Rupture history of the January 1, 1996, Ms 6.6 volcanic earthquake preceding the simultaneous eruption of Karymsky and Akademia Nauk volcanoes

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 103, NO. B8, PAGES 18,315-18,324, AUGUST 10, 1998 Rupture history of the January 1, 1996, Ms 6.6 volcanic earthquake preceding the simultaneous eruption of Karymsky and Akademia Nauk volcanoes in Kamchatka, Russia Vyacheslav M. Zobin Observatorio Vulcano16gico, Universidad de Colima, Colima, M6xico Valeria I. Levina Kamchatkan Seismological and Methodological Department, Geophysical Survey, Russian Academy of Sciences, Petropavlovsk-Kamchatsky, Russia Abstract. The intensive foreshock-mainshock-aftershock sequence of earthquakes preceded and accompanied the January 2, 1996, simultaneous eruption of the active Karymsky and the dormant Akademia Nauk volcanoes in Kamchatka, Russia. The finite fault, broad band teleseismic P waveform inversion was applied to the Ms 6.6 mainshock of this sequence which occurred on January 1, 1996, about 14 hours before the beginning of eruption. It was one of the strongest volcano-tectonic earthquakes in this century after the great Ms 7.0 volcanic earthquakes related to the Katmai, Alaska, and Sakurajima, Japan, volcanic eruptions. The main feature of the rupture process inferred from the inversion was the breaking of a sequence of four asperities with displacement greater than 200 cm which were situated at depths from 0 to 35 km just beneath the Akademia Nauk volcano. There were no broken asperities beneath the Karymsky volcano. The majority of small earthquake foci were located out of maximum asperities and filled a space between two zones of asperities at depths from 12 to 20 km. The position of broken asperities allows us to suggest that during the large earthquake rupturing an ancient magmaticolumn beneath the volcano was destroyed. Small earthquakes which were recorded mainly from 5 to 20 km depth were able only to destroy the intermediate-depth part of this column between 12 and 20 km. The main work for opening a way to move magma to the surface was made by a large shock which had broken the upper and deep parts of the ancient magma column. 1. Introduction The Kamchatka Peninsula is situated in the northwestern Copyright 1998 by the American Geophysical Union. Paper number 98JB /98/98JB About 6 km to the south of the Karymsky volcano is the ancient caldera of the Akademia Nauk volcano which is filled in its northern part by Karymsky Lake. The last part of the Pacific where the Kurile-Kamchatkan and eruption of this volcano took place C years ago Aleutian island arcs intersect. The eastern chain of 14 (tephrochronological data) (V.Yu.Kirianov, personal active Kamchatkan volcanoes goes along the Pacific coast communication, 1996), and this volcano was considered as of the peninsula. In its middle part is the Karymsky a dormant one. volcano, one of the most active Kamchatkan volcanoes (Figure lb). More than 20 eruptions of this volcano were These two volcanoes, Karymsky and Akademia Nauk, recorded during last 200 years. The stratovolcano together with several active and dormant volcanoes Karymsky is a regular cone with an absolute height of 1536 complete the 60-km-length Karymsky volcanic center of m and is in the center of the 7.5 thousand year old caldera km area. This center is characterized by a common The products of its eruptions were represented mainly by eruptive deep-seated riff-type NE-striking fault feeding the andesite-dacitic lava and pyroclastic flows. The last volcano eruptions (see Figure lb). The total volume of eruption of Karymsky began in 1970 and continued until erupted material in the Karymsky Volcanic Center amounts The eruption consisted mainly of vulcanian-type to 1700 km for 3 million years [Masurenkov, 1991 ]. explosions accompanied by explosion earthquakes The seismic activity of the Karymsky volcanic center in [Tokarev, 1989; Iranov et al., 1991 ] was related mainly to volcanic explosions of the Karymsky volcano and to earthquakes which have occurred within a 10-km-length zone extending south from 18,315 Karymsky volcano near Karymsky Lake. The shallow focus seismic sequence was recorded there in January- February 1978, when a magnitude Ms 5.2 mainshock was

