Giant Impact Induced Atmospheric Blow-Off

Previous calculations indicate that the Earth suffered impacts from objects up to Mars size. Such a giant impact may have produced a temporary ejecta-based ring that accreted to form the Moon. To simulate the surface waves from such events we approximated the cratering source as a buried pressurized sphere. For a 10^27 J impactor we calculated the resulting surface wave using the mode summation method of Sato et al.. For such an impact, the solid Earth free-surface velocity above, and antipodal to, the source achieves 2.6 and 1.9 km/s. Such large ground motions pump the atmosphere and result in upward particle motions which cause the atmosphere to be accelerated to excess of the escape velocity (11.2 km/s) at high altitudes. For a 1.3 × 10^32 J Moon-forming impact we calculate that ~50% of the Earth's atmosphere is accelerated to escape

Magnitude estimation for early warning applications using the initial part of P waves: A case study on the 2008 Wenchuan sequence

A period parameter τ_c and an amplitude parameter Pd determined from the very beginning of P wave are important for earthquake early warning (EEW), yet their dependence on source mechanism, focal depth and epicentral distance has not been fully studied. After the devastating Mw7.9 Wenchuan earthquake, hundreds of M4-6 earthquakes occurred with diverse focal mechanisms and depth range of 2–20 km. We calculate τ_c and Pd of these aftershocks and examine their dependence on magnitude, τ_c, distance, and depth. We find that τ_c correlates well with magnitude, but joint regression including distance and depth does not significantly improve the correlation. The effect of focal mechanism on the τ_c-magnitude correlation is not obvious. When P wave is nodal, τ_c measurement becomes inaccurate. Also, τ_c is systematically greater for slow earthquakes, leading to a possible false alarm. Thus, more studies are required to discriminate slow earthquakes for robust early warning

Probing an ultra-low velocity zone at the core mantle boundary with P and S waves

Recent studies of the core-mantle boundary (CMB) have revealed some very anomalous structures interpreted in terms of ultra low velocity zones (ULVZ). However, there remains considerable uncertainties about their physical descriptions or even if they occur above or below the CMB. They have only been detected in isolated situations using rather special techniques; these includes: distortions in SKS with the development of SKPdS and SPdKS, broadband PKP precursors, distinct ScS and S beyond 100 degree, and rapid changes in differential travel times of neighboring phases. Here we report on a situation where ray paths associated with PKP precursors and SKPdS sample the same ULVZ structure. The structure lies beneath central Africa and has been detected from WWSSN analog data (SKPdS) discussed previously. This data set has been enhanced with a collection of digital records sampling an elongated North-South zone roughly 800 km long. The entire SKPdS data set can be modeled with a ridge-shaped cross section with widths of 250 to 400 km and drops in P and S velocity of 10 and 30 percent. Fortunately, a new IRIS station (MSKU) located in Western Africa provided excellent PKP data from the New Britain Region events sampling the above structure. The PKP and strong precursors can be modeled by 2D synthetics generated from the same structure (used in modeling SKP dS) which provides a strong constraint on the definition characteristics of this particular ULVZ

Seismic Modeling Constraints on the South African Super Plume

Tomographic studies of the structure of the lower mantle beneath South Africa reveal large-scale low velocities above the core-mantle boundary. Predicted SKS delay patterns (up to 3 s) for some of these models fit observations (Kaapvaal Array data) quite well except for magnitude level, explaining less than one-half the observed anomaly. Moreover, the sharpness in travel-time offsets and waveform complications require that nearly vertical walls separate the anomalous structure from the normal preliminary reference Earth model (PREM) mantle. We present numerous record sections along with 2D and 3D synthetics displaying multipathing of arrivals (S_(d') SKS, SKKS, S, and ScS), based on a large-scale 3D structure. This kidney-shaped structure has one apex beneath the Indian Ocean (Kerguelen) and the other extending beneath the Mid-Atlantic (Cape Verde). The structure is about 1200 km wide beneath South Africa and extends upward to at least 1000 km through the lower mantle, similar to Grand's model but with an average uniform velocity decrease of about 3% relative to PREM. We have not found any evidences for ultra-low-velocity zones (ULVZ) beneath the main structure but ample evidence at some locations near the edges. We also analyzed Pd and the differentials between PcP travel times and P travel times (PcP-P) along the same great circle paths from the same events. The P-velocity is not very anomalous, perhaps -0.5%. The sharpness of the lateral boundaries (walls) and the large contrast in P and S velocities can be used in arguments for a thermochemical origin

Ridge-like lower mantle structure beneath South Africa

Recent (ScS-S) results from probing the deep structure beneath southern Africa display strong delays of up to 10 s at distances beyond 90°. Such delays could be explained by long-period tomographic models containing smooth (weak) features with the addition of rough (strong) D″ structure (3–9% drops in shear velocities). However, these structures cannot explain the (SKS-S) differentials sampling the same region. To explain the (SKS-S) and (ScS-S) data sets simultaneously requires instead a large-scale ridge-like structure with a relatively uniform 3% reduction in shear velocity. The structure is about 1000 km wide and extends at least 1200 km above D″. It is orientated roughly NW-SE and leans toward the east at latitudes from 15° to about 30°. It proves difficult to explain such sharp features with thermal effects alone and, thus, the importance of high-resolution waveform modeling to establish their existence. To derive the above results, we developed a special algorithm by matching simulated synthetics to observed broadband waveforms. This is achieved by computing the various arrivals separately using generalized ray theory for a reference model and allowing the arrivals to shift in relative times to match the data. Tomographic models can then be constructed, or existing tomographic models can be altered, to match these data, and new 2-D synthetics can be constructed as well to better fit the waveform data. These updated synthetics can again be decomposed and reassembled, and the process can be repeated. This algorithm is applied to a combination of analog and digital data along a corridor from South America, producing the high-resolution 2-D model described above

Approximate 3D Body-Wave Synthetics for Tomographic Models

We present a new method of generating analytical synthetics for tomographic-style models. These models are perturbations to a 1D layered model involving changes in block velocities producing 3D images. The procedure is broken into three steps: (1) construction of ray paths for the reference 1D layered model, (2) generation of perturbed paths and the construction of 2D synthetics in the plane containing the source and receiver, and (3) addition of out-of-plane contributions (2D) from virtual receivers weighted by diffraction operators. In step 1, the ray paths reflecting from the various interfaces are established with ray parameter (p_o) and travel time (t_o). Next, these values are corrected after adding the velocity perturbations where ray segments in faster blocks grow relative to slower blocks. This new set of ray parameters can be used to generate 2D Cagniard-deHoop synthetics or WKM synthetics. Contributions from virtual receivers at neighboring azimuths are added by convolving with diffraction operators that are defined by the source duration and travel time to the 3D structure. We suggest a particularly simple approximation based on four virtual receivers which produces synthetics in agreement with 3D numerical synthetics