<p>Supershear earthquakes are rare but powerful ruptures with devastating consequences. How quickly an earthquake rupture attains this speed, or for that matter decelerates from it, strongly affects high-frequency ground motion and the spatial extent of coseismic off-fault damage.&#160;Traditionally, studies of supershear earthquakes have focused on determining which fault segments sustained fully-grown supershear ruptures. Knowing that the rupture first propagated at subshear rupture speeds, these studies usually guessed&#160;an approximate location for the transition from subshear to supershear regimes.&#160;The rarity of confirmed supershear ruptures, combined with the fact that conditions for supershear transition are still debated, complicates the investigation of supershear transition in real earthquakes.&#160;Here, we find a unique signature of the location of a supershear transition: we show that, when a rupture accelerates towards supershear speed, the stress concentration abruptly shrinks, limiting the off-fault damage and aftershock productivity.&#160;First, we use theoretical fracture mechanics to demonstrate that, before transitioning to supershear, the stress concentration around the rupture tip shrinks, confining the region where damage & aftershocks are expected. Then, employing two different dynamic rupture modeling approaches, we confirm such reduction in stress concentration, further validating the expected signature in the transition region. We contrast these numerical and theoretical results with high-resolution aftershock catalogs for three natural supershear earthquakes, where we identify a small region with lower aftershock density near the supershear transition. Finally, using satellite optical image correlation techniques, we show that, for a fourth event, the transition zone is characterized by a diminution in the width of the damage zone.&#160;Our results demonstrate that the transition from subshear to supershear rupture can be clearly identified by a localized absence of aftershocks, and a decrease in off-fault damage, due to a transient reduction of the stress intensity at the rupture tip.</p>
Nuclear monitoring agencies often use seismic amplitudes to estimate the yields of underground nuclear tests. Any emplacement phenomena that can alter those amplitudes and lead to bias in estimated yields must be considered in the analysis. One condition that might cause such a bias is detonation in frozen rock. Laboratory analyses (Mellor, 1971; Miller and Florence, 1991) have shown that frozen rock has faster seismic velocity and greater compressive strength than unfrozen rock. This increased strength is hypothesized to reduce the seismically estimated yield of an explosion in frozen rock.
To test this hypothesis, we conducted the Frozen Rock Experiment (FRE), a series of explosions in frozen and unfrozen rock, in central Alaska during August 2006. Over 120 seismic instruments were deployed to record six detonations—three in frozen and three in unfrozen-dry media—at a wide range of distances and azimuths. The data acquired show that the frozen test site explosions had significantly larger amplitudes for all phases ( P , S , and surface waves) above 8–10 Hz. These data confirm that the frozen rock medium was stronger and resulted in a smaller seismic source radius for the explosions, thus increasing the corner frequency when compared to the unfrozen rock explosions. Between 3 and 9 Hz, the unfrozen shots produced slightly larger S and surface waves resulting in different P / S spectral ratio plots for the frozen and unfrozen shots, possibly affecting regional phase discrimination. We show that the observed amplitude differences for these shots can be effectively modeled using the Mueller-Murphy (1971) explosion source and the in situ P - and S -wave velocities for the two test sites. Differences in the velocities at the frozen and unfrozen rock test sites are caused by minor metamorphic facies changes, saturated versus dry conditions, and the presence of ice in the pores and fractures at the frozen test site. Extrapolation of the results of this study to synthetic nuclear explosions suggests there may not be significant coupling differences between explosions in frozen and unfrozen hard rock.
High-frequency bandpass filtering of broadband strong-motion seismograms recorded immediately adjacent to the fault plane of the 1999 Chi-Chi, Taiwan, earthquake reveals a sequence of distinct bursts, many of which contain quasi-periodic subbursts with repeat times on the order of a few tenths of a second. The subevents that produce these bursts do not appear in conventional slip-maps, presumably because of the low-pass filtering used in the waveform inversions. The origin times, locations, and magnitudes of of about 500 of these subevents were determined. Those closest to the hypocenter appear to have been triggered by the P wave, while the earliest subevents at greater distances are consistent with a rupture front propagating at about 2.0 km/sec. Later subevents at a given distance follow Omori9s law and may be interpreted as aftershocks that begin before the Chi-Chi rupture has terminated. The frequency-magnitude distribution of these subevents has a b -value equal to 1.1. The subevents are clustered in space, with most clusters located at shallow depth along the Chelungpu surface rupture. The larger subevents tend to occur at greater depth, while the small subevents are only located at shallower depths. Cluster locations generally coincide with the large-amplitude slip-patches found by source inversions at lower frequencies. Relocation of the deeper subevents suggests a nonplaner rupture surface where dip increases with depth at the southern end.
La zona de baja velocidad en regiones tectónicas y océanicas es demasiado pronunciada para ser únicamente producto de altos gradientes de temperatura. La fusión parcial es consistente con la baja velocidad, la baja Q y con los limites abruptos de esta región del manto superior y también es consistente con los valores medidos del flujo de calor. Las bajas temperaturas de fusión que se infieren parecen indicar que la presión del agua es suficientemente alta para bajar el punto de solidus de 200°C a 400°C por abajo de las determinaciones de laboratorio del punto de fusión de silicatos anhidros. La inestabilidad mecánica de una capa fundida parcialmente en el manto superior es probablemente una fuente importante de energía tectónica. La cima de la zona de baja velocidad se puede considerar como una superficie autolubricada sobre la cual pueden deslizarse la corteza y la cima del manto con muy poca fricción. El movimiento de alejamiento lateral de la corteza y del manto superior con respecto a los altos oceánicos se contrarresta con el flujo de material fundido en la capa de baja velocidad hacia el alto donde eventualmente emerge como corteza nueva. Si este flujo lateral del material fundido no es tan activo como la remoción del magma de la cima, entonces migrarán las regiones de extrusión tales como los altos oceánicos.
The motion along upper crustal faults in response to tectonic loading is controlled by both loading stresses and surface properties, for example, roughness. Fault roughness influences earthquake slip distributions, stress-drops and possible transitions from stable to unstable sliding which is connected to the radiation of seismic energy. The relationship between fault roughness and seismic event distributions is insufficiently understood, in particular, the underlying mechanisms of off-fault seismicity creation in the proximity of rough faults are debated. Here, we investigate the connection between roughness and acoustic emission (AE) density with increasing fault-normal distance during loading of surfaces with pre-defined roughness. We test the influence of fault roughness and normal stress variations on the characteristics of AE off-fault distributions. To this end, two sets of experiments were conducted: one to investigate the influence of initial surface roughness at constant confining pressure, and the other to investigate the influence of fault-normal stresses at constant roughness. Our experiments reveal a power-law decay of AE density with distance from the slip surface. The power-law exponents are sensitive to both fault roughness and normal stress variations so that larger normal stresses and increased roughness lead to slower AE density decay with fault-normal distance. This emphasizes that both roughness and stress have to be considered when trying to understand microseismic event distributions in the proximity of fault zones. Our results are largely in agreement with theoretical studies and observations of across-fault seismicity distributions in California suggesting a connection between off-fault seismicity and fault roughness over a wide range of scales. Seismicity analysis including a possible mapping between off-fault activity exponents, fault stresses and roughness, can be an important tool in understanding the mechanics of faults and their seismic hazard potential.
