Monitoring changes in seismic velocity related to an ongoing rapid inflation event at Okmok volcano, Alaska

Transcription

1 PUBLICATIONS Journal of Geophysical Research: Solid Earth RESEARCH ARTICLE 1.12/215JB11939 Key Points: ANI is a valuable method for monitoring volcanic activity at Okmok volcano Rapid inflation represents recharge of the shallow magma reservoir ANI offers the first seismic evidence of a rapid inflation event at Okmok Correspondence to: N. L. Bennington, ninfa@geology.wisc.edu Citation: Bennington, N. L., M. Haney, S. De Angelis, C. H. Thurber, and J. Freymueller (215), Monitoring changes velocity related to an ongoing rapid inflation event at Okmok volcano, Alaska, J. Geophys. Res. Solid Earth, 12, , doi:1.12/215jb Received 5 FEB 215 Accepted 13 JUL 215 Accepted article online 2 JUL 215 Published online 18 AUG 215 Monitoring changes velocity related to an ongoing rapid inflation event at Okmok volcano, Alaska Ninfa L. Bennington 1, Matthew Haney 2, Silvio De Angelis 3, Clifford H. Thurber 1, and Jeffrey Freymueller 4 1 Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin, USA, 2 Alaska Volcano Observatory, U.S. Geological Survey, Anchorage, Alaska, USA, 3 Department of Earth, Ocean, and Ecological Sciences, University of Liverpool, Liverpool, UK, 4 Department of Geology and Geophysics, University of Alaska Fairbanks, Fairbanks, Alaska, USA Abstract Okmok is one of the most active volcanoes in the Aleutian Arc. In an effort to improve our ability to detect precursory activity leading to eruption at Okmok, we monitor a recent, and possibly ongoing, GPS-inferred rapid inflation event at the volcano using ambient noise interferometry (ANI). Applying this method, we identify changes velocity outside of Okmok s caldera, which are related to the hydrologic cycle. Within the caldera, we observe decreases velocity that are associated with the GPS-inferred rapid inflation event. We also determine temporal changes in waveform decorrelation and show a continual increase in decorrelation rate over the time associated with the rapid inflation event. The magnitude of relative velocity decreases and decorrelation rate increases are comparable to previous studies at Piton de la Fournaise that associate such changes with increased production of volatiles and/or magmatic intrusion within the magma reservoir and associated opening of fractures and/or fissures. Notably, the largest decrease in relative velocity occurs along the intrastation path passing nearest to the center of the caldera. This observation, along with equal amplitude relative velocity decreases revealed via analysis of intracaldera autocorrelations, suggests that the inflation source may be located approximately within the center of the caldera and represent recharge of shallow magma storage in this location. Importantly, there is a relative absence of seismicity associated with this and previous rapid inflation events at Okmok. Thus, these ANI results are the first seismic evidence of such rapid inflation at the volcano American Geophysical Union. All Rights Reserved. 1. Introduction Okmok volcano is located on the NE portion of Umnak Island within the central Aleutian arc, Alaska. The volcano is a 3 km wide shield volcano containing a 1 km diameter caldera (Figure 1). Historically, eruptions at Okmok have occurred from 15 different vents within the caldera with eruptions typically forming cinder cones. Prior to the 28 eruption, the three most recent eruptions (1945, 1958, and 1997) emanated from Cone A (Figure 1) and were predominantly effusive [Larsen et al., 29]. The most recent eruption occurred on 12 July 28 at a series of new vents located NW of Cone D (Figure 1). Seismicity rates increased above background levels only 5 h prior to the eruption. The 28 eruption was highly explosive, with a volcanic explosivity index of 4, in part due to the magma interacting with groundwater and surface water. Larsen et al. [29] note that the 28 eruption was the first in the history of the Alaska Volcano Observatory (AVO) to go from aviation code green, indicating seismicity rates at background levels, directly to code red (imminent eruption with high rates of ash emission) within only several hours of time. The eruption was most energetic in the 1 h following the initial eruption with an estimated maximum ash column height of 16 km. Such frequent and, at times, unpredictable volcanic activity at Okmok poses a major hazard to North Pacific airtraffic and some of the worlds most productive fisheries. For this reason, it is imperative that we improve our ability to identify precursory activity leading to active volcanism at Okmok volcano. In the 7 years preceding the 28 eruption of Okmok volcano, two major rapid inflation events were observed within the caldera via GPS measurements. While Okmok inflated nearly continuously following the 1997 eruption at the volcano, those displacement rates were much lower than those rates observed during the two rapid inflation events. Fournier et al. [29] analyzed campaign and continuous GPS measurements from 21 to 27, and Biggs et al. [21] used the same data sets and time periods complemented by interferometric BENNINGTON ET AL. MONITORING RAPID INFLATION AT OKMOK 5664

