Precisely constraining the source parameters of large earthquakes is one of the primary objectives of seismology. However, the quality of the results relies on the quality of synthetic earth response. Although earth structure is laterally heterogeneous, particularly at shallow depth, most earthquake source studies at the global scale rely on the Green's functions calculated with radially symmetric (1-D) earth structure. To avoid the impact of inaccurate Green's functions, these conventional source studies use a limited set of seismic phases, such as long-period seismic waves, broad-band P and S waves in teleseismic distances (30° < ∆ < 90°), and strong ground motion records at close-fault stations. The enriched information embedded in the broad-band seismograms recorded by global and regional networks is largely ignored, limiting the spatiotemporal resolution. Here we calculate 3-D strain Green's functions at 30 GSN stations for source regions of 9 selected global earthquakes and one earthquake-prone area (California), with frequency up to 67 mHz (15 s), using SPECFEM3D_GLOBE and the reciprocity theorem. The 3-D SEM mesh model is composed of mantle model S40RTS, crustal model CRUST2.0 and surface topography ETOPO2. We surround each target event with grids in horizontal spacing of 5 km and vertical spacing of 2.0–3.0 km, allowing us to investigate not only the main shock but also the background seismicity. In total, the response at over 210 000 source points is calculated in simulation. The number of earthquakes, including different focal mechanisms, centroid depth range and tectonic background, could further increase without additional computational cost if they were properly selected to avoid overloading individual CPUs. The storage requirement can be reduced by two orders of magnitude if the output strain Green's functions are stored for periods over 15 s. We quantitatively evaluate the quality of these 3-D synthetic seismograms, which are frequency and phase dependent, for each source region using nearby aftershocks, before using them to constrain the focal mechanisms and slip distribution. Case studies show that using a 3-D earth model significantly improves the waveform similarity, agreement in amplitude and arrival time of seismic phases with the observations. The limitations of current 3-D models are still notable, dependent on seismic phases and frequency range. The 3-D synthetic seismograms cannot well match the high frequency (>40 mHz) S wave and (>20 mHz) Rayleigh wave yet. Though the mean time-shifts are close to zero, the standard deviations are notable. Careful calibration using the records of nearby better located earthquakes is still recommended to take full advantage of better waveform similarity due to the use of 3-D models. Our results indicate that it is now feasible to systematically study global large earthquakes using full 3-D earth response in a global scale.
The Vredefort impact structure, located in South Africa and formed 2.02 Ga, is the largest confirmed remnant impact crater on Earth. The widely accepted impactor diameter and velocity to form this crater are 15 km and 15 km/s, respectively, which produce a crater diameter of 172 km. This is much smaller than the most commonly cited estimates (250–280 km), and while previous results were able to match the geologic evidence known at that time, these impact parameters are not consistent with more recent geological constraints. Here, we conduct impact simulations to model the Vredefort crater formation with the shock physics code impact Simplified Arbitrary Lagrangian Eulerian (iSALE). Our numerical simulations show that combinations of the impactor diameter and impact velocity of 25 km and 15 km/s or 20 km and 25 km/s are able to recreate the larger crater size of ∼250 km. Moreover, these configurations can reproduce shock‐metamorphic features present in the impact structure today, including the distributions of breccia, shatter cones, planar deformation features in quartz and zircon, and melt. Our model also predicts that Vredefort and Karelia, Russia, where an ejecta layer from the impact was found, were approximately 2,000–2,500 km apart based on the layer thickness. Additionally, we use this model to predict the potential global effects of such a large impact by estimating the amount of climatically important gases released to the atmosphere at the time. Our work demonstrates the need to revisit previously estimated impactor parameters for large terrestrial craters in order to better characterize impact events on Earth and elsewhere.
more » « less- Award ID(s):
- 2102143
- NSF-PAR ID:
- 10372627
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Planets
- Volume:
- 127
- Issue:
- 8
- ISSN:
- 2169-9097
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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