47 results on '"Helmberger, Don V."'
Search Results
2. Evidence for a Chemical-Thermal Structure at Base of Mantle from Sharp Lateral P-Wave Variations beneath Central America
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Sun, Xinlei, Song, Xiaodong, Zheng, Sihua, and Helmberger, Don V.
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- 2007
- Full Text
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3. Partial rupture of a locked patch of the Sumatra megathrust during the 2007 earthquake sequence
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Konca, A. Ozgun, Avouac, Jean-Philippe, Sladen, Anthony, Meltzner, Aron J., Sieh, Kerry, Fang, Peng, Li, Zhenhong, Galetzka, John, Genrich, Jeff, Chlieh, Mohamed, Natawidjaja, Danny H., Bock, Yehuda, Fielding, Eric J., Ji, Chen, and Helmberger, Don V.
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- 2008
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4. Shallow Basin Structure and Attenuation Are Key to Predicting Long Shaking Duration in Los Angeles Basin
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Lai, Voon Hui, Graves, Robert W., Yu, Chunquan, Zhan, Zhongwen, and Helmberger, Don V.
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Ground motions in the Los Angeles Basin during large earthquakes are modulated by earthquake ruptures, path effects into the basin, basin effects, and local site response. We analyzed the direct effect of shallow basin structures on shaking duration at a period of 2–10 s in the Los Angeles region through modeling small magnitude, shallow, and deep earthquake pairs. The source depth modulates the basin response, particularly the shaking duration, and these features are a function of path effect and not site condition. Three‐dimensional simulations using the CVM‐S4.26.M01 velocity model show good fitting to the initial portion of the waveforms at periods of 5 s and longer but fail to predict the long shaking duration during shallow events, especially at periods less than 5 s. Simulations using CVM‐H do not match the timing of the initial arrivals as well as CVM‐S4.26.M01, and the strong late arrivals in the CVM‐H simulation travel with an apparent velocity slower than observed. A higher‐quality factor than traditionally assumed may produce synthetics with longer durations but is unable to accurately match the amplitude and phase. Beamforming analysis using dense array data further reveals the long duration surface waves have the same back azimuth as the direct arrivals and are generated at the basin edges, while the later coda waves are scattered from off‐azimuth directions, potentially due to strong, sharp boundaries offshore. Improving the description of these shallow basin structures and attenuation model will enhance our capability to predict long‐period ground motions in basins.
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- 2020
5. Low-velocity zone atop the 410-km seismic discontinuity in the northwestern United States
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Alex Song, Teh-Ru, Helmberger, Don. V., and Grand, Stephen P.
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Environmental issues ,Science and technology ,Zoology and wildlife conservation - Abstract
Author(s): Teh-Ru Alex Song (corresponding author) [1]; Don. V. Helmberger [1]; Stephen P. Grand [2] The seismic discontinuity at 410 km depth in the Earth's mantle is generally attributed to [...]
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- 2004
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6. Evidence for a Ubiquitous Seismic Discontinuity at the Base of the Mantle
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Sidorin, Igor, Gurnis, Michael, and Helmberger, Don V.
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- 1999
7. Seismic Evidence for an Inner Core Transition Zone
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Song, Xiaodong and Helmberger, Don V.
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- 1998
8. Lower Mantle Substructure Embedded in the Farallon Plate: The Hess Conjugate
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Ko, Justin Yen‐Ting, primary, Helmberger, Don V., additional, Wang, Huilin, additional, and Zhan, Zhongwen, additional
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- 2017
- Full Text
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9. Kursk Explosion
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Savage, Brian and Helmberger, Don V.
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Geophysics ,Geochemistry and Petrology - Abstract
On 12 August 2000 two explosions damaged the Russian submarine, the Kursk. The largest event was well recorded at seismic networks in northern Europe, which we then modeled. We developed a hybrid method based on generalized ray theory that treats an explosive source embedded in a fluid and recorded along continental paths. Matching record sections of observations with synthetics, we obtain an estimate of explosive size of slightly over 4 t. Several earth models determined previously, K8 and a Baltic model, were used to assess accuracy. These results are in general agreement with other investigators using more empirical methods. Knowing the conventional missile yield and the explosion size allows for an estimate of approximately five missiles exploded in the second larger explosion onboard the Kursk.
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- 2001
10. Seismic structure in central Mexico: Implications for fragmentation of the subducted Cocos plate
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Dougherty, Sara L., Clayton, Robert W., and Helmberger, Don V.
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The fine-scale seismic structure of the central Mexico subduction zone is studied using moderate-sized (M4-6) intraslab earthquakes. Regional waveforms from the Mapping the Rivera Subduction Zone (MARS) seismic array are complicated and contain detailed information about the subduction zone structure, including evidence of lateral heterogeneity. This waveform information is used to model the structure of the subducted plates, particularly along the transition from flat to normal subduction, where recent studies have shown evidence for possible slab tearing along the eastern projection of the Orozco Fracture Zone (OFZ). The lateral extent of a thin ultra-slow velocity layer (USL) imaged atop the Cocos slab in recent studies along the Meso America Subduction Experiment array is examined here using MARS waveforms. We find an edge to this USL which is coincident with the western boundary of the projected OFZ region. Forward modeling of the 2D structure of the subducted Rivera and Cocos plates using a finite difference algorithm provides constraints on the velocity and geometry of each slab's seismic structure in this region and confirms the location of the USL edge. We propose that the Cocos slab is currently fragmenting into a North Cocos plate and a South Cocos plate along the projection of the OFZ, in agreement with observations of variable Cocos plate motion on either side of the OFZ. This tearing event may be a young analogy to the 10 Ma Rivera-Cocos plate boundary, and may be related to the slab rollback in central Mexico.
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- 2012
11. Juan de Fuca subduction zone from a mixture of tomography and waveform modeling
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Chu, Risheng, Schmandt, Brandon, and Helmberger, Don V.
