine the crustal structureof Taiwan were made in the past. Yeh and Tsai (1981) usedan iterative damped least-squares inversion procedure to findP-wave velocities of a horizontally layered model for thecrust of central Taiwan. Their crustal velocity model has anupper crust of two sublayers whose thickness and velocityare 9 km and 5:8 km=sec for the upper layer and 8 km and6:1 km=sec for the lower layer, respectively. The thicknessand velocity of the lower crust are 19 km and 6:7 km=sec,respectively. The velocity of the upper mantle is 7:8 km=sec.Thus, the overall thickness of the crust in the Central Range

Figure 5. PGA residuals from records at different distances and site conditions. The Taipei Basin and the Ilan Plain, as expected for softsoil sites, have significantly positive residuals of 1.7 and 1.5 times, respectively, which are the median predictions. Surprisingly, the residualsfor some sites with dense and stiff soils, especially in Taoyuan (TCU007), Lungtan (TCU013), and Guansi (TCU021) areas, are about 3.4–8.1times the median predictions.


(CR) and Western Foothill (WF) regions is 36 km. Grav-ity data were also used to determine the crustal thick-ness of Taiwan. Liu and Yen (1975) calculated the crustalthickness by using Bouguer anomaly data and found thatit is thickest �� 40� 3 km� in north central Taiwan andthinnest �� 27� 2 km� in the southeastern and southerncoastal areas. The crustal thickness for Taiwan as a wholeis about 34� 2 km.

In order to ensure precise earthquake location in geo-logically complex areas and to provide information abouttectonic boundaries or the mechanics of deformation in theTaiwan area, many three-dimensional velocity models havebeen constructed (Roecker et al., 1987; Shin and Ho, 1994;Chen, 1995; Chen et al., 1995; Rau and Wu, 1995; Ma et al.,1996; Cheng, 1998; Shih et al., 1998; Yeh et al., 1998; Tom-fohrde and Nowack, 2000; Wang, 2004; Kim et al., 2004,2005; Liao, 2005). These studies all confirm the presenceof significant lateral heterogeneities of seismic velocity inTaiwan. Tomographic images of the crustal and mantle ve-locity structures under Taiwan were obtained by Rau and Wu(1995). They found a root under the CR, which is deeperin the north and becomes shallower toward the south. Simi-lar results were found by Ma and Song (1997), Song (1997),and Kim et al. (2005). Ma and Song (1997) investigatedthe Pn velocity and Moho depth variations beneath Taiwan.

The results showed that Pn velocity under the CR is about8:52 km=sec, which is about 5%–8% higher than the othertwo provinces: 7:79 km=sec under the Coastal Plain (CP)and 8:04 km=sec under the WF, respectively. The Mohodepths are about 31, 34.5, and 43 km, respectively, for theCP, WF, and CR.

Kim et al. (2004, 2005) used the receiver functionmethod (RFM) and tomographic inversion method (TIM) toinvestigate the complex crustal structures in the Taiwan re-gion. The depths of Moho discontinuity were determined asfollows: a trend of crustal thinning starting from the east (at50–52 km and 55 km by RFM and TIM, respectively) towardthe west (at 28–32 km and 35 km by RFM and TIM, respec-tively). This is in good agreement with the results from twoeast–west-trending deep seismic profiles previously obtainedusing airgun sources (Shin et al., 1998; Yeh et al., 1998). Insummary, the depth of Moho under Taiwan varies signifi-cantly, especially in the east–west direction. In the westernCP and WF, it is about 28–35 km, deepening gradually east-ward to reach a maximum depth of 50–55 km beneath theeastern CR.

