We have created a benchmark of spatial variations in shear wave anisotropy around Mount Ruapehu, New Zealand, against which to measure future temporal changes. Anisotropy in the crust is often assumed to be caused by stress-aligned microcracks, and the polarization of the fast quasi-shear wave (φ) is thus interpreted to indicate the direction of maximum horizontal stress, but can also be due to aligned minerals or macroscopic fractures. Changes in seismic anisotropy have been observed following a major eruption in 1995/96 and were attributed to changes in stress from the depressurization of the magmatic system. Three-component broadband seismometers have been deployed to complement the permanent stations that surround Ruapehu, creating a combined network of 34 three-component seismometers. This denser observational network improves the resolution with which spatial variations in seismic anisotropy can be examined. Using an automated shear wave splitting analysis, we examine local earthquakes in 2008. We observe a strong azimuthal dependence of φ and so introduce a spatial averaging technique and two-dimensional tomography of recorded delay times. The anisotropy can be divided into regions in which φ agrees with stress estimations from focal mechanism inversions, suggesting stress-induced anisotropy, and those in which φ is aligned with structural features such as faults, suggesting structural anisotropy. The pattern of anisotropy that is inferred to be stress related cannot be modeled adequately using Coulomb modeling with a dike-like inflation source. We suggest that the stress-induced anisotropy is affected by loading of the volcano and a lithospheric discontinuity. Copyright 2011 by the American Geophysical Union.
The dataset provided includes seismic anisotropy results using a 10-year catalogue around Taupō volcano (January 2010 to December 2019). For the seismic anisotropy measurements, we used seismic data from 23 stations managed by GeoNet. The earthquake catalogue for this dataset was determined by Illsley-Kemp et al. (2021) using matched-filtered earthquake detection (Chamberlain & Townend, 2018). The earthquake templates for the matched-filtered detection were obtained from the GeoNet catalogue with revised manual picks. In case you use this data, please cite the following publications: Bakkar, H. (2022). Seismic anisotropy and time-frequency analyses during Taupō's 2019 unrest [Master's thesis, Victoria University of Wellington]. Illsley-Kemp, F., Barker, S. J., Wilson, C. J. N., Chamberlain, C. J., Hreinsd ́ottir, S., Ellis, S., Hamling, I. J., Savage, M. K., Mestel, E. R., & Wadsworth, F. B. (2021). Volcanic unrest at Taupo ̄ volcano in 2019: Causes, mechanisms and implications. Geochemistry, Geophysics, Geosystems, e2021GC009803. The data is presented in .csv files, in the same format as MFAST output, (http://mfast-package.geo.vuw.ac.nz), in which each column is: 1. Name of the event. 2. Station code. 3. Station latitude. 4. Station longitude. 5. Event identification number. 6. Year. 7. Julian day on which the event occurred, with decimal digits giving the fraction of the day. 8. Earthquake latitude in degrees. 9. Earthquake longitude in degrees. 10. Distance between earthquake and station (km). 11. Earthquake depth (km). 12. Earthquake magnitude. 13. Back azimuth in degrees. 14. Initial polarisation of the shear wave in degrees. 15. Error of the initial polarisation in degrees, one standard deviation. 16. Start time of the selected measurement window in seconds, relative to the start of the seismogram at t = 0. 17. End time of the selected measurement window in seconds, relative to the start of the seismogram at t = 0. 18. Not used 19. Not used 20. Signal to noise ratio for this event. 21. Delay tome between fast and slow shear wave in seconds. 22. Delay time between fast and slow shear wave in seconds. 23. Angle of the orientation of the fast shear wave (φ), in degrees from North. 24. Error of φ in degrees, one standard deviation. 25. Angle of incidence at the station, measured against a horizontal plane in degrees, where 0 means vertical incidence. 26. Not used 27. Type of measurement. This field contains the measurement code that is used, the number of measurement window start times and the number of window end times. 28. Not used 29. Not used 30. Nyquist frequency of the event in Hz. 31. Evaluation of the measurement quality. 32. Lower corner frequency of the bandpass filter in Hz. 33. Higher corner frequency of the bandpass filter in Hz. 34. Angle between the initial polarisation and the fast orientation in degrees. 35. Not used 36. Not used 37. The maximum value of the eigenvalue of the corrected covariance matrix. 38. The number of degrees of freedom in the measurement. 39. The minimum value of the eigenvalue of the covariance matrix before it was scaled to have the 95% confidence level set to 1. 40. The S-wave travel time between the earthquake and the station. 41. The dominant frequency in the S wave, determined from the frequency at the maximum spectral amplitude.