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Seismic tomography images of the upper mantle structures beneath the Pacific Northwestern United States display a maze of high-velocity anomalies, many of which produce distorted waveforms evident in the USArray observations indicative of the Juan de Fuca (JdF) slab. The inferred location of the slab agrees quite well with existing contour lines defining the slab's upper interface. Synthetic waveforms generated from a recent tomography image fit teleseismic travel times quite well and also some of the waveform distortions. Regional earthquake data, however, require substantial changes to the tomographic velocities. By modeling regional waveforms of the 2008 Nevada earthquake, we find that the uppermost mantle of the 1D reference model AK135, the reference velocity model used for most tomographic studies, is too fast for the western United States. Here, we replace AK135 with mT7, a modification of an older Basin-and-Range model T7. We present two hybrid velocity structures satisfying the waveform data based on modified tomographic images and conventional slab wisdom. We derive P and SH velocity structures down to 660 km along two cross sections through the JdF slab. Our results indicate that the JdF slab is subducted to a depth of 250 km beneath the Seattle region, and terminates at a shallower depth beneath Portland region of Oregon to the south. The slab is about 60 km thick and has a P velocity increase of 5% with respect to mT7. In order to fit waveform complexities of teleseismic Gulf of Mexico and South American events, a slab-like high-velocity anomaly with velocity increases of 3% for P and 7% for SH is inferred just above the 660 discontinuity beneath Nevada.
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- 2012
12. Upper mantle P velocity structure beneath the Midwestern United States derived from triplicated waveforms
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Chu, Risheng, Schmandt, Brandon, and Helmberger, Don V.
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Upper mantle seismic velocity structures in both vertical and horizontal directions are key to understanding the structure and mechanics of tectonic plates. Recent deployment of the USArray Transportable Array (TA) in the Midwestern United States provides an extraordinary regional earthquake data set to investigate such velocity structure beneath the stable North American craton. In this paper, we choose an M_w5.1 Canadian earthquake in the Quebec area, which is recorded by about 400 TA stations, to examine the P wave structures between the depths of 150 km to 800 km. Three smaller Midwestern earthquakes at closer distance to the TA are used to investigate vertical and horizontal variations in P velocity between depths of 40 km to 150 km. We use a grid-search approach to find the best 1-D model, starting with the previously developed S25 regional model. The results support the existence of an 8° discontinuity in P arrivals caused by a negative velocity gradient in the lithosphere between depths of 40 km to 120 km followed by a small (∼1%) jump and then a positive gradient down to 165 km. The P velocity then decreases by 2% from 165 km to 200 km, and we define this zone as the regional lithosphere-asthenosphere boundary (LAB). Beneath northern profiles, waves reflected from the 410 discontinuity (410) are delayed by up to 1 s relative to those turning just below the 410, which we explain by an anomaly just above the discontinuity with P velocity reduced by ∼3%. The 660 discontinuity (660) appears to be composed of two smaller velocity steps with a separation of 16 km. The inferred low-velocity anomaly above 410 may indicate high water concentrations in the transition zone, and the complexity of the 660 may be related to Farallon slab segments that have yet to sink into the deep mantle.
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- 2012
13. Initiation of the great M_w 9.0 Tohoku–Oki earthquake
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Chu, Risheng, Wei, Shengji, Helmberger, Don. V., Zhan, Zhongwen, Zhu, Lupei, and Kanamori, Hiroo
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We determined the location, size, mechanism, and the frequency content of the first 4.0 s of the 2011 Tohoku–Oki earthquake. Since the beginning of this earthquake is very small, we develop a comparative approach against a near-by reference earthquake, the master event. We first determined the water depth near the master event using the differential timing between the water phase pwP reflected from the air–water interface and the depth phase pP reflected from the water–crust interface. Then we located the master event using the well-known ocean bathymetry in the area. After calibrating teleseismic arrays (Δ=30° to 90°) at short periods for timing and amplitude with respect to the master event, we were able to determine the initiation of themain event. It began as a small (M_w=4.9) thrust event located at 38.19°N, 142.68°E at a depth of 21 km, and, a few seconds later, evolved into a slower extremely large slip event up-dip.
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- 2011
14. Retrieval of Moho-reflected shear wave arrivals from ambient seismic noise
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Zhan, Zhongwen, Ni, Sidao, Helmberger, Don V., and Clayton, Robert W.
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Computer Science::Databases - Abstract
Theoretical studies on ambient seismic noise (ASN) predict that complete Green's function between seismic stations can be retrieved from cross correlation. However, only fundamental mode surface waves emerge in most studies involving real data. Here we show that Moho-reflected body wave (SmS) and its multiples can be identified with ASN for station pairs near their critical distances in the short period band (1–5 s). We also show that an uneven distribution of noise sources, such as mining activity and wind–topography interaction, can cause surface wave precursors, which mask weaker body wave phases.
- Published
- 2010
15. Rupture Process of the 1999 M_w 7.1 Duzce Earthquake from Joint Analysis of SPOT, GPS, InSAR, Strong-Motion, and Teleseismic Data: A Supershear Rupture with Variable Rupture Velocity
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Konca, A. Ozgun, Leprince, Sébastien, Avouac, Jean-Philippe, and Helmberger, Don V.
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We analyze the rupture process of the 1999 M_w 7.1 Duzce earthquake using seismological, remote sensing, and geodetic data. Ground deformation measured from the subpixel cross correlation of Satellite Pour l'Observation de la Terre (SPOT) images reveals a 55 km long fault trace and smooth surface-slip distribution peaking at 3.5–4 m. The westernmost segment overlaps for over 10 km with ruptures from the M_w 7.4 Izmit earthquake. The 15 km long easternmost segment, which cuts across mountainous topography, had not been reported previously. We determine a well-constrained source model using a four-segment fault geometry using constraints on surface fault slip and inverting Global Positioning System and Interferometric Synthetic Aperture Radar data along with strong-motion records. Our results show that some variability of the rupture velocity and an eastward supershear velocity are required to fit the strong-motion data. The rise time, up to 6 sec, correlates with cumulative slip, suggesting a sliding velocity of about 1 m/sec. The source model predicts teleseismic waveforms well, although early by 2 sec. This time shift is probably due to the weak beginning of the earthquake that is not observable at teleseismic distances. Strong-motion records are relatively well predicted from a source model derived from the teleseismic data using the fault geometry derived from the satellite images. This study demonstrates the benefit of using accurate fault geometries to determine finite-fault source models.
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- 2010
16. Mushy magma beneath Yellowstone
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Chu, Risheng, Helmberger, Don V., Sun, Daoyuan, Jackson, Jennifer M., and Zhu, Lupei
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A recent prospective on the Yellowstone Caldera discounts its explosive potential based on inferences from tomographic studies which suggests a high degree of crystallization of the underlying magma body. In this study, we show that many of the first teleseismic P-wave arrivals observed at seismic stations on the edge of the caldera did not travel through the magma body but have taken longer but faster paths around the edge. After applying a number of waveform modeling tools, we obtain much lower seismic velocities than previous studies, 2.3 km/sec (V_p) and 1.1 km/sec (V_s). We estimate the physical state of the magma body by assuming a fluid-saturated porous material consisting of granite and a mixture of rhyolite melt and water and CO_2 at a temperature of 800°C and pressure at 5 km (0.1 GPa). We found that this relatively shallow magma body has a volume of over 4,300 km^3 and is about 32% melt saturated with about 8% water plus CO_2 by volume.