2 18,316 ZOBIN AND LEVINA: NATURE OF A LARGE VOLCANIC EARTHQUAKE 1B3E 1BIE 1BgE 159.2E 1Bg.4E Figure 1. (a) The position of 1996 earthquake sequence (solid circles) within the Kamchatkan regional seismic network. Seismic stations are shown by solid triangles with numbers. (b) The epicenters of earthquakes (mb>3.5) which occurred from January 1 to 11, 1996, at depths from 0 to 60 km. The epicenter of the mainshock is shown by a solid hexagon; two volcanoes, Karymsky and Akademia Nauk (within Karymsky Lake), are shown by long triangles; and the seismic station KAR is shown by a smaller triangle. The heavy long dashed lines show the main ring and linear faults, and the heavy short lines show the southern border of the Akademia Nauk caldera, as determined by Masurenkov [ 1991 ]. felt at a seismic station KAR (Figure lb) with an intensity of VII (MSK-64 scale) [Zobin et al., 1983]. A new eruption cycle of the Karymsky volcano began with an intensive seismic sequence on January 1 at 0450 UT. The epicenters were located along a 40-km-length and 20-km-width NNE-SSW structure involving Karymsky Lake (Figure lb). The mainshock of this earthquake sequence (magnitude Ms 6.6) occurred at 0957 UT, January 1, 17 km to the south of the Karymsky volcano and 9 km to the south of the Akademia Nauk volcano (Table 1), and was felt with intensity IV-V (MSK-64 scale) at a distance of 80 km. It was the largest volcanic earthquake recorded in Kamchatka during the last 35 years, when the detailed seismic study was began. On the afternoon of January 2 (local time, UT plus12 hours), the two volcanoes, Karymsky and Akademia Nauk, began to erupt simultaneously (Figure 2). The powerful subaqueous explosions (up to 8 km altitude) in Karymsky Lake situated within the caldera of the Akademia Nauk volcano and a summit eruption of the Karymsky volcano formed an unusual double eruption. A new peninsula of 1 km width had appeared in Karymsky Lake, formed by about 106 m" of basaltic andesite and andesite-dacitic tephra during the 1-day eruption of the Akademia Nauk volcano. The eruption of the Karymsky volcano was represented by usual vulcanian-type explosions with a plume rising to 1 km above the crater [Fedotov et al., 1996] and continued for more than 2 years. This paper studies the rupture history of the Ms 6.6 mainshock. This Ms 6.6 earthquake was unusually large in magnitude for a volcano-tectonic earthquake. We can list only a few instrumentally recorded volcanic earthquakes of magnitude Ms > 6.5. Four of them (Ms 7.0, 6.9, 6.8, 6.6) were from the swarm accompanying the great calderamaking eruption of Katmai volcano, Alaska, June 1912 [Abe, 1992] and one event of magnitude Ms 7.0 was the mainshock of the foreshock-mainshock-aftershock sequence preceding and accompanying the great 1914 explosive eruption of Sakurajima volcano, Japan [Abe, 1979].

3 ZOBIN AND LEVINA: NATURE OF A LARGE VOLCANIC EARTHQUAKE 18,317 Table 1. Parameters of the Karymsky Earthquake, January 1, 1996 Origin Time Latitude, Longitude, Depth, Ms mb Energy, Reference øn øe km Nm 0957: : lxlo TM 2 References: 1, U.S. Geological Survey Earthquake Data Reports; 2, bulletin of Kamchatkan regional seismic network. A finite fault waveform inversion was applied to P wave teleseismic records of the January 1, 1996, earthquake to estimate the slip distribution along the earthquake fault plane. We discuss a relationship between the slip development during the mainshock rupturing and a course of earthquake sequence as well as a course of a double eruption. 2. Seismic Sequence Preceding the Eruption The events of the earthquake sequence were located by the Kamchatkan regional seismic network (Figure l a). This network consists of short-period three-component analog instruments with magnification of 5000 to at 1 Hz. The network configuration generally uses from 6 to 22 stations for location of Ms greater than 3.0 events. (The Kamchatkan seismic survey uses the energy class K as the main parameter of earthquake size. For better understanding, we use a regression between K and Ms K = 1.5 Ms obtained by Zobin [ 1996] for transforming energy class K into magnitude Ms). The mean errors in location are 2.8 km and 4.1 km in depth. Figure 2. Simultaneous eruptions of Karymsky (right) and Akademia Nauk (left) volcanoes, January 2, The distance between the summit of Karymsky and subaqueous vents in the Akademia Nauk caldera lake is 6 km. (Photo by the Institute of Volcanology, Petropavlovsk-Kamchatsky, Russia. Taken from Bulletin of the Global Volcano Network, 21 (5), 7, 1996.)

