Studies of Wadati-Benioff Zone Seismicity of the Anchorage, Alaska, Region

Bulletin of the Seismological Society of America, Vol. 97, No. 1B, pp. 52 62, February 2007, doi: 10.1785/0120060128 Studies of Wadati-Benioff Zone Seismicity of the Anchorage, Alaska, Region by Annette M. Veilleux and Diane I. Doser Abstract We have relocated over 8600 earthquakes occurring between 1964 and 1999 at depths 25 km within 100 km of Anchorage, Alaska, using the doubledifference technique. The relocated seismicity reveals important details of the subducting Pacific plate and Yakutat blocks including clustering of seismicity within the Pacific plate mantle, northeast- and northwest-striking near-vertical faults within the Pacific plate crust, and a region of intense deformation associated with the southwestern edge of the Yakutat block. Stress analyses from the direct inversion of firstmotion data and from inversion of focal mechanisms for 713 events indicates a rotation of the maximum compressive stress direction from a trench parallel direction (northeast) in the southwestern portion of the study area to an east west direction near the southwestern edge of the Yakutat block located just north of Anchorage. Regions of the Pacific plate mantle that produced M w 6.4 earthquakes in 1934 and 1954 continue to be seismically active. Introduction The Anchorage, Alaska, region is located in a complex tectonic environment where convergence of the Pacific plate and Yakutat block with the North American plate is occurring (Fig. 1). Within this region, the Pacific plate is subducting at a rate of about 5.4 cm/yr beneath North America (DeMets and Dixon, 1999). Subduction is complicated by the presence of the Yakutat block, which is partially accreted to North America and resists subduction as it converges with North America (DeMets and Dixon 1999; Fletcher and Freymueller, 1999). In the Anchorage region the subducted plate shallows to a dip of 3 from the trench to at least the mid- Kenai Peninsula (Brocher et al., 1994) and bends around Prince William Sound (Plafker et al., 1994). Ratchkovski and Hansen (2002) suggest this bending has created a tear in the plate just northeast of Anchorage. Thus, in this region a thick, shallowly subducting, lower plate representing a combination of the Yakutat block and Pacific plate creates a large asperity that ruptured during the 1964 great Alaska earthquake (Johnson et al., 1996). Global Positioning System (GPS) data (Zweck et al., 2002) indicate the plate interface in the Prince William Sound and eastern Kenai Peninsula region has been locked since 1964 but that there is plate relaxation and creep occurring to the west beneath Cook Inlet and the western Kenai Peninsula. Tectonics and Structure The trench of the Alaska Aleutian megathrust is located offshore about 200 km from the study area. The shallow dip of the subducting plate persists to at least the mid-kenai Peninsula, where a rapid increase in dip occurs to create partial melting of the slab and subsequent volcanoes on the western side of the Cook Inlet forearc basin. What is known of the geophysical structure of the Yakutat block is fairly limited but includes work by Brocher et al. (1994), Saltus et al. (2001), and Ye et al. (1997). Bruhn et al. (2004) suggest that the western portion of the Yakutat block could be a thicker oceanic plateau. The slope magnetic anomaly defined earlier by Griscom and Sauer (1990) and later by Saltus et al. (2001) is interpreted as the southern edge of the subducted Yakutat block (Fig. 1). Saltus et al. (2001) model a 40-km depth to the slab in mid-cook Inlet based on magnetic and gravity data, whereas seismicrefraction studies (Brocher et al., 1994) constrain the depth of the plate interface at 20 km in westernmost Prince William Sound. At the southern end of the Kenai Peninsula a seismic-refraction reflection study by Ye et al. (1997) shows a more steeply dipping ( 8 ) plate, but there possibly appears to be an underplated seamount subducting at this location. Most recently, a study by Eberhart-Phillips et al. (unpublished work, 2006) interpreted the 3D seismic-velocity structure of the transition from Aleutian subduction to Yakutat collision using local earthquakes and active source data. In their study the Yakutat slab is characterized by a thick lowvelocity zone and high V p /V s crust. The edge of the interpreted subducting Yakutat slab is shown in Figure 1. Note that their interpretation does not require a tear in the subducted slab as suggested by Ratchkovski and Hansen (2002). 52

