Peter M. Shearer

Global mapping of topography on transition zone velocity discontinuities by stacking SS precursors

We stack long-period, transverse-component seismograms recorded by the Global Digital Seismograph Network (GDSN) (1976–1996), Incorporated Research Institutions for Seismology-International Deployment of Accelerometers (IRIS-IDA) (1988–1996), and Geoscope (1988–1996) networks to map large-scale topography on the 410- and 660-km seismic velocity discontinuities. Underside reflections from these discontinuities arrive as precursors to the SS phase, and their timing can be used to obtain global variations of the depth to the reflectors. We analyze over 13,000 records from events mb>5.5, focal depth <75 km, and range 110° to 180° by picking and aligning on SS, then stacking the records along the theoretical travel time curves for the discontinuity reflections. Separate stacks are obtained for 416 equally spaced caps of 10° radius; clear 410- and 660-km reflections are visible for almost all of the caps while 520-km reflections are seen in about half of the caps. The differential travel times between the precursors and the SS arrival are measured on each stack, with uncertainty estimates obtained using a bootstrap resampling method. We then compute discontinuity depths relative to the isotropic Preliminary Reference Earth Model (PREM) at 40-s period, correcting for surface topography and crustal thickness variations using the CRUST5.0 model of Mooney et al. [1995], and for upper mantle S velocity heterogeneity using model S16B30 of Masters et al. [1996]. The resulting maps of discontinuity topography have more complete coverage than previous studies; observed depths are highly correlated between adjacent caps and appear dominated by large-scale topography variations. The 660-km discontinuity exhibits peak-to-peak topography of about 38 km, with regional depressions that correlate with areas of current and past subduction around the Pacific Ocean. Large-scale topography on the 410-km discontinuity is lower in amplitude and largely uncorrelated with the topography on the 660-km interface. The width of the transition zone, WTZ, as measured by the separation between the 410- and 660-km discontinuities, appears thickest in areas of active subduction (e.g., Kurils, Philippines, and Tonga) and thins beneath Antarctica and much of the central Pacific Ocean. Spatial variations in WTZ appear unrelated to ocean-continent differences but do roughly correlate with the S16B30 velocities in the transition zone, consistent with a common thermal origin for both patterns. The lower-amplitude 520-km reflector is more difficult to resolve but appears to be a global feature as it is observed preferentially for those bounce point caps with the most data.

A New Method for Determining First-Motion Focal Mechanisms

We introduce a new method for determining earthquake focal mecha- nisms from P-wave first-motion polarities. Our technique differs from previous meth- ods in that it accounts for possible errors in the assumed earthquake location and seismic-velocity model, as well as in the polarity observations. The set of acceptable focal mechanisms, allowing for the expected errors in polarities and takeoff angles, is found for each event. Multiple trials are performed with different source locations and velocity models, and mechanisms with up to a specified fraction of misfit po- larities are included in the set of acceptable mechanisms. The average of the set is returned as the preferred mechanism, and the uncertainty is represented by the dis- tribution of acceptable mechanisms. The solution is considered adequately stable only if the set of acceptable mechanisms is tightly clustered around the preferred mechanism. We validate the method by demonstrating that the well-constrained mechanisms found for clusters of closely spaced events with similar waveforms are indeed very similar. Tests on noisy synthetic data, which mimic the event and station coverage of real data, show that the method accurately recovers the mechanisms and that the uncertainty estimates are reasonable. We also investigate the sensitivity of focal mechanisms to changes in polarities, event depth, and seismic-velocity model, and we find that mechanisms are most sensitive to changes in the vertical velocity gradient.

Improving local earthquake locations using the L1 norm and waveform cross correlation: Application to the Whittier Narrows, California, aftershock sequence

Experiments with different earthquake location methods applied to aftershocks of the October 1, 1987, Whittier Narrows earthquake in California (ML=5.9) suggest that local event locations can be greatly improved through the use of the L1 norm, station corrections and waveform cross correlation. The Whittier Narrows sequence is a compact cluster of over 500 events at 12 to 18 km depth located within the dense station coverage of the Southern California Seismic Network (SCSN), a telemetered network of several hundred short-period seismographs. SCSN travel time picks and waveforms obtained through the Southern California Earthquake Center are examined for 589 earthquakes between 1981 and 1994 in the vicinity of the mainshock. Using a smoothed version of the standard southern California velocity model and the existing travel time picks, improved location accuracy is obtained through use of the L1 norm rather than the conventional least squares (L2 norm) approach, presumably due to the more robust response of the former to outliers in the data. A large additional improvement results from the use of station terms to account for three-dimensional velocity structure outside of the event cluster. To achieve greater location accuracy, waveforms for these events are resampled and low-pass filtered, and the P and S wave cross-correlation functions are computed at each station for every event pair. For those events with similar waveforms, differential times may be obtained from the cross-correlation functions. These times are then combined with the travel time picks to invert for an adjusted set of picks that are more consistent than the original picks and include seismograms that were originally unpicked. Locations obtained from the adjusted picks show a further improvement in accuracy. Location uncertainties are estimated using a bootstrap technique in which events are relocated many times for sets of picks in which the travel time residuals at the best fitting location are used to randomly perturb each pick. Improvements in location accuracy are indicated by the reduced scatter in the residuals, smaller estimated location errors, and the increased tendency of the locations to cluster along well-defined fault planes. Median standard errors for the final inversion are 150 m in horizontal location and 230 m in vertical location, although the relative locations within localized clusters of similar events are better constrained. Seismicity cross sections resolve the shallow dipping fault plane associated with the mainshock and a steeply dipping fault plane associated with a ML=5.3 aftershock. These fault planes appear to cross, and activity began on the secondary fault plane prior to the large aftershock.