Silicic caldera volcanoes are frequently situated in regions of tectonic extension, such as continental rifts, and are subject to periods of unrest and/or eruption that can be triggered by the interplay between magmatic and tectonic processes. Modern (instrumental) observations of deformation patterns associated with magmatic and tectonic unrest in the lead up to eruptive events at silicic calderas are sparse. Therefore, our understanding of the magmatic-tectonic processes associated with volcanic unrest at silicic calderas is largely dependent on historical and geological observations. Here we utilize existing instrumental, historical and geological data to provide an overview of the magmatic-tectonic deformation patterns operating over annual to 10 4 year timescales at Taupō volcano, now largely submerged beneath Lake Taupō, in the rifted-arc of the Taupō Volcanic Zone. Short-term deformation patterns observed from seismicity, lake level recordings and historical records are characterized by decadal-scale uplift and subsidence with accompanying seismic swarms, ground shaking and surface ruptures, many of which may reflect magma injections into and around the magma reservoir. The decadal-scale frequency at which intense seismic events occur shows that ground shaking, rather than volcanic eruptions, is the primary short-term local hazard in the Taupō District. Deformation trends near and in the caldera on 10 1 –10 4 yr timescales are atypical of the longer-term behavior of a continental rift, with magma influx within the crust suppressing axial subsidence of the rift basin within ∼10 km of the caldera margin. Examination of exposed faults and fissures reveals that silicic volcanic eruptions from Taupō volcano are characterized by intense syn-eruptive deformation that can occasionally extend up to 50 km outside the caldera structure, including ground shaking, fissuring and triggered fault movements. We conclude that eruption and unrest scenarios at Taupō volcano depend on the three-way coupling between the mafic-silicic-tectonic systems, with eruption and/or unrest events leading to six possible outcomes initially triggered by mafic injection either into or outside the magma mush system, or by changes to the tectonic stress state.
Research Article| September 27, 2017 Real‐Time Earthquake Monitoring during the Second Phase of the Deep Fault Drilling Project, Alpine Fault, New Zealand Calum J. Chamberlain; Calum J. Chamberlain aSchool of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, calum.chamberlain@vuw.ac.nz Search for other works by this author on: GSW Google Scholar Carolin M. Boese; Carolin M. Boese bInstitute of Earth Science and Engineering, University of Auckland, Auckland 1010, New ZealandiNow at Goethe University Frankfurt, Institute of Geosciences, Altenhöferallee 1, 60438 Frankfurt, Germany. Search for other works by this author on: GSW Google Scholar Jennifer D. Eccles; Jennifer D. Eccles cScience Centre, University of Auckland, 23 Symonds Street, Auckland 1010, New Zealand Search for other works by this author on: GSW Google Scholar Martha K. Savage; Martha K. Savage aSchool of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, calum.chamberlain@vuw.ac.nz Search for other works by this author on: GSW Google Scholar Laura‐May Baratin; Laura‐May Baratin aSchool of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, calum.chamberlain@vuw.ac.nz Search for other works by this author on: GSW Google Scholar John Townend; John Townend aSchool of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, calum.chamberlain@vuw.ac.nz Search for other works by this author on: GSW Google Scholar Anton K. Gulley; Anton K. Gulley cScience Centre, University of Auckland, 23 Symonds Street, Auckland 1010, New Zealand Search for other works by this author on: GSW Google Scholar Katrina M. Jacobs; Katrina M. Jacobs dGNS Science, P.O. Box 30‐368, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar Adrian Benson; Adrian Benson aSchool of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, calum.chamberlain@vuw.ac.nz Search for other works by this author on: GSW Google Scholar Sam Taylor‐Offord; Sam Taylor‐Offord dGNS Science, P.O. Box 30‐368, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar Clifford Thurber; Clifford Thurber eDepartment of Geoscience, University of Wisconsin–Madison, 1215W Dayton Street, Madison, Wisconsin 53706 U.S.A. Search for other works by this author on: GSW Google Scholar Bin Guo; Bin Guo eDepartment of Geoscience, University of Wisconsin–Madison, 1215W Dayton Street, Madison, Wisconsin 53706 U.S.A. Search for other works by this author on: GSW Google Scholar Tomomi Okada; Tomomi Okada