Body wave inversion of the 1970 and 1963 South American large deep-focus earthquakes

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. B12, PAGES 28,751-28,767, DECEMBER 10, 1999 Body wave inversion of the 1970 and 1963 South American large deep-focus earthquakes Charles H. Estabrook GeoForschungsZentmm Potsdam, Germany Abstract. A comprehensive set of teleseismic waveforms from two South American deep-focus earthquakes of the predigital era, the 1970 Colombia (Mw- 8.1) and 1963 Peru-Bolivia (Mw = 7.7) events, are inverted for source mechanism, seismic moment, rupture history, and cemroid depth. The P and SH wave inversion of the Colombia event confirms previous work, indicating that rupture occurred on a plane that dips steeply west. Rupture direction paralleled the trend of the Wadati-Benioff zone. We decompose the source into subevents, based on a source time function which shows two major moment release pulses separated by ~20 s. The first subevent is located near the initiation point at a depth of ~630 kin. The main moment release was located ~70 km to the southeast and ~20 km shallower. Rupture subsequently propagated farther southeast. The source time function has an initial subevent accounting for ~30% of the moment release of the entire event, whereas the long-period centtold momentensor (CMT) analysis [Russakoff el al., 1997] has the initial subevent yielding ~50%. The high-angle nodal plane rotated ~15 ø clockwise during the rupture, explaining the large compensated linear vector dipole (CLVD) component inferred from CMT solutions. Individual subevents have large CLVD and compressive isotropic components. A full moment tensor inversion of the Colombia and 1994 Bolivia events suggests that the initial subevents might contain a large non-double-couple (NDC) component. For the 1963 Peru-Bolivia event, using P waves, rupture propagated NNW for a distance of ~70 kin, parallel to the high-angle nodal plane and the trend of the Wadati-Benioff zone. The focal mechanism change dramatically after the second subevent, causing a very large NDC component. Both events, together with the 1994 Bolivia earthquake, have a precursor separated in space and time from the main rupture and show rupture velocities varying between 3 and 4 km/s between subevents, with <2.0 km/s on average for the entire event. Low seismic efficiencies and rupture velocitie support a highly dissipative, temperature-dependent rupture mechanism for large deepfocus South American earthquakes, compared with events in cold subducting slabs. 1. Introduction Since 1994 a series of very large deep-focus earthquakes have occurred [e.g., Kikuchi and Kanamori, 1994; Silver et al., 1995; Beck el al., 1995; Chen, 1995; Estabrook and Bock, 1995; Goes and Ritsema, 1995; Lundgren and Giardini, 1995; McGuire et al., 1997; Kanamori et al., 1998; Ihmld, 1998; Tinker et al., 1998; Tibi el al., 1999]. Although the amount of data and extent of the studies of these recent events is unprecedented, solid conclusions about the nature of deep earthquakes require extending our understanding to events of the predigital era. Therefore, to complementhe studies of the recent events, we analyze the largest events prior to 1994 using the same body wave inversion method [Ndb lek, 1984 that has been applied to the recent events. In this paper we use P and SH waves to invert for the rupture process of two large deep-focus South American earthquakes: the events of July 31, 1970, in Colombia (M w = 8.1, depth 623 km) and August 15, 1963, along the Peru-Bolivia border (M w = 7.7, 560 km). The issues which have been addressed with these events are (among others) whether a preference exists for rupture plane, rupture velocity, and mechanism for the triggering and Copyright 1999 by the American Geophysical Union. Paper number /99/1999JB continuation of rupture. Isacks and Molnar [1971] first observed that the focal mechanisms of deep earthquakes are predominantly normal faulting mechanisms with P axes parallel to the slab dip. Willemann and Frohlich [1987], Frohlich [ 1989], and Lundgren and Giardini [ 1994] attempted to determine whether a preference exists for one of the nodal planes to be the rupture plane. It is only, however, for the largest events that the rupture plane can conclusively be determined because of the smaller variance of waveform misfit for one nodal plane or because of an implied directivity vector which is perpendicular to the strike of one of the nodal planes. Studies of the 1994 Bolivia deep earthquake (Mw = 8.2, 651 km) found that deep events can have slow (<1.5 km/s) but variable rupture velocities [e.g., Silver e! al., 1995; Beck e! al., 1995; Chen, 1995; Estabrook and Bock, 1995; Lundgren and Giardini, 1995; McGuire et al., 1997; Kanamori e! al., 1998; Ihmld, 1998]. For this event the initial moment release was separated temporally and spatially from the main moment release. The 1994 Fiji deep earthquake (M w - 7.5, 568 km), however, had a much faster rupture velocity of 4-5 km/s [e.g., Goes and Ritsema, 1995; McGuire et al., 1997; Tibi et al., 1999]. McGuire et al. [1997] suggested that the Fiji earthquake was triggered in the cold core of the Pacific slab and propagated outside of the previously defined Wadati-Benioff zone. Because of a lack of preevent seismicity and aftershocks for large deep-focus South American events it is only possible 28,751

2 28,752 ESTABROOK: 1970 AND 1963 SOUTH AMERICAN DEEP EARTHQUAKES to draw conclusions about the relationship between mainshock rupture and the slab for the Fiji and the 1996 Flores Sea (M w = 7.9, 594 km) events, which occurred in regions of known seismicity. Silver et al. [1995] argued, however, that the 1994 Bolivia event also ruptured through the slab which is defined by locally recorded aftershocks [Myers et al., 1995]. McGuire et al. [1997] and Kanamori et al. [1998] suggested that different processes may act between the initiation (triggering) and subsequent rupture. 2. South American Deep Earthquakes The South American subduction zone is home to this century's largest deep earthquakes: the 1994 Bolivia and the 1970 Colombia events. Studies of the Colombia event have concentrated mainly on long- and ultralong-period radiation [Mendiguren, 1973; Dziewonski and Gilbert, 1974; Gilbert and Dziewonski, 1975; Mendiguren and Aki, 1978; Okal and Geller, 1979; Russakoff et al., 1997, hereinafter referred to as RET] primarily because the shorter-period body waves recorded by the Worldwide Standardized Seismograph Network (WWSSN) were off scale. Dziewonski and Gilbert [1974] and Gilbert and Dziewonski [1975] found that the tensional and intermediate moment tensor axes rotated clockwise about an Kikuchi [1987] found that rupture consisted of three isolated subevents with a total duration of -60 s and noted that it was necessary to introduce a long-period subevent of 60 s duration to increase the moment to that determined by Furumoto and Fukao [1976]. The study by Brt;istle and Mt¾11er [1987] is probably the most extensive of this event. They forward modeled P waves and found that rupture propagated along strike and downdip on a west dipping surface in a series of three subevents. Rupture began with a small subevent separated in space and time from later rupture. Rupture ended with a strong pulse of the opposite polarity to that of the main moment release, which Brt;istle and Maller[ 1987] interpreted as being a stopping phase. 3. Geometry of the South American Slab The epicenter of the 1970 Colombia earthquake (Plate 1) is located at the extreme northern end of a zone of very deep seismicity, separated by over 200 km from other known historical deep seismicity and over 400 km from more recent events. Thus an obvious question is whether the slab containing the 1970 event is continuous with events to the south. Okal and Bina [1994] relocated two historical events from 1921 (M w = 7.3,630 km) and 1922 (M w = 7.8, 660 km) to fall in a direct line between the recent seismicity (post 1963) south of 6øS and the 1970 epicenter, suggesting that the slab responsible for the southern seismicity might be continuous with the 1970 epicenter. Slab age (or thermal age [Kirby et al., 1996]) may play an important part in the invariant high-angle compressional axis during the rupture, suggesting a large compensated linear vector dipole (CLVD) component. Furumoto [ 1977] used on-scale P wave recordings from ~20 stations at epicentral distances less than 100 ø to obtain a rupture model by modeling pulse arrival times from deconvolved traces. Pulses located with a master event method tectonics and seismicity of the deep Colombian slab. On the cluster close to the west dipping nodal plane and show southward and upward migration with time. The high-angle west dipping nodal plane rotated -20 ø clockwise during the basis of bathymetry and seafloor age the Nazca Plate (Plate 1) can be divided into a relatively simple part south of the Carnegie Ridge (bathymetric trace of the Galapagos Hot Spot) rupture, in agreement with long-period studies. Rupture spread which becomes younger toward the northeast and a much out with a jerky motion over a curved rupture surface, and the overall rupture duration was -50 s. Fukao and Kikuchi [ 1987] used 12 P waves in a multiple-source inversion method in which rupture is constrained to lie on a single fault plane and identified three regions of high energy release: An initial subevent, a second occurring 20 s after initiation located km updip to the east, and a third starting 30 s after initiation and located up to 100 km to the south. The overall duration was again --50 s with an average rupture velocity of 1.3 km/s. Mendiguren and Aki [ 1978], using free oscillations, found that rupture propagated for ~150 km at an azimuth of 150 ø with a rupture velocity of 3.8 km/s. They also found that the intensity of the source increased as the square of the distance of the rupture, thus describing a ramp-shaped source time function or a fan-shaped fault. They found no evidence for precursory non-double-couple (NDC) moment release. The 1963 Peru-Bolivia earthquake has been studied at both surface wave and body wave periods [Chandra, 1970; Dziewonski and Gilbert, 1974; Gilbert and Dziewonski, 1975; Fukao and Kikuchi, 1987; Brt;istle and Mt¾11er, 1987]. Chandra younger section to the north of the Carnegie Ridge. Plate age is older downdip of the trench, but for a given depth it becomes younger toward the north [Pennington, 1981; Kirby et al., 1996]. [ sing the plate age data from Mt¾11er et al. [ 1997], the southern part of the Nazca Plate (west of northern Chile) is <40 Ma, with age increasing toward the northeast. Extrapolating to 600 km depth under Bolivia gives a plate age of ~50 Ma. Although Engebretson and Kirby [1992] and Kirby et al. [19951 suggest that the shallow and deep Nazca Plate might be discontinuous in age (but not broken) and that the deep plate is possibly 140 Ma, Okal and Bina [1994] contend that the age of the plate associated with the great Colombia earthquake is, at most, 55 Ma. Following the hypothesis that there is no age discontinuity in the subducted Nazca Plate to 650 km depth beneath southern Colombia and that the plate is continuous with the slab under Peru and Bolivia [Cahill and Isacks, 1992; Okal and Bina, 1994; Creager et al., 1995; Kirby et al., 1996; Gudmundsson and Sambridge, 1998, we can extrapolate to the region of the 1970 Colombia event to get an age of -40 Ma for the simple part of the Nazca Plate south of the Carnegie Ridge [1970] identified the event as a multiple shock and determined but significantl younger for the region north of the Carnegie average rupture properties including the location of the main energy release, which began 13 s after initiation and 46 km to the NNW (328ø). On the basis of rupture propagation to the NNW he suggested that the west dipping nodal plane was the rupture plane. Gilbert and Dziewonski 1975] found that there Ridge. The Carnegie Ridge may extend just north of the hypocentral region of the 1970 event and act as a suture between the simple Nazca Plate to the south and a much younger and more complicated plate to the north [e.g., van der Hilst and Mann, 1994]. was a 70 ø counterclockwise rotation of the horizontal stress Farther to the south (Plate 1), the subducted Nazca Plate is axes (tension and intermediate), suggesting a change in the stress regime and a large CLVD component. Fukao and thought to be continuous across the aseismic intermediate depth region between 300 and 530 km depth [James and