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- 2010
17. Rupture Kinematics of the 2005 M_w 8.6 Nias–Simeulue Earthquake from the Joint Inversion of Seismic and Geodetic Data
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Konca, A. Ozgun, Hjorleifsdottir, Vala, Song, Teh-Ru Alex, Avouac, Jean-Philippe, Helmberger, Don V., Ji, Chen, Sieh, Kerry, Briggs, Richard, and Meltzner, Aron
- Abstract
The 2005 M_w 8.6 Nias–Simeulue earthquake was caused by rupture of a portion of the Sunda megathrust offshore northern Sumatra. This event occurred within an array of continuous Global Positioning System (GPS) stations and produced measurable vertical displacement of the fringing coral reefs above the fault rupture. Thus, this earthquake provides a unique opportunity to assess the source characteristics of a megathrust event from the joint analysis of seismic data and near-field static co-seismic displacements. Based on the excitation of the normal mode data and geodetic data we put relatively tight constraints on the seismic moment and the fault dip, where the dip is determined to be 8° to 10° with corresponding moments of 1.24 x 10^(22) to 1.00 x 10^(22) N m, respectively. The geodetic constraints on slip distribution help to eliminate the trade-off between rupture velocity and slip kinematics. Source models obtained from the inversion of various combinations of the teleseismic body waves and geodetic data are evaluated by comparing predicted and observed long-period seismic waveforms (100–500 sec). Our results indicate a relatively slow average rupture velocity of 1.5 to 2.5 km/sec and long average rise time of up to 20 sec. The earthquake nucleated between two separate slip patches, one beneath Nias and the other beneath Simeulue Island. The gap between the two patches and the hypocentral location appears to be coincident with a local geological disruption of the forearc. Coseismic slip clearly tapers to zero before it reaches the trench probably because the rupture propagation was inhibited when it reached the accretionary prism. Using the models from joint inversions, we estimate the peak ground velocity on Nias Island to be about 30 cm/sec, an order of magnitude slower than for thrust events in continental areas. This study emphasizes the importance of utilizing multiple datasets in imaging seismic ruptures.
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- 2007
18. The 2005, M_w 7.6 Kashmir earthquake: Sub-pixel correlation of ASTER images and seismic waveforms analysis
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Avouac, Jean-Philippe, Ayoub, François, Leprince, Sébastien, Konca, Ozgun, and Helmberger, Don V.
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We analyze the M_w 7.6 Kashmir earthquake of October 8, 2005, using sub-pixel correlation of ASTER images to measure ground deformation, and modeling seismic waveforms. The surface rupture is continuous over a distance of 75 km and cuts across the Hazara syntaxis reactivating the Tanda and the Muzaffarabad faults. North of Muzaffarabad the surface rupture coincides approximately with the MBT, on the southwestern flank of the syntaxis, although the two faults have opposite dip angles. The rupture terminates abruptly at the hairpin turn of the MBT showing a strong structural control. The fault offset is 4 m on average and peaks to 7 m northwest of Muzaffarabad. The rupture lasted about 25 s and propagated updip and bi-laterally ~2 km/s, with a rise time of 2–5 s. The shallowness and compactness of the rupture, both in time and space, provide an explanation for the intensity of destructions. This kind of analysis could be achieved as soon as a post-earthquake image is available, and would provide key information for early assessment of damages. The study sheds some light on seismic hazard in the Himalaya, and raises concern regarding the possibility of a repetition of the 1555 event which presumably ruptured the Himalayan front south of the Kashmir basin and may have reached a magnitude M_w > 8.
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- 2006
19. Ridge-like lower mantle structure beneath South Africa
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Ni, Sidao and Helmberger, Don V.
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Recent (ScS-S) results from probing the deep structure beneath southern Africa display strong delays of up to 10 s at distances beyond 90°. Such delays could be explained by long-period tomographic models containing smooth (weak) features with the addition of rough (strong) D″ structure (3–9% drops in shear velocities). However, these structures cannot explain the (SKS-S) differentials sampling the same region. To explain the (SKS-S) and (ScS-S) data sets simultaneously requires instead a large-scale ridge-like structure with a relatively uniform 3% reduction in shear velocity. The structure is about 1000 km wide and extends at least 1200 km above D″. It is orientated roughly NW-SE and leans toward the east at latitudes from 15° to about 30°. It proves difficult to explain such sharp features with thermal effects alone and, thus, the importance of high-resolution waveform modeling to establish their existence. To derive the above results, we developed a special algorithm by matching simulated synthetics to observed broadband waveforms. This is achieved by computing the various arrivals separately using generalized ray theory for a reference model and allowing the arrivals to shift in relative times to match the data. Tomographic models can then be constructed, or existing tomographic models can be altered, to match these data, and new 2-D synthetics can be constructed as well to better fit the waveform data. These updated synthetics can again be decomposed and reassembled, and the process can be repeated. This algorithm is applied to a combination of analog and digital data along a corridor from South America, producing the high-resolution 2-D model described above.
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- 2003
20. Analyses and Simulation of 3D Scattering Due to Heterogeneous Crustal Structure and Surface Topography on Regional Phases, Magnitude Discrimination
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URS GROUP INC GAITHERSBURG MD, Pitarka, Arban, Helmberger, Don V., Ni, Sidao, URS GROUP INC GAITHERSBURG MD, Pitarka, Arban, Helmberger, Don V., and Ni, Sidao
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The main purpose of this study is the understanding of scattering and its effects through step-by-step numerical experiments and waveform modeling, with the goal of providing useful insights into ongoing research for the development of simple empirical models of scattering that can be used in reducing the scatter in measures of Lg and coda wave magnitude and for discrimination purposes as well. We are performing anelastic 3D finite-difference simulations of wave propagation in highly heterogeneous media for a range of source depths, receiver distances and source types. In the first stage of our study we investigated the effect of small-scale crustal heterogeneities on wave propagation scattering. During the second stage we investigated the effect of surface topography combined with crustal heterogeneities on wave propagation scattering. In our simulations of regional wave propagation we used a finite difference computer program using our computer cluster. The seismograms were calculated at up to 3.5 Hz for regional distances of up to 300 km. The topography elevation was simulated using correlated random variations along the free surface. Our simulations show that the surface topography increases the wave-path scattering effects. The combined effects of crustal heterogeneities and surface topography produce Lg, P, and S coda waves with significant energy even for explosion sources. The energy of Lg coda waves depends on the source depth. P/Lg ratios estimated at different frequencies indicate that this ratio could be a good discriminant between explosions and earthquakes when calculated at high frequencies. The wave-path scattering effects were also investigated by simulating observed high frequency source directivity effects, as well as fault zone wave trapping in highly fractured conditions from aftershocks of the Big Bear earthquake sequence., Proceedings of the Monitoring Research Review(30th)-Ground-Based Nuclear Explosion Monitoring Technologies, held on 23-25 Sep 2008 in Portsmourth, VA. Pub. in Ground-Based Nuclear Explosion Monitoring Technologies, v1, p201-212, 2008.
