NEAR-SOURCE STRONG GROUND MOTION: CHARACTERISTICS AND DESIGN ISSUES

George P. Mavroeidis1 and Apostolos S. Papageorgiou2

ABSTRACT

In this study, a large number of near-source strong ground motions have been collected from various tectonic environments worldwide creating an extensive data set. Time histories from different fault types and earthquake magnitudes recorded at various distances from the causative fault are compared and their special features are discussed. A comprehensive review and study of the factors that influence the near-fault ground motions is attempted. The variation and interrelation of the kinematic and dynamic parameters pertaining to the rupture process are examined. Furthermore, a simple mathematical expression for the representation of near-source ground motions is proposed. The input parameters of the mathematical expression have a clear physical meaning and can be related to basic features of the fault rupture. This expression facilitates the thorough study of the elastic and inelastic response of structures subjected to near-fault seismic excitations as function of its input parameters. Finally, a discussion regarding the implications of near-source ground motions on the engineering design of long-period structures is presented in an effort to gain insight regarding those features of near-field ground motions that control the dynamic response of such structures.

Introduction A challenging research topic in engineering seismology and earthquake engineering is the characterization of near-fault seismic motions and their effects on the performance of special structures, such as base-isolated buildings and bridges, long-span bridges and tall buildings. The problem under consideration is twofold: the first aspect of the problem is related to the physical understanding, modeling and simulation of near-fault ground motions, while the second one is associated to the characteristics of the structure itself that control its behavior under near-source dynamic excitations. While there is a good understanding of the latter aspect of the problem, there are still uncertainties and difficulties in understanding, describing and predicting the near-fault ground motions. The Station No. 2 (C02) record obtained from the 1966 Parkfield, California earthquake was essentially the first recorded time history where the decisive near-source effect was identified. This ground motion record is characterized by strong velocity and displacement pulses of relatively long periods (see Fig. 1) that clearly distinguish it from typical far-field recordings. Housner and Trifunac (1967) first made this significant observation. A year later, Aki (1968) successfully simulated the motion at Station No. 2 using a simple moving dislocation model in a paper that began a new era in earthquake

1Graduate Research Assistant, Department of Civil, Structural & Environmental Engineering, State University of New York at Buffalo, Buffalo, NY 14260 2Professor, Department of Civil, Structural & Environmental Engineering, State University of New York at Buffalo, Buffalo, NY 14260

seismology. Since then, the occurrence of well-recorded major events has verified the existence of these pulses in near-fault regions, as well as their destructive potential when the causative fault is in the immediate vicinity of large metropolitan areas. Bertero et al. (1978) were the first engineers to recognize the problem and analyze its effects on flexible structures. However, it was only after the 1994 Northridge, California earthquake that the majority of engineers recognized the severe implications of such pulses on the performance of long-period structures and started considering methods to incorporate the near-source effects in engineering codes and design.

Figure 1. Characteristic example of intense long-period near-fault velocity and displacement pulses: the

N65E component of Station No. 2 (C02) from the 1966 Parkfield, California earthquake. The solid line indicates the fault trace, while the star and triangle denote the locations of the epicenter and station, respectively.

In the present work, a comprehensive review and study of the factors that influence the near-fault ground motions is attempted using a large number of near-source strong ground motions recorded in various tectonic environments around the world. The interrelation of important kinematic and dynamic parameters pertaining to the rupture process is addressed. In addition, a simple mathematical expression that adequately describes the nature of the impulsive near-fault ground motions both qualitatively and quantitatively is proposed. Finally, a discussion regarding the implications of near-field ground motions on the design of long-period structures is presented in an effort to gain insight regarding the near-source ground motion features that control structural behavior.