2 1.12/215JB11939 Latitude (deg) Longitude (deg) Figure 1. A map of Okmok volcano, which is located along the Aleutian Arc, Alaska. White squares indicate the distribution of seismic instruments. Red triangles denote the location of Cones A and D. Permanent GPS stations are indicated as green hexagons. Black circles represent earthquake epicenters observed on Okmok s seismic network in the 2 months preceding as well as during the current rapid inflation event (4/1/213 and 8/4/214). Earthquake epicenters are also plotted for the major rapid inflation events in and in 24, which preceded the 28 Okmok eruption (magenta circles). synthetic aperture radar observations. In both studies, the authors reported two main periods of rapid inflation: summer of 22 through late 23 and spring through summer of 24. By the summer of 24, volume recovery of magma storage was estimated to be 75 85% of the 1997 erupted volume of magma [Lu et al., 21]. Following rapid inflation in 23 24, inflation slowed dramatically with magma storage reaching 85 1% of the 1997 eruption volume by July 28 [Lu et al., 21]. Lu et al. [21] note a similar pattern of initially rapid and then dramatically slowed inflation leading up to the 1997 eruption. The authors attribute this inflation pattern to an initial large-pressure gradient between a deeper magma source and shallow magma storage. According to their interpretation, as the pressure equalizes between the two regions, the rate of inflation within the magma reservoir decreases. However, if magma influx from the deeper magma region continues, a threshold pressure value is reached and eruption occurs. The above studies demonstrate that geodesy is an excellent tool for monitoring volcanic activity at Okmok. Though still in its infancy, seismic interferometry with ambient noise offers a novel approach to observing temporal changes at active volcanoes. A relatively small number of studies endeavor to use ambient noise interferometry (ANI) to study volcanoes. An early study by Sens-Schonfelder and Wegler [26] relates changes velocity at Merapi volcano to seasonal variations in annual rainfall. The bulk of ANI studies of volcanoes have been conducted at Piton de la Fournaise (PdF) volcano. These studies connect changes in the seismic velocity field to preeruption inflation and coeruptive deflation of the volcanic edifice for a series of eruptions between 1999 and 211 [Brenguier et al., 28; Duputel et al., 29; Brenguier et al., 212; Obermann et al., 213]. Obermann et al. [213] expand on this ANI work by also quantifying temporal changes in the correlation between seismic waveforms recorded on PdF s permanent seismic network. Small changes between the seismic waveforms result from temporal changes in the scattering properties of the subsurface. Baptie [21] applies ANI to Montserrat volcano and identifies changes in velocity associated with lava dome collapse. Mordret et al. [21] observe a preeruption decrease in relative velocity at the edifice of Mount Ruapehu, which they suggest represents intrusion of new magma into the magma storage zone. Finally, Anggono et al. [212] connect the increase in velocity they determine at Miyakejima volcano with deflation of the volcano s magma reservoir during the 2 eruption. Following Okmok s 28 eruption, we have identified a rapid inflation event via the volcano s permanent GPS network. This network is composed of two stations within (OKCE and OKNC) and two stations outside (OKSO and OKFG) the caldera (Figure 1). Figure 2 shows displacement data observed on Okmok s permanent GPS network between early January 213 and mid-october 214. GPS stations within the caldera show a rapid increase in the rate of vertical and horizontal displacement beginning in early June 213 (day 15 of Figure 2) whereas stations outside the caldera show little to no change in displacement. In this study, we take advantage of continuous waveforms recorded on the permanent seismic network at Okmok (Figure 1) to apply ANI to search for temporal changes velocity associated with this rapid inflation event. Our study focuses on the time period between early January 213 and mid-october 214. Figure 3 shows data availability for AVO s permanent seismic network for years We also determine temporal changes BENNINGTON ET AL. MONITORING RAPID INFLATION AT OKMOK 5665

3 1.12/215JB11939 (a) Vertical (cm) OKCE (b) OKNC East (cm) North (cm) (c) Vertical (cm) OKFG (d) OKSO East (cm) North (cm) Days since 1/1/ Days since 1/1/213 Figure 2. Displacement data, in centimeter, collected on Okmok s permanent GPS network at stations (a) OKCE, (b) OKNC, (c) OKFG, and (d) OKSO. Blue, open circles represent GPS-observed displacements in centimeter. Red lines indicate best fit straight lines for displacement data at time intervals: 15, 15 25, 25 35, and days. The time axis is with reference to 1/1/213, :: AM. in waveform decorrelation associated with this event. Interpretation of these changes velocity and decorrelation rate allow us to roughly constrain the location of the rapid inflation event at Okmok. 2. Data Processing We identify temporal changes velocity by measuring subtle differences between reference and current noise correlation functions (CF) for permanent seismic stations and station pairs at Okmok volcano (Figure 1). The reference CF is determined over a period of quiescence at the volcano. For each station or station pair, the reference CF is calculated as the average of all available hour-long ambient noise autocorrelation or cross correlation, respectively, from 212. Similarly, the current CF is determined as the average of available hour-long autocorrelation or cross correlations in a 1 day window in 213 and 214. The presence of data gaps in the seismic record precludes calculation of our CFs using a continuous record of hour-long ambient noise signal (Figure 3). BENNINGTON ET AL. MONITORING RAPID INFLATION AT OKMOK 5666