Studies of Wadati-Benioff Zone Seismicity of the Anchorage, Alaska, Region 53 Figure 1. Regional map showing location of Anchorage study area (box). White arrow shows direction of Pacific plate convergence relative to North America (DeMets and Dixon, 1999) and gray arrow shows direction of convergence of Yakutat block relative to North America (Fletcher and Freymueller, 1999). Star is the epicenter of the 1964 great Alaska earthquake. The thin dashed line is the rupture area of 1964 event. CI is Cook Inlet, KP is Kenai Peninsula, and SMA is slope magnetic anomaly (shown by bold dashed line). Thin solid lines denote the boundaries of the Kodiak, Kenai, and McKinley blocks (Ratchkovski and Hansen, 2002). Triangles are volcanoes of the Alaska Range. Dark-gray region is the subducted portion of the Yakutat block from Eberhart-Phillips et al. (unpublished work, 2006). Seismicity The 1964 Great Alaskan earthquake rupture produced a maximum slip of 20 m (Johnson et al., 1996) in a region that appears to be associated with the location of the subducting Yakutat block. The first comprehensive study of the deeper ( 40 km) seismicity of the south-central Alaskan subduction zone (Kodiak Island to eastern Prince William Sound) was conducted by Ratchkovsky et al. (1997a) using a joint hypocenter determination relocation technique with 6 years of data. Their results showed the presence of a double seismic zone beneath Kodiak Island and the southern Kenai Peninsula with a separation of 10 15 km between zones and convergence of the zones at 80 100 km depth. Inversion of first-motion data for the relocated earthquakes by Ratchkovsky et al. (1997b) found that r 3 is oriented downdip in the upper seismic zone and r 1 is perpendicular to the dip of the Wadati Benioff zone. In the lower zone r 1 and r 3 are rotated 40 60 from the downdip direction due to bending of the slab. A subsequent study by Ratchkovsky et al. (1998) focused on relocations and stress-field orientations from shallow earthquakes ( 50 km) occurring between 155 145 W and 59 63 N over the same period as the 1997 studies. This study determined that the majority of the focal mechanisms for the shallower events had west-northwest east-southeast orientated T axes, indicating that the subducting slab was undergoing downdip extension at shallow depths. Ratchkovski and Hansen (2002) used the joint hypocenter determination (JHD) method with 10 years of phase data to relocate over 14,000 events in south-central Alaska. This study revealed the finer details of the regional shape of the Wadati Benioff zone and evidence for plate segmentation within the McKinley block (see Fig. 1). They used 50- to 100-km-wide cross-sectional views to examine seismicity of the shallowly subducting slab and to analyze the tear between the Kenai and McKinley block segments (Fig. 1). They also used Harvard centroid moment tensor (CMT) focal mechanisms to demonstrate that down-dip extension is occurring within the slab except near the tear where rapid stress changes occur. A further study by Ratchkovski et al. (2002) relocated earthquakes from the Alaska Earthquake Information Center catalog occurring between 1971 and 2001 using the double-difference technique of Waldhauser and Ellsworth (2000), including events from the Anchorage, Alaska, region. Flores and Doser (2005) relocated shallow ( 40 km deep) seismicity for the Anchorage region for the 35 years following the 1964 Great Alaska earthquake by using the double-difference technique. Inversion of first-motion data for events at depths of 20 to 40 km (occurring within the lower crust of North America and/or the upper portion of the subducting plate(s)) for stress-field orientation showed r 3 near the inferred tear was parallel to the Wadati Benioff zone and then rotated to a downdip direction to the southwest. r 1 was oriented 60 90 from the direction of plate motion. Data Analysis Relocations In this study we use the double-difference (HypoDD program) relocation technique of Waldhauser and Ellsworth (2000) with the velocity model of Fogleman et al. (1993). We separated the data into three periods based on network operation and analysis procedures. Data from 1964 to 1971 were obtained from the International Seismological Centre (www.isc.ac.uk); data from October 1971 to July 1988 were from the U.S. Geological Survey (K. Fogelman, written comm., 2001); and data from August 1988 to December 1999 were from the Alaska Earthquake Information Center (AEIC) (N. Ruppert [Ratchkovski], written comm., 2001). We relocated a total of 8667 events (Fig. 2) with a maximum separation distance of events within a cluster set at 10 km. A minimum of eight observations were required for each event pair. After relocation we used the software package ZMAP (Wiemer, 2001) to examine seismicity in cross section and map view. Data for the three periods were relocated separately. For

