Seminar Abstracts

Quantifying Near-Field Ground deformation of Large Magnitude Earthquakes Using Subpixel Optical Image Correlation

January 31st, 2017

Speaker: Chris Milliner
Berkeley Seismological Laboratory

Measurements of surface deformation from large magnitude earthquakes provide useful insight into faulting mechanics and the rupture process at depth. However, standard geodetic and field techniques are significantly limited in capturing complex tectonic surface deformation, particularly in the near-field (< 2 km from surface ruptures). InSAR data typically decorrelates in such regions where phase gradients are too high, leaving 1-2 km wide data gaps, while GPS has limited spatial coverage, and geologic field surveys typically cannot observe the subtle off-fault, distributed deformation accommodated over wide fault zones. Sub-pixel correlation of optical images taken before and after earthquakes, however, is well optimized to retrieve the full surface displacement close to the earthquake rupture, providing a complementary technique to other geodetic methods. In this talk I will present optical correlation results that reveal the near-field deformation pattern of the 1992 Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine earthquakes. I will demonstrate how such measurements can deepen our understanding of fault zone deformation, from how the magnitude of distributed strain may vary between fault systems to explaining the spatial heterogeneity of co-seismic surface slip that has eluded previous studies. These results have important implications for understanding the dynamics of rupture, the nature of diffuse deformation in continental regions, and improving estimates of slip at shallow depth in finite-fault inversions.


Mega-earthquakes and fault properties

Febrauary 7th, 2017

Speaker: Quentin Bletery
University of Oregon

Earthquakes are the results of rapid slip on active faults loaded in stress by the tectonic plates motion. The distribution of this rapid slip along the rupturing faults is heterogeneous. Imaging the complexity of such slip distributions is one the main challenges in seismology because it can be used to assess characteristics of faults associated with high concentration of slip which could be tracked on other faults to better anticipate future large earthquakes. To improve the imaging of such co-seismic slip distributions, three axes may be followed : increase the constraints on the source models by including more observations into the inversions, improve the physical modeling of the forward problem and improve the formalism to solve the inverse problem. We follow these three axes to better image the rupture of the two largest earthquakes of the 21st century, the 2004 Sumatra-Andaman (Mw 9.3) and the 2011 Tohoku-Oki (Mw 9.0) earthquakes. Analysis of the imaged rupture characteristics reveals spatial relationships with aftershocks distribution, heterogeneities on the megathrusts imaged by tomography and geological structures in the wedge at the scale of a few tens of kilometers. At a broader — global — scale, we investigate a potential link between mega-earthquake occurrence and fault geometry. We find that most mega-earthquakes occurred on the most planar subduction faults. A simplified analytic model demonstrates that heterogeneity in shear strength increases with fault curvature. Shear strength on planar megathrusts is more homogeneous, and hence more likely to be exceeded simultaneously over large areas, than on highly curved faults.


The 2016 Kumamoto Earthquake Sequence:how the main shock starts and stops

February 14th, 2017

Speaker: Han Yue
CalTech

Faults and volcanos are significant structures hosting devastating natural disasters, i.e. great earthquakes and volcanic eruptions. The interaction between fault and volcano behaviors provides a good opportunity to understand both the rupture and eruption mechanisms. On April 16th 2017, the Kumamoto earthquake (Mw = 7.1) shocked the vicinity area of the Aso volcano on the Kyushu island, Japan. The mainshock is preceded by an active foreshock sequence including two major (Mw > 6) foreshocks. GPS stations recorded an aseismic slow-slip transient following the major foreshocks, which happened close to the mainshock hypocenter and appears to have triggered the mainshock rupture. In this study, we use seismological, geodetic and remote-sensing data to reconstruct a kinematic rupture model of the Kumamoto earthquake sequence. The aftershocks complement to the southern segment of the co-seismic rupture, in agreement with a Coulomb stress triggering relationship. A seismicity gap is detected near the northern end of the co-seismic rupture disagree with Coulomb stress triggering. This seismicity gap colocates with a low velocity anomaly near the volcano, which may be related with a local magma chamber. Such consistency indicates the dynamic rupture is terminated by a weak material barrier introduced by the geothermal condition of the Aso volcano.


