Export 10 results:
Sort by: Author [ Title  (Desc)] Type Year
A B C D E F G H I J K L M N O P Q R S [T] U V W X Y Z   [Show ALL]
Shearer, PM.  1996.  Transition zone velocity gradients and the 520-km discontinuity. Journal of Geophysical Research-Solid Earth. 101:3053-3066.   10.1029/95jb02812   AbstractWebsite

Stacks of long-period Global Digital Seismograph Network (GDSN) seismograms at 110 degrees to 180 degrees epicentral distance reveal precursors to SS that result from underside reflections off upper mantle seismic discontinuities. The 410- and 660-km discontinuities are obvious in these stacks, but identification and modeling of other transition zone discontinuities are complicated by sidelobes from the 410- and 660-km reflections. These sidelobes result from the limited bandwidth of the GDSN instrument responses and the effect of crustal reverberations on the SS reference phase. The crustal effects can be minimized by restricting the records to oceanic bounce points where the NG-km-thick crust has little effect on the long-period waveforms. Over 2000 long-period, transverse-component seismograms with oceanic SS bounce points recorded by the GDSN from 1976 to 1991 are manually edited, aligned on SS, and then stacked using a new procedure that weights the records by data quality. The resulting image shows a clear reflection from a 520-km discontinuity that cannot be explained as a sidelobe artifact, confirming earlier results of Shearer [1990, 1991] and Revenaugh and Jordan. [1991]. By stacking along the expected travel time curves for discontinuity phases, the time versus range image of the precursor wave field is reduced to a single trace that measures upper mantle reflectivity versus time. The features in this reflectivity profile are sensitive to the brightness and depth of the transition zone discontinuities and to the steepness of the velocity gradients between the interfaces. Using geometrical ray theory and assuming a constant velocity versus density scaling relationship, I fit this reflectivity profile with velocity models of the upper mantle using both forward modeling and direct inversion. The inverse problem. is addressed by performing a deconvolution of the profile with the SS reference phase (after a correction for attenuation), followed by a direct mapping of reflectivity versus time into velocity versus depth. Velocity-depth profiles resulting from these procedures are roughly in agreement with standard upper mantle velocity models, except that the SS precursor data require a minor discontinuity near 520 km and a steeper gradient just below the 660-km discontinuity. Estimated discontinuity shear impedance changes are 6.7 +/- 0.1% at 420 km, 2.9 +/- 0.7% at 519 km, and 9.9 +/- 1.5% at 663 km. The impedance change near 520 km is consistent with current mineral physics results for the olivine beta to gamma phase change and places constraints on the fraction of olivine in the transition zone.

Minster, BJ, Day SM, Shearer PM.  1991.  The transition to the elastic regime in the vicinity of an underground explosion. Explosion Source Phenomenology. :229–238.: American Geophysical Union Abstract
Flanagan, MP, Shearer PM.  1998.  Topography on the 410-km seismic velocity discontinuity near subduction zones from stacking of sS, sP, and pP precursors. Journal of Geophysical Research-Solid Earth. 103:21165-21182.   10.1029/98jb00595   AbstractWebsite

We stack the teleseismic depth phases sS, sP, and pP produced by deep focus earthquakes to image precursory arrivals that result from near-source, underside reflections off the 410-km seismic velocity discontinuity (hereinafter referred to as the 410) and use differential time measurements between these phases and their precursors to compute discontinuity depths near seven subduction zones around the Pacific Ocean margin. We begin by selecting seismograms with high-quality depth phase arrivals recorded by, several long-period networks between the years 1976 and 1996. Filtering the waveforms and stacking them along theoretical travel-time curves reveals dear precursors which vary in shape and timing. We compute confidence levels to evaluate the reliability of the observed precursory features using a bootstrap method that randomly resamples the seismograms prior to stacking. We measure the differential travel time between the reference pulse and the precursor using a cross-correlation technique and convert this time to an apparent discontinuity depth using the isotropic Preliminary Reference Earth Model (PREM) at 25-s period corrected to an oceanic crustal thickness. The lateral resolution of our long-period stacks for 410 topography is limited compared to that sometimes achieved in short-period analyses but is much higher than that obtained from global SS precursor studies. For most subduction zones the results indicate little change in the average depth to the 410-km discontinuity in the local areas sampled by the precursor bounce points compared td broad regional depths inferred from SS precursor results. This implies that any large variations in depth to the 410-km discontinuity neat subduction zones are limited to a narrow zone within the slab itself where they may be difficult to resolve with long-period data. Coverage for the Tonga and Peru-Chile subduction zones is sufficiently dense that we can observe lateral variations in 410 depths. In Tonga the results suggest depth variations perpendicular to the slab of up to 33 km, after correcting for probable lateral heterogeneity in velocity above 400 km depth, and variations parallel to the Slab orientation as large as 13 km. The cross-slab variation is consistent with the elevation of olivine phase transformations in cold regions; the variation along strike suggests a more complex thermal heterogeneity that may be related to the subduction history of this region. We see evidence for additional reflectors above the 410 in some of the waveform stacks, but the inconsistency and weak amplitude of these features preclude definitive interpretations.

