A Preliminary Study on the Near-source Strong-Motion Characteristics of the Great 2008 Wenchuan Earthquake in China

Transcription

1 A Preliminary Study on the Near-source Strong-Motion Characteristics of the Great 28 Wenchuan Earthquake in China Ming LU, Xiao Jun LI China Earthquake Administration, Beijing, China and John X. ZHAO GNS Science, Lower Hutt, New Zealand

2 Abstract The great 28 Wenchuan Earthquake with a moment magnitude of 7.9 and a surface-wave magnitude of 8. in Shichuan, China, caused unprecedented loss of human life and widespread severe damage to many types of structures. 32 strong motion records were obtained within a source distance of 3km and 3 near-source records were obtained within a source distance of 2km. We present the preliminary results on the characteristics of the near-source records and the strong-motion aspects of this great earthquake. This earthquake may be divided into four sub-events, according to the rupture time history and the final slip distribution. One station recorded strong-ground motions from two sub-events in two wellseparated time windows and this allows us to examine the effect of earthquake parameters for each of the sub-event. We find that, in the spectral period range of.5-2s, the response spectra of the near-source records from the Wenchuan earthquake are significantly less than those of buried-fault earthquakes, such as the 989 Loma Prieta earthquake and 994 Northridge earthquake that have a much smaller moment magnitude than the Wenchuan earthquake. In the fault-normal direction the displacement spectra at long period for the closest station are similar to those of the Lucerne record from the 992 Landers earthquake, but significantly smaller than those of the TCU52 and TCU68 records from the 999 Chichi, Taiwan earthquake. At short and intermediate period, the near-source spectra are much larger than the design spectra in the previous version of the Chinese design code for the heavily damaged area, but they are comparable at long spectral periods. Finally an attenuation model developed using Japanese strong motion records predicts the horizontal and the vertical peak ground accelerations of the Wenchuan earthquake records surprisingly well. Keywords: Wenchuan earthquake, near-source records, near-source response spectra, strongmotion attenuation. Introduction The great Wenchuan earthquake with a moment magnitude of 7.9 and a surface magnitude of 8. struck the western part of Shichuan province, China on 2 May 28, resulting in unprecedented human casualties and infrastructural damage. Large ground-surface ruptures along the Longmenshan fault have been reported by Xu et al. []. Figure shows the relative vertical and horizontal displacement at ground surface possibly caused by fault slip, and the largest horizontal displacement is 5.3m. Both the main fault (Longmenshan fault) and the secondary fault (Hanwang-Bailu fault) have a relatively shallow dipping angle towards to the north-west. At the northern part of the fault, the ground-surface relative displacement reported by Xu et al. [] suggests that the dominant displacement occurred along the strike of the fault. Fault-rupture models derived from inversion analyses on teleseismic records have been reported by Wang et al. [2], Zhang et al. [3], Institute for Research on Earth Evolution [4], Chen and Hayes [5] and Koketsu et al. [6]. The rupture time history, moment release functions and the final slip distribution differs significantly among these inversion models. The model by Koketsu et al. [6] used three near-source strong-motion records and produced large final vertical slip along the Hanwang-Blailu surface rupture consistent with that reported by Xu et al. []. However, Koketsu et al. [6] did not provide detailed moment release function. In the present study, we choose the fault rupture model of Wang et al. [2] with a correction on the fault slip distribution according to the Koketsu et al. [6] model because, Their fault-displacement distributions on the ground surface are broadly consistent with those reported by Xu et al. []; 2

3 2 Their fault rupture time history is reasonably consistent with the strong-motion record in one particular station that well separately recorded two sub-events; and 3 Their locations of large fault slips are consistent with the area of the worst damage apart from the inconsistent final vertical slip distribution along the Hanwang-Bailu surface. Figure 2 shows the fault surface projection, the slip distribution derived from the inversion analyses by Wang et al. [2], the locations of two major asperities where a large fault slip was concentrated, and the locations of three strong motion stations along the strike of the fault. The total length of the fault-rupture plane reported by Wang et al. [2] is just over 3km. The final slip distribution and the fault slip time history by Wang et al. [2] show a number of important aspects:. The fault ruptured to the ground surface at many locations; 2. After the rupture initiation, the rupture appears to propagate from the hypocentre to the south-west end of the fault in the first s; 3. A secondary fault parallel to the main fault and about 5km to the east ruptured almost simultaneously with the part of fault close to the first major asperity; 4. There are two major asperities with slip displacement as large as 2m; 5. The slip on the fault at the first major asperity and the secondary fault started at about s after the initiation time; 6. The slip at the second asperity started about 2s after the slip at the first asperity ended; 7. The large fault slip at the first asperity close to Wenchuan is located at a depth of 5-25km; 8. The second asperity close to Beichuan has two areas of large slip concentration with one area being located at a depth of -5km and the other at a depth of -5km. These characteristics of the source slip distribution may have strong influence on the nearsource ground motions. In the light of rupture models derived by Koketsu et al. [6], we made a simple modification to the fault rupture model by Wang et al. [2]. The final slip distribution of the sub-event 2 (see next section about the division of sub-event) at the Long Men Shan fault should be assigned to the secondary fault (Hanwang Bailu surface rupture) in Figure 2 and the slip distribution at the secondary fault should assigned to the main fault. This modification leads to that the SFB station is on the hanging wall of the secondary fault and the area of the SFB station is likely to have experienced large vertical up lift, consistent with the results reported by Xu et al. [] shown in Figure. Somerville and Pitarka [7] suggested that large crustal earthquakes (M w 7., such as the 992 Landers earthquake, California, USA, the 999 Kocaeli, Turkey earthquake and the 999 Chichi, Taiwan earthquake) with a large surface fault-rupture displacement tended to produce smaller ground motions in a period range of -2s than those from earthquakes without large surface rupture (M w <7., such as the 994 Northridge earthquake, and the 989 Loma Prieta earthquake, California, USA). Dalguer et al. [8] showed that, using a set of dynamic model parameters, numerical modelling was able to produce the observed difference in short- and intermediate-period ground motions for surface-rupture and buried-fault earthquakes. Abrahamson and Silva [9], Boore and Atkinson [] and other modellers for the Next Generation of Attenuation models (NGA) used a term of depth to the top of rupture to predict response spectra from surface-rupture and buried-fault earthquakes (see Earthquake Spectra, Vol 24, No., 28). In their models the response spectra with spectral period less than 2s are proportional to the depth to the top of rupture. 3

