Attenuation of P and S waves in a magma chamber in Long Valley Caldera, California

Ryall, F; Ryall, Alan

doi:10.1029/gl008i006p00557

Cited by 58 publications

(21 citation statements)

References 11 publications

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“…Their results show a zone of low P-wave velocity beneath the southern part of the resurgent dome and the south moat of the caldera between 5 and 9 km below the surface. Evidence for the physical properties of the upper crust was provided by the detailed mapping of ray paths with anomalously high attenuation of S-waves (Ryall and Ryall, 1981;Sanders, 1984). These latter observations are generally interpreted to indicate a plexus of magma bodies.…”

Section: The Region Separating Central and Southern California (Garlomentioning

confidence: 97%

Chapter 9: Regional crustal structure and tectonics of the Pacific Coastal States; California, Oregon, and Washington

Mooney¹,

Weaver²

1989

Geological Society of America Memoirs

106

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The Pacific Coastal States form a complex geologic environment in which the crust and lithosphere have been continuously reworked. We divide the region tectonically into the southern transform regime of the San Andreas fault and the northern subduction regime, and summarize the geophysical framework with contour maps of crustal thickness, lithospheric and seismicity cross sections, and results from site-specific geophysical studies. The uniformity of crustal thickness (30 ± 2 km) in southern California is remarkable, and appears to be primarily the result of crustal extension in the Mojave Desert and ductile shear of the lower crust along the plate transform boundary. Southern California seismicity defines a broad zone of deformation that extends from the Borderland to the Mojave Desert (about 300 km). The geophysical framework of central and northern California records magmatism and accretion associated with the Mesozoic and Cenozoic subduction, late Cenozoic transform faulting, and in the Basin and Range to the east, extension. The crust thickens from about 20 km at the coast to as much as 55 km in the Sierra Nevada, and thins to about 30 km in the Basin and Range. Cross sections of the crust show that seismic velocities and densities vary significantly over short distances perpendicular to the coast, reflecting processes that include the accretion of oceanic sediments and igneous crust, and significant lateral motion of crustal blocks. Maximum hypocentral depths in central California become deeper as the crust thickens to the west, but seismicity is low beneath the Great Valley and Sierra Nevada, which together appear to form a relatively undeforming block. The lower crust of the Pacific Coastal States has a high average seismic velocity (6.7 km/sec or greater), which probably is the product of tectonic underplating of oceanic crust and/or magmatic underplating by a basaltic melt. The geophysical framework of the subduction regime is dominated by the subduction of the Gorda and Juan de Fuca plates, arc magmatism in the Cascade Range, and plateau volcanism and rifting in the back arc. As defined by earthquake hypocenters, the Juan de Fuca plate dips at a shallow angle (3°) within 50 km of the trench, increases to10° beneath the continental shelf and coastal province, and plunges more steeply (25° dip) a short distance west of the Cascade Range. Whereas a true continental Moho exists from the Cascade Range to the east, the Moho is that of the subducting oceanic lithosphere west of the range. Crustal thickness increases from about 18 km at the coast to about 42 km beneath the Cascades Range, a distance of about 200 km. The crustal velocity structure and crustal thickness of the Cascades Range is relatively uniform along its axis. The velocity structure shows high velocities (greater than 6.5 km/sec) at Mooney, W. D., and Weaver, C. S., 1989, Regional crustal structure and tectonics of the Pacific Coastal States; California, Oregon, and Washington, in Pakiser, L. C., and Mooney, W. D., Geophysical framework of the...

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Section: The Region Separating Central and Southern California (Garlomentioning

confidence: 97%

Chapter 9: Regional crustal structure and tectonics of the Pacific Coastal States; California, Oregon, and Washington

Mooney¹,

Weaver²

1989

Geological Society of America Memoirs

106

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“…At greater depth, extension would take place mainly through faulting instead of dike formation. The existence of magma at shallow depth, at least within the caldera, is indicated by teleseismic P delays [Steeples and lyer, 1976], possible reflections on an explosion profile [Hill, 1976], lack of $ waves for paths through the caldera [Ryall and Ryall, 1981b], uplift of the resurgent dome [Savage and Clark, 1982], and spasmodic tremor in a small area near Mammoth Lakes [Ryall and Ryall, 1983]. Such a tectonic/volcanic process would be consistent with an earlier suggestion by Lachenbruch and $ass [1978] that lithospheric extension in the Basin and Range province takes place through a combination of normal faulting and magmatic intrusion of the brittle crust.…”

