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Reconsidering earthquake scaling (2016)

J. Gomberg, A. Wech, K. Creager, K. Obara, D. Agnew

Wiley

In: Geophysical Research Letters

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Publication Date: 2016-06-16

Description: The relationship (scaling) between scalar moment, M 0 , and duration, T , potentially provides key constraints on the physics governing fault slip. The prevailing interpretation of M 0 -T observations proposes different scaling for fast (earthquakes) and slow (mostly aseismic) slip populations, and thus fundamentally different driving mechanisms. We show that a single model of slip events within bounded slip zones may explain nearly all fast and slow slip M 0 -T observations and both slip populations have a change in scaling, where the slip area growth changes from 2-dimensional when too small to sense the boundaries to 1-dimensional when large enough to be bounded. We present new fast and slow slip M 0 -T observations that sample the change in scaling in each population, which are consistent with our interpretation. We suggest a continuous, but bimodal distribution of slip modes exists and M 0 -T observations alone may not imply a fundamental difference between fast and slow slip.

Print ISSN: 0094-8276

Electronic ISSN: 1944-8007

Topics: Geosciences , Physics

Published by Wiley on behalf of American Geophysical Union (AGU).

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The global seismographic network surpasses its design goal (2004)

R. Butler ; T. Lay ; K. Creager ; [et al.]

In: Eos, Trans., Am. Geophys. Un., Potsdam, ZIPE, vol. 85, no. 23, pp. 229, pp. 5091692, (ISBN: 0-12-018847-3)

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Publication Date: 2004

Keywords: Seismic networks ; GSN ; Project report/description ; Seismology ; NOISE ; dynamic ; range ; Broad-band ; Seismometer ; EOS

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BIBLIOGRAPHY SEISMOLOGY

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Earthquake spectra and near-source attenuation in the Cascadia subduction zone (2012)

J. Gomberg, K. Creager, J. Sweet, J. Vidale, A. Ghosh and A. Hotovec

Wiley

In: Journal of Geophysical Research JGR - Solid Earth

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Publication Date: 2012-05-23

Description: Models of seismic source displacement spectra are flat from zero to some corner frequency, fc, regardless of source type. At higher frequencies spectral models decay as f−1 for slow events and as f−2 for fast earthquakes. We show that at least in Cascadia, wave propagation effects likely control spectral decay rates above ∼2 Hz. We use seismograms from multiple small-aperture arrays to estimate the spectral decay rates of near-source spectra of 37 small ‘events’ and find strong correlation between source location and decay rate. The decay rates (1) vary overall by an amount in excess of that inferred to distinguish slow sources from fast earthquakes, (2) are indistinguishable for sources separated by a few tens of km or less, and (3) separate into two populations that correlate with propagation through and outside a low-velocity zone imaged tomographically. We find that some events repeat, as is characteristic of low-frequency earthquakes (LFEs), but have spectra similar to those of non-repeating earthquakes. We also find no correlation between spectral decay rates and rates of ambient tremor activity. These results suggest that earthquakes near the plate boundary, at least in Cascadia, do not distinctly separate into ‘slow’ and ‘fast’ classes, and correctly accounting for propagation effects is necessary to characterize sources.

Print ISSN: 0148-0227

Topics: Geosciences , Physics

Published by Wiley on behalf of American Geophysical Union (AGU).

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Tidal Modulation of Nonvolcanic Tremor (2008)

Rubinstein, J. L. ; La Rocca, M. ; Vidale, J. E. ; [et al.]

American Association for the Advancement of Science

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Publication Date: 2017-04-04

Description: Episodes of nonvolcanic tremor and accompanying slow slip recently have been observed in the subduction zones of Japan and Cascadia. In Cascadia, such episodes typically last a few weeks, and differ from “normal” earthquakes in their source location and momentduration scaling. The three most recent episodes in the Puget Sound/Southern Vancouver Island portion of the Cascadia subduction zone have been exceptionally well recorded. In each episode, we see clear pulsing of tremor activity with periods of 12.4 and 24-25 hours, the same as the principal lunar and lunisolar tides. This indicates that the small stresses associated with the solid-earth and ocean tides influence the genesis of tremor much more effectively than they do “normal” earthquakes. Because the lithostatic stresses are 105 times larger than those associated with the tides, we argue that tremor occurs on very weak faults.

Description: Published

Description: 186 -189

Description: 3.1. Fisica dei terremoti

Description: JCR Journal

Description: reserved

Keywords: Nonvolcanic ; tremor ; 04. Solid Earth::04.06. Seismology::04.06.01. Earthquake faults: properties and evolution ; 04. Solid Earth::04.06. Seismology::04.06.02. Earthquake interactions and probability ; 04. Solid Earth::04.06. Seismology::04.06.03. Earthquake source and dynamics

Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)

Type: article

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Cascadia Tremor Located Near Plate Interface Constrained by S Minus P Wave Times (2009)

La Rocca, M. ; Creager, K. C. ; Galluzzo, D. ; [et al.]

the American Association for the Advancement of Science

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Publication Date: 2017-04-04

Description: Nonvolcanic tremor is difficult to locate because it does not produce impulsive phases identifiable across a seismic network. An alternative approach to identifying specific phases is to measure the lag between the S and P waves. We cross-correlate vertical and horizontal seismograms to reveal signals common to both, but with the horizontal delayed with respect to the vertical. This lagged correlation represents the time interval between vertical compressional waves and horizontal shear waves. Measurements of this interval, combined with location techniques, resolve the depth of tremor sources within T2 kilometers. For recent Cascadia tremor, the sources locate near or on the subducting slab interface. Strong correlations and steady S-P time differences imply that tremor consists of radiation from repeating sources.