4 1.12/215JB11939 Station OKAK OKCF OKER OKID OKRE OKSP OKTU OKWE OKWR OKCE OKFG OKNC OKSO Figure 3. Data availability for the AVO s permanent seismic network at Okmok volcano between 1 January 212 and 8 October 214. Black lines indicate when data were successfully telemetered, and white lines indicate when telemetry was down. We obtain each hour-long autocorrelation or cross correlation following a procedure similar to the ambient noise tomography study of Masterlark et al. [21]. Hour-long seismic data at each station are initially low-pass filtered at 2 Hz. Automatic gain control is then applied via running RMS normalization using a time window length of 1 s. Both Bensen et al. [27]and Masterlark et al. [21] note that RMS normalization is comparable to sign bit normalization. Following application of automatic gain control, we whiten the signal over the frequency band.1 to 1.8 Hz. Individual stations or station pairs are then autocorrelated or cross-correlated, respectively, for each hour-long segment of seismic data. Once the averaged current CF is determined from a set of hour-long correlations, it is filtered from.3 to 1 Hz, which includes the frequency band of microseismic ocean noise. The same filtering is applied for the reference CF, which is the yearlong average of hourly CFs for the year 212. When determining a reference CF that involves a short-period instrument, we exclude those correlations determined for dates 17 August through 8 October 212. It was determined that high levels of low-frequency (~.7 Hz) noise are present on short-period stations during this time period. Figure 4 shows a comparison between the reference and current CFs for the intracaldera station pair OKCE- OKNC. At time = s, the CFs match well; however, later portions of the CFs show that the current CF becomes a time-shifted version of the reference CF. Snieder et al. [22] demonstrate that a relative shift in time, Δt/t, between a reference and current CF can be used to estimate the relative change in the bulk or average seismic velocity of the medium, Δv/v: Δt t ¼ Δv v : (1) We cross correlate the reference and current CFs over a 2 s long moving time window. Our moving time window is carried out between 15 and 75 s of the CFs. For each 2 s time window, the maximum crosscorrelation coefficient (CC) is found. (a) Following Haney et al. [215], if the CC.9, the associated lag time between CFs is determined and the center time of the moving window is noted. Once the CC is found for the Time (s) 2 s time window, and the lag time (b) (c) and associated center time possibly determined, the time window is shifted by a single sample, or.1 s, in an attempt to calculate a lag time for the new time window s center time Time (s) Time (s) Figure 4. (a) Comparison between the reference and current CF for station pair OKCE-OKNC. Early and late periods of comparison are shown in Figures 4b and 4c, respectively. Reference and current CF are shown as dashed and solid black lines, respectively. The current correlation is computed over a 1 day window with center time of 7::, 11/8/213. After all lag times and moving window center times are collected, the lag time, Δt, versus the center time of each 2 s window is plotted and a best fit line with slope, Δt/t, is determined. The absolute value of BENNINGTON ET AL. MONITORING RAPID INFLATION AT OKMOK 5667

5 1.12/215JB11939 Table 1. Parameters for Ambient Noise Interferometry Study at Okmok Volcano Parameter Value Length of time window considered s Band-pass frequencies.3 1Hz Moving window length 2 Minimum correlation to accept Δt measurement.9 Minimum Pearson s coefficient to accept Δt measurement.8 Temporal averaging window ±5 days Pearson s coefficient, R, is used to quantify the strength of this best fit line to the observed lag times [Haney et al., 215]. We can then use equation (1) to calculate Δv/v for the current CF. If R <.8, indicating a poor linear relationship, no estimate of relative velocity is made for the current CF [Haney et al., 215]. Error bars for the resulting Δv/v value are calculated following Brenguier et al. [28]. Those Δv/v values with associated error bars 1% are not plotted. While perturbations velocity reflect changes in long-scale changes in the elastic parameters of the medium, we can also monitor short-scale changes at the volcano by calculating the decorrelation between the reference and current CF. The decorrelation, or distortion between a reference and current CF, is used to identify changes in the scattering properties of the medium [Obermann et al., 213; Haney et al., 215]. The decorrelation coefficient (DC) is determined only for times when Δv/v is calculated and meets the criteria previously discussed. DC is calculated as DC ¼ 1 CC (2) The DC is determined coincident with the solution for each Δt. We determine the DC value for each 2 s moving window and solve for the decorrelation rate, which is the change in the DC over time, for the entire current CF. After Δt/t and decorrelation rate are determined, a new current CF is formed for a 1 day window that is 5 h after the center time of the previous window, and new changes in relative seismic velocity and decorrelation rate are determined by repeating the above process. We note that ambient noise correlations are determined for the frequency band of.3 to 1 Hz. Table 1 summarizes the parameters used for our ambient noise interferometry study of Okmok. 