iPlex Lunch - Winter-2018
The HayWired Scenario—How Can the San Francisco Bay Region Bounce Back Better?
Jan. 10, 2018
noon - 12:50 p.m.
Geology 1707
Presented By:
- Dr. Kenneth Hudnut - USGS
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The HayWired scenario is a hypothetical yet scientifically realistic and quantitative depiction of a moment magnitude (Mw) 7.0 earthquake (mainshock) occurring on April 18, 2018, at 4:18 p.m. on the Hayward Fault in the east bay part of the San Francisco Bay area, California. The hypothetical earthquake has its epicenter in Oakland, and strong ground shaking from the scenario causes a wide range of severe impacts throughout the greater bay region. In the scenario, the Hayward Fault is ruptured along its length for 83 kilometers (about 52 miles). Building on a decades-long series of efforts to reduce earthquake risk in the San Francisco Bay region, the hypothetical HayWired earthquake is used to examine the well-known earthquake hazard of the Hayward Fault, with a focus on newly emerging vulnerabilities. After a major earthquake disaster, reestablishing water services and food-supply chains are, of course, top priorities. However, problems associated with telecommunication outages or “network congestion” will increase and become more urgent as the bay region deepens its reliance on the “Internet of Things.” Communications at all levels are crucial during incident response following an earthquake. Damage to critical facilities (such as power plants) from earthquake shaking and to electrical and telecommunications wires and fiber-optic cables that are severed where they cross a fault rupture can trigger cascading Internet and telecommunications outages, and restoring these services is crucially important for emergency-response coordination. Without good communications, emergency-response efficiency is reduced, and as a result, life-saving response functions can be compromised. For these reasons, the name HayWired was chosen for this scenario to emphasize the need to examine our interconnectedness and reliance on telecommunications and other lifelines (such as water and electricity). Earthquake risk in the San Francisco Bay region has been greatly reduced as a result of previous concerted efforts; for example, a roughly $50 billion investment in strengthening infrastructure was motivated in large part by the 1989 magnitude (M) 6.9 Loma Prieta earthquake. The earthquake hazard from the Hayward Fault remains high, however, and work still needs to be done to ensure that the region is ready for an earthquake like that in the HayWired scenario. Already, there is a renewed commitment from the newly formed HayWired Coalition—consisting of numerous government, academic, utility-provider, and community stakeholders—to bring new, varied perspectives to bear on the problems that remain.
New Frontiers of Planetary Seismology.
Jan. 17, 2018
noon - 12:50 p.m.
Geology 1707
Presented By:
- Pr. Philippe LOGNONNE - Institut de Physique du Globe de Paris Université Paris Diderot-Sorbonne Paris Cité, France, Institut Universitaire de France
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About 45 years ago seismology started its escape from Earth, with not only the first successful installation of a seismometer on the Moon by the Apollo missions but also with the first observations of seismic waves in the ionosphere, 250 km or more above Earth surface. Our journey to today’s research at these frontiers of seismology will start with the Moon and the 40 years old Apollo data and will then move to Mars and finally Venus or Europa, both targets of concept studies for the 2020-2030. We first present the most recent results obtained in the re-processing of the Apollo data since 2000: re-estimation of the lunar crustal thickness, discovery of the Lunar core reflected seismic waves, characterization of the dynamics of the deep moon quake and impacts. We then move to Mars, where data will wait for the launch in May 2018 of the NASA InSight mission, which will carry to the Martian surface a 3 axis Very Broad Band and a 3 axis Short Period seismometer. We present the scientific perspectives of the mission and the technical challenges associated to the robotic installation of VBB instruments in an hostile and windy environment. We then conclude with possible future missions in planetary seismology, which concepts are presently worked by the international Planetary seismology. These might either enable the seismic discovery of new bodies, like Euopa, one of the icy moon of Jupiter with an underground ocean or Venus, with remote sensing perspectives based on airglow observations, or might lead to the deployment of a new seismic network on the Moon.
Machine learning on the 5200 element Long Beach array:
Jan. 24, 2018
noon - 12:50 p.m.
Geology 1707
Presented By:
- Peter Gerstoft - UCSD
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Machine learning on the 5200 element Long Beach array: Localizing small sources and sparse travel-time tomography
In this talk, I will focus on two unsupervised learning methods for analyzing the Long Beach data. Unsupervised machine learning we infer a function to describe hidden structure from "unlabeled" data. An example of unsupervised machine learning is dictionary learning. Dictionary leaning can improve SSP resolution by generating a dictionary of shape functions for sparse processing (e.g., compressive sensing) that optimally compress SSPs; both minimizing the reconstruction error and the number of coefficients. Dictionary learning can also be used for travel time tomography. A 2D travel time tomography method which regularizes the inversion by modelling sparsely patches of slowness pixels from discrete slowness map, and adapts sparse dictionaries to the slowness data.