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- 2008
21. Advanced Waveform Simulation for Seismic Monitoring Events
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LAWRENCE LIVERMORE NATIONAL LAB CA, Helmberger, Don V., Tromp, Jeroen, Rodgers, Arthur J., LAWRENCE LIVERMORE NATIONAL LAB CA, Helmberger, Don V., Tromp, Jeroen, and Rodgers, Arthur J.
- Abstract
Comprehensive nuclear-test-ban monitoring in terms of location and discrimination has progressed significantly in recent years. However, the characterization of sources and the estimation of low yields remains a particular challenge. As the recent Korean shot demonstrated, we can probably expect to have a small set of teleseismic, far-regional and high-frequency regional data to analyze in estimating the yield of an event. Since stacking helps to bring signals out of the noise, it becomes useful to conduct comparable analyses on neighboring events, earthquakes in this case. If these auxiliary events have accurate moments and source descriptions, we have a means of directly comparing effective source strengths. Although we will rely on modeling codes, 1D, 2D, and 3D, we will also apply a broadband calibration procedure to use longer periods (P >5 s) of waveform data to calibrate short-period (P between 0.5 to 2 Hz) and high-frequency (P between 2 to 10 Hz) as path-specific station corrections from well-known regional sources. We have expanded our basic cut-and-paste (CAP) methodology to include not only timing shifts but also amplitude (f) corrections at recording sites. The name of this method was derived from source inversions that allow timing shifts between "waveform segments" (or cutting the seismogram up and re-assembling) to correct for crustal variation. For convenience, we will refer to these f-dependent refinements as CAP+ for (short period, SP) and CAP++ for still higher frequency. These methods allow the retrieval of source parameters using only P-waveforms where radiation patterns are obvious as demonstrated in this report and are well suited for explosion P-wave data. The method is easily extended to all distances because it uses Green's function although there may be some changes required in t* to adjust for offsets between local vs. teleseismic distances., Presented at the Monitoring Research Review (29th): Ground-Based Nuclear Explosion Monitoring Technologies held in Denver, Colorado on 25-27 September 2007. Published in the Proceedings of the Monitoring Research Review (29th): Ground-Based Nuclear Explosion Monitoring Technologies, p80-90, 2007. Sponsored by the National Nuclear Security Administration (NNSA) and the Air Force Research Laboratory (AFRL). Performed in cooperation with the California Institute of Technology, Padadena, CA.The original document contains color images.
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- 2007
22. Ground Truth Locations Using Synergy Between Remote Sensing and Seismic Methods
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AIR FORCE TECHNICAL APPLICATIONS CENTER PATRICK AFB FL, Ichinose, Gene A., Thio, Hong K., Helmberger, Don V., AIR FORCE TECHNICAL APPLICATIONS CENTER PATRICK AFB FL, Ichinose, Gene A., Thio, Hong K., and Helmberger, Don V.
- Abstract
In this study, we have relocated a set of 50 ground truth (GT) earthquakes as determined from regional modeling, to estimate the performance of the station correction methodology. We constructed station-specific source correction or 'station correction' surface from 760 station correction (global distribution) based on a small set of five well and closely located earthquakes in the central Tibetan Plateau. The station corrections were interpolated to develop a global surface for P-wave travel time corrections. As a test, we relocated a set of 50 GT earthquakes, with some seismic events located about 800 km from the center of the PASSCAL array across the Tibetan Plateau and surrounding regions. When the corrections were applied to the iasp91 velocity model, the mislocations decreased by an average by 37% for 37 out of the 50 GT events. This indicates significant improvement in seismic event locations relative to using the iasp91 model without P-wave travel-time corrections. Surprisingly, the distribution of mislocations did not vary or correlate with distance and some of the farthest events from the center of the Tibetan Plateau were improved more than the locations in the center where the travel time corrections are more relevant. Myers and Schultz (2000) noted that correction surfaces for nearby stations are similar, suggesting coherent assessment of the results, corrections could be developed from a small cluster of earthquakes and transported for use with seismic events outside of the area out to a radius of 800 km. Additional tests should be performed to further validate these claims especially when crossing tectonic boundaries., Presented at the Proceedings of the Monitoring Research Review (29th): Ground-Based Nuclear Explosion Monitoring Technologies held in Denver, CO on 25-27 September 2007. Published in Proceedings of the Monitoring Research Review (29th): Ground-Based Nuclear Explosion Monitoring Technologies, p406-413, 2007. Sponsored by the National Nuclear Security Administration (NNSA) and the Air Force Research Laboratory (AFRL). The original document contains color images.
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- 2007
23. Seismic Modeling Constraints on the South African Super Plume
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van Der Hilst, R. D., Bass, J. D., Matas, J., Trampert, J., Helmberger, Don V., Ni, Sidao, van Der Hilst, R. D., Bass, J. D., Matas, J., Trampert, J., Helmberger, Don V., and Ni, Sidao
- Abstract
Tomographic studies of the structure of the lower mantle beneath South Africa reveal large-scale low velocities above the core-mantle boundary. Predicted SKS delay patterns (up to 3 s) for some of these models fit observations (Kaapvaal Array data) quite well except for magnitude level, explaining less than one-half the observed anomaly. Moreover, the sharpness in travel-time offsets and waveform complications require that nearly vertical walls separate the anomalous structure from the normal preliminary reference Earth model (PREM) mantle. We present numerous record sections along with 2D and 3D synthetics displaying multipathing of arrivals (S_(d') SKS, SKKS, S, and ScS), based on a large-scale 3D structure. This kidney-shaped structure has one apex beneath the Indian Ocean (Kerguelen) and the other extending beneath the Mid-Atlantic (Cape Verde). The structure is about 1200 km wide beneath South Africa and extends upward to at least 1000 km through the lower mantle, similar to Grand's model but with an average uniform velocity decrease of about 3% relative to PREM. We have not found any evidences for ultra-low-velocity zones (ULVZ) beneath the main structure but ample evidence at some locations near the edges. We also analyzed Pd and the differentials between PcP travel times and P travel times (PcP-P) along the same great circle paths from the same events. The P-velocity is not very anomalous, perhaps -0.5%. The sharpness of the lateral boundaries (walls) and the large contrast in P and S velocities can be used in arguments for a thermochemical origin.