54 A. M. Veilleux and D. I. Doser variation we observed was an increase in seismicity to the east and southeast of Anchorage between 1983 and 1984 that may be related to the aftershock sequence of the 1983 Columbia Bay earthquakes within northern Prince William Sound. Note that the HypoDD algorithm is a relative location technique, although in regions where the depth to the plate interface has been independently determined by other geophysical techniques our focal depths are comparable to these estimates. Figure 2. Relocated seismicity of the Anchorage study area. The filled square is Anchorage. Solid lines indicate positions of cross sections shown in Figure 3. Thin dashed line represents the inferred tear in the subducting plate (Ratchkovski and Hansen, 2002) and bold dashed line is the edge of the Yakutat block from Eberhart-Phillips et al. (unpublished work, 2006). KA is Knik Arm, TA is Turnagain Arm, and TL is Tustumena Lake. 1964 1971 we relocated 180 of an initial 218 events with magnitudes of 3.1 to 5.5. The relocations moved an average of 0.017 latitude, 0.039 longitude, and 5.3 km (upward) in depth relative to the original locations. For 1971 1988 we relocated 4443 of 5198 original events with magnitudes of 0.4 to 4.6. These relocations shifted an average of 0.012 latitude, 0.019 longitude, and 2.8 km (upward) in depth relative to the original locations. For events between 1988 and 1999 we were able to relocate 4044 of 4575 events of magnitude 1.5 to 5.6, with the relocations moving an average of 0.009 latitude, 0.018 longitude, and 0.83 km (upward) in depth relative to the original locations. Events from these periods will have relative shifts with respect to each other. Tests with data subsets, however, suggest that relative shifts are comparable to the location uncertainties. We also examined spatial variations in the original and relocated hypocenters to determine whether any bias was introduced in the relocation process. We saw no evidence that the relocation process dropped earthquakes from any particular portion of our study area; although the relocation process did sharpen clusters and lineations in the seismicity when compared with the more diffuse seismicity observed in the original locations. The only notable spatial/temporal Focal Mechanisms/Stress Analyses We determined 713 focal mechanisms from magnitude 2.0 to 5.5 events occurring between 1989 and 1999 using HASH (Hardebeck and Shearer, 2002) and first motions from the AEIC database. There were 2.4% quality A, 17.7% quality B, 21% quality C, and 58.9% quality D events (see Hardebeck and Shearer, 2002, for a description of quality factor). The focal mechanisms were then inverted using a ZMAP algorithm (based on Michael, 1984, 1987) to determine stress-field orientation. For comparison, we also directly inverted first-motion polarity information for the stress-field orientation using GetStress (Robinson, 1999). GetStress bypasses the need to determine focal mechanisms and confidence limits are determined by repeatedly inverting random subsets of the firstmotion data. First motions were taken from the AEIC database and only events with more than eight first-motion observations were used in analysis. Data were selected in rectangular volumes that were bounded by gaps in seismicity. The first-motion data were randomly resampled 1000 times to determine the range of stress orientations consistent with the first-motion data and their 95% confidence intervals. Results Relocations Figure 2 shows the relocated earthquakes in map view with depths ranging from 25 and 124 km. The most striking features seen in Figure 2 are a large cluster of seismicity located northwest of Knik Arm and apparent northeast southwest and north-northwest south-southeast trending lineations in seismicity observed to the southwest of the inferred edge of the Yakutat block. To further investigate the finer details of the shape of the subducted slab, cross-sectional views of seismicity are shown in Figure 3, with cross-section locations shown in Figure 2. Cross-section NN shows a 60-km-wide projection of seismicity. Other cross sections show 30-km-wide projections of seismicity. Figure 3a (cross-section NN ) is oriented parallel to the strike of the Wadati Benioff zone, whereas Figures 3b e are oriented parallel to the dip of the subduction zone. For clarity the cross sections start at 20- km depth to exclude any seismicity related to upper crustal deformation. Arrows on the cross sections indicate the in-

