Earthquake Science Center Seminars U.S. Geological Survey

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.

Taiyi Wang, Stanford University

All instrumented basaltic caldera collapses generate Mw > 5 very long period earthquakes. However, previous studies of source dynamics have been limited to lumped models treating the caldera block as rigid, leaving open questions related to how ruptures initiate and propagate around the ring fault, and the seismic expressions of those rupture dynamics.

In the first part of my talk, I will present the first 3D numerical model capturing the nucleation and propagation of ring fault rupture, the mechanical coupling to the underlying viscoelastic magma, and the associated seismic wavefield. I demonstrate that seismic radiation, neglected in previous models, acts as a damping mechanism reducing coseismic slip by up to half, with effects most pronounced for large magma chamber volume, high magma compressibility, or large caldera block radius. Viscosity of basaltic magma has negligible effect on collapse dynamics. In contrast, viscosity of silicic magma significantly reduces ring fault slip.

In the second part of my talk, I compare simulation results with the 2018 Kīlauea caldera collapse. Three stages of collapse, characterized by ring fault rupture initiation and propagation, deceleration of the downward-moving caldera block and magma column, and post-collapse resonant oscillations, in addition to chamber pressurization, are identified in simulated and observed (unfiltered) near-field seismograms. A detailed comparison of simulated and observed displacement waveforms corresponding to collapse earthquakes with hypocenters at various azimuths of the ring fault reveals a complex nucleation phase for earthquakes initiated on the northwest.

At the end of my talk, I will show ongoing work in deriving rigorous seismic representations of caldera collapse earthquakes from dynamic rupture simulations. The theory is fully general and can be applied to other volcanic processes, enabling parameterization of seismic inverse problems consistent with source physics.

Shinji Toda, Tohoku University

The 1 Jan 2024 Noto Hanto earthquake launched a plethora of ills on the Noto Hanto population, taking 200 lives, and causing $25B in damage, only $5B of which was insured. These ills include a tsunami that arrived within a few minutes of the mainshock, as well as unexpectedly strong shaking throughout the Noto peninsula. In addition to direct shaking damage, the shaking triggered massive landslides in steep terrain, and caused extensive liquefaction in coastal marshes and estuaries. Coastal uplift of up to 4 m also lifted fishing harbors out of the water. Because the affected population locates nearly on top of the epicenter, neither the earthquake early warning nor the tsunami warning were effective. The earthquake was preceded by an extremely intense 3-year-long seismic swarm, and so efforts are under way to discern if the swarm triggered the earthquake, and if so, how. Whether swarms can trigger great quakes is a key question with which we must now grapple, as swarms are common in California, and elsewhere in Japan, as well. Sadly, the previously mapped offshore fault that ruptured was not used in the Japanese HERP hazard assessment, and so the Noto peninsula hazard had been greatly underestimated.

Mong-Han Huang, University of Maryland

The Ross Ice Shelf (RIS) in Antarctica is the largest ice shelf in the world. As the RIS flows toward the Ross Sea, a buildup of tensile stress due to increasing ice flow velocity develops a series of flow-perpendicular rift zones. Although these rifts are essential in contributing to future calving and reduction in size of the ice shelf, their material properties and mechanical response to external stress in the rift zone scale (~10-100 km) are poorly understood, partly due to a lack of high spatial-temporal scale in-situ observations to characterize key rift processes. Using a deployment of seismometers and GPS stations from the NSF DRRIS project and recently by our team, we further explore the link between seismicity, tidal cycles, and air temperature at two rifts of different ages. We found that icequakes along the major rift zones on RIS are modulated with the oscillating tidal stressing rate, and icequakes have a stronger modulation with stress rate in older rifts. We adopted the theory proposed by Heimisson and Avouac (2020) about seismicity rate due to oscillating stresses for icequakes. On RIS, the characteristic time scale from elevated icequake seismicity rate to background rate is much shorter than the periodicity of the tidal stresses, and therefore the seismicity rate is proportional to the stressing rate. This also suggests that how stress varies in time, rather than the total quantity of stress, has a higher contribution to brittle fractures in ice shelves. Constraining the current behavior of ice shelf rifts and their modulation by oscillating stresses will help inform their future stability in a changing climate.