iPlex Lunch - fall-2017

Oct. 4, 2017
noon - 12:50 p.m.
Geology 1810

Presented By:

• Zachary E. Ross - Caltech

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The relationship between the internal structure of fault zones and the earthquake source is poorly understood. To investigate possible connections, I examine three active earthquake sequences in southern California and Japan (2010 Mw 7.3 El Mayor-Cucapah, 2016 Mw 5.2 Borrego Springs, and 2016 Mw 6.2 Tottori) that exhibit complex spatiotemporal patterns. A template matching algorithm is used to construct expanded high-resolution seismicity catalogs to characterize the geometry of the fault systems at depth. The precisely located seismicity delineates highly-complex active fault structures consisting of orthogonal or subparallel faults with frequent segmentation and branching. Slip models are derived for the Borrego Springs and Tottori mainshocks, along with the seismic energy, stress drop, and radiation efficiency. These events occurred in complex immature fault zones and are found to have very low radiation efficiency values; this suggests that most of the available strain energy was dissipated as off-fault brittle fracturing. More than 155,000 aftershocks of El Mayor-Cucapah in the Yuha Desert and postseismic geodetic strain reveal two distinct processes driving the evolution of the sequence. Early aftershocks spread away from the mainshock rupture terminus with the logarithm of time, and were likely driven by afterslip. A second wave of aftershocks swept back across the region with the square-root of time and swarm-like characteristics, and were likely driven by fluid diffusion. The discussed studies provide links between properties of fault zone structures, earthquake source processes, and driving mechanisms behind aftershock production.

Oct. 11, 2017
noon - 12:50 p.m.
Geology 1707

Presented By:

• Luis Antonio Domínguez Ramírez - UNAM

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Recent earthquakes along the Mexican subduction zone challenge our current understanding about where and how the next destructive earthquake will nucleate. For example, the September 7th, 2017 M8.2 Juchitan earthquake ruptured a segment along the coast known as the Tehuantepec Gap, where no strong earthquakes have been reported for more than 100 years. Although the location and magnitude of this event was expected, the focal mechanism revealed a normal interplate event, instead of a megathrust subduction zone earthquake. Few days after this event, and even more unlikely event caused vast damage in Mexico City and nearby populations. A M7.1 earthquake occurred scarcely 170km from the city, coincidentally during the 32 anniversary of the M8.1, 1985 Michoacan Earthquake that caused more than 10,000 casualties according to official records. In the same way, this event had normal focal mechanism, likely due to the bending of the flat segment of the Cocos Plate. Current research, focused on understanding what is the contribution of different seismic and aseismic mechanisms mainly along the Guerrero Gap, the closest aseismic gap to Mexico City. Large slow slip events periodically rupture this area and are suspected to triggered large earthquakes along this segment. Characteristic repeating earthquakes show large variations along the coast, suggesting areas of locking and creeping along the trench. Therefore, the Mexican subduction zone still poses a large amount of uncertainties about how large and moderate earthquakes nucleate. Future research must focus on comprehensive studies beyond the traditional areas of research to better understand the seismic risk along the subduction zone and inner areas. If you want to meet with the presenter, please sign up here.

Oct. 18, 2017
noon - 12:50 p.m.
Geology 1707

Presented By:

• Alex Beer - ETH & Caltech

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Bedrock outcrops set the boundaries of mountain streams and confine channel-hillslope evolution. Acquisition of current spatial erosion data is challenging, so few local measurements exist and it is hard to assess process mechanic concepts/models on various spatio-temporal scales. I will present insights from several spatial erosion measurement techniques and speculate on the dependence of erosion process dominance. If you want to meet with him, please sign up here.