4 For the Wenchuan earthquake, two strong-motion records were obtained within a source distance (the closest distance from a recording site to the rupture plane) of 5km and five records within a source distance of 3km. The closest record is from a station within about 2km from the fault rupture plane and is the closest record from such a large earthquake (the only other record is from the Mw= Denali earthquake in Alaska, USA, at a source distance of 3km). Near-source records usually have two types of velocity pulse: forward-directivity pulse (Somerville et al. []) and fault-fling pulse (Abrahamson [2]), which have distinct shapes. A forward-directivity pulse is caused by the constructive interference between the waves generated by the current fault rupture and those propagated from the previous rupture locations. This leads to a large velocity in the direction normal to the direction of faultrupture propagation, when the fault-rupture propagation velocity is close to the shear-wave velocity of the crustal rock. The forward directivity effect occurs only at stations on the receiving side of the fault-rupture propagation. This velocity pulse is typically in two directions (positive and negative) and the ground surface displacement due to the velocity pulse reaches nearly zero after the velocity pulse passes the recording station. A fault-fling pulse is a result of permanent fault displacement and this velocity pulse usually occurs in one direction. The ground surface displacement is almost constant after the fault-fling pulse passes the recording site. In practice, a near-source record may contain both types of velocity pulses. Three near-source records from the Wenchuan earthquake, Mian Zu Qing Ping (MZQ), Si Fang Ba Jiao (SFB) and Wolong (WL) (Li et al. [3]) have peak ground accelerations (PGA) over.55g and some of these records appear to have one or both types of velocity pulse. Characteristics of the Wenchuan earthquake Wang et al. [2] presented a moment release function and snapshots of the rupture propagation along the fault plane. These snapshots show many sub-events in the main shock. We divided the rupture plane into roughly four parts for the convenience of explaining the characteristics of the strong-motion records, though the division is somewhat subjective (see Figure 2b). Figure 3 shows the magnitude variation with increasing time computed from the moment release function by Wang et al. [2]. The first part of the rupture occurred in the south-western part of the fault, close to the area of Dujiangyan city where some buildings suffered major damage and collapsed. This sub-event has a moment magnitude a moment magnitude of 7.. The slip in the hypocentral area occurred at a depth range of 8-24km. The moment magnitude for the second sub-event, with two parallel faults being ruptured, is possibly about 7.6. A large asperity is located in the secondary fault according to Kotetsu et al. [6], at a depth between 6 and 24km, and the surface slip displacement is typically 2-5m along this fault. The second large asperity is located in the area close to Beichuan (Wang et al. [2]) at a depth between and 5km with a magnitude about 7.6. This asperity can be divided into two parts, one part in a depth range of -5km and the other in a depth range of -5km. The surface slip displacement in this part of the fault varies between and m. The moment release between 4 and 55s is mainly from the deep part and the moment release in the time range of 55-7s is mainly from the shallow part of the large slip concentration. The last sub-event in Qingchuan has a moment magnitude about 7.2. This part of the fault is separated from the other parts because this segment of the fault possibly has a strike-slip focal mechanism as suggested by Xu et al. []. 4

