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Fan, WY, Shearer PM.  2017.  Investigation of Backprojection Uncertainties With M6 Earthquakes. Journal of Geophysical Research-Solid Earth. 122:7966-7986.   10.1002/2017jb014495   AbstractWebsite

We investigate possible biasing effects of inaccurate timing corrections on teleseismic P wave backprojection imaging of large earthquake ruptures. These errors occur because empirically estimated time shifts based on aligning P wave first arrivals are exact only at the hypocenter and provide approximate corrections for other parts of the rupture. Using the Japan subduction zone as a test region, we analyze 46 M6-M7 earthquakes over a 10year period, including many aftershocks of the 2011 M9 Tohoku earthquake, performing waveform cross correlation of their initial P wave arrivals to obtain hypocenter timing corrections to global seismic stations. We then compare backprojection images for each earthquake using its own timing corrections with those obtained using the time corrections from other earthquakes. This provides a measure of how well subevents can be resolved with backprojection of a large rupture as a function of distance from the hypocenter. Our results show that backprojection is generally very robust and that the median subevent location error is about 25km across the entire study region (approximate to 700km). The backprojection coherence loss and location errors do not noticeably converge to zero even when the event pairs are very close (<20km). This indicates that most of the timing differences are due to 3-D structure close to each of the hypocenter regions, which limits the effectiveness of attempts to refine backprojection images using aftershock calibration, at least in this region.

Lin, GQ, Amelung F, Shearer PM, Okubo PG.  2015.  Location and size of the shallow magma reservoir beneath Klauea caldera, constraints from near-source V-p/V-s ratios. Geophysical Research Letters. 42:8349-8357.   10.1002/2015gl065802   AbstractWebsite

We present high-resolution compressional wave to shear wave velocity ratios (V-p/V-s) beneath Klauea's summit caldera by applying an in situ estimation method using waveform cross-correlation data for three similar earthquake clusters. We observe high V-p/V-s ratios (1.832 and 1.852) for two event clusters surrounded by the low background V-p/V-s value of 1.412 at similar to 2.1km depth below the surface. These high and low V-p/V-s ratios can be explained by melt- and CO2-filled cracks, respectively, based on a theoretical crack model. The event cluster with the highest V-p/V-s ratio consists of long-period events that followed the 1997 East Rift Zone eruption, indicating their association with fluid and magma movement. The depths of the two clusters with high V-p/V-s ratios are consistent with the magma reservoir location inferred from geodetic observations. Their locations east and north of Halemaumau crater suggest a horizontal extent of a few kilometers for the reservoir.

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.

Kaneko, Y, Shearer PM.  2015.  Variability of seismic source spectra, estimated stress drop, and radiated energy, derived fromcohesive-zonemodels of symmetrical and asymmetrical circular and elliptical ruptures. Journal of Geophysical Research-Solid Earth. 120:1053-1079.   10.1002/2014jb011642   AbstractWebsite

Large variability of earthquake stress drops and scaled energy has been commonly reported in the literature, but it is difficult to assess how much of this variability is caused by underlying physical source processes rather than simply observational uncertainties. Here we examine a variety of dynamically realistic rupture scenarios for circular and elliptical faults and investigate to what extent the variability in seismically estimated stress drops and scaled energy comes from differences in source geometry, rupture directivity, and rupture speeds. We numerically simulate earthquake source scenarios using a cohesive-zone model with the small-scale yielding limit, where the solution approaches a singular crack model with spontaneous healing of slip. Compared to symmetrical circular source models, asymmetrical models result in larger variability of estimated corner frequencies and scaled energy over the focal sphere. The general behavior of the spherical averages of corner frequencies and scaled energy in the subshear regime extends to the supershear regime, although shear Mach waves generated by the propagation of supershear rupture lead to much higher corner frequency and scaled energy estimates locally. Our results suggest that at least a factor of 2 difference in the spherical average of corner frequencies is expected in observational studies simply from variability in source characteristics almost independent of the actual stress drops, translating into a factor of 8 difference in estimated stress drops. Furthermore, radiation efficiency estimates derived from observed seismic spectra should not be directly interpreted as describing rupture properties unless there are independent constraints on rupture speed and geometry.