fResearch Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980‐8578, Japan Search for other works by this author on: GSW Google Scholar Ryota Takagi; Ryota Takagi fResearch Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980‐8578, Japan Search for other works by this author on: GSW Google Scholar Keisuke Yoshida; Keisuke Yoshida gNational Research Institute for Earth Science and Disaster Prevention, 3‐1, Tennodai, Tsukuba, Ibaraki 305‐0006, Japan Search for other works by this author on: GSW Google Scholar Rupert Sutherland; Rupert Sutherland aSchool of Geography, Environment and Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand, calum.chamberlain@vuw.ac.nz Search for other works by this author on: GSW Google Scholar Virginia G. Toy Virginia G. Toy hDepartment of Geology, University of Otago, Dunedin 9054, New Zealand Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2017) 88 (6): 1443–1454. https://doi.org/10.1785/0220170095 Article history first online: 27 Sep 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Calum J. Chamberlain, Carolin M. Boese, Jennifer D. Eccles, Martha K. Savage, Laura‐May Baratin, John Townend, Anton K. Gulley, Katrina M. Jacobs, Adrian Benson, Sam Taylor‐Offord, Clifford Thurber, Bin Guo, Tomomi Okada, Ryota Takagi, Keisuke Yoshida, Rupert Sutherland, Virginia G. Toy; Real‐Time Earthquake Monitoring during the Second Phase of the Deep Fault Drilling Project, Alpine Fault, New Zealand. Seismological Research Letters 2017;; 88 (6): 1443–1454. doi: https://doi.org/10.1785/0220170095 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search ABSTRACT The Deep Fault Drilling Project (DFDP) is a multinational scientific drilling effort to study the evolution, structure, and seismogenesis of the Alpine fault, New Zealand, via in situ measurements of fault rock properties. The second phase of drilling (DFDP‐2), undertaken in the Whataroa Valley in late 2014, was intended to intersect the Alpine fault at a depth of around 1 km. In conjunction with the drilling and on‐site science activities, a real‐time seismic monitoring scheme and traffic‐light response protocol were established to detect, locate, and if necessary respond to seismicity within 30 km of the drill site. This network was operated around the clock between late August 2014 and early January 2015, and we detected and located 493 earthquakes of ML 0.6–4.2. None of these earthquakes occurred within 3 km of the drill site, and nor did any of the seismicity detected require changes to drilling operations. The monitoring was undertaken using open‐source software operated by an international team of 16 seismologists (including eight postgraduate students) working in 7 institutions and 3 countries to provide rapid on‐ and off‐site manual checking and relocating of events. The team's standard response time between detection and final location was less than 30 min under normal background seismicity conditions and up to 1 hr during swarm activity and for low‐priority, distant (≥30 km epicentrally from the drill site) earthquakes. This article documents the methodology, infrastructure, protocols, outcomes, and key lessons of this monitoring. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
We quantify foreshock occurrence probabilities by applying the empirical technique of Jones (1985) to the western Nevada and eastern California earthquake catalog compiled by the University of Nevada, Reno, from 1934 through 1991. The foreshock occurrence rates depend heavily on the parameters used to remove aftershocks from the catalog. It is necessary to separate the Mammoth/Mono region from the rest of the catalog to determine the parameters that most effectively remove the aftershocks from the catalog. The probability that an M ≧ 3.0 earthquake will be followed by an earthquake of larger magnitude within 5 days and 10 km is 10% in the Mammoth/Mono region and 6% in the Nevada region, and seems to be independent of the magnitude of the proposed foreshock. The probability that an earthquake will be followed by another one at least one magnitude unit larger is 1 to 2% in each region. These probabilities imply that the occurrence of an earthquake M ≧ 4.0 increases the possibility of a damaging earthquake of M ≧ 5.0 by several orders of magnitude above the low background probability. Most mainshocks occur within a few hours after a possible foreshock, and the probability that a mainshock will still occur decreases logarithmically with time after the proposed foreshock. These foreshock properties are similar to those in southern California and in other parts of the world, with the exception that the Mammoth/Mono region, a volcanic area, exhibits more swarm-like behavior than does the southern California or Nevada region.
