The Source‐Scanning Algorithm: mapping the distribution of seismic sources in time and space
01 May 2004-Geophysical Journal International (Oxford University Press)-Vol. 157, Iss: 2, pp 589-594
TL;DR: The Source Scanning Algorithm (SSA) as discussed by the authors is a method for imaging the distribution of seismic sources in both time and space, using trial locations and origin times, the method calculates the "brightness" function by summing the absolute amplitudes observed at all stations at their respective predicted arrival times.
Abstract: SUMMARY We introduce a new method, which we call the Source-Scanning Algorithm (SSA), for imaging the distribution of seismic sources in both time and space. Using trial locations and origin times, the method calculates the ‘brightness’ function by summing the absolute amplitudes observed at all stations at their respective predicted arrival times. The spatial and temporal distribution of sources is then identified by a systematic search throughout the model space and time for the maximum brightness. The greatest advantages of this method are that: (1) it exploits waveform information (both arrival times and relative amplitudes) without the need to calculate highfrequency synthetic seismograms; and (2) it requires neither pre-assembled phase-picking data nor any a priori assumptions about the source geometry. A series of tests using synthetic data have shown that this method is robust and can faithfully recover the input source configuration to within 1 grid interval. Finally, we demonstrate the value of the algorithm by locating a typical tremor event with emergent waveforms that occurred during the recent episodic tremor and slip (ETS) sequence in the northern Cascadia subduction zone.
Citations
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TL;DR: In this paper, the authors show that if interstitial fluids are present and pore pressure is near-lithostatic around and downdip from the frictional stability transition, transients with recurrence intervals of ∼1 year are predicted on the basis of laboratory friction parameters and their temperature (hence depth) variations.
Abstract: [1] Aseismic deformation transients can emerge as a natural outcome of the rate and state friction processes revealed in laboratory fault-sliding experiments. When that constitutive formulation is applied to model subduction earthquake sequences, transients can arise spontaneously for certain effective stress () variations with depth. We show that if interstitial fluids are present and pore pressure is near-lithostatic around and downdip from the frictional stability transition, transients with recurrence intervals of ∼1 year are predicted on the basis of laboratory friction parameters and their temperature (hence depth) variations. The recurrence interval decreases with and reaches 14 months when is ∼2–3 MPa. Dimensional analysis and numerical studies show that the fault response primarily depends on a parameter W/h*. Here the high pore pressure zone extends distance W updip from the stability transition, and h* is the stable patch size for steady sliding. Evidence that such fluid conditions may actually be present is independently provided by the occurrence of nonvolcanic tremors as apparent responses to extremely small stress changes and by petrological constraints on expected regions of dehydration for the shallow dipping subduction zones where transients are observed. Transient sequences can also be triggered by a modest, one-time, step-like interseismic stress perturbation on the subduction fault, due to nearby earthquakes, or to pore pressure changes, e.g., during episodes of metamorphic fluid release. Properties of triggered transients and future thrust earthquakes depend on the interseismic time when the perturbation is introduced, its relative location along the subduction fault, and its magnitude.
437 citations
Cites methods from "The Source‐Scanning Algorithm: mapp..."
...correlation method [Obara, 2002] and the Source Scanning Algorithm [Kao and Shan, 2004] to locate the tremor activity on the south Vancouver Island in May 2005....
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...Hirose et al. [2006] applied both the Envelope Crosscorrelation method [Obara, 2002] and the Source Scanning Algorithm [Kao and Shan, 2004] to locate the tremor activity on the south Vancouver Island in May 2005....
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TL;DR: In this article, a family of slow earthquakes that occur as shear slip on the downdip extensions of fault zones in a regime that is transitional between a frictionally locked region above and a freely slipping region below are identified.
Abstract: Nonvolcanic tremor is observed in close association with geodetically observed slow-slip events in subduction zones. Accumulating evidence points to these events as members of a family of slow earthquakes that occur as shear slip on the downdip extensions of fault zones in a regime that is transitional between a frictionally locked region above and a freely slipping region below. By virtue of their locations and their properties, slow earthquakes are certain to provide new insights into the behavior of earthquakes and faulting and into the hazard they embody.
409 citations
Cites methods from "The Source‐Scanning Algorithm: mapp..."
...Locations based on the source-scanning algorithm (Kao & Shan 2004), which is a form of Kirchhoff migration, indicate a wide range of depths, with a somewhat higher concentration near the plate interface (Kao et al. 2005)....
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TL;DR: In this article, the authors present an approach to localize very weak events, not visible in the individual seismogram of the network, where no picking of events in the seismograms of the recording network is required.
