1:45 PM - 2:00 PM
[SCG70-01] Physics of Near-Source Strong Ground Motions
It has been thought that rupture directivity occurs if a strike-slip or dip-slip rupture propagates to a site in the along-strike or updip direction, respectively. Its effects appear in the fault-normal component of seismic ground motion. However, though the 2015 Gorkha earthquake was a dip-slip event, the along-strike rupture propagation generated large ground motion pulses. For the 2016 Kumamoto earthquake, large ground motion pulses were found in the fault-parallel component. Accordingly, the fling step effect and faulting motion theory have been advocated, but our studies showed the rupture directivity effect to be a main cause of the large pulses. The first key point is that a ‘large’ ground motion pulse has to occur for the identification of rupture directivity. Although constructive interference can occur in any rupture direction, a large ground motion pulse is not generated if subfaults along the direction radiate only small ground motions or their focal mechanisms vary largely. For a typical dip slip of 45 degree, large ground motions are not generated along the strike direction because of a nodal plane in the S-wave radiation pattern, and therefore, the rupture directivity is not visible during the along-strike rupture propagation of a typical dip-slip earthquake. Since the Gorkha earthquake is a low-angle dip-slip event of 10 degree, the strike direction got away from the nodal plane and into a zone of large ground motion in the radiation pattern, causing the rupture directivity effect. The second key point is that an overall rupture direction is not always similar to the rupture direction around a zone of large slip. The Kumamoto earthquake is this case so that the upward rupture propagation around its large-slip zone generated directivity pulses in the fault-parallel component.
To compare the rupture directivity and fling step contributions to ground motions in the near field of the Gorkha earthquake, we calculated ground motions using the far- and intermediate-field terms, or only the near-field term of the analytical solution in an infinite medium. The results of this calculation in the figure show that, even in the near field, the rupture directivity effect is mostly larger than the fling step effect. It is noted the most that the shape of a fling step pulse is mainly controlled by constructive interference like a rupture directivity pulse.