Signature of transition to supershear rupture speed in the coseismic off-fault damage zone

Most earthquake ruptures propagate at speeds below the shear wave velocity within the crust, but in some rare cases, ruptures reach supershear speeds. The physics underlying the transition of natural subshear earthquakes to supershear ones is currently not fully understood. Most observational studies of supershear earthquakes have focused on determining which fault segments sustain fully grown supershear ruptures. Experimentally cross-validated numerical models have identified some of the key ingredients required to trigger a transition to supershear speed. However, the conditions for such a transition in nature are still unclear, including the precise location of this transition. In this work, we provide theoretical and numerical insights to identify the precise location of such a transition in nature. We use fracture mechanics arguments with multiple numerical models to identify the signature of supershear transition in coseismic off-fault damage. We then cross-validate this signature with high-resolution observations of fault zone width and early aftershock distributions. We confirm that the location of the transition from subshear to supershear speed is characterized by a decrease in the width of the coseismic off-fault damage zone. We thus help refine the precise location of such a transition for natural supershear earthquakes.

Aftershock signature of the M7.5 Palu 2018 supershear rupture from a rapidly deployed nodal array

<p>Supershear earthquakes have significant implications for seismic hazard, in terms of  ground shaking and aftershock pattern. It has been suggested that supershear ruptures are associated with fewer aftershocks on the supershear rupture segment, however this needs to be tested using high resolution event locations. Current aftershock catalogues for the M7.5 Palu 2018 supershear rupture are not of sufficient resolution to identify any characteristic aftershock pattern. Additionally it is unclear whether the supershear rupture speed occurred from the time of earthquake initiation, or at a later time on a certain segment of the fault.</p><p>We deployed a nodal array to record aftershocks following the main event. The array comprised of twenty short-period nodes, which can be deployed rapidly, making them ideal for post-rupture investigations in areas of sparse coverage. We expand the earthquake catalogue by applying template matching to the nodal array data. We then relocate seismicity recorded by the array using a double difference method. We also relocate seismicity that occurred before the array was active, using a relative relocation method. To do this, we calibrate the more distant permanent stations using events well-located by the nodal array. We further derive moment tensors for the largest events by waveform modelling using short-period and broadband records.</p><p>Our results show that the aftershocks cluster at the northern and southern extents of rupture. There is a relative dearth of aftershocks in the middle part of the rupture, particularly in the Palu valley, where rupture terminated to the surface. The fault here is a long and straight distinctive geomorphic feature. Many secondary faults were triggered, particularly in the southern Sapu valley fault system. An earthquake swarm was triggered 1 month after the main event on a strike-slip fault 200km away.</p>

Earthquake rupture properties in presence of thermal -pressurization of pore fluids

<p>Frictional heat generated in the fault core during earthquake rupture can raise the fluid pressure in the slip zone. Such increase of fluid pressure decreases the effective normal stress and thereby lowers the frictional strength of the fault. Therefore, thermal pressurization (TP) of pore fluid affects earthquake rupture processes including nucleation, propagation, and arrest. While the effects of pore pressure and fluid flow rate on dynamic weakening of faults are qualitatively understood, a detailed analysis of how TP affects  earthquake rupture parameters is needed to further deepen our understanding. </p><p>In this study, we investigate the role of two key TP parameters -- hydraulic diffusivity and shear-zone half-width -- earthquake dynamics and kinematic source properties (slip, peak slip-rate, rupture speed and rise time). We conduct  a suite of 3D dynamic rupture simulations applying a rate-and-state dependent friction law (with strong velocity weakening) coupled with thermal-pressurization of pore fluids. Simulations are carried out with the open source software SeisSol (www.seissol.org). The temporal evolution of rupture parameters over ~1’000 randomly  distributed on-fault receivers is statistically analyzed in terms of  mean variations of rupture parameters and correlations among rupture parameters. </p><p>Our simulations reveal that mean slip decreases with increasing hydraulic diffusivity, whereas mean peak slip-rate and rupture speed remain nearly constant. On the other hand, we observe only a slight decrease of mean slip with increasing shear-zone half-width, whereas mean peak slip-rate and rupture speed show clear decrease. The faster diffusion of pore pressure as hydraulic diffusivity increases promotes faster increase of the effective normal stress (and fault strength) behind the main rupture front, reducing the rise time and, therefore also affecting mean slip. An increase in shear-zone half- width represents a heat source distributed over larger fault normal distance causing a second-order effect on mean slip. Additionally, our simulations reveal correlations among rupture parameters: 1) slip has weak negative correlation with peak slip-rate and negligible correlation with rupture speed, but a positive correlation with rise time, 2) peak slip-rate has a strong positive correlation with rupture speed, but a strong negative correlation with rise time, 3) rupture speed has strong negative correlation with rise time. We observe little or negligible effects of variations of hydraulic diffusivity and shear-zone half- width on the correlations between rupture parameters. Overall, our study builds a fundamental understanding on how thermal pressurization of pore fluids affects dynamic and thereby kinematic earthquake rupture properties. Our findings are thus important for the earthquake source modeling community, and particularly, for assessing seismic hazard due to induced events in geo-reservoirs.</p>