Fan, WY, Shearer PM, Gerstoft P.  2014.  Kinematic earthquake rupture inversion in the frequency domain. Geophysical Journal International. 199:1138-1160.   10.1093/gji/ggu319   AbstractWebsite

We develop a frequency-based approach to earthquake slip inversion that requires no prior information on the rupture velocity or slip-rate functions. Because the inversion is linear and is performed separately at each frequency, it is computationally efficient and suited to imaging the finest resolvable spatial details of rupture. We demonstrate the approach on synthetic seismograms based on the Source Inversion Validation Exercise 1 (SIV1) of a crustal M-w 6.6 strike-slip earthquake recorded locally. A robust inversion approach is obtained by applying a combination of damping, smoothing and forcing zero slip at the edge of the fault model. This approach achieves reasonable data fits, overall agreement to the SIV1 model, including slip-rate functions of each subfault, from which its total slip, slip time history and rupture velocity can be extracted. We demonstrate the method's robustness by exploring the effects of noise, random timing errors, and fault geometry errors. The worst effects on the inversion are seen from errors in the assumed fault geometry.

Kaneko, Y, Shearer PM.  2014.  Seismic source spectra and estimated stress drop derived from cohesive-zone models of circular subshear rupture. Geophysical Journal International. 197:1002-1015.   10.1093/gji/ggu030   AbstractWebsite

Earthquake stress drops are often estimated from far-field body wave spectra using measurements of seismic moment, corner frequency and a specific theoretical model of rupture behaviour. The most widely used model is from Madariaga in 1976, who performed finite-difference calculations for a singular crack radially expanding at a constant speed and showed that (f) over bar (c) = k beta/a, where (f) over bar (c) is spherically averaged corner frequency, beta is the shear wave speed, a is the radius of the circular source and k = 0.32 and 0.21 for P and S waves, respectively, assuming the rupture speed V-r = 0.9 beta. Since stress in the Madariaga model is singular at the rupture front, the finite mesh size and smoothing procedures may have affected the resulting corner frequencies. Here, we investigate the behaviour of source spectra derived from dynamic models of a radially expanding rupture on a circular fault with a cohesive zone that prevents a stress singularity at the rupture front. We find that in the small-scale yielding limit where the cohesive-zone size becomes much smaller than the source dimension, P- and S-wave corner frequencies of far-field body wave spectra are systematically larger than those predicted by the Madariaga model. In particular, the model with rupture speed V-r = 0.9 beta shows that k = 0.38 for P waves and k = 0.26 for S waves, which are 19 and 24 per cent larger, respectively, than those of Madariaga. Thus for these ruptures, the application of the Madariaga model overestimates stress drops by a factor of 1.7. In addition, the large dependence of corner frequency on take-off angle relative to the source suggests that measurements from a small number of seismic stations are unlikely to produce unbiased estimates of spherically averaged corner frequency.

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.

Buehler, JS, Shearer PM.  2014.  Anisotropy and Vp/Vs in the uppermost mantle beneath the western United States fromjoint analysis of Pn and Sn phases. Journal of Geophysical Research-Solid Earth. 119:1200-1219.   10.1002/2013jb010559   AbstractWebsite