3 ß._. ESTABROOK 1970 AND 1963 SOUTH AMERICAN DEEP EARTHQUAKES 28,753 90øW. 80øW I 70øW I 60øW 50ow 10øN. ;'. - o 10øS ' ß :F.-' -., ß ';'- : ;,, American Pla-' 20øS 30øS <=, - 30oS 40øS...,' 40os 90øW 80øW 70øW... 60øW 50 o W Plate 1. Seismic setting in South America. Subduction of the Nazca Plate (red arrows) varies in relative velocity from 8.4 cm/yr in the south to 6.8 cm/yr in the north (NUVELI: DeMets et al. [1990]). Topography and bathymetry (in km) is from ETOPO5. Relocated International Seismological Centre (ISC) earthquakes [Engdahl et al., 1998] appear as dots (0-200 km), squares ( km), and triangles ( km). The relocated earthquakes were contoured by Gudmundsson and Sambridge [1998] in 50 km intervals. Dates indicate the locations of this century's major deep earthquakes. Seafloor age isochrons (in Myr) are from Miiller et al. [ 1997].

4 28,754 ESTABROOK: 1970 AND 1963 SOUTH AMERICAN DEF, P EARTHQUAKES Snoke, 1990; Engdahl et al., 1995; Okal and Talandier, 1997] but highly contorted horizontally [e.g., Creager et al., 1995] as the plate conforms to the S-shaped Wadati-Benioff zone beneath Peru, Bolivia, and Argentina. The 1963 Peru-Bolivia earthquake occurred where the Wadati-Benioff zone changes from linear north-south trending beneath Peru to nearly eastwest trending beneath Bolivia. 4. Body Wave Inversion of the 1970 Colombia Earthquake Obtaining data for the 1970 Colombia earthquake presents an immediate problem: How can one find enough on-scale body wave data to perform a meaningful body wave inversion? In Furumoto's [1977] body wave study he presented seismograms corrected for instrument response, geometrical spreading, and attenuation, but, unfortunately, those values are not presently known. Thus in order to use an equivalent data set it was necessary to obtain the original seismograms or copies thereof. We digitized many full-sized seismograms from the WWSSN obtained for the original study by Dziewonski and Gilbert [1974] and Gilbert and Dziewonski [1975] and microfilm copies obtained from the U.S. Geological Survey and Lamont-Doherty Earth Observatory. Seismograms from the critical stations KBS (Kings Bay, Spitzbergen, Norway) and WIN (Windhoek, Namibia) are only available from Furumoto's paper. To use these data in the inversion, it was necessary to convolve the seismograms with the WWSSN response and correct them for geometrical spreading. Because of these uncertainties, KBS and WIN are down weighted in the inversions. Following the method of NdbOlek [1984], we use teleseismic P and SH waves to determine focal mechanism, source depth, source time function, and seismic moment for the 1970 Colombia earthquake. Nfib lek's method is an interative least squares waveform inversion scheme in which the rootmean-square of the misfit between the observed and synthetic seismograms is minimized. Complex events can be modeled by several subevents, for which focal mechanisms, depths, source time functions (moment release rate), and subevent locations are determined simultaneously. Estabrook and Bock [1995] altered Nfib lek's original algorithm by increasing the array sizes to accommodate a longer time series in the inversion window and more layers in the velocity model. Focal mechanisms are initially constrained to have a doublecouple radiation pattern: This assumption is in agreement with the results of RET which suggest that the non-doublecouple (NIX2) component is <10% of the total moment for the 1970 Colombia event. Later we perform a full moment tensor inversion. Copies of P and S waves with their depth phases (if on scale) were scanned using the program NXSCAN (Incorporated Research Institutions for Seismology (IRIS) Data Center). The seismograms were then dehelixed, resampled at 2 samples/s and low-pass filtered at 2 s with a three-pole causal Butterworth filter to remove digitizing noise. Stations recording the P and S waves are restricted to lie between 30 ø and 90 ø from the source to minimize the interference of phase arrivals from the uppermost and lowermost mantle. Mantle attenuation is modeled with a t* operator (travel time divided by average quality factor (daverage) of 1 s and 4 s for P and S waves, respectively. Although these values are a bit high for crustal events using broadband seismograms (0.7 s and 2.8 s for shallow Alaskan events [Estabrook et al., 1994]) and deep events in Tonga using short-period seismograms (0.7 s [Bock and Clements, 1982]), they are middle values between high attenuation for the depth phases, which travel three times through the highly attenuating upper mantle, and low attenuation for the direct phases, which sample the upper mantle only once. We have compared our synthetic seismograms with synthetics calculated with the reflectivity method [Kind, 1979, 1985] for the preliminary reference Earth model (PREM)[Dziewonski and Anderson, 1981]. For the pp and sp depth phases relative to the direct P wave we find no systematic pulse broadening and amplitude changes compared to the reflectivity synthetics at 5-10 s period, which suggests that Ndb lek's [ 1984] method correctly models the depth phase amplitudes despite the depth phases traveling three times through the upper mantle. Time delays for the depth phases are <0.25 s. This can be easily understood when one considers that damping is strongest at high frequencies, and, given the amount of time that body waves spend in the asthenosphere (high attenuation), amplitude and frequency content of longperiod teleseismic body waves is hardly affected. Also, because attenuation in the upper mantle under South America is small in magnitude (as compared with the western Pacific island arcs) [e.g., Sacks and Okada, 1974; Barazangi et al., 1975; Bock and Clements, 1982; Flanagan and Wiens, 1994], effects on the depth phases should be small. For the velocity and density model at the source, we use the International Association of Seismology and Physics of the Earth's Interior 1991 (IASP91) [Kennett and Engdahl, 1991] and AK135 [Kennett et al., 1995] Earth models, respectively. The source model is approximated by 12 layers above a depth of 660 km. The receiver structure at all stations is a half-space where P and S wave velocity and density are 6.0 km/s, 3.46 km/s, and 2.75 g/cm 3, respectively Results The best fitting centroid solution (Cen) appears in Table 1. The Colombia earthquake has a normal faulting focal mechanism and occurred at a depth of 630 km located 48 km at an azimuth of 127 ø from the nucleation point, taken to be the relocated International Seismological Centre (ISC)-determined epicenter [Engdahl et al., 1998]. This indicates that dominant rupture was to the SE, which is in agreement with previous studies [Furumoto, 1977; Mendi guren and Aki, 1978; Fukao and Kikuchi, 1987; RET]. We have tested the azimuth of the Cen solution and find that equally valid solutions have azimuths between 120 ø and 150 ø. We next model the waveforms with a propagating point source (PPS) propagating to the SE. To determine the preferred rupture azimuth, we systematically alter the rupture azimuth, fixing the rupture velocity and inverting for mechanism, depth, and moment distribution. Figure la shows the variance of misfit plotted with respect to rupture azimuth. The minimum variance is for an azimuth of ~160 ø, roughly along the strike of the high-angle nodal plane, though the range of 150 ø to 200 ø is also equally valid. The misfit curve for average rupture velocity (Figure lb) indicates that the rupture velocity is well constrained to lie between 1 and 3 km/s with a minimum at 2.0 km/s. We also determined a depth of 627 km for the PPS solution, which is less than the 651 km determined by the ISC (645 km in the relocation by Engdahl et al. [ 1998]) but similar to the centroid moment tensor (CMT) centroid depth of 623 km (RET). The strike of the high-angle plane is