5 We note that work on fault rupture plane is still in progress and the model by Wang et al. [2] or others may need to be modified in many parts of the fault. However, our interpretations based on the Wang et al. [2] and Koketsu et al. [6] may be adequate for engineering aspects of the near-source records. Characteristics of near-source strong motion records The fault-surface displacements reported by Xu et al. [] suggest that reverse mechanism dominated at least 2/3 of the main fault from the southern end. This type of fault produces much larger fault slip in the fault-normal direction than in the fault-parallel direction. Forward-directivity only occurs in the displacement direction normal to the rupture propagation direction and at the receiving side of the fault rupture propagation. We present the strong motion records in the fault-normal component (perpendicular to the fault strike) and the fault-parallel component (along the strike), and the fault-normal component may contain both fault-fling and forward-directivity pulses. Velocity and displacement time histories are obtained following the processing method described by Boore [4] to retain the permanent ground displacement. Velocity and acceleration time histories are then recomputed from the processed displacement time history using simple numerical differentiation and these time histories are referred to as corrected records. Because of the location of the near-source stations (SFB and MZQ) and dip-slip faulting, the fling step and the directivity pulse both appear on the fault-normal component, which makes them harder to separate than in the case of strike-slip faulting, where forward directivity pulse is in fault-normal direction and fault fling pulse is in the fault parallel directions.. Wolong record Figures 4-6 show the corrected acceleration, velocity and displacement time histories of the record from Wolong site for the fault-parallel, fault-normal and vertical components respectively. This record does not appear to contain any permanent displacements and has been processed using a band-pass filter with the corner frequency of.7-5hz. This station is at about km away from the fault rupture plane at the southwest end (see Figure 2b). The PGA in the fault-normal and fault-parallel directions are similar, just over.6g, but the vertical component has a PGA of.94g, the largest vertical PGA among the three near-source records, as shown in Table. The peak ground velocity (PGV) for this record is moderate, just over 3cm/s for the horizontal components and 23cm/s in the vertical direction. Because of the location, this record shows two sub-events very clearly. The first sub-event in the time window of 25-35s is likely to be associated with the fault rupture in the hypocentral area, consistent with the rupture duration for this part of the fault reported by Wang et al. [2]. The rupture in this part of the fault initiated in the hypocentre and then propagated in the southwest and northeast directions largely along the strike of the fault. The second sub-event is likely to be associated with the fault rupture in the Wenchuan area and the secondary fault Hanwang-Bailu fault that is parallel to the main Longmenshan fault. The centre of the first asperity on the secondary fault (Kotetsu et al. [6]) is just over 8 km from the Wolong station. The rupture further away from the Wolong station, such as the large fault slip concentration in the Beichuan area (at a distance over 7km away) generated a PGA much less than.g, except for three peaks at 8s (.g), 85s (.2g) and 5s (.g) (not shown in Figures 4-6). In the fault-normal direction, PGA and PGV associated with the first sub-event are much larger than those associated with the second sub-event. However, the peak ground displacement (PGD) associated with the second sub-event is much larger than that associated with the first 5

6 sub-event in the two horizontal directions (Figures 4 and 5). Figure 6 shows that, in the vertical direction, the PGA is larger than those of the horizontal components (associated with the first sub-event), and that the PGD is similar to those of the horizontal components. The PGD associated with the first sub-event is also similar to that of the second sub-event. Figure 7 shows the response spectra of the Wolong record. The acceleration spectra of the fault-normal and fault-parallel components are generally similar. The vertical spectra are considerably larger than those of the horizontal components within the spectral period of.5s and decrease rapidly with increasing spectral period (Figure 7a). Figure 7 shows that the velocity spectra of the vertical component in a period range of.5-3s are smaller than those of the horizontal components and the three components have similar spectra in the spectral period range of 3-6s. Figure 7(c) shows that, at long period, the vertical spectra are similar to those of the fault-normal component and are larger than those of the fault-parallel component in the period range of 3-9s. The response spectra of the fault-normal component are considerably larger than those of the fault-parallel component in the spectral period range of 5-8s. To explore the possible different characteristics of the two sub-events, we present the response spectra of the Wolong record computed separately for two sub-events. The left panel of Figure 8 shows the spectra of the first sub-event (the rupture in the Dujiangyan area), and the right panel shows the spectra of the second sub-event (the rupture in the Wenchuan area). The acceleration and velocity spectra of the first sub-event within the spectral period of 3.s (Figures 8a, 8c and 8e) are nearly identical to those computed from the full record as shown in Figure 7. The displacement spectra of the vertical component associated with the first sub-event are similar to those of the full record, and the vertical displacement spectra associated with the second sub-event are also comparable to those of the full record at all periods. The large vertical spectra of the Wolong records may be a result of the slip direction of the two sub-events, the up-east slip in both the first (Figure 2b, in Dujiangyan area) and the second sub-events (in Wenchuan area). The displacement spectra of the fault-normal and fault-parallel components associated with the second sub-event are considerably larger than those associated with the first sub-event, a possible result of the large magnitude associated with the second sub-event. The large vertical displacement spectra over 5s spectral period (Figure 8e) are nearly 3 times that of the horizontal component, a possible result of the large vertical displacement in the time window in Figure 6c. The relatively large vertical displacement spectra associated with the second sub-event possibly result from the large vertical displacement in the 45-6s time window. Note that a large vertical fault-fling effect (large vertical permanent displacement) can also produce large vertical spectra (comparable with or larger than the horizontal spectra) as for the TCU52 and TCU68 records from the 999 Chichi, Taiwan earthquake. However the large vertical spectra for the Wolong record do not appear to be caused by fault-fling effect. 2. MZQ record Figure 9- shows the corrected time histories from the MZQ site with a source distance less than 2km. PGA, PGV and PGD for this record are presented in Table. The faultparallel component has a PGA of.83g, larger than that of the fault-normal component (.67g). The fault-normal component has a much larger PGV (.39m/s) and PGD (2.35m) than those of the fault-parallel component, a possible result of the large permanent fault displacement of.5m in the fault-normal direction and forward-directivity effect (the rupture of the fault for sub-event 2 at the main fault appears to propagate towards to the MZQ station). 6