We develop a model-free technique to identify weak sources within dense sensor arrays using graph clustering. No knowledge about the propagation medium is needed except that signal strengths decay to insignificant levels within a scale that is shorter than the aperture. We then reinterpret the spatial coherence matrix of a wave field as a matrix whose support is a connectivity matrix of a graph with sensors as vertices. In a dense network, well-separated sources induce clusters in this graph. The geographic spread of these clusters can serve to localize the sources. The support of the covariance matrix is estimated from limited-time data using a hypothesis test with a robust phase-only coherence test statistic combined with a physical distance criterion. The latter criterion ensures graph sparsity and thus prevents clusters from forming by chance. We verify the approach and quantify its reliability on a simulated dataset. The method is then applied to data from a dense 5200 element geophone array that blanketed 7km×10km of the city of Long Beach (CA). The analysis exposes a helicopter traversing the array and oil production facilities.
Deciphering Dynamic Crustal Faulting from Interseismic Phenomena
Jan. 31, 2018
noon - 12:50 p.m.
Geology 1707
Presented By:
- Junle Jiang - UCSD
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Geophysical observations from major crustal faults are often limited to short interseismic periods, while most of these faults are capable of hosting large earthquakes. Understanding the seismic behavior of faults from interseismic observations is hence crucial to hazard assessments. In this talk, I will present results from studies of some major crustal faults, based on modeling dynamic earthquake sequences and aseismic slip, and integrating laboratory-based insights on rock friction with observations of microseismicity and geodetic locking and creep. Such a framework is used to illuminate the depth extent of large earthquakes on the seismically quiescent San Andreas fault in California, reveal the possible occurrence of deep slow slip on the San Jacinto fault at Anza, and discern physical mechanisms responsible for unsteady shallow creep on the Superstition Hills fault in California and the North Anatolian fault at Ismetpasa, Turkey. These applications demonstrate the value of observation-driven, physics-based approaches in providing constraints on the fault zone rheology and potential behavior of past and future earthquakes.
Probing the paleoseismic record on the San Andreas Fault System: Patterns and Prognoses
Feb. 7, 2018
noon - 1 p.m.
Geology 1707
Presented By:
- Kate Scharer - USGS
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In this talk I will review the types of data collected to document the prehistoric record of large, ground-rupturing earthquakes, and how these data are used to understand fault behavior patterns. Earthquake evidence and radiocarbon dating at individual sites along the southern San Andreas Fault suggest that earthquake recurrence is not clustered; rather it is more regular than would be expected from a random distribution. Comparing records along the fault system, simple models of the maximum rupture length, and thus estimates of moment release, show interesting patterns that differ from current probabilistic hazard models and suggest the most recent event in 1857 was larger than average. Finally, an evaluation of the most robust, long records from around the plate boundary system reveals that fine-scale variations that appear in the historic record are not demonstrated in the paleoseismic record.
Source Attributes and Strong Ground Motion Characteristics from 2016-2017 Central Italy Earthquake Sequence
Feb. 14, 2018
noon - 1 p.m.
Geology 1707
Presented By:
- Paolo Zimmaro - UCLA Civil Engineering
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The Central Italy 2016-2017 earthquake sequence has generated three mainshocks: M6.1 24 August, M5.9 26 October, and M6.5 30 October 2016. These events, along with a relevant number of aftershocks, were well recorded by Italian networks. This earthquake sequence occurred in a gap between two earlier damaging events, the 1997 M6.1 Umbria-Marche earthquake to the north-west and the 2009 M6.1 L’Aquila earthquake to the south-east. Analysis of data from this earthquake sequence will likely have worldwide relevance. For example, many recent ground motion models are based on global databases in which the large majority of recordings for normal fault events with M>5.5 are from Italy. This presentation focuses on source attributes and ground motion characteristics of this earthquake sequence. Significant evidences, including surface rupture patterns and inverted finite fault models, point to the earthquakes having been generated on the Mt. Vettore-Mt. Bove fault system (all three mainshocks) and on the northern segment of the Laga Mountains fault system (portion of 24 August event). This information could be used to improve earthquake rupture forecast models taking into account multi-fault ruptures. Ground motion characteristics of mainshocks and selected aftershocks are also presented. Stations at close distance, including near the hanging wall, exhibit fling step in some cases but, remarkably, no obvious rupture directivity. Overall, the data exhibit fast anelastic attenuation at large distances (> 100 km). This feature seems to be persistent throughout all of the recent Italian earthquakes and it is consistent with recent Italy adjusted global ground motion models, but not with a recent Italy-specific model. Outcomes from this study could provide useful information on how to improve future probabilistic seismic hazard analyses in Italy and elsewhere.
Resolving the up-dip extent of slip in large subduction zone earthquakes and its influence on aftershock distributions
Feb. 21, 2018
noon - 1 p.m.