Moho Reflection

In this study, we first considered the one-dimensional(1D) velocity model of Chen (1995), which is used for rou-

Figure 6. PGV residuals from records at different azimuths. Many stations in azimuth ranging from 270° to 335°, especially those inTaoyuan (TCU007), Lungtan (TCU013), and Guanshi (TCU021) areas, had logarithmic residuals of about 0.55–1.19, corresponding to afactor of 1.73–3.30 times the predicted median values. These high residual stations were not located at azimuths oriented perpendicular to thefault plane. The focal mechanism of the Niu Dou earthquake was determined byMa andMori (1998) with a dip of 76.2°, a rake of 34.6°, and astrike of 133.5°.


tine earthquake location by the Central Weather Bureau asthe reference model for calculating the travel times.

Because the hypocenter of the Niu Dou earthquake waslocated in northern Taiwan, the crustal velocity structuresunder northern Taiwan are discussed in more detail. Shen(2000) used an iterative damped least-squares inversion pro-cedure to find a P-wave velocity model for the crust of north-ern Taiwan. This crustal velocity model consists of foursublayers. The total thickness of the crust varies significantlyfrom 35 to 43 km, especially along the southwest–northeastdirection. In the western CP and WF, it is about 35 km. Itgradually deepens northeastward to reach a maximum depthof 43 km beneath the Ilan Plain. Lin (2001) and Lin et al.(2004) used a tomographic inversion method to determinethe three-dimensional VP and VS velocity structures and tooutline the Moho depth under northern Taiwan. They found aMoho depth of about 42 km in the area around the Niu Douearthquake. Similar results were found by Song (1997) andLiao (2005). Based on these studies, a velocity model is con-structed, incorporating the following features (1) the crustalthickness for Taiwan as a whole is about 33 km and (2) atrend of crustal thinning from east (under the Central Moun-tain Range) toward west (under the Western Coastal Plainand WF). In addition, the Moho discontinuity under theNiu Dou area is set at a depth of 42 km and deepens towardsouth. The Moho depth under the eastern Central MountainRange reaches a maximum depth.

Based on the previous discussion, we have constructed acrustal velocity model, as given in Table 3 and shown in Fig-ure 7. Basically, this crustal velocity model, following Chen(1995), consists of 10 layers, except the thicknesses of theeighth and ninth layers are modified. Thus, the total thick-ness of the crust, to the base of layer 8, is 42 km beneaththe epicenter of the Niu Dou earthquake. In order to verifythat the Niu Dou earthquake was indeed located within thecrust, we have relocated the earthquake by using the alter-native velocity model. By comparing the results in Table 4for the location of the Niu Dou earthquake, as determined byusing two different velocity models, that is, one previouslydeveloped by Chen (1995) and the other constructed in thisstudy, as given in Table 3, we conclude that the hypocenter of

the Niu Dou earthquake is indeed located within the crust(C.-H. Chang, personal comm., 2006).

In order to exclude the strong-motion records in theTaipei Basin, which are known to be subjected to large siteamplification due to soft soils, a profile of recorded timehistories, as shown in Figure 8, is compiled by selecting53 velocity seismograms restricted in the azimuth range of245°–335° and epicenter distance range of 5–100 km. Therecordings are from a variety of site conditions. The calcu-lated travel-time curves and the range of distances for thedirect P, S, PmP, and SmSwaves are also shown in Figure 8.The calculated travel-time curves and the range of distancesfor the direct P, S waves and the reflected PmPh (SmSh),PmPi (SmSi) waves that represent the waves reflected fromthe horizontal and inclined portions of Moho discontinuity,respectively, are also shown in Figure 8. These curves werecomputed using the 2D crustal velocity model whose originalmodel was given in Table 3; the modified model is shown inFigure 9.

The source-time function of the Niu Dou earthquakeas teleseismically determined has a pulse with a durationof about 3 sec (Lin and Ma, 1996). Accordingly, the SmSarrival-time curves in Figure 8 reappear at a delay of 3 secto represent the duration of the strong source pulse teleseis-mically seen. In Figure 8, the onset of the largest waves atmost stations coincided with the arrival time of the Mohoreflection SmS at distances beyond 50 km. The move-outof this onset with distance clearly follows the SmS arrival-time curve and not that of direct S. The observed duration ofstrong motion following the SmS arrival-time curves is about3 sec, which is compatible with the 3 sec duration of strongsource pulse, as determined by Lin and Ma (1996).