Near-Source Strong Ground Motion Database The near-fault strong ground motion database that we have compiled consists of a large number of processed near-field strong ground motions from a variety of tectonic environments around the globe, including those generated by the recent 1999 Izmit, Turkey, 1999 Chi-Chi, Taiwan and 1999 Duzce, Turkey earthquakes. Namely, more than 160 recorded ground motion time histories from different fault types (i.e., strike-slip, reverse, oblique, normal) and earthquake magnitudes (i.e., Mw=5.6 to 8.1) recorded at various distances (i.e., 0 to 20 km) from the causative fault have been gathered from well known and extensively studied seismic events that have occurred in the USA, Canada, Mexico, Japan, Greece, Turkey, Romania, USSR, Iran, India and Taiwan. This set of data serves as basis for the analysis and discussion presented next. Detailed information regarding this near-fault strong ground motion database will be provided in the near future in a comprehensive MCEER report that is currently under preparation. The intense velocity and displacement pulses described in the previous section for the 1966 Parkfield, California earthquake are not observed in all database records. Although all stations included

in the database are located in the vicinity of their respective causative faults, these typical pulses characterize less than half of the database recordings. The existence of these pulses mainly depends on the relative position of the station that recorded the motion with respect to the direction of propagation of the rupture front on the fault plane as explained in detail in the next section. In other words, even though “killer pulses” are typical of near-fault ground motions, they are not a universal near-field feature (Papageorgiou, 1998). Furthermore, whenever they are present, they are not always generated by the same effect. “Directivity” and “Fling” are by far the most common causes of such strong pulses, but they are not the only ones. This will become apparent in the next sections where the key features that control near-source strong ground motions are discussed using examples found in the literature.

Characteristics of Near-Source Strong Ground Motions Directivity Effect Forward directivity occurs when the fault rupture propagates toward a site with rupture velocity approximately equal to the shear wave velocity. In this case, most of the energy arrives in a single large long-period pulse at the beginning of the record representing the cumulative effect of almost all the seismic radiation from the fault. This phenomenon is even more pronounced when the direction of slip on the fault plane points toward the site as well. These conditions are easily satisfied in strike-slip faulting, but frequently met in dip-slip faults as well (e.g., Somerville, 2000). On the other hand, if the fault rupture propagates away from the site, no strong velocity or displacement pulses are observed. Fig. 2 confirms the above statement for the fault-normal velocity and displacement time histories recorded during the 1992 Landers, California earthquake at Lucerne Valley (LUC) and Joshua Tree (JSH) stations located in the forward and backward direction with respect to the propagating rupture front.

Figure 2. Characteristic example of the “forward directivity” effect from the 1992 Landers, California

earthquake; the fault rupture initiates at the epicenter (i.e., star) and propagates along the strike-slip fault (i.e., thick solid line). Note the intense fault-normal velocity and displacement pulses at Lucerne Valley (LUC) station located in the forward direction. In contrast, the ground motion at Joshua Tree (JSH) station, lying in the backward direction, is very weak.

The intense directivity pulse is oriented in the fault-normal direction due to the radiation pattern of the shear dislocation on the fault plane. Somerville and Graves (1993) have presented a schematic diagram illustrating this feature. As a result, the fault-normal peak velocity is larger than the fault-parallel peak velocity. It should be mentioned that for strike-slip faults the strike-normal direction usually coincides with the fault-normal direction. In addition, for dip-slip faults with large dip angles, the strike-normal component might still adequately approximate the fault-normal component. Fig. 3 shows the Pacoima Dam (PCD) velocity records generated by the 1971 San Fernando, California earthquake. The rotation of the recorded velocity from the original components to the strike-normal and strike-parallel

directions yields a significant velocity pulse in the direction perpendicular to the fault strike. On the other hand, for low-dip dip-slip faults the fault-normal component of ground motion may be more adequately represented by the vertical component.

Figure 3. Characteristic example of forward directivity velocity pulse in the strike-normal direction

generated by the 1971 San Fernando, California earthquake at Pacoima Dam (PCD) station; no pulse is observed in the strike-parallel direction. Note the significant difference between the peak ground velocity values of the two horizontal components.

A similar conclusion applies to the corresponding response spectra of forward directivity pulses; the response spectrum of the fault-normal component is the predominant one for the entire frequency range, while the response spectrum of the fault-parallel component is significantly weaker. A typical example of near-source acceleration, velocity and displacement response spectra in the strike-normal and strike-parallel directions is illustrated in Fig. 4 for the ground motion recorded at Pacoima Dam (PCD) during the 1971 San Fernando, California earthquake.

Figure 4. Near-fault acceleration, velocity and displacement response spectra (5% damping) in strike-

normal and strike-parallel directions for the ground motion recorded at Pacoima Dam (PCD) station during the 1971 San Fernando, California earthquake.