Data Mining Microseismicity using PageRank

February 21st, 2017

Speaker: Ana Aguiar Moya
Lawrence Livermore National Laboratory

Data mining methods have often been used to explore the similarities of individual seismic events that comprise an earthquake sequence. A relatively new example is PageRank (Google’s initial search algorithm), which can be used for measuring connectivity between two seismograms. PageRank was originally developed for webpage search engines and was subsequently adapted for use in seismology to detect low-frequency earthquakes (Aguiar and Beroza, 2014). PageRank links seismograms directly through cross correlation and indirectly when two seismograms have a high correlation coefficient (CC) with one another but only one of those has a high CC with a 3rd seismogram. We expand on this initial application of PageRank by using it to define signal-correlation topology for micro-earthquakes in a geothermal environment, including the identification of signals that are connected to the largest number of other signals. We have focused on the Newberry Volcano in the Deschutes National Forest in Central Oregon, which has been stimulated two times using high-pressure fluid injection to study the Enhanced Geothermal Systems (EGS) technology. Initial locations of the 2012 stimulation found that events occurred in two distinct depth ranges and microseismicity did not clearly outline subsurface structures (Foulger and Julian, 2013). We explored the spatial and temporal development of the 2012 events to better understand how the stimulation modifies stress, fractures rock, and affects permeability. By applying PageRank, we created signal families and simultaneously relocated events within families using the Bayesloc approach (Myers et al., 2007). After relocation, event families are tightly clustered spatially, and some events determined to be linked by PageRank but not spatially clustered in the initial locations are indeed relocated to the same cluster. We also found that signal similarity (linkage) at several stations, not just one or two, is needed to confidently determine if events are in close proximity to one another. Indirect linkage of signals using PageRank is a reliable way to increase the number of events that are confidently determined to be located close to one another and have a similar focal mechanism. We are currently analyzing the second stimulation, performed in 2014, and will compare these results to clusters found in the initial stimulation. This will allow us to determine whether changes in the state of stress and/or changes in the generation of subsurface fracture networks can be detected using PageRank topology. Ultimately, automating and applying this method in real time could be used in adaptive approaches to enhance production at wellinstrumented geothermal sites.This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-720039.


Multi-frequential periodogram and correlation analyses of earthquake numbers and hypocenter depths in central California

February 28th, 2017

Speaker: Pierre Dutilleul
McGill University

Earthquake catalog data include the hypocentral spatial coordinates (longitude, latitude, depth) and the time of earthquake occurrence as spatio-temporal information about recorded events. In central California, periodicities are studied and have been found in earthquake occurrence, targeting a possible relationship with annual water loading. In this talk, I will mainly follow the statistical approach based on time-series analysis in the frequency domain. Specifically, I will present results for the multi-frequential periodogram analysis (MFPA) of time series of monthly earthquake numbers and time series of monthly values of mean depth and median depth of the hypocenters, and their correlation analysis in the time domain (cross correlograms) and the frequency domain (squared coherencies). Earthquake magnitude is taken into account as a possible source of variation, with data analyses performed for different intervals of magnitude values (i.e., 2.5+, 2.5-3.0 and 3.0+), and original catalog data as well as data declustered in 2D and 3D are analyzed. The MFPA shows a strong 6-month periodicity in most of the depth time series, together with a 4-month periodicity and to a lesser extent, 12-month and 14.24-month periodic components; similar results are obtained for the time series of earthquake numbers. Correlation between the two types of time series is strongest for the original catalog data, followed by the data reduced by 3-D declustering. At the interface with seismology and geophysics, the statistically significant periodicities and correlations are discussed in relation to seasonal loading models.

This is joint work with Chris Johnson and Roland Bürgmann, UC Berkeley Seismology Lab.


GPS Imaging of Earth’s Vertical Motion: From Sierra Nevada to North America

March 7th, 2017

Speaker: Bill Hammond
University of Nevada, Reno

Vertical land motion attributable to deep solid Earth processes has some of the most profound impacts on Earth’s topography, but its geodetic signals are elusive and among the most difficult to observe directly. In this presentation I will show how we are using a new data visualization technique ‘GPS Imaging’ to enhance the interpretation of vertical motions. Like a doctor’s x-ray of the Earth, reminiscent of seismic tomography, the images show snapshots of the dynamic response to various events and loads from past and present large scale geodynamic processes. The images reveal in detail the effects of present day shallow mantle flow and flexure of the lithosphere associated with processes such as uplift of the Sierra Nevada, hydrological loading of the lithosphere, glacial isostatic adjustment, interseismic strain and post-earthquake relaxation. GPS Imaging focuses attention on the component of uplift that is driven by these large-scale geodynamic effects, providing a new window to view mantle processes. Creating these images is possible by development of GPS networks which are undergoing exponential proliferation across the planet, increasing coverage, lengthening time series, and transforming our view of Earth’s shape and how it changes over time. Advances in data processing have made it possible to cope with this new torrent of data. At the Nevada Geodetic Laboratory we now process data from over 15,500 stations globally distributed, and together with UNAVCO are now promoting the Plug and Play GPS concept to make the data products more accessible, discoverable, and useful for all. Our system now presents a portal through which it is possible to view ongoing Earth deformation with unprecedented scope and ease. The presentation will include a short introduction to the Plug and Play GPS data products.