Studies of Wadati-Benioff Zone Seismicity of the Anchorage, Alaska, Region 55 Figure 3. Cross sections along strike (a) and dip (b e) of subducting slab. Locations are shown in Figure 2. For clarity only earthquakes with depths 25 km are shown. Open stars are events with M 5.5; filled stars are historic events of M w 6.4 from Doser and Brown (2001). (continued on next page) ferred location of the edge of the Yakutat block (from Eberhart-Phillips et al., unpublished work, 2006), the location of the inferred tear between the McKinley and Kenai blocks (from Ratchkovski and Hansen, 2002), and the edge of the Prince William Sound asperity (from Johnson et al., 1996). Open stars indicate the locations of magnitude 5.0 earthquakes occurring since 1964 and solid stars are magnitude 6.4 events occurring prior to 1964 taken from Doser and Brown (2001). Figure 3a shows along strike breaks in seismicity both at the inferred edge of the Yakutat block and the tear. A possible double seismic zone is observed between 0 and 30 km along the profile. Cross-section CC shown in Figure 3b is taken across the region where both the Pacific plate and Yakutat block should be subducting. The Yakutat block has a total thickness of 10 11 km in Prince William Sound (Brocher et al., 1994), whereas the Pacific plate crust is estimated to be 6 to 8 km thick in Prince William Sound (Brocher et al., 1994) and the southern Kenai Peninsula region (Ye et al., 1997). A portion of the subducting slab appears to have concave curvature, with an increase in deeper earthquakes ( 50 km) observed near the point of maximum curvature. Figure 3c is a cross section along FF that was constructed to pass through Anchorage and cut perpendicular to northeast southwest lineations in seismicity observed just south of the inferred edge of the Yakutat Block (Fig. 2). Prominent lineations in seismicity are observed within the subducting plate, as indicated by arrows in Figure 3c. These patterns are analyzed in greater detail in a subsequent section of this article. Figure 3d,e are cross-sections HH and LL that cut across the portion of the study area where only the Pacific plate may be subducting. Here there appears to be less con-

56 A. M. Veilleux and D. I. Doser

Studies of Wadati-Benioff Zone Seismicity of the Anchorage, Alaska, Region 57 centration of seismicity at depths of 40 to 60 km. The seismogenic portion of the slab also appears thinner, more steeply dipping, and less warped than toward the northeast. Figure 3d shows the position of an M w 6.8 (Doser and Brown, 2001) earthquake that occurred in 1949. This event occurred beneath a locked portion of the plate interface where the slab is currently quiescent. Thus we do not know if this region is only active immediately prior to megathrust earthquakes or will become more active as the stress shadow of the 1964 mainshock (e.g., Doser et al., 2006) fades. The location of an M w 6.6 earthquake (Doser and Brown, 2001) in 1954 that caused intensity VIII damage in the Anchorage region (Stover and Coffman, 1993) is also shown on the cross section. The 1954 event appears to be located within the mantle of the subducting Pacific plate and has a strikeslip focal mechanism similar to several mechanisms of recent nearby events (Fig. 4). At least three other earthquakes of M 5.5 have occurred in the hypocentral region of the 1954 event since the 1930s, suggesting that this portion of the Pacific plate could serve as an important seismic source zone for the Anchorage region. Figure 3e shows an unusual cluster of seismicity within the Pacific plate mantle located directly beneath the northwestern end of Lake Tustumena (Fig. 2). This seismicity does not appear to reflect a double seismic zone, as it does not continue downdip along the slab when observed in either 2D or 3D perspectives. Focal mechanisms for events in this cluster (Fig. 4) suggest reverse faulting along the east west striking faults. An M w 6.8 event in 1934 (Doser and Brown, 2001) appears to be associated with this cluster of activity, although the 1934 event had a mechanism indicating oblique-normal faulting. Intensities for the 1934 event were higher to the northeast and east (Stover and Coffman, 1993), suggesting either a northeastward and upward-directed rupture, or focusing effects within the slab. Focal Mechanisms Focal mechanisms determined using the HASH technique are shown in Figure 4. The squares denote regions used to invert for the orientation of the stress field directly from first-motion data. A wide variety of mechanisms are observed, but the majority (50%) shows reverse to reverseoblique faulting (rakes of 90 to 150 ). The sizes of the mechanisms are proportional to magnitude. In the upper portion of the slab (30 to 40 km) reverse to oblique-reverse faulting is common, whereas normal, normal-oblique, and strike-slip faulting are more common in the deeper (70 to 90 km) portion of the slab. Many (48%) mechanisms