5 ESTABROOK: 1970 AND 1963 SOUTH AMERICAN DEEP EARTHQUAKES 28,755 Table 1. Double-Couple Inversion Results Strike, deg Dip, Rake, Depth, Moment, Duration, Delay, Di stance, deg deg km 1019 N m s s km Azimuth, deg Variance Reduction Cen 179 _ _ _ _+ 3 PPS Sub S1 166_+8 60_+3-90_+7 635_+3 S2 181 _ _+ 4 S Sub Sum S1 195 _ _ _ _+ 5 S _ _ _+ 10 S _ S Sum Sub S S _ _+ 5 S3 115_ _ S S _ _+ 4 Sum Colombia _ _ _ _ _ _ _ _ _ _ _ _ Peru-Bolivia P} Cen _ Sub S1 217 _ _ _ _ _ S Sum Sub S1 212_ _23-135_ _ _ S _ S3 151 _ _ _ _ _ _+21 Sum _ _ O.3625 O Cen, centroid solution; PPS, propagating point source. Colombia, 2.0 km/s at 160ø; Peru-Bolivia, 2.0 km/s at 340 ø. S1, S2, and S3 are subevents of three-point-source solution (3Sub); Sum, vector sum of subevents 1, 2, and 3. Fault plane convention follows Aki and Richards [ 1980]. The one standardeviation errors are multiplied by 5 (strike, dip, rake, delay, distance, and azimuth) and 10 (depth)[nd lek, 1984]. well constrained to be -180 ø. Given the similarity between the strike of the high-angle plane and the rupture direction, we conclude that rupture occurred on the plane striking ø and dipping ~60 ø to the west. Although it is conceivable that rupture could also occur on the east dipping plane, subevent locations and depths are more consistent with the west dipping plane as discussed in the following. Point sources are then added to match features in the PPS source time function. The best fitting solution for the 1970 Colombia earthquake (see Figure 2 and Table 1) has three point sources (3Sub): The first, S 1, occurred during the first 20 s of rupture and is located near the nucleation point. Two more point sources (S2 and S3) were added to model the later episode of moment release and are located SE of the initial event. Figure 3 compares the direct Pwaves from four stations for the different inversions. Individual pulses are better matched at various azimuths with the increasingly complex source models (compare 3Sub with Cen at GDH and WIN). The addition of subevents beyond three causes the variance to decrease but with little improvement at the stations GDH, COP, and WIN. Improvements are seen at the nodal stations (Figure 2) and at SPA for the initial pulse. Figure 4 illustrates the rupture history of the 1970 Colombia earthquake. Rupture extent is -100 km, in agreement with the results of Fukao and Kikuchi [ 1987] but smaller than that of Mendiguren and Aki [ 1978]. The subevent locations to the south of the initiation point and shallowing toward the east suggest that the high-angle west dipping planes of the subevents ruptured in the event. The inversion results imply that the focal mechanism changed in strike from the first to the later subevents and that the rupture velocity was highly variable. The strikes of the high-angle nodal planes of S2 and S3 are rotated 11 ø to 15 ø clockwise relative to the strike of S 1, with a corresponding rotation of the tensional and null axes. Earlier studies [Dziewonski and Gilbert, 1974; Gilbert and Dziewonski, 1975; Furumoto, 1977] had a larger clockwise rotation (~20 ø ) of the high-angle nodal plane during the course of rupture. The variable rupture velocity is inferred from comparison of the results for the PPS and 3Sub solutions. The average rupture velocity for PPS was -2.0 km/s at 160ø; the locations of S2 and S3 suggest that rupture between subevents was higher ( km/s). We tried an inversion scheme in which a propagation velocity was applied to the individual subevents, but the preferred velocity was <1 km/s, implying either that rupture velocity within the subevents was indeed very low, or that subevent ruptures were not unilateral and could not be modeled with a directivity function [Aki and Richards, 1980]. The observation of a variable rupture

6 28,756 ESTABROOIC 19'70 AND 1963 SOUTH AMERICAN DEEP EARTHQUAKES (a) (b) 0.50 Rupture Velocity (km/s) Figure 1. Variance of the misfit between observed and synthetic seismograms from the 1970 Colombia earthquake plotted as a function of (a) azimuth and (b) velocity of the propagating point source (PPS). velocity is supported by recent results from Chen [1995], Estabrook and Book [1995], and Ihmld [1998] for the Bolivia deep earthquake and from Bouchon [ 1998] for shallow events. The observations from Ihmld [ 1998] and Bouchon [ 1998], that apparent rupture velocities are lower within episodes of moment release than between episodes, are opposite to Chen's suggestion that the rupture velocity was higher within subevents for the Bolivia event. Certainly, more examples are needed to resolve this issue. The west dipping nodal plane cuts nearly perpendicular to the subducting slab as contoured by Gudmundsson and Sambridge [ 1998], analogous to the rupture surface of the 1994 Bolivia earthquake. The positions of S2 and S3 relative to S1 are ø counterclockwise from the strike of the slab, which might indicate that rupture propagated toward the upper surface of the slab as defined by S2 and S3. While the exact position of the slab is not known (because of the absence of any other nearby seismicity), the positions of S2 and S3 suggesthat rupture paralleled the strike of the slab after the initial event. This agrees with the suggestion from Mendiguren and Aki [1978] that rupture was trapped by the slab. This has also been suggested for the 1994 Flores Sea event [e.g., Tibi et al., 1999]. The rupture model is consistent with initiation in the cold slab core and with most of the moment release in the warmer outer region of the slab. Figure 5 shows a comparison of displacement seismograms recorded at station SPA (South Pole) for the 1970 Colombia and 1994 Bolivia earthquakes. The duration of the Colombian e 0.45 J ß ß - event is nearly 20 s longer than that of the Bolivian event, and the initial pulse is also much larger and well separated from the main pulse. SPA, in the rupture direction for the Colombia event but opposite to the rupture direction for the Bolivian event, is the only P wave from the 1970 event which shows such a large initial pulse (see Figures 2 and 3). The main 0.40 pulses of the two events, on the other hand, are very similar in amplitude, complexity, and duration. The pp-p time is slightly smaller for the Colombian event, indicating a Rupture Azimuth (deg) shallower depth. The SPA seismogram for the Colombian event also shows a nonzero amplitude between the P and pp waves, which, in the case of the Bolivian event, had been argued by Vidale e! al. [1995] to indicate a permanent offset, 0.50 though Estabrook and Bock [ 1995] found that they cotfid model the nonzero amplitude without a near-field term. Kanamori [1973] argued that SPA is an anomalous station, which often shows amplitude several times smaller than other stations at long periods; on the other hand, our experience from modeling the 1994 Bolivia earthquake suggests that at _ body wave periods (5-20 s) the waveforms can be modeled and ß ß that anomalies in the signal are most likely due to SPA being ß often the only station at southern azimuths. Furthermore, ß ß ß synthetic seismograms calculated for a crustal model with 3 km of ice are smoother, having damped short periods, but the overall amplitudes are reduced by only 10%. Thus the thickness of ice under SPA cannot explain the larger 0.40,, amplitudes which we observe in the P wave Comparison With Long-Period CMT by Russakoff et al. [1997] RET inverted for the moment tensor of the 1970 Colombia earthquake using long-period surface waves and normal modes in an effort to resolve the controversy over the large precursory isotropic (compressional) component postulated by Dziewonski and Gilbert [1974]. RET concluded that the large precursory isotropic component was an artifact caused when normal mode splitting and coupling between toroidal and spheroidal modes were not incorporated into the inversion. Figure 6 comparesource time functions determined from body wave modeling and from normal mode inversion of the frequency-dependent total moment function. The source time functions are remarkably similar considering the different analysis methods involved. Although all of the source time functions show that rupture occurred in two episodes, the source time function determined by RET has an initial pulse during the first 15 s of rupture which is larger than the remaining pulses. All of the body wave source time functions show that the later pulses had greater moment release. The difference between our source time function and that of RET is further confirmed by comparing centroid times (24.7 s for RET and 34.1 s for the Cen inversion). The centroid location from RET implies that the large moment release pulse at the beginning must be located between the initiation point and the centroid. Our inversion shows that the first subevent is small

