## 4 Discussion
### Discussion on the Trampoline Effect  
In seismic design, the influence of vertical ground motion on structural responses may need to be considered, and this influence is often specified by a certain ratio of horizontal ground motion [40]. Generally, the vertical acceleration is about 1/2 to 2/3 of the horizontal one [41]. With the development of strong motion technology, the number of strong motion stations has increased rapidly, and more near-field strong motion records have been obtained, giving people a deeper and more comprehensive understanding of strong motion records. Especially in recent years, more and more earthquake records with larger vertical peak accelerations have drawn people's attention.  
Figure 12: Distribution of high-value PGA near locked fault segments.  
The trampoline effect refers to the phenomenon that when seismic waves reach the ground surface, the surface layer will be compressed and bounced up, just like a trampoline. When the surface layer recovers its elasticity, it will produce an upward reaction force, which will increase the ground acceleration (as shown in Figure 13). The trampoline effect was first observed in the 2008 Mw6.9 Iwate-Miyagi earthquake in Japan [42], and later reported in other earthquakes, such as the 2010 Mw7.2 El Mayor-Cucapah earthquake in Mexico [43], and the 2016 Mw7.8 Kaikoura earthquake in New Zealand [44]. In these earthquakes, it was observed that the vertical acceleration was significantly larger than (or even several times larger than) the horizontal acceleration, and the vertical upward acceleration was significantly larger than the downward acceleration. [PERSON] et al. studied the 3866 cm/s\({}^{2}\) vertical acceleration produced by the Mw6.9 Iwate-Miyagi earthquake on 14 June 2008 and found that the vertical ground acceleration showed a clear asymmetry, with the peak value in the upward direction being about 1.6 times that in the downward direction.  
The main factors that affect the trampoline effect are the seismic wave characteristics, the ground properties, and the topography. The seismic wave characteristics include the frequency content, amplitude, duration, and incidence angle of the seismic waves. Some site conditions may enhance or reduce the trampoline effect. For example, soft soils with low stiffness and high damping may amplify seismic waves more than hard rocks with high stiffness and low damping. Layered grounds with impedance contrasts may reflect or refract seismic waves more than homogeneous grounds.  
This paper calculates the ratio of vertical ground motion to horizontal ground motion peak acceleration (V/H) and its relationship with the number of stations, as shown in Figure 14. The analysis shows that there were 172 stations with a vertical-to-horizontal acceleration ratio greater than 2/3 in this earthquake motion, accounting for 48% of the total number of records. By further analyzing typical vertical and horizontal acceleration records, the paper finds that station 3123 located in Hatay city (coordinates 36.160\({}^{\circ}\) E, 36.214\({}^{\circ}\) N, fault distance 1.72 km) has a vertical peak acceleration of 867.58 cm/s\({}^{2}\), which was 1.46 times that of the north-south direction (593.94 cm/s\({}^{2}\)); more importantly, its vertical upward peak acceleration of 867.58 cm/s\({}^{2}\) was much larger than its downward peak acceleration of 470.46 cm/s\({}^{2}\) (the upward direction was 1.84 times that of the downward direction), as shown in Figure 15, is was a typical manifestation of the trampoline effect in this earthquake.  
Figure 13: Schematic diagram of the trampoline effect [42].