7 The vertical component has a moderate PGA of.47g. It appears that two major asperities in the 2 nd and the 3rd sub-events (both with a moment magnitude of 7.6) both contributed to the strong shaking in a time range of 35-55s because of the location of the MZQ station and the progress in fault rupture. The accelerations (over.2g) in the time window of 65-75s may be associated with the rupture at the northern end of the fault for the second major asperity (subevent 3). The velocity pulse at 38s in Figure has a period about 6s, which may have had relatively little effect on the response of most structures in the heavily damaged area, as many multi-storey buildings and bridges with moderately long span would have a natural period less than 2.s. Figure shows the displacement time histories, and the permanent ground displacement for the fault-normal component at this site is about.5m (of a similar order to the slip distribution suggested by Wang et al. [2] in Figure 2b),.75m for the fault-parallel component and.7m in the vertical component. The survey by Xu et al. [] does not suggest large surface-fault displacement (Figure ) in this area and one possible reason for this is that the.5m displacement may have occurred over a -3m zone without a sudden and obvious relative displacement. Note that the permanent ground displacements estimated from acceleration time histories are known to have poor accuracy (Boore, [4]) and these values are the probable ones rather than the corrected ones. The accuracy of permanent ground displacement has virtually no effect for response spectra within about s spectral period, and the inaccurate estimates for permanent ground displacement hence has little effect on the response of most structures in the heavily damaged area. Figure 2 shows the response spectra for the MZQ record. The fault-parallel component has much larger spectral accelerations within the spectral period.2s than the fault-normal component, and the vertical component has about half of the spectral accelerations of the fault-normal component in a very wide spectral period band (.-2s). The fault-normal component has a much larger spectral velocity and displacement at spectral periods over 2.5s and none of the displacement spectra reaches constant-displacement period range within the spectral period of s. The large displacement spectra of the fault-normal component are a possible result of fault-fling (permanent ground displacement about.5m) and forward directivity effects with PGD being about m larger than the permanent ground displacement. Note that the MZQ station is located between the two asperities, and the fault slip and the area of moderately large slip in the immediate area (km in diameter) of the MZQ station are relatively small (see Figure 2a). The PGD, PGV, permanent ground displacement and response spectra derived this station are unlikely to be the representative values for the nearsource ground motions for immediate areas about the two asperities. 3. SFB record Figure 3 shows the acceleration time histories of the SFB record at a source distance of 3 kilometres. The duration of strong shaking (over.3g) is nearly 3s and is about 6s for accelerations over.2g. The PGA in the fault-normal and fault-parallel direction is moderate, only about.5g, and the vertical PGA is.56g, as shown in Table. A notable feature of this record is that the vertical peak values, including PGV and PGD, are all larger than the two horizontal components. Figure 4 shows the velocity time histories for the fault-parallel, fault-normal and the vertical components, with the velocity in the time range of 35-6s in the fault-normal component being considerably larger than those of the fault-parallel component. The vertical velocity time-history in the time window of 38-48s have positive values that lead to the large vertical displacement shown in Figure 5. The permanent vertical displacement is about 3.8m while the permanent displacements in the two horizontal directions are likely to be within the error range of the processing method. This site is on the hanging wall and a 7

8 large vertical (upward) permanent displacement is plausible. The fault-fling is the possible reason for the large peak ground-motion values for the vertical component. Figure 6 shows the response spectra of the SFB records, and a striking feature is that the vertical spectra in the period range of.3-.8s are considerably larger than those of both faultnormal and fault-parallel components, a possible result of vertical fault fling effect. Another feature is the unsmoothed raise of displacement from zero to over 37cm within about 2s, leading to large accelerations and the long duration of the strong shaking. The vertical velocity and displacement spectra in the period range of.5-7s are comparable with those of the fault-normal component and are considerably larger than those of the fault-parallel component in spectral periods over 4s. The displacement spectra of the fault-parallel component appear to reach nearly constant values for periods over 3s and for period over about 5s for the vertical component, while the displacement spectra for the fault-normal component linearly increase with increasing spectral period up to s as shown in Figure 6(c). The displacement spectra of the fault-normal component are much larger than those of the fault-parallel and vertical components in the spectral period over 6s, as a result of longperiod ground motions in the 35-45s time window. We do not have a plausible explanation why fault fling step cannot be obtained in the displacement time-history of the SFB record. One possible reason is that the fault dip angle changed quickly with increasing depth with fault close to the ground surface is nearly vertical according to Wang et al [2] inversion model. Further refinement in the fault-rupture model in the near future may provide reasonable explanations. Comparison with near-source strong motion records from other large earthquakes Table 2 lists the PGA, PGV and PGD from large earthquakes in the NGA dataset and the records are separated into two groups characterized by the depth to the top of the fault rupture. Near-source PGAs from moderate and large (M w over 6.5) events do not appear to depend on earthquake magnitude significantly. PGVs and PGDs listed in Table 2 have no obvious correlation with either magnitude or site class, though regression analyses on the near-source records from the NGA data set show strong magnitude-dependent PGV and PGD (Zhao 29, unpublished results). For surface-rupture earthquakes, PGV and PGD at stations close to the fault are largely determined by forward-directivity and/or fault-fling effect which means that the magnitude scaling for near-source PGV and PGD cannot be reliably estimated. The longperiod spectra caused by fault-fling effect may be sensitive to the source distance because permanent ground displacements are likely to decease reasonably quickly with increasing distance from the fault rupture plane. Among the near-source records from surface-rupture earthquakes listed in Table 2, only TCU52 and TCU68 records from the 999 Chichi, Taiwan earthquake (due to fault-fling effect) and the Lucerne record from the 992 Landers earthquake (a combined effect of faultfling and forward-directivity effect) have a larger PGV and PGD values than the fault-normal component of the MZQ record. The vertical PGV and PGD for the SFB record are also less than those of TCU52 and TCU68 records but larger than all other records in Table 2. The Denali record, the only record from an M w =7.9 earthquake in Table 2, has comparable PGV and PGD with those of the MZQ record but has a much smaller PGV than that of the MZQ record. The records from the buried-fault earthquakes in Table 2 have comparable PGA with 8

