David Schwartz, Scientist Emeritus, USGS
            The Mw 7.9 Denali fault, Alaska earthquake of Nov. 3, 2002 was one of only nine earthquakes in shallow continental crust, worldwide, larger than magnitude 7.8 since 1900. The complex faulting, which involved the Susitna Glacier thrust fault, the principal Denali strike-slip fault, and the Totschunda branch of the Denali fault occurred along 351 km of the 1200 km-long Denali fault system. Right-lateral surface offsets of 4.5&#8211;5.1 m, with a maximum offset of 8.8 m, were measured. The rupture traversed the Trans-Alaska pipeline, which had been successfully designed to accommodate Denali slip. The three-fault rupture complexity has given impetus to the inclusion of multi-fault ruptures in seismic hazard analysis. The seminar will provide an overview of the coseismic faulting and the post-seismic geologic investigations, including implications of the 2002 event for the behavior of long strike-slip faults.

The November 3, 2002 Mw 7.9 Denali Fault Earthquake, Alaska: It’s Geological, Seismological, and Engineering Legacy

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Jeanne Hardebeck, U.S. Geological Survey
            Aftershock triggering is commonly attributed to static Coulomb stress changes from the mainshock.  A Coulomb stress increase encourages aftershocks in some areas, while in other areas termed &#8220;stress shadows&#8221; a decrease in Coulomb stress suppresses earthquake occurrence.  While the predicted earthquake rate decrease is rarely seen, lower aftershock rates are observed in the stress shadows compared to stress increase regions.  However, the question remains why some aftershocks occur in the stress shadows.  I examine three hypotheses: (1) Aftershocks appear in shadows because of inaccuracy in the computed stress change. (2) Aftershocks in the shadows occur on faults with different orientations than the model receiver faults, and these unexpected fault orientations experience increased Coulomb stress. (3) Aftershocks in the shadows are triggered by other physical processes, specifically dynamic stress changes. For the 2016 Kumamoto, Japan, and 2019 Ridgecrest, California, sequences, the first two hypotheses seem unlikely.  Over many realizations of the stress calculations with different modeling inputs, numerous aftershocks consistently show negative static Coulomb stress changes both on the model receiver faults and the individual event focal mechanisms.  Hypothesis 3 appears more likely, as the spatial and temporal distribution of aftershocks in the stress shadows are consistent with the expectations of dynamic triggering: the aftershocks occur mainly in a burst over the first few days to weeks, and decay with distance like near-field body waves.  The time series of dynamic stress can be modeled, and numerous metrics explored, such as the maximum dynamic Coulomb stress change, and the period and duration of the stressing.  Determining which metrics correspond to aftershock occurrence in the stress shadows may be useful in discriminating between various proposed physical mechanisms of dynamic stress triggering.

Stress Shadows: Insights into the Physics of Aftershock Triggering

Theresa Sawi, U.S. Geological Survey
            Repeating earthquakes sequences are widespread along California&#8217;s San Andreas fault (SAF) system and are vital for studying earthquake source processes, fault properties, and improving seismic hazard models. In this talk, I&#8217;ll be discussing an unsupervised machine learning‐based method for detecting repeating earthquake sequences (RES) to expand existing RES catalogs or to perform initial, exploratory searches. This method reduces spectrograms of earthquake waveforms into low-dimensionality &#8220;fingerprints&#8221; that can then be clustered into similar groups independent of initial earthquake locations, allowing for a global search of similar earthquakes whose locations can afterwards be precisely determined via double-difference relocation. We apply this method to ∼4000 small (⁠Ml 0&#8211;3.5) located on a 10-km-long creeping segment of SAF and double the number of detected RES, allowing for greater spatial coverage of slip‐rate estimations at seismogenic depths. This method is complimentary to existing cross‐correlation‐based methods, leading to more complete RES catalogs and a better understanding of slip rates at depth.

Detecting Repeating Earthquakes on the San Andreas Fault with Unsupervised Machine-Learning of Spectrograms

Andres Pena Castro, University of New Mexico
            The seismicity detected in the Antarctic continent is low compared with other continental intraplate regions of similar size. The low seismicity may be explained by (i) insufficient strain rates to generate earthquakes, (ii) scarcity of seismic instrumentation for detecting relatively small earthquakes, (iii) lack of comprehensive data mining for tectonic seismicity, or a combination of all the aforementioned. There have been ∼ 200 earthquakes in the interior of the Antarctic continent in the past two decades according to the International Seismological Centre (ISC) and other global catalogs. Previous studies in Antarctica have used seismometers installed for relatively short periods of time (∼days to months) to detect icequakes and/or tectonic earthquakes but a thorough integration of temporary and permanent network data is needed. Additionally, most of the reported seismicity was detected using classic earthquake detection techniques such as short-term-average/long-term-average or other energy detectors. State-of-the-art detection techniques, including machine learning, have proven to outperform classic detection techniques in different seismic sequences around the world and enable automated re-analysis of large volumes of data.
Here I will present a new seismic catalog for the southernmost continent. We use a Machine Learning phase picker technique on over 21 years of seismic data from on-continent temporary and permanent networks to obtain the most complete catalog of seismicity in Antarctica to date. The new catalog contains 60,006 seismic events within the Antarctic continent between January 1, 2000 to January 1, 2021, with event magnitudes between −1.0 to 4.5. Most of the detected seismicity occurs near Ross Island, large ice shelves, ice streams, ice-covered volcanoes, or in distinct and isolated areas within the continental interior. Their locations and waveform characteristics indicate volcanic, tectonic, or cryospheric sources. The catalog shows that Antarctica is more seismically active than prior catalogs would indicate. This catalogue provides a resources for more specific targeting with other detection and analysis methods such as template-matching or transfer learning, to further discriminate event types and investigate diverse seismogenic processes across the continent.

