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1. How the changes of fault zone material properties influence earthquake nucleation and rupture

   Huang, Yihe; Thakur, Prithvi (October 2021, AGU Fall Meeting 2021)

   null (Ed.)

   The fault damage zone is a well-known structure of localized deformation around faults. Its material properties evolve over earthquake cycles due to coseismic damage accumulation and interseismic healing. We will present fully dynamic earthquake cycle simulations to show how the styles of earthquake nucleation and rupture propagation change as fault zone material properties vary temporally. First, we will focus on the influence of fault zone structural maturity quantified by near-fault seismic wave velocities in simulations. The simulations show that immature fault zones promote small and moderate subsurface earthquakes with irregular recurrence intervals, whereas mature fault zones host pulse-like earthquake rupture that can propagate to the surface, extend throughout the seismogenic zone, and occur at regular intervals. The interseismic healing in immature fault zones plays a key role in allowing the development of aseismic slip episodes including slow-slip events and creep, which can propagate into the seismogenic zone, and thus limit the sizes of subsequent earthquakes by releasing fault stress. In the second part, we will discuss how the precursory changes of seismic wave velocities of fault damage zones may affect earthquake nucleation process. Both laboratory experiments and seismic observations show that the abrupt earthquake failure can be preceded by accelerated fault deformation and the accompanying velocity reduction of near-fault rocks. We will use earthquake cycle simulations to systematically test the effects of timing and amplitudes of such precursory velocity changes. Our simulations will provide new insights into the interplay between fault zone structure and earthquake nucleation process, which can be used to guide future real-time monitoring of major fault zones. 

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2. The competitive effects of on-fault normal stress and off-fault seismic velocity change on seismic cycles

   Zhai, Peng; Huang, Yihe (April 2023, 2023 SSA Annual Meeting)

   The temporal variation of elastic property of the bulk material surrounding the fault is considered an important contribution to the observed co-seismic velocity reduction and interseismic healing. Paglialunga et al. [2021] found that as fault normal stress increases, co-seismic velocity reduction becomes larger because more cracks reopen with higher stress drops. Larger normal stress can lead to smaller nucleation size and contribute to larger co-seismic slip. By contrast, with larger co-seismic velocity reduction and interseismic healing, more slow slip events can propagate in the seismogenic zone [Thakur and Huang, 2021], because the temporal velocity change related to fault zone damage modulates earthquake nucleation. Hence, fault normal stress and temporal damage zone structure evolution have opposite influences on the spatial distribution and recurrence intervals of earthquakes. We conducted 2-D anti-plane fully-dynamic seismic cycle simulations and explored the effects of fault normal stress on seismic cycle when there is coseismic damage and interseismic healing in the fault damage zone. The normal stress is in a range of 40-70 MPa and the co-seismic rigidity reduction is in a range of 5-8%. We find larger normal stress results in larger co-seismic slip and fewer slow slip events, while more co-seismic velocity reduction and interseismic healing leads to more partial ruptures as well as slow slip events. With the increase of both normal stress and seismic velocity change, more regular earthquakes occur and slow slip events gradually disappear. For the selected parameter space, the influence of seismic velocity change is not as significant as the effect of normal stress. However, fault zone maturity or the initial rigidity of fault damage zones should also affect the competitive relationship between normal stress and seismic velocity change, and we will characterize earthquakes and slow-slip events in immature and mature fault damage zones when both on-fault normal stress and off-fault seismic velocity vary over earthquake cycles. 