3. Concurrent GPS Data The permanent GPS network at Okmok consists of four stations, OKNC, OKCE, OKFG, and OKSO, which are spatiotemporally coincident with four identically named permanent seismic stations at the volcano (Figures 1 and 2). These GPS time series are in the ITRF28 reference frame. Thus, changes in displacement at these stations are influenced by local activity at Okmok volcano as well as the motion of the North American Plate. Following Okmok s 28 eruption, we have identified a rapid inflation event, beginning in early June 213, via the volcano s permanent GPS network. In the time leading up to and during this rapid inflation event, for the two intracaldera GPS stations, we observe four separate time intervals that each appear relatively linear with respect to changes in displacement with time (days 15, 15 25, 25 35, and of Figure 2). For each of these time intervals, we estimate the rate of displacement as the best fit linear trend representing each subset of displacement data versus time. Over the entire time interval of the study (early January 213 to mid-october 214), GPS stations outside the caldera show a relatively constant rate of displacement with little to no change in displacement with time (Figure 2). Thus, for these stations, we estimate the rate of displacement as the linear trend that best fits the entire GPS time series. The best fit linear trends representing the estimated rates of displacement are shown in Figure 2. Table 2 summarizes the estimated rates of vertical and horizontal displacement determined for Okmok s permanent GPS network between early January 213 and mid-october 214. We express the misfit of each estimated rate of displacement by calculating the reduced χ 2 misfit between the determined linear trend and the observed displacement data. A reduced χ 2 misfit of 1 would indicate that the estimated displacement rate fits the observed GPS data to within its uncertainties. For the set of displacement rates indicated in Table 2, the reduced χ 2 misfit ranges from 1.1 to 3.2 with a mean value of 2.2, suggesting that the estimated displacement rates fit the GPS data to acceptable levels. Between early January and late May 213 (days 15 of Figure 2), vertical and horizontal displacements appear relatively unchanged at all GPS stations with an average rate of displacement of.3 cm/d. BENNINGTON ET AL. MONITORING RAPID INFLATION AT OKMOK 5668

6 1.12/215JB11939 Table 2. Rates of Displacement Are Determined Using Displacement Data Observed on Okmok Volcano s Permanent GPS Network During the Noted Time Intervals a Time interval (days since 1/1/213) Station Component Estimated rate of displacement (cm/d) Reduced χ 2 misfit for rate of displacement 15 OKCE Vertical OKCE Vertical OKCE Vertical OKCE Vertical OKCE East OKCE East OKCE East OKCE East OKCE North OKNC Vertical OKNC Vertical OKNC Vertical OKNC Vertical OKNC East OKNC North OKNC North OKNC North OKNC North OKFG Vertical OKFG East 2 65 OKFG North OKSO Vertical OKSO East OKSO North a Each rate of displacement is determined using a straight line fit to each noted time interval of GPS data. The misfit between the straight line and the observed GPS data is determined using reduced χ 2. However, beginning in early June 213 (day 15 of Figure 2) we see the start of a rapid inflation event that appears to be constrained to Okmok s caldera. The two intracaldera GPS stations show an observable increase in vertical (OKCE and OKNC), westward (OKCE-only), and northward (OKNC-only) displacement. Relative to the rates of displacement between early January and late May 213, these GPS stations/components observe an approximate factor of 1 increase in the rates of vertical and horizontal displacements (day of Figure 2 and Table 2). The average displacement rate for this time period is.3 cm/d. By early September 213 (day 25 of Figure 2), these increases in vertical and horizontal displacements begin to occur more rapidly with rates of displacement 2 to 3 times larger than pre-june 213 displacement rates and an average displacement rate of.6 cm/d (days of Figure 2). However, starting in mid-december 213 (day 35 of Figure 2) the inflation event appears to slow with an average displacement rate of.2 cm/d for these intracaldera stations. This trend continues through mid- October 214 (day 65 of Figure 2) at which point the intracaldera stations telemetry is down. As noted previously, over these same time intervals (days 65 of Figure 2), GPS stations OKSO and OKFG, located outside the caldera, show little to no change in vertical and horizontal displacement with an average displacement rate of only.4 cm/d. 4. ANI Results and Discussion Cross correlation and autocorrelation of ambient noise recorded on the AVO s permanent seismic network are analyzed in order to determine temporal changes velocity and decorrelation rate related to the GPS-inferred rapid inflation event at Okmok volcano. As noted previously, the rapid inflation event began in early June 213. The time period of our study begins in early January 213, months preceding the rapid inflation event, and extends through mid-october 214, the date when Okmok s seismic data were no longer successfully telemetered. Notably, intracaldera stations (OKNC and OKCE) were successfully telemetered only from late July 213 to early January 214 and the summer of 214, yielding a shorter time interval for ANI analysis of stations within the caldera (Figure 3). For this reason, we are unable to BENNINGTON ET AL. MONITORING RAPID INFLATION AT OKMOK 5669