7 ESTABROOK: 1970 AND 1963 SOUTH AMERICAN DEEP EARTHQUAKES 28,757 GDH P waves AAM '" STUh o 25 5o 75 loo 125 sec! E Ool,,,!!! O O175 sec Figure 2. Observed (solid lines) and synthetic (dotted lines) P and SH waveforms and focal mechanism of the 1970 Colombia earthquake for inversion 3Sub (Table 1). The focal mechanism shown in the center is the weighted average of the three subevents. The small vertical dotted lines on the seismograms mark the beginning point and endpoint of the inversion window. The source mechanism a lower hemisphere, equalarea projection with dark shading representing compressional first motions. Station distances and gains are normalized to 40 ø and 1, respectively. The time and amplitude scales for the P and SH waves are shown in the bottom left. PandSH waves were digitized from vertical and horizontal seismograms, respectively. An "h" following the station code means that the horizontal component was used. and that additional subevents in the first 20 s are located both to the east and south but not directly between the initial point and the centroid location in Cen. The centers of moment release (or centroids) for inversions 3Sub, 4Sub, and 5Sub (Figure 3 and Table 1) differ substantially from Cen with the centroid being located near the later subevents as finer details of the rupture are modeled. The difference in the amount of moment release at the beginning of rupture might reflect differing inversion techniques or frequency bands of the data. Our inversion is a constraine double couple. RET performed full moment tensor inversions with and without constraints on the isotropic component, both of which showed little difference in centroid times and moment tensor. Could this difference between the body wave and surface wave results be related to the rupture process? RET suggesthat at frequencies lower than 12 mhz (83 s) there is nearly double the moment at the beginning of rupture compared with body wave inversion. The initial subevent might also have preferentially radiated longer-period waves (a slow event) compared with the later rupture. Qualitatively, the shape of the source time function from RET requires shorter periods to arrive before longer periods (the first pulse is narrower than the second). To test this, we lowpass filter the source time function and find that the centroid time is progressively later for longer periods. Thus, unless the longer-period waves violated causality, which RET

8 28,758 ESTABROOK: 1970 AND 1963 SOUTH AMERICAN DEEP EARTHQUAKES STF GDH (72 ø, 7 ø) COP (88 ø 34 ø) WIN (890, 112 ø ) SPA (880, 180 ø ) PPS 3Sub 4Sub 5Sub Time (sec) Time (sec) Figure 3. Comparison of observed (solid lines) and synthetic (dashed lines) P waves for the different inversions of the 1970 Colombia earthquake. STF, combined source time function; GDH, Godhavn, Greenland; COP, Copenhagen; WIN, Windhoek, Namibia; and SPA, South Pole. For inversion name acronyms, refer to Table 1. Numbers next to station codes are epicentral distance and azimuth in degrees. Only direct P waves are shown; depth phases were used in the inversions at COP and SPA. demonstrate was not the case, we can discount the possibility that the initial subevent preferentially radiated longer-period energy. In section 4.3 we try to resolve the time variations in NDC moment release The Non-Double-Couple Component: Full Moment Tensor Inversion To investigate the hypothesis described in section 4.2, we calculate the non-double components from the various inversions. We use the following parameters to describe the NDC component, calculated following Jost and Herrmann [1989]: The CLVD component e is defined as the ratio of the smallest to the largest eigenvalue (in absolute sense) of the moment tensor M, and the isotropic component I is expressed as the percentage of the best double-couple seismic moment. The phrase tr=0 (try0) means that the constraint trace M = 0 (no isotropic component) has (has not) been applied in the inversion. The moment tensor calculated by RET (RET try0 in Table 2) yields e = and I = -9% (implosive). The combined moment tensor for the double couple 3Sub solution gives e = (3Sub DC in Table 2). This relatively small value is the result of only a minor change in the focal mechanism during rupture. By adding more subevents to the first 20 s of rupture (4Sub and 5Sub in Figure 3 and Table 2) we can increase the magnitude of e and the amount of energy partitioned into the beginning of rupture, but it does not reach the amount obtained by RET. Furthermore, subevent location becomes highly unstable, which leads us to conclude that the addition of subevents beyond three may cause us to model correlated structural phases (converted/reflected P waves) or noise. The surface wave data which RET used might be biased because of the lack of the lower orbit (R1 and G1) surface waves. Although the trend with additional subevents at the beginning is to increase the CLVD component and the partitioning of moment toward the initial pulse, we cannot match the results from RET. Hara et al. [1995] and RET have shown that the isotropic component of deep earthquakes is <10%, and, if the isotropic component was >20%, it should be resolved. However, if the largest isotropic component occurs only at the beginning of rupture, where it might be an order of magnitude larger than that later in the rupture, its effect on the combined moment tensor might still be unresolvably small. We have modified Ndb lek's [1984] code to perform a full moment tensor inversion. As a starting solution (initial guess) we start with the double-couple solutions and locations shown in Table 1 and then allow the moment tensor elements to find a new minimum. We perform constrained (five elements with trace M = 0) and unconstrained (six independent moment tensor elements) inversions for the Cen, PPS, and 3Sub double-couple solutions. The new solutions can be found in Table 2. To test the reliability and resolvability of the moment tensor code, we perform a full moment tensor inversion of the June 9, 1994, Bolivia earthquake using waveforms from Estabrook and Bock [1995]. The centroid solutions (Cen tr=0 and Cen try0) both have a small CLVD component e of The isotropic component I is +1.6% (explosive), but this is smaller than the standard error of the seismic moment (2%). In all inversions of the Bolivia event, I is smaller than the standard error of the seismic moment, so it is probably unreliable. The initial subevent (S1) of the 1994 Bolivia event has a large NDC component both in the constrained and unconstrained moment tensor inversions (I = -22% and e =-0.18 to -0.25), but because of the large errors

