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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. Influence of Fault Zone Maturity on Fully Dynamic Earthquake Cycles

   https://doi.org/10.1029/2021GL094679  

   Thakur, Prithvi ; Huang, Yihe ( September 2021 , Geophysical Research Letters)

   We study the mechanical response of two‐dimensional vertical strike‐slip fault to coseismic damage evolution and interseismic healing of fault damage zones by simulating fully dynamic earthquake cycles. Our models show that fault zone structure evolution during the seismic cycle can have pronounced effects on mechanical behavior of locked and creeping fault segments. Immature fault damage zone models exhibit small and moderate subsurface earthquakes with irregular recurrence intervals and abundance of slow‐slip events during the interseismic period. In contrast, mature fault damage zone models host pulse‐like earthquake ruptures that can propagate to the surface and extend throughout the seismogenic zone, resulting in large stress drop, characteristic rupture extents, and regular recurrence intervals. Our results suggest that interseismic healing and coseismic damage accumulation in fault zones can explain the observed differences of earthquake behaviors between mature and immature fault zones and indicate a link between regional seismic hazard and fault structural maturity.

    

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3. 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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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. Insights on fault damage zone structures from the azimuthal variation in the stacked spectra of earthquake clusters

   Neo, Jing Ci ; Huang, Yihe ; Yao, Dongdong ( January 2022 , AGU Fall Meeting)

   Fault damage zones can influence various aspects of the earthquake cycle, such as the recurrence intervals and magnitudes of large earthquakes. The properties and structure of fault damage zones are often characterized using dense arrays of seismic stations located directly above the faults. However, such arrays may not always be available. Hence, our research aims to develop a novel method to image fault damage zones using broadband stations at relatively larger distances. Previous kinematic simulations and a case study of the 2003 Big Bear earthquake sequence demonstrated that fault damage zones can act as effective waveguides, amplifying high-frequency waves along directions close to fault strike via multiple reflections within the fault damage zone. The amplified high-frequency energy can be observed using the stacked P-wave spectra of earthquake clusters with highly-similar waveforms (Huang et al., 2016). We attempt to identify the high-frequency peak associated with fault zone waves in stacked spectra by conducting a large-scale study of small earthquakes (M1.5–3). We use high quality broadband data from seismic stations at hypocentral distances of 20-100km in the 2004 Parkfield and 2019 Ridgecrest earthquake regions. First, we group earthquakes in clusters by their locations and their waveform similarity, and then stack their velocity spectra to average the source effects of individual earthquakes. We applied our method to the 2019 Ridgecrest earthquake sequence, and our preliminary results show that stations close to the fault strike tend to record more high-frequency energies around the characteristic frequency of fault zone reflections. The frequency bands in which amplified high-frequency energies are observed may be used to estimate the width and velocity contrast of the fault damage zone. We aim to develop a robust and versatile method that can be used to search for fault damage zone structures and estimate their material properties, in order to shed light on earthquake source processes. 

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