P and $ velocity structure of the Yellowstone volcanic field from local

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. B7, PAGES 15,105-15,121, JULY 10, 1999 P and $ velocity structure of the Yellowstone volcanic field from local earthquake and controlled-source tomography Douglas S. Miller and Robert B. Smith Department of Geology and Geophysics, University of Utah, Salt Lake City Abstract. The three-dimensional P and S velocity (from VF/V s ratios) distribution and improved hypocenters for the Yellowstone volcanic field have been determined from inversion of first arrival times from 7942 local earthquakes and 16 controlled-source explosions. The P velocity model has an rms residual of ñ 0.09 s, whereas the Vp/Vs ratio model (calculated from 511 earthquakes) has an rms residual of ñ 0.29 s. High P and S velocities outside the Yellowstone caldera represent thermally undisturbed basement and sedimentary rocks. A caldera-wide 15% decrease from regional P velocities at depths of 6 to 12 km is coincident with a -60 mgal gravity anomaly and is interpreted as a hot, subsolidus, granitic batholith with a quasi-plastic rheology. Localized 30% reductions from regional seismic velocities and higher Vp/Vs ratios 8 km beneath Yellowstone's resurgent domes are interpreted as partial melts and vestigial magma systems associated with youthful (less than 2 Ma) silicic volcanism. Additional low seismic velocities and Vp/Vs ratios and a 20 mgal gravity low less than 4 km beneath the northeast caldera rim are interpreted as a hydrothermal fracture zone thermally driven by underlying partial melt. Hypocenters relocated with the three-dimensional P velocity model show subparallel alignment with NW-SE trending postcaldera volcanic vents and normal faults northwest of the caldera. Focal depths of relocated earthquakes decrease from more than 11 km outside the caldera to less than 6 km within the caldera, reflecting thinning and heating of the seismogenicrust. 1. Introduction observations have been interpreted as evidence for a magma body(s) in the shallow crust (<10 km depth) of Yellowstone (see The 6500 km 2 Yellowstone volcanic field, Wyoming Smith and Braile [ 1994] for a recent review of previous geological (hereinaftereferred to as Yellowstone), is one of the most active and geophysical studies of the Yellowstone region and volcano-tectonic provinces in the world. This area encompasses Christiansen [1999] for a summary of Yellowstone's volcanic Yellowstone National Park and is often associated with its world history), despite limited resolution and uncertainties with regard to renowned display of geysers, hot springs, and fumaroles. It is also the size and shape of the suspected magma chamber(s). characterized by recent explosive silicic volcanism, including The primary objective of this paper is to clarify the area's upper three cataclysmic, caldera-forming eruptions at 2.0, 1.2, and 0.6 crustal structure, including the form and distribution of potential Ma (Figure 1). Twenty-nine postcaldera silicic and basaltic flows, magma and hydrothermal fluid bodies, based on the seismic some as young as 70 ka, have occurred since the last explosive properties of the crust beneath the Yellowstone volcanic field. eruption and demonstrate that Yellowstone remains volcanically Previous seismic studies have focused on the primary (P) wave active. velocity structure rather than the shear (S) wave velocity structure Large normal faults associated with Basin-Range extension, of Yellowstone because very little S wave data have been including the Rocky Mountains' largest historic earthquake (M s available [see Benz and Smith, 1984]. This study is the first 7.5 at Hebgen Lake, Montana) and widespread earthquake determination of the three-dimensional P and S velocity structure swarms, characterize its seismotectonic setting. Unprecedented of Yellowstone from an integrated set of local earthquake and crustal deformation of up to 1 m of caldera uplif from 1923 to controlled source data from regional seismic refraction 1984, reversing to subsidence of up to 15 cm by 1995, in experiments [Smith et al., 1982]. Our data consist of (1) first P combination with its extraordinarily high thermal flux of up to arrivals from local earthquakes recorded by the Yellowstone mw m '2, confirms the fundamental role of active igneous Seismic Network, operated from 1973 to 1981 by the U.S. and tectonic processes at Yellowstone. Geological Survey (USGS) [Pitt, 1987] and by the University of Yellowstone's youngest caldera (0.6 Ma) spans 75 km by 45 Utah Seismograph Stations (UUSS) since 1983 [see Nava and km and is interrupted by two - 10 km by - 20 km resurgent Smith, 1996], (2) refraction first P arrival data from the 1978 and structural domes in its northeast (Sour Creek) and west central 1980 Yellowstone-Snake River Plain seismic experiments [Smith (Mallard Lake) regions (Figure 1). A regional 60 mgal gravity et al., 1982], and (3) new, digital, broadband S wave data acquired low and low seismic velocities coincide with the caldera. These specifically for this study. 1 Now at Exxon Exploration Company, Houston, Texas. Copyright 1999 by the American Geophysical Union. Paper number 1998JB /99/1998JB Data and Methodology The Yellowstone earthquake data are from a compilation of travel time picks and event locations determined by the USGS [Pitt, 1987] and the University of Utah [Peyton and Smith, 1990]. At the time of this study the Yellowstone Seismograph Network 15,105

2 15,106 MILLER AND SMITH: YELLOWSTONE P AND $ VELOCITY 4.5 ø 00' N EXPLANATION QUATERNARY Surficial ideposits lnte.rmediaie Volcanic Rocks.. PRE-..1.RTI ARY Rhyotite Flows... Basalt Flows : Crystalline and! Sedi'rrenlary irocks Welded Ash Flow SYMBOLS Tuffs.//' Normal Fault: '" Ca.Jdera Rim 44 ø 00' N, Vo.lcanic Vent '111 ø 15' W I '10 ø.00' W '.j jj J km Figure 1. Generalized geologic map of the Yellowstone volcanic field and surrounding region. Major surface rock types, caldera boundaries, Quaternary normal faults, Pleistocene volcanic vents, resurgent domes, and major lakes are shown. Number and letter symbols denote the following: I, Huckleberry Ridge caldera (2.0 Ma); II, Henry's Fork caldera (1.2 Ma); III, Yellowstone caldera (0.6 Ma); ML, Mallard Lake resurgent dome; SC, Sour Creek resurgent dome; HSB, Hot Springs Basin; HF, Hebgen Lake Fault; GF, Gallatin Fault; TF, Teton Fault; JL, Jackson Lake; HL, Hebgen Lake; YL, Yellowstone Lake; CJ, Canyon Junction; LJ, Lake Junction; MJ, Madison Junction; MHS, Mammoth Hot Springs; NJ, Norris Junction; and OF, Old Faithful, and YNP, Yellowstone National Park boundary. comprised 18 vertical-component and three three-component stations, all of which were digitally recorded at the University of Utah via radio telemetry. Digital three-component seismic data were collected with broadband seismometers at six sites (Figure 2) that were established for this study from late 1990 to early 1994 to augment the existing vertical-component seismic database archived by the USGS and UUSS Data P wave data. The P wave data comprised 66,637 observations from 7942 earthquakes, recorded from November 1972 to December 1993, and 462 Pg (critically refracted P) arrivals from 16 explosions recorded by a total of 254 stations (Figure 3). Seismograms, including UUSS and portable broadband records, were timed to s, and a minimum estimate of the average timing error for the whole P data set is s. Reading quality factors for the arrival time picks vary from 0 to 4, with 0 being a clear, impulsive arrival and 4 representing an ambiguous, emergent arrival. The timing errors of the explosion data are s to s [Schilly et al., 1982] S wave data. To invert for Vp/V s ratios, we used a second data set composed of 3871 P and 677 S observations from 512 earthquakes recorded by 41 stations from May 1990 to July Fewer high-quality S arrivals were obtained because only the nine stations specifically operated for this project were equipped with three-component seismometers (Figure 2). The P arrivals recorded prior to 1994 were taken from the P data set, and seismograms from two telemetered, short-period, UUSS threecomponent sensors and the six portable broadband stations were used to pick S arrival times. Representative samples of the threecomponent digital seismic data are shown in Figure 4. Identifying first S arrivals is more complicated than recognizing first P arrivals, principally due to the difficulties of separating the $ wave from the P wave coda, increased shear wave attenuation in regions of low velocity, and the arrival of nearsource P-to-$ and/or near-receiver $-to-p converted phases between the direct P and $ arrivals. Mistaking a converted phase for the true $ arrival reduces both the absolute $ and differential $- P (S minus P) travel times, resulting in increased apparent shear wave velocities and artificially low Vv/V s ratios. To minimize the likelihood of misidentifying converted phases as the dominant secondary arrival, S arrivals were picked only from horizontal component seismograms [Andrews et al., 1985; Eberhart-Phillips, 1993]. Many of the horizontal component seismogramshowed no clear secondary arrival, however, adding uncertainty to the picks (compare Figures 4a and 4b). Overall, the minimum estimated picking error for the $ arrivals is s.

