Embarazo y Estreptococo del grupo B (EGB)

We design 20 slip histories (with hypocentre fixed, same as in chapter 7) with which ground rotation rates are calculated. The aim is to investigate the composite effect of the medium and the source complexity on the ground rotation rates, and to provide a possible range of the ground rotation rates introduced by the possible M7 earthquakes inside our study area. The results are shown and analyzed in the following sections.

8.4.1 Average PGRR Characteristics

The mean values of all 20 resulting peak ground rotation rates (PGRRs) on the surface are shown in Fig. 8.9for three components. The area, with the mean PGRR (x-component) larger than0.9×10−4 rad/s (black rectangle, Fig. 8.9 left), is the area towards which the rupture propagates and where the directivity plays the most important role (as explained in chapter 5). We name that area as A. The mean PGRR in this area for thex-component (fault parallel) is larger than other parts of the study area. Meanwhile, for another horizontal component, no such PGRR distribution is found. Elevated rotation rates are observed inside the basin but outside area A both for the y-component (largest value is located at the basin edge) and the vertical component. For the y-component, inside the region B, the rotation rates are even compatible to those right on the fault trace. Thus it can be concluded that the x- and

z-components (fault-parallel) are dominated by the directivity effect and they-component has significant contributions from the 3D structure (basin effects) and slip history.

8.4 Slip History Effect 81

Figure 8.9: Mean values of peak ground ro- tation rates from 20 different slip histories. Top left. x-component. Top right. y- component. Bottom left. z-component. The thick white lines are the fault traces on which red asterisks mark the epicenters. The thin white lines are the contours of the shear- wave velocity isosurface at 2 km/s. Area A is the region where different directivity effect on different rotation rate components is ob- served. Area B is picked up for discussion. Note the difference of the color scale.

8.4.2 Source Dependent PGRR Variations

Four representative slip histories are taken as examples2 and the resulting rotation rates are shown and compared in this section to investigate the effects of slip history on the ground rotation rate in more detail. The results are shown in Fig. 8.10. The final slip distributions are shown in the top line. These slip models are considered to be representative because of the following reasons: slip 5 has a distinct asperity area right in the middle of the fault plane; slip 7 has a smaller asperity area with very large slip close to the hypocentre; slip 10 is more uniform then the other three examples; slip 16 has two asperity areas and the major broken part occurs in the bottom half part of the fault.

For slip 16, over the entire study area, the rotation rates are smaller compared to the other three slip models in all three components. Right on the fault trace, at point D where high slip asperity is nearby, large x-component rotation rate is observed for the slip model 7, but not for the other three slip models (Fig. 8.10 second line). The z-component rotation rates are elevated by the slip asperity in the region C for the slip model 7 (Fig. 8.10 fourth line). There are not such elevations for the slip models of 10 and 16. For they-component, at the point B, close to the asperity area of slip 5, large rotation rate is observed, compared to the other three slip models (Fig. 8.10 third line). Comparison between different components of PGRRs inside area A (Fig 8.9), suggests that the directivity plays a more important role on thex-component (the fault parallel one).

The characteristic properties of the peak ground rotation rates are summarized and shown here. The maximum, the standard deviation and the relative variations are adopted. Two ratios are calculated and shown to characterize the relative variations: RSD – the one of the standard deviation relative to the mean value, and Rmax – the one between the maximum value and the mean value. The aim is to provide the rotation rate range excited by these hypothetic M7 earthquakes. We show those ground rotation rate characteristics in Fig. 8.11 for different rotation rate components.

Both the largest rotation rate and the largest standard deviation of the entire study area for the vertical component are 4.5 times larger than those of the x-component and 6 times larger than those of the y-component. These largest ratios are located either right above (x-

and z-component) or very close to the fault trace (y-component). For the vertical component, very large gradients of rotation rate are observed at the two sides of the fault trace. For the

y-component, the largest rotation rate happens around the epicentre while that for the other two components is located a bit further from the epicentre (right on the fault trace, at the far end of the fault trace (with respect to the epicentre)). Similar spatial distributions are found in terms of the standard deviation, too. A bit further from the fault trace, e.g., at station E, there are still large standard deviations and maximum values for the y-component (Figure 8.11, middle) which are compatible to those on the fault trace. Large rotation rate values and standard deviations are also observed for the vertical component at this station. But for the

x-component, these two variables are quite small compared to those on the fault trace. The spatial distributions of ratio RSD between the standard deviations and the mean values of all 20 simulations are shown in Fig. 8.11 (bottom) to characterize the variations in a relative meaning. The largest RSD of the entire study area is 420% for the x-component, 50% for the y-component and 45% for the vertical component, respectively. The difference is where it happens. For the x-component,RSDis very big inside the region F which is centered

2

8.4 Slip History Effect 83

Figure 8.10: Peak ground rotation rate distributions from slip model 5, 7, 10 and 16. The corresponding slip distributions are shown in the top line. The thick white line is the fault trace on which red asterisk marks the epicentre. The thin white lines are the contours of the shear-wave velocity isosurface at 2 km/s. Region B, C, and D are picked up for discussion.

Figure 8.11: Peak ground rotation rate variation – maximum, standard deviation and varia- tions (measured by two ratios: the one between the standard deviation and the mean value; and the one between the maximum value and the mean value). Top. Maximum value. Sec- ond line. Standard deviation. Third line. Ratio between the standard deviation and the mean value. Bottom. Ratio between the maximum value and the mean value of the indi- vidual PGRRs. From left to right are the x-, y-, and z-components. Fault trace is marked as black dashed rectangle and epicentre is shown with red asterisk. Thin white lines are the contours of the shear velocity isosurface at 2.0 km/s. Station E and G, and region A and F are picked up for more detailed discussion. Note the color scale difference.

8.5 Comparison with Translations 85