Pn and Sn phases are valuable for resolving velocity structure in the mantle lid, as they propagate horizontally right below the Moho. Relatively few Sn tomography attempts have been made compared to Pn, because Sn is often highly attenuated or buried in P wave coda. USArray has greatly increased data coverage for regional phases, and both Pn and Sn are routinely picked by network analysts. Here we jointly invert Pn and Sn arrival time residuals with a modified time-term analysis and a regularized tomography method and present new maps of crustal thickness, uppermost mantle P velocity perturbations, Vp/Vs ratios, and azimuthal anisotropy strength and orientation beneath the western United States. The results indicate partially molten mantle below the Snake River Plain and the Colorado Plateau. The seismic structure of the top approximate to 40 km of the mantle below the Colorado Plateau differs from that seen at greater depths in other studies, such as surface wave or teleseismic body wave tomography, whereas the Snake River Plain anomaly just below the Moho is comparable to structures seen at about approximate to 200 km depth. Pn fast axes provide complementary information to SKS shear wave splitting observations, and our analysis indicates that in several regions in the western United States the orientation of azimuthal anisotropy changes with depth in the upper mantle. However, we have so far been unable to resolve shear wave splitting directly in Sn waveforms, which seem to be dominated by Sn-SV energy.

Kane, DL, Shearer PM, Goertz-Allmann BP, Vernon FL.  2013.  Rupture directivity of small earthquakes at Parkfield. Journal of Geophysical Research-Solid Earth. 118:212-221.   10.1029/2012jb009675   AbstractWebsite

Theoretical modeling of strike-slip ruptures along a bimaterial interface suggests that earthquakes initiating on the interface will have a preferred rupture direction. We test this model with 450 small earthquakes (2 < M < 5) from Parkfield, California, to look for evidence of consistent rupture directivity along the San Andreas Fault. We analyze azimuthal variations in earthquake source spectra after applying an iterative correction for wave propagation effects. Our approach avoids directly modeling source spectra because these models generally assume symmetric rupture; instead, we look for azimuthal variations in the amplitudes of the source spectra over specified frequency bands. Our overall results show similar proportions of events exhibiting characteristics of rupture directivity toward either the southeast or northwest. However, the proportion of events with southeast rupture directivity increases as we limit the data set to larger magnitudes, with 70% of the 46 events M > 3 exhibiting southeast rupture characteristics. Some spatial and temporal variability in rupture directivity is also apparent. We observe a higher proportion of northwest directivity ruptures following the 2004 M 6 Parkfield earthquake, which ruptured toward the northwest. Our results are generally consistent with the preferred southeast rupture directivity model but suggest that directivity is likely due to several contributing factors. Citation: Kane, D. L., P. M. Shearer, B. P. Goertz-Allmann, and F. L. Vernon (2013), Rupture directivity of small earthquakes at Parkfield, J. Geophys. Res. Solid Earth, 118, 212-221, doi: 10.1029/2012JB009675.

Shearer, PM, Stark PB.  2012.  Global risk of big earthquakes has not recently increased. Proceedings of the National Academy of Sciences of the United States of America. 109:717-721.   10.1073/pnas.1118525109   AbstractWebsite

The recent elevated rate of large earthquakes has fueled concern that the underlying global rate of earthquake activity has increased, which would have important implications for assessments of seismic hazard and our understanding of how faults interact. We examine the timing of large (magnitude M >= 7) earthquakes from 1900 to the present, after removing local clustering related to aftershocks. The global rate of M >= 8 earthquakes has been at a record high roughly since 2004, but rates have been almost as high before, and the rate of smaller earthquakes is close to its historical average. Some features of the global catalog are improbable in retrospect, but so are some features of most random sequences-if the features are selected after looking at the data. For a variety of magnitude cutoffs and three statistical tests, the global catalog, with local clusters removed, is not distinguishable from a homogeneous Poisson process. Moreover, no plausible physical mechanism predicts real changes in the underlying global rate of large events. Together these facts suggest that the global risk of large earthquakes is no higher today than it has been in the past.

