Geophysics and Tectonics Seminar - fall-2021
The Salton Trough Natural Earthquake Laboratory
Sept. 29, 2021
noon - 1 p.m.
Slichter 3853
Presented By:
- Thomas Rockwell - Emeritus Professor of Geology, SDSU
Subscribe to Calendar
The Salton Trough is a natural laboratory for the study of earthquake sequences and fault interaction, as well as the study of fault zones and their associated damage structure. Sediments associated with ancient Lake Cahuilla, which filled the Trough six times in the past 1100 years, can be correlated around the basin and cross elements of many of the major faults of the southern San Andreas fault (SAF) system. High precision dating of the lakes allows for placing the earthquakes into a regional sequence. Depending on their occurrence before, during or after individual lakes, which allows for a precision of dating that is far beyond the simple correlation of earthquakes based on radiocarbon dating alone. The Salton Trough is also a hyper-arid region with little to no soil cover, which allows for the natural exposure of the fault core and damage zone of most elements of the SAF system. This talk will be an update on the ages of the late Holocene lakes, the recalculation of ages of the larger regional earthquakes, and their sequence of occurrence, with discussion on fault damage structure and extent and its possible relationship to Mmax.
On the role of thermal stress and fluid pressure in deformation and seismicity of geothermal reservoir
Oct. 6, 2021
noon - 1 p.m.
Slichter 3853
Presented By:
- Kyungjae (KJ) Im - Caltech
Subscribe to Calendar
Geothermal areas are particularly prone to induced seismicity and earthquake triggering. In this work, we numerically investigate two interesting observations related to seismicity at Coso and Brawley geothermal field in California. The Coso geothermal field lies just north of the surface ruptures driven by the 2019 Ridgecrest M7.1 earthquake in an area where coseismic stress changes should have triggered aftershocks, but a gap of aftershocks is observed there. Brawley swarm occurred 2 years after the onset of geothermal activity. Intriguingly the swarm, including M5.4 event occurred at a >5km depth, much larger than the ~1km reach of the geothermal wells. Both area experiences strong surface subsidence during geothermal operation, implying significant stress change and deformation in the reservoir. To model accurate field scale thermo-hydro-mechanical response, we set a permeable reservoir embedded in a large hosting domain using a Tough-FLAC coupled simulator. The reservoir is assumed to fail according to the Mohr-Coulomb criteria. Additionally, for the Brawley field, a critically stressed normal fault is embedded through the reservoir and host rock, as observed in a previous study. All simulation parameters are determined to reproduce the observed production, pressure and surface deformation of each field. Both simulations result in comparably rapid poro-elastic stress change over a broad area of reservoir and host, followed by slow but strongly accumulating thermal stress change within localized areas near injection and production zones. The simulation successfully reproduces the flow rates and surface deformation of the fields. In Coso simulation, we found that thermal contraction of the reservoir induces significant stress depletion and contributed to impede Ridgecrest aftershock triggering. In Brawley simulation, deep earthquake is triggered by stress transition from normal fault reactivation driven by thermo-poro-elastic stress. Our study shows how a geothermal operation can, in principle, contribute to seismic hazard mitigation through the aseismic release of tectonic stresses within a geothermal field but points to the difficulty of mitigating the hazard posed by stress transfers in the surrounding area.
Grains versus plates: the competition between tectonic and sedimentary processes in creating topography
Oct. 13, 2021
noon - 1 p.m.
Slichter 3853
Presented By:
- Chris Paola - U. Minnesota
Subscribe to Calendar
Earth's topography generally represents the outcome of a contest between creation of relief by tectonics and destruction, or filling, of relief by surface processes. We'll review studies our group has done over many years in which we attempt to parameterize the outcome of this competition in terms of tectonic versus sedimentary rates, based mostly on lab experiments with imposed deformation fields. A prevailing theme is the importance of dimensionless numbers involving tectonic versus sedimentary mass input rates.
Seismic Evidence for an Intermediate Phase during the Olivine-Wadsleyite Transformation within the Subducting Pacific Slab in Kuril
Oct. 20, 2021
noon - 1 p.m.