9 those of the MZQ records though the records from the buried-fault events have a larger source distance and much smaller moment magnitude. Figures 7 and 8 compare the response spectra of the fault-normal component from the 28 Wenchuan earthquake with a number of the near-source records listed in Table 2. Figure 7(a) shows that the acceleration spectra (the left panel) of the MZQ record are generally much less than those from the buried-fault earthquakes in the spectral band of.5-2s, especially in the period range of.5-s, even though the buried-fault earthquakes have much smaller moment magnitudes, consistent with the findings by Somerville and Pitarka [7]. Figure 7 shows that the displacement spectra for the MZQ fault-normal component are generally smaller that those of the buried-fault earthquakes in the period range of -2.5s and much smaller than the LGPC record of the 989 Loma Prieta earthquake. At spectral periods over 3.5s the displacement spectra of the MZQ record are larger than those of the Northridge records. The acceleration spectra for the fault-normal component of the SFB record within a period range of.2s are comparable to those of the buried-fault earthquakes, such as the Sylmar 36, the Rinali 226 (994 Northridge), Tabas fault-normal component (978 Tabas, Iran) and the LGPC records (989 Loma Prieta) as shown in Figure 7(c). Figure 7(c) also shows that the acceleration spectra of the SFB record in the spectral period range of.5-.2s are much smaller than those of the buried-fault earthquakes. Figure 7(d) shows that the displacement spectra for the SFB record are much smaller than those of the buried-fault records in the period range of 2-4s, and the relatively small long-period spectra for the SFB record (from the surface-rupture earthquake) is not only caused by the relatively long source distance of about 4km from the fault rupture plane. To scale the spectra from 4km to 2km distance, a factor about.3 can be derived from attenuation models (Zhao et al. [5]) at a.8s period. The spectra from buried-fault earthquakes are well above 2 times the SFB record. Figure 7(e) shows that the spectra within the period range of.3s are comparable with those of the buried-fault earthquakes. However, the Wolong spectra in the period band of.5-2s are much less than those from the buried-fault records, even though the sub-event at the southwest end of the fault appears to have a large area of the fault plan below about km. The low long-period energy of the first sub-event (because of the relatively small magnitude) and the large distance between Wolong station and the fault plane of the second sub-event (about 4-5km) are possible causes for the relatively small long-period spectra of the full Wolong records. The spectra ratios at long periods between the Wolong record and the near-source record at MZQ site are reasonably consistent with the geometric attenuation rates from the Abrahamson and Silva [9] and the Zhao et al. [5] models. Figure 8 compares the near-source spectra of the Wenchuan earthquake with those from surface-rupture earthquakes. The acceleration and displacement spectra of the fault-normal component of the MZQ record are generally very similar to those of the Lucerne record from the 992 Landers earthquake at most spectral periods, as shown in Figures 8(a) and 8, even though these two records have different type of velocity pulse in the fault-normal direction. The MZQ acceleration spectra at short periods (PGA and at periods less than.2s) of the fault-normal component are considerably larger than those of the TCU52, TCU68 (EW component is the fault-normal component) and TAPS Pumping Station. The displacement spectra of the MZQ record over s spectral period are considerably smaller than those of the TCU52 and TCU68 records (caused by very large fault permanent displacement) and the record from TAPS Pumping Station. Note that the long-period 9

10 spectra from the Lucerne record are the result of both forward-directivity and fault-fling effects. Figure 8(c) compares the acceleration spectra of the fault-normal component of the SFB record with those of the surface-rupture earthquakes. The short-period spectra for this record within about.5s spectral period are comparable with those of the Lucerne record but are much larger than those from the TCU52, TCU68 and the Denali record. However, the displacement spectra of the SFB record over 2s spectral period are considerably less than those of the spectra from the surface-rupture earthquake records (Figure 8d). The ground motion at this station is contributed by the 2 nd and the 3 rd sub-events that did not rupture simultaneously, and the relatively large source distance of just over 4km (from the fault rupture plane) may be the reason for the relatively small displacement spectra. Figures 8(e) and (f) compares the response spectra of the fault-normal component at Wolong station. The short-period acceleration spectra in a spectral period range of.2-.5s for this record are considerably higher than those of the surface-rupture earthquakes but the spectra over.6s are much smaller. The small displacement spectra at the Wolong station in the spectral period over 3s are a possible result of the large distance from the fault of the second sub-event (the long period spectra of the full record is from the 2 nd sub-event). In summary, the response spectra of the near-source record MZQ (fault-normal component) has the third largest PGV and PGD among the near-source records in the NGA dataset, with only the TCU52 and TCU68 record from the 999 Chichi, Taiwan earthquake having larger values. The near-source response spectra in the period range of.5-2s are much smaller than those from the buried-fault earthquakes in the NGA dataset. The acceleration spectra of one near-source record (MZQ) in the period range of.5-2s are comparable with those of the near-source records from surface-rupture earthquakes. There are too few near-source records for us to have a complete picture of possible ground shaking in the near-source region. However, very small PGA and short-period spectra, seen in some of the near-source records from the 999 Chichi, Taiwan earthquake and the 22 Denali earthquake have not been recorded for the Wenchuan earthquake. We cannot be certain wether the area with a large fault displacement experienced very small PGA and short period spectra. The fault-fling effect probably occurred in many areas of the near-source region, where large ground surface displacements due to fault slip or even ground failure (over a very large area) have been observed (Xu et al. []). The hanging-wall side of the fault may also have experience large vertical response spectra because of the large vertical uplift. Displacement response spectra comparable with those of the TCU52 and TCU68 records may also have occurred in areas close to the large surface rupture of the Wenchuan earthquake. Comparison with Design Spectra in the Design Code in China For nearly all parts of the heavily damaged area, the design intensity level is VII according to the Chinese code GB 5-2 (Standard of People s Republic of China, 2 edition [6]). The equivalent acceleration at a zero spectral period is.g only for a design intensity of VII. Using Tables 3.2.2, and and Figure 5..5 in the Chinese design code, the 5% damped elastic spectra in the design intensity VII zone for rare earthquakes (equivalent to 475 year return period) are shown in Figure 9, together with those before 28 being given in (a) and with those of the revised version (28 edition, revised to VIII) in