4 18,318 ZOBiN AND LEVINA: NATURE OF A LARGE VOLCANIC EARTHQUAKE It is a common feature of volcanic activity that the seismic events precede the beginning of eruption [e.g., Minakami, 1960; Gorelchik et al., 1990; McNutt, 1996]. Usually, these seismic events occur as a swarm of earthquakes with no event standing out in magnitude. The 1996 seismic sequence was unusual for a volcanic region, because it was a foreshock-mainshock-aftershock sequence with a difference in magnitude Ms between the mainshock and the largest aftershock equal to 1.5, according to the U.S. Geological Survey (USGS) Earthquake Data Report (EDR) of January Figure 3 shows the temporal variation in numl er of events during the first 10 days of activity when there were more than 800 earthquakes located with magnitude Ms (calculated from K) greater than 1.0, and 13 of them were of magnitude mb greater than 4.5. After January 4 the number of events sharply decreased, while the low level of activity continued during the year, with shallow events concentrated mainly beneath Karymsky Lake. The epicentral zone of the earthquake sequence represented an elliptical area with a long 40-km axis oriented about 20 ø NNE and a width of 20 km. This area is limited by the system of NE striking faults (see Figure lb). The events were distributed in the depth interval from 0 to 60 km, with the majority of events situated at the depths from 5 to 20 km (Figure 4). The mainshock occurred about 6 hours after the beginning of the seismic sequence and 14 hours before the beginning of eruption (Figure 3). The regional network located the epicenter at 53.90øN and øE with a good accuracy (+3 km). The depth of the event was estimated as 0 km, and this estimate may be inaccurate due to the position of the nearest stations at distances of km from the epicenter. The nearest station KAR was out of operation during January 1. The depth distribution for foreshocks (Figure 4a) suggests a depth of 10 km as likely for the mainshock. The focal mechanism of the mainshock was estimated using the P wave first motions recorded at regional stations as well as the teleseismic data published in EDR. The mechanism solution used 11 regional records and 69 teleseismic records (Figure 5). One can see that the data of regional stations were decisive in the construction of nodal planes. We got strike-slip faulting with a near-vertical fault plane oriented 14 ø NINE, similar to the orientation of the long axis of epicentral area (Table 2). This focal mechanism is similar to the strike-slip focal mechanism of the magnitude Ms 5.2 event from the 1978 sequence, which had practically the same epicenter (53.90øN and øE) [Zobin et al., 1983]. 3. P Waveform Inversion of Mainshock The fault parameterization and modeling procedure for the finite-fault, waveform inversion technique was fully described by Hartzell and Heaton [1983] and Mendoza 160 beginning Ms [of eruption 8Ol- ' o JANUARY 1996 Figure 3. Temporal variation in number of events (mb>2.3) during the first 10 days of the earthquake sequence. Arrows indicate the beginning of simultaneous eruption of two volcanoes and the appearance of mainshock. The numbers of events for each 6-hour interval is shown.

