Seismicity of the Kodiak Island Region ( ) and Its Relation to the 1964 Great Alaska Earthquake

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1 Bulletin of the Seismological Society of America, Vol. 92, No. 8, pp , December 2002 Seismicity of the Kodiak Island Region ( ) and Its Relation to the 1964 Great Alaska Earthquake by Diane I. Doser, Wesley A. Brown, and Monique Velasquez* Abstract Earthquake relocation, first-motion, and waveform modeling studies of M 5.5 earthquakes are used to examine the seismicity associated with the Kodiak Island region since the 1964 great Alaska earthquake. Our results indicate that the most intense seismicity and moment release in this region occurred within the Pacific plate and along the plate interface of the southwestern edge of the Kodiak segment, where slip during the 1964 mainshock was less than 5 m. Few earthquakes have occurred within the North American plate. The intense seismicity correlates well with the northern edge of a zone of high plate interface coupling detected by Global Positioning System/geodesy studies. Our studies suggest that during , faulting within the Pacific plate was characterized by normal and normal-oblique faulting. However, since 1974 reverse-oblique faulting has become more common in the Pacific plate. Seismic moment release has been over 6 times greater in the Kodiak region than in the Prince William Sound Cook Inlet region during the past 37 years. Unlike the Prince William Sound region, there is also a lack of down-dip migration of seismicity since Introduction South-central Alaska is the site of complicated subduction of the Pacific plate beneath North America along the Aleutian trench. In 1964 a M w 9.2 earthquake produced slip on two separate asperities along an 800-km length of this subduction interface (Christensen and Beck, 1994). The northern, the Prince William Sound (PWS) asperity, had 18-m average slip, and the southern, Kodiak asperity had 10-m average slip (Johnson et al., 1996) (Fig. 1). The difference in slip along the two asperities reflects important differences in the style of subduction along the south-central Alaskan margin. Within PWS, the Yakutat block, an allochthonous terrane (Pflaker, 1987), is loosely coupled to the Pacific plate. This gives rise to a thicker, buoyant plate, leading to a very low angle of subduction (3 to 4 ) (Brocher et al., 1994), a strongly locked zone, and long repeat times (several centuries) (Nishenko and Jacob, 1990). Along the Kodiak asperity, the dip of subduction increases to 8 (Johnson et al., 1996), and repeat times for failure appear to be much shorter ( 60 years for magnitude events) (Nishenko and Jacob, 1990). Recent studies of crustal structure, regional seismicity, and the rupture process of the 1964 mainshock provide a tectonic framework that has led us to reexamine the larger earthquakes occurring in south-central Alaska since 1928 in *Present address: Conoco Inc., 400 E. Kaliste Saloom Rd., Lafayette, Louisiana order to relate the seismicity to the 1964 mainshock rupture process. Our first studies (Doser et al., 1999; Doser and Brown, 2001) focused on seismicity related to the PWS rupture zone. This article focuses on the post-1964 seismicity of the Kodiak rupture zone and surrounding regions. Our study area extends from the southern end of the PWS rupture zone south to the northern end of the 1938 Semidi (M w 8.2) rupture zone (Johnson and Satake, 1994) and from the Aleutian trench westward to the Alaska Peninsula (Fig. 2). Regional Tectonic Setting Pacific plate motion in the Kodiak region is north-northwestward at about 6 cm/yr (DeMets et al., 1990), a direction that is left oblique to the plate margin (Fig. 1). Von Huene et al. (1999) have suggested that rupture during the 1964 earthquake was primarily controlled by structures within the subducting plate and by the amount of subducted sediment along the plate margin. The greatest structural control on the 1964 rupture zone is the presence of the Yakutat block, leading to the lower dip and stronger coupling of the plate interface within the PWS region. The southern edge of the Yakutat block is delineated by the slope magnetic anomaly (Fig. 2b). Other controlling features within the subducted plate beneath the Kodiak region include the 58 N fracture zone, the Kodiak seamount chain, and the Aja fracture zone (Fig. 2b) (von Huene et al., 1999). 3269