Lin, GQ, Shearer PM, Matoza RS, Okubo PG, Amelung F.  2014.  Three-dimensional seismic velocity structure of Mauna Loa and Kilauea volcanoes in Hawaii from local seismic tomography. Journal of Geophysical Research-Solid Earth. 119:4377-4392.   10.1002/2013jb010820   AbstractWebsite

We present a new three-dimensional seismic velocity model of the crustal and upper mantle structure for Mauna Loa and Kilauea volcanoes in Hawaii. Our model is derived from the first-arrival times of the compressional and shear waves from about 53,000 events on and near the Island of Hawaii between 1992 and 2009 recorded by the Hawaiian Volcano Observatory stations. The V-p model generally agrees with previous studies, showing high-velocity anomalies near the calderas and rift zones and low-velocity anomalies in the fault systems. The most significant difference from previous models is in V-p/V-s structure. The high-V-p and high-V-p/V-s anomalies below Mauna Loa caldera are interpreted as mafic magmatic cumulates. The observed low-V-p and high-V-p/V-s bodies in the Kaoiki seismic zone between 5 and 15 km depth are attributed to the underlying volcaniclastic sediments. The high-V-p and moderate- to low-V-p/V-s anomalies beneath Kilauea caldera can be explained by a combination of different mafic compositions, likely to be olivine-rich gabbro and dunite. The systematically low-V-p and low-V-p/V-s bodies in the southeast flank of Kilauea may be caused by the presence of volatiles. Another difference between this study and previous ones is the improved V-p model resolution in deeper layers, owing to the inclusion of events with large epicentral distances. The new velocity model is used to relocate the seismicity of Mauna Loa and Kilauea for improved absolute locations and ultimately to develop a high-precision earthquake catalog using waveform cross-correlation data.

Lin, G, Shearer PM, Hauksson E, Thurber CH.  2007.  A three-dimensional crustal seismic velocity model for southern California from a composite event method. Journal of Geophysical Research-Solid Earth. 112   10.1029/2007jb004977   AbstractWebsite

[1] We present a new crustal seismic velocity model for southern California derived from P and S arrival times from local earthquakes and explosions. To reduce the volume of data and ensure a more uniform source distribution, we compute "composite event" picks for 2597 distributed master events that include pick information for other events within spheres of 2 km radius. The approach reduces random picking error and maximizes the number of S wave picks. To constrain absolute event locations and shallow velocity structure, we also use times from controlled sources, including both refraction shots and quarries. We implement the SIMULPS tomography algorithm to obtain three-dimensional dimensional (3-D) V-p and V-p/V-s structure and hypocenter locations of the composite events. Our new velocity model in general agrees with previous studies, resolving low-velocity features at shallow depths in the basins and some high-velocity features in the midcrust. Using our velocity model and 3-D ray tracing, we relocate about 450,000 earthquakes from 1981 to 2005. We observe a weak correlation between seismic velocities and earthquake occurrence, with shallow earthquakes mostly occurring in high P velocity regions and midcrustal earthquakes occurring in low P velocity regions. In addition, most seismicity occurs in regions with relatively low V-p/V-s ratios, although aftershock sequences following large earthquakes are often an exception to this pattern.

Lin, GQ, Shearer P.  2005.  Tests of relative earthquake location techniques using synthetic data. Journal of Geophysical Research-Solid Earth. 110   10.1029/2004jb003380   AbstractWebsite

We compare three relative earthquake location techniques using tests on synthetic data that simulate many of the statistical properties of real travel time data. The methods are (1) the hypocentroidal decomposition method of Jordan and Sverdrup (1981), (2) the source-specific station term method (SSST) of Richards-Dinger and Shearer (2000), and (3) the modified double-difference method (DD) of Waldhauser and Ellsworth (2000). We generate a set of synthetic earthquakes, stations, and arrival time picks in half-space velocity models. We simulate the effect of travel time variations caused by random picking errors, station terms, and general three-dimensional velocity structure. We implement the algorithms with a common linearized approach and solve the systems using a conjugate gradient method. We constrain the mean location shift to be zero for the hypocentroidal decomposition and double-difference locations. For a single compact cluster of events, these three methods yield very similar improvements in relative location accuracy. For distributed seismicity, the DD and SSST algorithms both provide improved relative locations of comparable accuracy. We also present a new location technique, termed the shrinking box SSST method, which provides some improvement in absolute location accuracy compared to the SSST method. In our implementation of these algorithms, the SSST method runs significantly faster than the DD method.

