The Hanging Wall/Footwall Effects Of Near-Fault Ground Motions

Study on the near-fault ground motion has been the hot research topic in the engineering seismological community for more than 10 years, which results from the near-fault ground motion having the different characteristics from far-fault ground motion and its serious damage on the structures. Beside the conventional influence factors on the ground motions, such as source, path and the site condition, both the hanging wall/foot wall effects (the HW/FW effects) and rupture directivity have great influences on the near-fault ground motions. For the large earthquakes occurred on non-vertical faults, such as the Northridge earthquake, Chi-Chi earthquake and Wenchuan earthquake, there are obvious differences between the ground motions on the hanging wall side and those on the foot wall side. This study focuses on the ground motion differences between the hanging wall and foot wall, i.e., the HW/FW effects on the near-fault ground motions. Three aspects involved are mainly studied, the characteristics, influence factors and reasons of the HW/FW effects. To deal with the three issues, we conduct analyses of both the real near-fault recordings and simulated ground motions. Through comparing the peak values, response spectra and durations of near-fault ground motions between the hanging wall and foot wall, some conclusions can be drawn:1. Analyses of the observed ground motions from the Chi-Chi and Northridge earthquakes indicate that the HW/FW effects have important influences on the peak values, response spectra and durations of ground motions. First, the three-component peak ground accelerations (PGA), peak ground velocities (PGV) and peak ground displacements (PGD) on hanging wall are larger than those on foot wall with the same rupture distances. Second, the tri-component spectral ordinates of accelerations at almost all period ranges are larger on the hanging wall than on the foot wall at the similar rupture distances. Third, the tri-component durations of ground motions on hanging wall are smaller than those on the foot wall. Last, the HW/FW effects have higher influences on horizontal component than the vertical component of near-fault ground motions, and the HW/FW effects on peak values of ground motions becomes lower and lower in terms of PGA, PGV and PGD.2. The commonly used site-to-source distance measures, such as rupture distance Drup, epicentral distance Depc, hypocentral distance Dhyp, Joyner-Boore distance DJB and seismological distance Dseis can not describe the general proximity of a site to the rupture plane. The root-mean-square distance Drms with the meaning of weighted-average is introduced to accurately represent the proximity between an observer and the rupture plane, and examine the HW/FW effects on the near-fault ground motions. The result shows that there is no obvious bias towards positive for the hanging wall residuals when the Drms is used as distance measure, which is opposite to the results when other distance measures are used. The HW/FW effects are demonstrated to be a geometric effects caused by the asymmetry of dipping fault.3. By numerical simulation of strong ground motions from different thrust-fault events, the influence factors of the HW/FW effects are studied, such as the depth to the top of rupture (Ztop), the fault-dip (Dip), the rupture velocity (Vr), the location of initial point of rupture and magnitude.(1)Variation of Ztop has an obvious effect on the ground motions differences between hanging wall and footwall.①With increasing of Ztop, the ratio between hanging wall and footwall (HW/FW ratio) for amplitudes (PGA, PGV and PGD) of ground motions becomes larger and the affected area of the HW/FW effects extends greatly. The variation of Ztop does not change the HW/FW effects on FN component, but UD and FP components of peak values.②The influence of Ztop on the HW/FW effects on response spectra acceleration (RSA) depends on component and period. For short period, the UD- and FP- components HW/FW ratio for RSA become larger and affected area extends if Ztop increases. For long-period, the UD-component HW/FW ratio decreases but the FP-component HW/FW ratio increases with the increasing of Ztop.③With the increase of Ztop, the difference of the duration between hanging wall and footwall reduces in the near-fault region.(2)Variation of Dip has a great effect on the ground motion differences between hanging wall and footwall.①For the amplitudes of ground motion, the HW/FW ratios for PGA, PGV and PGD increase with the decreasing of Dip.②For the RSA, the variation of Dip on the HW/FW effects depends on component and period. However, in general, the HW/FW ratios for three-component RSA increase and the affected areas of HW/FW effects extend greatly if Dip decreases.③For the duration of ground motions, the difference of duration between HW and FW enlarges when Dip reduces.