Abstract: SUMMARY
The localization of seismic events is of utmost importance in seismology and exploration. Current techniques rely on the fact that the recorded event is detectable at most of the stations of a seismic network. Weak events, not visible in the individual seismogram of the network, are missed out. We present an approach, where no picking of events in the seismograms of the recording network is required. The observed wavefield of the network is reversed in time and then considered as the boundary value for the reverse modelling. Assuming the correct velocity model, the reversely modelled wavefield focuses on the hypocentre of the seismic event. The origin time of the event is given by the time where maximum focussing is observed. The spatial extent of the focus resembles the resolution power of the recorded wavefield and the acquisition. This automatically provides the uncertainty in the localization with respect to the bandwidth of the recorded data. The method is particularly useful for the upcoming large passive networks since no picking is required. It has great potential for localizing very weak events, not detectable in the individual seismogram, since the reverse modelling sums the energy of all recorded traces and, therefore, enhances the signal-to-noise ratio similar to stacking in seismic exploration. The method is demonstrated by 2-D and 3-D numerical case studies, which show the potential of the technique. Events with a S/N ratio smaller than 1 where the events cannot be identified in the individual seismogram of the network are localized very well by the method.
231 citations
TL;DR: The observed depth range implies that tremors could be associated with the variation of stress field induced by a transient slip along the deeper portion of the Cascadia interface or, alternatively, that episodic slip is more diffuse than originally suggested.
Abstract: The Cascadia subduction zone is thought to be capable of generating major earthquakes with moment magnitude as large as M(w) = 9 at an interval of several hundred years. The seismogenic portion of the plate interface is mostly offshore and is currently locked, as inferred from geodetic data. However, episodic surface displacements-in the direction opposite to the long-term deformation motions caused by relative plate convergence across a locked interface-are observed about every 14 months with an unusual tremor-like seismic signature. Here we show that these tremors are distributed over a depth range exceeding 40 km within a limited horizontal band. Many occurred within or close to the strong seismic reflectors above the plate interface where local earthquakes are absent, suggesting that the seismogenic process for tremors is fluid-related. The observed depth range implies that tremors could be associated with the variation of stress field induced by a transient slip along the deeper portion of the Cascadia interface or, alternatively, that episodic slip is more diffuse than originally suggested.
217 citations
TL;DR: Brown et al. as mentioned in this paper used a network autocorrelation approach to detect and locate low-frequency earthquakes within tremor recorded at three subduction zones characterized by different thermal structures and levels of interplate seismicity: southwest Japan, northern Cascadia, and Costa Rica.
Abstract: Deep tremor under Shikoku, Japan, consists primarily, and perhaps entirely, of swarms of low-frequency earthquakes (LFEs) that occur as shear slip on the plate interface. Although tremor is observed at other plate boundaries, the lack of cataloged low-frequency earthquakes has precluded a similar conclusion about tremor in those locales. We use a network autocorrelation approach to detect and locate LFEs within tremor recorded at three subduction zones characterized by different thermal structures and levels of interplate seismicity: southwest Japan, northern Cascadia, and Costa Rica. In each case we find that LFEs are the primary constituent of tremor and that they locate on the deep continuation of the plate boundary. This suggests that tremor in these regions shares a common mechanism and that temperature is not the primary control on such activity. Citation: Brown, J.R., G. C. Beroza, S. Ide, K. Ohta, D. R. Shelly, S. Y. Schwartz, W. Rabbel, M. Thorwart, and H. Kao (2009), Deep low-frequency earthquakes in tremor localize to the plate interface in multiple subduction zones, Geophys. Res. Lett., 36, L19306, doi:10.1029/2009GL040027.
189 citations
References
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TL;DR: In this paper, a least square solver is found by iteratively adjusting the vector difference between hypocentral pairs to minimize residuals between observed and theoretical travel-time differences.
Abstract: We have developed an efficient method to determine high-resolution hypocenter locations over large distances. The location method incorporates ordinary absolute travel-time measurements and/or cross-correlation P-and S-wave differential travel-time measurements. Residuals between observed and theoretical travel-time differences (or double-differences) are minimized for pairs of earthquakes at each station while linking together all observed event-station pairs. A least-squares solu- tion is found by iteratively adjusting the vector difference between hypocentral pairs. The double-difference algorithm minimizes errors due to unmodeled velocity struc- ture without the use of station corrections. Because catalog and cross-correlation data are combined into one system of equations, interevent distances within multiplets are determined to the accuracy of the cross-correlation data, while the relative lo- cations between multiplets and uncorrelated events are simultaneously determined to the accuracy of the absolute travel-time data. Statistical resampling methods are used to estimate data accuracy and location errors. Uncertainties in double-difference locations are improved by more than an order of magnitude compared to catalog locations. The algorithm is tested, and its performance is demonstrated on two clus- ters of earthquakes located on the northern Hayward fault, California. There it col- lapses the diffuse catalog locations into sharp images of seismicity and reveals hor- izontal lineations of hypocenters that define the narrow regions on the fault where stress is released by brittle failure.
2,891 citations
"The Source‐Scanning Algorithm: mapp..." refers background in this paper
...There have been major efforts to improve the precision of relative locations within a cluster of seismic events using the traveltime differences between pairs of events (Waldhauser & Ellsworth 2000) or stations (Zhou 1994)....
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TL;DR: Deep long-period tremors were recognized and located in a nonvolcanic region in southwest Japan, indicating that the tremors may have been caused by fluid generated by dehydration processes from the slab.
