Earthquake Science Center Seminars

Dara Goldberg, USGS Geologic Hazards Science Center

Following a significant earthquake, the USGS National Earthquake Information Center (NEIC) publishes a spatiotemporal estimate of the earthquake’s slip pattern, known as a kinematic finite fault model. These models are critical to informing downstream response products such as ShakeMap ground motion estimates and PAGER loss estimates. Because large earthquakes can involve slip over tens to hundreds of kilometers along a fault, it is vital to rapidly assess the amount and location of slip along the fault. Since its introduction to NEIC’s rapid response capabilities in late 2007, the finite fault product has been computed in the first several hours after a significant earthquake using only teleseismic data, for which it is generally possible to obtain a reliable model for earthquakes of magnitude 7 and larger. In this talk, I will highlight some recent and anticipated updates to the NEIC’s rapid finite fault modeling capabilities to include regional-distance seismic and geodetic observations. These new features allow rapid modeling of earthquakes as small as magnitude 6, and reduce the overall time required to produce a reliable model of slip.

Wenbo Wu, Woods Hole Oceanographic Institution

As the major buffer of Earth’s energy imbalance, the ocean plays a key role in regulating global climate and temperature changes. However, accurate estimation of global ocean temperature change remains a challenging sampling problem. To complement existing point measurements, we have developed a novel and low-cost method of using travel time changes of acoustic waves from repeating natural earthquakes to infer basin-scale average ocean temperature changes. Using the land-based seismometers and CTBTO hydrophones, we have detected not only seasonal signals, which are generally consistent with that in previous oceanographic datasets of ECCO and Argo, but also interesting signals missing in ECCO and Argo. Recently, we are working on the frequency and mode dependent travel time changes of these acoustic signals, which could reveal depth information of ocean property changes. I will also talk about the challenges of the multiple frequency measuring and mode tomography and the opportunities of Distributed Acoustic Sensing in T-wave study.

Shanna Chu, USGS ESC

The faults on which earthquakes occur sometimes form complex interconnected patterns. The level of this complexity may increase high-frequency ground motions from earthquakes occurring on such faults. In this talk, I describe ways of quantifying the complexity of groups of faults based on how they are aligned and how densely they are spaced. I found that high-frequency ground motions in Southern California tend to correlate with misaligned faults, suggesting that structural interactions between different parts of the fault system may play a role in generating the ground motions felt during earthquakes. I introduce a physical model to explain one possibility of how fault interactions could generate high-frequency ground motion and the settings where such interactions could be expected to occur. Finally, I explore some of the ways in which data-driven initiatives like the Ridgecrest stress drop validation project can help us gain understanding of the relation between fault complexity and high-frequency radiation.

Janet Watt, USGS Pacific Coastal and Marine Science Center

Distinguishing between seismic and aseismic fault slip in the geologic record is difficult, yet fundamental to estimating the seismic potential of faults and the likelihood of multi-fault ruptures. We integrated chirp sub-bottom imaging with targeted cross-fault coring and core analyses of sedimentary proxy data to characterize vertical deformation and slip behavior within an extensional fault bend along the Hayward-Rodgers Creek fault system in northern San Pablo Bay. We identified and traced four key seismic horizons (R1–R4), all younger than approximately 1400 CE, that cross the fault and extend throughout the basin. A stratigraphic age model was developed using detailed down-core radiocarbon and radioisotope dating combined with measurements of anthropogenic metal concentrations. The onset of hydraulic mining within the Sierra Nevada in 1852 CE left a clear geochemical and magnetic signature within core samples. This key time horizon was used to calculate a local reservoir correction and reduce uncertainty in radiocarbon age calibration and models. Vertical fault offset of strata younger than the most recent surface-rupturing earthquake on the Hayward fault in 1868 CE suggest near-surface vertical creep is occurring along the fault in northern San Pablo Bay at a rate of approximately 0.4 mm/yr. In addition, we present evidence of at least one and possibly two coseismic events associated with growth strata above horizons R1 and R2, with median event ages estimated to be 1400 CE and 1800 CE, respectively. The timing of both these events overlaps with paleoseismic events on adjacent fault sections, suggesting the possibility of multi-fault rupture.

Daniel Faulkner, University of Liverpool

Estimating the size and frequency of earthquakes during subsurface fluid injection is currently not possible. Understanding the fundamental controls on earthquake-size distributions is the first step to making these predictions. This study shows that the laboratory measured frictional stability parameter (a-b) relates closely to fault-related induced earthquake-size distributions (b-values) in a shale gas play. Fluid injection promoted seismicity along faults that cut lithologically distinct horizons, each with characteristic frictional stability parameters and earthquake-size distributions, are compared. Results indicate material properties play a key role in earthquake-size distributions. As such, it may be possible to constrain the expected seismicity from laboratory measurements prior to any fluid injection into the subsurface.