Slichter 3853
Presented By:
- Jaiqi Li - Department of Earth, Planetary, and Space Sciences, UCLA
Subscribe to Calendar
At the top of the mantle transition zone, olivine (α) transforms to wadsleyite (β) at about 410 km depth under equilibrium conditions, i.e., a pressure around 14 GPa and a temperature of about 1350 °C. The subsequent wave speed increase upon the α-β phase transition led to the discovery of the “410-km discontinuity” as a global feature thanks to seismology. In contrast, lower temperatures (< 1000°C) within the slab inhibit diffusive processes, thus diffusionless transformations might occur, such as the transition from α-olivine to ω-olivine (ε*-phase), i.e. new high-pressure polymorphy recently discovered in heavily shocked wadsleyite and ringwoodite meteorites. Our seismic waveform inversion results of the triplicated P wave datasets (in the Kuril subduction zone) show drastic variations of P-wave velocity inside the slab: a zone of extremely low wave speed (wave speed reduction < -20%) is located between 383 and 415 km depth, close to the cold core of the slab. These observations indicate that a layer of destabilized olivine exists within the cold slab, which highlights the transient (meta)stability of the ε*-phase, under substantial shear stress. Nonetheless, any phase transformation at relatively low temperatures necessarily induces long-lived grain-size reduction, which is also known to reduce P wave velocities. Whichever the transformation, we propose that the extremely low wave speed zone corresponds to a layer of partially transformed material, possibly consisting of a mixture of α, ω, and β-olivines.
Accurate Prediction of Ocean Basins Using Upper Mantle Potential Temperatures
Oct. 27, 2021
noon - 1 p.m.
Slichter 3853
Presented By:
- Xiyuan Bao - Department of Earth, Planetary, and Space Sciences, UCLA
Subscribe to Calendar
The mid-ocean ridges represent a very visible consequence of mantle convection and plate tectonics, and exhibit distinct large-scale geophysical and geochemical patterns. Previous studies try to understand the differences by examining the correlation among basin-distinct spreading rate, ridge depth, and geochemistry of MORB. (Klein and Langmuir 1987, Gale et al. 2014, Yaoling Niu 2016, Brandl et al. 2013), but they usually focus on shallow processes associated with partial melting and melt transport to explain the observations. However, MORB observations, especially geochemistry and associated estimates of mantle temperatures are strongly affected by shallow melting processes, this makes it difficult to understand any potential contributions of the deep mantle flow (Krein et al. 2021). In this study, we examine the role of the deep mantle and large-scale tectonics on the mid-ocean ridge system, using just the seismically-inferred (Bao et al. 2021) deep (260-600 km) upper mantle temperatures. Using a robust machine learning model (Random Forest), we show that it is possible to predict the ocean basin of each ridge segment (data: Gale et al. 2014) up to 90% accuracy, with the mantle temperature at depth alone, in the absence of any other information. We find that the two features that provide more than half of the discriminative power in the random forest model are the temperature difference between the mid-layer (320-500 km) and other depths, as well as average temperature over all depths. Our result implies that the temperature of the upper mantle may record the 100s Ma (mantle overturn timescale) convection and tectonic history. These processes have long-term impact on the upper mantle flow, and the (thermal) heterogeneity in each basin. We posit that the mantle convection and tectonic processes play a significant role in explaining the large-scale geophysical and geochemical differences in the mid-ocean ridge system.
Subduction Initiation and the Pacific Hemisphere at 50 Ma
Nov. 3, 2021
noon - 1 p.m.
Slichter 3853
Presented By:
- Michael Gurnis - Caltech
Subscribe to Calendar
Subduction initiation is a fundamental component of the plate tectonic cycle and is especially relevant to understanding the changing force balance on plates. The last several years has witnessed a surge of effort aimed at understanding this process. I will address recent observational programs, including two ocean drilling, IODP, expeditions and the South Island Subduction Initiation Experiment, SISIE, in the context of the mechanics of subduction initiation. I will address the major change in plate and plume motion associated with the Pacific Plate at around 50 Ma within the context of a new generation of ultra high-resolution global models of plate tectonics and mantle flow. I will show that the traditional model of subduction initiation in the western Pacific fails to change plate motion fast enough and offer a new reconstruction of the Pacific. The reconstructions, when combined with the dynamic models, can explain the rapid change in Pacific Plate motion.