Focusing Effects of an Inclined Moho

In the previous record profile, the Moho reflection fromthe base of layer 8 is a strong arrival at the epicentral distancebetween 50 and 75 km. It is visible on most of the stations inthis distance range. In the following analysis, we will focuson the effects of S-wave reflection from the base of an in-

Table 3The Crustal Velocity Model Used in This Study

Thickness (km) Depth (km) VP �km=sec� VS �km=sec� QP QS Density (g=cm3)

2 0–2 3.48 1.96 150 75 2.12 2–4 4.48 2.62 300 150 2.55 4–9 5.25 3.03 300 150 2.64 9–13 5.83 3.35 300 150 2.64 13–17 6.21 3.61 500 250 2.88 17–25 6.41 3.71 500 250 2.85 25–30 6.83 3.95 500 250 2.812 30–42 7.29 4.21 600 300 3.08 42–50 7.77 4.49 1500 750 3.2

half-space >50 8.24 4.76 2200 1100 3.6


clined Moho discontinuity to explain the enhanced PGA andPGV recorded in northwestern Taiwan.

We use a ray-tracing method to calculate the travel timeof reflected waves from the Moho discontinuity. As the in-cident angle increases to 69.7° corresponding to a distance of76 km, reflection of shear waves from the interface at thelower crust will reach the critical angle and undergo totalinternal reflection. In order to explain and fit the large am-plitudes observed at a distance less than 76 km, a refinedtwo-dimensional (2D) velocity model with inclined Mohodiscontinuity is developed.

Let us consider that the Moho discontinuity surface isnot parallel to the free surface but has a dip angle ψ. Forup-dip propagation, the surface is inclined upward from eastto west. The thickness of the crust is not uniform: it is 42 kmbeneath the northern Central Mountain Range and 33 kmunder the western CP and WF. Thus, the final 2D model forthe velocity structure along the east–west direction consistsof 10 layers, as shown in the upper left part of Figure 9. TheMoho discontinuity is at a depth of 42 km beneath the epi-center of the Niu Dou earthquake. It extends horizontally to-ward the west for 6.8 km. It then turns up-dip at 24.2° with aratio of 20 km horizontally to 9 km vertically. Beyond thatpoint westward, the Moho discontinuity is kept flat at a con-stant depth of 33 km.

The curves for the travel time and the incident angle(theta) as a function of epicentral distance for the dippingMoho model are shown in Figure 9. Curves A and B showthe relation between incident angle versus the distance forwaves reflected from the horizontal and inclined portions ofthe Moho discontinuity, respectively. Curves C and D showthe relation between travel time versus distance for wavesreflected from the horizontal and inclined portions of theMoho discontinuity, respectively. It is noted that there existsan overlap distance range from 48 to 86 km over which re-flected waves from both discontinuities will simultaneouslyarrive, when the theta angle increases from 55.6° to 92.1°. Inthe intervals of theta from 55.6° to 72.9° and from 72.9° to92.1°, the S waves are reflected from the horizontal and dip-ping portions of the Moho discontinuity, respectively.

In the overlap distance range from 48 to 86 km, simul-taneous arrival of the SmS waves reflected from the hori-zontal and dipping portions of the Moho discontinuities willcause focusing and interference resulting in enhancementof ground-motion amplitudes. Figure 10 shows a crustal ve-locity model for northern Taiwan with ray traces for directS waves (in red) and SmS (in blue) reflected from the Mohodiscontinuity. We can see that focusing and interference be-tween SmS waves reflected from both the horizontal and in-clined portions of the Moho discontinuity in the epicentraldistance range of 48 to 76 km. This is taken as the primarycause for significantly enhanced PGA and PGV at a group ofstations distributed in this narrow distance range in north-western Taiwan.