9 ESTABROOK: 1970 AND 1963 SOUTH AMERICAN DEEP EARTHQUAKES 28,759 1øS 73øW i 72øW I 1øS Source time Function Depth (km) Sl (635 km) S2 (625 km) 'co :' ' J 100 c,,,,, S3 (618 km) 2øS 73øW 620 _72øW ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: :: ::::5 I o W-E (km) 5o Figure 4. Map of epicentral region of the 1970 Colombia earthquake showing our interpretation of the rupture process. The initiation (star) and subevents of inversion 3Sub (triangles) are plotted in map and cross section. Source time function (moment release rate) of the inversion 3Sub is shown at the top along with subevent source time functions (shaded). Arrows depict directions to subevents along with rupture velocities between subevents. Cross is centtold location from body wave inversion 3Sub, and the hexagon is from the centtold momentensor (CMT) analysis (RET). Wadati-Benioff zone contours are from Gudrnundsson and Sambridge [1998]. In the EW cros section the high-angle plane of the centroid focal mechanism shown as a heavy line along with the dip of the Wadati-Benioff zone (slab) from Gudmundsson and Sambridge [ 1998]. Read as July 31, associated with the seismic moment of S 1 we are unsure of its significance. Moment tensor inversions of the Colombia earthquake all have large implosive isotropic and CLVD components. Of the three South American earthquakes only the Colombia event has a NDC component (isotropic and CLVD) larger than the standard error of the seismic moment. We note that there is a trade-off between the isotropic and the CLVD components: Application of the deviatoric constraint (trace M = 0) causes the NDC moment release to be taken from the isotropic to the CLVD component. Thus it is unclear if the NDC moment release occurs in the isotropic or CLVD components. Our results agree with that of RET that the isotropic component is not resolvable, but, additionally, we conclude that there is a significant NDC component with the tendency for a higher percentage of NDC moment release at the beginning of rupture. 5. Body Wave Inversion of the 1963 Peru-Bolivia Earthquake P wave seismograms for the 1963 event were digitized (following the method outlined in section 4.1 for the Colombian earthquake) from microfilm, full-sized copies of original records and from seismograms presented by Briistle and Maller [1987]. We then inverted the seismograms using Ndb lek's [1984] code Results Shown in Figure 7 is the best solution for the 1963 Peru- Bolivia earthquake. The solution, 3Sub, has three subevents. Table 1 shows the inversion restfits. The first two subevents are stable in their results; the third subevent is necessary for the inversion, but we are not confident of its location. The

10 28,760 ESTABROOK: 1970 AND 1963 SOUTH AMERICAN DEEP EARTHQUAKES p P 31-JUL-1970 SPA Dist=88.5 ø I p lp 9-JUN-1994 SPA Dist=76.3 ø ' I I I I I I I I I I I I I Time (sec) Figure 5. Comparison of vertical P wave displacement seismograms from the 1970 Colombia and 1994 Bolivia earthquakes recorded at SPA (South Pole). Worldwide Standardized Seismograph Network (WWSSN) instrument response has been deconvolved from the 1970 seismogram. The higher-frequency content of the 1994 seismogram probably an artifact of the use of the broadband recording system. Vertical lines indicate arrival times of the main phases from the IASP91 model for a source at 650 km depth. improvement in fit (variance reduction) compared to a solution with two subevents, 2Sub, is mainly at the southern stations WEL and SBA in the time interval starting ~20 s after initiation. The Cen solution has a smaller variance than the PPS solution (Table 1), indicating that moment release was not unilateral but rather was released behind the rupture front in a manner better approximated by a point source. For the Colombia event, on the other hand, the PPS solution has a somewhat smaller variance than the Cen solution. Our results for the Peru-Bolivia earthquake identify a change in focal mechanism and moment release in a nonunilateral fashion late in the rupture: This might correspond to the suggestion by Brt stle and Mailer [1987] that rupture accelerated after the second subevent and ended with a strong "stopping phase" of opposite polarity to that of the main phase. The large positive polarity phase observed at the European stations (40-50 s after rupture initiation) is the instrumental backswing from the WWSSN seismometer and should not be confused with a source effect. The high-angle nodal plane rotates counterclockwise between S1 and S2. S3 still has a steeply dipping pressure axis, but its mechanism is very different from those of S 1 and S2. The drastic mechanism change is not reflected in the PPS and Cen solutions, which both have smaller moments than the 3Sub solution and shorter durations. From the combined moment tensor for the 3Sub solution (3Sub DC in Table 2) the CLVD component e is -0.26, in agreement with Dziewonski and Gilbert [ 1974], Gilbert and Dziewonski [ 1975], Houston [ 1993], and Huang et al. [ 1997]. The CLVD component using the momen tensor calculated by Huang et al. [1997] is These results suggest that the large non-double-couple component of the 1963 Peru-Bolivia earthquake is caused by a double-couple focal mechanism which changes during the rupture. We performed a full moment tensor inversion but found that only the centroid constrained and unconstrained inversions were stable (Cen tr--0 and Cen try0 in Table 2). The NDC component in the centroid inversions are small, and it appears that the large e comes from S3. Shown in Figure 8 are focal mechanisms of deep-focus events in the Peru-Bolivia region. Using a common station set, the 1963 earthquake (S1) is relocated relative to the January 10, 1994, earthquake using the master event technique (R. Tibi, personal communication, 1998). Antolik et al. [1999] chose the shallow-dipping nodal plane of January 10, 1994, event as the rupture plane. Estabrook and Book [ 1995], along with many' other workers, determined that the horizontal plane was the rupture surface for the great 1994 Bolivia event. Choosing a rupture surface for the 1963 event is more difficult, but on the basis of the subevent location and the prelerred rupture directivity direction of the PPS solution (330 ø) we choose the high-angle, west dipping plane as the rupture plane, similar to the interpretations of Chandra [1970] and

11 ESTABROOIC 1970 AND 1963 SOUTH AMERICAN DEEP EARTHQUAKES 28,761 Fukao and Kikuchi [1987] M o = 1.22 x 10 ] N In T O = 32.0 s Furumoto [1977] M o = 2.1 x 10 z] N In T O = 37.2 s This study (Cen) M o = 1.45 x 10a1 N In T o = 34.1 s Russakoff et al. [1997] M o = 1.42 x!0 z N In T o = 24.7 s Time (sec) Figure 6. Source time functions of the 1970 Colombia earthquake from the CMT inversion [Russakoff et al., 1997], centroid body wave inversion (Cen), and summed deconvolved nonnodal P waves from Furumoto [1977] and Fukao and Kikuchi [1987]. Centroid times T O have been calculated for all the source time functions. M o, seismic moment. P waves TRN, BHP v sec Figure 7. Observed (solid lines) and synthetic (dotted lines) waveforms for the Peru-Bolivia earthquake of August 15, 1963, for solution 3Sub (Table 1). Symbols are same as in Figure 2.

12 28,762 ESTABROOK: 1970 ANT) 1963 SOUTH AMERICAN DF_ P EARTHQUAKES o oo ddd d I I I I I I I I i i i i ddd odd dd do ddd -H-H -H ddd o,. o,- o, o,- o,- o,' o, o,- o,' o,' o, o, o,' o,'?- o,' o,' o,' o,' o, o,' o,' csd -H -H -H dd dddd ddddddddo dodd d ddd o d ddd d dddd ddddd dddd do