5 ZOBIN AND LEVINA: NATURE OF A LARGE VOLCANIC EARTHQUAKE 18,319 A before the mainshock 120[ l after the mat)sljock DEPTH, KM lo 2O O Figure 4. Depth distribution of the earthquakes (mb>2.3) which occurred (a) before the mainshock of January 1, at 0957 and (b) after the mainshock, from January 1, 0958 to January 11, The number of events for each 2.5 km of depth is shown. [1996] and has been developed in numerous papers by windows. The synthetic waveforms and observed data Hartzell with colleagues. The complete list of references constitute an overdetermined system of linear equations of used for calculations of synthetics for teleseismic records the form Ax = b, where A is an m-by-n matrix of the was published by Hartzell et al. [ 1994]. synthetics and b is an m-length vector containing the The method proposes that the fault plane of fixed seismic observations. The vector x, of length n, contains the dimensions and orientation is embedded at the appropriate subfault dislocation weights required of each componento depth in the crustal structure of the earthquake source reproduce the observed data. The main constraints on the region. The fault parameterization includes the choice of solution used to minimize instability are the requirements the focal depth, the coordinates of epicenter, the fault that there be a nonnegative solution and a smooth variation length and width, the fault strike, dip, and rake, the rupture of slip across the fault. We find the smoothest solution with velocity, and the shape and duration of the source time the smallest moment that fits the data. function. Also, we have to introduce the optimal number of In this study we use a location made by the local rectangular, equal-area subfaults with uniformly distributed Kamchatkan seismic network. A depth estimation is not point sources over each subfault which would model the sufficient. The USGS EDR gives a fixed depth of 33 km, seismic radiation produced by the fault. The subfault and the Kamchatkan local network gives a depth estimation dimensions and the total number of subfaults may be of 0 km, which is very shallow for the magnitude 6.6 event. chosen after consideration of the following factors: the Therefore, we took for the initial inversion a depth of 10 bandwidth of the data, the size of the fault plane, and the km according to the depth distribution of the foreshocks computational limitations. (see Figure 4a). The nonnegative least squares inversion generates the The records of the study earthquake were overlapped solution vector, which then yields the moment release with the recordings of the Mw 7.9 earthquake from function and the slip distribution for a number of time Minahassa Peninsula which occurred 01 hour and 52 min

6 18,320 ZOBIN AND LEVINA: NATURE OF A LARGE VOLCANIC EARTHQUAKE N The length of fault was taken to be too large for the Ms 6.6 event to include all significant displacements along the fault. The fault width was taken in accordance with the aftershock depth distribution. The size of subfaults was taken to be small enough to discriminate small area asperities and large enough to avoid the problems with our computation capabilities. The hypocenter was embedded 40 km from the left edge of the fault. The top of the fault is 1 situated at a depth of 0.13 km; the bottom is situated at 59.3 km. Practically, the top of fault plane was situated at the Earth's surface. An elementary boxcar source time function of width 1 s was used for each discrete rupture 1 interval of each subfault. The amplitude of each boxcar in the discretization is solved for in the inversion and lagged in time by the width of the boxcar. The inversion was T conducted for up to three consecutive 1-s time windows. Mendoza and Hartzell [1988] pointed out that rupture velocity values generally remain within the range of 0.8 to P 0.9 of the shear wave velocity for most earthquakes. Therefore, the rupture velocity was taken equal to 2.7 km, which is approximately 80% of the shear wave speed in the layer containing the hypocenter. Table 4 presents a local Figure 5. The P wave first motion focal mechanism solution of the mainshock. Open symbols denote Kamchatkan velocity structure [Kuzin, 1974] used for compressions; solid symbols denote dilatations. Large inversion. symbols correspond to the data which were read from the regional records, small symbols correspond to the data which were