7 1.12/215JB a b c d e Days since 1/1/213 OKWE-OKRE OKWR auto OKAK-OKNC OKWE auto OKTU auto Figure 5. s velocity related to seasonal effects are observed at station pair (a) OKWE-OKRE, and at individual stations (b) OKWR, (c) OKAK-OKNC, (d) OKWE, and (e) OKTU. Relative velocity changes determined for the positive and negative sides of the CF are shown as black and grey filled circles (Figure 5a). Dashed vertical line represents GPS inferred start of rapid inflation event. estimate changes velocity preceding and during the first 2 months of the rapid inflation event. For our study period, data gaps also exist for stations and station pairs with intrastation paths outside the caldera. These data gaps preclude the determination of changes velocity for certain time periods seen in Figure 3. Finally, we note that ANI results are determined and displayed only for stations and station pairs meeting the criteria discussed in the Data Processing section. For 213, robust changes velocity are determined for three individual stations located outside the caldera (OKWR, OKWE, and OKTU) and two station pairs with intrastation paths that sample primarily or entirely outside the caldera (OKAK-OKNC and OKWE-OKRE). These stations and station pairs exhibit a quasi-sinusoidal pattern of changes velocity with average peak seismic velocity changes of ±.2% (Figure 5). This agrees with the pattern and magnitude of seasonal velocity changes observed using coda wave interferometry at Mount St. Helens, which are attributed to annual variations in snowpack, variations in groundwater level, and/or fluid saturation [Hotovec-Ellis et al., 214]. Thus, we attribute these changes velocity observed by stations and station pairs outside of Okmok s caldera to the seasonal hydrologic cycle. Notably, stations and station pairs on the NW side of the caldera show changes velocity that are in phase with one another, but out of phase with station OKTU, located on the SE side of the caldera (Figures 1 and 5). At OKTU, seismic velocity begins to decrease by early June 213, which is ~75 days earlier than the other stations/station pairs. Risien and Chelton [28] show that prevailing winds near Okmok travel NW-SE in the winter months (December, January, and February). Thus, it is reasonable to assume that more snow accumulates on the windward (NW) side of the caldera. We suggest that the out of phase velocity changes determined at station OKTU, relative to stations/station pairs on the NW side of the volcano, may be due to less snow being deposited in this region of the volcano. A smaller accumulation of snow would melt more quickly resulting in a shorter period of increased seismic velocity. Unfortunately, no precipitation data exist at Okmok to corroborate this hypothesis. BENNINGTON ET AL. MONITORING RAPID INFLATION AT OKMOK 567

8 1.12/215JB a b OKNC-OKCE OKNC-OKTU c d e OKCE-OKTU OKWR-OKNC OKNC auto (grey) OKCE auto (black) Days since January 1 Figure 6. Changes in relative seismic velocity are determined for station pairs (a) OKNC-OKCE, (b) OKNC-OKTU, (c) OKCE-OKTU, (d) OKWR-OKNC and for individual stations (e) OKNC and OKCE. For our time interval of interest, 1 January 213 through 23 August 214, perturbations to seismic velocity determined between station pairs are shown as black (positive side of the CF) and grey (negative side of the CF) filled circles (Figures 6a 6d), and changes velocity at individual stations OKNC and OKCE are shown as grey and black filled circles (Figure 6e), respectively. Average seasonal changes velocity are determined for the time interval 1 January 212 through 31 December 212. For station pairs (Figure 6a 6d), these seasonal changes in velocity are indicated by overlaid red solid lines. Red and cyan solid lines represent 212 seasonal changes velocity for individual stations OKNC and OKCE (Figure 6e), respectively. Since our time interval of interest encompasses multiple years, 213 and 214, we place the 212 seasonal pattern of velocity perturbations at day and again at day 366. This allows for direct comparison between 212 seasonal velocity changes and those perturbations velocity determined for 1 January 213 through 23 August 214. The dashed vertical line in the figure represents the GPS inferred start of rapid inflation event. Unlike year 213, the set of relative velocity changes determined at stations outside the caldera in 214 is not continuous throughout the entire year. This makes it difficult to identify possible seasonal trends velocity changes. However, for the small set of days in 214 where robust changes velocity are determined, we see seismic velocity perturbations that follow the same seasonal pattern as seen for those same dates in 213 (Figure 5). We also determine robust changes velocity for station pairs with intrastation paths that traverse the caldera (OKNC-OKCE, OKNC-OKTU, OKCE-OKTU, and OKWR-OKNC) and individual stations located within the caldera (OKNC and OKCE) (Figure 6). For these stations and station pairs, we extend ANI analysis into 212. In 212, Okmok volcano showed no obvious signs of magmatic activity; thus, changes velocity determined for this year should be related only to the hydrologic cycle. We find that individual stations and station pairs show a small initial decrease and then continued increase velocity in 212 (Figure 6) similar to the seasonal trend observed at stations and station pairs outside the caldera in the latter half of 213 (Figure 5). For 213, intracaldera intrastation paths and individual stations show ongoing decreases velocity unlike the 212 seasonal trends (Figure 6). While it would be most appropriate BENNINGTON ET AL. MONITORING RAPID INFLATION AT OKMOK 5671