3 MILLER AND SMITH: YELLOWSTONE P AND S VELOCITY 15, ø 10' N 111ø20'W [] YNP A Vellical-component I 40 I seismograph km [] Temporary ' -'l Broadband 44 ø 00' N-Jr ø 50' W :J l Network seismograph Three-component Three-component seismograph seismograph represent critically refracted rays or turning rays with deep inflection points, the maximum distance cutoff of 130 km was selected to retain information provided by deeper rays in the inversions. The 130 km value corresponds to the longest possible diagonalength of the model (110 km), with an additional 20 km allowed to include peripheral events and receivers outside the model volume to reduce edge effects and enhance azimuthal coverage. A more restrictive set of distance weighting cutoffs, 30 and 100 km, was used for final inversions to reduce the influence of possibly poorly approximated head wave ray paths to the most distant stations. Reducing the allowable source-receiver offsets also helped downweight distant observations that might have contributed to initially poor hypocenter locations. For the P velocity inversion, initial and final travel time residual cutoffs of 0.5, 2.0, and 3.5 s and 0.2, 1.0, and 1.5 s, respectively, were used. Cutoffs of 1.5, 4.0, and 5.0 s and 1.0, 2.5, and 4.0 s were chosen for the initial and final Vt,/V s ratio inversions. Events with travel time residuals less than the minimum cutoff for each case were given full weight (1), whereas events with travel time residuals between the minimum and intermediate and intermediate and maximum cutoffs were linearly weighted between 1 and 0.02 and 0.02 and 0, respectively. Events with residuals greater than the maximum cutoff were excluded from the respective inversion. The selection of cutoff values was based on testing with synthetic data (Appendix A) and experience gained from earlier studies that used a similar methodology. For example, the first sets of residual weighting parameters were deemed appropriate to prevent erroneous observations from biasing the solution while not excluding important structural Figure 2. Permanent seismograph network and portable seismograph distribution in the Yellowstone region, information represented by travel times with large residuals [see Shaded areas and letter symbols are defined in Figure 1. Eberhart-Phillips, 1993]. The S picks were assigned reading weights between 2 and 4, based on the quality of the arrival and the presence of arrivals from potentially converted phases. The S arrivals were converted to S-P travel times and incorporated into a final S-P data set containing the P arrivals and S-P times for all events recorded on the three-component seismometers. The S-P data are thus susceptible to timing errors in the P and S picks, but because the estimated error for the S arrivals is much greater than that for the P arrivals, the error in these data were assumed to be that of the S data set Data selection. Our data selection strategy was to use as much of the data as possible to compute an initial solution and remove extreme errors in the data by averaging in an initial, onedimensional velocity model (see Appendix A, Table A1) and by weighting constraints. We automatically rejected thos events with less than five recorded arrivals and events with less than four nonzero weighted arrivals. Station corrections were not included t,. o to avoid inadvertently suppressing structural variations. o 'ø5 ß.. Each observation was normalized by the reciprocal of the 'o o square of its reading quality factor and then weighted according to ß ß o its epicentral distance and travel time residual. Initial coarse inversions were carried out with distance weighting cutoffs of 50 AoNrruD.S o :o 4o and 130 km; that is, distance weights of 1 and 0 were given to + o.o+... I 4no0o. + o 2.0+ km 109 ø 50' W events with epicentral distances of 50 km or less and 130 km or SHOT POINT 4.0+ more, respectively. Events with epicentral distances between 50 km and 130 km were assignedistance weights linearly scaled Figure 3. Seismicity map of the Yellowstone plateau showing between 1 and 0. These values were chosen to be sufficiently epicenters of 7942 local earthquakes and locations of 16 large to prevent exclusion of valuable structural information, controlled source-explosions used for the three-dimensional P especially observations from sources located on opposite sides of velocity inversion analysis. Shaded areas and letter symbols are the model. Also, since first arrivals at the most distant stations defined in Figure 1.

4 15,108 MILLER AND SMITH' YELLOWSTONE P AND S VELOCITY (a) West Yellowstone :47: N-S Component - 1, 3 16:48:08:048 seconds (b) Lake Junction L ' ' I,,^ ' ' I ' _ P p s , < _ N-S Component 16:48: I 16:48' I I seconds Figure 4. Seismograms for a typical Yellowstone earthquake recorded at (a) the West Yellowstone broadband seismic station and (b) the Lake Junction broadband seismic station Inversion Procedure We used the damped least squares, iterative local earthquake tomography (LET) SIMULPS algorithm developed by Thurber [1981, 1983, 1993] and Eberhart-Philtips [1989] to simultaneously invert for P velocity structure, Vp/V s ratios, and hypocenters. Detailed discussions of LET are given by Thurber [1992] and Evans et al. [ 1994] and will not be repeated here. Solution quality is described by full matrix resolution and a weighted hit count called the "derivative weight sum," an estimate of ray density near a grid point scaled' by ray-node separation and ray path length near the node. Larger derivative weight sum values indicate more data contributed to the estimation of the corresponding model parameters. Nonuniform P and S data distribution prompted us to solve for the P velocity and Vp/Vs ratio (and hence S velocity) structures separately. Significant differences between the P and S sampling densities and dominant ray path directions are illustrated by the ray path coverages of the Yellowstone P and S data sets (Figure 5). These dissimilarities are a function of different P and S source-receiver geometries and are too pronounced to allow direct, simultaneous determination of the P velocity and Vp/Vs ratio structure at the same scale. The large number and much greater density of generally higher-quality P observations allow