In a limited number of cases, a direction other than the fault-normal may exist, where the most

severe directivity pulse occurs. Many factors may contribute to this situation. For instance, the fault-parallel direction usually defined based on the strike direction of an ideal fault plane may not locally coincide with the actual fault direction. Furthermore, anisotropy of the wave propagation medium may also result in rotation of the direction along which strong pulses are observed. Fling Effect The fling effect is related to the permanent tectonic deformation of the ground at a specific site due to an earthquake (see Abrahamson, 2000). It appears in the form of step displacement and one-sided velocity pulse in the strike-parallel direction for strike-slip faults (e.g., stations SKR and YPT from the 1999 Izmit, Turkey earthquake) or in the strike-normal direction for dip-slip faults (e.g., stations TCU052 and TCU068 from the 1999 Chi-Chi, Taiwan earthquake). In the latter case, directivity and fling effects

“build up” in the same direction and should be decoupled. In general, the fling effect is noticeable only when excessive tectonic deformation occurs associated with significant slip on the fault plane. Fig. 5 illustrates fling pulses recorded during the 1999 Izmit, Turkey earthquake. The displacement offsets recorded at Sakarya (SKR) and Yarimca (YPT) stations are compatible with the permanent tectonic deformations measured in the EW direction of the North Anatolian fault at the specific sites. The amplitude of the fault-parallel velocity pulse at SKR has been further intensified due to a supershear rupture velocity as explained in another section.

Figure 5. Characteristic examples of “fling” effects from the 1999 Izmit, Turkey earthquake. Note the

significant fault-parallel velocity pulses and displacement offsets at SKR and YPT stations. Shear Dislocation vs. Crack-Type Rupture

Aki and Richards (1980; Fig. 15.3, page 859) explain that a constant stress-drop crack model and a uniform dislocation model can be distinguished from the shape of the transverse displacement; that is from the shape of the displacement perpendicular to the fault-surface (i.e., vertical component for low-dip faults and strike-normal component for vertical strike-slip faults). This statement is directly applicable to actual seismic events. Namely, the observed transverse displacement waveforms of low-dip earthquakes are characterized by a simple smooth ramp with a very slight overshoot as shown in Fig. 6 for the 1985 Michoacan, Mexico and 1999 Chi-Chi, Taiwan earthquakes. On the other hand, the recorded transverse displacement waveforms of vertical strike-slip earthquakes display a strongly impulsive shape as illustrated in Figs. 1 and 6 for the 1966 Parkfield, California, 1992 Landers, California and 1999 Izmit, Turkey earthquakes.

Figure 6. Typical sets of near-source displacement time histories for strike-slip (i.e., 1992 Landers,

California; 1999 Izmit, Turkey) and dip-slip (i.e., 1985 Michoacan, Mexico; 1999 Chi-Chi, Taiwan) earthquakes where significant displacement offsets were observed. Note the waveform differences and similarities.

In general, dislocation models can adequately simulate strike-slip earthquakes, while large dip-slip events, especially in subduction zones, require crack-like slip functions. This is an important remark related to the mechanics of the rupture and results in significant waveform differences in the near-fault ground motions. A more extended discussion can be found in Yomogida (1988), Campillo et al. (1989) and Ruppert and Yomogida (1992) where the Caleta de Campos (CAL) record obtained during the 1985 Michoacan, Mexico earthquake is analyzed. Pulse Waveform Characteristics The pulse duration (or period), the pulse amplitude, as well as the number and phase of half cycles are the principal parameters that define the waveform characteristics of near-field pulses. These four parameters are discussed next.

There is no unique method to define the duration or period of the pulse. The simplest way is to determine the pulse duration using the zero crossings of the pulse waveform. A more complicated way is the employment of an equivalent pulse width based on energy considerations (signal spectrum) and basic concepts from the signal processing theory. An alternative approach is the utilization of the peak value of the velocity response spectrum to indirectly define the pulse period. Therefore, an “objective” definition of the pulse duration is needed, compatible with the physical aspects of the problem. We will further discuss this important issue in another section where a mathematical expression appropriate for near-source ground motions is proposed.

Figure 7. A group of narrow band near-source directivity pulses from a number of moderate-to-large

seismic events whose period increases with earthquake magnitude.