- Published
- 2005
24. A Teleseismic Study of the 2002 Denali Fault, Alaska, Earthquake and Implications for Rapid Strong-Motion Estimation
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Ji, Chen, Helmberger, Don V., Wald, David J., Ji, Chen, Helmberger, Don V., and Wald, David J.
- Abstract
Slip histories for the 2002 M7.9 Denali fault, Alaska, earthquake are derived rapidly from global teleseismic waveform data. In phases, three models improve matching waveform data and recovery of rupture details. In the first model (Phase I), analogous to an automated solution, a simple fault plane is fixed based on the preliminary Harvard Centroid Moment Tensor mechanism and the epicenter provided by the Preliminary Determination of Epicenters. This model is then updated (Phase II) by implementing a more realistic fault geometry inferred from Digital Elevation Model topography and further (Phase III) by using the calibrated P-wave and SH-wave arrival times derived from modeling of the nearby 2002 M6.7 Nenana Mountain earthquake. These models are used to predict the peak ground velocity and the shaking intensity field in the fault vicinity. The procedure to estimate local strong motion could be automated and used for global real-time earthquake shaking and damage assessment.
- Published
- 2004
25. Retrieval of Moho-reflected shear wave arrivals from ambient seismic noise
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Zhan, Zhongwen, primary, Ni, Sidao, additional, Helmberger, Don V., additional, and Clayton, Robert W., additional
- Published
- 2010
- Full Text
- View/download PDF
26. Mushy magma beneath Yellowstone
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Chu, Risheng, primary, Helmberger, Don V., additional, Sun, Daoyuan, additional, Jackson, Jennifer M., additional, and Zhu, Lupei, additional
- Published
- 2010
- Full Text
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27. Determination of earthquake focal depths and source time functions in central Asia using teleseismic P waveforms
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Chu, Risheng, primary, Zhu, Lupei, additional, and Helmberger, Don V., additional
- Published
- 2009
- Full Text
- View/download PDF
28. Adaptive Techniques for Estimating and Locating Small Events Using Regional Waveform Data
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CALIFORNIA INST OF TECH PASADENA SEISMOLOGICAL LAB, Helmberger, Don V., Ji, Chen, Savage, Brian, CALIFORNIA INST OF TECH PASADENA SEISMOLOGICAL LAB, Helmberger, Don V., Ji, Chen, and Savage, Brian
- Abstract
Attempts at locating and identifying small events could benefit greatly from a better appreciation of regional seismograms. Essentially, we need more information from the few regional seismograms available than just arrival times and amplitude ratios. Here we discuss a method of estimating source parameters using as few as one broadband station in conjunction with travel-time and polarities from at least one more station. The method employs an adaptive grid-search of matching three-component waveform records against synthetics to establish source location and depth. The basic matching procedure contains a trade-off of source mechanism with location. The better the constraint on mechanism and depth, the better the location. It appears that refined depths and origin times can be obtained for GT5 events if the first-motion polarities are available to constrain the focal plane. The locations of GT10 and GT25 and EHB (Engdahl, Kennett, and Buland) events can also be improved if CMT's and other independent depth estimates are available. Preliminary results applied to the PASSCAl data recorded in Pakistan and Tibet (China) and TriNet (CAL) proved highly effective when locating events located approximately between two stations. If both stations have usable three-component data, we can determine depth and source mechanism and refine the location. If we assume the mechanism and depth are known (Master Reference Event), we can relocate the event using just one broadband station and the P-wave arrival time pick from the other, or we can refine the crustal model. We find good agreement when testing these results against those from the entire array., Proceedings of the Annual DoD/DOE Seismic Research Symposium (22nd): Planning for Verification of and Compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT) held in New Orleans, Louisiana on 13-15 September 2000. U.S. Government or Federal Rights. The original document contains color images.
- Published
- 2000
29. Validating tomographic model with broad-band waveform modelling: an example from the LA RISTRA transect in the southwestern United States
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Alex Song, Teh-Ru, primary and Helmberger, Don. V., additional
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- 2007
- Full Text
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30. Construction of Synthetics for 2D structures; core phases
- Author
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Boschi, Enzo, Ekström, Göran, Morelli, A., Helmberger, Don V., Zhao, Lian-She, Garnero, Edward J., Boschi, Enzo, Ekström, Göran, Morelli, A., Helmberger, Don V., Zhao, Lian-She, and Garnero, Edward J.
- Abstract
Recent studies have emphasized the prominence of anomalous high and low velocity regions in the lower mantle. For example, slower than average seismic shear wave velocities beneath the central Pacific accompanied by surrounding higher than average velocities is common to most tomographic studies (e.g., see Tanimoto, 1990; Masters et al., 1992; Su and Dziewonski, 1994). Images from these reports indicate long wavelength (> 3000 km) structures with velocity anomalies of up to ± 2.5%. A particularly prominent slow region occurs just north of the Fiji-Tonga region in all of these models and has been noted by many authors with respect to differential times of body waves, such as ScS-S by Sipkin and Jordan (1980), S-SKS by Garnero and Helmberger (1993) and others. The latter study demonstrates that S-SKS time predictions made from present 3D models do not agree with all of the observations. Since this anomaly may well be associated with up-welling currents in the mantle, the seismic details become particularly important especially at the shorter wavelengths associated with body waves (5-100 km). Thus, better methods of generating realistic synthetics for these types of complex models would be useful and are addressed in this study.