5 MILLER AND SMITH: YELLOWSTONE P AND S VELOCITY 15,109 (a) + + (b) Model Vertical-component Three-component MAGNITUDI :,S sei,mograph ß 0.0+ seismograph o 2.0+ () o..0+ km Figure 5. Representative ray path distributions in (a) the P wave data set and (b) the S wave data set. Dominant ray paths are characterized by concentrated rays. Shaded areas and letter symbols are defined in Figure 1. the P velocity model to mask smaller yet potentially significant The graded inversion scheme used to solve for the P velocity Vp/V s ratio and S velocity variations. Because of the denseray model was not applied to the Vp/V s ratio inversion. Instead, a path distribution of the P data set, we used a parameterization for single-step inversion using the intermediate P model grid, with x, the P velocity model twice as fine as that used for the Vp/Vs ratio y, and z node spacings of 20, 18.5, and 4 km, respectively, was and S velocity models. calculated. Direct inversion for the Vp/Vs field should give Our study area comprised a rectangular area of 80 km (E-W) reasonable results in regions of large P and S velocity variation and 74 km (N-S) circumscribed by the station network and because the Vp/V s ratio distribution likely much smoother than encompassing the entire Yellowstone caldera and most of the corresponding P or S velocity structure [Evans et al., 1994]. Yellowstone's volcanic field and earthquake activity. Some The Vp/V s ratio inversion was performed after the P velocity stations outside the model boundaries were included in the inversion was completed, allowing use of the final threeinversion to enhance azimuthal ray path distribution and increase dimensional P velocity model in the computation of S velocities hypocenter stability. Preliminary analysis of the P wave data for raytracing and subsequent estimation of Vp/Vs ratios. P showed little ray path sampling to depths greater than about 15 velocities were not allowed to vary during the Vp/Vs ratio km, so 16 km was chosen as the base of the model. The model inversion since they were considered well determined from was then parameterized by dividing the volume into evenly spaced inversion of the P data set alone. rectangular cells. The large P data set allowed use of a finer The model volume was not uniformly sampled due to the horizontal grid spacing (---10 km) than that used in previous three- irregular source-receiver geometry. In general, sparsely sampled dimensional inversion attempts for P velocities (---20 km) [Benz areas correspond to sparse seismic activity and/or coarse station and Smith, 1984]. coverage, resulting in concomitant poor resolution. P wave Relocated hypocenters and updated travel times from each velocities at poorly resolved nodes were considered representative inversion were used as starting data for the next iteration. The averages of surrounding regions of the model because they were number of velocity grid nodes for each successive iteration was first determined from a coarser grid with better resolution and doubled by interpolating the locations and velocities of new E-W tended not to change as the grid became finer and the resolution and N-S nodes halfway between existing nodes in each layer. decreased. Similarly, nodes in new layers were interpolated halfway between For convenience, we discuss the resolution of the inversion existing layers and assigned the average P velocities of the upper results in terms of the diagonal element of the resolution matrix, and lower bounding layers in the preceding model. The initial, referred to hereafter simply as resolution. The three-dimensional coarse P velocity grid had node spacings of 40, 37, and 8 km P velocity model shows the highest derivative weight sum hlong the x, y, and z axes, respectively, followed by intermediate (sampling density) values and is best resolved (Plates l a and lb) and fine grids with half the node spacing of the previous grid. northwest of the caldera where the concentration of ray paths is This type of "progressive" or "graded" inversion scheme allows greatest (Figure 5). The resolution values for nodes in this area at determination of large-scale regional trends followed by more depths of 2 to 10 km are as high as 0.96, indicating that the precise tuning of the model [Eberhart-Phillips, 1990, 1993; Evans corresponding P velocities are mostly independently determined. et al., 1994]. The resolution falls off to less than 0.4 below 10 km because of

6 15,110 MILLER AND SMITH: YELLOWSTONE P AND $ VELOCITY poor ray path sampling. High resolution generally greater than 0.8 is also shown for the central caldera above 10 km. Nodes in the northeastern and southwestern caldera, on the other hand, have poorer resolution of 0.4 to <0.1. Prediction of P velocities in these parts of the model is thus largely dependent on surrounding P velocities because of relatively low ray path density and sparse dimensional P velocity model (Appendix B ), with the rms residual reduced from 0.13 s (in the initial model) to 0.09 s. P velocities on the northern, eastern, and southern margins of the model are 6-7 km/s (Plates 2a and 2b), representing a 10-25% increase relative to the starting, one-dimensional model (Appendix A, Table A1). Five low-velocity anomalies are identified in the seismic activity. Moderate to high derivative weight sum and model. A broad low-velocity zone of km/s, best resolution values in the extreme northeastern comer of the model are interpreted as an artifact of the inversion because that part of the model is not constrained by the P data set. The derivative weight sum distribution and resolution of the final V Vs model (Plates 1 c and 1 d) also mimics the density of ray path sampling (Figure 5). The highest resolution, as much as 0.42 northwest of the caldera, corresponds to areas with the most earthquakes recorded at three-component seismograph stations. developed between depths of 6 and 12 km, coincides approximately with the caldera. A smaller, -10 km x km, but more pronounced, pearshaped low-velocity zone underlies the northeast caldera rim from depths <2 km to >4 km. P velocities in this zone are as low as 3.4 km/s at 4 km depth, a 37% reduction from the starting P velocity of 5.4 km/s. Since this zone of low P velocities is nearly coincident with Hot Springs Basin, we call it the Hot Springs Regions with few or no earthquakes and three-component Basin velocity anomaly. seismographs display poor resolution, i.e., the southern and eastern edges of the model. The sparser source-receiver geometry and ray path distribution of the $ data set compared to the P data set (Figure 5) accounts for the difference of more than a factor of Below 4 km, the Hot Springs Basin anomaly diffuses and migrates south and west until it is almost completely beneath the northeastern caldera, where it merges with a third distinct lowvelocity zone extending from depths of 6 to at least 12 km. P 2 between the highest resolution of the P and V /V s models. The velocities as low as 4.0 km/s in this-20 km x-20 km lowapparently low-resolution values, however, may not be a robust indication of how well determined the V /Vs ratios really are; for example, Thurber and Atre [1993] consider values greater than velocity zone show a P velocity decrease at 8 km depth of as much as 32% from an initial value of 5.88 km/s. This zone of low velocities is roughly coincident with the Sour Creek resurgent 0.15 acceptable resolution for their Vp/Vs ratio model of the dome. It broadens into a diffuse zone of P velocities as low as Parkfield, California, area. Some smearing of the velocity model and degradation of resolution is likely along the dominant ray path direction [Eberhart-Phillips, 1986, 1990; Walck and Clayton, 1987]. For the Yellowstone P and S data sets, the dominant ray path direction is E-W or SE-NW (Figure 5), indicating increased uncertainty in the relative E-W spatial dimensions of anomalies and implying velocities and Vp/V s ratios in parts of the model sampled by few N-S trending rays may be more representative of adjacent regions. This kind of smearing or leaking of anomalies is a result of an uneven data distribution and imposing a discrete grid on the continuous Earth. The quality of the P velocity solution was also evaluated by relocating the 16 explosions using the fixed, final threedimensional P velocity model. Average changes in the known explosion locations were 0.31 km (west), 0.01 km (south), and 0.01 km (shallower); the maximum shift was -2 km, and the average rms residual was 0.14 s. These results vindicate the reliability of the P velocity model and add confidence to its interpretation km/s under the central caldera at depths between 8 and 12 km. A fourth but less distinct low-velocity zone characterized by P velocities as low as 4.1 km/s at 8 km depth (a 30% reduction from the starting model) is present beneath the southwestern caldera in the vicinity of the Mallard Lake resurgent dome. A N-S trending band of low velocities in the north central P velocity model marks the Norris-Mammoth corridor. P velocities in this region vary from 4.5 to 4.3 km/s at 4 and 8 km below the surface, respectively. The low P velocity anomalies below -6 km depth are imaged in poorly resolved parts of the model, raising the possibility that the velocity anomalies are inversion artifacts and have no geologic meaning. In this case, poor resolution in the vicinity of the anomalies indicates averaging or smearing due to resolving kernels larger than the node spacing. This hypothesis was tested by inverting an artificial data set (no noise added) generated from the one-dimensional starting P velocity model without the low velocity zones (Appendix A, Table A1). The initial model was reproduced, as expected, but with significantly higher resolution than models with low-velocity zones (as much as twice as high at depths greater than -4 km beneath the caldera). These results 2.3. Velocity Models and Relocated Hypocenters To enhance stability of the ray tracer and smooth unwarranted near-surface velocity oscillations, an additionalayer of velocity nodes at z = -1 km, fixed at 3.39 km/s, was added as an upper boundary layer for the final inversion. All of the models suffer from sparse sampling and hence low resolution in the top (0 km) and bottom (16 km) boundary layers, so these layers are omitted from the following discussion. The remaining layers, at 2, 4, 6, 8, 10, 12, and 14 km for the P velocity model and 4, 8, and 12 km for suggesthe lower resolution of the three-dimensional P velocity model in the vicinity of low-velocity anomalies below-6 km depth is coupled to the presence of low-velocity bodies, i.e., anomalous composition and structure, and not strictly a function of the P data set and model parameterization Relocated hypocenters: Hypocenters relocated with the new three-dimensional P velocity model (Figure 6) moved an average of 1.0 km upward, 0.3 km west, and 0.4 km north, and the average rms residual was reduced from 0.10 s to 0.04 s. The most visible effect of the hypocente relocation is the clustering of the Vp/Vs ratio and S velocity models, are included in the interpretation, but we show only the 4 km and 8 km layers for convenience. The one-dimensional starting models for the P and l/p/v s ratio inversions were the same as the background velocities 1 Supporting Appendices B-D are available on diskette or via Anonymous FTP from kosmos.agu.org, directory APEND (Username = anonymous, used in the synthetic tests (Appendix A). Password = guest). Diskette may be ordered from American Geophysical Three-dimensional P wave velocity model. A total of Union, 2000 Florida Avenue, N.W., Washington, DC or by phone 66,637 P travel times were inverted to derive the final three- at ; $15.00 Payment must accompany order.

