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Journal of Volcanology and Geothermal Research 296 (2015) 1930
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Journal of Volcanology and Geothermal Research
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Seismic velocity structure and earthquake relocation for the magmaticsystem beneath Long Valley Caldera, eastern California
Guoqing LinDepartment of Marine Geosciences, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA
E-mail address: [email protected]
http://dx.doi.org/10.1016/j.jvolgeores.2015.03.0070377-0273/ 2015 Elsevier B.V. All rights reserved.
a b s t r a c ta r t i c l e i n f o
Article history:Received 9 January 2015Accepted 11 March 2015Available online 18 March 2015
Keywords:Long ValleyCrustal structureMagma chamber
A new three-dimensional (3-D) seismic velocity model and high-precision location catalog for earthquakesbetween 1984 and 2014 are presented for Long Valley Caldera and its adjacent fault zones in eastern California.The simul2000 tomography algorithm is applied to derive the 3-D Vp andVp/Vsmodels using first-arrivals of 1004composite earthquakes obtained from the original seismic data at the Northern California Earthquake DataCenter. The resulting Vpmodel reflects geological structures and agrees with previous local tomographic studies.The simultaneously resolved Vp/Vs model is a major contribution of this study providing an important comple-ment to the Vp model for the interpretation of structural heterogeneities and physical properties in the studyarea. The caldera is dominated by low Vp anomalies at shallow depths due to postcaldera fill. High Vp and lowVp/Vs values are resolved from the surface to ~3.4 km depth beneath the center of the caldera, correspondingto the structural uplift of the Resurgent Dome. An aseismic body with low Vp and high Vp/Vs anomalies at4.26.2 km depth below the surface is consistent with the location of partial melt suggested by previousstudies based on Vp models only and the inflation source locations based on geodetic modeling. The Sierrancrystalline rocks outside the caldera are generally characterized with high Vp and low Vp/Vs values. The newlyresolved velocity model improves absolute location accuracy for the seismicity in the study area and ultimatelyprovides the basis for a high-precision earthquake catalog based on similar-event cluster analysis and waveformcross-correlation data. The fine-scale velocity structure and precise earthquake relocations are useful for inves-tigating magma sources, seismicity and stress interaction and other seismological studies in Long Valley.
2015 Elsevier B.V. All rights reserved.
Long Valley Caldera in eastern California is well-known for produc-ing numerous volcanic eruptions over the past 3 Myr. It is sandwichedby the Sierra Nevada and the Basin and Range Province. Major geologi-cal structures in this area include Mammoth Mountain, situated at thesouthwest rim of Long Valley caldera, and ENE-dipping range-frontnormal faults including the Hartley Springs Fault north of the calderaand the Hilton Creek and the Round Valley Faults to the south (Fig. 1).Since the 1980s, this region has shown episodic unrest including earth-quake swarmsand uplift of the central portion of the caldera (theResur-gent Dome) along with variations in thermal springs and gas emissions(Hill, 2006). Hill (2006) also illustrated schematically the interactionsbetween the unrest in the magmatic system underlying the calderaand the seismic activity in the adjacent Sierra Nevada block. This inter-action could benefit from improved knowledge of the seismic velocitystructure and earthquake relocations.
