Geophysics and Tectonics Seminar - spring-2023

April 5, 2023
noon - 1 p.m.
Geophysics Seminar Room

Presented By:

• John Vidale - USC

Seminar Description coming soon.

April 12, 2023
noon - 1 p.m.
Geology 1707

Presented By:

• Caroline Beghein - UCLA

In this informal talk, I will present our recently published results on the presence of seismic anisotropy in the Martian crust from surface waves and our newer results that include higher-mode surface wave data. I would like this presentation to serve as the starting point for discussions and hopefully, we will collectively come up with possible interpretations of the observed signal.

April 19, 2023
noon - 1 p.m.
Geology 1707

Presented By:

• Prof. Joel Scheingross - University of Nevada Reno

The role of tectonics in shaping the morphology of Earth’s surface is well established. In this talk, I will present two twists that add to, and in cases challenge, our understanding of how to read the expression of tectonic processes in topography. First, I will present new work using a well-constrained case study in the Andean foreland to illustrate how lithospheric flexure can modulate patterns of fluvial erosion and deposition in low-relief landscapes. This tectonic- regulation of geomorphic processes has a cascading effect, ultimately dictating hydrology and river water geochemistry in low relief basins. Second, I will synthesize new and existing work from my group documenting how erosional feedbacks in steep landscapes can modify and obscure the morphology of river knickpoints. River knickpoints are classically interpreted as markers of a landscape responding to a change in tectonic forcing; however, our work shows that knickpoints can self-emerge in the absence of changes in tectonic forcing, and that geomorphic feedbacks can modify the morphology of knickpoints formed via perturbations in tectonic forcing.

April 26, 2023
noon - 1 p.m.
Geophysics Seminar Room

Presented By:

• Chen Gu - Tsinghua U.

In this presentation, we explore the source mechanisms of repeating earthquakes from fracturing systems in multi-scales, from induced seismicity in oil/gas fields to pico-seismicity in the laboratory. Utilizing a Bayesian source mechanism inversion method, we analyze field and laboratory data and observe the importance of regional stress fields and local fault networks in generating micro-seismicity. We also study rupture processes in a saw-cut Lucite sample through monitoring ultrasonic acoustic emissions (AEs), revealing a correlation between the proximity of low-frequency events and the subsequent triggering of large high-frequency repeating events. Furthermore, we propose a game engine-based digital twin planet designed for smart hazard mitigation, emphasizing key modules that focus on deciphering, forecasting, and visualizing time series signals from ground motions to building vibrations. Integrating Bayesian inference and generative AI techniques, we model real-world time series data by combining physical information and observed data. We demonstrate the digital twin planet's capabilities in reconstructing earthquake hazard scenarios using the M7.8 Kahramanmaras earthquake as a case study, interpreting unexpectedly high near-fault peak ground accelerations (PGAs) and showcasing the entire hazard progression from the earthquake's inception to the eventual collapse of buildings.

May 3, 2023
noon - 1 p.m.
Geology 1707

Presented By:

• Allen Husker - Caltech

This talk explores the theme of detecting unusual seismic signals and the analysis of the output. I also relate my journey from a graduate student at UCLA to professor in Mexico to professor at Caltech. The first part of the talk focuses on slow slip phenomena in the Mexican subduction zone by the Meso American Seismic Experiment (MASE). I helped install and run MASE as a grad student. The experiment was incredibly successful with 100+ publications to date utilizing the data. We used the data to massively detect low frequency earthquakes and tectonic tremor with template matching techniques. The insight from those detections associated with geodetic movement showed the direct link of the phenomena in Mexico. The next part of the talk moves to some of my current research at Caltech. We use a combination of machine learning and cross correlation techniques to analyze moonquakes. Specifically, we analyze the Lunar Seismic Profiling Experiment (LSPE) which was a mini-array of geophones at the lunar surface. Most of the thousands of moonquakes detected are thermally driven at the surface. Most events occur at sunrise or sunset and follow temperature changes of the regolith. Events also repeat with a very high correlation coefficient. We have found 30 repeating families of moonquakes to date within the data set, with most families only found exclusively during sunset or sunrise suggesting different sources for each. The lunar lander module also presents a significant source of seismic events with most occurring during sunrise as the module heats from expansion.