7 MILLER AND SMITH' YELLOWSTONE P AND $ VELOCITY 1:5,111 N (a) kin37 i iiii IIIIlllll 0 ' P velocity model (b) N ! km! I, i i i i i i i i i is Derivative Weight Sum Diagonal Element of Resolution Matrix N (c) 0 74 I Vp/V s model (d). '0 k m3 7, 0. _ 7-8. o i 16 i! km km.f: =4kin ]! I I i i / I... i't... s s... "i.'l ß!3 ß 0! Derivative Weight Sum Diagonal Element of Resolution Matrix Plate 1. (a) Derivative weight sum and (b) diagonal element of the resolution matrix for the three-dimensional P velocity model, superimposed on an outline of the Yellowstone caldera. (c) Derivative weight sum and (d) diagonal element of the resolution matrix for the three-dimensional Vp/Vs ratio model, superimposed on an outline of the Yellowstone caldera.

8 15,112 MILLER AND SMITH: YELLOWSTONE P AND S VELOCITY 4- o 4-

9 MILLER AND SMITH: YELLOWSTONE P AND $ VELOCITY 15,113 (a) Depth slices for S velocity model (b) ii i V s (km/s) N (c), Sources of crustal deformation... Saturated and overpressured 37 km facture zone 7 Dome I Outlm I -- gram I. I I P',aial m l I hcrmaliy i 8 -- I undisturbed I - basement I Depth limit 6-- / " :am,in -4.0 ' km Vp (k s) Inte retive geologic cross section from P wave model of Yellowstone caldera Plate 3. Depth slices from the three-dimensional S velocity model at (a) 4 km and (b) 8 km. Outlines of the Yellowstone caldera, resurgent domes, and Yellowstone Lake are shown on both images. (c) Interpretive SW-NE geologicros section, parallel to the SW-NE axis of the Yellowstone caldera, showing the P velocity model and crustal deformation source volumes. The interpretation of peripheral high velocities in poorly resolved parts of the model as thermally undisturbed basement rocks is based in part on previou seismic studies [e.g., Lehman et al., 1982; Benz and Smith, 1984; and Brokaw, 1985]. Crustal deflation source volumes are from inversion of volumetrichanges that matched the crustal deflation pattern derived from GPS measurements [Meertens et al.,

10 15,114 MILLER AND SMITH: YELLOWSTONE P AND S VELOCITY -[- 45i 10' N 111 i 20' W o o Model Normal fault Volcanic Vent km MAGN!TUDES ' i 00' N-Ji i 50' W Figure 6. Map of the Yellowstone plateau showing 7942 epicenters relocated with the three-dimensional P velocity model. Also shown are Quatemary faults and postcaldera volcanic vents. Bold letters denote locations of focal depth cross sections shown in Figure 7. Shaded areas and letter symbols are defined in Figure 1. events in the southwestern and east central caldera as well as east of the caldera. The most intense seismicity in Yellowstone occurs northwest of the caldera in an E-W band from the eastern end of Hebgen Lake to the Norris Geyser Basin. A smaller NW-SE trending cluster of events -10 km east of West Yellowstone is mainly the result of a swarm in 1985 [Nagy and Smith, 1988]. Swarm activity is also common in the Norris Junction area. A small cluster of events just inside the caldera rim southeast of Norris is more diffuse but has a similar NW-SE alignment. A broad, N-S trending zone of seismicity dominates the east central caldera from Canyon Junction to the southern part of Yellowstone Lake. Significant earthquake activity is also represented by a 40 km long band of earthquakes in the southwestern caldera from the caldera boundary to just south of Old Faithful. This band of seismicity is truncated by the caldera rim and changes orientation from a N-S to a NW-SE trend about halfway along its length. The rest of the Yellowstone volcanic field is characterized by diffuse seismicity, except for small clusters of events-20 km east of the caldera and on the caldera boundary -20 km northwest of Old Faithful. A conspicuous lack of seismicity distinguishes the northeastern and central caldera Focal depth distribution: The representative depth distribution of earthquake loci relocated with the threedimensional P velocity model is shown in four cross sections (Figure 7). Most earthquakes in the central caldera occur at depths less than 5 km except for several events in the northeast caldera as deep as 9 km (A-A'). Two cross sections (B-B' and B'-B") show a pattem of rapidly shallowing earthquake foci toward and within the caldera. Maximum focal depths near the eastern end of Hebgen Lake are more than 11 km, whereas the majority of earthquake foci in and around the Norris Geyser Basin and the northem caldera rim are less than 8 km deep. Focal depths within the caldera are mostly less than 6 km but deepen to -10 km beneath Yellowstone Lake. The northem caldera boundary is marked by an abrupt 4 km decrease in focal depths, and the relative aseismicity of the caldera is also clearly illustrated. A pronouncedecrease in focal depths and seismicity also shown in the southwestem caldera (C-C'). These pattems are not as clear in the eastem caldera (D-D') where focal depths increase 1-2 km to the southeast Contribution of controlled source data: The importance of using explosion data to determine the final threedimensional P velocity solution was investigated by inverting the P wave data without the 462 refraction observations. Inversion of the earthquake data alone gave a P velocity model with a final rms residual of 0.09 s, reduced from 0.12 s. In general, the P velocity model determined without the controlled source data shows the same pattem of low-velocity anomalies as the final P velocity model (Plates 2a and 2b; Appendix B) but lacks a clearly defined, prominent low-velocity zone above 4 km depth beneath the northeastem caldera, and the velocity anomalies near Sour Creek and Mallard Lake are -3-4 km deeper. These results demonstrate the upper 4--5 km of the P velocity model is primarily constrained by the explosion data. This conclusion is consistent with the P data distribution, which comprises more explosions and temporary seismographs than earthquakes and permanent network stations in the northeastem caldera (Figures 2 and 3). It also agrees with the depth coverage of locally recorded ray paths generateduring the Yellowstone controlled-source experiments, since critically refracted ray paths were largely restricted to the upper 5-6 km (the average depth of the Pg refractor reported by Lehman et al. [1982], and Brokaw [1985]) in contrasto the 5+ km depths of typical near-source Yellowstonearthquake ray paths. The explosion data, which represent Pg waves propagating the heterogeneous shallow crust, provide unique information not available from earthquakes. These data also help average out near-surface errors because of its density and known source parameters. Without the controlled source data, low velocities in the upper few kilometers are not recovered, thus forcing lowvelocity zones deeper in the model to accommodate travel time delays required by the earthquake data Three-dimensional VI,/V s ratio model. The final threedimensional Vp/Vs ratio model (Appendix C) is much smoother and shows less detail than the P velocity model because of its coarser grid parameterization. Although greater uncertainty the 673 S-P travel times inverted to derive the Vp/Vs ratio model resulted in a higher rms residual of 0.29 s (reduced from 0.5 s) compared to the final P velocity model, unique information is provided by this model. Vp/V s ratios do not vary greatly from the starting model. Northwest and north of the caldera, ratios increase 4% to as high as 1.75 and 1.71 at depths of 4 and 8 km, respectively (Plates 2d and 2e), and correspond to the regional Vp/V s ratio for the Yellowstone volcanic field independently determined by Chatterjee et al. [1985]. A NW-SE trending zone of Vp/Vs ratios as low as 1.61 divides the region of higheratios. This low Vp/Vs ratio zone broadens with increasing depth and probably corresponds to low P velocities in the Norris-Mammoth corridor but is offset by-10 km to the west due to the coarser grid spacing.