(Re)Discovering the seismicity of Antarctica: A new seismic catalog for the southernmost continent

Zachary Smith, University of California Berkley
            Intense dynamic stresses during earthquakes can activate numerous subsidiary faults and generate off-fault damage that alters fault properties and can impact the source processes and rupture dynamics of future earthquakes. Distinguishing how much damage accumulates during a single earthquake versus multiple earthquake cycles and determining how the magnitude of earthquakes impacts off-fault damage remains challenging. We combine geodetic, field, and experimental observations to evaluate the relationship between slip and off-fault deformation during a single earthquake and to assess how deformation evolves through successive earthquake cycles. Our study is focused on distributed faults that ruptured during the 2019 Mw 6.4 and 7.1 Ridgecrest earthquake sequence. Coseismic surface offsets are well imaged by satellite observing systems and the faults cut dikes of the extensive Independence dike swarm which serve as excellent linear cumulative displacement markers and records of near-fault damage exposed at Earth&#8217;s surface. Geodetic observations allow us to constrain slip and off-fault deformation due to a single event and the dikes enable us to constrain cumulative displacements.
We bridge the gap between space geodetic observations of deformation and laboratory scale deformation by collecting sub-millimeter resolution ground-based imagery and LiDAR across offset dikes and along bedrock sections of the major faults. Using these high-resolution scans, we compare fault slip with mesoscale fault-damage properties (e.g., fracture density and fragment size). Optical grain size analysis shows that fault damage is lithology dependent and that asymmetric grain size reduction of bedrock across faults is common. Fault zone asymmetry may result from slip on geometrically complex faults, preferred rupture directivity on subsidiary faults, or by distributed off-fault shearing, as observed in geodetic studies. We performed successive dynamic loading rock mechanics experiments to investigate how deformation may evolve over multiple earthquake cycles leading to the development of damage zone asymmetry and pulverized zones. Integration of geodetic, field, and laboratory observations provides a multiscale view of off-fault deformation to better inform the interpretation of inelastic strain accumulation in geodetic data, damage accumulation along large strike-slip faults, and seismic hazards associated with distributed shallow faulting.

Fault damage zone evolution across distributed fault systems: Insights from Ridgecrest, California

Yifang Cheng, Tongji University, Shanghai
            Earthquake focal mechanisms offer insights into the architecture, kinematics, and stress at depth within fault zones, providing observations that complement surface geodetic measurements and seismicity statistics. We have improved the traditional focal mechanism calculation method, HASH, through the incorporation of machine learning algorithms and relative earthquake radiation measurements (REFOC). Our improved approach has been applied to over 1.5 million catalog earthquakes in California from 1980 to 2021, yielding high-quality focal mechanisms for more than 50% of these events. In this presentation, I will elucidate how analyzing the focal mechanisms of small earthquakes advances our understanding of fault zone behaviors at varying scales, from major plate boundaries to microearthquakes.
We integrate focal mechanism data with geodetic observations, and seismicity analysis to elucidate the fine-scale fault zone structure, stress field, as well as local variations of on-fault creep rate and creep direction. All observed fine-scale kinematic features can be reconciled with a simple fault coupling model, inferred to be surrounded by a narrow, mechanically weak zone. This comprehensive analysis can be applied to other partially coupled fault zones for advancing our understanding of fault zone kinematics and seismic hazard assessment.
Additionally, we utilized the new focal mechanism catalog to construct a statewide stress model for California, shedding light on stress accumulation and release dynamics within this complex fault system. Our analysis suggests that local stress rotations in California are predominantly influenced by major fault geometries, slip partitioning, and inter-fault interactions. Major faults not optimally oriented for failure under the estimated stress regime are characterized by limited stress accumulation and/or recent significant stress release.
Finally, I will present ongoing work that employs focal mechanisms and P-wave spectra to determine microearthquake source properties, including fault orientation, slip direction, stress drop, and 3D rupture directivity. This approach markedly improves microearthquake source characterization, thereby offering an extensive dataset for probing fine-scale fault mechanics and earthquake source physics.

The November 3, 2002 Mw 7.9 Denali Fault Earthquake, Alaska: It’s Geological, Seismological, and Engineering Legacy

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