The nucleation of the Izmit and Düzce earthquakes: Some mechanical logic on where and how ruptures began

Summary In spite of growing evidence that many earthquakes are preceded by increased seismic activity, the nature of this activity is still poorly understood. Is it the result of a mostly random process related to the natural tendency of seismic events to cluster in time and space, in which case there is little hope to ever predict earthquakes? Or is it the sign that a physical process that will lead to the impending rupture has begun, in which case we should attempt to identify this process. With this aim we take a further look at the nucleation of two of the best recorded and documented strike-slip earthquakes to date, the 1999 Izmit and Düzce earthquakes which ruptured the North Anatolian Fault over ∼200 km. We show the existence of a remarkable mechanical logic linking together nucleation characteristics, stress loading, fault geometry and rupture speed. In both earthquakes the observations point to slow aseismic slip occurring near the ductile-to-brittle transition zone as the motor of their nucleation.

Feed Additive of Binder Seaweed Grass in Fish Feed Formulation on Physical Characteristics and Efficiency

The study was designed experimentally using a Completely Randomized Design, five treatments were repeated three times included the use of carboxymethyl cellulose as a control as much as 5%, and the treatment of feed additives of seaweed as much as 5%, 75%, 10% and 12.5%. The parameters measured were feed efficiency, rupture speed test, sink speed test, durability test, stability test and moisture content test using Analysis of Variance and continued with Duncan's Multiple Distance Test. The results showed that seaweed binder additive feed increased (p <0.05) durability and stability in water, but did not show a significant difference (p> 0.05) on feed efficiency, breaking speed and sinking speed. Stability of pellets in water with the addition of seaweed binders of 10% is significantly higher (p <0.05) than other pellets, i.e. after 10 - 60 minutes ranging from 82.70% -97.40% with pellet durability of 98.24% and feed efficiency of 42.93%.

Tsunami Generation due to Supershear Earthquakes: A Case Study

&lt;p&gt;The 28 September 2018 Mw 7.5 Sulawesi strike-slip earthquake generated an unexpected tsunami with devastating consequences.&amp;#160;Since&amp;#160;such strike-slip earthquakes are not expected to generate large tsunamis, the latter&amp;#8217;s origin remains much debated. A key notable feature of this earthquake is that it ruptured at supershear speed, i.e.,&amp;#160;with a&amp;#160;rupture speed greater than the shear wave speed of the host medium. Dunham and Bhat (2008) showed that such supershear ruptures, in half-space, produce two shock fronts (or Mach fronts) corresponding to&amp;#160;an&amp;#160;exceedance of shear and Rayleigh wave speeds. The Rayleigh Mach front carries significant vertical velocity along its front. We couple the ground motion produced by such a supershear earthquake to a 1D non-linear shallow water wave equation that accounts for both the&amp;#160;time-dependent&amp;#160;bathymetric displacement&amp;#160;as well its&amp;#160;velocity. We use an extension of Fourier-based PDE solvers called the Fourier Continuation (FC)&amp;#160;method&amp;#160;to numerically solve the system. The FC&amp;#160;method enables&amp;#160;high-order&amp;#160;convergence of Fourier series approximations of non-periodic functions by resolving the well-known Gibbs &amp;#8220;ringing&amp;#8221; effect. &amp;#160;FC-based solvers offer&amp;#160;limited&amp;#160;numerical&amp;#160;dispersion, high-order accuracy and mild CFL&amp;#160;conditions&amp;#8212;making&amp;#160;them ideal to solve this system. Using the local bathymetric profile of Palu bay around the Pantoloan harbor tidal gauge, we&amp;#160;have been&amp;#160;able to clearly reproduce the observed tsunami with minimal tuning of parameters. We conclude that the Rayleigh Mach front,&amp;#160;generated&amp;#160;by a supershear earthquake combined with the Palu bay geometry, caused the tsunami.&lt;/p&gt;

