P-Wave Back-Azimuth and Slowness Anomalies Observed by an IMS Seismic Array LZDM

2010 ◽  
Vol 100 (2) ◽  
pp. 657-669
Author(s):  
C. Hao ◽  
Z. Zheng
Keyword(s):  
Seismic Array ◽  
P Wave ◽  
Back Azimuth

P-wave backazimuth anomalies observed by a small-aperture seismic array at Pinyon Flat, southern California: Implications for structure and source location

1996 ◽  
Vol 86 (2) ◽  
pp. 470-476 ◽  
Author(s):  
Cheng-Horng Lin ◽  
S. W. Roecker
Keyword(s):  
Southern California ◽  
Seismic Array ◽  
P Wave ◽  
Small Aperture ◽  
Data Set ◽  
Single Interface ◽  
Seismic Networks ◽  
Dense Seismic Array ◽  
Beamforming Technique

Abstract Seismograms of earthquakes and explosions recorded at local, regional, and teleseismic distances by a small-aperture, dense seismic array located on Pinyon Flat, in southern California, reveal large (±15°) backazimuth anomalies. We investigate the causes and implications of these anomalies by first comparing the effectiveness of estimating backazimuth with an array using three different techniques: the broadband frequency-wavenumber (BBFK) technique, the polarization technique, and the beamforming technique. While each technique provided nearly the same direction as a most likely estimate, the beamforming estimate was associated with the smallest uncertainties. Backazimuth anomalies were then calculated for the entire data set by comparing the results from beamforming with backazimuths derived from earthquake locations reported by the Anza and Caltech seismic networks and the Preliminary Determination of Epicenters (PDE) Bulletin. These backazimuth anomalies have a simple sinelike dependence on azimuth, with the largest anomalies observed from the southeast and northwest directions. Such a trend may be explained as the effect of one or more interfaces dipping to the northeast beneath the array. A best-fit model of a single interface has a dip and strike of 20° and 315°, respectively, and a velocity contrast of 0.82 km/sec. Application of corrections computed from this simple model to ray directions significantly improves locations at all distances and directions, suggesting that this is an upper crustal feature. We confirm that knowledge of local structure can be very important for earthquake location by an array but also show that corrections computed from simple models may not only be adequate but superior to those determined by raytracing through smoothed laterally varying models.

scholarly journals Separating the scattered wavefield from teleseismic P using curvelets on the long beach array data set

2019 ◽  
Vol 220 (2) ◽  
pp. 1112-1127
Author(s):  
Jia Zhang ◽  
Charles A Langston
Keyword(s):  
Wave Propagation ◽  
Signal To Noise Ratio ◽  
Spherical Wave ◽  
Curvelet Transform ◽  
Structural Heterogeneity ◽  
Seismic Array ◽  
P Wave ◽  
Long Beach ◽  
Data Set ◽  
Array Data

SUMMARY A dense seismic array, composed of over 5000 stations with an average spacing close to 120 m was deployed in Long Beach, CA, by NodalSeismic and Signal Hill Petroleum as part of a survey associated with the Long Beach oilfield. Among many interesting wave propagation effects that have been reported by others, we observe that the coda of teleseismic P waves display waves caused by obvious local scattering from the Signal Hill popup structure between strands of the Newport-Inglewood fault. The density of the seismic array allows space-based methods, such as the Curvelet transform, to be investigated to separate the teleseismic and local scattered wavefields. We decompose a synthetic wavefield composed of a teleseismic plane wave and local scattered spherical wave in the curvelet domain to test the plausibility of our curvelet analysis and then apply the technique to the Long Beach array data set. Background noise is removed by a soft thresholding method and a declustering technique is applied to separate the teleseismic and local scattered wavefield in the curvelet domain. Decomposed results illustrate that the signal-to-noise ratio of the teleseismic P wave can be significantly improved by curvelet analysis. The scattered wavefield is composed of locally propagating Rayleigh waves from the pop-up structure and from the Newport Inglewood fault itself. Observing the wavefield both in space and time clearly improves understanding of wave propagation complexities due to structural heterogeneity.