A Global View of the Lithosphere-Asthenosphere Boundary

Seismic imaging reveals a global velocity anomaly at depths of 70 to 100 kilometers that may be from some melt or a strong fabric in Earth’s mantle. Boundary Issues of the Lithosphere The depth of Earths tectonic plates is defined by the lithosphere-aesthenosphere boundary (LAB), but its seismic signature is more subtle compared with other deeper boundaries within Earth (see the Perspective by Romanowicz). Under oceanic plates, the LAB is often defined by where temperatures are hot enough to cause some melting. This boundary has been hard to detect in older oceanic plates, but it is important for understanding how these plates thicken with age or distance from ocean ridges, and for assessing heat flow through the oceanic crust. Kawakatsu et al. (p. 499) use a detailed seismic array to detect a seismic velocity reduction beneath the Philippine Sea and Pacific plates. The data imply that 5%, or less, melt forms horizontal layers, and that oceanic plate thicknesses do indeed deepen with age. Rychert and Shearer (p. 495) used 15 years of seismic data to explore the global distribution of an anomaly imaged by conversion of pressure waves to shear waves (waves associated with a sharp velocity drop). The data reveal a broad signal at depths of 70 kilometers (km) beneath ocean islands to 95 km beneath Precambrian shields. It is not clear whether this boundary is the lithosphere-aesthenosphere boundary or a layer with a distinct horizontal fabric. The lithosphere-asthenosphere boundary divides the rigid lid from the weaker mantle and is fundamental in plate tectonics. However, its depth and defining mechanism are not well known. We analyzed 15 years of global seismic data using P-to-S (Ps) converted phases and imaged an interface that correlates with tectonic environment, varying from 95 ± 4 kilometers beneath Precambrian shields and platforms to 81 ± 2 kilometers beneath tectonically altered regions and 70 ± 4 kilometers at oceanic island stations. High-frequency Ps observations require a sharp discontinuity; therefore, this interface likely represents a boundary in composition, melting, or anisotropy, not temperature alone. It likely represents the lithosphere-asthenosphere boundary under oceans and tectonically altered regions, but it may constitute another boundary in cratonic regions where the lithosphere-asthenosphere boundary is thought to be much deeper.

Constraints on upper mantle discontinuities from observations of long‐period reflected and converted phases

Stacked images combining over 5 years of long-period Global Digital Seismograph Network data reveal many phases associated with reflections and conversions from upper mantle discontinuities. These images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking. Results obtained for P, S, SS, and PP reference arrivals resolve numerous phases from discontinuities near 410 and 660 km, while some of the stacks also show evidence for a weaker discontinuity near 520 km. Phases of particular interest include P and SH multiples resulting from topside reflections, precursors to SS from underside reflections, and P-to-SV converted phases. These phases can be clearly seen both in the waveform stacks and in cross-correlation analysis of individual seismograms. Travel times for these arrivals are converted to discontinuity depths relative to velocities in the Preliminary Reference Earth Model, and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle. Average apparent depths to the discontinuities for the different phases agree to within ±3 km for the 410-km discontinuity, to within ±4 km for the 520-km discontinuity, and to within ±8 km for the 660-km discontinuity. The best global averages are obtained from the SS precursor data which indicate discontinuities at 415, 519, and 659 km. Discontinuity depths obtained from the P-to-SV converted phases at over 35 individual seismic stations exhibit variations of less than ±20 km. Apparent depths to the 660-km discontinuity consistently show greater variability than depths to the 410-km discontinuity, supporting recent laboratory results which indicate that the Clapeyron slope for the 660-km discontinuity is significantly larger in magnitude than the slope for the 410-km discontinuity. Precursors to SS (seen between 110° and 180°) are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point Apparent discontinuity depths computed from SS precursors for different tectonic regions agree to within about ±5 km. The SS precursors have especially good coverage near the subducting slabs in the northwest Pacific. Analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000- to 1500-km-wide region near the slab in which the 660-km discontinuity is depressed by about 20 km. Measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking. Relative amplitude analysis suggests that the P and S wave impedance changes at 410 km are about 0.8–1.1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0.3 and 0.6 of the changes at 410 km.