Yao, HJ, Shearer PM, Gerstoft P.  2012.  Subevent location and rupture imaging using iterative backprojection for the 2011 Tohoku Mw 9.0 earthquake. Geophysical Journal International. 190:1152-1168.   10.1111/j.1365-246X.2012.05541.x   AbstractWebsite

Knowledge of the rupture speed and spatialtemporal distribution of energy radiation of earthquakes is important for earthquake physics. Backprojection of teleseismic waves is commonly used to image the rupture process of large events. The conventional backprojection method typically performs temporal and spatial averaging to obtain reliable rupture features. We present an iterative backprojection method with subevent signal stripping to determine the distribution of subevents (large energy bursts) during the earthquake rupture. We also relocate the subevents initially determined by iterative backprojection using the traveltime shifts from subevent waveform cross-correlation, which provides more accurate subevent locations and source times. A bootstrap approach is used to assess the reliability of the identified subevents. We apply this method to the Mw 9.0 Tohoku earthquake in Japan using array data in the United States. We identify 16 reliable subevents in the frequency band 0.21 Hz, which mostly occurred around or west of the hypocentre in the downdip region. Analysis of Tohoku aftershocks shows that depth phases can often produce artefacts in the backprojection image, but the position and timing of our main shock subevents are inconsistent with depth-phase artefacts. Our results suggest a complicated rupture with a component of bilateral rupture along strike. The dominant energy radiation (between 0.2 and 1 Hz) is confined to a region close to the hypocentre during the first 90 s. A number of subevents occurred around the hypocentre in the first 90 s, suggesting a low initial rupture speed and repeated rupture or slip near the hypocentre. The rupture reached the coastal region about 106 km northwest of the hypocentre at 43 s and the region about 110 km north of the hypocentre at 105 s with a northward rupture speed similar to 2.0 km s-1 at 60110 s. After 110 s, a series of subevents occurred about 120220 km southwest of the hypocentre, consistent with a 3 km s-1 along-strike rupture speed. The abundant high-frequency radiation in the downdip region close to the coast suggests intermittent rupture probably in the brittleductile transition zone. The lack of high-frequency radiation in the updip region suggests the rupture near the trench was more continuous, probably due to more homogeneous frictional properties of the shallow slab interface. The lack of early aftershocks in the updip region indicates that most of the accumulated slip in the updip region during the interseismic period was probably released during the main shock.

Castro, RR, Shearer PM, Astiz L, Suter M, Jacques-Ayala C, Vernon F.  2010.  The Long-Lasting Aftershock Series of the 3 May 1887 M-w 7.5 Sonora Earthquake in the Mexican Basin and Range Province. Bulletin of the Seismological Society of America. 100:1153-1164.   10.1785/0120090180   AbstractWebsite

We study local and regional body-wave arrival times from several seismic networks to better define the active regional fault pattern in the epicentral region of the 3 May 1887 M-w 7.5 Sonora, Mexico (southern Basin and Range Province) earthquake. We determine hypocenter coordinates of earthquakes that originated between 2003 and 2007 from arrival times recorded by the local network RESNES (Red Sismica del Noreste de Sonora) and stations of the Network of Autonomously Recording Seismographs (NARS)-Baja array. For events between April and December 2007, we also incorporated arrival times from USArray stations located within 150 km of the United States-Mexico border. We first obtained preliminary earthquake locations with the Hypoinverse program (Klein, 2002) and then relocated these initial hypocenter coordinates with the source-specific station term (SSST) method (Lin and Shearer, 2005). Most relocated epicenters cluster in the upper crust near the faults that ruptured during the 1887 earthquake and can be interpreted to be part of its long-lasting series of aftershocks. The region of aftershock activity extends, along the same fault zone, 40-50 km south of the documented southern tip of the 1887 rupture and includes faults in the epicentral region of the 17 May 1913 (I-max VIII, M-I 5.0-0.4) and 18 December 1923 (I-max IX, M-I 5.7-0.4) Granados-Huasabas, Sonora, earthquakes, which themselves are likely to be aftershocks of the 1887 event. The long aftershock duration can be explained by the unusually large magnitude of the mainshock and by the low slip rates and long mainshock recurrence times of the faults that ruptured in 1887.