Description: Published

Description: 620-623

Description: 3.1. Fisica dei terremoti

Description: 3.2. Tettonica attiva

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: JCR Journal

Description: reserved

Keywords: Cascadia Tremor ; Plate Interface Constrained ; 04. Solid Earth::04.06. Seismology::04.06.03. Earthquake source and dynamics ; 04. Solid Earth::04.06. Seismology::04.06.06. Surveys, measurements, and monitoring ; 04. Solid Earth::04.07. Tectonophysics::04.07.04. Plate boundaries, motion, and tectonics ; 04. Solid Earth::04.07. Tectonophysics::04.07.06. Subduction related processes

Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)

Type: article

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Short‐term and long‐term tremor migration patterns of the Cascadia 2004 tremor and slow slip episode using small aperture seismic arrays (2010)

McCausland, W. A. ; Creager, K. C. ; La Rocca, M. ; [et al.]

American Geophysical Union

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Publication Date: 2017-04-04

Description: An edited version of this paper was published by AGU. Copyright (2010) American Geophysical Union.

Description: Tectonic tremor has been recorded at many subduction zones, including the Nankai, Cascadia, Mexican, and Alaskan subduction zones. This study, the first to use small aperture seismic arrays to track tremor, deployed three small aperture seismic arrays along the Cascadia subduction zone during a tremor and slow slip episode in July 2004. The tremor was active during virtually all (up to 99%) minutes of the analyzed tremor episode using 5 min sample windows. Individual wave phases were tracked across the arrays and used to derive slowness vectors. These were compared with slowness vectors computed from a standard layered Earth model to derive tremor locations. Locations were stable within a volume roughly 250 km2 in epicenter and 20 km in depth for hours to days before moving to a new volume. The migration between volumes was not smooth, and the movement of the sources within the volume followed no specific pattern. Overall migration speeds along the strike of the subduction zone were between 5 and 15 km/d; smaller scale migration speeds between volumes reached speeds up to 2 km/min. Uncertainties in the best locations were 5 km in epicenter and 10 km in depth. For this data set and processing methodology, tremor does not locate predominately on the primary subduction interface. Our favored model for the generation of tectonic tremor signals is that the tremor is triggered by stress and fluid pressure changes caused by slow slip and is composed, at least in part, of low‐frequency earthquakes broadly distributed in location

Description: Published

Description: B00A24

Description: 3.2. Tettonica attiva

Description: 3.3. Geodinamica e struttura dell'interno della Terra

Description: JCR Journal

Description: reserved

Keywords: tremor migration ; Cascadia 2004 ; 04. Solid Earth::04.06. Seismology::04.06.03. Earthquake source and dynamics ; 04. Solid Earth::04.06. Seismology::04.06.06. Surveys, measurements, and monitoring ; 04. Solid Earth::04.06. Seismology::04.06.09. Waves and wave analysis ; 04. Solid Earth::04.07. Tectonophysics::04.07.04. Plate boundaries, motion, and tectonics

Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)

Type: article

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Magma reservoirs from the upper crust to the Moho inferred from high-resolution Vp and Vs models beneath Mount St. Helens, Washington State, USA (2016)

Kiser, E., Palomeras, I., Levander, A., Zelt, C., Harder, S., Schmandt, B., Hansen, S., Creager, K., Ulberg, C.

Geological Society of America (GSA)

In: Geology

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Publication Date: 2016-05-24

Description: The size, frequency, and intensity of volcanic eruptions are strongly controlled by the volume and connectivity of magma within the crust. Several geophysical and geochemical studies have produced a comprehensive model of the magmatic system to depths near 7 km beneath Mount St. Helens (Washington State, USA), currently the most active volcano in the Cascade Range. Data limitations have precluded imaging below this depth to observe the entire primary shallow magma reservoir, as well as its connection to deeper zones of magma accumulation in the crust. The inversion of P and S wave traveltime data collected during the active-source component of the iMUSH (Imaging Magma Under St. Helens) project reveals a high P-wave (Vp)/S-wave (Vs) velocity anomaly beneath Mount St. Helens between depths of 4 and 13 km, which we interpret as the primary upper–middle crustal magma reservoir. Beneath and southeast of this shallow reservoir, a low Vp velocity column extends from 15 km depth to the Moho. Deep long-period events near the boundary of this column indicate that this anomaly is associated with the injection of magmatic fluids. Southeast of Mount St. Helens, an upper–middle crustal high Vp/Vs body beneath the Indian Heaven Volcanic Field may also have a magmatic origin. Both of these high Vp/Vs bodies are at the boundaries of the low Vp middle–lower crustal column and both are directly above high Vp middle–lower crustal regions that may represent cumulates associated with recent Quaternary or Paleogene–Neogene Cascade magmatism. Seismicity immediately following the 18 May 1980 eruption terminates near the top of the inferred middle–lower crustal cumulates and directly adjacent to the inferred middle–lower crustal magma reservoir. These spatial relationships suggest that the boundaries of these high-density cumulates play an important role in both vertical and lateral transport of magma through the crust.

Print ISSN: 0091-7613

Electronic ISSN: 1943-2682

Topics: Geosciences

Published by Geological Society of America (GSA)

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