exhibit high-angle nodal planes (dip 60 ) and 38% have nodal planes that strike 140 to 180. Note that the orientation of reverse faulting varies from east west-striking nodal planes in the southwestern portion of the study area to northwestand north-northeast-striking nodal planes in the central portion of the study area. Figure 4. Focal mechanisms determined using the HASH technique. Boxes indicate regions used in the GetStress stress inversion. The sizes of the mechanisms are proportional to magnitude, as indicated. SL is Skilak Lake and TL is Tustumena Lake. The filled square is Anchorage. Seismicity of the Upper Cook Inlet Region We have constructed a map view of upper Cook Inlet centered approximately on Anchorage, Alaska, that more clearly illustrates the lineations in the seismicity (Fig. 5). The stars are events within this region that are associated with the indicated focal mechanisms. These mechanisms indicate reverse and reverse-oblique faulting along either northwest- or north-northeast-striking fault planes. Du et al. (2004) found localized linear zones of faulting in the subducting slab of the North Island, New Zealand, when analyzing double-difference relocation data, although normal faulting was occurring, not high-angle reverse faulting as indicated in Figure 5. Hansen and Ratchkovski (2001) also found high-angle normal faulting occurring within the subducting Pacific plate in the Kodiak Island region. Figure 6a shows a cross-sectional view of the seismicity from northwest to southeast (A B), whereas Figure 6b shows a northeast southwest striking cross section (C D). Note that the cross sections are primarily located southwest of the inferred edge of the Yakutat block and thus reflect seismicity within the Pacific plate. Figure 6a suggests fault-

58 A. M. Veilleux and D. I. Doser ing along high-angle planes dipping slightly to the northwest, except at the eastern end of the profile where there may be dip to the southeast. The faulting appears to extend through the entire crust of the Pacific plate ( 10 km). Figure 6b also suggests high-angle, near-vertical faulting. As the inferred edge of the Yakutat block is approached (toward point D) the seismicity becomes more concentrated and no apparent linear structures are observed. Stress Orientations We inverted first-motion data for stress orientations in the five regions shown in Figure 4 using the GetStress technique (Table 1). These regions were selected based on clustering of events. Our initial inversions gave results that were too scattered in regions 2 and 9 to produce reliable estimates of r 1 and r 3 directions. We then partitioned region 2 by depth to determine whether there were notable depth variations in the stress field (40 50, 50 60, 60 70 km). The results (Fig. 7) remained scattered for 40 to 50 and 50 to 60 km depth, but results at 60 to 70 km indicate near-horizontal r 3 and near-vertical r 1. The scattered results of region 9 (Fig. 7) could reflect mixing of first-motion data for the upper and lower portions of the slab (see Fig. 3e) because focal mechanisms in this region (Fig. 4) indicate the deeper ( 60 km) events are occurring on west-southwest east-northeast reverse faults, whereas shallower events appear to be occurring on northwest- and northeast-striking reverse-oblique faults; Figure 5. Map view of Anchorage region. The filled stars indicate events with focal mechanisms. Dashed line is the inferred edge of Yakutat block. The filled square is Anchorage. Bold lines are positions of cross sections shown in Figure 6. Figure 6. Cross sections along lines AB (a) and CD (b) shown in Figure 5. Events located within 15 km of profiles are shown. Arrows point to lineations in seismicity. The filled square shows location of Anchorage. however, not enough first-motion observations were available to further subdivide the region by depth. The results for regions 1, 2 (60 70 km), 3, and 4 (solid lines) are compared with previous studies (dashed lines) in Figure 8. The black lines indicate the orientation of r 1 and the gray lines the orientation of r 3. The lengths of the lines are proportional to plunge as indicated. Results of Flores and Doser (2005) were obtained from the inversion of first-motion data from earthquakes with focal depths of 30 to 40 km. The results of Lu et al. (1997) were obtained from inversion of focal mechanisms of earthquakes with magnitude 3.0 and depths 40 km in subregions selected to parallel the contours of the downgoing slab. The orientation of r 1 in regions 3 and 4 is consistent with that observed in previous studies. Region 1 differs from the orientations of Flores and Doser (2005), but may reflect variation of stress field with depth, as our results are for events deeper than 40 km. These results suggest east west to northeast southwest-directed r 1. r 3 is generally oriented in the downdip direction, in agreement with previous studies (e.g., Ratchkovsky et al., 1997b, 1998; Ratchkovski and Hansen, 2002). Next the focal mechanisms shown in Figure 4 were inverted with the ZMAP stress-inversion algorithm (based on Michael, 1984, 1987) to determine stress-field orientation