11 ,. (a) SW Yellowstone Caldera NE (c) NW e llowst eee ee ß ß 0 - f e e ß. ß 15'! I 15 o km 75 ß. ß I I 0 km 74 (b) Yellowstone Caldera NW Yellowstone Caldera 9 A-A', -...,tat' :,. ß....._... 'e ß ee ß ß ß1 ß g ß ß :i',.-.':,,"t... ß ß ß ß ;0,. ;.'. SE 15 - ' I 0 km 58 Figure 7. Yellowstone focal depth cross sections: (a) SW-NE, parallel to the long axis of the Yellowstone caldera, (b) composite W-E and NW-SE, north of and across the caldera, (c) NW-SE, across the southwestern caldera, and (d) NW-SE, across the northeastern caldera. o 'N 'W 4-4- (b) oo $o.. ß O O o O MAGNITUDES ß I ,f05' N-+- o km Figure 8. Map of the Yellowstone plateau showing epicenters of 511 earthquakes (a) used for the threedimensional V.,/V s ratio inversion analysis and (b) relocated with the three-dimensional S velocity model. Shaded areas and lette symbols are defined Figure 'W

12 15,116 MILLER AND SMITH: YELLOWSTONE P AND S VELOCITY The northeastern caldera, in the vicinity of the P velocity anomalies at Hot Springs Basin and the Sour Creek dome, shows only a hint of Vp/Vs ratio variation, varying from 1.63 at 4 km depth to 1.67 at 8 and 12 km depths. Relative to initial ratios of 1.68 and 1.65 at the same depths, respectively, the changes may not be significant. However, compared to average caldera Vff7/s ratios of 1.69 at 4 km depth and 1.66 at 8 and 12 km depths, they suggest a nominal perturbation in the V Vs field is required by the data. Furthermore, consideration of the synthetic Vp/V s ratio test results (Appendix A) suggests the slight changes are minimum estimates of the differences between the starting and true ratio distributions. Vp/Vs ratios in the northeastern calderare thus likely lower than the caldera average above 4 km depth but higher velocity model and persist to a depth of 14 km, the limit of the than the caldera average below 4 km depth. data-model parameterization's resolving power. The final Vff/Vs ratio model does not show any anomalous The caldera-wide low P and S velocity anomaly below 4 km variation in the southwestern caldera, possibly because the 20 km depth coincides with the prominent 60 mgal negative Bouguer x 18.5 km grid spacing smoothed a Vff/Vstructure associated with gravity anomaly (Plate 2), implying a common source for these the slightly smaller P velocity anomaly near the Mallard Lake features. The gravity low can be explained only by a thick dome. In addition, the caldera as a whole shows slightly higher accumulation of low-density caldera fill and/or a low-density Hypocenters recalculated with the S-P data (Figure 8b) showed, on average, small shifts of 0.9 km (west), 0.0 km (north), and 1.0 km (shallower). The heterogeneity of the rocks underlying the caldera, however, is demonstrated by declustering of events south and east of the caldera. Alternatively, the hypocenter declustering may indicate inaccurate locations caused by misidentification of S arrivals due to attenuation, phase conversion, and anisotropy along propagation paths through and/or beneath the caldera. 3. Comparison of Seismic Velocity Distributions With Volcano-Tectonic Features 3.1. Velocity Models In general, the P and S velocity models (Plates 2 and 3; Appendices B and D) correlate with Yellowstone's tectonic and volcanic features. High P velocities north, east, and south of the caldera coincide with outcrops of higher-velocity Precambrian metamorphic and Paleozoic sedimentary rocks (Figure 1), whereas lower P and S velocities within and southwest of the caldera are coincident with ash flow sheets and/or postcaldera rhyolite flows. These patterns are well developed in the upper 8 km of the P Vp/V s ratios than surrounding rocks, buthe increase is only a few volume of rock underlying the caldera. However, the magnitude percent Three-dimensional S velocity model. The threeof the observed gravity anomaly and estimated velocity reductions cannot be attributed solely to caldera fill, making an increase in dimensional S velocity model (Appendix D) was calculated from temperature, an increase in the fracture and microcrack density, the final P velocity and V Vs ratio models, and is therefore and the presence of hydrothermal fluids, partial melts, and/or dependent on both models. In contrasto the P velocity model, magma more attractive interpretations. the S velocity structure does not show a distinct pattem delineating Northeastern caldera. The northeastern caldera is the caldera above 8 km depth (Plates 3a and 3b). From depths of 8 km to at least 12 km, however, a caldera-wide low S velocity anomaly of km/s is well developed relative to regional S velocities of km/s. Within the caldera, a prominent km by -40 km zone of S velocities as low as km/s between 8 and 12 km depths roughly coincides with the P velocity anomaly under the Sour Creek dome. Compared to initial S velocities (Appendix A, Table A1), this anomaly represents a decrease of as much as 33%; if the comparison is made to background S velocities determined from the final P velocity and Vv/V s ratio models, the S velocity reduction is nearly 43%. A broad low S velocity zone of less than 3.0 km/s is imaged in the upper 4 km beneath the northeast caldera rim. Although this zone is approximately coincident with the Hot Springs Basin P velocity anomaly, it is wider and more pronounced in the nearcharacterized by Hot Springs Basin, the largest and most active element of the Yellowstone hydrothermal system, and coincident low P and S velocity anomalies, lower Vp/Vs ratios, and an additional gravity low. The strong spatial correlation between the Hot Springs Basin P velocity anomaly and the -20 mgal gravity anomaly suggests a common origin for these features. All of these observations can be explained by the fractured, fluid (gas or gas/liquid) saturated, and possibly hydrothermally altered volume of rock proposed by Lehman et al. [ 1982] at depths as shallow as 2 km and as deep as 4 km. This curved, saturated fracture zone is likely related to the normal fault zone northeast of Hot Springs Basin (Figure 1). Most of the faults in this area strike NW and dip SW toward the caldera, implying they extend directly under the Hot Springs Basin area and possibly intersecthe zone of low P velocities and the caldera rim fracture zone. Postcaldera surface northeast of the caldera. The Norris-Mammoth corridor is slip along these faults could have extensively fractured this region, marked by low S velocities of 3.2 and 2.5 km/s at 4 and 8 km depths, respectively. No S velocity anomaly is apparent in the creating a highly permeable conduit for hydrothermal fluids. During periods of tectonic quiescence along the faults, minerals southwestern caldera, probably for the same reasons a Vv/V s ratio precipitated from hydrothermal fluids could clog the upper anomaly was not imaged in this part of the model. Greater uncertainty in the final locations of earthquake foci portions of the fracture network, eventually resulting in higher pore pressures and a significant velocity reduction. In addition, calculated from the Vp/Vs ratio inversion resulted from ambiguities the low densities are consistent with characteristics of in the S-P data. Initial hypocenters for the S data set (Figure 8a) were taken from the hypocenters relocated with the threedimensional P velocity model. The average rms residual for the 511 events with at least one S arrival was 0.14 s, comparable to the estimated observational error of the S-P data set. hydrothermal alteration, including increased secondary porosity, microcracks, and low-density replacement mineral assemblages. The Sour Creek resurgent dome has concomitant low P and S velocities and higher Vp/V s ratios but does not show a strong correlation with the observed gravity field. The lack of a distinct, large-amplitude gravity low coincident with the anomalous velocities argues against a significant upper crustal magma accumulation beneath the eastern half of the caldera. A moderate volume(s) of partially melted rock is a reasonable interpretation, however, of the P and S velocity reductions, Vp/V s ratio increases, and the intermediate gravity low beneath the Sour Creek resurgent dome. Alternatively, magmatic and/or hydrothermal liquids produced by cooling of partially melted rocks could locally