Based on the available recorded data, a clear visual observation can be made: the rupture directivity produces a narrow band pulse in near-fault strong motion recordings whose period increases with earthquake magnitude. Fig. 7 displays a group of near-source fault-normal velocity records from a number of moderate-to-large earthquakes, where the above statement is being confirmed. On the other hand, the amplitude of the pulse (e.g., peak ground velocity) appears to be a function of the earthquake magnitude and distance of the station to the causative fault. Under specific assumptions and constraints (e.g., effect of distance on pulse duration, self-similarity), empirical relationships may be derived to relate the period of the pulse with the moment magnitude, as well as the amplitude of the pulse with the

distance and moment magnitude (e.g., Somerville, 1998; Alavi and Krawinkler, 2001).

The number and phase of half cycles are additional basic parameters necessary to adequately define the principal pulse waveform characteristics. Fig. 7 illustrates that actual velocity time histories are composed of different combinations of half cycles. This is an important issue that usually is being overlooked in practice. However, it may be significant for structural response (in particular inelastic structural response) and should be considered when near-source modeling and simulations are performed. Response Spectra and Earthquake Magnitude

We have generated the pseudo-acceleration, pseudo-velocity and displacement response spectra for the entire set of the near-fault records included in our database in order to directly compare them and identify possible trends associated with the effect of earthquake magnitude.

Figs. 8a, 8b and 8c present those sets of the generated response spectra that exhibit the highest peak spectral acceleration, velocity and displacement values, respectively. Note that the maximum peak spectral acceleration occurs in a moderate quake (i.e., PCD station from the 1971 San Fernando, California earthquake with Mw=6.6), the maximum peak spectral displacement is associated with a very large seismic event (i.e., TCU052 station from the 1999 Chi-Chi, Taiwan earthquake with Mw=7.6) and the maximum peak spectral velocity occurs in a large quake (i.e., TAK station from the 1995 Kobe, Japan earthquake with Mw=6.9).

Figure 8. Near-fault pseudo-acceleration, pseudo-velocity and displacement response spectra (5%

damping) in strike-normal, strike-parallel and vertical directions for the ground motions recorded at: (a) PCD station (1971 San Fernando, California earthquake), (b) TAK station (1995 Kobe, Japan earthquake), and (c) TCU052 station (1999 Chi-Chi, Taiwan earthquake).

The above observation associated with maximum peak spectral values may be extended to the

majority of the records included in our database. Namely, the pseudo-acceleration response spectra of very large earthquakes (i.e., Mw = 7.2-8.1) are weak in short periods; the 1992 Landers, California, 1999 Izmit, Turkey, 1999 Chi-Chi, Taiwan, 1999 Duzce, Turkey and 1985 Michoacan, Mexico earthquakes are such characteristic examples. On the other hand, the pseudo-acceleration response spectra of many smaller seismic events (i.e., Mw = 6.7-7.1) are in general stronger than the corresponding spectra of larger earthquakes for the same period range (see also Somerville, 2000). For example, the pseudo-acceleration

response spectra of time histories recorded during the 1971 San Fernando, California, 1978 Tabas, Iran, 1989 Loma Prieta, California, 1994 Northridge, California, and 1995 Kobe, Japan comply with this observation. Fig. 8 verifies the above remark as well. Even though this empirical observation, at a first glance, may come as a surprise, simple analytical models can provide rigorous mathematical interpretation and physical insight regarding this issue (see Mavroeidis and Papageorgiou, 2002). Other Near-Source Effects on Strong Ground Motions

The preceded analysis and discussion pertains to directivity and fling effects. In this section, other near-source effects, not necessarily related to directivity or fling, are briefly addressed. Hanging Wall Effect

Stations located on the hanging wall of a dip-slip fault usually get larger ground motions than stations lying on the footwall at the same closest distance (Abrahamson and Somerville, 1996). The overall proximity of the hanging wall sites to the fault plane, as opposed to sites located on the footwall, is an apparent explanation. Oglesby et al. (1998) have proposed additional physical arguments to interpret this empirical observation. Surface or Interface P-wave

Another significant near-fault effect is the SP-wave (also known as surface or interface P-wave). This phase originates at the source as a shear wave, undergoes critical reflection at the surface of the elastic half-space, and subsequently propagates along the surface with the P-wave velocity (Bouchon, 1978). Kawase and Aki (1990) invoke the SP-wave to explain the damage pattern observed after the 1987 Whittier Narrows, California earthquake. Mavroeidis and Papageorgiou (2000) explore the possibility that the intense pulses recorded on the horizontal components at AEG station during the 1995 Aigion, Greece earthquake may be associated with the SP-wave. Special Cases

