First, the ground velocities of different components simulated with two example hypocentres, H1 and H2 (Fig. 6.1), are shown2. H1 is located at the right edge of the fault plane and thus a unilateral rupture propagation is implemented into the source, whereas H2 is close to the middle of the fault plane and the rupture propagates bilaterally. The peak ground velocity (PGV) distributions are shown in Fig. 6.3.
The maximum peak ground velocities of the two horizontal components of the entire study area are equivalent and three times larger than that of the z-component. The y-component PGVs inside the small basin, region B, are larger than its neighboring area (Fig. 6.3bottom). In the area surrounding the north-west (NW) tip of the fault trace where the rupture propagates towards, for the example of H1, larger PGVs are observed compared with the example of H2 in y- and z-component (Fig. 6.3 middle and bottom). Whereas for the x-
component, in this area, H2 leads to larger PGVs than H1.
For H1 (Fig. 6.3Left), for all three components, the largest PGV location is close to both the NW edge of the fault plane and to the edge of the basin. The large PGV area is also very close the slip asperity area on the fault plane (Fig. 6.1). The joint contribution of the three
2
6.5 Two Hypocentre Examples 47
Figure 6.2: Velocity snapshots (y-component) at different times from hypocentre H3 in Fig. 6.1. Black thin lines show the contours of isosurface of shear wave velocity at 2 km/s. Thick black line shows the fault trace. Note the change of the color scale. Areas A, B and C are depicted to illustrate the structure effect on wave propagation.
Figure 6.3: PGV distributions for two example hypocentres. Left. H1. Right. H2. From top to bottom arex-, y-and z-components, respectively. The epicentres are indicated as red asterisks. The straight white line indicates the fault trace. Thin curved white lines are contours of the seismic velocity model. Note the color scale difference between different components. Region A and B are chosen for illustration.
6.5 Two Hypocentre Examples 49 major effects, i.e., the forward directivity effect, the basin amplification effect and the high slip elevation, dominate the PGV distribution for this hypocentre location. Small velocity amplitudes are observed in the area behind the rupture propagation which can be explained by the backward directivity effect.
For the hypocentre H2 (Fig. 6.3 right), there are large PGVs inside the entire basin for thex-component. In the two regions surrounding the two tips of the fault trace, large PGVs are observed (y-component, Fig. 6.3 middle right ). In this case the rupture starts from the middle of the fault trace and propagates bilaterally to these two regions.
Two profiles are chosen and the velocity seismograms on them are shown. The first profile (Fig. 6.4) is parallel to the fault trace, located aty=28.80 km and across the basin edge. For the hypocentre H1 (unilateral rupture process), in the range ofx∈[32,48] km (far from the basin edge), they-component velocity seismograms are characterized with one major impulse. Basin effects such as amplitude amplification and duration time elongation are clearly observed in the range ofx∈[15,30]km for all the three components. And for they-component velocity, the duration time and the amplitude change less than for the other two components. For H1, there are almost no perturbations in the range of x >50 km where the backward directivity effect controls the seismic motion generation (this range is far from the fault plane and outside the basin).
Figure 6.4: Fault parallel velocity profile for two ex- ample hypocentres. From top to bottom are the x-
, y- and z-components, re- spectively. The shear wave velocity isosurface depth (at 2.0 km/s) is depicted in the bottom as the shadowed area. The maximum ve- locity amplitude across this profile is shown with the in- let number.
The second profile (Fig. 6.5) is perpendicular to the fault trace and located at x=33.60 km. This profile is across the fault plane, the bigger basin and the small basin (in the range of y ∈ [50,65] km). We first focus in a small range: y ∈ [15,20] km. The x-component seismograms are characterized with one major impulse while the y-component seismograms
are with two impulses. The largest velocity amplitude of thex-component (fault parallel) is 2.16 m/s which is larger than that of they-component (fault perpendicular), 1.92 m/s. Again the basin amplification effect on the amplitude and the duration time elongation are observed for all three components. For the x- and z-component, there is a phase change of the first arrival when moving across the fault trace. The velocity amplitudes on the fault trace are very small both for H1 and H2.
Figure 6.5: Fault perpendic- ular velocity profile for two example hypocentres. From top to bottom are the x-
, y- and z-components, re- spectively. The shear wave velocity isosurface depth (at 2.0 km/s) is depicted in the bottom as the shadowed area. The maximum ve- locity amplitude across this profile is shown with the in- let number.