It should be noted that at distances beyond 48 km, thePGA and PGV occurred at times corresponding to strongreflections from the Moho discontinuity, instead of directS waves. Because of the presence of an inclined Moho dis-continuity, the amplitudes of recorded ground motion didnot significantly decrease at hypocentral distances between66 and 84 km, but decreased rapidly beyond 84� 2 km, asshown in Figure 3.

Discussion

In the preceding analysis, we have demonstrated the fo-cusing and interference effects of reflected S waves from aninclined Moho discontinuity to be the primary cause for sig-nificantly enhanced PGA and PGV recorded in northwesternTaiwan during the Niu Dou earthquake. However, ground-motion amplitudes can also be affected by a variety of otherfactors, such as source or site effects, some of which maycontribute together. It is therefore necessary to distinguish

Figure 7. A crustal velocity model for this study. This crustalvelocity model, basically following Chen (1995), consists of 10 lay-ers, except the thicknesses of the eighth and ninth layers are mod-ified. Thus, the total thickness of the crust, to the base of layer 8, is42 km beneath the epicenter of the Niu Dou earthquake instead of35 km as proposed by Chen (1995).

Table 4Comparison of the Niu Dou Earthquake Location by Using Two Crustal Velocity Models

Velocity Model Latitude (°N) Longitude (°E) Focal Depth (km) Moho Depth (km) ML Mw

Chen (1995) 24.606 121.669 39.88 35 6.50 6.35This study 24.603 121.666 39.77 42 6.50 6.35


the predominant factor that caused the significant enhance-ment of recorded ground-motion amplitudes.

Seismic radiation patterns can result in geographicasymmetry of ground motion due to the faulting process thatis closely related to the focal mechanism of the earthquake.The focal mechanism of the Niu Dou earthquake was deter-mined by Lin and Ma (1996) using teleseismic waveforms. Itwas an oblique faulting mechanism with the dip at 76.2°, therake at 34.6°, and the strike at 133.5° (Ma and Mori,1998). Inorder to understand the relation between the station residualsof PGV and the seismic radiation patterns, we now refer toFigure 6 where the residuals of the horizontal PGVare plottedas a function of site azimuth, that is, the direction from theseismic source to a recording station. The data confirm thatmany Class E soil stations in Chianan Plain (CHYarea) havelarge positive PGV residuals, above 0.8, corresponding to anamplification factor of 2.2 relative to the predicted medianvalues. The large residuals of these stations at azimuth rang-

ing from 220° to 240°, which were close to the azimuths per-pendicular to the fault plain, were most likely due to seismicradiation effects. In the meantime, some of the stations be-long to Class E soft soil sites that might have contributed topart of the large amplification.

Figure 6 also shows that another group of stations atazimuth ranging from 270° to 331°, especially those in Tao-

Figure 8. A reduced travel-time plot of the north–south-component velocity seismograms for stations located in the azimuthrange of 245°–335° and in the epicenter distance range of 5–100 km.The calculated travel-time curves and the range of distances forthe direct P, S, PmP, and SmS waves are also shown. The calcu-lated travel-time curves and the range of distances for the direct P,S waves and the reflected PmPh (SmSh), PmPi (SmSi) waves thatrepresent the waves reflected from the horizontal and inclined por-tions of Moho discontinuity, respectively, are also shown.

Figure 9. The travel time and incident angle (theta) versus theepicentral distance curves of a dipping Moho model. Curves A andB stand for the incident angle versus distance curves of waves re-flected from the horizontal and inclined portions of the Moho dis-continuity, respectively. Curves C andD stand for the traveling timecurves of wave reflected from the horizontal and inclined disconti-nuities, respectively. The crustal velocity model is also shown in theupper left part of the figure.