The extreme acceleration (�2g) recorded at Cape Mendocino during the 1992 Petrolia, California earthquake was a case where special geometrical conditions were satisfied. Oglesby and Archuleta (1997) postulate a circular barrier on the fault plane so that the above station is located on the axis that is normal to the fault plane and passes through the center of the circle that defines the barrier. Such an axis is the locus of “caustics” as demonstrated by Papageorgiou and Aki (1983). The coherent addition of seismic waves at a station characterized by this specific geometrical set up can explain the amplified ground acceleration.

High amplitude velocity pulses recorded in the strike-parallel direction may be due to a supershear rupture velocity as well. Mendez and Luco (1988) have modeled a supershear rupture using a steady-state dislocation model embedded in a layered half-space. The strike-parallel component of SKR record from the 1999 Izmit, Turkey earthquake (see Fig. 5) is a characteristic example of supershear rupture (Bouchon et al., 2001). Strong indications of supershear rupture velocity have been identified for the 1979 Imperial Valley and 1992 Landers, California earthquakes as well. However, the existence of supershear rupture velocity is rather an unusual situation [for the conditions favorable to supershear rupture propagation see the discussion in Bouchon et al. (2001) and references therein].

Mathematical Expression for the Representation of Near-Source Ground Motions

A great challenge for engineering seismologists is to develop reliable analytical models to effectively describe the nature of impulsive near-source ground motion and provide engineers with the necessary information to design structures able to withstand near-field seismic excitations. Thus, there is a clear need to complement empirical data by theoretical models that relate the kinematic and dynamic parameters associated with the fault rupture to principal pulse characteristics (see Papageorgiou, 1998).

In this direction, we propose a five-parameter mathematical expression for the near-fault ground velocity pulses. It generates synthetic signals as the product of a harmonic oscillation and a bell-shape envelope (i.e., shifted haversed sine function). That is:

� � � �� �

��

��

��������

��

����

����

���

�

otherwise

withf

ttf

tttfttfAtv PP

PP

,0

122

,2cos2cos121

)( 0000 ���

���

�

(1)

where A is the amplitude of the signal, fP is the frequency of the amplitude-modulated harmonic (or the prevailing frequency of the signal), � is the phase of the amplitude-modulated harmonic (i.e., �=0 and �=��/2 define symmetric and anti-symmetric signals, respectively), � is a parameter defining the oscillatory character of the signal (i.e., for small � the signal approaches a delta-like pulse, for larger � more oscillations appear), and t0 specifies the epoch of the envelope’s peak. The inverse of the prevailing frequency (fP) provides an “objective” definition of the pulse duration (TP). That is:

P

P fT 1

� (2)

The input parameters of the proposed mathematical expression coincide with the key features

that determine the waveform characteristics of the near-source ground velocity pulses (i.e., amplitude, pulse duration, phase and number of half cycles) analyzed in a previous section. It is important that all input parameters have an unambiguous physical interpretation. In general, the use of artificial input parameters may give rise to more flexible waveform shapes; however, the lack of any physical significance of these parameters makes difficult to link the analytical model with the physical aspects of the problem. It should be noted that an exponential function (i.e., Gaussian envelope) might substitute for the shifted haversed sine function of Eq. 1. However, the existence of an exponential function in the expression of the proposed mathematical model introduces unnecessary complexity to further analytical derivations regarding the elastic and inelastic response of single-degree-of-freedom (SDOF) systems subjected to synthetic near-fault ground motions produced by the mathematical expression. We have calibrated our analytical model using the entire set of near-fault records with distinct velocity pulses included in the compiled database. We have simultaneously fitted the displacement, velocity and acceleration time histories, as well as their corresponding elastic response spectra. A sample of this work is illustrated in Fig. 9. Detailed analysis regarding the proposed analytical model, a technique to synthesize reliable near-source ground motions, as well as a study of the elastic and inelastic response of SDOF systems subjected to synthetic near-fault motions generated by the proposed model are presented in Mavroeidis and Papageorgiou (2002) and Mavroeidis et al. (2002).