Lin, GQ, Shearer PM.  2009.  Evidence for water-filled cracks in earthquake source regions. Geophysical Research Letters. 36   10.1029/2009gl039098   AbstractWebsite

We identify lowered Vp/Vs ratios near earthquake source regions in southern California using observations from a seismic tomography model and high-resolution local Vp/Vs estimates using waveform cross-correlation data from within similar event clusters. The median tomographic Vp/Vs ratio is 1.716 +/- 0.008 at all the relocated crustal earthquake locations, compared to the background median value of 1.729 +/- 0.007 for the tomography model, although the error estimates overlap slightly. The median in situ Vp/Vs ratio of 1.673 +/- 0.022 within the similar event clusters suggests that tomographic studies are overestimating Vp/Vs at source regions. Interpretation of Vp/Vs anomalies is complicated by the scatter in values obtained for individual clusters and in comparisons to absolute Vp and Vs velocities in the tomography model. However, the low Vp/Vs ratios measured for the seismicity clusters are hard to explain with known rocks and suggest the presence of water-filled cracks with several percent porosity in earthquake source regions in southern California, which likely has an effect on faulting and earthquake activity. Citation: Lin, G., and P. M. Shearer (2009), Evidence for water-filled cracks in earthquake source regions, Geophys. Res. Lett., 36, L17315, doi: 10.1029/2009GL039098.

Allmann, BP, Shearer PM.  2007.  A high-frequency secondary event during the 2004 Parkfield earthquake. Science. 318:1279-1283.   10.1126/science.1146537   AbstractWebsite

By using seismic records of the 2004 magnitude 6.0 Parkfield earthquake, we identified a burst of high-frequency seismic radiation that occurred about 13 kilometers northwest of the hypocenter and 5 seconds after rupture initiation. We imaged this event in three dimensions by using a waveform back-projection method, as well as by timing distinct arrivals visible on many of the seismograms. The high-frequency event is located near the south edge of a large slip patch seen in most seismic and geodetic inversions, indicating that slip may have grown abruptly at this point. The time history obtained from full-waveform back projection suggests a rupture velocity of 2.5 kilometers per second. Energy estimates for the subevent, together with long-period slip inversions, indicate a lower average stress drop for the northern part of the Parkfield earthquake compared with that for the region near its hypocenter, which is in agreement with stress-drop estimates obtained from small-magnitude aftershocks.

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.

Prieto, GA, Thomson DJ, Vernon FL, Shearer PM, Parker RL.  2007.  Confidence intervals for earthquake source parameters. Geophysical Journal International. 168:1227-1234.   10.1111/j.1365-246X.2006.03257.x   AbstractWebsite

We develop a method to obtain confidence intervals of earthquake source parameters, such as stress drop, seismic moment and corner frequency, from single station measurements. We use the idea of jackknife variance combined with a multitaper spectrum estimation to obtain the confidence regions. The approximately independent spectral estimates provide an ideal case to perform jackknife analysis. Given the particular properties of the problem to solve for source parameters, including high dynamic range, non-negativity, non-linearity, etc., a log transformation is necessary before performing the jackknife analysis. We use a Student's t distribution after transformation to obtain accurate confidence intervals. Even without the distribution assumption, we can generate typical standard deviation confidence regions. We apply this approach to four earthquakes recorded at 1.5 and 2.9 km depth at Cajon Pass, California. It is necessary to propagate the errors from all unknowns to obtain reliable confidence regions. From the example, it is shown that a 50 per cent error in stress drop is not unrealistic, and even higher errors are expected if velocity structure and location errors are present. An extension to multiple station measurement is discussed.

Lin, GQ, Shearer P.  2007.  Estimating local V-P/V-s ratios within similar earthquake clusters. Bulletin of the Seismological Society of America. 97:379-388.   10.1785/0120060115   AbstractWebsite

We develop and test a method to estimate local VP/V, ratios for compact similar earthquake clusters using the precise P and S differential times obtained using waveform cross-correlation. We demonstrate how our technique works using synthetic data and evaluate likely errors arising from near-source takeoff angle differences between P and S waves. We use a robust misfit function method to compute VPA, ratios for both synthetic data sets and several similar event clusters in southern California, and use a bootstrap resampling approach to estimate standard errors for real data. Our technique has higher resolution for near-source VP/V, ratios than typical tomographic inversion methods and provides constraints on near-fault rock properties.