11 for the same geographic area. The code stipulates that buildings under the design load for rare earthquakes should not collapse. Note that the displacement demand of the Wolong record in the spectral period range of -2s is marginally larger than the code design spectra (Figure 9) and lower than the design spectra over 2.3s. A moment-resisting frame building with 5-5 storeys (for design intensity VII zone, the natural period of this type of building tends to be close to.-.5 times the number of storeys) may have a good chance of surviving this level of ground shaking. The revised design spectra after the Wenchuan earthquake are presented in Figures 9(c & d) and they appear to be reasonable for longperiod structures. Note that the wide-spread collapses of buildings and bridges are not only caused by ground motions that are much larger than the design spectra. Many structures close to large groundsurface ruptures (caused by fault rupture or ground failure) were torn apart by differential foundation displacement. Many of the other collapses caused by dynamic vibration or ground shaking may be a direct result of design failures. For example, nearly 9% of the plastic hinges in the collapsed reinforced concrete structures occurred in either columns or column-beam joints, suggesting possibly serious flaws in either the design code and/or inappropriate design practice in rural areas or small town and cities in China. Attenuation of peak ground accelerations Figure 2 compares the recorded PGA from the Wenchuan earthquake with the attenuation models developed by Zhao et al. (26) for crustal earthquakes using mainly Japanese data. The recorded PGA in Figure 2 is from Yuan et al. [7]. The site conditions for the strongmotion stations are not known and the model for site class II (soil site with a natural period between.2s and.4s in the Zhao et al. model [5], corresponding to site class C in the NEHRP site classes BSSC 2) is used. Figure 2(a) shows the geometric mean of two horizontal components, and the Zhao et al. model [5] fits the PGA data surprisingly well. Figure 2 compares the vertical PGA with an unpublished model by Zhao (28), derived from mainly Japanese data and the fit is also surprisingly good. The largest vertical PGA is from the SFB record and is a possible result of the large vertical fault-fling effect. Summary We have presented the characteristics of three near-source strong-motion records from the Great Wenchuan 28 earthquake in China. The length of the fault-rupture plane is over 3km with a reverse faulting mechanism for 2/3 of this fault from the south-west end and apparent strike-slip faulting for/3 of the fault from the northeast end. Using the inversion results from Wang et al. [2] we have divided the earthquake into 4 sub-events so as to explain the near-source ground-motion characteristics. The first sub-event was located at the southern end of the Longmenshan fault in the hypocentral area measuring 5x3km. The second subevent occurred on the main fault and on a secondary fault close to Wenchuan area with a fault-rupture length of 96km. This part of the secondary fault, according to the Koketsu et al. model [6] has one of the two major asperities. The third sub-event was located in the Beichuan area with a fault length of km and the last sub-event was located in the Qingchuan area with a fault length of 96km. Moment magnitude is calculated for each subevent, based on the moment release functions and snapshots of rupture propagation by Wang et al. [2]. The first sub-event has a moment magnitude of 7.. The second and the third subevents have a possible moment magnitude of 7.6 and the fourth sub-event has a possible