9 1.12/215JB11939 Latitude (deg) Longitude (deg) Figure 7. Summary figure of ANI results at Okmok volcano where changes in relative velocity determined from cross correlation of station pairs and autocorrelation of individual stations are shown as arrows and filled circles, respectively. Seasonal changes are indicated in blue and marked decreases in velocity outside these seasonal changes are shown in red. Nonseasonal decreases velocity are calculated for the time interval between early August 213 and late December 213. Over this time interval, we are unable to calculate the total decrease velocity for station pair OKCE-OKTU. Changes velocity are not recovered for this station pair until 26 October 213. For this reason, we use a dashed red line to indicate that OKNC displays a nonseasonal decrease velocity. The dashed yellow line represents the hypothesized plane that a shallow magma body is centered on. The magenta and orange stars represent the location of a long-term deformation source at Okmok based on Fournier et al. [29] and Lu et al. [21]. to compare these decreases velocity in 213 to seasonal velocity changes obtained via averaging over multiple years, nearly all intracaldera station pairs and stations examined in this study incorporate station OKNC, which was not installed until September 21. Due to persistent data gaps and timing issues at OKNC in 21 and 211, we are limited to obtaining seasonal trends velocity changes only for 212. During the ongoing inflation event, station pair OKNC-OKCE and stations OKNC and OKCE show the largest decreases in relative velocity with cumulative decreases of.47% (OKNC-OKCE) and.44% (OKCE and OKNC), between early August and late December 213 (days of Figure 6). For the same time interval, station pair OKNC-OKTU shows similar but smaller changes in seismic velocity with a total decrease of.36%. A smaller cumulative decrease of.27% is observed at station pair OKNC- OKWR (Figure 6). We note that station pairs OKTU-OKNC, OKNC-OKCE, and OKAK-OKNC and individual stations OKCE and OKNC all show marked increases in the rate at which seismic velocity decreases beginning in early September 213 and extending through late December 213 (days of Figure 6). This time coincides with the start of the most rapid rates of displacement seen at Okmok s intracaldera GPS stations OKCE and OKNC (Figure 2 and Table 2). In Figure 7, we summarize the resulting observed changes velocity at Okmok volcano during 213. As discussed previously, intrastation paths and individual stations located in the caldera show marked decreases in relative seismic velocity, whereas stations and intrastation paths outside the caldera show only seasonal trends in velocity changes. This provides evidence that the marked decreases in relative velocity are associated with the GPS-inferred rapid inflation event occurring within the caldera. The magnitude of these intracaldera decreases in relative velocity are of comparable magnitude to those changes observed during rapid inflation preceding the July 26 eruption at PdF volcano. At PdF, rapid inflation of the volcano s main cone, as inferred via GPS, in the month preceding its July 26 eruption resulted in a.3% decrease in relative velocity for station pairs with intrastation paths traversing the volcano s edifice [Duputel et al., 29]. Temporal variations of the decorrelation rate at individual stations and station pairs are also examined (Figure 8). We only observe a consistent trend in the rate of decorrelation over time for a single station pair and one individual station. However, the station pairs and individual station demonstrate progressively increasing decorrelation rate with time. Both station pair OKCE-OKNC and individual station OKNC show total increases in decorrelation rate of.4 s 1 over the time of analysis. These total increases in decorrelation rate for the intrastation path and individual station within Okmok s caldera (Figure 8) are comparable to those found at PdF by Obermann et al. [213]. In the 45 days leading up to the October 21 eruption at PdF, Obermann et al. [213] show total decorrelation rate increases of.3,.6, and.1 s 1 for three intrastation paths near the eruption site. As discussed above, we find comparable levels of anomalies to those of Duputel et al. [29] and Obermann et al. [213]. Following these studies, we associate observed decreases velocity and increases in decorrelation rate, coincident in time and space with GPS-inferred rapid inflation at Okmok, with increased BENNINGTON ET AL. MONITORING RAPID INFLATION AT OKMOK 5672