Studies of Wadati-Benioff Zone Seismicity of the Anchorage, Alaska, Region 59 Table 1 Results from Inversion of First-Motion Data Region No. of First Motions % Correct r 1 azm, plng ( )* r 3 azm, plng ( )* Region 1 11,745 83 220,40 100,31 61.4 61.8 N 149.5 150.5 W Region 2 384 81 320,40 110,46 61.55 61.9 N 150.5 151.75 W (40 50 km) Region 2 545 87 40,50 150,16 61.55 61.9 N 150.5 151.75 W (50 60 km) Region 2 447 92 270,80 110,9 61.55 61.9 N 150.5 151.75 W (60 70 km) Region 3 591 88 250,50 120,28 61.0 61.4 N 148.75 150.25 W Region 4 8,732 80 250,60 60,30 61.2 61.55 N 150.25 151.75 W Region 9 620 87 280,60 80,9 60.1 60.4 N 150.5 151.5 W *azm, azimuth; plng, plunge. (Fig. 9). A 0.1 by 0.1 grid was used for the inversion and a minimum of five mechanisms was required to determine a stress-field orientation. The stress-regime characterization is according to Zoback (1992). The solid lines indicate reverse faulting predominates and that r 1 rotates from the northeast southwest (trench parallel) to east west as the edge of the subducting Yakutat block is approached. Maximum compressive stress directions mimic the bend in the subducting slab and indicate compression normal to the southwestern edge of the Yakutat block. The bold single-headed arrows in Figure 9 indicate r 1 directions determined from the inversion of first-motion data (Figs. 7 and 8) and are comparable to results obtained from the ZMAP algorithm. Discussion High-angle faulting along conjugate faults has been observed in several other subduction zones (e.g., Jiao et al., 2000; Du et al., 2004; Rietbrock and Waldhauser, 2004; Ranero et al., 2005). Ranero et al. (2005) suggest that many intraslab events represent reactivation of pre-existing faults, independent of the state of stress in the slab. The faults may not always move with the same sense of motion as the conditions under which they originally formed (Ranero et al., 2005). Inversion of first-motion data for regions 1, 3, and 4 (Table 1) suggests that the stress states of these regions are optimum for either high-angle ( 70 ) reverse and reverseoblique faulting along faults striking north south to northwest or moderate-angle ( 30 to 50 ) normal and normaloblique faulting along faults striking southwest. Focal mechanisms (Fig. 4), however, indicate that reverse and reverse-oblique faulting predominates in these regions, perhaps because of pre-existing weaknesses in the Pacific plate. In deeper portions of the slab, such as region 2 (at 60 70 km depth), the stress state becomes optimum for higher-angle (50 to 70 ) normal faulting along north south-striking faults, in agreement with some focal mechanisms (Fig. 4). This suggests that the same north south- to northwesttrending pre-existing features in the Pacific plate could move as normal or reverse faults, depending on the stress state in the slab. Conclusions Relocation of over 8600 earthquakes occurring during the past 40 years shows nearly flat subduction of the Yakutat/Pacific lower plate in the northern part of the study area. We observe intensely concentrated seismicity, warping of the subducted slab, and the occurrence of deeper ( 50 km) events in regions of maximum slab curvature where both the Pacific plate and Yakutat block are being subducted immediately downdip of the currently locked portion of the plate interface. Southwest of the edge of the Yakutat block near Anchorage, seismicity patterns and focal mechanisms suggest northeast- and northwest-striking, near-vertical reverse

60 A. M. Veilleux and D. I. Doser Figure 7. Stress-inversion results from GetStress for 1000 iterations using random subsets of first-motion data for regions shown in Figure 4. Filled symbols indicate maximum compressive stress; open symbols indicate minimum compressive stress. faults are active within the crust of the Pacific plate. In other portions of the slab, however, the stress field is optimal for high-angle normal faulting with a similar orientation. In the southern portion of the study area we observe an unusual cluster of deeper (60 70 km seismicity) in the Pacific plate mantle. This region was the site of an M w 6.8 earthquake in 1934. In this region a high-velocity zone has been imaged within the lower crust of North America (Eberhart-Phillips et al., unpublished work, 2006), suggesting that density and rheological differences within the upper plate may serve to concentrate seismic deformation in the lower Pacific plate. An M w 6.6 in 1954 also appears to have occurred within the Pacific plate mantle. The fact that larger (M 6) earthquakes appear to occur within the mantle of the subducting plate is consistent with observations of intraslab seismicity in other subduction zones such as the central Andes (Rietbrock and Waldhauser, 2004) or Cascadia (Ichinose et al., 2006). Over 700 focal mechanisms indicate compression lead-