Figure 9. Proposed mathematical expression (i.e., black line) fitted to recorded ground motions (i.e., gray

line). Both time histories (i.e., displacement, velocity, and acceleration series), as well as the corresponding 5% damping elastic response spectra are illustrated.

Design Issues

The near-source ground motions are not adequately represented in modern codes. Code provisions have historically been developed based on recorded ground motions not sufficiently close to the causative fault. The current design guidelines usually employ the concept of response spectrum for the design of structures subjected to ground motion dynamic excitations. However, it seems that the response spectrum is not capable of capturing and incorporating the seismic demands introduced by intense near-fault ground motions. For instance, a suite of different ground motions (i.e., with or without near-source pulses) can match a specific design spectrum. Thus, if we perform non-linear time history analyses using this suite of ground motions, we will get significantly different responses depending on the variability of the employed recordings. On the other hand, if we perform a response spectrum analysis, we will get the same output for all time histories, as all recordings have the same spectrum. In other words, the use of any near-fault time histories whose response spectra are compatible with the design spectrum is not the best solution to proceed, even if the design spectrum incorporates near-source effects (i.e., remember that a signal consists of amplitude and phase). Near-fault time histories that include forward directivity effects should be chosen. Somerville (2000) discusses this issue in detail.

Both experimental and analytical studies have been conducted to gain a better understanding of the behavior of long-period structures (e.g., tall buildings, long-span bridges, base-isolated structures) subjected to near-fault ground motions. It is apparent that near-source pulses may be detrimental for these structures. Kasalanati and Constantinou (1999) conducted a systematic experimental study regarding the behavior of bridge elastomeric and energy dissipation systems under high velocity near-source seismic

excitations. They concluded that the addition of linear or non-linear viscous dampers in elastomeric isolation systems improves significantly the performance of structures subjected to near-fault ground motions. Hall et al. (1995) presented the effects of near-source ground motions on flexible buildings including a high-rise building and a base-isolated structure. In addition, Hall and Ryan (2000) performed numerical simulations for isolated buildings designed in compliance with the 1997 UBC near-source factors. Both actual earthquake records and synthetic ground motions were employed in these analyses. An interesting conclusion is that displacement pulses can be effective in causing damage if the duration of the pulse is comparable to the fundamental period of vibration of the structure. Alavi and Krawinkler (2001) presented a detailed study and established a methodology regarding the design of buildings under near-fault seismic excitation. Many other researchers, not listed herein, have also studied the response of simple or complex structures subjected to near-source ground motions.

Finally, it should be mentioned that the recorded peak ground displacements often used in design are not always accurate or representative of actual situations. The standard processing procedures of “raw” strong motion data remove part of the peak shear-wave displacement and essentially the whole static displacement through filtering. Besides the recent 1999 Turkey and Taiwan earthquakes, the only other exemption available in the literature is the Lucerne Valley (LUC) record of the 1992 Landers, California earthquake (Iwan and Chen, 1994) where special processing was employed to preserve the displacement information. However, the procedure proposed by Iwan and Chen (1994), as well as any other similar technique, can be applied for correction of time histories recorded by digital instruments only, as these displacements cannot be recovered with analog instruments. Therefore, extra caution is required when displacement time histories of near-fault records are compared to choose the most appropriate for reliable aseismic design of structures.

Conclusions We have presented a comprehensive review and study of the factors that influence the character of near-source ground motions with emphasis on directivity effects. Although forward directivity is by far the most common cause of the strong pulses observed in near-field regions, other important near-fault effects also exist. Nevertheless, the near-source motions, primarily influenced by source effects, exhibit characteristics that essentially reflect the complexity of the source process. In addition, we have proposed a simple mathematical expression for the representation of near-field ground motions. The input parameters of the mathematical expression have an unambiguous physical meaning. This mathematical model enables engineers to generate reliable near-source time histories appropriate for engineering design and applications. Furthermore, it facilitates the thorough study of the elastic and inelastic response of structures subjected to near-fault seismic excitations as function of the model input parameters. Finally, a discussion regarding the implications of near-field ground motions on the engineering design of long-period structures has been presented.

Acknowledgments

This work was supported by FHWA Contract DTFH61-98-C-00094, Task C2-1, under the auspices of the Multidisciplinary Center for Earthquake Engineering Research.

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