12 moment magnitude of 7.2. The fourth sub-event has a strike-slip dominant rupture type and the other 3 sub-events have a reverse focal mechanism. Station Wolong is km from the south-west end of main Longmenshan fault and the record clearly shows two sub-events, one associated with the fault rupture around the hypocentre and one from the second sub-event associated with one of the two major asperities(close to Wenchuan area). The second large asperity in the Beichuan section of the Longmenshan fault generated relatively small strong-ground motion (considerably less than.g) at the Wolong site. The short-period spectra of horizontal components for this record from the first subevent (close to this station with a possible moment magnitude about 7. or less) are considerably larger than those from the second sub-event, but the long-period spectra generated by the second sub-event are much larger than those by the first sub-event (both have a possible moment magnitude of 7.6). The vertical displacement spectra of the first subevent at periods over 3s are much larger than those of the horizontal component but this is not a result of vertical fault-fling effect, as it is not possible to derive permanent ground displacement from this record. Station MQZ is located about 2km from the fault-rupture plane and has a PGA of.67g, a PGV of 39cm/s and a PGD of 235cm in the fault-normal direction, the third largest record among the world-wide strong-motion data in terms of PGV and PGD. The permanent displacement at this station in the fault-normal direction is about 5cm, 75cm in the faultparallel component and about 7cm for the vertical component. Fault-fling effect is evident in the fault-normal component with a relatively large displacement demand on long-period structures. Station SFB is located about 3km from the secondary fault rupture plane (with large vertical fault slip as shown by Koketsu et al. [6]) and on the hanging wall side. The duration of strong shaking for this station is longer than that of the MQZ station and the two sub-events with major asperities appear to contribute to the strong ground shaking. The PGA for the faultnormal component is.49g and.5g for the fault-parallel component. The PGV for the fault-normal component is only 38cm/s and 79cm/s for the fault-parallel component. The PGD is 76cm for the fault-normal component and 52cm for the fault-parallel component. The striking feature of this record is that the vertical PGA (.56g), PGV (85cm/s) and PGD (376cm) are much larger than those of the horizontal component. A large vertical permanent ground displacement of about 37cm is the third largest vertical PGD among the world-wide near-source records (after TCU52 and TCU68 records from the 999 Chichi, Taiwan earthquake). The vertical acceleration spectra in the spectral period range of.3-.9s are considerably larger than those of the horizontal component and the displacement spectra over 4s spectral period are considerably larger than the fault-parallel component. The large vertical PGV and PGD at the SFB site are likely caused by fault-fling effect a large vertical permanent displacement. We compared the near-source spectra of the Wenchuan earthquake with those of buried-fault and surface-rupture earthquakes. In the spectral period range of.5-2s, the near-source spectra at the MZQ site (about 2km from the fault-rupture plane) are generally much smaller than those of the buried-fault earthquakes. The response spectra from the MZQ site are comparable with those of the surface-rupture earthquakes, consistent with the finding by Somerville and Pitarka [7]. Very small PGA and short-period spectra, such as some of the near-source records from the 999 Chichi, Taiwan earthquake and the 22 Denali earthquake were not recorded from the Wenchuan earthquake. Therefore it is not possible to be certain 2

13 wether the area with a large fault displacement experienced very small short-period ground shaking. Displacement response spectra comparable with those of the TCU52 and TCU68 records may also have occurred in areas close to the large surface rupture of the Wenchuan earthquake but were not recorded because of too few strong-motion stations in the nearsource region. Because the MZQ and SFB stations are located between the two asperities, the PGD, PGV, permanent ground displacement and response spectra derived these two stations are unlikely to be the representative values for the near-source ground motions for immediate areas about the two asperities. The fault-fling effect appears to be the reason for large long-period spectra in the fault-normal and/or vertical direction. Many areas on the hanging-wall side of the main fault are likely to have experienced vertical ground shaking stronger than horizontal shaking. We compared the near-source response spectra with the design spectra in the previous and current design code for the area that was heavily damaged in the great Wenchuan earthquake. The near-source spectra at MZQ station greatly exceed the design spectra (before the revision in 28) within the spectral period of 3s, while the other two near-source records have comparable long-period spectra to the code design spectra. The revised design spectra in the current Chinese design code for the heavily damaged area appear to be reasonable for long period structures. Code design spectra usually do not envelope the most severe conditions, but just represent a level closer to the median value, and that the change in the code has moved a previously deficient level close to the median. However, there will be places where the design code is exceeded, such as in the area of the MZQ Station. The attenuation models developed by Zhao et al. [5] for crustal earthquakes using mainly Japanese data match the attenuation of horizontal and vertical peak ground accelerations from the Wenchun earthquake very well. 3

14 Acknowledgements The strong-motion records used in the present study were kindly provided by the National Strong Motion Observation Network System, China Earthquake Administration. Jane Forsyth and Dr. Caroline Holden reviewed this manuscript with invaluable comments. We also would like to thank Professor Kazuki Koketsu for his explanation of the rupture model his team developed. The research reported here is partially supported by Foundation for Research Science and Technology of New Zealand, Contract No. C5X42. References. Xu,Xi-wei, Wen, Xue-ze, Ye,Jian-qing, Ma, Bao-qi, Chen, Jie, Zhou, Rong-Jun, He, Hong-lin, Tian,Qin-Jian, He,Yu-Lin, Wang,Zhi-cai, Sun, Zhao-min, Feng, Xi-Jie, Yu, Shen-e, Ran, Yong-kang, Li, Xi-guang, Li, Chen-Xia and An,Yan-Fen, The Ms 8. Wenchuan Earthquake Surface Rupture and its Seismogenic Structure, Seismology and Geology, 3(3) , in Chinese with English abstract, Wang, W.M., Zhao, L.F., Li, J. and Yao, Z.X., Rupture process of the 28 Ms 8. Wenchuan Earthquake of Shichuan, China, Chinese Journal of Geophysics, 5(5), 43-4, in Chinese with English abstract, Zhang Y., Feng, W.P., Xu L.S., Zhou C.H. and Chen Y.T., Spatio-temporal rupture process of the 28 great Wenchuan earthquake, Science in China Series D: Earth Sciences, 52(2), 45-54, Institute for Research on Earth Evolution, Simulation of 28 Wenchuan, China earthquake using the Earth Simulator Chen, J., and Hayes, G., Preliminary Result of the May 2, 28 Mw 7.9 Eastern Sichuan, China Earthquake, Koketsu, K., Yokota, Y., Ghasemi, H., Hikima, K., Miyake, H., and Wang, Z., Source process and ground motions of the 28 Wenchuan earthquake, Proceedings of International Conference on Earthquake Engineering - The First Anniversary of Wenchuan Earthquake, -2 May 29, Southwest Jiao Tong University, Chengdu, Shichuan, China, pp Somerville, P. G. and Pitarka, A., Differences in earthquake source and ground motion characteristics between surface and buried earthquakes, Proc. Eighth National Conf. Earthquake Engineering, Paper No. 977, Dalguer, L. A., Miyake, H., Day, S.M. and Irikura, K, Surface Rupturing and Buried Dynamic-Rupture Models Calibrated with Statistical Observations of Past Earthquakes Bulletin of the Seismological Society of America, Vol. 98, No. 3, pp. 47 6, Abrahamson, N. A. and Silva, W. J., Summary of the Abrahamson & Silva NGA Ground-Motion Relations, Earthquake Spectra, Vol 24(), 67-97, 28. Boore, D.M. and Atkinson, G.M., Ground-Motion Prediction Equations for the Average Horizontal Component of PGA, PGV, and 5%-Damped PSA at Spectral Periods between. s and. s, Earthquake Spectra, Vol 24(), 99-38, 28. Somerville, P. G., N. F. Smith, R. W. Graves and N. A. Abrahamson, Modification of empirical strong ground motion attenuation relations to include the amplitude and 4