The M7.1 2019 Ridgecrest Earthquake: Time-Varying Ground and Tall Building Response Enabled by the Community Seismic Network
Nov. 10, 2021
noon - 1 p.m.
Slichter 3853
Presented By:
- Monica D. Kohler - Caltech
Subscribe to Calendar
The Ridgecrest earthquakes of July 2019 were recorded on hundreds of Community Seismic Network (CSN) accelerometers in greater Los Angeles, and showed unexpected patterns and large spatial variations in long-period (≥ 3 s) shaking amplification. CSN is a low-cost, strong-motion seismic network consisting of over 700 accelerometers deployed in southern California, taking advantage of small-form-factor MEMS sensing technologies, on-site computing, and cloud infrastructure. Seismic interferometric techniques inspired by seismological applications are applied to data from CSN sensors permanently installed on nearly every floor of a 52-story steel moment-and-braced frame building in downtown Los Angeles. Wavefield data from before and during the 2019 M6.4 foreshock and M7.1 mainshock are processed for time domain impulse response functions which are generalized to show that a building’s nonlinear response can be quantified through time-varying measurements of representative pseudo-linear systems. Shear-wave velocities, found from linear regression to the impulse response function’s direct and scattered energy arrivals, are reduced by up to 10% during building shaking. The time-varying observations are resolved over time scales of seconds, throughout the entire duration of building shaking. A numerical application of Helmholtz tomography is applied to simulated impulse response functions from finite-element models of the same building to show how variations in wave properties can be used to map small-scale damage, such as fractures, in a building.
The Environmental Master Variable: Watershed Suspended Sediment Dynamics from Headwaters to the Harbor
Nov. 17, 2021
noon - 1 p.m.
Slichter 3853
Presented By:
- Andrew Gray - UC Riverside
Subscribe to Calendar
All natural aquatic systems transport fine sediments in suspension, where they play important physical, chemical and biological roles, perhaps most valued as substrate for floodplain, beach, and wetland maintenance. However, over-abundance of suspended sediment and contamination with human generated pollutants together represent the most prevalent impairment of water bodies in the US. Human development has resulted in spatio-temporal patterns of both sediment impairment and resource deficiencies within individual watersheds. Efforts to define thresholds of acceptable sediment abundance and composition, and develop watershed-scale management plans to achieve these goals, are further complicated by highly variable and poorly characterized natural sediment regimes. We know that the provenance, travel time and transport path of fine sediments largely control their composition and pollutant load. Yet these watershed-scale dynamical characteristics are usually not considered in assessments of sediment as resource or impairment, which tend to rely on ‘snapshot’ ambient characterizations of suspended and/or bed sediments. How long will ambient impairments or sediment starvation persist? What is the sediment impairment or accretionary/erosional trajectory for a given site? We will explore recent advancements in the study of sediment dynamics, and consider exciting potential applications that may help address these questions and others critical to the management of aquatic systems at the watershed scale.
How Plants Shape Mountains
Dec. 1, 2021
noon - 1 p.m.
Slichter 3853
Presented By:
- Todd Ehlers - U. Tübingen
Subscribe to Calendar
Earth surface processes are modulated by fascinating interactions between climate, tectonics, and biota. These interactions are manifested over diverse temporal and spatial scales ranging from seconds to millions of years, and microns to thousands of kilometers, respectively. Investigations into Earth surface shaping by biota have gained growing attention over the last decades and are a research frontier. In this lecture, I present an integration of new observational and numerical modeling research on the influence of vegetation type and cover on the erosion of mountains. I do this through an investigation of millennial timescale catchment denudation rates measured along the extreme climate and ecologic gradient of the western margin of South America.