Waveform Relocated Earthquake Catalog for Southern California (1981 to June 2011)

We determine a new relocated catalog, HYS_catalog_2011, for southern California from 1981 through June 2011. About 75.3% of the hypocenters are calculated with absolute and differential travel‐time picks, and 24.7% could be relocated only by using absolute travel‐time picks with 3D or 1D velocity models. The total catalog consists of more than 502,000 earthquakes in the region extending from Baja California in the south to Coalinga and Owens Valley in the north. The catalog consists of three M 7.1, M 7.2, and M 7.3 mainshocks; their foreshocks and aftershocks; and background seismicity caused by tectonic and other processes in the southern California crust. Hypocenters in the new relocated catalog exhibit tighter spatial clustering of seismicity than does the routinely generated catalog, and the depth distribution is tighter and reflects the thickness of the seismogenic zone more accurately. Compared to the standard catalog, the relocated hypocenters are more easily related to other data sets, such as mapped late Quaternary faults.

Comprehensive analysis of earthquake source spectra in southern California

We compute and analyze P wave spectra from earthquakes in southern California between 1989 and 2001 using a method that isolates source-, receiver-, and path-dependent terms. We correct observed source spectra for attenuation using both fixed and spatially varying empirical Greens function methods. Estimated Brune-type stress drops for over 60,000 M_L = 1.5 to 3.1 earthquakes range from 0.2 to 20 MPa with no dependence on moment or local b value. Median computed stress drop increases with depth in the upper crust, from about 0.6 MPa at the surface to about 2.2 MPa at 8 km, where it levels off and remains nearly constant in the midcrust down to about 20 km. However, the results at shallow depths could also be explained as reduced rupture velocities near the surface rather than a change in stress drop. Spatially coherent variations in median stress drop are observed, with generally low values for the Imperial Valley and Northridge aftershocks and higher values for the eastern Transverse ranges and the north end of the San Jacinto fault. We find no correlation between observed stress drop and distance from the San Andreas and other major faults. Significant along-strike variations in stress drop exist for aftershocks of the 1992 Landers earthquake, which may correlate with differences in main shock slip.

Southern California Hypocenter Relocation with Waveform Cross-Correlation, Part 2: Results Using Source-Specific Station Terms and Cluster Analysis

We obtain precise relative relocations for more than 340,000 southern California earthquakes between 1984 and 2002 by applying the source-specific station-term (SSST) method to existing P- and S-phase picks and a differential lo- cation method to about 208,000 events within similar-event clusters identified with waveform cross-correlation. The entire catalog is first relocated by using existing phase picks, a reference 1D velocity model, and the SSST method of Richards-Dinger and Shearer (2000). We also perform separate relocations of Imperial Valley events by using a velocity model more suited to this region. Next, we apply cluster analysis to the waveform cross-correlation output to identify similar-event clusters. We re- locate earthquakes within each similar-event cluster by using the differential times alone, keeping the cluster centroid fixed to its initial SSST location. We estimate standard errors for the relative locations from the internal consistency of differential locations between individual event pairs; these errors are often as small as tens of meters. In many cases the relocated events within each similar-event cluster align in planar features suggestive of faults. We observe a surprising number of such faults at small scales that strike nearly perpendicular to the main seismicity trends. In general, the fine-scale details of the seismicity reveal a great deal of structural com- plexity in southern California fault systems.

Using S/P Amplitude Ratios to Constrain the Focal Mechanisms of Small Earthquakes

We test whether S-wave/P-wave amplitude ratio data can improve the computed focal mechanisms of small earthquakes, using events from two southern California aftershock sequences. The observed S/P ratios are generally consistent with the expected mechanisms, implying that S/P ratios can in fact be useful in constraining the focal mechanisms of small events. However, we also find that noise in the observations leads to scatter in the S/P ratios of factors of 2, and sometimes higher. This scatter limits the usefulness of the S/P ratios in two ways: (1) the focal mechanism cannot simply be fit to S/P amplitude data alone without accounting for the noise in a more sophisticated focal mechanism inversion process; (2) while the amplitude ratios may improve poorly constrained mechanisms, they are less useful in refining solutions that are already relatively well constrained.

Applying a three-dimensional velocity model, waveform cross correlation, and cluster analysis to locate southern California seismicity from 1981 to 2005

We compute high-precision earthquake locations using southern California pick and waveform data from 1981 to 2005. Our latest results are significantly improved compared to our previous catalog by the following: (1) We locate events with respect to a new crustal P and S velocity model using three-dimensional ray tracing, (2) we examine six more years of waveform data and compute cross-correlation results for many more pairs than our last analysis, and (3) we compute locations within similar event clusters using a new method that applies a robust fitting method to obtain the best locations satisfying all the differential time constraints from the waveform cross correlation. These results build on the relocated catalogs of Hauksson and Shearer (2005) and Shearer et al. (2005) and provide additional insight regarding the fine-scale fault structure in southern California and the relationship between the San Andreas Fault (SAF) and nearby seismicity. In particular, we present results for two regions in which the seismicity near the southern SAF seems to align on dipping faults.