Section: Variations In Fault Dip a Decrease In Dip Of The Fault Planmentioning

confidence: 99%

Systematic change of focal mechanism with depth in the Western Great Basin

Vetter¹,

Ryall²

1983

J. Geophys. Res.

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For most areas in the western Great Basin, focal mechanisms show a consistent pattern of primarily strike‐slip motion for shallow events and oblique or normal slip for deeper events. However, orientation of the axis of least principal stress (T axis) is different for different areas: NW–SE for western Nevada and the Mono Lake region, and NE–SW for the Mammoth Lakes area. Along the remainder of the Sierra Nevada frontal fault zone, T axes show both orientations. In general the change in mechanism with depth is interpreted as being caused by increasing overburden pressure, resulting in rotation of the maximum compressive stress (P axis) from horizontal at depths less than about 6 km to vertical at depths greater than about 9 km. The absence of normal‐slip events at depths greater than 10 km in one area (Mono‐Excelsior‐Luning zone) may be explained by a larger horizontal compressive stress than exists in areas that do have normal faulting at such depths. In some areas conjugate right‐ and left‐lateral shear on nearly vertical fractures may be associated with the formation of clusters of magma‐filled dikes at shallow depths. Assuming strike‐slip faulting to be characteristic of earthquakes with depth less than 6 km and normal faulting for events deeper than 10 km, extrapolation of measured shear stress in the upper few kilometers of the crust provides a rough estimate of the maximum and minimum principal stress as a function of depth. At about 3 km depth we find that S1 and S3 are both horizontal, with values of about 104 and 55 MPa, respectively. At depth of 20 km, S1 is vertical and equal to the overburden pressure, about 530 MPa; S3 is horizontal and about 260–300 MPa.

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“…Bailey et al (1976) and Hill (1976) discussed the geological and geophysical structure of Long Valley Caldera and its eruption history. A persistent focus of research has been the location and activity of magma (Hildreth, 2004;McConnel et al, 1995;Peacock et al, 2016;Ryall & Ryall, 1981). A combination of volcanic and tectonic forces generates high rates of seismicity, including frequent earthquake swarms.…”

Section: Introductionmentioning

confidence: 99%

Imaging a Crustal Low‐Velocity Layer Using Reflected Seismic Waves From the 2014 Earthquake Swarm at Long Valley Caldera, California: The Magmatic System Roof?

Nakata

Shelly

2018

Geophysical Research Letters

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The waveforms generated by the 2014 Long Valley Caldera earthquake swarm recorded at station MLH show clear reflected waves that are often stronger than direct P and S waves. With waveform analyses, we discover that these waves are reflected at the top of a low‐velocity body, which may be residual magma from the ∼767 ka caldera‐forming eruption. The polarity of the reflection compared to direct P and S waves suggests that the reflection is SP waves (S from hypocenters to reflector and then convert to P waves to the surface). Because the wavefields are coherent among different earthquakes and hold high signal‐to‐noise ratios, we apply them to a wavefield migration method for imaging reflectors. The depth of the imaged magmatic system roof is around 8.2 km below the surface. This is consistent with previous studies. Even though we use only one station and waveforms from one earthquake swarm, the dense cluster of accurately located earthquakes provides a high‐resolution image of the roof.

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Attenuation of P and S waves in a magma chamber in Long Valley Caldera, California

Cited by 58 publications

References 11 publications

Chapter 9: Regional crustal structure and tectonics of the Pacific Coastal States; California, Oregon, and Washington

Chapter 9: Regional crustal structure and tectonics of the Pacific Coastal States; California, Oregon, and Washington

Systematic change of focal mechanism with depth in the Western Great Basin

Imaging a Crustal Low‐Velocity Layer Using Reflected Seismic Waves From the 2014 Earthquake Swarm at Long Valley Caldera, California: The Magmatic System Roof?

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