High-resolution Velocity Structure Research Articles

High-resolution velocity structure and seismogenic potential of strong earthquakes in the Bamei-Kangding segment of the Xianshuihe fault zone

High-resolution velocity structure and seismogenic potential of strong earthquakes in the Bamei-Kangding segment of the Xianshuihe fault zone

Three-dimensional high-resolution velocity structure imaging and seismicity study of Yangbi Ms6.4 earthquake

Three-dimensional high-resolution velocity structure imaging and seismicity study of Yangbi Ms6.4 earthquake

Three-dimensional high-resolution velocity structure imaging and seismicity study of the Tangshan fault

Based on the 4415 earthquakes body wave arrival times recorded by the 47 fixed and 33 temporary stations in Tangshan and its surrounding areas, the Tangshan fault’s area focal parameters and three-dimensional fine velocity structure model were inversed by using the consistency-constrained double-difference tomography method. The results revealed that the relocated earthquakes were roughly vertical to the fault’s strike, with the maximum depth exceeding 25 km. The Tangshan fault’s area Vp, Vs and Vp/Vs values demonstrate obvious inhomogeneity, and high speed bodies prone to the brittle fault are dominant on both sides of the fault in the upper crust. Small low velocity bodies appeared initially at the Ms7.8 Tangshan earthquake source in the 14–16 km range of the middle crust, followed by a gradual expansion of the low velocity area with an increase in depth. Variation in the velocity structure below the Tangshan earthquake source shows that the high temperature mantle source invading the crust has weakened the upper crust’s high-speed area, leading to stress accumulation in the local area, which is an important factor for the occurrence of earthquakes in the Tangshan region. The relationship between the regional seismic structure and Tangshan seismic sequence, causing migration of surrounding blocks and seismic activity in recent years was analyzed based on the Tangshan fault’s 3D highresolution velocity structure imaging. It is considered that the Tangshan area unique velocity structure not only plays a pivotal role in promoting horizontal tectonic stress during seismic activity, but is also the root cause of the aftershock evolution process and the fault’s morphological characteristics.

Seismic envelope inversion and modulation signal model

We recognized that the envelope fluctuation and decay of seismic records carries ultra low-frequency (ULF, i.e., the frequency below the lowest frequency in the source spectrum) signals that can be used to estimate the long-wavelength velocity structure. We then developed envelope inversion for the recovery of low-wavenumber components of media (smooth background), so that the initial model dependence of waveform inversion can be reduced. We derived the misfit function and the corresponding gradient operator for envelope inversion. To understand the long-wavelength recovery by the envelope inversion, we developed a nonlinear seismic signal model, the modulation signal model, as the basis for retrieving the ULF data and studied the nonlinear scale separation by the envelope operator. To separate the envelope data from the wavefield data (envelope extraction), a demodulation operator (envelope operator) was applied to the waveform data. Numerical tests using synthetic data for the Marmousi model proved the validity and feasibility of the proposed approach. The final results of combined [Formula: see text] (envelope-inversion for smooth background plus waveform-inversion for high-resolution velocity structure) indicated that it can deliver much improved results compared with regular full-waveform inversion (FWI) alone. Furthermore, to test the independence of the envelope to the source frequency band, we used a low-cut source wavelet (cut from 5 Hz below) to generate the synthetic data. The envelope inversion and the combined [Formula: see text] showed no appreciable difference from the full-band source results. The proposed envelope inversion is also an efficient method with very little extra work compared with conventional FWI.

Seismic structure of the source region of the 2007 Chuetsu-oki earthquake revealed by offshore-onshore seismic survey: Asperity zone of intraplate earthquake delimited by crustal inhomogeneity

The 2007 Niigata-ken Chuetsu-oki earthquake occurred on July 16, 2007 with a magnitude of 6.8. Immediately after the mainshock, a dense network of ocean-bottom seismometers and temporary land seismic stations were deployed to obtain the accurate aftershock location. A seismic survey using ocean-bottom seismometers, land stations and controlled sources at sea and on land was also conducted along three profiles to estimate the detailed velocity structure of the source region. A thick sedimentary layer covers the crust, and this layer is thickest near the coast. The upper crust has a P-wave velocity of 6km/s and a large lateral heterogeneity with respect to thickness and velocity. The lower crust, with P-wave velocity of 7km/s, has a thickness of approximately 10km. The thickness of the crust is estimated to be approximately 24km. The precise aftershock distribution was obtained by using the high-resolution velocity structure. The aftershocks in the upper crust form a plane dipping to the southeast. Most of the aftershocks are located in the upper crust; in addition, a small number of aftershocks in the lower crust seem to be positioned on the same plane formed by the aftershocks in the upper crust. The mainshock fault estimated from the aftershock distribution is positioned in the high-velocity body of the upper crust. A large deformation of the sediments above the epicentral region is interpreted to be due to the repeated occurrence of large earthquakes. The lower crust immediately below the fault has low velocity. The source region of the 2007 event in the upper crust is sandwiched by a weakening structure. We suggest that the ductile deformation of both the sediments and the lower crust causes stress accumulation in the source region.