Oct. 25, 2017
noon - 12:50 p.m.
Geology 1707

Presented By:

• Alexander Robel - Caltech & Georgia Tech

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Iceberg mélange is a dense granular aggregation of icebergs and sea ice that is found floating in the ocean in front of many marine-terminating glaciers in Greenland and Antarctica. Recent observations indicate that the decline of local sea ice cover within mélange has been accompanied by an increase in nearby iceberg calving and associated ice sheet mass loss. The leading for this association is that mélange exerts a mechanical force directly on the glacier, lowering extensional stresses, preventing fracture propagation and iceberg calving near the glacier terminus. In this talk, I will show how sea ice thickness and concentration play a critical role in setting the material strength of mélange. I adapt a discrete element model to simulate mélange as a cohesive granular material. In these simulations, mélange laden with thick, dense, landfast sea ice can produce enough resistance to shut down calving at the terminus. When sea ice thins, mélange weakens, reducing the mechanical force of mélange on the glacier terminus, and increasing the likelihood of calving. When calving events occur, jamming waves are initiated within mélange, propagating away from the glacier and causing the fracture of the sea-ice matrix that bonds mélange, increasing the likelihood of subsequent calving events. I compare these simulations to observations of mélange from ground-based photogrammetry and interferometric synthetic aperture radar. I then discuss whether longer periods of sea-ice-free conditions in winter may lead to a transition from currently slow calving, predominantly occurring in the summer, to rapid calving, occurring throughout the year, accelerating ice sheet mass loss and sea level rise. The results in this talk can mostly be found in the following paper from earlier this year: LINK TO PAPER

Nov. 1, 2017
noon - 12:50 p.m.
Geology 1707

Presented By:

• Yingdi Luo - Caltech

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Geological, seismological, geodetic and experimental studies provide evidence of the heterogeneous structure of natural faults. To advance our understanding of the mechanical role of fault heterogeneity on the diversity of earthquake slip behavior, we conduct a theoretical and computational study of heterogeneous fault models. We consider faults with a mixture of frictionally stable and unstable materials and spatial contrasts of fault zone pore fluid pressure, akin to hydraulically sealed brittle asperities embedded in a ductile fault zone matrix. We first study faults with a regular alternation of materials, using linear stability analysis and quasi-dynamic rate-and-state simulations. We find transitions in fault behavior from fast to slow earthquakes to steady slip, and determine how these transitions depend on the composition and strength contrast of the material mixture. Based on these results, we develop rate-and-state models with stochastic distributions of brittle asperities in a ductile matrix to study slow slip and tremor phenomena. We focus on the hierarchical patterns of tremor migration observed in subduction zones, which feature distinct tremor propagation speeds in different directions. Our models are in quantitative agreement with observations of episodic slow slip and tremor events in Cascadia. We discovered that, in contrast to a common view, slow slip might well be a result of tremor activity rather than its cause. The collective interaction of asperities with a broad range of material properties, mediated by creep, is a novel and robust mechanism for the generation of slow slip events. We find that the hierarchical patterns of tremor migration and the nucleation locations of tremor swarms provide constraints on fault rheology. Our study also shows that, despite multiple asperity interactions, there is a close relation between tremor rate and the underlying slip rate which supports an approach to constrain slow slip rate via observed tremor rates.
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Nov. 8, 2017
noon - noon
Geology 1707

Presented By:

• Men-Andrin - Caltech

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The temporal evolution of earthquake rupture is relatively well constrained by seismological observations, and contains a wealth of information on earthquake rupture processes. Here we study i) short distance (<25km) seismic recordings of shallow crustal earthquakes, and ii) source time functions derived from teleseismic recordings of shallow subduction megathrust events. We use these observations to compile an empirical description of rupture evolution that includes both the typical rupture behavior, as well the deviations from it. With both data types we find well-defined typical patterns, which are incompatible with widely used standard source models, such as self-similar pulses and cracks. In particular, observed moment growth rates are systematically lower than the standard models predict. We discuss the possible physical origins of the observed temporal rupture behavior, and the implications for rupture predictability and earthquake early warning systems

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Nov. 15, 2017
noon - 12:50 p.m.
Geology 1707

Presented By:

• James Dolan - USC

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On the constancy (or not) of fault slip: Potential controls, implications for seismic hazard and plate boundary mechanics, and the need for systems-level analysis