Effects of Barriers on Fault Rupture Process and Strong Ground Motion Based on Various Friction Laws

A barrier may induce a supershear rupture on a fault. This paper focuses on two questions: One is whether the existence of a barrier accelerates the propagation speed of a whole fault rupture, and the other is what are the effects of friction laws and strength of a barrier on the rupture propagation process. For these purposes, classical slip-weakening, rate-state, and modified slip-weakening friction laws are employed to simulate the effect of a barrier on the fault rupture process. The simulation results showed that the rupture speed of the fault obviously decreases when the rupture front propagates to the barriers, and the rupture speed obviously increases when the rupture front leaves barriers. It was also found that a barrier on a fault may induce a supershear rupture via the rate-state friction law. The simulation results also showed that with the increase of barrier strength, the rupture speed near barriers fluctuates more and more; when the barrier strength exceeds a certain level, a supershear rupture area appears on the fault; with the increase of barrier strength, the propagation distance of the rupture at supershear wave velocity correspondingly increases. In addition, with the increase of barrier strength, the overall rupture duration of the fault slightly increases. This indicates that a barrier cannot shorten the total duration of a fault rupture. Though a barrier will lead to a supershear rupture, it just regulates the distribution of the rupture speed on the fault surface. Moreover, with the increase of barrier strength, the peak ground acceleration caused by rupture through the barrier also increases, indicating that the existence of a barrier may lead to the intensification of seismic hazards.

Diverse rupture processes in the 2015 Peru deep earthquake doublet

Earthquakes in deeply subducted oceanic lithosphere can involve either brittle or dissipative ruptures. On 24 November 2015, two deep (606 and 622 km) magnitude 7.5 and 7.6 earthquakes occurred 316 s and 55 km apart. The first event (E1) was a brittle rupture with a sequence of comparable-size subevents extending unilaterally ~50 km southward with a rupture speed of ~4.5 km/s. This earthquake triggered several aftershocks to the north along with the other major event (E2), which had 40% larger seismic moment and the same duration (~20 s), but much smaller rupture area and lower rupture speed than E1, indicating a more dissipative rupture. A minor energy release ~12 s after E1 near the E2 hypocenter, possibly initiated by the S wave from E1, and a clear aftershock ~165 s after E1 also near the E2 hypocenter, suggest that E2 was likely dynamically triggered. Differences in deep earthquake rupture behavior are commonly attributed to variations in thermal state between subduction zones. However, the marked difference in rupture behavior of the nearby Peru doublet events suggests that local variations of stress state and material properties significantly contribute to diverse behavior of deep earthquakes.

Calculation of the local rupture speed of dynamically propagating earthquakes

The velocity at which a propagating earthquake advances on the fault surface is of pivotal importance in the contest of the source dynamics and in the modeling of the ground motions generation. In this paper the problem of the determination of the rupture speed (<em>v__r</em>) is considered. The comparison of different numerical schemes to compute <em>v_r</em> from the rupture time (<em>t__r</em>) shows that, in general, central finite differences schemes are more accurate than forward or backward schemes, regardless the order of accuracy. Overall, the most efficient and accurate algorithm is the five–points stencil method at the second–order of accuracy. It is also shown how the determination of <em>t__r</em> can affect <em>v_{_r}</em>; numerical results indicate that if the fault slip velocity threshold (<em>v__l</em>) used to define <em>t__r</em> is too high (<em>v_{_l}</em> ≥ 0.1 m/s) the details of the rupture are missed, for instance the rupture tip bifurcation occurring for 2–D supershear rupture. On the other hand, for <em>v__l</em> ≤ 0.01 m/s the results appear to be stable and independent on the choice of <em>v__l</em>. Finally, it is demonstrated that in the special case of the linear slip–weakening friction law the definitions of <em>t__r</em> from the threshold criterion on the fault slip velocity and from the achievement of the maximum yield stress are practically equivalent.