13 ,. ESTABROOK: 1970 AND 1963 SOUTH AMERICAN DEEP EARTHQUAKES 28,763 12øS 70øW 69øW 68øW 67øW 12øS "" ' Peru ",, '",, '",," i' q6 km) Bolivia S2 (562 km) 13øS (651 km) 13øS "' (603 km) 14øS 15øS I, ', ' " ' " "½ '"... m W m ',7,, >,.,-' ',,, '-. ¾.._ '-.. -,z 70øW 69øW 68øW 67øW Figure 8. Focal mechanisms of deep events in the Peru-Bolivia region. The initiation (star) and subevents (triangles) of the 1963 event (630815) were obtained via body wave modeling. Lower hemisphere focal mechanisms are shown for an inversion with three subevents along with mechanisms for the great 1994 Bolivia earthquake (940609) [Estabrook and Book, 1995], the M w = 6.8 event of January 10, 1994 (940110) (R. Tibi, personal communication, 1998) and the M w = 6.6 event of November 28, 1997 (971128) (Harvard quick CMT). Wadati-Benioff zone contours are from Gudrnundsson and Sambridge [1998]. Cross is centroid location from Ix)dy wave inversion of the 1963 event, and the hexagon is from CMT analysis of Huang et al. [1997]. The location shown for the 1963 Peru-Bolivia event has been calculated relative to the location of the January 10, 1994, event [Engdahl et al., 1998] using the master event technique. Brastle and Mailer [1987]. The similarity between the mechanisms of the January 10 and June 9, 1994, events suggests that for events deeper than 600 km the horizontal nodal plane is the rupture plane along this part of the subduction zone. The change in preferred rupture plane as a function of depth might be significant. 6. Comparison With Other Large Deep Events We compare the rupture behavior and dynamic parameters of the five largest deep earthquakesince 1963 in Figure 9 and Table 3. Source time functions of the 1994 Bolivia, 1970 Colombia, and 1963 Peru-Bolivia earthquakes show a small precursor separated by between 5 and 15 s from the main moment release. The precursor is not observed for the 1994 Fiji and 1996 Flores Sea earthquakes. This initial subevent increases in moment and duration, and the main moment release episode is delayed, as a function of temperature. Fukao [1972] and Fukao and Kikuchi [1987] did not observe a precursor for the deep-focus western Brazil earthquake of 1963 (M w = 7.7, 596 km [Huang et al., 1997]); they did, however, observe that rupture occurred on the east dipping nodal plane with a low rupture velocity ( km/s). Estabrook et al. [!992, 1994] found that large shallow-subduction-zone earthquakes often begin gradually, but they do not have precursors as do these South American deep events. Because event durations are expected, to first order, to be a function of seismic moment, Vidale and Houston [ 1993] suggest making comparisons among earthquakes by first adjusting the source time function durations by the cube root of their seismic moment. The resulting durations are 12.9 s for Colombia, 11.8 s for Peru-Bolivia, 8.1 s for Bolivia, 4.8 s for Flores Sea, and 5.3 s for the Fiji events. Thus, whether using adjusted or absolute durations, the South American events have longer durations. Radiated seismic energies ER are calculated from the differentiated square of the centroid source time functions following Vassiliou and Kanamori [1982]. For the older events, ER may be underestimated because long-period WWSSN instruments lack energy at short periods [Kikuchi and Fukao, 1988]. Addressing the band-limiting effects on seismic energy estimates, Abercrombie [1995] suggested that the frequency content f of the seismic signal should be both much higher and lower than the corner frequency fc of the earthquake (fmax>5fc and fmin<fc/2). Newman and Okal [ 1998] suggesthat fmax should be ~2 Hz. With the hand-digitized long-period WWSSN data this poses a problem: For the Peru- Bolivia and Colombia events, in which fc (1/duration) are 0.03

14 l 28,764 ESTABROOK: 1970 AND 1963 SOUTH AMERICAN DEEP EARTHQUAKES o o o x o 1 / Colombia Precursor / Main 3 Peru-Bolivia Bolivia Flores Sea _ 10-, - 11 fi 9 o3o9 o 1 o Time (sec) Figure 9. Source time functions of the three largest South American deep-focus earthquakes since 1963 (Colombia and Peru-Bolivia are from the Cen inversions; 1994 Bolivia is from Estabrook and Bock [ 1995]) together with the 1994 Fiji and 1996 Flores Sea events [from Tibi et al., 1999]. Source time functions are plotted as a function of thermal parameter (plate age times vertical descent velocity). The 1994 Bolivia and 1963 Peru-Bolivia events should have about the same thermal parameter (as pointed out by the arrows) but are only plotted at different positions for clarity. Thermal parameter value appropriate for Fiji-Tonga is 11,800 km and for Indonesia is 9400 km [Wiens and Gilbert, 1996]. Taking the age of the deep Nazca Plate as, at most, 60 Ma and a convergence rate of cm/yr for a subduction zone dipping at 45 ø, the thermal parameter for the deep Nazca Plate is between 2900 and 3600 km. See text for further discussion. Hz (1/34 s)and Hz (1/68 s), respectively, the values of fmax following Abercrombie [1995] are 0.15 Hz (7 s) and 0.07 Hz (14 s). Thus the short-period end of the long-period WWSSN seismograms could cause the seismic energy of the Peru-Bolivia event to be underestimated. To determine just how significantly we might underestimate ER, we calculate ER by low-pass filtering the source time function of the 1994 Bolivia event at 5 s and find that E is only ~40% of the unfiltered estimate. Thus source time functions deficient in the shorter periods could underestimate E. Kikuchi and Fukao [1988] calculated radiated energies of 2.1x1015 J for the Colombia and 1.7x1015 J for the Peru-Bolivia events. Their value for the Colombia event is in agreement with our value, but their estimate for the Peru-Bolivia event is larger, though we note that their energy estimate is based on a seismic moment which is nearly double our value. For the Flores Sea event, Tibi et al. [ 1999] (which we report here) estimate E R = 2.1x10 6 J: this is in good agreement with the values of 1.9x10 6 J reported by Houston [1996] and 1.8x1016 J reported by the U.S. Geological Survey. Following Kanamori et al. [1998], we calculate static stress drop Ao, minimum frictional stress C fmin, and seismic efficiency lmax- The three events from the South American subduction zone (a warm slab) all have very small values of ]max, indicating that the sources were highly dissipative, while the events from the colder slabs have larger ]max. The energy-to-moment ratio (log (E Mo) [Newman and Okal, 1998]) also shows a trend similar to ]max but avoids the additional uncertainty associated with stress drop. Kanamori et al. [ 1998] proposed for the 1994 Bolivia event, on the basis of the small ]max and some reasonable estimates of thermodynamic parameters, that melting should occur along the fault surface. We find, also, following the assumptions of Kanamori et al. [ 1998], that melting should have occurred for the 1994 Bolivia and 1970 Colombia events but not for the other events. From Figure 9 it appears that deep earthquake rupture durations are temperature dependent: Slabs with smaller thermal parameters [Kostoglodov, 1989; Kirby et al., 1996; Wiens and Gilbert, 1996; Okal and Kirby, 1998] and thus higher temperatures have longer overall durations, longer delay times between the initiation of rupture and the initiation of the main episode of moment release, and an increasing percentage of total moment in the initial rupture episode. The temperature dependence of the differential time between initial and main rupture suggests that higher temperatures delay the initiation of main rupture via a slower rupture velocity. The three South American events all have very small seismic efficiencies and average rupture velocities, indicating that the sources were highly dissipative, while the events from the colder slabs have greater seismic efficiencies and rupture velocities. This temperature dependency suggests that plastic instability or shear-induced melting [Griggs and Baker, 1969; McKenzie and Brune, 1972; Ogawa, 1987; Hobbs and Ord, 1988; Lomnitz-Adler, 1990; Spray, 1993; Kikuchi and Kanamori, 1994] caused by phase-transformation-induced grain size reduction [Riedel and Karato, 1997] is a viable mechanism of deep earthquake rupture. Kanamori et al. [ 1998] and McGuire et al. [ 1997] suggested that the mechanisms of rupture initiation and propagation might differ, possibly initiating as transformational faulting but propagating farther via shear-induced melting or plastic instability. Our results lend support to such an interpretation because of the differing rupture velocities between and within subevents, the occurrence of a small initiation phase separated in space and time from the main rupture, and the preference for NDC moment release at the initiation of rupture. 7. Conclusions Examination of the largest deep-focus earthquakes in recent times using the same technique reveals several interesting features. Large South American events often occur as multiple ruptures. Their source time functions are of longer duration than for events of comparable size that occur in colder slabs.

