A Silent Slip Event on the Deeper Cascadia Subduction Interface

Dragert, H.; Wang, Kelin; James, Thomas S.

doi:10.1126/science.1060152

Cited by 857 publications

(816 citation statements)

References 23 publications

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“…Similar seismic signals have been recently observed in the Cascadia subduction zone. Here periods of deep tremor are clearly correlated both in space and time with the slip episodes observed every 14 ± 2 months by continuous GPS measurement on Vancouver Island and northern Washington [Dragert et al, 2002;Rogers and Dragert, 2003;McCausland and Malone, 2003]. Data from the widely-spaced stations of the Pacific Northwest Seismic Network (PNSN) have been used to infer rough estimates about the source location, using either the signal's envelopes [McCausland and Malone, 2004] or a modified beam-forming technique [Kao and Shan, 2004].…”

Section: Introductionmentioning

confidence: 99%

Array measurements of deep tremor signals in the Cascadia subduction zone

Rocca

McCausland

Galluzzo

et al. 2005

Geophysical Research Letters

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Preliminary analysis of deep tremor recorded during July, 2004, in the Cascadia Subduction zone shows that small aperture arrays can resolve the slowness and back azimuth of seismic waves with a useful resolution. Data were collected by three dense arrays of short‐period seismometers specifically deployed in the Puget Sound area under an US‐Italy‐Canada cooperative effort. Slowness analyses at the three arrays indicate that the 2–4 Hz tremor wave‐field is composed of waves propagating with apparent velocities higher than 4 km/s. Combining this with polarisation analysis show these waves to be transverse (SH) waves. However, P‐waves, though smaller in amplitude, can be detected by different slowness values obtained for the radial and transverse components. The intersection of wave vectors determined by the back azimuth and slowness values measured at the three arrays provides a preliminary estimate of source location for a sample of the recorded deep tremor.

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Section: Introductionmentioning

confidence: 99%

Array measurements of deep tremor signals in the Cascadia subduction zone

Rocca

McCausland

Galluzzo

et al. 2005

Geophysical Research Letters

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“…[70] The five PBO sites B003, B004, B005, B007, and B018 are of special interest because they have recorded strain transients associated with the episodic tremor and slip ("ETS") episodes that appear to originate on the Cascadia subduction thrust at depths of 35 to 40 km [Dragert et al, 2001;Melbourne et al, 2005]. At these sites, the theoretical tidal strains are dominated by load tides from the Pacific Ocean and the Straits of Georgia and Juan de Fuca (Table 3).…”

Section: Northern Washington: Difficulty Matching Theoretical Tides Amentioning

confidence: 99%

Tidal calibration of Plate Boundary Observatory borehole strainmeters: Roles of vertical and shear coupling

Roeloffs

2010

J. Geophys. Res.

102

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A multicomponent borehole strainmeter directly measures changes in the diameter of its cylindrical housing at several azimuths. To transform these measurements to formation strains requires a calibration matrix, which must be estimated by analyzing the installed strainmeter's response to known strains. Typically, theoretical calculations of Earth tidal strains serve as the known strains. This paper carries out such an analysis for 12 Plate Boundary Observatory (PBO) borehole strainmeters, postulating that each of the strainmeters' four gauges responds (“couples”) to all three horizontal components of the formation strain tensor, as well as to vertical strain. Orientation corrections are also estimated. The fourth extensometer in each PBO strainmeter provides redundant information used to reduce the chance that coupling coefficients could be misleadingly fit to inappropriate theoretical tides. Satisfactory fits between observed and theoretically calculated tides were obtained for three PBO strainmeters in California, where the calculated tides are corroborated by other instrumentation, as well as for six strainmeters in Oregon and Washington, where no other instruments have ever recorded Earth tidal strain. Several strainmeters have unexpectedly large coupling coefficients for vertical strain, which increases the strainmeter's response to atmospheric pressure. Vertical coupling diminishes, or even changes the sign of, the apparent response to areal strain caused by Earth tides or deep Earth processes because near the free surface, vertical strains are opposite in sign to areal strain. Vertical coupling does not impair the shear strain response, however. PBO borehole strainmeters can provide calibrated shear strain time series of transient strain associated with tectonic or magmatic processes.

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“…Recent studies using GPS and high-density seismic networks extended the measurable period range to days, months and years, which led to the discovery of slow and silent earthquakes 8 . From early seismological studies, some earthquakes were known to be slow, with a timescale longer than a few minutes, but these recent studies demonstrated the existence of seismic events with even longer timescales, which are often associated with small tremors 9 .…”

Section: Slow and Silent Earthquakesmentioning

confidence: 99%

Earthquake physics and real-time seismology

Kanamori

2008

Nature

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Simply stated, an earthquake is caused by slip on a fault. However, the slip motion is complex, reflecting the variation in basic physics that governs fault motion in different tectonic environments. Seismologists can learn a great deal about earthquakes from studying the details of slip motion. The size of great earthquakesSeismic slip motion involves a broad 'period' (or frequency) range, at least from 0.1 s to 1 hour, and a wide range of amplitudes, roughly from 1 µm to 30 m. Most seismographs available before the 1960s could record ground motions over only short periods -less than 30 s -which prevented seismologists from studying important details of earthquake processes. In the 1960s, longer-period analogue seismographs became available, allowing seismologists to study great earthquakes (in general, magnitude ≥ 8) over an extended period range; this has resulted in substantial progress in our understanding of earthquakes. For example, being able to measure long-period waves of up to 1 hour has made it possible for seismologists to establish the overall size of great earthquakes accurately. With the old instruments, wave amplitudes were measured over only a short period range, leading to underestimates of the magnitude of great earthquakes (Fig. 1). The monitoring of longperiod waves, combined with rigorous use of wave theory, rectified this problem, and, as a result, our perception of global seismicity changed drastically during the twentieth century. With the old estimates, global seismic activity seemed to have been relatively constant over the century, but according to the new estimates, a burst of activity occurred between 1952 and 1965; about 40% of the total seismic energy released during the century was released during this period (see ref. 1 for a review).Another important finding is that most earthquakes involve a relatively low stress change, 1-10 MPa. This contrasts with the much higher stress -100 MPa or greater -involved in fracture of rocks at high confining pressure. This indicates that the fracture process in Earth's crust involves special physics, and this is currently the subject of extensive research.A better understanding of great earthquakes has also contributed to an improved understanding of the relationship between earthquakes and global plate motion.

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A Silent Slip Event on the Deeper Cascadia Subduction Interface

Cited by 857 publications

References 23 publications

Array measurements of deep tremor signals in the Cascadia subduction zone

Array measurements of deep tremor signals in the Cascadia subduction zone

Tidal calibration of Plate Boundary Observatory borehole strainmeters: Roles of vertical and shear coupling

Earthquake physics and real-time seismology

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