Lin, GQ, Shearer P.  2006.  The COMPLOC earthquake location package. Seismological Research Letters. 77:440-444.   10.1785/gssrl.77.4.440   Website
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.

Wolfe, CJ, Okubo PG, Ekstrom G, Nettles M, Shearer PM.  2004.  Characteristics of deep (<= 13 km) Hawaiian earthquakes and Hawaiian earthquakes west of 155.55 degrees W. Geochemistry Geophysics Geosystems. 5   10.1029/2003gc000618   AbstractWebsite

[ 1] High precision relocation of earthquakes recorded by the Hawaiian Volcano Observatory (HVO) seismic network provides new information on the characteristics of seismic faulting at this oceanic hot spot. Using waveform cross correlation, we have measured correlation coefficients and travel time differences for a set of 14,605 deep (greater than or equal to 13 km) earthquakes recorded from 1988 to 1998. We find that about half of the analyzed earthquakes are in similar event clusters that delineate fault zones in the lower crust and upper mantle. We suggest that much of this deep seismicity reflects rupture in the brittle lithosphere away from the magma pathways, although at Kilauea the stresses from magma movement may additionally help trigger mantle earthquakes on preexisting faults in regions with high differential ambient stresses. Focal mechanisms of similar event clusters throughout Hawaii display characteristic patterns and appear consistent with the hypothesis that deep earthquakes on preexisting faults reflect the stresses due to volcano loading and flexure. We also present the results of applying cross correlation analyses and relocation to -7000 earthquakes at all depths located west of 155.55degreesW and recorded from 1988 to 1998. The pattern of relocated earthquakes at the Kealakekua fault zone is consistent with the presence of a lowangle detachment on the west flank of Mauna Loa.

Wolfe, CJ, Okubo PG, Shearer PM.  2003.  Mantle fault zone beneath Kilauea volcano, Hawaii. Science. 300:478-480.   10.1126/science.1082205   AbstractWebsite

Relocations and focal mechanism analyses of deep earthquakes (greater than or equal to13 kilometers) at Kilauea volcano demonstrate that seismicity is focused on an active fault zone at 30-kilometer depth, with seaward slip on a low-angle plane, and other smaller, distinct fault zones. The earthquakes we have analyzed predominantly reflect tectonic faulting in the brittle lithosphere rather than magma movement associated with volcanic activity. The tectonic earthquakes may be induced on preexisting faults by stresses of magmatic origin, although background stresses from volcano loading and lithospheric flexure may also contribute.

Fialko, Y, Sandwell D, Agnew D, Simons M, Shearer P, Minster B.  2002.  Deformation on nearby faults induced by the 1999 Hector Mine earthquake. Science. 297:1858-1862.   10.1126/science.1074671   AbstractWebsite

Interferometric Synthetic Aperture Radar observations of surface deformation due to the 1999 Hector Mine earthquake reveal motion on several nearby faults of the eastern California shear zone. We document both vertical and horizontal displacements of several millimeters to several centimeters across kilometer-wide zones centered on pre-existing faults. Portions of some faults experienced retrograde (that is, opposite to their long-term geologic slip) motion during or shortly after the earthquake. The observed deformation likely represents elastic response of compliant fault zones to the permanent co-seismic stress changes. The induced fault displacements imply decreases in the effective shear modulus within the kilometer-wide fault zones, indicating that the latter are mechanically distinct from the ambient crustal rocks.