Figure 10. A crustal velocity model for northern Taiwan withray traces of direct S waves (in red) and SmS waves (in blue) re-flected from the Moho discontinuity. We can see that focusing andinterference between SmS waves reflected from both the horizontaland inclined portions of the Moho discontinuity in the epicentraldistances ranged from 48 to 76 km. The P and S velocities of eachlayer are also shown in the left part of the figure.


yuan (TCU007), Lungtan (TCU013), Guanshi (TCU021),Hsinchu (TCU095), and Miaoli (TCU047) areas, had loga-rithmic residuals of about 0.55–1.19, corresponding to anamplification factor of 1.73–3.30 over the predicted medianvalues. These large PGV sites were not located in azimuthsperpendicular to the fault plane. This suggests that the ob-served large-amplitude late-arrival S waves were unlikely tobe caused by seismic radiation effects.

Moreover, site effects can also play an important role inamplifying seismic ground motions. A simple way to esti-mate site effects is made in terms of soil-type classification(Kawase, 2004). Figures 11 and 12 show a reduced timeplot of the north–south-component acceleration and veloc-ity waveforms, respectively, for the nine stations with PGAgreater than 100 gal, as extracted from Figure 8. Accordingto Table 5, these nine stations with large PGA and PGV am-plitudes are located on Class C or D dense and stiff soils.Obviously, the unusually large amplification phenomenon

cannot be explained by site effects, either. This is supportedby normal site response at these stations found in our pre-vious studies using a large data set (Liu and Tsai, 2005).In summary, we can discount either seismic radiation or localsite response effects as a major cause for unusual amplifica-tion observed at these stations.

Furthermore, McGarr et al. (1991) pointed out that if thehighest-amplitude portion of the S-wave train is SmS in thehorizontal seismogram, then the highest-amplitude portionof the P-wave train should also be found in the PmP phaseon the vertical seismogram. Figure 13 plots a profile of re-corded time histories of the vertical component, as compiledby selecting 53 velocity seismograms. The ranges of azimuthand epicenter distance of these records are the same as Fig-ure 8. The calculated travel-time curves and the range of dis-tances for the direct P, S, PmP, and SmS waves are alsoshown in Figure 13. In the figure, the portion of PmP wavetrains shows similarly enhanced amplitudes just as the en-hanced SmS wave trains shown in Figure 8.

Figure 11. A reduced travel-time plot of the north–south-component accelerograms for the nine stations with PGA greaterthan 100 gal. The calculated arrival times for direct P, S, PmP,and SmS phases are also shown. Station codes and relevant PGAare marked at the heads and tails of the traces, respectively. Therelevant PGA data of these records are also shown in Figures 1,3, and 5.

Figure 12. A reduced travel-time plot of the north–south-component velocity seismograms for the nine stations with PGAgreater than 100 gal, as extracted from Figure 8. The calculated ar-rival times for direct P, S, PmP, and SmS phases are also shown.Station codes and relevant PGV are marked at the heads and tails oftraces, respectively. The relevant PGV data of these records are alsoshown in Figures 2, 4, and 6.


Summary

Based on preceding results and discussion, we can sum-marize as follows:

1. In the present study, we have identified focusing andinterference between SmS waves reflected from a hori-zontal and inclined portions of the Moho discontinuity

as the primary cause for unusually large peak groundmotion recorded at distances between 50 and 75 km innorthwestern Taiwan. This interpretation is supported byconstructing an inclined Moho model in which the am-plitudes and arrival times of the largest shear waves werein close agreement with the data.

2. The residuals of horizontal PGA and PGV recorded fromthe Niu Dou earthquake confirmed that many Class Esoft soil stations in the Taipei Basin and the Ilan Plainhad the expected large amplification of about 1.7 and1.5 times, respectively, the predicted median values. Sur-prisingly, residuals at many Class C or D dense and stiffsoil sites, especially those in Taoyuan (TCU007), Lung-tan (TCU013), Guanshi (TCU021), Hsinchu (TCU095),and Miaoli (TCU047) areas located in northwesternTaiwan, had residuals about 3.4–8.1 times the predictedmedian values. This is shown primarily due to effects ofreflection at the Moho discontinuity.