RETREAT - a REal-time TREmor Analysis Tool

2020 ◽  
Author(s):  
Patrick Smith ◽  
Chris Bean
Keyword(s):  
Real Time ◽  
Volcanic Tremor ◽  
Seismic Array ◽  
Analysis Tool ◽  
Time Data ◽  
Array Analysis ◽  
Array Data ◽  
Real Time Data ◽  
Back Azimuth ◽  
Tremor Analysis

<p>The EUROVOLC project aims to promote an integrated and harmonised European volcanological community, and one of its main themes focuses on understanding sub-surface processes. Early identification of magma moving towards the surface is very important for the mitigation of risks from volcanic hazards, and joint research activities within the project aim to develop and improve volcano pre-eruptive detection schemes. Volcanic tremor is a sustained seismic signal associated with eruptions and is often linked to movement of magmatic fluids in the subsurface. However, it can occur pre-, syn- and post-eruption, and signals with similar spectral content can also be generated by several other processes (e.g. flooding, rockfalls). Hence one of the best ways of distinguishing between the processes underlying tremor generation is through tracking the evolution of its spatial location. Due to its continuous nature tremor cannot be located using classical seismological methods and so its source must be determined using alternatives such as seismic array analysis.</p><p>This work presents RETREAT, a REal-time TREmor Analysis Tool developed under EUROVOLC, that uses seismic array data and array processing techniques to detect, quantify and locate volcanic tremor signals. It is an open-source python-based tool that utilizes existing routines from the open-source <em>obspy</em> framework to carry out analysis of seismic array data in real-time. The tool performs f-k (frequency-wavenumber) analysis using beamforming to calculate the back azimuth and slowness in overlapping time windows, which can be used to detect and track the location of volcanic tremor sources.</p><p>A graphical and web-based interface has been developed which allows adjustment of highly configurable input parameters. These include options for setting the data source, pre-processing, timing and update options as well as the parameters for the seismic array analysis which must be carefully selected and tuned for the specified array. Once configured the tool fetches waveform data in real time and updates its output accordingly, returning plots of the array processing results (slowness and back azimuth values) as well as plots of the seismic waveform, envelope and spectrogram. The tool has been tested on real-time data using the <em>obspy</em> FDSN (International Federation of Digital Seismograph Networks) client to fetch data from the IRIS datacenter, using example array data from the small aperture SPITS seismic array in Spitsbergen, Svalbard. A 'replay’ mode using existing archive (non real-time) data has also been implemented and tested on array data from the 2014 eruption at Holuhraun and Bardarbunga volcano in Iceland, collected as part of the FUTUREVOLC project. The RETREAT tool is now ready for testing and implementation in a volcano monitoring setting at observatories. It will also be made freely available to download as a EUROVOLC community tool.</p>

scholarly journals The Effects of Local Structure and the Adjacent Upper Mantle on Short-Period P Wave Arrivals Recorded at the Warramunga Seismic Array

1974 ◽  
Vol 36 (2) ◽  
pp. 295-319 ◽  
Author(s):  
C. Wright ◽  
J. R. Cleary ◽  
K. J. Muirhead
Keyword(s):  
Upper Mantle ◽  
Local Structure ◽  
Seismic Array ◽  
P Wave ◽  
Short Period

Epicentral uncertainties and detection probabilities from the Yellowknife seismic array data

1968 ◽  
Vol 58 (5) ◽  
pp. 1359-1377
Author(s):  
E. B. Manchee ◽  
D. H. Weichert
Keyword(s):  
Automatic Detection ◽  
Seismic Array ◽  
P Wave ◽  
Wave Number ◽  
Detection Level ◽  
Average Deviation ◽  
Short Period ◽  
Detection Probabilities ◽  
The Difference ◽  
The Individual

Abstract Analog recording tapes from the Yellowknife seismic array have been processed diqitally in Canada for over a year, with concentration on the automatic detection and epicenter location of events between 26° and 90° distance, using short period P-wave arrivals. For the purpose of detection, signals from the individual seismometers in the two arms of the cross array are analog band-pass filtered, digitized at 20 samples/s, multiplexed into a digital computer, velocity and azimuth filtered and correlated, using an exponentially weighted integration over time with an equivalent width of 1.6 s. The correlograms for up to 168 phased beams are scanned for values exceeding a preset trigger level and an event is recorded when the level is passed consistently a number of times. In late 1966, during a seismically quiet period and with the array fully operational, the 50 per cent automatic detection level achieved by this method for events in the Third Zone to Yellowknife was m4.0 ± 0.1, slightly better than the level of an analog trigger operated at the station which uses the correlogram method for a single unphased beam only. The 50 per cent detection level of the Yellowknife standard station is about m4.4 and thus the array-computer automatic detection method gives about Δm0.4 improvement, which is expected from the processing method used if the noise is largely uncorrelated. No significant variations in the detection level with azimuth have yet been observed. Approximate epicenter locations are determined from the best apparent arrival vector. The best vector is assumed to be given by the maxima of parabolas at constant azimuth and wave number interpolated between the highest values of the correlograms. USCGS P.D.E. information is used in conjunction with the J-B tables to calculate an expected apparent arrival vector. The difference between the expected and best interpolated arrival vectors has an average deviation of about 6 ms/km. Their distribution does not suggest a simple crustal or upper mantle cause under the array station.

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