Nikolaidis, RM, Bock Y, de Jonge PJ, Shearer P, Agnew DC, vanDomselaar M.  2001.  Seismic wave observations with the Global Positioning System. Journal of Geophysical Research-Solid Earth. 106:21897-21916.   10.1029/2001jb000329   AbstractWebsite

We describe the direct measurement of ground displacement caused by the Hector Mine earthquake in southern California (M-w 7.1, October 16, 1999). We use a new method of instantaneous positioning, which estimates site coordinates from only a single epoch of Global Positioning System (GPS) data, to measure dynamic as well as static displacements at 24 stations of the Southern California Integrated GPS Network (SCIGN), with epicentral distances from 50 to 200 km. For sites outside the Los Angeles basin the observed displacements are well predicted by an elastic half-space model with a point shear dislocation; within the sedimentary basin we observe large displacements with amplitudes up to several centimeters that last as long as 3-4 min. Since we resolve the GPS phase ambiguities and determine site coordinates independently at each epoch, the GPS solution rate is the same as the receiver sampling rate. For the SCIGN data this is 0.033 Hz (once per 30 s), though sample rates up to 2 Hz are possible with the SCIGN receivers. Since the GPS phase data are largely uncorrelated at I s, a higher sampling rate would offer improved temporal resolution of ground displacement, so that in combination with inertial seismic data, instantaneous GPS positioning would in many cases significantly increase the observable frequency band for strong ground motions.

Schulte-Pelkum, V, Masters G, Shearer PM.  2001.  Upper mantle anisotropy from long-period P polarization. Journal of Geophysical Research-Solid Earth. 106:21917-21934.   10.1029/2001jb000346   AbstractWebsite

We introduce a method to infer upper mantle azimuthal anisotropy from the polarization, i.e., the direction of particle motion, of teleseismic long-period P onsets. The horizontal polarization of the initial P particle motion can deviate by > 10 degrees from the great circle azimuth from station to source despite a high degree of linearity of motion. Recent global isotropic three-dimensional mantle models predict effects that are an order of magnitude smaller than our observations. Stations within regional distances of each other show consistent azimuthal deviation patterns, while the deviations seem to be independent of source depth and near-source structure. We demonstrate that despite this receiver-side spatial coherence, our polarization data cannot be fit by a large-scale joint inversion for whole mantle structure. However, they can be reproduced by azimuthal anisotropy in the upper mantle and crust. Modeling with an anisotropic reflectivity code provides bounds on the magnitude and depth range of the anisotropy manifested in our data. Our method senses anisotropy within one wavelength (250 km) under the receiver. We compare our inferred fast directions of anisotropy to those obtained from P-n travel times and SKS splitting. The results of the comparison are consistent, with azimuthal anisotropy situated in the uppermost mantle, with SKS results deviating from P,, and Pp., in some regions with probable additional deeper anisotropy. Generally, our fast directions are consistent with anisotropic alignment due to lithospheric deformation in tectonically active regions and to absolute plate motion in shield areas. Our data provide valuable additional constraints in regions where discrepancies between results from different methods exist since the effect we observe is local rather than cumulative as in the case of travel time anisotropy and shear wave splitting. Additionally, our measurements allow us to identify stations with incorrectly oriented horizontal components.

Shearer, PM.  2001.  Improving global seismic event locations using source-receiver reciprocity. Bulletin of the Seismological Society of America. 91:594-603.   10.1785/0120000238   AbstractWebsite

The leading source of error in seismic event locations is travel-time perturbations caused by three-dimensional Earth structure. The reciprocity of travel times between sources and receivers provides a method for testing the effectiveness of empirical methods for improving event locations that rely on nearby calibration events of known location. We apply this approach to travel-time residuals obtained by Engdahl et al, (1998) for almost 100,000 teleseismic events. By analyzing the residual patterns at thousands of seismic stations of known location, we characterize the spatial coherence of station/event mislocation vectors. We find that, on average, calibration events are likely to improve locations only if they are located within 100-150 km of the target events. For 84 events of known location, we find that applying source-receiver reciprocity can significantly reduce location errors by correcting for the teleseismic residual pattern observed at stations close to the target events. These results have implications for efforts to improve event locations for nuclear explosion monitoring purposes.