Earthquake Science Center Seminars U.S. Geological Survey

Theresa Sawi, U.S. Geological Survey

Repeating earthquakes sequences are widespread along California’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’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 “fingerprints” 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 0–3.5) 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.

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.

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’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.

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.

Travis Alongi, U.S. Geological Survey

Many of the world’s most damaging faults are offshore, presenting unique challenges and opportunities for studying earthquakes and faults. This talk explores how earthquake-generated (passive) and human-made (active) marine seismic methods improve our knowledge of on-fault slip behavior and off-fault damage.

The first part of my talk explores coupling along the poorly resolved shallow offshore portion of the southernmost Cascadia subduction zone plate interface using microseismicity patterns. Knowledge of coupling provides information about the spatial distribution and magnitude of elastic strain accumulated interseismically, presumably to be released in future earthquakes. We develop a high-quality seismic catalog using a dense amphibious seismic array and advanced location techniques to provide constraints on the coupling here. We reveal an absence of shallow plate interface seismicity, suggesting high coupling.

The second part of my talk focuses on the in-situ spatial distribution of secondary faults surrounding the main fault identified using marine-controlled source seismic reflection imaging. Through secondary faulting, the damage zone provides a window into the inelastic response of the Earth’s crust to strain. To better understand the damage zone, we develop a workflow to automate fault detections in seismic images, with dense sampling, over large distances (~10 km from the fault). Using this method, we find a peak in fault damage occurring at the location of the active main fault strand and a decay of damage with lateral distance. We found that rock type influences damage patterns and controls near-fault fluid flow. Additionally, accumulated fault slip determines the overall width of the damage zone, and along-strike variations in damage are controlled by fault obliquity.

Thomas Rockwell, San Diego State University

The Salton Basin was free of significant water between about 100 BCE and 950 CE but has filled to the sill elevation of +13 m six times between ca 950 and 1730 CE. Based on a dense array of cone penetrometer (CPT) soundings across a small sag pond, the Imperial fault is interpreted to have experienced an increase in earthquake rate and accelerated slip in the few hundred years after re-inundation, an observation that is also seen on the southern San Andreas and San Jacinto faults. This regional basin-wide signal of transient accelerated slip in interpreted to result from the effects of increased pore pressure on fault strength resulting from the ~100 m of water load during full lake inundations. If the relationship between co-seismic subsidence in the sag depression and horizontal slip through the sag is even close to constant, the slip rate on the Imperial fault may have exceeded the plate rate for a few hundred years due to excess stored elastic strain that accumulated during the extended dry period prior to ca 950 CE.