15 , ESTABROOK: 1970 AND 1963 SOUTH AMERICAN DEEP EARTHQUAKES 28,765 Table 3. Dynamic Parameters of Large Deep Earthquakes Earthquake Depth, M o, Rupture S, D, E R, log (ER?V[o)/ cl, km 1020 N m Velocity, m 2 m 1015 J MPa MPa km/s elfrain, l'lmax 1970 Colombia x Peru-Bolivia x Bolivia* x _ Flores Sea? x Fijit x Formulas are from Kanamori et al. [ 1998]: Slip D = Mo/( ts); event radius r = (S/ r)i/2; static stress drop ho -- 7 r td/(16r) = 2.4MoS-3/2; seismic efficiency 'lmax - 2(ER/Mo) t/ao; minimum frictional stress Jfmin = (1- 'lraax)/ t /2, where t is the shear modulus, S is the fault area, and E is the radiated seismic energy. Value of t is 116 GPa (Peru-Bolivia, Flores Sea, and Fiji) and 120 GPa (Colombiand Bolivia). * Estabrook and Bock [ 1995].? Tibi et al. [ 1999]. The ruptures begin with an episode of low moment release Aki, K., and P. Richards, Quantitative Seisinology, Theory and Methods, followed s later by a much larger subevent. There is vol. 1,558 pp,, W. H. Freeman, New York, Antolik, M., D. Dreger, and B. Romanowicz, Rupture processes of large often an initiation phase which is separated in both space and deep-focus earthquakes from inversion of moment rate functions, J. time from the main, larger component of rupture. These large Geophys. Res., 104, , earthquakes can be decomposed into a sequence of subevents. Barazangi, M., W. Pennington, and B. Isacks, Global study of seismic For the 1970 Colombia event, rupture began at ~630 km depth wave attenuation in the upper mantle behind island arcs using pp and propagated southward for ~100 km on a high-angle plane that dips steeply to the west. Although rupture was essentially waves, J. Geophys. Res., 80, , Beck, S. L., P. Silver, T. C. Wallace, and D. James, Directivity analysis unilateral, moment was released in a series of subevents, where of the deep Bolivia earthquake of June 9, 1994, Geophys. Res. Lett., 22, , the first was located near the initiation point and the main Bock, G., and J. R. Clements, Attenuation of short-period P, PcP, ScP and moment release began ~20 s later and 70 km to the southeast. pp waves in the Earth's mantle, J. Geophys. Res., 87, , The duration of rupture was 60 s. For the 1963 Peru-Bolivia 1982, event, rupture began at ~545 km depth and propagated toward Bouchon, M., Stress and friction on earthquake faults inferred from near-source seismic data, (abstract), Eos Trans. AGU, 79(45), Fall the NNW for a distance of ~70 km to a depth of ~570 km. The Meet. Suppl., F609, rupture direction was parallel to the trend of the Wadati-Benioff Brtistle, W., and G. Mailer, Stopping phases in seismograms and the zone as in the 1970 Colombia event. For both the Colombia spatiotemporal extent of earthquakes, Bull. Seismol. Soc. Am., 77, 47- and Peru-Bolivia events there was a large CLVD (non-double- 68, Cahill, T., and B. L. Isacks, Seismicity and shape of the subducted couple) component caused by a change in focal mechanism Nazca Plate, J. Geophys. Res., 97, 17,503-17,529, during rupture. This was not evident with the 1994 Bolivia Chandra, U., The Peru-Bolivia border earthquake of August 15, 1963, earthquake. A full moment tensor inversion of the Colombia Bull. Seismol. Soc. Am., 60, , and 1994 Bolivia events might suggest that the initial Chen, W.-P., En echelon ruptures during the great Bolivian earthquake subevents contained a non-double-couple component. With of 1994, Geophys. Res. Lett., 22, , Creager, K. C., L.-Y. Chiao, J.P. Winchester, and E. R. Engdahl, all three large South American events the average rupture Membrane strain rates in the subducting plate beneath South velocities are slow (<2.0 km/s), but from the timing of the America, Geophys. Res. Lett., 22, , subevents the rupture velocity between subevents may be higher (3-4 km/s). Seismic efficiencies and rupture velocities DeMets, C., R. G. Gordon, D. F. Argas, and S. Stein, Current plate motions, Geophys. J. Int., 101, , Dziewonski, A.M., and D. L. Anderson, Preliminary reference Earth are also much lower compared with the large Fiji and Flores Sea model, Phys. Earth Planet. Inter., 25, , events, which is consistent with a highly dissipative, Dziewonski, A.M., and F. Gilbert, Temporal variation of the seismic temperature-dependent rupture mechanism such as plastic momentensor and the evidence of precursive compression for two instability or shear-induced melting. deep earthquakes, Nature, 257, , Engdahl, E. R., R. D. van der Hilst, and J. Berocal, Tomographic Acknowledgments. I thank R. Abercrombie, G. Bock, T. Boyd, W. Brtistle, T. Dahm, G. Ekstrbm, H, Gbdde, C. Haberland, R, Kind, F. KrOger, D. Ramesh, M. Riedel, B. Schurr, and R. Tibi for fruitful discussions. G. Bock, F. Krtiger, E. Okal, R. Russo, R. Tibi, and an anonymous referee provided valuable suggestions on the manuscript. W. Brtistle, M. Furumoto, G. Ekstr6m, and J. Taggart helped me obtain the seismograms. Microfilm copies were obtained from the U.S. Geological Survey and Lamont-Doherty Earth Observatory. The GMT software [Wessel and Smith, 1995] was used in creating many of the figures. NXSCAN software was obtained from the IRIS Data Management Center. This research was supported by the Deutsche imaging of subducted lithosphere beneath South America, Geophys. Res. Lett., 22, , Engdahl, E. R., R. van der Ifilst, and R. Buland, Global teleseismic earthquake relocation with improved travel times and procedures for depth determination, Bull. Seismol. Soc. Am., 88, , Engebretson. D.C., and S. H. Kirby, Deep Nazca slab seismicity: Why is it so anomalous? (abstract), Eos Trans. AGU, 73(43), Fall Meet. Suppl., 379, Estabrook, C. H., and G. Bock, Rupture history of the great Bolivian earthquake: Slab interaction with the 660-km discontinuity?, Geophys. Res. Lett., 22, , Forschungsgemeinschaft (grant Es129/1) and the GeoForschungs- Estabrook, C. H., J. L. Nfib lek, and A. L. Lerner-Lam, Tectonic model Zentrum Potsdam. of the Pacific-North American plate boundary in the Gulf of Alaska from broadband analysis of the 1979 St. Elias, Alaska, earthquake References and its aftershocks, J. Geophys. Res., 97, , Estabrook, C. H., K. H. Jacob, and L. R. Sykes, Body wave and surface Abercrombie, R. E., Earthquake source scaling relationships from -1 to 5 Mœ using seismograms recorded at 2.5-km depth, J. Geophys. Res., I00, 24,015-24,036, wave analysis of large and great earthquakes along the eastern Aleutian are, : Implications for future events, J. Geophys. Res., 99, 11,643-11,662, 1994.