We observe significant spatial variations in shear wave splitting from deep teleseismic S and regional ScS recorded on an array in the fore‐arc region of the Hikurangi subduction zone, New Zealand. We also observe variations from different events recorded at the same station. The fast polarization directions range between 23° and 72° and the delay times range between 0.75 s and 1.80 s. These observations contrast with previous SKS studies in New Zealand where little spatial variation is seen, and highlights the importance of using a variety of phases with different periods. Our observations are consistent with a model in which a lateral anisotropic boundary runs ∼NE‐SW in a zone between Kapiti Island and ∼40 km east of the Tararua mountain range. The anisotropic boundary is possibly related to a prominent vertical velocity feature near Kapiti Island.
Abstract A new method based on the joint inversion of receiver functions and surface-wave phase velocities results in well-determined shear-velocity structures that are consistent with the compressional-wave structure, gravity, heat flow, and elevation data in the northern Basin and Range. This new inversion method takes advantage of average-velocity information present in the surface-wave method and differential velocity information contained in the receiver function method, thus minimizing the nonuniqueness problem that results from the velocity-depth trade-off. An unusually thick (38 km) and relatively faster crust and upper mantle are found in central and eastern Nevada compared to the thin (28 to 34 km) and slower crust and upper mantle of the western Basin and Range. We interpret the regions of thicker and faster crust and upper mantle as zones that have undergone less Cenozoic extension relative to the surrounding regions to the west and north. The thick crust and consequently greater depth to the dense mantle material is consistent with the gravity low pattern in central and eastern Nevada. Simple gravity modeling shows both local and regional isostatic compensation occur within 40 km of the surface, indicating a near-classical Airy type of compensation in the province. We analyze in detail the shear-wave (S-wave) velocity model derived from the receiver functions at station BMN and compressional-wave (P-wave) velocity models derived from the 1986 PASSCAL experiment in northwestern Nevada. The most interesting feature of these models is the presence of negative-velocity gradients in the S-wave model with no corresponding velocity decrease in the P-wave models between depths of 10 and 24 km. This combined velocity model may be explained by high pore fluid pressures at these depths. This model favors a layered fluid porosity model proposed in the literature to explain extensive middle- to lower-crust continental seismic reflections and high electrical conductivity. An upper-mantle, gradational low-velocity zone is present between 32 and 38 km in the S-wave model. This upper-mantle, shear-wave, low-velocity zone is consistent with partial melt, which may be the source material for magmatic underplating in this region.
We have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western United States. Fast polarization directions (ϕ) and delay times (δ t ) show spatial variations that are coherent within geologic provinces. Stations located near the San Andreas fault show clear evidence for fault‐parallel anisotropy in the crust and upper mantle (115–125 km thickness). This can be explained by the finite strain associated with the relative plate motion between the North American and Pacific plates. The lateral extent of this strain field is probably narrow to the west, because stations 55 km west of the San Andreas fault do not show fault‐parallel anisotropy in southern California. Station LAC located 80 km east of the San Andreas fault shows large fault‐parallel anisotropy. This suggests that the Pacific‐North American plate boundary in the mantle might be displaced to the east in southern California. A deeper E‐W oriented fast direction of anisotropy underlies the fault‐parallel anisotropic layer in the vicinity of the San Andreas fault. An E‐W fast feature is also present beneath the western Basin and Range and the foothills of the Sierra‐Nevada, although local variations are present. The magnitude of delay times suggests that this feature resides in the asthenosphere. We interpret this feature as the asthenospheric flow in the slabless window left behind the Farallon plate. The flow‐induced anisotropy may partially be frozen‐in at shallow depths. Station ORV is located near the southern edge of the Gorda slab where no anisotropy is detected. The absence of anisotropy at this location could therefore mark a boundary between Farallon associated flow and regions where E‐W oriented asthenospheric flow did not occur. The lack of evidence for NE‐SW fast orientation within the Walker Lane Shear Belt of western Nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the San Andreas fault. Stations located over the young subducting Gorda plate mark a change in the fast direction to nearly NE‐SW. This direction aligns well with the maximum compressive stress direction in the overlying North American plate and the NE‐SW directed internal shearing of the Gorda plate. The anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions. This implies that the anisotropic thickness may include some of the asthenosphere. Alternatively, using a higher anisotropy of 8% can bring thicknesses in line with other measures of lithospheric thicknesses. The correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method, and that asthenospheric flow may be a significant contributor to seismic anisotropy.
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