3. The propagation path effects, especially those due tocrustal structures, can play an important role in producinglarge ground motion while site conditions are clearly notresponsible. Our results suggest that when a damagingearthquake occurs at depth near the Moho boundary,the reflected SmS waves can cause significantly ampli-fied ground motions in distances beginning at around50 km. The exact distance range will depend on the thick-ness of the crust and the dip angle of an inclined Mohodiscontinuity.

Data and Resources

Seismograms used in this study were collected as partof the Taiwan Strong-Motion Instrumentation Program(TSMIP) using 16-bit accelerographs. The digital TSMIPstrong-motion data can be obtained from the Central WeatherBureau of Taiwan at www.cwb.gov.tw, available at http://e-service.cwb.gov.tw/i-sales-web2/Services%20Application/services_application.htm.

Acknowledgments

We thank the Central Weather Bureau of Taiwan for providing excel-lent strong-motion data. We are grateful to Joe Litehiser and Art McGarr for

Table 5Station Code, Site Classification, Amplification Factor, and Related Data Used in Figures 1–6, 11, and 12

StationNumber

StationCode

Latitude(°N)

Longitude(°E)

EpicentralDistance (km)

HypocentralDistance (km)

Azimuth(°)

SiteClass NS

PGA (cm=sec =sec) PGV (cm=sec)

H Residual Factor H Residual Factor

1 TAP038 25.02 121.41 53.07 66.38 330.7 D 179.9 155.9 1.53 4.60 10.80 1.19 3.302 TCU013 24.87 121.20 55.04 67.97 301.5 C 163.1 200.9 1.81 6.12 8.95 1.02 2.763 TCU021 24.79 121.17 55.05 67.98 292.2 D 123.1 160.0 1.58 4.87 9.77 1.10 3.024 TCU007 25.00 121.31 56.87 69.46 320.3 D 286.8 259.1 2.10 8.13 10.46 1.19 3.305 TCU014 25.05 121.31 60.90 72.80 323.2 D 96.7 100.7 1.21 3.36 5.23 0.55 1.736 TCU024 24.74 121.08 61.46 73.26 283.6 C 115.2 132.6 1.50 4.47 8.67 1.06 2.897 TCU095 24.69 121.01 66.98 77.95 278.1 C 137.9 176.7 1.87 6.48 9.10 1.17 3.238 TCU043 24.69 120.95 73.11 100.8 277.3 C 99.2 100.0 1.39 4.03 4.87 0.61 1.859 TCU047 24.62 120.94 73.91 140.7 271.2 C 121.2 130.6 1.67 5.32 5.54 0.75 2.12

Figure 13. A reduced travel-time plot of the vertical velocityseismograms for stations located in the azimuth range of 245°–335° and in the epicenter distance range of 5–100 km. The calcu-lated travel-time curves and the range of distances for the direct P,S, PmP, and SmS waves are also shown. The calculated travel-timecurves and the range of distances for the direct P, S waves and thereflected PmPh (SmSh), PmPi (SmSi) waves that represent thewaves reflected from the horizontal and inclined portions of Mohodiscontinuity, respectively, are also shown.


their critical comments and suggestions that have led to a significant im-provement of the article. We are also grateful to Y. H. Tseng, G. S. Chang,H. C. Pu, and Mr. S. K. Wang for their computer programs. This researchwas supported by the Taiwan Earthquake Research Center (TEC) fundedthrough National Science Council (NSC) with Grant Number NSC96-2625-Z-244-001 and Number NSC97-2625-M-244-001. The TEC contributionnumber for this article is 00037.

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(K.-S.L.)

Pacific Gas and Electric CompanySan Francisco, [email protected]

(Y.-B.T.)

Manuscript received 10 October 2007


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Large Effects of Moho Reflections (SmS) on Peak Ground ...€¦ · Large Effects of Moho Reflections (SmS) on Peak Ground Motion in Northwestern Taiwan by Kun-Sung Liu and Yi-Ben