Multi-band imaging of seismic source rupture process

<p>The standard method to image the source rupture process of a large earthquake is finite fault inversion, which uses the low-frequency signal to invert the slip distribution of the fault. However, in different stages of the source rupture process of a large earthquake, the seismic waves radiated by the source have different dominant frequencies, such as high frequency seismic waves excited by the rupture front. If we can analyze seismic waves in different frequency bands, it is expected to obtain a more detailed source rupture process of large earthquakes. Therefore, we respectively adopted the high frequency signal back-projection imaging method and the low frequency signal finite fault inversion method, and took the 2016 Kaikoura <em>M</em><sub>W</sub>7.8 earthquake as an example to obtain the history of rupture propagation and fault slip distribution.The calculated results show that the high-frequency energy radiation of the earthquake can be divided into three stages, and the low-frequency energy radiation can be divided into two stages. The energy release process in different frequency bands is complementary in time and space. The rupture process of the whole source can be explained by the asperity model and the barrier model.</p>

Source rupture process of the 2018 Hokkaido Eastern Iburi earthquake deduced from strong-motion data considering seismic wave propagation in three-dimensional velocity structure

The source rupture process of the 2018 Hokkaido Eastern Iburi earthquake (MJMA 6.7) was analyzed by a kinematic waveform inversion method using strong-motion data in 0.04–0.5 Hz. This earthquake occurred close to the Hidaka Collision Zone and the Ishikari depression, where the crustal structure is rather complex. Thus, we used a three-dimensional velocity structure model to compute the theoretical Green’s functions by the finite difference method. A source fault model with strike-angle variation was set based on the spatial distribution of the early aftershocks. The strong-motion stations used for the source inversion were selected based on the result of forward ground motion simulation of a moderate aftershock. The slip in the first 5 s was relatively small, but an area of significant slip with peak slip of 1.7 m was found in the depth range from 22 to 32 km. The rupture propagated upward mainly in the southwest direction. Based on the regional crustal structure and the configuration of the Moho discontinuity, the large-slip area was thought to be located in the lower crust, and its rupture did not reach the upper part of the continental crust. Most of the early aftershocks occurred around the large-slip area. The later aftershocks at the depth shallower than 20 km occurred outside the causative source fault of the mainshock. Three-dimensional ground motion simulation demonstrated that the heterogeneous source process and the three-dimensional basin and crustal velocity structure brought a large velocity pulse to an area to the southwest of the source fault, where the largest PGV was observed during the mainshock. The spatial distribution of the simulated PGV resembled the observed PGV distribution except some sites located inside the Ishikari depression where thick Quaternary soft low-velocity sediments exist at the top of the basin.

Further studies on the focal mechanism and source rupture process of the 2012 Haida Gwaii, Canada, 7.8 moment magnitude earthquake

The 28 October 2012 Haida Gwaii, British Columbia, Canada, earthquake with a moment magnitude (MW) of 7.8 occurred along an east-dipping poorly known thrust fault beneath the Queen Charlotte Terrace. It was the largest thrust event ever recorded in this dominated by strike-slip motion region. We studied the focal mechanism and the source rupture process for the event. The retrieved geometric parameters of the fault plane were a strike of 329°, dip of 24°, and slip of 114°. The isotropic moment was negative, and its value was about one-fifth of the total seismic moment released. The earthquake ruptured an area of about 160 km × 60 km, and major slip occurred in an area of about 100 km × 60 km. The maximum slip was about 5.8 m. The slip distribution on the fault plane was highly heterogeneous, with four slip patches. The main slip lay on a large zone above the hypocentre to the sea floor. The maximum and average stress drops calculated using the Brune model were 16.5 and 4.6 MPa, respectively. The major rupture occurred about 10 s after the rupture initiation, and lasted about 25 s. During a subducting earthquake, the leading edge of the overriding plate is assumed to spring seaward and upward, while the landward portion is assumed to extend and drop down, and the generated rapid motions set off a tsunami. The falling-down process seems to be consistent with a negative isotropic moment.