Publications

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2014
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.

2006
Prieto, GA, Parker RL, Vernon FL, Shearer PM, Thomson DJ.  2006.  Uncertainties in earthquake source spectrum estimation using empirical Green functions. Earthquakes; radiated energy and the physics of faulting. 170( Abercrombie RE, McGarr A, Kanamori H, Di Toro G, Eds.).:69-74., Washington: American Geophysical Union   10.1029/170gm08   Abstract

We analyze the problem of reliably estimating uncertainties of the earthquake source spectrum and related source parameters using Empirical Green Functions (EGF). We take advantage of the large dataset available from 10 seismic stations at hypocentral distances (10 km < d <50 km) to average spectral ratios of the 2001 M5.1 Anza earthquake and 160 nearby aftershocks. We estimate the uncertainty of the average source spectrum of the M5.1 target earthquake by performing propagation of errors, which, due to the large number of EGFs used, is significantly smaller than that obtained using a single EGF. Our approach provides estimates of both the earthquake source spectrum and its uncertainties, plus confidence intervals on related source parameters such as radiated seismic energy or apparent stress, allowing the assessment of statistical significance. This is of paramount importance when comparing different sized earthquakes and analyzing source scaling of the earthquake rupture process. Our best estimate of radiated energy for the target earthquake is 1.24×1011 Joules with 95% confidence intervals (0.73×1011, 2.28×1011). The estimated apparent stress of 0.33 (0.19, 0.59) MPa is relatively low compared to previous estimates from smaller earthquakes (1MPa) in the same region.

2003
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.

2002
Shearer, PM.  2002.  Parallel fault strands at 9-km depth resolved on the Imperial Fault, Southern California. Geophysical Research Letters. 29   10.1029/2002gl015302   AbstractWebsite

[1] Precision relocation of hundreds of small earthquakes occurring along the Imperial Fault in Southern California during the last two decades reveals parallel steaks of seismicity at 9-km depth. These strands are spaced about 0.5 km apart within a 2 km wide zone of earthquakes near the brittle-ductile transition between the shallow locked part of the fault and a creeping zone at depth. These results suggest that the lower crustal shear zone below the Imperial Fault, site of major earthquakes in 1940 and 1979, must be at least two kilometers wide.

2001
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.

2000
Astiz, L, Shearer PM.  2000.  Earthquake locations in the inner Continental Borderland, offshore southern California. Bulletin of the Seismological Society of America. 90:425-449.   10.1785/0119990022   AbstractWebsite

The inner Continental Borderland region, offshore southern California, is tectonically active and contains several faults that are potential seismic hazards to nearby cities. However, fault geometries in this complex region are often poorly constrained due to a lack of surface observations and uncertainties in earthquake locations and focal mechanisms. To improve the accuracy of event locations in this area, we apply new location methods to 4312 offshore seismic events that occurred between 1981 and 1997 in seven different regions within the Borderland. The regions are defined by either temporal or spatial clustering of seismic activity in the Southern California Seismic Network (SCSN) catalog. Obtaining accurate locations for these events is difficult, due to the lack of nearby stations, the limited azimuthal coverage, and uncertainties in the velocity structure for this area. Our location procedure is based on the L-l norm, grid search, waveform cross-correlation method of Shearer (1997), except that we use a nearest neighbor approach (Astiz et al., 2000) to identify suitable event pairs for waveform cross-correlation and we explore the effect of different velocity models on the locations and associated station terms. In general, our relocated events have small estimated relative location errors and the events are more clustered than the SCSN catalog locations. A quarry on the south tip of Catalina Island provides a test of our location accuracy and suggests that, under ideal conditions, offshore events can be located to within 1 to 2 km of their true locations. Our final locations for most clusters are well correlated with known local tectonic features. We relate the 1981 Santa Barbara Island (M-L = 5.3) earthquake with the Santa Cruz fault, the 13 July 1986 Oceanside (M-L = 5.3) sequence with the San Diego Trough fault zone, and events near San Clemente Island with the known trace of the San Clemente fault zone. Over 3000 of the offshore events during this time period are associated with the 1986 Oceanside earthquake and its extended aftershock sequence. Our locations define a northeast-dipping fault plane for the Oceanside sequence, but in cross-section the events are scattered over a broad zone (about 4-km thick). This could either be an expression of fault complexity or location errors due to unaccounted for variations in the velocity structure. Events that occur near Coronado Bank in the SCSN catalog are relocated closer to the San Diego coast and suggest a shallow-angle, northeast-dipping fault plane at 10 to 15 km depth.

1998
Shearer, PM.  1998.  Evidence from a cluster of small earthquakes for a fault at 18 km depth beneath Oak Ridge, southern California. Bulletin of the Seismological Society of America. 88:1327-1336. AbstractWebsite

A swarm of about 50 small earthquakes (M similar to 1.5) occurred for a month during 1989 beneath Oak Ridge, southern California. Location accuracy using conventional analysis of arrival-time picks is limited for these events by the weak, emergent nature of arrivals on the available seismograms. However, waveform crosscorrelation techniques are found to provide precise relative event locations due to the similarity of the waveforms recorded at individual stations. The relocated events form a small cluster about 1 km across at a depth of similar to 18 km and are aligned along a plane that dips 35 degrees to the northwest. Estimated standard errors for the locations are generally less than 50 m. The time evolution of the sequence shows a gradual migration of activity away from its initiation point. Three additional events occurred several months later; these align along the same plane but are displaced about 500 m to the southeast from the main swarm. Reliable fault-plane solutions are difficult to obtain for these events due to the small number of station records available, the limited range of takeoff angles, and the weak initial arrivals on many of the seismograms, Stacking the records at each station over the different events greatly reduces prearrival noise levels and assists in resolving the average P first motions. Analysis of these first-motion data indicates that the slip planes of probable focal mechanisms are not in agreement with the plane defined by the seismicity. The seismicity alignment may represent the extension of the Simi fault, in which case some shallowing of the fault dip would be required to match the observed 35 degrees dip at 18 km.

1995
Shearer, PM.  1995.  Seismic Studies of the Upper-Mantle and Transition Zone. Reviews of Geophysics. 33:321-324.   10.1029/95rg00186   Website