Slichter 3853
Presented By:
- Thorne Lay - UCSC
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Slip distributions during large underthrusting earthquakes on subduction zone plate boundaries are now commonly determined using seismic, geodetic, and/or tsunami observations. A persistent challenge has been to constrain the up-dip extent of rupture, or how close to the trench coseismic sliding occurs. This issue is important for several reasons. Assessing the potential for shallow tsunami earthquakes that rupture the megathrust near the trench requires knowledge of whether prior deeper ruptures have extended to the trench. This is illustrated by the 2010 Mentawai tsunami earthquake, which ruptured up-dip of the 2007 Sumatra event. Earthquake slip at shallow depth also characterizes the frictional properties of the shallow megathrust and its seismogenic potential. On-land geodetic observations have little resolution of rupture if it occurs relatively far off-shore, as was demonstrated by analyses of the great 2011 Tohoku earthquake. Seismological observations alone also may have a quite limited resolution of very shallow faulting on the up-dip portion of the megathrust. New approaches using water reverberations in the P coda hold the promise of improving seismological resolution. Off-shore seafloor geodetic observations (GPS-Acoustic or ocean-bottom pressure sensors) significantly improve the determination of trenchward rupture extent, as shown for the 2011 Tohoku event, but only a few regions currently have sufficient instrumentation. For a number of recent large earthquakes, joint analysis of seismic and tsunami observations, in some cases including on-shore and off-shore geodetic data, appears to provide reliable estimates of up-dip slip extent (corroborated by direct imaging of seafloor offsets for 2010 Maule, Chile and 2011 Tohoku earthquakes). For the tsunami data to be best interpreted, corrections for path effects that have commonly been ignored must be included.
How Faults Wake Up: Insights from Induced Earthquakes in the U. S. Midcontinent
Feb. 28, 2018
noon - 1 p.m.
Geology 1707
Presented By:
- William Ellsworth - Stanford University
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Much of Oklahoma and Southern Kansas has seen widespread seismic activity in the last decade that is attributed to large-scale wastewater disposal into the Arbuckle group. Using a waveform-relocated earthquake catalogue, we perform a systematic study of the activity on several hundreds of identified faults. We use 93 sequences with at least 30 events for a detailed analysis of their spatio-temporal evolution. For most awakened faults, seismicity tends to initiate at shallower depth and migrates deeper along the faults as the sequence proceeds. No major sequence starts with the largest earthquake and many sequences initiate months before they rise to peak activity. We study temporal clustering as a means to quantify earthquake interactions. Some sequences show no temporal clustering similar to Poissonian background seismicity but at a much higher rate than the natural background. Other sequences exhibit strong temporal clustering akin to mainshock-aftershock sequences. We conclude that once initiated by anthropogenic forcing, portions of the activated faults in the Oklahoma/Kansas area are close enough to failure to continue failing through earthquake interactions. In many sequences, including those with the largest earthquakes, seismicity continues within the previously activated region rather than by growing the activated area. Therefore, monitoring seismicity with a low magnitude threshold and high location precision has the potential to detect minor activity as it initiates failure on specific faults and thus provides time to take actions to mitigate the occurrence of potentially damaging earthquakes. A Systematic Assessment of the Spatio‐Temporal Evolution of Fault Activation Through Induced Seismicity in Oklahoma and Southern Kansas.
Teleseismic Reflection Imaging for Understanding the Thermal, Chemical and Dynamical Evolution of the Earth: Two Case Studies from the Western United States and the Hawaiian Hotspot
March 7, 2018
noon - 1 p.m.
Geology 1707
Presented By:
- Chunquan Yu - Caltech
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Knowledge of Earth’s interior structures is important to understand its thermal, chemical, and dynamical evolution. We developed and applied seismic imaging methods based on teleseismic reflected waves to study discontinuities in the crust and mantle. Using a novel, curvelet-based, array analysis technique, we improve both the quantity and quality of SS precursors (SS waves reflecting at mantle transition zone (MTZ) discontinuities), including their travel time picks and amplitude measurements. Amplitude-distance trends in the reflectivity of SS precursors provide important constraints on the wavespeed and density contrasts across MTZ discontinuities. In conjunction with thermal dynamic modeling, our results suggest lateral variation in mantle composition near the base of the MTZ, from average pyrolitic mantle beneath Hawaii to a mixture with more melt-depleted harzburgite southeast of the hotspot. This discovery corroborates petrological predictions and numerical convection models showing compositional segregation near the 660 in high temperature, low viscosity environments produced by lower mantle upwelling. For crustal structure studies, we further developed the virtual deep seismic sounding (VDSS) method. We used data from ~1,000 broadband seismic stations to provide high-resolution estimates of crustal structure in the western Cordillera of the United States. The most robust result is the geographic distribution of residual topography (that is, the difference between observed elevation and that expected from crustal buoyancy alone) and, by implication, thermal or petrologic anomalies in the mantle. Residual topography of the western US Cordillera varies considerably. Overall, in regions to the east of the Wasatch hinge line (the eastern limit of significant extension in the North American cratonic basement) patterns of high residual topography and anomalies of low seismic wave-speeds in the upper mantle are similar, suggestive of a common, thermal origin. In contrast, such a similarity is absent in regions to the west of the hinge line, suggesting substantial effects of petrological heterogeneities in the mantle.