Studies of Wadati-Benioff Zone Seismicity of the Anchorage, Alaska, Region 61 Figure 8. Stress-inversion results for regions 1 4, compared with previous studies. Black lines are maximum compressive stress; gray lines are minimum compressive stress. Lengths of lines are proportional to plunge as indicated. Thin dashed lines are from Flores and Doser (2005) and bold dashed lines are from Lu et al. (1997). ing to high-angle reverse and reverse-oblique faulting in the shallow portion of the subducted slab and more (10 15%) normal and strike-slip faulting in the deeper ( 70 km) slab. Stress-field analysis from the inversion of focal mechanisms and directly from the inversion of first-motion data indicate that maximum compressive stress rotates from northeast to east west across the region, while minimum compressive stress is generally oriented downdip. This mimics the bending of the slab across the region and indicates compression perpendicular to the edge of the Yakutat block. Acknowledgments We thank N. Ruppert (Ratchkovski) for sharing results of her ongoing studies of the regional seismicity with us, for her helpful comments, and her thorough review of this manuscript. The comments of an anonymous reviewer also improved this manuscript. Discussions with P. Haeussler are also appreciated. D. Eberhart-Phillips generously provided us with a preprint of her paper on the velocity structure of south-central Alaska. This research was supported by Grants 03HQGR099 and 05HQGR0101 from the U.S. Geological Survey s National Earthquake Hazards Reduction Program. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government. Figure 9. Stress-inversion results showing orientation of maximum compressive stress for 0.1 by 0.1 regions of the study area. Nodes with undefined stress are not shown. Plain bold black lines represent reverse faulting (R), bold black lines with arrows are obliquereverse faulting (OR), thin black lines are strike-slip faulting (SS), bold gray lines with double arrows are oblique-normal faulting (ON), and plain bold gray lines are normal faulting (N) using the convention of Zoback (1992). Single thick arrows indicate maximum compressive stress orientations obtained from the inversion of first-motion data for regions 1 4 (see Fig. 8). Dashed line is the inferred edge of the Yakutat block. References Brocher, T. M., G. S. Fuis, M. A. Fisher, G. Plafker, M. J. Moses, J. J. Taber, and N. I. Christensen (1994). Mapping the megathrust beneath the northern Gulf of Alaska using wide-angle seismic data, J. Geophys. Res. 99, 11,663 11,685. Bruhn, R. L., T. L. Pavlis, G. Plafker, and L. Serpa (2004). Deformation during terrane accretion in the Saint Elias orogen, Alaska, GSA Bull. 116, 771 787. DeMets, C., and T. H. Dixon (1999). New kinematic models for Pacific- North America motions from 3 Ma to present, 1: Evidence for steady motion and biases in the NUVEL-1A model, Geophys. Res. Lett. 26, 1921 1924. Doser, D. I., and W. A. Brown (2001). A study of historic earthquakes of the Prince William Sound, Alaska, region, Bull. Seism. Soc. Am. 91, 842 857. Doser, D. I., A. M. Veilleux, C. Flores, and W. A. Brown (2006). Changes in seismic moment rates along the rupture zone of the 1964 Great Alaska earthquake, Bull. Seism. Soc. Am. 96, 1545 1550. Du, W., C. H. Thurber, M. Reyners, D. Eberhart-Phillips, and H. Zhang (2004). New constraints on seismicity in the Wellington region of New Zealand from relocated earthquake hypocenters, Geophys. J. Int. 158, 1088 1102. Fletcher, H. J., and J. T. Freymueller (1999). New GPS constraints on the motion of the Yakutat block, Geophys. Res. Lett. 26, 3029 3032.