13 MILLER AND SMITH: YELLOWSTONE P AND S VELOCITY 15,117 increase pore pressures and create more microcracks. These within the calder are less than 6 km deep compared to more than mechanisms would enhance seismic velocity reduction and Vp/Vs 11 km deep northwest of the caldera. The shallowest focal depths ratio increases yet have small effects on the density and hence occur in the west central caldera where maximum focal depths are gravity field. The heat source driving the system is represented by the Sour Creek dome velocity anomaly. In this interpretation, the local consistently <5 km deep. Variations in the thickness of the seismogenic crust, or depth of the brittle-ductile transition, are likely represented by the pattern of relatively shallow seismicity hydrothermal convection cell is recharged with meteoric water within the calder and deeper seismicity outside the caldera. The from within the caldera, which percolates to depths of 4-6 km, is 80th percentile of focal depths is thought to represent the brittleheated sufficiently to make it more buoyant than the host rock, and ductile transition and corresponds to temperatures as high as 350' ascends along the fracture/fault zone in a northeasterly direction 'C in many crustal regimes [Smith and Bruhn, 1984] or as The rising hot water contributes to the velocity reduction in the low as 250øC in young volcanic areas [e.g., Hill, 1992]. An curved, diffuse zone of low P velocities between the Sour Creek average geothermal gradient of-70'c km 'l in the west central dome and Hot Springs Basin P velocity anomalies. In addition, the locus of recent caldera deformation is less than 20 km south of the Sour Creek dome-hot Springs Basin velocity caldera is implied by these data Regional spatial comparisons of velocity distributions. Seismic activity northwest of the caldera and in the southwestern and gravity anomalies. A likely cause of the observed crustal caldera is concentrated in sublinear zones with the same dynamics in the northeastern caldera is injection and expulsion of hydrothermal fluids and/orhyolitic/basaltic melts associated with the inferred shallow, possibly overpressured, fracture zone in the vicinity of Hot Springs Basin. This anomaly has depth similar to the main deformation sources (3-6 km) modeled by Vasco et al. [1990] and Meettens et al. [1993] Southwestern caldera. The low P velocities beneath the Mallard Lake resurgent dome can also be interpreted as a zone of partially melted rock despite a lack of corresponding S velocity, orientation as the Hebgen Lake and Teton fault zones, respectively. Some faults northwest of the caldera have been mapped as splays of the Hebgen Lake fault zone and likely correspond to at least some of the seismic activity there. Seismicity in the southwestem caldera may reflect movement along an extension or splay of the Teton fault zone now buried by ash flow sheets and postcaldera rhyolite flows (Figure 1). Faults north and south of the caldera may have once been continuous [Ruppel, 1972]. If so, cooling and solidification of the Vp/Vs ratio, and gravity anomalies. The smaller breadth and depth granitic body beneath the caldera has allowed brittle deformation, extent of the Mallard Lake dome P velocity anomaly implies a manifest as faulting and earthquakes, to again occur within the smaller volume of partial melt than that beneath the Sour Creek caldera as the fault zones north and south of the caldera grow dome. A reduced, perhaps hardly distinguishable, density contrast toward each other along a prominent zone of weakness. Such a would be expected from less partial melt in a smaller area. In addition, the coarse parameterization used to invert for the Vp/V s zone of weakness is implied not only by the seismicity in the western caldera but by a linear, NW-SE trending zone of ratio model may have smoothed out any anomalous Vp/V s ratio postcaldera volcanic vents (Figure 1). Isolated swarm activity on and S velocity features with horizontal dimensions less than the 20 the caldera boundary northwest of Old Faithful is probably related km by 18.5 km grid interval Northern caldera. The low P and S velocities and Vp/V s to current movements along the adjacent segment of the caldera rim fracture zone, possibly as an incipient hydrothermal cell (hot ratios imaged in the Norris-Mammoth corridor can be interpreted springs and/or geysers). as a deep graben bounded on the west by the Gallatin fault and filled with low-velocity alluvium and colluvium. This The seismic activity in and southeast of the Norris Junction area is also spatially associated with a band of volcanic vents. interpretation is compatible with previous seismic investigations Extrapolation of this trend south and east across the caldera of Yellowstone [e.g., Lehman et al., 1982; Benz and Smith, 1984]. suggestsplays of the Gallatin fault and the large normal faults In addition, fluid- and/or gas-saturated sediments associated with near the southern end of Yellowstone Lake extend beneath the hydrothermal features in this region could contribute to the observed velocity reduction. central caldera (Figure 1). An alternative explanation for the Norris seismicity is a large hydrothermal/fracture system at the intersection of the southern end of the Gallatin fault zone and the 3.2. Hypocenters and Volcano-Tectonic Patterns The distribution of hypocenters relocated with the new threedimensional P velocity model shows markedly less intense seismicity within the caldera. This result is consistent with the northern edge of the caldera rim fracture zone. Seismicity in the east central caldera could also be associated with the transition between brittle, thermally undisturbed rock east of the caldera and more plastic, possibly hydrothermally altered interpretations of the velocity and Vp/Vs ratio models. The rocks rocks beneath the caldera. No interpretations of the small cluster underlying the caldera at depths greater than a few kilometers may have a quasi-plastic rheology primarily related to abnormally high temperatures and vestigial pockets of partial melt. In such a regime, stresses and strain accumulation are relieved by ductile flow and aseismic creep. This would result in relatively few of events east of the caldera can be made because the velocity models do not extend far enough to the east. Taken together, these data imply a significant heat source in the upper crust as shallow as 8 km and possibly deeper than 12 km. The preferred geologic model inferred from the velocity inversion earthquakes within the caldera and the sharp focal depth decrease results and supported by other geologic and geophysical data coincident with the caldera rim (Figure 7). High temperatures in the shallow crust immediately northeast of the caldera could also result in reduced seismic activity Recaiculated focal depths. Computation of the focal depths' 85th percentile, i.e., the depth above which 85% of the earthquake foci occur, shows the majority of earthquake foci (Plate 3c) includes (1) a large volume of hot but solid granitic rock beneath the caldera and extending 10 km radially beyond the caldera rim, set in a matrix of metamorphic basement and sedimentary cover rocks less thermally disturbed by recent magmatic processes; (2) an oblong, partially overpressured zone of fractured rock in the vicinity of Hot Springs Basin at depths of