- Published
- 1996
31. Regional Source Parameters, Seismic Energy, and Discrimination
- Author
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Husebye, Eystein Sverre, Dainty, A. M., Helmberger, Don V., Woods, Brad, Husebye, Eystein Sverre, Dainty, A. M., Helmberger, Don V., and Woods, Brad
- Abstract
We have examined broadband waveforms from a large number of NTS explosions and earthquakes throughout the southwestern United States in order to characterize seismic sources. Explosions were found to be richer in coda energy than earthquakes. Most earthquakes show relatively little long-period (T>4 sec) coda energy and tend to be richer in long-period and shear-wave energy than explosions. We have developed several seismic discriminants based on these observations and our modeling experience. One promising discriminant is the ratio of short-period vertical component, P-wavetrain energy, to long-period surface wave energy, averaged over three components. Explosions tend to have a higher ratio than do earthquakes, essentially an extension of m_b:M_s. Magnitude threshold for this discriminant is about 3.5. Another useful discriminant is based on the total broadband energy to moment ratio where explosions are distinguished by their stronger energy levels relative to their long-period amplitudes. This approach requires Green's functions, a source estimator program, and processes all events as earthquakes. For this method to be effective requires the calibration of the region using relatively large earthquakes, M>5, but does not require calibrations of explosions.
- Published
- 1996
32. Probing an ultra-low velocity zone at the core mantle boundary with P and S waves
- Author
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Ni, Sidao, primary and Helmberger, Don V., additional
- Published
- 2001
- Full Text
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33. Development of Regional Discriminants
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CALIFORNIA INST OF TECH PASADENA SEISMOLOGICAL LAB, Helmberger, Don V., Song, Xi, CALIFORNIA INST OF TECH PASADENA SEISMOLOGICAL LAB, Helmberger, Don V., and Song, Xi
- Abstract
With the installation of broadband, high dynamic range instruments, it has become possible to compare the regional waveforms of earthquakes and explosions at magnitudes 3 to 6. These waveforms are similar for event sequences in many situations and can be inverted for source mechanisms. We find that flat-layered models are sufficient for inverting seismograms at periods greater than a few seconds. This paper presents three studies aimed at determining crustal models, source finiteness and modeling complex structure near a receiver. We have conducted a set of sensitivity tests on the parameters of 1-D models to compare their impact on different segments of regional seismograms. We found that P sub nl waves (extended P-waves) are controlled in broadband character by the mid-crust while the top layer contributes to the long-period motions. The SV wave is mostly controlled by the shear wave velocity of the lower crust, especially the crustal layer just below the source depth. The top crustal layer controls the shape of the surface waves at ranges from 300 to 600 km, and the upper crust, especially the crustal layer just above the source depth, controls their timing. Applying these tests in modeling three earthquakes in the Basin-and-Range province, we found that a simple two-layer crustal model could effectively explain the data both in timing and in shape. The main crustal layer has P and S velocities of 6.1 km/sec and 3.6 km/sec, similar to those found by Langston and Helmberger (1974). A surface layer of thickness 2.5 to 3.5 km is required to fit the Rayleigh waves. Fast estimation of point-source parameters for earthquakes has witnessed much progress in recent years due to the development of broadband seismic networks., This article is from 'Proceedings of the Annual Seismic Research Symposium on Monitoring a Comprehensive Test Ban Treaty (17th) Held in Scottsdale, Arizona on 12-15 September, 1995', 1996 0607 035, p58-67.
- Published
- 1995
34. Constructing synthetics from deep earth tomographic models
- Author
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Ni, Sidao, primary, Ding, Xiaoming, additional, and Helmberger, Don V., additional
- Published
- 2000
- Full Text
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35. PKP differential travel times: Implications for 3‐D lower mantle structure
- Author
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Song, Xiaodong, primary and Helmberger, Don V., additional
- Published
- 1997
- Full Text
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36. anisotropy of Earth's inner core
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Song, Xiaodong, primary and Helmberger, Don V., additional
- Published
- 1993
- Full Text
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37. Effect of velocity structure in D″ on PKP phases
- Author
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Song, Xiaodong, primary and Helmberger, Don V., additional
- Published
- 1993
- Full Text
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38. Retrieval of Moho-reflected shear wave arrivals from ambient seismic noise.
- Author
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Zhongwen Zhan, Ni, Sidao, Helmberger, Don V., and Clayton, Robert W.
- Subjects
SHEAR waves ,SURFACE energy ,SURFACE waves (Fluids) ,GREEN'S functions ,DIFFERENTIAL equations - Abstract
Theoretical studies on ambient seismic noise (ASN) predict that complete Green's function between seismic stations can be retrieved from cross correlation. However, only fundamental mode surface waves emerge in most studies involving real data. Here we show that Moho-reflected body wave ( SmS) and its multiples can be identified with ASN for station pairs near their critical distances in the short period band (1–5 s). We also show that an uneven distribution of noise sources, such as mining activity and wind–topography interaction, can cause surface wave precursors, which mask weaker body wave phases. [ABSTRACT FROM AUTHOR]
- Published
- 2010
- Full Text
- View/download PDF
39. Validating tomographic model with broad-band waveform modelling: an example from the LA RISTRA transect in the southwestern United States.
- Author
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Teh-Ru Alex Song and Helmberger, Don. V.
- Subjects
- *
WAVES (Physics) , *TOMOGRAPHY , *OPTICAL diffraction , *FINITE differences ,RIO Grande Rift - Abstract
Traveltime tomographic models of the LA RISTRA transect produce excellent waveform fits if we amplify the damped images. We observe systematic waveform distortions across the western edge of the Great Plains from South American events, starting about 300 km east of the centre of the Rio Grande Rift. The amplitude decreases by more than 50 per cent within array stations spanning less than 200 km while the pulse width increases by more than a factor of 2. This feature is not observed for the data arriving from the northwest. While the S-wave tomographic image shows a fast slab-like feature dipping to the southeast beneath the western edge of the Great Plains, synthetics generated from this model do not reproduce the waveform characteristics. However, once we modify the tomographic image by amplifying the velocity contrast between the slab and adjoining mantle by a factor of 2–3, the synthetics produce observed amplitude decay and pulse broadening. In addition to the traveltime delay, amplitude variation due to wave phenomena such as slab diffraction, focusing and defocusing provide much tighter constraints on the geometry of the fast anomaly and its amplitude and sharpness as demonstrated by a forward sensitivity test and snapshots of the seismic wavefield. Our preferred model locates the slab 200 km east of the Rio Grande Rift dipping 70°–75° to the southeast, extending to a depth near 600 km with a thickness of 120 km and a velocity of about 4 per cent fast. In short, adding waveform and amplitude components to regional tomographic studies can help validate and establish structural geometry, sharpness and velocity contrast. [ABSTRACT FROM AUTHOR]
- Published
- 2007
- Full Text
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40. Evidence for a chemical-thermal structure at base of mantle from sharp lateral P-wave variations beneath Central America.