62 A. M. Veilleux and D. I. Doser Flores, C. F., and D. I. Doser (2005). Shallow seismicity of the Anchorage, Alaska region (1964 1999), Bull. Seism. Soc. Am. 95, 1865 1879. Fogelman, K. A, J. C. Lahr, C. D. Stephens, and R. A. Page (1993). Earthquake locations determined by the southern Alaska network for Oct. 1971 May 1989, U.S. Geol. Surv. Open-File Rept. 93-309. Global Centroid Moment Tensor (CMT) Project catalog search, www. globalcmt.org/cmtsearch.html (last accessed June 2006). Griscom, A., and P. E. Sauer (1990). Interpretation of magnetic maps of the northern Gulf of Alaska, with emphasis on the source of the Slope Anomaly, U.S. Geol. Surv. Open-File Rept. 90-0348. Hansen, R. A., and N. A. Ratchkovski (2001). Sequence of strong intraplate earthquakes in the Kodiak Island region, Alaska in 1999 2001, Geophys. Res. Lett. 28, 3729 3732. Hardebeck, J. L., and P. M. Shearer (2002). A new method for determining first-motion focal mechanisms, Bull. Seism. Soc. Am. 92, 2264 2276. Ichinose, G. A., H. K. Thio, and P. G. Somerville (2006). Moment tensor and rupture model for the 1949 Olympia, Washington, earthquake and scaling relations for Cascadia and global intraslab earthquakes, Bull. Seism. Soc. Am. 96, 1029 1037. Jiao, W., P. G. Silver, Y. Fei, and C. T. Prewitt (2000). Do intermediateand deep-focus earthquakes occur on preexisting weak zones? An examination of the Tonga subduction zone, J. Geophys. Res. 105, 28, 125 28, 138. Johnson, J. M., K. Satake, S. R. Holdahl, and J. Sauber (1996). The 1964 Prince William Sound earthquake: joint inversion of tsunami and geodetic data, J. Geophys. Res. 101, 523 532. Lu, Z., M. Wyss, and H. Pulpan (1997). Details of stress directions in the Alaska subduction zone from fault plane solutions, J. Geophys. Res. 102, 5385 5402. Michael, A. J. (1984). Determination of stress from slip data: faults and folds, J. Geophys. Res. 89, 11,517 11,526. Michael, A. J. (1987). The use of focal mechanisms to determine stress: a control study, J. Geophys. Res. 92, 357 368. Plafker, G., J. C. Moore, and G. R. Winkler (1994). Geology of southern Alaska margin, in The Geology of North America, Vol. G-1. The Geology of Alaska, G. Plafker and H. C. Berg (Editors), Geological Society of America, Boulder, Colorado, 389 449. Ranero, C. R., A. Villaseñor, J. P. Morgan, and W. Weinrebe (2005). Relationship between bend-faulting at trenches and intermediate-depth seismicity, Geochem. Geophys. Geosys. 6, Q12002, doi 10.1029/2005 GC000997. Ratchkovski, N. A., and R. A. Hansen (2002). New evidence for segmentation of the Alaska subduction zone, Bull. Seism. Soc. Am. 92, 1754 1765. Ratchkovski, N. A., R. A. Hansen, and J. T. Freymueller (2002). What can we learn from the double-difference relocation of the Alaska earthquake catalog? (abstract). EOS Trans. AGU 83, S22B 1014. Ratchkovsky, N. A., J. Pujol, and N. N. Biswas (1997a). Relocation of earthquakes in the Cook Inlet area, South Central Alaska, using the Joint Hypocenter Determination method, Bull. Seism. Soc. Am. 87, 620 636. Ratchkovsky, N. A., J. Pujol, and N. N. Biswas (1997b). Stress pattern in the double seismic zone beneath Cook Inlet, south-central Alaska, Tectonophysics 281, 163 171. Ratchkovsky, N. A., J. Pujol, and N. N. Biswas (1998). Relocation of shallow earthquakes in southern Alaska using Joint Hypocenter Determination method, J. Seism. 2, 87 102. Rietbrock, A., and F. Waldhauser (2004). A narrowly space double-seismic zone in the subducting Nazca plate, Geophys Res. Lett. 31, L10608, doi 10.1029/2004GL019610. Robinson, R. (1999). Get Stress: A computer programme for inverting first motion observations to a regional stress tensor, Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand, 7 pp. Available at: ftp.gns.cri.nz/pub/robinson/getstress (last accessed January 2006). Saltus, R. W., P. J. Haeussler, R. E. Bracken, J. P. Doucette, and R. C. Jachens (2001). Anchorage Urban Region Aeromagnetics (AURA) Project: preliminary geophysical results, U.S. Geol. Surv. Open-File Rept. 01-0085, 21pp. Stover, C. W., and J. L. Coffman (1993). Seismicity of the United States, 1568 1989 (revised), U.S. Geol. Surv. Profess. Pap. 1527, 418 pp. Waldhauser, F., and W. L. Ellsworth (2000). A double-difference earthquake location algorithm: method and application to the northern Hayward fault, Bull. Seism. Soc. Am. 90, 1353 1368. Wiemer, S. (2001). A software package to analyze seismicity: ZMAP, Seism. Res. Lett. 72, 374 383. Ye, S., E. R. Flueh, D. Klaeschen, and R. von Huene (1997). Crustal structure along the EDGE transect beneath the Kodiak shelf off Alaska derived from OBH seismic refraction data, Geophys. J. Int. 130, 283 302. Zoback, M. L. (1992). First and second-order patterns of stress in the lithosphere: the World Stress Map Project, J. Geophys. Res. 97, 11,703 11,728. Zweck, C., J. T. Freymueller, and S. C. Cohen (2002). Three-dimensional elastic dislocation modeling of the postseismic response to the 1964 Alaska earthquake, J. Geophys. Res. 107, no. B4, doi 1029/2001 JB000409. Department of Geological Sciences University of Texas at El Paso El Paso, Texas 79968-0555 (D.I.D, A.M.V.) GX Technology Corporation 2101 CityWest Blvd. Building III, Suite 900 Houston, Texas 77042 (A.M.V.) Manuscript received 12 June 2006.