14 15,118 MILLER AND SMITH: YELLOWSTONE P AND $ VELOCITY 2-4 km; (3) a zone of partial melt beneath the northeastern caldera how well the combined earthquake and refraction data could at 8 km depth, coincident with the Sour Creek dome and adjacent constrain the major velocity anomalies reported in previous to and underlain by bodies with lesser partial melt concentrations studies. An idealized three-dimensional P velocity model was and/or quasi-plastic rheology to a depth of at least 12 km; and (4) constructed to approximate these velocity anomalies in a target a separate zone of partial melt-8 km beneath the southwestern volume 80 km wide by 74 km long by 16 km thick. The model caldera in the vicinity of the Mallard Lake dome. The zones of volume was parameterized with a grid composed of 729 nodes in partial melt beneath the resurgent domes are likely remnants of increments of 10.0, 9.25, and 2 km along the x (E-W), y (N-S), two magma chambers related to the eruption of the Lava Creek and z (vertical) axes, respectively, giving nine planes of nodes in Tuff and related to the loci of recent crustal deformation in the all three coordinate axis directions (origin in the center). A onedimensional P velocity depth function (Table A1) was assigned to Yellowstone volcanic plateau. the nodes to form a planar, four-layer velocity model used by the UUSS for routine locations of Yellowstone earthquakes [Peyton 4. Concluding Remarks and Smith, 1990]. P velocity reductions of 26-32% and 23% were incorporated into the model beneath the northeastern and P and S wave tomography using local earthquakes and southwestern portions of the caldera, respectively (Figure A1), to explosions has been used to elucidate important details of the simulate upper crustal velocity anomalies determined from Yellowstone volcanic plateau's crustal structure with greater previous controlled source [Lehman et al., 1982] and tomographic resolution than previous geophysical studies. A three-dimensional studies [Benz and Smith, 1984]. A synthetic P data set of? velocity model (average rms residual of q s) for the calculated earthquake and shotravel times was generated using Yellowstone plateau has been determined, and three-dimensional Thurber's [1981, 1983] three-dimensional ray-tracing algorithm Vp/Vs ratio (average rms residual of q s) and S velocity and the idealized P velocity model with the same source-receiver models for the region have been constructed for the first time. Exclusion of the controlled source data results in smeared or geometry of the actual P data set. Four inversions were performed with the synthetic P data set to nonimaged P velocity features in the near-surface (upper 4 km), evaluate the effects of observational noise, model demonstrating that the explosion data constrain shallow structure parameterization, hypocenter location, and source-receiver in the P velocity model and provide important informationot coverage. All of the test inversions started with the idealized onecontained in the earthquake data. dimensional P velocity model (Table A1) withouthe low-velocity The P and S velocity structure of the Yellowstone caldera zones. In the source-receiver coverage test, no noise was added to shows prominent low-velocity anomalies in the upper crust likely the travel times but each hypocenter was perturbed by q- 5 km to related to Yellowstone's youthful volcanism. Concomitant low simulate a worst-case scenariof poorly located hypocenters. The seismic velocities, relatively lower Vp/Vs ratios, and low densities inversion converged to a final solution in four iterations, reducing beneath the northeast rim of the caldera in the vicinity of Hot the initial rms residual of 0.1 s to 0.06 s. The average difference Springs Basin identify a saturated and hydrothermally between the relocated, perturbed hypocenters and the unperturbed, metamorphosed fracture zone characterized by potentially high actual hypocenters was 0.93 km, illustrating the ability of the pore pressures at least as shallow as 2 km and extending to a depth inversion technique to produce satisfactory results even with data of 4 km. Low P and S velocities coupled with relatively higher contaminated by large errors. The velocity results of this Vp/Vs ratios below a depth of 6 km in the east central caldera are inversion (Figure A2) show relatively low velocities are imaged in interpreted as a zone of partially melted rock underlying the Sour the correct locations, but the magnitude of the velocities within the Creek resurgent dome. Low P velocities at 8 km depth in the anomalies is higher by -1 km/s than the true velocities. southwestern caldera imply small concentrations of partial melt For the noise test, normally distributed (Gaussian) random beneath the Mallard Lake resurgent dome. A broad, lowernoise with a standard deviation of q- 0.1 s was added to the amplitude P and S low-velocity anomaly coincides with a calderacalculated P travel times to simulate actual recording noise and wide regional gravity low and is thought to represent a batholithic, reading error. Inversion of these data gave a final rms residual of quasi-plastic, granitic volume of rock at high but subsolidus 0.10 s, equal to the added noise level, after two iterations. Two temperatures. additional inversions of the synthetic P data set (no noise added) Earthquake foci relocated with the three-dimensional P velocity model shallow to less than -6 km within the caldera and show were performed with different model parameterizations to characterize the effect of imposing the three-dimensional grid on subjacent alignment with postcaldera volcanic vents and the the model space. First, the grid was modified by reducing the orientation of large normal faults north and south of the caldera. These data combined with observations of recent crustal deformation suggest crustal volcano-tectonic processes are still active in Yellowstone and emphasize the importance of continued geophysical monitoring of the Yellowstone volcanic field, particularly since renewed volcanic activity would likely be preceded by earthquakes with unusual space-time relationships as well as measurable changes in surface deformation [Smith and Braile, 1994; Christiansen, 1999]. Appendix A: Testing With Synthetic Data A1. Synthetic P Velocity Models A series of tests was carried out to determine the coverage and resolving power of the Yellowstone P and S data sets, particularly Table A1. Starting One-Dimensional P Velocity and Vp/V s Ratio Models Depth, km P Velocity, km/s Vp/V s Ratio S Velocity, km/s