published in Earthquake Data Reports. The 4. Rupture Model of Earthquake deviation from the average positions of the P and T axes determined at the 85% confidence level is 5 ø. The inversion was begun from the run using a focal depth of 10 km. The best solution for this rupture model was chosen by smoothing the solution with the simultaneous minimization of the seismic moment and the Euclideanorm II b- Ax II, which is the difference between before our event. It was impossible to resolve a Harvard the matrix of synthetics Ax and the vector b containing centroid moment tensor for the Karymsky earthquake (M. seismic observations. Figure 7a illustrates this process of Salganik, personal communication, 1996). This limited the finding the best solution with optimal smoothing for the 10- possible number of worldwide records of the event which km-depth hypocenter. might be used for inversion and made very complicated Then we ran the inversions for depths of 5 and 15 km. records of the $ waveforms. Nevertheless, the nine For better comparison between the three solutions, all broadband digital records of GDSN (Global Digital parameters of the model used for 10-km-depth hypocenter Seismograph Network) were selected (Table 3). For inversion, we used the P waveforms (vertical component) recorded at distances from 50 ø to 75 ø. We used 30-s Table 2. Focal Mechanism of the Mainshock samples (Figure 6). This rather short duration of sample allowed us to neglecthe influence of the very long period oscillations of the previous large Minahassa earthquake but Parameter Value, deg was enough for the reconstruction of the Ms=6.6 event slip process (e.g., Mw 6.7 Northridge 1994 earthquake, Tplunge 16 California, Wald et al. [1996]). To better isolate the T azimuth 238 influence of long-period oscillations of the Minahassa N plunge 74 N azimuth 45 event, the records were high-pass filtered at Hz with P plunge 4 a zero-phase, third-order Butterworth filter (Figure 6). P azimuth 147 For our model we took the NNE striking near-vertical Nodal plane 1 Strike 14 nodal plane of focal mechanism coinciding with a long axis Dip 81 of the aftershock area (see Figure 5 and Table 2) as the Slip 14 actual fault for inversion. The fault plane was taken as the Nodal plane 2 Strike x60 km 2 near-vertical rectangle divided by 225 subfaults Dip 76 of 5x4 km 2. This fault goes along the epicentral area Slip 171 including the Karymsky and Akademia Nauk volcanoes.

7 ZOBIN AND LEVINA: NATURE OF A LARGE VOLCANIC EARTHQUAKE 18,321 Table 3. Broadband Records Used in Source Inversion Station Code Distance, deg Azimuth, deg Whiskeytown Dam WDC Arti ARU Frobisher Bay FFC Wild Horse Valley WVOR Lhasa LSA Chiang Mai CHTO Borgarnes BORG Yorkshire YSNY Lisbon LBNH were repeated including the value of smoothing. The comparison of the relation between the synthetics and observed records showed that the model of 10 km depth was the best with the highest average value of relation and miniinure deviation from the average (Table 5). Figure 7b shows that the version with a focal depth of 10 km was the best also from the point of view of minimization of seismic moment and Euclidean norm. Therefore, the 10-km-depth model was taken as final. Figure 8 shows the comparison of synthetics and observed records which were obtained for the model. The rupture model is presented in terms of a contour map of the final slip on the fault (Figure 9) and the integrated moment release function (Figure 10). The contour map of the final slip on the fault (Figure 9) was obtained t sing three 1.0-s time windows and represents all fault displacements occurring within 3.0 s after the passage of a rupture front propagating at 2.7 km/s away from the hypocenter. One can see that the distribution of displacements for two zones of rupturing was obtained, the upper, surrounding the hypocenter, and the lower, arctype broad zone extending to depths of 40 km. These two zones of rupturing were separated by a narrow (about 10 km) layer without significant displacements. The main slips (with displacements greater than 125 cm) occurred within a fault of 50 km length and 40 km width. Figure 10 shows that there was a two-stage energy release during the rupturing with a total duration of 24 s. A! B WDC DUG ELK LSA 60 sec Figure 6. The examples of records (a) before and (b) after the filtering. Arrows indicate the length of sample for inversion.