May 10, 2023
noon - 1 p.m.
Geology 1707

Presented By:

• Baoning Wu - USC

Rate- and state-dependent friction laws are commonly used to explain seismic observations in both laboratory and natural settings. However, the underlying physics of state dependence remains an active area of debate. In this talk, we present a physical model that links the evolution of the real area of contact on a rough interface to the state variable in the rate-and-state friction law. Our model proposes that the real area of contact changes due to creep or rejuvenation at the micro-contacts, which corresponds to changes in the state variable. To validate our model, we conduct numerical simulations on two laboratory friction experiments at the interface of transparent materials. The first experiment is a classic velocity-step and slide-hold-slide test on ~1cm^2 acrylic plastic conducted by Dieterich and Kilgore (1994), while the second experiment involves the dynamic rupture of laboratory earthquakes along a 5 mm x 200 mm PMMA interface conducted by Svetlizky, Bayard, and Fineberg (2019). Our simulations reproduce the laboratory data reasonably well, suggesting a valid constitutive relationship between the real area of contact and the size and age of micro-asperities at contact junctions. Our findings demonstrate a direct link between the state-variable and an observable quantity in the laboratory, providing new insights into the physical mechanisms underlying rate- and state-dependent friction laws.

May 17, 2023
noon - 1 p.m.
Geophysics Seminar Room

Presented By:

• Alexander Handwerger - JPL/UCLA

Landslides are often triggered by precipitation and as a result are sensitive to local climate conditions. Climate change is impacting precipitation patterns worldwide and therefore will likely have a profound influence on landslide activity over the coming decades. Here we use standardized open-access satellite radar data to assess landslide sensitivity to precipitation across a large rainfall gradient in California between 2015 and 2020. During this time period, our study area experienced some of the wettest and driest years on record, which is a precipitation pattern that is predicted to become the norm over the next century in California. We found that landslides in both wet regions of northwestern California and dry regions of southwestern California were similarly sensitive to seasonal and multi-year changes in precipitation. These landslides moved faster than average during wet years and slower than average during dry years. Our findings further confirm landslide sensitivity to climate change under diverse hydroclimate conditions and highlight the need to establish a long time series of landslide behaviors that can be used to better predict future landslide activity

May 24, 2023
noon - 1 p.m.
Geology 1707

Presented By:

• Guoliang Li - USC

Establishing a baseline of ongoing secular velocity variations at the subsurface can improve the accuracy of detecting and interpreting short-term velocity changes and advance the understanding of observed seismic motions and the behavior of subsurface materials. Toward these goals, we develop and apply a deconvolved autocorrelation (DA) method to estimate regional daily and seasonal changes of seismic velocities in southern California. The DA method combines advantages of traditional autocorrelation and Horizontal-to-Vertical Spectral Ratio, and is used to analyze over 10 years of data recorded by 50 stations. The results indicate widespread daily and seasonal changes of up to 10% and 4%, respectively, in the top tens of meters of the crust. The thickness of the surface layer, distance from the coast, and topographic variations are important factors controlling the amplitudes of the resolved velocity variations. The results suggest that changes of soil moisture and thermoelastic strain are likely dominant factors affecting the daily and seasonal variations, respectively. The developed DA method can improve the accuracy and robustness of estimated changes of subsurface materials at other locations.

May 31, 2023
noon - 1 p.m.
Geophysics Seminar Room

Presented By:

• Jezabel Curbelo - Universitat Politècnica de Catalunya (UPC)

numerical modeling of planetary and stellar convection, taking into account compressibility effects is crucial. However, using the exact equations may not be feasible due to the generation of fast acoustic waves, which distract from the slower convective motions caused by buoyancy. The Oberbeck-Boussinesq model simplifies the calculations by suppressing the acoustic waves making it easier for numerical simulations, but is so simple and pressure effects are relegated to a secondary role. Intermediate models, such as the anelastic and anelastic liquid models, have also been proposed to balance simplicity and accuracy. Our study examines compressible convection using various approximations and exact equations, considering different equations of state (EoS). Our findings are mostly discussed in the framework of mantle convection but the EoS is flexible enough to be applied to the inner core or icy planets as well.

June 7, 2023
noon - 1 p.m.
Geophysics Seminar Room

Presented By:

• Chen ji - UCSB

It has been long recognized that the source parameters of earthquakes, such as rupture area, fault length, the corner frequency of source spectra, etc., scale with earthquake magnitudes, often exhibiting self-similarities but also considerable between-event deviations (e.g., Aki, 1967; Kanamori and Anderson, 1975). In this study, we introduce a double-corner-frequency (DCF) source spectral model JA19_2S, which, in conjunction with a stochastic ground-motion model, can reproduce the mean peak ground acceleration (PGA) and mean peak ground velocity (PGV) of the NGA West2 database for magnitudes 3.3 to 7.3. The JA19_2S displacement amplitude spectrum remains constant for frequencies less than 𝑓_c1 , decays as 𝑓^-1 between 𝑓_c1 and 𝑓_c2 , and decays as 𝑓^-2 for frequencies greater than 𝑓_c2. The 𝑓_c1 and 𝑓_c2 scaling relations with the moment magnitude (M) reflect the scaling relations of total rupture duration and average rise time. After combining the JA19_2S model with static fault geometry scaling relations proposed by Leonard (2010), we estimate the inferred seismic radiated energy, average stress drop, and rupture velocity for 5.3