A long-standing question in the Long Valley region is the location,size and geometry of magma bodies beneath the caldera. Both geodeticand gravity data show the presence of magma sources (e.g., Carle, 1988;Battaglia et al., 1999; Langbein, 2003; Battaglia et al., 2003a, b; Tizzani etal., 2009). The best-fitting inflation source models based on surface de-formation observations are prolate ellipsoids with the depth rangingbetween 5.5 and 7.6 km beneath the Resurgent Dome (Langbein et al.,1995; Marshall et al., 1997;Thatcher & Massonnet, 1997;Fialko et al.,2001;Battaglia et al., 2003a;Tizzani et al., 2009). These ellipsoids haveaspect ratios between 0.25 and 0.66 with the semi-major axis about1 km size. However, studies based on seismic data show conflictingresults. Reflection and refraction studies (Hill, 1976; Luetgert &Mooney, 1985; Zucca et al., 1987) identified low velocity zones atdifferent depth ranges, which are usually interpreted as magma bod-ies. Evidence for the presence of magma underlying the caldera aremainly supported by teleseismic data in the form of low P-wave ve-locities, high attenuation, and anomalous ray paths (Steeples & Iyer,1976;Ryall & Ryall, 1981; Sanders & Ryall, 1983; Sanders, 1984;Dawson et al., 1990; Sanders, 1993; Sanders et al., 1995; Sanders &Nixon, 1995;Steck, 1995; Weiland et al., 1995; Seccia et al., 2011;Menendez & Thurber, 2011). The regional Vp tomography for northernCalifornia by Thurber et al. (2009) resolved a mid-crustal (10 to
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1500 2000 2500 3000 3500 4000Elev (m)
Fig. 1.Major geological structures in the study area, including Long Valley Caldera, the Resurgent Dome (light gray area), MammothMountain (dark gray areawith arrow), and fault lines.The background is the topography base map from the U. S. Geological Survey. The red box in the inset map shows the location of our study area in California.
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20 km depth) low-velocity zone beneath Long Valley with velocitiesabout 10 % lower than the surrounding region. A low Vp body between6 and 11 km depth beneath the caldera appears in the statewide ve-locity model for California by Lin et al. (2010). Seismic velocity andattenuation studies based on local earthquake data, however, didnot find evidence for significant low-velocity volumes in the upper10 to 15 km of the crust (Kissling, 1988; Hauksson, 1988;Black et al.,1991; Romero et al., 1993; Ponko & Sanders, 1994; Tryggvason, 1998).
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Fig. 2. Event and station distributions in our study. Blue triangles: network stations; yellowEarthquake Data Center catalog; red area: the Resurgent Dome; and orange area at the soutcolor in this figure legend, the reader is referred to the web version of this article.)
The aim of this study is to use newly developed high-resolutionthree-dimensional (3-D) Vp and Vp/Vs models and precise earth-quake relocations based on local seismic data to study the magmat-ic system beneath Long Valley Caldera and the structuralheterogeneities associated with adjacent fault zones. The 3-D velocitymodel and earthquake location catalog based on waveform cross-correlation data are made available at http://www.rsmas.miami.edu/users/glin/LV.html.
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triangles: temporary stations; gray dots: local earthquakes in the Northern Californiahwest rim of the caldera: Mammoth Mountain. (For interpretation of the references to
reek Fault2 4 6 8
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Fig. 3. Composite earthquakes (gray dots), explosion data (big blue dots), and inversion grid nodes (pink squares) used in the tomographic inversion. Yellow straight lines are profiles forthe cross sectional views of the relocation comparison in Fig. 7 and the velocity models in Figs. 12 and 13. Red and orange areas represent the Resurgent Dome andMammoth Mountain,respectively. The inset figure shows the 1-D initial Vpmodel for the inversion. (For interpretation of the references to color in this figure legend, the reader is referred to theweb version ofthis article.)
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2. Data set
The seismic data used in this study are obtained from the NorthernCalifornia Earthquake Data Center (NCEDC, 2014), including first-arrival picks and waveform data for both compressional (P) and shear(S) waves of all local earthquakes between January 1984 and August2014 in the study area (Fig. 2). The data set consists of 181,809 earth-quakes (gray dots in Fig. 2) with over 2.8 million P- and 0.17 million S-picks recorded by the regional seismic network stations (blue trianglesin Fig. 2), including the Northern California Seismic Network and theUniversity of Nevada, Reno Seismic Network. Because the number of Spicks is only 6% of P picks in the NCEDC catalog, I applied the compositeevent method presented by Lin et al. (2007a) to incr