- Author
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Xinlei Sun, Xiaodong Song, Sihua Zheng, and Helmberger, Don V.
- Subjects
CORE-mantle boundary ,TOMOGRAPHY ,AZIMUTH ,CHEMICAL structure ,INTERNAL structure of the Earth - Abstract
Compressional waves that sample the lowermost mantle west of Central America show a rapid change in travel times of up to 4 s over a sampling distance of 300 km and a change in waveforms. The differential travel times of the PKP waves (which traverse Earth's core) correlate remarkably well with predictions for S-wave tomography. Our modeling suggests a sharp transition in the lowermost mantle from a broad slow region to a broad fast region with a narrow zone of slowest anomaly next to the boundary beneath the Cocos Plate and the Caribbean Plate. The structure may be the result of ponding of ancient subducted Farallon slabs situated near the edge of a thermal and chemical upwelling. [ABSTRACT FROM AUTHOR]
- Published
- 2007
- Full Text
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41. Three-dimensional structure of the African superplume from waveform modelling.
- Author
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Sidao Ni, Helmberger, Don V., and Tromp, Jeroen
- Subjects
- *
MANTLE plumes , *PLUMES (Fluid dynamics) , *THREE-dimensional imaging , *SEISMIC waves , *GEOLOGICAL modeling , *MID-ocean ridges , *EARTH'S mantle , *EARTHQUAKES - Abstract
Previous 2-D modelling of seismic waveforms and traveltimes has revealed a large-scale ridge-like velocity anomaly beneath Africa, which is usually referred to as the African superplume. The structure starts from the southern Indian Ocean and extends northwestwards into the Atlantic Ocean, with its base on the core–mantle boundary. The structure has relatively sharp lateral boundaries with the shear velocity inside 3 per cent lower than the ambient mantle, while the compressional velocity is almost normal. The 3-D structure is best illustrated by seismic waveforms recorded by the South Africa Array generated by earthquakes in the western Pacific Ocean. The diffractedSphase travels along the axis of the structure for over 60°, with the northern- and southernmost paths sampling its edges. TheSwaveforms are simple but delayed by up to 20 s for paths sampling the middle of the structure, whereas they display two arrivals for paths along the boundaries. These complex waveforms can be explained by 3-D multipathing due to rapid lateral variations in shear-wave velocity along the edges of the structure. These sharp features are confirmed by modelling broadband records associated with the proposed ridge structure with two independent methods: the spectral element method and a ray-based 3-D code (DWKM). [ABSTRACT FROM AUTHOR]
- Published
- 2005
- Full Text
- View/download PDF
42. Low-velocity zone atop the 410-km seismic discontinuity in the northwestern United States.
- Author
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Song, Teh-Ru Alex, Helmberger, Don. V., and Grand, Stephen P.
- Subjects
- *
EARTHQUAKE zones , *SPEED , *EARTH (Planet) , *OLIVINE , *SEISMIC waves - Abstract
The seismic discontinuity at 410?km depth in the Earth's mantle is generally attributed to the phase transition of (Mg,Fe)2SiO4 (refs 1, 2) from the olivine to wadsleyite structure. Variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations. For example, a cold anomaly would elevate the 410-km discontinuity, because of its positive Clapeyron slope, whereas a warm anomaly would depress the discontinuity. But trade-offs between seismic wave-speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities, and structure often appears very complicated. Here we simultaneously model seismic refracted waves and scattered waves from the 410-km discontinuity in the western United States to constrain structure in the region. We find a low-velocity zone, with a shear-wave velocity drop of 5%, on top of the 410-km discontinuity beneath the northwestern United States, extending from southwestern Oregon to the northern Basin and Range province. This low-velocity zone has a thickness that varies from 20 to 90?km with rapid lateral variations. Its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic ‘receiver-function’ observations observed above the region. We interpret the low-velocity zone as a compositional anomaly, possibly due to a dense partial-melt layer, which may be linked to prior subduction of the Farallon plate and back-arc extension. The existence of such a layer could be indicative of high water content in the Earth's transition zone. [ABSTRACT FROM AUTHOR]
- Published
- 2004
- Full Text
- View/download PDF
43. Constructing synthetics from deep earth tomographic models.
- Author
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Sidao Ni, Xiaoming Ding, and Helmberger, Don V.
- Subjects
TOMOGRAPHY ,EARTH (Planet) - Abstract
Examines the construction of two-dimensional synthetics from deep earth tomographic models. Assessment on the heterogeneity of deep mantle structure; Performance of high-resolution waveform modeling; Localization of ray segments.
- Published
- 2000
- Full Text
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44. The effects of tectonic release on short-period P waves from NTS explosions
- Author
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Lay, Thorne, Wallace, Terry C., and Helmberger, Don V.
- Abstract
The first cycle (ab amplitude) of teleseismic short-period P waves from underground nuclear explosions at Pahute Mesa (NTS) show a systematic azimuthal amplitude pattern that can possibly be explained by tectonic release. The amplitudes vary by a factor of three, with diminished amplitudes being recorded at azimuths around N25°E. This azimuthal pattern has a strong sin(2φ) component and is observed, to varying degrees, for 25 Pahute Mesa events, but not for events at other sites within the NTS. Events that are known to have large tectonic release have more pronounced sin(2φ) amplitude variations. A synthesis of long-period body and surface wave investigations of tectonic release for Pahute Mesa events shows that, in general, the nonisotropic radiation is equivalent to nearly vertical, right-lateral strike-slip faulting trending from N20°W to due north. Long-period P waves at upper mantle distances demonstrate that there is a significant high-frequency component to the tectonic release. Using the long-period constraints on orientation, moment, and frequency content of the tectonic release, the expected short-period P wave effects are predicted. For models in which the downgoing P wave from the explosion triggers tectonic release within a few kilometers below the shot point, a factor of 2.5 amplitude variation with azimuth is predicted for the short-period ab amplitudes, with the lowest amplitudes expected near N25°E. Rather subtle azimuthal variations in the waveforms are expected, particularly for downward propagating ruptures, which is consistent with the absence of strong variations in the data. The occurrence of the azimuthal pattern, albeit with varying strength, for all of the Pahute Mesa events suggests a tectonic release model in which the shatterzone surrounding the explosion cavity is extended preferentially downward by driving a distributed network of faults and joints underlying the Mesa several kilometers beneath the surface. In this model, all events could have a component of tectonic release which would reflect the regional stress regime, although there may be slight spatial and temporal variations in the tectonic release contribution. Some events may trigger slip on larger throughgoing faults as well. While it is shown that tectonic release can affect teleseismic short-period signals significantly, and may contribute to the Pahute Mesa amplitude pattern, other possible explanations are considered.