15 MILLER AND SMITH: YELLOWSTONE P AND S VELOCITY 15,119

16 . ß 15,120 MILLER AND SMITH: YELLOWSTONE P AND $ VELOCITY km W iiiiiii i iii i ill ; iiii1 ii i!._ill! 1.90 ;.' I.,gO... : :.. ' 1.7o 138 Vp/V' s Ratio ratio increases of 12% and 15% were added to the model at 8 km depth (Figure A3). A set of synthetic S-P travel times was generated using the idealized Vff'V s ratio model, the sourcereceiver geometry of the actual S data set, and the final threedimensional P velocity model determined from actual P observations (Appendix B). No modifications were made to the grid used to compute the synthetic S-P data set, but random noise (On = s) was added to the calculated S-P travel times. P velocities were held fixed during the inversion to isolate the character of the S data set. Only two iterations were required to converge to a solution with a final rms residual of 0.08 s. The results of the test S-P inversion (Figure A4) show relatively higher Vff'V s ratios in the northeast part of the model, corresponding to the true model. However, only a hint of the southwest anomaly is imaged, perhaps because some of the misfit due to the anomaly was accommodated by placing higher Vff'V s ratios at 10 km depth in the center of the model. All of the Vp/V s ratio anomalies have smaller imaged amplitudes than the true amplitudes, similar to the synthetic P results. Thus the Vff'V s ratio variation obtained by inversion of the real S data set represents a minimum estimate of the actual variation. The same reasoning applies to the S velocity structure calculated from the Yellowstone P velocity and Vff'V s ratio models. 74 Figure A3. Idealized V, V s ratio model used as the starting model for the synthetic S-P data tests: (a) SW-NE cross section, (b) depth slice at 8 km, and (c) W-E slice at y = 37 km. Outlines of the Yellowstone caldera and Yellowstone Lake are shown on Figures A3a and A3b. km 37 -'16 N number of nodes in each of the nine layers from 81 to 49. The initial rms residual of 0.08 s was reduced to 0.03 s after five iterations. A second modified grid comprising nine layers that varied in thickness from 3 km at z < 7 km to I km for z > 9 km was used to investigate the effect of varying the vertical position of the nodes. After three iterations, the rms residual was reduced from 0.09 s to 0.05 s. The velocity results of these tests are similar to those of the source-receiver coverage test; both lowvelocity zones were imaged in the correct spatial position, although the amplitude of the anomalies is lower than those in the true model, especially for depths less than 8 km. The results of the test inversionsupporthree conclusions: (1) the Yellowstone P data set and source-receiver geometry are adequate to resolve low velocity anomalies in the northeastern and southwestern caldera to depths of at least 12 km; (2) the effects of noise, model parameterization, and poorly located hypocenters (up to ñ 5 km errors) are not expected to significantly degrade the P velocity solution; and (3) any low velocities imaged by inversion of the P data set must be considered a minimum estimate of their true magnitude, due in part to rays bending around rather than passing through the low-velocity bodies. A2. Synthetic Vt,/V s Ratio Model For the S data setest an idealized one-dimensional Vp/V s ratio model (Table A1) was constructed. Rectangular areas with Vp/V s b) c.) 40 0 ken 190 Vp/V s Ratio Figure A4. Inversion results for the synthetic S-P data set with ñ 0.1 s Gaussian noise added: (a) SW-NE cross section, (b) depth slice at 8 km, and (c) W-E slice at y = 37 km. Outlines of the Yellowstone caldera and Yellowstone Lake are shown on Figures A4a and A4b.

17 MILLER AND SMITH: YELLOWSTONE? AND $ VELOCITY 15,121 Acknowledgments. This research was sponsored through Graduate contribution by R. A. Hutchinson: Summary of felt earthquakes in Fellowships awarded to the author, Doug Miller, by the National Science Yellowstone National Park, , special publication, pp. 1-62, Foundation, the University of Utah Research Council, and the Society of Univ. of Utah Seismogr. Stn., Salt Lake City, Exploration Geophysicists Foundation. We thank the staff of the Peyton, S. L., and R. B. Smith, Earthquake data for the Yellowstone University of Utah Seismograph Stations for facilitating use of the National Park region, January 1, 1989 to December 31, 1989, special Yellowstone earthquake data and assisting with field operations. My publication, pp. 1-26, Univ. of Utah Seismogr. Stn., Salt Lake City, father, Calvin Miller, is particularly thanked for his assistance with May rigorous winter field work. Thanks are due Yellowstone National Park Pitt, A.M., Catalog of earthquakes in the Yellowstone National Parkstaff for facilitating field work including, the late Rick Hutchinson, John Hebgen Lake region, Wyoming, Montana, and Idaho, for the years Varley, and John Lounsbury. Field work was supported by the National , U.S. Geol. Surv. Open File Rep., , Park Service, U.S. Geological Survey, grant A-0975, and the Ruppel, E. T., Geology of pre-tertiary rocks in the northern part of National Science Foundation, research grant EAR Yellowstone National Park, Wyoming, U.S. Geol. Surv. Prof. Pap., , 66 pp., Schilly, M. M., R. B. Smith, L. W. Braile, and J. Ansorge, The 1978 References Yellowstone-eastern Snake River Plain seismic profiling experiment: Data and upper crustal structure of the Yellowstone region, d. Geophys. Andrews, M. C., W. D. Mooney, and R. P. Meyer, The relocation of Res., 87, , microearthquakes in the northem Mississippi embayment, d. Geophys. Smith, R. B., and L. W. 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Taylor, Inversion of Yellowstone SIMULPS12 for imaging Vt, and V_e/Vs: A derivative of the Thurber to nographic inversion SIMUL3 for local earthquakes and explosions, vertical displacements and gravity changes, 1923 to to U.S. Geol. Surv. Open File Rep., , , d. Geophys. Res., 95, , Hill, D. P., Temperatures at the base of the seismogenicrust beneath Walck, M. C., and R. W. Clayton, P wave velocity variations in the Coso Long Valley caldera, California, and the Phlegrean Fields caldera, region, California, derived from local earthquake travel times, J. Italy, in Volcanic Seismology, edited by P. Gasparini, R. Scarpa, and K. Geophys. Res., 92, , ki, pp , Springer-Verlag, New York, Lehman, J. A., R. B. Smith, M. M. Schilly, and L. W. Braile, Upper crustal structure of the Yellowstone caldera from delay time analyses and gravity correlations, d. Geophys. Res., 87, , Meertens, C.M., R.B. Smith, and D.W. Vasco, Kinematics of crustal D. S. Miller, 818 Kip Lane, Pinehurst, TX deformation of the Yellowstone hotspot using GPS, Eos Trans..4GU, (dsmille eec.exxon.com) 74 (43), Fall Meet. Suppl., 63, R. B. Smith, Department of Geology and Geophysics, 702 WBB, Nagy, W. C., and R. B. Smith, Seismotectonic implications of the University of Utah, Salt Lake City, UT (rbsmith moose.utah.edu) Yellowstone earthquake swarm from velocity and stress inversion, Seismol. Res. Lett., 59, 20, Nava, S. J., and R. B. Smith, Earthquake catalog for the Yellowstone Received September 11, 1997; revised August 11, 1998; National Park region, January 1, 1992 to December 31, 1994, with a accepted November 16, 1998.)