(3) Variation of Vr has an effect on the ground motion differences between hanging wall and footwall.①For the amplitudes of ground motions, the HW/FW ratios for PGA, PGV and PGD decrease with the increase of Vr②For the RSA, the variation of Vr on the HW/FW effects depends on component and period. However, generally speaking, the HW/FW ratios for RSA decrease when Vr increases, but the affected area of HW/FW effects does not change a lot.③For the duration of ground motions, the difference of duration between hanging wall and footwall enlarges with the increase of Vr.④The HW/FW effects on PGA are more obvious under super-shear rupture(The velocity of rupture is larger than the velocity of S wave) status than under sub-shear rupture(The velocity of rupture is smaller than the velocity of S wave) status, however, the HW/FW effects on PGV and PGD are opposite. The HW/FW effects on RSA at short-period are more obvious under super-shear rupture status than under sub-shear rupture statu, however, the HW/FW effects on RSA at long-period are opposite.(4) The location of initial point of rupture has an effect on the ground motions difference between HW and FW. In general, the variation of location for initial point of rupture along fault-parallel direction does not influence the HW/FW effects, but the variation along the down-dip direction changes the HW/FW effects a lot.①For the amplitude of ground motions, with the initial point ascending from the bottom of the rupture, to the center of down-dip direction, and then the top of rupture, the HW/FW ratios of PGA, PGV and PGD increases and the affected area of HW/FW effects extends.②For the short-period RSA, with the initial point changing from the top of rupture, to the bottom of the rupture, and then the center of down-dip direction, the HW/FW ratio for RSA decreases. However, for the long-period RSA, the HW/FW effects become more and more obvious when the initial point changes from the bottom of the rupture, to the center of down-dip direction, and then the top of rupture.③For the duration of ground motion, the difference of duration between HW and FW monotonously enlarges with the initial point changing from the top of the rupture, to the center of down-dip direction, and then the bottom of rupture.(5) Variation of Magnitude has an effect on the ground motion differences between hanging wall and footwall.①For the amplitudes of ground motion, the HW/FW ratios for PGA, PGV and PGD increase with the increasing of Magnitude, but the affected areas of HW/FW effects extend.②For the RSA, the variation of Magnitude on the HW/FW effects depends on component and period. However, in general, the HW/FW ratios for three-component RSA increase if Magnitude increases, but the affected areas of HW/FW effects extend.③For the duration of ground motions, the difference of duration between HW and FW reduces when Magnitude reduces.(6)Using root-mean-square distance, we compare the influencing factors of the HW/FW effect. The result indicates the factors which greatly change the asysmetry of dipping fault such as depth to the top of fault, fault dip and magnitude are also the major influencing factors of HW/FW effects.4. By simulating the ground motions from the non-vertical strike-slip and normal events, the hanging wall/footwall effects on near-fault ground motions are examined. The focal mechanism on the hanging wall/footwall effects is also studied. The result show:①The HW/FW effects on near-fault ground motions also exist in the strike-slip event as long as the earthquake occurs on non-vertical fault.②The HW/FW effects on near-fault ground motions also exist in the normal event as long as the earthquake occurs on non-vertical fault.③For the normal and thrust event, the HW/FW effects mainly embody on UD and FP component of ground motion. However, for the strike-slip event, the HW/FW effects embody on three components of ground motion④For the strike-slip, normal and thrust event with the same earthquake magnitude and the same fault dip, the HW/FW effects on near-fault ground motion are most obvious in strike-slip event, and least obvious in the normal event.5. Based on the observed ground motions from Wenchuan earthquake, the hanging wall/footwall effects on near-fault ground motions are studied, and the comparisons between the observed motions from this event and the predicted motions from contemporary attenuation relations are also conducted.①The HW/FW effects on amplitudes of ground motion mainly embodies on PGA.②The HW/FW effects on response spectra accelerations mainly embody on the short-period (T≤0.5s) ground motions.③The HW/FW effects on duration mainly embodies on the duration of acceleration.④The observed short-period (T≤0.5s) ground motions from the Wenchuan earthquake are under-predicted, however, the long-period (T>1s) ground motions are greatly over-estimated by the contemporary attenuation relations world-wide.