10 1.12/215JB11939 Decorrelation Rate (sec -1 ).1.5 a OKCE_OKNC Days since 1/1/213 b OKNC auto Days since 1/1/213 Figure 8. A relative increase in decorrelation rate is observed at station pair (a) OKNC-OKCE and at individual stations (b) OKNC. Relative increases in decorrelation rate are determined for the positive and negative sides of the CF and are shown as black and grey lines (Figure 8a). production of volatiles and/or magma influx into the magma reservoir, and associated opening of fractures and/or fissures. While previous ANI studies at PdF have succeeded in localizing the changes velocity and/or decorrelation rate using the dense station coverage at the volcano [Brenguier et al., 28; Duputel et al., 29; Obermann et al., 213], we do not have a sufficient number of usable station pairs to carry out a similar analysis at Okmok. For example, at PdF, Obermann et al. [213] invert seismic velocity changes determined at 14 station pairs in order to precisely map the locations of these perturbations in velocity. In our study, we determine robust changes velocity at only 11 stations/station pairs. Although we obtain only a small set of seismic velocity changes, these observations can be used to put some constraints on the location of the inflation source within Okmok s caldera. Relative velocity changes derived from autocorrelations at stations OKNC and OKCE show equal amplitude decreases over the entire time period of analysis (Figure 6e). For example, in Figure 6e, on days 225, 277, 3, and 35, both stations show relative velocity changes of.2%,.3%,.13%, and.2%, respectively. This suggests that the inflation source is centered along a plane that is equidistant from both stations. The projection of this plane, shown in Figure 7, intersects the approximate center of the caldera. Importantly, while both stations show a total decrease in relative velocity of.44%, the intrastation path between the two stations, OKNC-OKCE, shows a slightly larger total decrease in relative velocity of.47%. The intrastation path for this station pair lies nearest to the approximate center of the caldera further supporting an hypothesized inflation source centered along a plane between the two stations (Figures 7 and 8). Also, as noted above, station pair OKNC-OKTU shows a smaller decrease velocity. Its intrastation path passes near to the center of the caldera, but also extends outside of the caldera. Finally, the smallest total decrease velocity is at station pair OKNC-OKWR, which has an intrastation paths located furthest from the caldera s center (Figures 6 and 7). Taken together, these pieces of evidence suggest that the current inflation event may represent recharge to a shallow magma storage region located near the center of the caldera. This agrees with previous geodetic studies, which model GPS data or interferometric synthetic aperture radar images of Okmok collected between 1997 and 28 to obtain a long-term deformation source located within the geometric center of the volcano s caldera [Lu et al., 23, 21; Fournier et al., 29] (Figure 7). Notably, no locatable seismicity is observed near the center of the caldera in the 2 months leading up to rapid inflation, nor during the rapid inflation event (April 213 to August 214 seismicity shown in Figure 1). This suggests that rapid inflation is occurring aseismically in this region. Alternatively, the health of Okmok s seismic network may not allow for the observation of lower magnitude events associated with inflation. Dixen et al. [213] report a magnitude of completeness (M c ) of 1.7 for Okmok s earthquake catalog in 212. More recent publications detailing the M c for the 213 and 214 earthquake catalogs do not exist. However, data availability in 213 and 214 is of similar frequency to year 212, suggesting a similar M c for those years (Figure 3). We also examine seismicity observed during the previous major rapid inflation events in and 24, which preceded Okmok s 28 eruption (Figure 1). However, there are only six earthquakes BENNINGTON ET AL. MONITORING RAPID INFLATION AT OKMOK 5673

11 1.12/215JB11939 Center Frequency of Narrow Bandwidth (Hz) Relative Velocity Change (%) Figure 9. Relative velocity changes at station OKNC were determined via filtering over a series of narrow bandwidths. The center frequency of each narrow bandwidth is plotted versus the relative velocity change. Specifically, we plot changes velocity for days 1/1/213 (red), 1/17/213 (blue), 11/11/213 (black), 11/21/213 (magenta), and 12/16/ 213 (cyan). recorded in this region during these time periods. This relative absence of seismicity further suggests that inflation within Okmok s caldera occurs aseismically. Thus, ANI provides the only seismic evidence of rapid inflation at Okmok volcano. In the above discussion, we capitalize on the magnitude and distribution of seismic velocity changes determined at 11 stations/station pairs in order to infer the location of the inflation source at Okmok (Figure 7). To estimate the depth at which these changes velocity predominantly occur, we carry out an ANI analysis using the same reference and current CFs as described in the Data Processing section. However, the CFs are filtered over narrow bandwidths of.2 Hz. The center frequencies of the narrow bandwidths are.3,.4,.5,.6,.7,.8,.9, and 1 Hz. ANI analysis is carried out for each narrow bandwidth and the center frequency versus relative velocity change for a particular date is plotted. From Figure 9, we see that the largest decreases velocity occur