8 18,322 ZOBIN AND LEVINA: NATURE OF A LARGE VOLCANIC EARTHQUAKE Table 4. Velocity Structure Vp, km/s Vs, km/s Thickness, km 2E+20 A Z 1E+20 The total seismic moment was equal to 9.0x1019 Nm, or corresponded to an event with magnitude Mw 7.1. o E+20 The comparison of Figures 9 and 10 showed that at the first stage of rupturing (from 0 to 8 s) the development of a rupture around the hypocenter was observed, with breaking of two small-area asperities with displacement greater than 200 cm. These two asperities were situated just beneath the 1E+20 Akademia Nauk volcano at depths between 0 and 12 km. Then we had a rupturing with the absence of significant (more than 125 cm) displacements at a depth between 12 and 20 km under the volcano. During the second stage (from 8 to 24 s) the development of a rupture within a h=15krn h=5 krn h=10 km '(. B OE+O broad arc-type area was observed, but with only two small- ' ' I area maximum (more than 200 cm) asperities destroyed. They were situated beneath the Akademia Nauk volcano at depths from 25 to 35 km. The second stage was about times more intensive in moment release than the initial one. Figure 7. Illustration (a) of the choice of the best solution for different smoothings of the 10-km-depth model and (b) 5. Results and Discussion of the choice of the best focal depth for the source. The circle marks the best solution. The finite fault, broadband teleseismic P waveform inversion was applied to the large volcanic earthquake of January 1, 1996, which was the mainshock of the sequence wave arrivals. Figure 6 shows that after filtration we got the preceding the simultaneous eruption of the volcanoes distinct P phases. The probable error in timing would not Karymsky and Akademia Nauk in Kamchatka, Russia. This exceed 0.5 s, which could produce an error of about 1.4 km inversion revealed the spatiotemporal rupture history of this in position of the asperity, assuming a rupture velocity of event. 2.7 km/s. This error can not effect any significant The main feature of the rupture process was the breaking disagreement in our speculations. of the sequence of four asperities with displacements Figure 11 shows the depth distribution of main asperities greater than 200 cm which were situated the depths from together with the focal distribution of small earthquakes 0 to 35 km just beneath the Akademia Nauk volcano. No recorded before the beginning of eruption. One can see broken asperities were recorded beneath the Karymsky that the majority of small-earthquake foci were located out volcano. of maximum asperities and filled a space between two The analysis of our choice of the best solution (Figure 7) zones of asperities at depths from 12 to 20 kin. The main showed that within the framework of the method we had asperities were situated within the Earth's crust beneath the obtained a rather reliable model. The error in position of volcano and formed a column from the crust bottom to the asperities may depend mainly on the errors in timing of P surface. o Table 5. Comparison of Models with Different Depth and Similar Smoothing Depth, Seismic Euclidean Average Relation Deviation From km Moment, Nm Norm of Synthetics and Observed Average Relation E20* % E % E % * Read 1.31 E20 as 1.3 l x1020.

9 ZOBIN AND LEVINA' NATURE OF A LARGE VOLCANIC EARTHQUAKE 18,323 We can propose some speculations about the role of ssw large earthquake in eruptive processes. The position of o broken asperities allows us to suggest that during the large earthquake rupturing an ancient magmatic column beneath the volcano was destroyed that went through the crust from a depth of 35 km. The depth between 20 and 40 km, which 2o extends from upper mantle to the crest, is the most likely place for magma collecting [Fedotov, 1991]. Small earthquakes which were recorded mainly from depths of 5 to 20 km were able to destroy only the intermediate-depth part