There is mounting evidence that the occurrence of large earthquakes on both single faults and fault systems is not a random process. Earthquakes often cluster in both space and time, leading to episodic increases in slip that can span multiple earthquake cycles and tens of meters of fault slip. Comparisons between geodetic and longer-term geologic rates demonstrate that such clusters and lulls may in some instances coincide with transiently elevated or decreased periods of elastic strain accumulation. Moreover, recent observations from several plate boundaries that I will discuss in this talk suggest the coordinated waxing and waning of slip on mechanically complementary regional fault systems. Although a thorough understanding of both the causes and generality of such emergent phenomena is of basic importance for fault mechanics and earthquake physics, as well as for more accurate assessment of seismic hazard, our ability to evaluate the importance of these behaviors has been severely data limited, demonstrating the necessity of documenting additional examples of incremental fault behavior. In addition to discussing examples of potentially coordinated fault system behavior from southern California and New Zealand, I will describe several potential mechanisms that have been proposed to explain such behavior, including possible temporal variations in fault strength, which in kinematically complex fault systems may result in the weakest part of any system accommodating faster-than-average rates while mechanically complementary parts of the system move more slowly, and potential variations in local plate boundary rate. These results reinforce the need for system-level analyses of incremental fault slip patterns in efforts to more fully understand the controls on earthquake occurrence, with obvious implications for everything from seismic hazard assessment (including the “P” word) to plate boundary mechanics to the proper interpretation and use of both geologic and geodetic rate data.

Nov. 22, 2017
noon - 12:50 p.m.
Geology 1707

Presented By:

• Shen, Zheng Kang - UCLA

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It has been almost ten years since the 2008 Mw7.9 Wenchuan, China earthquake, and a lot has been learnt about its tectonic origin, rupture process, and physical mechanisms. Some important questions, however, are still being debated. For example, for the earthquake rupture process, what controlled the slip distribution, and how the rupture was developed? Can we learn something about faulting styles of continental thrust faults from this quake? A large reservoir was built right on the seismogenic Longmen Shan fault 2.7 years before the Wenchuan earthquake, was the quake triggered by water impoundment of the reservoir? If so, by what triggering mechanism? A Mw6.6 Lushan earthquake occurred 5 years after Wenchuan and ~70 km southwest of the Wenchuan surface rupture, was this quake an aftershock of the Wenchuan earthquake? What should be the right criterions to be used for aftershock assessment anyway? In this talk, I will present our undertakings of these questions and some of the answers we have found for them.

Nov. 29, 2017
noon - 12:50 p.m.
Geology 1707

Presented By:

• Joann Stock - Caltech

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The Salton Trough and Imperial Valley contain pull-apart basins formed by the on-going oblique extension between the Pacific and North America plates, where new crust is being formed along the plate boundary. Much of the plate boundary slip is distributed along the San Andreas and San Jacinto Fault systems, heading southward to a complex set of connections with the Imperial fault and other faults in Mexico. To better understand the structure in this region, we conducted an active source seismic survey (the 2011 Salton Seismic Imaging Project, or SSIP) using explosions in shallow boreholes to derive the crustal structure and fault orientations. This talk will highlight some recently published results from the SSIP and other geological studies, as well as summarize observations from Mexico about the faulting and basin history south of the border. Velocity models and geological interpretations of the crust beneath the Salton Sea indicate that there is a layer of new mafic crust beneath the lowest part of the basin, which is not detected farther south in the Imperial Valley or in the Mexicali Valley. The 3D seismic velocity models also reinforce the correlation of some NE-trending seismicity lineaments with major structural boundaries. These results have important implications for how this part of the Pacific-North America plate boundary evolved, as well as for societal concerns such as earthquake shaking in the LA Basin and future ground ruptures in the Mexicali Valley.

Dec. 6, 2017
noon - 12:50 p.m.
Geology 1707

Presented By:

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Seminar Description coming soon.