Sumiejski, LE, Shearer PM.  2012.  Temporal Stability of Coda Q(-1) in Southern California. Bulletin of the Seismological Society of America. 102:873-877.   10.1785/0120110181   AbstractWebsite

Some studies of coda Q(-1) have found temporal changes that may be associated with earthquake activity, but these analyses are subject to biases due to differences in source locations and other nonstationary behavior in earthquake catalogs. These biases can be greatly reduced by using clusters of repeating earthquakes; studies using this approach have generally found no resolvable changes in coda Q(-1). We examine coda Q(-1) variations across southern California using 22 similar event clusters identified from a recent large-scale waveform cross-correlation project to improve earthquake locations. These clusters are found across the region and span the time period between 1981 and 2005. We apply the method of Beroza et al. (1995) to compute differential coda Q(-1) using waveforms from similar earthquake pairs and analyze the results to constrain any possible temporal variations. Results from individual event pairs show a great deal of scatter in differential coda Q(-1), but exhibit no clear temporal variations or changes associated with large earthquakes. Application of a median filter to smooth the results shows that any persistent large-scale changes in coda Q(-1) during this time period are less than about 30%.

Bulow, RC, Johnson CL, Bills BG, Shearer PM.  2007.  Temporal and spatial properties of some deep moonquake clusters. Journal of Geophysical Research-Planets. 112   10.1029/2006je002847   AbstractWebsite

Using the event search method of Bulow et al. ( 2005), we have found 503 new deep moonquakes among the eight largest ( in terms of total number) nearside source regions, increasing the number of identified events for each cluster an average of 36% over the existing catalog. These new events provide an improved deep event catalog, with which we explore some temporal and spatial aspects of deep moonquakes. First, we examine the spectra of moonquake occurrence times at each deep source region, and observe known tidal periodicities, notably those at similar to 27 days and 206 days. Application of spectral methods for the analyses of point processes ( discrete events) allows us to resolve closely spaced tidal periods not previously seen in moonquake data. Second, we pick seismic phase arrival times from optimized stacks of events from each source region. We use these picks, along with published velocity models, to relocate the nine source regions. Source regions A1 and A18 are the best located, with 95% confidence bounds of less than +/- 5 degrees in latitude and longitude, and consistent with estimates from different studies. The locations of source regions A8 and A9 are poorly constrained, with uncertainties in latitude of up to +/- 28 degrees resulting from the absence of clear phase arrivals at station 15. Large trade-offs exist between relocation estimates and choice of velocity model, and the lack of reliable seismic phase arrivals severely affects location error.

Ishii, M, Shearer PM, Houston H, Vidale JE.  2007.  Teleseismic P wave imaging of the 26 December 2004 Sumatra-Andaman and 28 March 2005 Sumatra earthquake ruptures using the Hi-net array. Journal of Geophysical Research-Solid Earth. 112   10.1029/2006jb004700   AbstractWebsite

Seismograms from a dense, high-quality seismic network in Japan are used to investigate the characteristics of the 26 December 2004 Sumatra-Andaman and the 28 March 2005 Sumatran earthquakes. The onset of the P waveforms are aligned through cross correlation, and a simple concept of back-projecting seismic energy to a grid of potential source locations is applied. The waveform alignment removes the effects due to lateral variations in wave speed between the hypocenter and each station. To better approximate the effects of three-dimensional heterogeneity for paths originating from grid points away from the hypocenter, cross-correlation results of the P waveforms from aftershocks are introduced. This additional information leads to improved resolution of smaller-scale features near many of the aftershocks by reducing wavefront distortion. The back-projection analysis provides a quick assessment of the spatiotemporal extent and variability of relative high-frequency energy release, which can be translated into an estimate of the moment magnitude, as well as an unparalleled view of high-frequency rupture propagation. The results are, in general, consistent with those obtained from more involved source inversion methods. The 2004 Sumatra-Andaman earthquake released most energy in a region northwest of the Sumatra island and the rupture extended to the northern Andaman islands, about 1300 km from the epicenter. This northern portion of the rupture radiated a considerable amount of energy, but there is little evidence of slow slip. The 2005 event is imaged to have bilateral rupture with northwestern slip occurring for about 50 s before it moved to the southeast of the epicenter.

Buehler, JS, Shearer PM.  2015.  T phase observations in global seismogram stacks. Geophysical Research Letters. 42:6607-6613.   10.1002/2015gl064721   AbstractWebsite

The T phase, conversion of acoustic to seismic energy, is typically observed as a high-frequency wave train at hydrophones or coastal seismic stations. Here we show that the T phase can be observed in broadband waveform stacks of similar to 5200 earthquakes recorded by the Global Seismic Network. To enhance the phase arrivals in stacks, we apply short-time window average over long-time window average filtering to individual traces before stacking. Although the T phase arrival is visible in stacks from seismograms filtered at 0.5-5 Hz, it appears much stronger at higher frequencies (2-8 Hz) and is further enhanced by only stacking seismograms from oceanic paths. Stacking only subsets of the data depending on continental path lengths on the receiver side shows that the T phase can be observed at stations up to 4 degrees inland from the coast, and changes in the T phase arrival time correspond to reasonable crustal velocities.