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3. Earthquake behaviors in earthquake cycle simulations with fault damage zones

   Huang, Yihe; Thakur, Prithvi (January 2022, SSA Annual Meeting)

   Faults are usually surrounded by damage zones associated with localized deformation. Here we use fully dynamic earthquake cycle simulations to quantify the behaviors of earthquakes in fault damage zones. We show that fault damage zones can make a significant contribution to the spatial and temporal seismicity distribution. Fault stress heterogeneities generated by fault zone waves persist over multiple earthquake cycles that, in turn, produce small earthquakes that are absent in homogeneous simulations with the same friction conditions. Shallow fault zones can produce a bimodal depth distribution of earthquakes with clustering of seismicity at both shallower and deeper depths. Fault zone healing during the interseismic period also promotes the penetration of aseismic slip into the locked region and reduces the sizes of fault asperities that host earthquakes. Hence, small and moderate subsurface earthquakes with irregular recurrence intervals are commonly observed in immature fault zone simulations with interseismic healing. To link our simulation results to geological observations, we will use simulated fault slip at different depths to infer the timing and recurrence intervals of earthquakes and discuss how such measurements can affect our understanding of earthquake behaviors. We will also show that the maturity and material properties of fault damage zones have strong influence on whether long-term earthquake characteristics are represented by single events. 

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4. Imminent seismic velocity changes in earthquake cycle simulations

   Thakur, Prithvi; Huang, Yihe (October 2021, AGU Fall Meeting 2021)

   null (Ed.)

   Earthquake prediction is the holy grail of seismology. Many previous studies have searched for robust precursory signals to inform us of imminent earthquakes, the most significant of which are seen in laboratory experiments as temporal changes in pressure and shear wave velocities during the seismic cycle. Similar changes are seen in natural faults and the surrounding structurally complex network of fractures with nested hierarchy of localized deformation, referred to as fault damage zone. However, little is known whether such temporal changes in material properties contains any precursory signals for imminent earthquakes.Conversely, the effect of precursory velocity changes on the seismic cycle is not well understood. By imposing shear wave velocity changes in fault damage zones, we investigate the effects of these precursors on multiple stages of the seismic cycle, including nucleation, coseismic, postseismic, and interseismic stages. We perform 2D fully dynamic earthquake cycle simulations with a fault-parallel damage zone for strike-slip fault systems with antiplane geometry. The fault is governed by rate-state-dependent friction laws, and the fault damage zone material is considered elastic. Our preliminary results show that the temporal onset of shear wave velocity drop causes a reduction in earthquake recurrence intervals over the seismic cycle. Furthermore, a dynamic earthquake rupture within the seismic cycle terminates much faster and abruptly in models with precursory velocity changes. We will also discuss how the precursory velocity changes affect the fault-slip behavior, including fast-slip, slow-slip, and aseismic creep, for different amplitudes of shear wave velocity changes at different compliance contrast of the fault damage zones. Our results highlight the importance of short and long-term monitoring of fault zone structures for better assessment of regional seismic hazard. 

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5. The Effects of Precursory Seismic Velocity Changes on Earthquake Sequence Simulations

   Thakur, Prithvi; Huang, Yihe (January 2022, SSA Annual Meeting)

   Predicting the onset and timing of fault failure is one of the ultimate goals of seismology. However, our current understanding of the earthquake preparation and nucleation process is limited. One direction towards understanding this process is looking at precursory signals preceding large earthquakes. Previous laboratory experiments have studied robust precursory signals, observed as temporal changes in pressure and shear wave velocities during the seismic cycle. The effects of such precursory velocity changes on the seismic cycle are not well understood. We use numerical models to simulate fully-dynamic earthquake cycles in 2D strike-slip fault systems with antiplane geometry, surrounded by a narrow fault-parallel damage zone. By imposing shear wave velocity changes inside fault damage zones, we investigate the effects of these precursors on multiple stages of the seismic cycle, including nucleation, coseismic, postseismic, and interseismic stages. Our modeling results show a wide spectrum of fault-slip behaviors including fast earthquakes, slow-slip events, and variable creep. One primary effect of the imposed velocity precursor is the facilitation of the otherwise slow-slip event to grow into a fully dynamic earthquake. Furthermore, the onset time of these precursors have significant effects on the nucleation phase of the earthquakes, and earlier onset of precursors causes the earthquakes to nucleate earlier with a smaller nucleation size. Our results highlight the importance of short and long-term monitoring of fault zone structures for better assessment of regional seismic hazard. 

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