- Published
- 1984
45. A broadband study of the 13 August 1978 Santa Barbara earthquake
- Author
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Wallace, Terry C., Helmberger, Don V., and Ebel, John E.
- Abstract
The Santa Barbara earthquake of 13 August 1978, provides an opportunity to perform a broadband investigation of body waves for a well-recorded, moderate size (M_L = 5.1) event. The long- and short-period teleseismic body waves are modeled in the time domain to construct a source time function which is consistent in the period range of 1 to 20 sec. The long-period records indicate an overall duration of 6 sec while the short-period records reveal the fine-scale character of the slip history consisting of two sharp pulses separated by about 1 sec. The source mechanism determined from this analysis is a moderately dipping (30°NE) thrust with significant left-lateral slip. The moment was determined to be 1.1 × 10^(25) dyne-cm. The earthquake was also reasonably well recorded on accelerographs in the near-field. The modeling of the strong motion displacements was a two step procedure: (1) the displacements were modeled alone, and (2) in an attempt to achieve consistency between the local and far-field time functions, the qualitative features of the teleseismic short-period time function were used to predict the displacements. If the two sources in the short-period time function are allowed to have different mechanisms, the displacements can be modeled quite well. This suggests that the overall faulting process was rough, and the multiple source character suggested at high frequencies is due to high-stress drop asperities. The two sources are modeled as asperities separated by 1.5 km; the first source has a mechanism consistent with the teleseismic solution while the second source is more steeply dipping. The total moment determined from the strong motion data is 3.5 × 10^(24) dyne-cm or one-third the long-period moment. This is consistent with other recent studies which suggest that the high-frequency strong ground motion is controlled by the distribution of asperities even though the sum of their moments may be small compared to the overall moment. This study also shows the importance of teleseismic short periods in predicting the local displacements.
- Published
- 1981
46. Source models and yield-scaling relations for underground nuclear explosions at Amchitka Island
- Author
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Lay, Thorne, Helmberger, Don V., and Harkrider, David G.
- Abstract
Source models are determined for the three underground nuclear explosions at the Amchitka test site using seismic observations in the period range 0.5 to 20.0 sec. Empirical yield-scaling relations are inferred from the source models and compared with the predictions of the Haskell, Mueller-Murphy, and finite difference numerical models. Several recent studies of high-frequency, near-field signals and teleseismic short-period P waves for LONGSHOT, MILROW, and CANNIKIN constrain the source functions at periods of 0.5 to 2.0 sec. Teleseismic pS and Rayleigh wave observations are used to constrain the source functions at longer periods. Using a modified Haskell source time function representation given by ψ(t) = ψ_∞ {1-e^(-kt)[1 + Kt + (Kt)^2/2-B(Kt)^3]}, the data are best-fit if the corner frequency parameter, K, scales as predicted by the Mueller-Murphy model, and if the amount of overshoot in the reduced displacement potential, which is proportional to B, decreases with increasing yield (depth of burial). The latter behavior is opposite to that predicted by the Mueller-Murphy model and follows from the observation that the long-period level of the explosion potential, ψ_∝, increases with yield, W, by ψ_∝ ∝ W^(0.90), or with yield and depth by ψ_∝ ∝ W/h^(1/3). This long-period and overshoot scaling is consistent with that found for some numerical models, and allowing for the depth dependence of the Rayleigh wave excitation, results in the observed M_S versus log(W) slope of ∼1. The decrease in overshoot with increasing depth of burial may be the result of the increase in shear strength with increasing overburden pressure. If yield or depth dependence of the source potential overshoot proves to be a general phenomenon, a possibility supported by a preliminary investigation of Pahute Mesa observations, accurate yield estimation will require broadband seismic data. The source function representation adopted is shown to provide an excellent fit to the rise time of very near-in velocity recordings to the rise time with frequencies of 10 Hz and higher.
- Published
- 1984
47. The structure of the lowermost mantle determined by short-period P-wave amplitudes
- Author
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Ruff, Larry J. and Helmberger, Don V.
- Abstract
The seismic velocities in the D″ region (lowermost 200 km of the mantle) are recognized to be anomalously low, though the details of the velocity structure are not known. The structure of D″ is important, in particular whether a smooth velocity model is appropriate or not. A smooth decrease in the seismic velocities would be consistent with a thermal boundary layer at the base of the mantle. We have used the amplitudes of short-period (T=1 s) P- and diffracted P-waves to investigate the internal structure of D″. A short-period amplitude data set is obtained by using underground nuclear events as sources and applying receiver corrections to the amplitudes. Receiver effects are largely responsible for the factor of ∼8 scatter in the amplitudes of the North American WWSSN stations. Applying receiver corrections reduces the scatter to a factor of ∼2, thereby providing a quantitatively useful amplitude profile into the core shadow. Using Soviet events and North American WWSSN stations, the D″ layer beneath the north polar region is well sampled. The core shadow (at T=1 s) begins sharply at a distance of Δ=95.5 and the slope of the amplitude decay is well defined. Also, the amplitudes decrease slightly from Δ∼87 to ∼90, then increase to Δ∼95. Synthetic seismograms are used to test various earth models, with the important conclusion that the amplitudes from smooth D″ models with a small velocity gradient in D″ decay too slowly in the shadow. This mismatch cannot be satisfactorily explained by random forward scattering or a thin low Q layer within D″. Anelastic calculations show that a thin low Q layer in D″ decreases the amplitudes before the shadow, with little effect on the decay slope in the shadow. All of the features of the observed amplitude profile can be explained as the interference effects of a model that has a low-velocity zone in the upper part of D″ followed by a normal velocity gradient in the lower part of D″. This type of model (POLAR series) also explains the scatter often observed in dT/dΔ beyond Δ∼90. The interference effects and required velocity changes in D″ are small, and long-period amplitudes will respond only to the averaged velocity gradient in D″. The POLAR models imply a compositional and/or phase change at the top of D″. Thus, the preferred seismological model does not allow the D″ region to be interpreted as a single thermal boundary layer between the mantle and core.
- Published
- 1982
- Full Text
- View/download PDF
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