15 duration effects of rupture directivity, Seismological Research Letters, Vol. 68, , Abrahamson, N., Introduction to Strong Motion Seismology, Norm Abrahamson, Pacific Gas & Electric Company SSA/EERI Tutorial 4/2/6, Li, X.J., Zhou, Z.H., Yu, H.Y., Wen, R.Z., Lu, D.W., Huang, M., Zhou, Y.N., and Cu, J.W. Strong motion observations and recordings from the great Wenchuan Earthquake, Earthquake Engineering and Engineering Vibration, 7(3), , Boore, D.M., Effect of Baseline Corrections on Displacements and Response Spectra for Several Recordings of the 999 Chi-Chi, Taiwan, Earthquake, Bulletin of the Seismological Society of America, 9(5) 99 2, October 2 5. Zhao, J.X., J. Zhang, A. Asano, Y. Ohno, T. Oouchi, T. Takahashi, H. Ogawa, K. Irikura, H.K. Thio, P.G. Somerville, Y. (Yasuhiro) Fukushima, and Y. (Yoshimitsu) Fukushima, Attenuation relations of strong ground motion in Japan using site classification based on predominant period. Bulletin of the Seismological Society of America, , Standard of People s Republic of China, (2) Code for seismic design of buildings GB 5-2, 2 and 28 Editions 7. Yuan, Y.F. et al., General introduction of engineering damage of Wenchuan Ms 8. earthquake, Earthquake Engineering and Engineering Vibration, Vol 28 Supplement, 5

16 Table Peak ground acceleration, velocity and displacement for 3 near-source records of the Wenchuan earthquake PGA (g) PGV (cm/s) PGD (cm) Wolong (Soil) Fault-normal Fault-parallel Vertical MZQ (Soil) Fault-normal Fault-parallel Vertical SFB (Soil) Fault-normal Fault-parallel Vertical

17 Table 2 Information of large near-source records from NGA strong-motion dataset Earthquake Name Year Station Name Mw FM Depth (km)* Dist (km) Site class Surface-rupture earthquakes CHY8 2.7 C TCU52.7 C R Chi-Chi, Taiwan TCU65.6 D TCU68.3 C TCU2.5 C Denali, Alaska 22 TAPS Pump Station 7.9 SS 2.7 D Landers, US 992 Lucerne 7.28 SS 2.2 C Buried-fault earthquakes Tabas, Iran 978 Tabas 7.35 R 2. B Loma Prieta, US 989 LGPC 6.93 OB C Sylmar Conv. Sta 5.4 D Northridge- 994 Sylmar Olive View Hosp R C Rinaldi Rec. Sta. * Depth to the top of fault rupture R reverse faulting SS strike-slip faulting OB oblique faulting 6.5 D Table 2 Information of large near-source records from NGA strong-motion dataset (continuous) First horizontal Second horizontal Vertical Station Name PGA (g) PGV (cm/s) PGD (cm) PGA (g) PGV (cm/s) PGD (cm) PGA (g) PGV (cm/s) PGD (cm) Surface-rupture earthquakes CHY TCU TCU TCU TCU TAPS Pump Station Lucerne Buried-fault earthquakes Tabas LGPC Sylmar Conv. Sta Sylmar Olive View Hosp Rinaldi Rec. Sta

18 (a) Figure The ground-surface rupture displacement, possibly caused by fault rupture and/or ground failure, (a) vertical; and horizontal. The vertical black lines illustrate the displacement amplitude. The locations of the 3 near-source stations are marked by triangles in (a). The fault extending from Dujiangyan up to Beichuan is referred to as the Longmenshan fault which connects the Beichuan- Yingxiu surface rupture (both are referred to as the main fault). Hanwang-Bailu surface rupture is referred to as the secondary fault. 8

19 Asperity # 2 Sub-event 4 Sub-event 3 Asperity # Sub-event MZQ SFB Sub-event 2 Wolong Hypocentre (a) Figure 2 The surface fault projection and total fault slip distribution in (a); and subevents with corresponding moment magnitude and approximate location of the 3 strong motion stations in. The locations of dark areas on the fault surface projection are labelled as the first and second asperities the concentrations of large fault slip. The large numbers, 2 and 3 in are for the fault segments used by Wang et al. (28), with for the fault from Dujiangyan up to the Beichuan segment (sub-event 3), 2 for the secondary fault and 3 for the Qingchuan segment (sub-event 4). Note that Koketsu et al [6] suggested that the first major asperity should be located on the secondary fault. SFB station is about 3km from the secondary fault and is on the hanging wall side 9