at center frequencies of.6 to.7 Hz. If we assume that the upper crust beneath Okmok s caldera has an average shear velocity of 2.25 km/s (determined via velocity model of Masterlark et al. [21]), we can estimate wavelengths of 3.75 and 3.21 km associated with center frequencies of.6 and.7 Hz, respectively. Brenguier et al. [27] note that CFs determined in our frequency bandwidth (.1 to 1 Hz) consist mainly of Rayleigh waves. Lowrie et al. [27] explain that with increasing depth, the amplitude of Rayleigh waves decreases exponentially. The authors estimate the maximum depth of penetration of the surface wave as Z p ¼ λ=e (3) where λ is the wavelength and e is the exponential. Following equation (3), we estimate a Z p of 1.4 to 2.7 km. This is the estimated depth at which the largest decreases velocity occur. Seismic velocity models of Okmok suggest the presence of weak, fluid saturated materials at <2 km depth and the presence of a magma reservoir at >4 km depth [Masterlark et al., 21; Ohlendorf et al., 214]. Thus, we estimate the largest decreases velocity occurring within and immediately below this region of weak, fluid saturated materials. For the current rapid inflation event at Okmok, we cannot say definitively whether the event is still in progress due to seismic and GPS network telemetry extending only until late September 214 and mid-october 214, respectively. We can, however, analyze the state of this inflation event immediately preceding the end of successful telemetry in 214. ANI analysis of the 214 seismic data set reveals robust changes velocity for autocorrelations at stations OKCE and OKNC. Unfortunately, these were the only two results from the 214 seismic data set that showed well-resolved changes velocity. At these stations, changes velocity determined between May and September 214 (days of Figure 6e) remain constant relative to those values determined in late December 213 (days of Figure 6e). This plateau in relative velocity changes would suggest that the rate at which magma and/or volatiles enters the magma reservoir has slowed dramatically relative to the time period between early September and late December 213 when peak decreases velocity were observed (days of Figure 6e). Complementary to these seismic observations, displacement rates determined between mid-december 213 and mid-october 214 (days of Figure 2 and Table 2) are less than half the peak rates of displacement determined between mid-september and mid-december 213 (days of Figure 2 and Table 2), but of the same magnitude as displacement rates determined for the beginning of the rapid inflation BENNINGTON ET AL. MONITORING RAPID INFLATION AT OKMOK 5674

12 1.12/215JB11939 event (days of Figure 2 and Table 2). Taken together, these seismic and geodetic observations suggest that by fall 214, Okmok s rapid inflation event was slowing dramatically. 5. Conclusions We carry out ANI in order to determine subtle changes velocity associated with a recent, and possibly ongoing, rapid inflation event at Okmok volcano. Outside of Okmok s caldera, individual stations and station pairs show changes velocity inferred to be related to the seasonal hydrologic cycle. Intrastation paths that traverse the caldera and individual stations located within the caldera display continuous decreases velocity. For the same time period, temporal variations in the decorrelation rate are also determined. For an individual station and intrastation path located within the caldera, we see a constant increase in the value of the decorrelation rate. The intracaldera decrease in seismic velocity and increase in decorrelation rate are coincident in both space and time with the ongoing GPS-inferred rapid inflation event. We suggest that such perturbations velocity and decorrelation rate represent the production and/or influx of magmatic fluids into the shallow magma reservoir. Finally, using the distribution of seismic velocity changes, we constrain the location of this rapid inflation event. Decreases velocity are largest for the intrastation that is nearest to the approximate center of Okmok s caldera. Also, the two intracaldera seismic stations show the same amplitude decrease velocity over time, which suggests that the source of rapid inflation is centered along a point equidistance from the two stations. Notably, the equidistant plane between the two the stations approximately intersects the center of the caldera. Our inferred inflation source location agrees with previous geodetic studies, which model a long-term deformation source near the geometric center of the caldera. For these reasons, we suggest that the ongoing inflation event may represent recharge of shallow magma storage, which is located in the approximate center of the caldera. Notably, we find an absence of seismicity coincident in time with this and past rapid inflation events at Okmok, suggesting that such events occur aseismically. Thus, ANI offers the first seismic evidence of such rapid inflation events at the volcano. We also estimate that the largest decreases velocity occur above the inferred top of the magma reservoir and immediately below a region of weak, fluid saturated material in the shallow caldera. For the most recent rapid inflation event at Okmok, results from ANI analysis and geodetic observations suggest that the event slowed dramatically by fall 214. 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