Exploring processes of induced seismicity from mesoscale field experiments

April 4th, 2017

Speaker: Yves Guglielmi
Lawrence Berkeley National Laboratory

Designing effective and reliable strategies to prevent and/or manage fluid-injection induced seismicity of concern while minimizing economic impact depend on an in-depth understanding of the physics of the earthquake source and how it is influenced by the changes in subsurface fluid pressures, stresses, and geochemistry resulting from injection and their interactions. Fault and fractures dynamics studies face two key problems (1) the up-scaling of laboratory determined properties and constitutive hydromechanical laws to the reservoir/crustal scale which is not straightforward when considering faults and fractures heterogeneities, (2) the difficulties to control both the induced seismicity and the stimulated zone geometry when a fault is reactivated. Using instruments developed to measure coupled pore pressures and deformations downhole and a surrounding seismicity sensors network, we conducted field academic experiments to characterize fault hydromechanical dynamic behavior during the earthquake nucleation process. We show experiments where different fault zones geologies under contrasted state of stresses were explored in three different underground research laboratories (IRSN-Tournemire-France, LSBB-Apt-France, Mt-Terri-Ste-Ursanne-Switzerland) where experimental conditions can be optimized. Experiments consisted in pressurizing intervals in different fault zone facies (core, fractured damage zone, etc.) to induce changes in effective stresses high enough to produce measurable fault movements and seismicity. Shear and normal displacements respectively of 0.05 to 1.5 10-3m were measured at velocities of 0.1 to >10 micrometer per seconds. The induced seismic events computed magnitude ranged between -4.3 and ~-2.5. Their spatio-temporal distribution, compared with the measured displacement at the injection points, shows that most of the deformation induced by the injection is aseismic, and that there is a similarity in the way faults of contrasted geologies activate under effective stress variations. As no seismicity is observed in the close vicinity of the injection areas, the presence of fluid seems to prevent seismic slips, because of a high “mostly” poroelastic dilatant effect. Therefore, the seismic behavior seems to be strongly sensitive to the structural heterogeneity, including permeability of the fault zone, which leads to an heterogeneous stress response in and propagating away from the pressurized volume.


The Denali Fault as a Plate Boundary: New Results from Double-difference Tomography, Receiver Functions, and Fault Zone Head Waves

April 11th, 2017

Speaker:Amir Allam
University of Utah

We examine the structure of the Denali fault in the crust and upper mantle using double-difference tomography, P-wave receiver functions, and analysis (spatial distribution and moveout) of fault zone head waves. The three methods have complementary sensitivity; tomography is sensitive to 3D seismic velocity structure but smooths sharp boundaries, receiver functions are sensitive to (quasi) horizontal interfaces, and fault zone head waves are sensitive to (quasi) vertical interfaces. The results indicate that the Mohorovičić discontinuity is vertically offset by 10 to 15km along the central 600km of the Denali fault in the imaged region, with the northern side having shallower Moho depths around 30km. An automated phase picker algorithm is used to identify ~1400 events that generate fault zone head waves only at near-fault stations. At shorter hypocentral distances head waves are observed at stations on the northern side of the fault, while longer propagation distances and deeper events produce head waves on the southern side. These results suggest a reversal of the velocity contrast polarity with depth, which we confirm by computing average 1D velocity models separately north and south of the fault. Using teleseismic events with M>=5.1, we obtain 31,400 P receiver functions and apply common-conversion-point stacking. The results are migrated to depth using the derived 3D tomography model. The imaged interfaces agree with the tomography model, showing a Moho offset along the central Denali fault and also the sub-parallel Hines Creek fault, a suture zone boundary 30 km to the north. To the east, this offset follows the Totschunda fault, which ruptured during the M7.9 2002 earthquake, rather than the Denali fault itself. The combined results suggest that the Denali fault zone separates two distinct crustal blocks, and that the Totschunda and Hines Creeks segments are important components of the fault and Cretaceous-aged suture zone structure.


Ocean Microseisms and Antarctic Lithosphere: Seismological and Interdisciplinary Investigations

May 9th, 2017

Speaker:Anya Reading
University of Tasmania

Research is presented on two projects, array seismology analysis of ocean microseisms, and investigations of the Antarctic lithosphere, that combine seismology with other lines of information. Ocean microseisms are explored using new algorithm implementations which reveal less dominant noise sources and a wider variety of seismic phases. Antarctica’s former neighbours in Gondwana, such as Australia, combined with plate tectonic reconstructions provide candidate tectonic scenarios for the lithosphere of East Antarctica. Computation-based approaches, seismological and geological observations are used to make best use of diverse, incomplete datasets relating to the deep 3D structure of the Antarctic Continent. This structure, often surprisingly variable, is the dynamic foundation for ice sheet and sea-level changes.

Bio:
Anya Reading leads the 'Compute Earth' research group at the School of Physical Sciences (Earth Sciences), University of Tasmania as a Professor in the Science, Engineering and Technology Faculty. As an undergraduate at the University of Edinburgh, UK, she studied astrophysics and took Honours in geophysics. Through PhD research at the University of Leeds, UK, focused on New Zealand seismology, she began a journey of discovery of the southern hemisphere continents, their tectonic origins and evolution. She held a post-doc position with British Antarctic Survey, and moved to the Australian National University in 2000. In 2007, she joined the academic faculty at University of Tasmania, Hobart: Australia’s international hub for Antarctic and Southern Ocean science and logistics. She was awarded a Fulbright Senior Scholarship in 2016, based at CU Boulder, CO, to further research on the 3D structure of the Antarctic continent and build collaborations between Australia and US Antarctic scientists.



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