of this column between 12 and 20 km. The main work 4o for opening a way to move magma to the surface from the bottom of crust was made by a large shock which would break the upper and deep pans of the ancient magma 6ol column. The comparison of the 1996 earthquake sequence with the 1978 earthquake sequence of 103 events of magnitude DISTANCE ALONG STRIKE, KM Ms from 2 to 5.2 (calculated from K) [Zobin et al., 1983] Figure 9. The contours of final slip in centimeters on the shows that they had many similar features. They both were fault plane. The fault plane has a dip of 81 ø and has its top depth of 0.01 km; therefore, the distance downdip of the fault and the depth are the same for this model. The hypocenter is indicated by a solid hexagon. Cumulative slip is contoured at 75-cm intervals beginning from 50 cm. We have projected on the fault plane the two volcanoes, :, Karymsky and Akademia Nauk. A.N.V. is the Akademia WVOR Nauk volcano, K.V. is the Karymsky volcano. The solid ' arrows indicate the position of volcanoes FFC foreshock-mainshock-aftershock sequences according to the classification of the Kamchatkan earthquakes given by Zobin and Ivanova [1992]. Their mainshocks had similar ARU positions within the aftershock zones. The epicentral area of the 1996 sequence overlapped the smaller 1978 ' epicentral area. The only difference was in the depth! ' CHTO distribution, and perhaps this difference determined the activation of the Akademia Nauk volcano. The events of,., the 1978 sequence were distributed at depths from 0 to 10 ",', BMN km, while be the supposed events of thathe 1996 were deeper from and 0 to more 60 km intensive depth It can seismotectonic activity of 1996 moved greater amounts of magma from km depth to the surface. Both -', WDC sequences occurred together with the progressive extension DUG , ELK 'ss. s LSA sec Figure 8. The comparison of observed (solid line) and synthetic (dashed line) P waveforms corresponding to the slip distribution inferred for the Karymsky earthquake. The epicentral distance (in degrees) and azimuth from epicenter (in degrees) are indicated for each seismogram pair. Figure 10. The moment release as a function of time.

10 18,324 ZOBIN AND LEVINA: NATURE OF A LARGE VOLCANIC EARTHQUAKE SSW o ß ß -x ß -125 ' '_ 2oo 20 ß øø -- ø' ø 4O References CR US T MANTLE ß ß A.N.V. K.V..! Fedotov and Yu.P Masurenkov, pp , Nauka, Moscow, Fedotov, S.A., M.A. Maguskin, V.A. Saltykov, and R.A. Shuvalov, Simultaneous eruption of Karymsky Volcano and awakening of the Academy of Sciences Caldera Volcano, related seismicity and deformation, January-May, 1996, Kamchatka, paper presented at the XXVth General Assembly, ESC, Reykjavik, Iceland, Gorelchik, V.I., V.M. Zobin, and P.I. Tokarev, Volcanic earthquakes of Kamchatka: Classification, nature of source and spatio-temporal distribution, Tectonophysics, 180, ,1990. Hartzell, S.H., and T.H. Heaton, Inversion of strong ground motion and teleseismic waveform for the fault rupture history of the 1979 Imperial Valley, California, earthquake, Bull. Seismol. Soc. Am., 73, , õ0 I - ß Hartzell, S.H., C. Langer, and C. Mendoza, Rupture history of eastern North American earthquakes, Bull. Seismol. Soc. Am., 84, , DISTANCE ALONG STRIKE, KM Ivanov, B.V., O.A. Braitseva, and M.I. Zubin, Karymsky Volcano. In Active Volcanoes of Kamchatka, Vol. 2, edited by Figure 11. The comparison of dislocation model with S.A. Fedotov and Yu.P Masurenkov, pp , Nauka, seismic and volcanic characteristics. Solid circles denote Moscow, the hypocenters of earthquakes of magnitude Ms>3.0 Kuzin, I.P., The Focal Zone and Upper Mantle Structure of the recorded before the beginning of eruption. The hypocenter East Kamchatka (in Russian), 127 pp, Nauka, Moscow, of the mainshock is indicated by a solid hexagon. Maguskin, M.A., V.B. Enman, B.V. Seleznev, and V.I Shkred, Cumulative slip is contoured at 75-cm intervals