16 28,766 ESTABROOK: 1970 AND 1963 SOt I'H AMERICAN DEEP EARTHQUAKES Flanagan, M.P., and D. A. Wiens, Radial upper mantle attenuation Kirby, S. H., E. A. Okal, and E. R. Engdahl, The 9 June 94 Bolivian deep structure of inactive back are basins from differential shear wave earthquake: An exceptional event in an extraordinary subduction measurement, J. Geophys. Res., 99, 15,469-15,485, Frohlich, C., The nature of deep-focus earthquakes, Annu. Rev. Earth Planet. Sci., 17, , Fukao, Y., Source process of a large deep-focus earthquake and its tectonic implications: The western Brazil earthquake of 1963, Phys. Earth Planet. Inter., 5, 61-76, Fukao, Y., and M. Kikuchi, Source retrieval for mantle earthquakes by iterative deconvolution of long-period P waves, Tectonophysics, 144, , Furumoto, M., Spacio-temporal history of the deep Colombia earthquake of 1970, Phys. Earth Planet. Inter., 15, 1-12, Furumoto, M., and Y. Fukao, Seismic moment of great deep shocks, Phys. Earth Planet. Inter., 11, , zone, Geophys. Res. Let& 22, , Kirby, S. H., S. Stein, E. A. Okal, and D.C. Rubie, Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere, Rev. Geophys., 34, , Kostoglodov, V. V., Maximum depth of earthquakes and phase transformation within the lithospheric slab descending the mantle, in Physics and Interior Structure of the Earth, edited by V. A. Magnitsky, Nauka, Moscow, Lomnitz-Adler, J., Am deep focus earthquakes caused by a Martensitic transformation?, J. Phys. Earth, 38, 83-98, Lundgren, P. R., and D. Giardini, Isolated deep earthquakes and the fate of subduction in the mantle, J. Geophys. Res., 99, 15,833-15,842, Gilbert, F., and A.M. Dziewonski, An application of normal mode Lundgren, P., and D. Giardini, The June 9 Bolivia and March 9 Fiji deep theory to the retrieval of structural parameters and source earthquakes of 1994, 1, Source processes, Geophys. Res. Lett., 22, mechanisms from seismic spectra, Philos. Trans. R. Soc. London, Ser , A, 278, , McGuire, J. J., D. A. Wiens, P. J. Shore, and M. G. Beyis, The March 9, Goes, S., and J. Ritsema, A broadband P-wave analysis of the large deep 1994 (M w 7.6), deep Tonga earthquake: Rupture outside the Fiji Island and Bolivia earthquakes of 1994, Geophys. Res. Lett., 22, seismically active slab, J. Geophys. Res., 102, 15,163-15,182, , McKenzie, D. P., and J. N. Brune, Melting on fault planes during large Griggs, D. T., and D. W. Baker, The origin of deep-focus earthquakes, earthquakes, Geophys. J. R. Astron. Soc., 29, 65-78, in Properties of Matter Under Unusual Conditions, edited by H. Mark Mendiguren, J. A., Identification of free oscillation spectral peaks for and S. Fernbach, pp. 2342, Wiley-Interscience, New York, July 31, Colombian deep shock using the excitation criterion, Gudmundsson, O., and M. Sambridge, A regionalizod upper mantle Geophys. J. R. Astron. Soc., 33, , (RUM) seismic model, J. Geophys. Res., 103, , Mendiguren, J. A., and K. Aki, Source mechanism of the deep Hara, T., K. Kuge, and H. Kawakatsu, Determination of the isotropic Colombian earthquake of 1970 July 31 from the free oscillation data, component of the 1994 Bolivia deep earthquake, Geophys. Res. Let& Geophys. J. R. Astron. Soc., 55, , , , Mailer, R. D., W. R. Roest, J.-Y. Royer, L. M. Gahagan, and J. G. Hobbs, B. E., and A. Ord, Plastic instabilities: Implications for the origin Sclater, Digital isochrons of the world's ocean floor, J. Geophys. of intermediate and deep focus earthquakes, J. Geophys. Res., 93, Res., 102, , ,521-10,540, Myers, S.C., T. C. Wallace, S. L. Beck, P. G. Silver, G. Zandt, J. Houston, H., The non-double-couple component of deep earthquakes Vandecar, and E. Minaya, Implications of spatial and temporal and the width of the seismogenic zone, Geophys. Res. Lett., 20, development of the aftershock sequence for the M w 8.3 June 9, , deep Bolivian earthquake, Geophys. Res. Lett., 22, , Houston, H., Radiated seismic energy of large deep earthquakes Nfib lek, J. L., Determination of earthquake source parameters from (abstract), Eos Trans. AGU, 77(46), Fall Meet. Suppl., F491, inversion of body waves, Ph.D. dissertation, 361 pp., Mass. Inst. of Huang, W.-C., E. A. Okal, G. Ekstr6m, and M.P. Salganik, Centroid Technol., Cambridge, moment tensor solutions for deep earthquakes predating the digital Newman, A. V., and E. A. Okal, Teleseismic estimates of radiated era: The World-Wide Standardized Seismograph Network dataset seismic energy: The E/M o discriminant for tsunami earthquakes, J. ( ), Phys. Earth Planet. Inter., 99, , Geophys. Res., 103, 26,885-26,898, Ihml6, P. F., On the interpretation of subevents in teleseismic Ogawa, M., Shear instability in a viscoelastic material as the cause of waveforms: The 1994 Bolivia deep earthquake revisited, J. deep focus earthquakes, J. Geophys. Res., 92, 13,801-13,810, Geophys. Res., 103, 17,919-17,932, Okal, E. A., and C. R. Bina, The deep earthquakes of in Isacks, B., and P. Molnar, Distribution of stresses in the descending northern Peru, Phys. Earth Planet. Inter., 87, 33-54, lithosphere from a global survey of focal-mechanism solutions of Okal, E. A., and R. J. Geller, On the observability of isotropic seismic mantle earthquakes, Rev. Geophys., 9, , sources: The July 31, 1970 Colombian earthquake, Phys. Earth James, D. E., and J. A. Snoke, Seismic evidence for continuity of the Planet. Inter., 18, , deep slab beneath central and eastern Peru, J. Geophys. Res., 95, , Okal, E. A., and S. H. Kirby, Deep earthquakes beneath the l:iji Basin, Jost, M. L., and R. B. Herrmann, A student's guide to and review of SW Pacific: Earth's most intense deep seismicity in stagnant slabs, moment tensors, Seismol. Res. Let& 60, 37-57, Phys. Earth Planet. Inter., 109, 25453, Kanamori, H., Source mechanism of the Alaskan earthquake of 1964 Okal, E. A., and J. Talandier, T waves from the great 1994 Bolivian from amplitudes of free oscillations and surface waves: Comments, deep earthquake in relation to channeling of S wave energy up the Phys. Earth Planet. Inter., 7, , slab, J. Geophys. Res., 102, 27,421-27,437, Kanamori, H., D. L. Anderson, and T. H. Heaton, Frictional melting Pennington, W. D., Subduction of the eastern Panama basin and during the rupture of the 1994 Bolivian earthquake, Science, 279, seismotectonics of northwestern South America, J. Geophys. Res., , , 10,753-10,770, Kennett, B. L. N., and E. R. Engdahl, Traveltimes for global earthquake location and phase identification, Geophys. J. Int., 105, , Kennett, B. L. N., E. R. Engdahl, and R. Buland, Constraints on seismic velocities in the Earth from traveltimes, Geophys. J. Int., 122, , Kikuchi, M., and Y. Fukao, Seismic wave energy inferred from longperiod body wave inversion, Bull. Seismol. Soc. Am., 78, , Kikuchi, M., and H. Kanamori, The mechanism of the deep Bolivia earthquake of June 9, 1994, Geophys. Res. Lett., 21, , Kind, R., Extensions of the reflectivity method, J. Geophys., 45, , Kind, R., The reflectivity method for different source and receiver Riedel, M. R., and S. Karato, Grain-size evolution in subducted oceanic lithosphere associated with the olivine-spinel transformation and its effects on rheology, Earth Planet. Sci. Let& 148, 27-43, Russakoff, D., G. Ekstr6m, and J. Tromp, A new analysis of the great 1970 Colombia earthquake and its isotropic component, J. Geophys. Res., 102, 20,423-20,434, Sacks, I. S., and H. Okada, A comparison of the anelasticity structure beneath western South America and Japan, Phys. Earth Planet. Inter., 9, , Silver, P. G., S. L. Beck, T. C. Wallace, C. Meade, S.C. Myers, D. E. James, and R. Kuehnel, Rupture characteristics of the deep Bolivian earthquake of 9 June 1994 and the mechanism of deep-focus earthquakes, Science, 268, 69-73, Spray, J. G., Viscosity determinations of some frictionally generated silicate melts: Implications for fault zone rheology at high strain structures and comparison with GRF data, J. Geophys., 58, , rates, J. Geophys. Res., 98, , Tibi, R., C. H. Estabrook, and G. Book, The 1996 June 17 Flores Sea and

17 ESTABROOK: 1970 AND 1963 SO[rI'H AMERICAN DEEP EARTHQUAKES 28, March 9 Fiji-Tonga earthquakes: Source processes and deep on distant broadband seismograms, Geophys. Res. Lett., 22, 1-4, earthquake mechanisms, Geophys. J. Int., 138, , Tinker, M. A., S. L. Beck, W. Jiao, and T. C. Wallace, Mainshock and Wessel, P., and W. H. F. Smith, New version of the generic mapping aftershock analysis of the June 17, 1996, deep Flores Sea earthquake tools released, Eos Trans. AGU, 76, 329, sequence: Implications for the mechanism of deep earthquakes and Wiens, D. A., and H. J. Gilbert, Effect of slab temperature on deepthe tectonics of the Banda Sea, J. Geophys. Res., 103, ,002, earthquake aftershock productivity and magnitude-frequency relations, Nature, 384, , van der Hilst, R., and P. Mann, Tectonic implications of tomographic Willemann, R. J., and C. Frohlich, Spatial patterns of aftershocks of deep images of subducted lithosphere beneath northwestern South focus earthquakes, J. Geophys. Res., 92, 13,927-13,943, America, Geology, 22, , Vassiliou, M. S., and H. Kanamori, The energy release in earthquakes, C. H. Estabrook, GeoForschungsZentrum Potsdam, T elegrafenberg, Bull. Seisrnol. Soc. Am., 72, , Potsdam, Germany. (chuck@gfz-potsdam.de) Vidale, J. E., and H. Houston, The depth dependence of earthquake duration and implications for rupture mechanisms, Nature, 365, 45-47, (Received December 1, 1998; revised May 31, 1999; Vidale, J. E., S. Goes, and P. G. Richards, Near-field deformation seen accepted July 12, 1999.)