Chapter 6 Multimessenger Physics

Combining observations of multi-messengers help in boosting the sensitivity of astrophysical source searches, and probe various aspects of the source physics. In this chapter we discuss how LHAASO observations of very high energy (VHE) gamma rays in combination with telescopes for the other messengers can help in solving the origins of VHE neutrinos and galactic and extragalactic cosmic rays.

Evaluation of Physics Frameworks for Building Web-Based Games

Recently, WebGL technology has shown a lot of potential for developing games. Since this technology is still relatively new, there is still much potential in the game development area that has not been explored yet. This paper explores the development of a game engine made with WebGL technology that integrates some physics frameworks for developing web-based 2D or 3D games. Specifically, we integrated three open source physics frameworks, which are Bullet, Cannon, and JigLib, into a WebGL-based game engine. We assessed these frameworks using some experiments, in terms of their correctness or accuracy, performance, completeness and compatibility. The results show that it is possible to integrate open source physics frameworks into a WebGL-based game engine, and Bullet is the best physics framework to be integrated into a WebGL-based game engine.

Imaging the Shallow Structure of the Yucca Flat at the Source Physics Experiment Phase II Site with Horizontal-to-Vertical Spectral Ratio Inversion and a Large-N Seismic Array

The Source Physics Experiment (SPE) is a series of chemical explosions at the Nevada National Security Site (NNSS) with the goal of understanding seismic-wave generation and propagation of underground explosions. To understand explosion source physics, accurate geophysical models of the SPE site are needed. Here, we utilize a large-N seismic array deployed at the SPE phase II site to generate a shallow subsurface model of shear-wave velocity. The deployment consists of 500 geophones and covers an area of, approximately, 2.5×2 km. The array is located in the Yucca Flat in the northeast corner of the NNSS, Nye County, Nevada. Using ambient-noise recordings throughout the large-N seismic array, we calculate horizontal-to-vertical spectral ratios (HVSRs) across the array. We obtain 2D seismic images of shear-wave velocities across the SPE phase II site for the shallow structure of the basin. The results clearly image two significant seismic impedance interfaces at ∼150–500 and ∼350–600 m depth. The shallower interface relates to the contrast between Quaternary alluvium and Tertiary volcanic rocks. The deeper interface relates to the contrast between Tertiary volcanic rocks and the Paleozoic bedrock. The 2D subsurface models support and extend previous understanding of the structure of the SPE phase II site. This study shows that the HVSR method in conjunction with a large-N seismic array is a quick and effective method for investigating shallow structures.

Source Separation and Medium Change of Contained Chemical Explosions from Coda Wave Interferometry

Differences in the seismic coda of neighboring events can be used to investigate source location offsets and medium change with coda wave interferometry (CWI). We employ CWI to infer the known relative location between two chemical explosions in Phase I of the Source Physics Experiment (SPE). The inferred displacement between the first, SPE-1, and second, SPE-2, chemical explosion is between 6 and 18 m, with an expectation of 9.2 m, where the known separation is close to 9.4 m. We also employ CWI to find any velocity perturbation due to damage from SPE-2, by comparing its coda with the collocated third SPE chemical explosion, SPE-3. We find that damage due to SPE-2 must be confined to a spherical region with radius less than 10 m and velocity perturbation less than 25%.

The dynamics of earthquakes rupture : A view from the laboratory

<p> </p><p>Earthquakes are spectacular natural disasters, with for example the recent disastrous Sumatra and Tohoku-Oki earthquakes (2004 and 2011, respectively). Presently, predicting earthquakes remains one of the biggest societal challenges in natural science. While seismological observations have much improved in recent years, our understanding of earthquake source physics remains limited due to the scarcity of monitored seismic rupture along similar fault systems, making long- or short-time scale predictions impossible. Friction and fracture are the two keys to understanding earthquakes. Laboratory experiments could be a robust solution to study earthquakes under safe and controlled conditions, which is mandatory to understand and compare the details of earthquake source physics. Conversely to common friction experiments conducted at both slow and seismic slip rates, the stick-slip mechanism is associated to the propagation of a rupture front, i.e. the radiation of seismic waves. Using stick-slip as an earthquake analog coupled to a state-of-the-art high frequency acoustic monitoring system, we demonstrated in the past that accelerations recorded in the kilohertz range on centimeter-sized samples were self-similar to the ones one can expect at the kilometric scale for a large earthquake. Based on this laboratory earthquakes catalogue, we highlighted that acoustic and strain measurements can be used to (i) locate and follow seismicity, (ii) estimate the energy budget of laboratory earthquakes, (iii) discriminate the mode of slip and the rupture speed. Lately, using medium scale experiments, we studied the scale dependence of rupture processes. These new results, notably in term of weakening of faulting and energy balance allowed us to initiate a bridge between laboratory earthquakes, fracture mechanics and natural seismicity. We discuss here how these experimental results can be upscaled to natural earthquakes.</p>

Effect of Random 3D Correlated Velocity Perturbations on Numerical Modeling of Ground Motion from the Source Physics Experiment

ABSTRACT Explosions are traditionally discriminated from earthquakes, using the relative amplitude of compressional and shear waves at regional and teleseismic distances known as the P/S discriminant. Pyle and Walter (2019) showed this technique to be less robust at shorter distances, in detecting small-magnitude earthquakes and low-yield explosions. The disparity is largely due to ground motion from small, shallow sources being significantly impacted by near-surface structural complexities. To understand the implications of wave propagation effects in generation of shear motion and P/S ratio during underground chemical explosions, we performed simulations of the Source Physics Experiment (SPE) chemical explosions using 1D and 3D velocity models of the Yucca Flat basin. All simulations used isotropic point sources in the frequency range 0–5 Hz. We isolate the effect of large-scale geological structure and small-scale variability at shallow depth (<5 km), using a regional 3D geologic framework model (GFM) and the GFM-R model derived from the GFM, by adding correlated stochastic velocity perturbations. A parametric study of effects of small-scale velocity variations on wave propagation, computed using a reference 1D velocity model with stochastic perturbations, shows that the correlation length and depth of stochastic perturbations significantly impact wave scattering, near-surface wave conversions, and shear-wave generation. Comparisons of recorded and simulated waveforms for the SPE-5 explosion, using 3D velocity models, demonstrate that the shallow structure of the Yucca Flat basin contributes to generation of observed shear motion. The inclusion of 3D wave scattering, simulated by small-scale velocity perturbations in the 3D model, improves the fit between the simulated and recorded waveforms. In addition, a relatively low intrinsic attenuation, combined with small-scale velocity variations in our models, can confirm the observed wave trapping and its effect on duration of coda waves and the spatial variation of P/S ratio at basin sites.