beginning Peculiarities of the surface deformations of Karymsky Volcano from 125 cm. A.N.V. is the Akademia Nauk volcano, K.V. from geodetic and photogrammetric data in (in Russian), Volcanol. Seismol., 4, 49-64, is the Karymsky volcano. The border between the crust and Masurenkov, Yu.P., Tectonic position and general history and mantle is shown by the dashed line. evolution of Eastern Kamchatka volcanoes. In Active Volcanoes of Kamchatka, Vol. 2, edited by S.A. Fedotov and Yu.P Masurenkov, pp. 8-15, Nauka, Moscow, McNutt, S.R., Seismic monitoring and eruption forecasting of of the Akademia Nauk caldera in 1976 revealed by volcanoes: A review of the state-of-the-art and case histories. geodetic observations [Maguskin et al., 1982; see also In Monitoring and Mitigation of Volcano Hazards, edited by Karymsky in Bulletin of the Global Volcano Network, 21 R. Scarpa and R. Tilling, Springer-Verlag, New York, pp , (3), 9-10, and 21 (5), 5-8, 1996] and marked the initial and Mendoza, C., Rapid derivation of rupture history for large final stages of the volcanic eruption preparation. earthquakes, Seismol. Res. Lett., 67, 19-26, The inversion has shown that all significant Mendoza, C., and S.H. Hartzell, Inversion for slip distribution using teleseismic P waveforms: North Palm Springs, Borah displacements during the large event were recorded beneath Peak, and Michoacan earthquakes, Bull. Seismol. Soc. Am, the Akademia Nauk volcano and practically no 78, , displacements beneath the Karymsky volcano. Therefore, Minakami, T., Fundamental research for predicting volcanic eruptions (I), Bull. Earthquake Res. Inst. Univ. Tokyo, 38, 497- we can propose that the seismic activity related to the large 544, earthquake of January 1 was the precursor to the eruption Tokarev, P.I., Eruptions and seismicity of Karymsky Volcano in of the Akademia Nauk volcano, and the eruption of (in Russian), Volcanol. Seismol., 2, 3-13, Wald, D.J, T.H. Heaton, and K.W. Hudnut, The slip history of the neighboring Karymsky volcano was only triggered by this 1994 Northridge, California, earthquake determined from seismic sequence. strong-motion, teleseismic, GPS, and leveling data, Bull. Seismol. Soc. Any., 86, S49-S70, Acknowledgements. The broadbandigital seismic records of Zobin, V.M., Apparent stress of earthquakes within the shallow the Global Digital Seismograph Network were received by subduction zone near Kamchatka peninsula, Bull. Seismol. Soc. Any., 86, , from AutoDRM of the U.S. Geological Survey. D. Droznin had Zobin, V.M., and E.I Ivanova, Aftershocks of shallow prepared the software for analysis of seismicity, and E.I. Ivanova earthquakes near Kamchatka Peninsula, Geophys. d. Int., 108, had calculated a focal mechanism solution. The comments by Fred Klein and an anonymous reviewer were very valuable. We thank T. Domingues, C. Famozo, A. Santillan, G. Marmolejo, and M. Mayoral for help in the computational problems , Zobin, V.M., P.P. Firstov, and E.I. Ivanova, Earthquake swarm in the region of Karymsky Volcano in Jan.-Feb., 1978 (in Russian), Volcanol. Seismol., 5, 64-73, Volcanol. Seismol., Engl. Transl., 5, , Abe, K., Magnitudes of major volcanic earthquakes of Japan 1901 V.I. Levina, Kamchatkan Seismological Experimental and to 1925, d. Fac. Sci. Hokkaido Univ., Set. VII, 6, , Methodological Department, Geophysical Survey, Russian Academy of Sciences, Petropavlovsk-Kamchatsky, Abe, K., Seismicity of the caldera-making eruption of Mount Russia. ( levina omsp.kamchatka. su) Katmai, Alaska in 1912, Bull. Seismol. Soc. Am., 82, , V.M. Zobin, Observatorio Vulcano16gico, Universidad de Colima, Colima, M6xico. ( vzobin cgic.ucol.mx) Fedotov, S.A., On the mechanism of volcanic activity in Kamchatka, Kurile-Kamchatka arc and in similar structures. In (Received July 3, 1997; revised February 12, 1998; Active Volcanoes of Kamchatka, Vol. 1, edited by S.A. acceptedmarch25, 1998.)