Abstract: Deep long-period tremors were recognized and located in a nonvolcanic region in southwest Japan. Epicenters of the tremors were distributed along the strike of the subducting Philippine Sea plate over a length of 600 kilometers. The depth of the tremors averaged about 30 kilometers, near the Mohorovic discontinuity. Each tremor lasted for at most a few weeks. The location of the tremors within the subduction zone indicates that the tremors may have been caused by fluid generated by dehydration processes from the slab.
1,227 citations
"The Source‐Scanning Algorithm: mapp..." refers background in this paper
...Nonvolcanic deep tremor associated with subduction in southwest Japan, Science, 296, 1679–1681....
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...4 A P P L I C AT I O N T O A T R E M O R E V E N T I N N O RT H E R N C A S C A D I A Seismic tremor activities are documented in both the southwest Japan (Obara 2002) and Cascadia subduction zones (Rogers & Dragert 2003)....
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TL;DR: It is proposed that ETS activity can be used as a real-time indicator of stress loading of the Cascadia megathrust earthquake zone and is called episodic tremor and slip (ETS).
Abstract: We found that repeated slow slip events observed on the deeper interface of the northern Cascadia subduction zone, which were at first thought to be silent, have unique nonearthquake seismic signatures. Tremorlike seismic signals were found to correlate temporally and spatially with slip events identified from crustal motion data spanning the past 6 years. During the period between slips, tremor activity is minor or nonexistent. We call this associated tremor and slip phenomenon episodic tremor and slip (ETS) and propose that ETS activity can be used as a real-time indicator of stress loading of the Cascadia megathrust earthquake zone.
1,129 citations
"The Source‐Scanning Algorithm: mapp..." refers background in this paper
...4 A P P L I C AT I O N T O A T R E M O R E V E N T I N N O RT H E R N C A S C A D I A Seismic tremor activities are documented in both the southwest Japan (Obara 2002) and Cascadia subduction zones (Rogers & Dragert 2003)....
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TL;DR: In this paper, a least-squares point-by-point inversion of strong ground motion and teleseismic body waves is used to infer the fault rupture history of the 1979 Imperial Valley, California, earthquake.
Abstract: A least-squares point-by-point inversion of strong ground motion and teleseismic body waves is used to infer the fault rupture history of the 1979 Imperial Valley, California, earthquake. The Imperial fault is represented by a plane embedded in a half-space where the elastic properties vary with depth. The inversion yields both the spatial and temporal variations in dislocation on the fault plane for both right-lateral strike-slip and normal dip-slip components of motion. Inversions are run for different fault dips and for both constant and variable rupture velocity models. Effects of different data sets are also investigated. Inversions are compared which use the strong ground motions alone, the teleseismic body waves alone, and simultaneously the strong ground motion and teleseismic records. The inversions are stabilized by adding both smoothing and positivity constraints.
The moment is estimated to be 5.0 × 10^(25) dyne-cm and the fault dip 90° ± 5°. Dislocation in the hypocentral region south of the United States-Mexican border is relatively small and almost dies out near the border. Dislocation then increases sharply north of the border to a maximum of about 2 m under Interstate 8. Dipslip motion is minor compared to strike-slip motion and is concentrated in the sediments. The best-fitting constant rupture velocity is 80 per cent of the local shear-wave velocity. However, there is a suggestion that the rupture front accelerated from the hypocenter northward. The 1979 Imperial Valley earthquake can be characterized as a magnitude 5 earthquake at the hypocenter which then grew into or triggered a magnitude 6 earthquake north of the border.
979 citations
"The Source‐Scanning Algorithm: mapp..." refers background in this paper
...The basic principle is that the complete source configuration can be recovered from the constructive and/or destructive interference observed at stations along different azimuths/distances (Hartzell & Heaton 1983)....
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TL;DR: In this paper, a method was developed to compute seismic reflection traveltimes in complex 3D velocity models with complex 3-D reflector geometry, which is faster and more accurate than several alternative schemes, and was incorporated in a procedure to compute reflection travel times.
Abstract: SUMMARY A method has been developed to compute seismic reflection traveltimes in complex 3-D velocity models with complex 3-D reflector geometry. An existing finitedifference algorithm for calculating first-arrival traveltimes was modified to handle large, sharp velocity contrasts properly. The modified algorithm is faster and more accurate than several alternative schemes, and was incorporated in a procedure to compute reflection traveltimes. Snell's law for reflections is used in the vicinity of the reflecting interface. The reflector model is allowed to vary smoothly in depth, increasing the accuracy compared with a discretized reflector model. Reflection traveltimes are computed simultaneously for all revivers, requiring only two applications of the finite-difference algorithm for each shot. This results in a significant saving in computation time in comparison with other algorithms which require one pass of the finite-difference algorithm for each shot and each receiver. The reflection traveltime procedure is well suited for incorporation in inversion schemes for 3-D velocity and reflector structure.
367 citations
"The Source‐Scanning Algorithm: mapp..." refers methods in this paper
...D ray tracing code developed by Hole & Zelt (1995) and stored as ‘reference files’....
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