Joshua Crozier, USGS

Wavelet based seismic event detection yields a catalog of thousands of very-long-period seismic signals over the over the 2008-2018 eruption of Kīlauea Volcano, HI, USA. These represent impulsively triggered magma oscillations within the shallow conduit and lava lake, and thus provide a direct probe of the magma system. We invert for these events, along with geodetic and lava lake data, using a petrologically informed model of magma dynamics. The inversions reveal significant variation in magma temperature and highly disequilibirum volatile contents over days to years, and suggest an evolving magma system geometry. Such changes can modulate eruption style and hazards, making in situ inference of their temporal evolution vital for volcano monitoring.

Laura Pinzon-Rincon, Université Grenoble Alpes

Seismologists eagerly seek new and preferably low-cost ways to map and monitor the complex structure of the top few kilometers of the crust. Passive seismic imaging appears as a novel, low-cost, and environmentally-friendly approach for exploring the sub-surface. Usually, passive seismic interferometry relies on blind correlations within long time series of seismic noise or coda waves. Here, we propose a complementary approach: seismic interferometry using opportune sources, specifically moving sources that are not stationary in time. This new approach relies on an accurate understanding of the seismic source's mechanism, a careful signal time-window and station pairs selection, and seismic phase identification. For example, massive freight trains were only recently recognized as a persistent, powerful cultural (human activity-caused) seismic source. Thus, these train signals can be considered an opportune seismic source for passive seismic interferometry because they are readily available, detectable, repeatable, and generate high-frequency broadband energy. To illustrate this novel method's potential, we show a case study in a mineral exploration context at the Marathon site, Ontario, Canada, where we deployed a dense nodal array of 1020 sensors. We retrieve high-frequency energy using train signals only, and we use these arrivals to generate a 3D shear-wave velocity model. We discuss the pros and cons of the method compared to more standard approaches with the help of numerical modeling. We showed that by correlating train tremors, we retrieved high-frequency arrivals with higher quality than using the standard method while using fewer data. Far from being restrained to near-surface imaging, this new way of analyzing opportune seismic sources can be applied in various contexts and scales using natural or man-generated signals.

Eva Kostyu, American Geophysical Union

This presentation will introduce the audience to the AGU Bridge Program and the program's guiding principles, origin, and impact. The AGU Bridge Program is part of the multi-disciplinary coalition, IGEN (Inclusive Graduate Education Network), which aims to increase representation of historically marginalized populations in graduate STEM education through the promotion of equitable practices and student supports. The AGU Bridge Program has a dual focus working with both graduate geoscience departments and individuals interested in pursuing an advanced degree. Forty-six departments across the US partner with AGU in this effort, with new department cohorts selected each year through a competitive application process. Students who apply through the shared IGEN application and accept an offer of admission from a partner department enter the program as Bridge Fellows with a cohort of peers and access to AGU resources. With this two-part focus on departmental adaptations and direct student support, the AGU Bridge Program is pushing for graduate education practices rooted in equity resulting in improved educational experiences and greater diversity in the geosciences.

Haoran Meng, University of California, San Diego

Resolving earthquake rupture processes is essential for understanding earthquake physics and determining seismic hazards. However, it remains challenging to directly solve the finite-source attributes of moderate and small earthquakes; rupture processes of such events are traditionally resolved using spectral methods in the frequency domain. In this study, we apply a time-domain approach that makes minimal assumptions to estimate the second-degree seismic moments of 39 M 3.9 to 5.5 earthquakes from the 2019 Ridgecrest, California sequence. The second moments can characterize earthquake finite-source attributes, including the rupture length, width, duration, velocity, and directivity. We also quantify the finite-source model uncertainties by examining both model assumptions and data imperfections. These evaluations show that our second moments method can robustly resolve the earthquake rupture length and duration. The rupture width, velocity, and directivity ratio may suffer uncertainties when the network configuration is less optimal. The resolved finite-source attributes of the 39 earthquakes show no clear spatial patterns. Most of these earthquakes ruptured bilaterally with a median scaled rupture-speed of 69% of the local shear-wave speed. On average, the rupture length is 3.0 times the rupture width, deviating away from a circular crack. With the kinematic attributes, we compute the earthquake static stress-drop and obtained a median stress-drop estimate of 35.2 MPa with a median lower bound of 1.8 MPa assuming an elliptical crack model. We show that model assumptions can lead to up to two orders of magnitude differences in the stress-drop estimates for the same earthquakes with the same set of measurements.

Timothy Clements, USGS

Extreme natural events, including magnitude 7+ earthquakes, heavy rainfall and flooding from atmospheric rivers, and multi-year droughts, can alter the elastic properties of the near-surface on time scales ranging from seconds to decades. Tracking the influence these events on the near-surface at a variety of temporal and spatial scales, however, is difficult to achieve with a single dataset. Here, I show that the change in seismic velocity (dv/v) derived from the auto-correlation of the ambient seismic field can be used to track the state of California's aquifers and the post-seismic relaxation following the Hector Mine, Baja, and Ridgecrest earthquakes over the past two decades. I leverage the computing language Julia and cloud computing resources to make daily measurements of dv/v for every available broadband seismometer in California since 1999. I will show the dominant components of dv/v time series for locations across California and their comparisons to measured groundwater levels, modeled groundwater levels from precipitation data, gravitational data from the GRACE satellite, and ground displacement from GPS stations.