2 3270 D. I. Doser, W. A. Brown, and M. Velasquez Figure 1. Earthquakes of the Kodiak and PWS regions from this study (diamonds) and Doser et al. (1999) (circles). Boxes indicate slip model obtained for the 1964 great Alaska earthquake by Johnson et al. (1996) and for the 1938 Semidi earthquake by Johnson and Satake (1994). Slip in meters for each patch of the plate interface is given in italic numbers. Bold arrow shows Pacific plate motion direction from DeMets et al. (1990). Box labeled ANC is Anchorage. Upper-plate structure within both the Kodiak and PWS regions is dominated by a Mesozoic and Tertiary accretionary complex. This complex has been described as being composed of two principal terranes, the Chugach terrane, consisting of Mesozoic through latest Cretaceous turbidites and melange (e.g., Plafker et al., 1994b), and the Prince William terrane, consisting of a Paleogene turbidite-fan complex with minor interleaved volcanics (Plafker, 1987) (Fig. 2a). Von Huene et al. (1999) suggested that the seaward edge of the Prince William terrane serves as the backstop for present-day subduction within the Kodiak region. The Chugach and Prince William terranes are often shown as being separated by the Contact fault zone, although von Huene et al. (1999) indicated that within the Kodiak Island region there appear to be no marked changes in velocity, density, or magnetic properties across the Contact fault zone. This suggests little physical difference between the terranes. The southern edge of both the Prince William and Chugach terranes appears to define the southern end of Kodiak Island and the Kodiak rupture zone (von Huene et al., 1999). Northwest of the Border Ranges fault is the Peninsular terrane, a Mesozoic island arc complex built on metasedimentary and metavolcanic basement (Plafker et al., 1994b). There is no evidence of movement post-50 Ma along the Border Ranges fault (Haeussler et al., 1995). Present-day subduction gives rise to a modern volcanic arc trending northeast southwest within the Alaska Peninsula, with a marked change to a north-northwest southsoutheast trend at the southern end of Cook Inlet. This bend may be related to the change in the dip of the plate interface between the Kodiak and PWS regions. Previous Geophysical Studies Many geophysical studies were conducted in the 5 years following the 1964 mainshock and are summarized in a report by the National Academy of Sciences (1972). These studies focused primarily on the PWS region, which had greater accessibility and a higher population density. Stauder and Bollinger (1966) determined focal mechanisms from P- wave first-motion and S-wave polarization analysis for a number of early aftershocks (Table 1). Studies of seismicity have been hampered by the lack of a regional seismograph network in the Kodiak Island area. Over the past five years, the installation of seismic networks for monitoring volcanoes on the Alaska Peninsula has increased the ability to detect and locate earthquakes within the Kodiak region (e.g., Hansen and Ratchkovski, 2001), but coverage on Kodiak and surrounding islands remains sparse. Ratchkovsky et. al. (1997a, 1998) relocated events as far south as 58 N using the joint hypocenter determination (JHD) method. Hansen and Ratchkovski (2001) relocated events of the 1999 M w 7 Karluk Lake sequence, as well as regional events occurring since They also conducted waveform modeling studies of the 1999 mainshock and a M w 6.4 aftershock using broadband regional seismograms. The offshore 2001 M w 7 Albatross Basin sequence is discussed by Ratchkovski and Hansen (2001). Tichelaar and Ruff (1993) inverted long-period P waves

3 Figure 2. Earthquakes analyzed in this study (diamonds) and in Doser et al. (1999) (circles). Nine earthquakes not shown on the following detailed maps of the Kodiak region are indicated by numbers (see Table 1 and text for details). (a) Thin dashed lines indicate major faults/terrane boundaries from Plafker et al. (1994a). Italics indicate terrane names. Thick dashed lines indicate boundaries of the Upper and Lower Kodiak study regions. The focal mechanism for event 38 (with M w 6.0) is also shown. KE, Kennedy Entrance. (b) Bold dashed lines show major structural features of the Pacific plate from von Huene et al. (1999). Asterisks following some event numbers indicate less reliable CMT solutions. SMA, slope magnetic anomaly; 58 N, 58 N fracture zone; KSMC, Kodiak seamount chain; AFZ, Aja fracture zone. Numbered black symbols are events below the plate interface; open numbered symbols are events above the plate interface. Boxes indicate the slip model obtained for the 1964 great Alaska earthquake by Johnson et al. (1996) and for the 1938 Semidi earthquake by Johnson and Satake (1994). Slip in meters for each patch of the fault is given in italic numbers. 3271

4 3272 D. I. Doser, W. A. Brown, and M. Velasquez Table 1 Focal Mechanisms from First-Motion and Waveform-Modeling Studies Event* Date Time Depth (km) Magnitude Focal Mechanism (strike, dip, rake) Moment N m Reference 1 L m b, 6.5 M w , 73 10, BW 25 67, 90, 99 SB 2 L m b, 5.9 M w 60 30, 68 16, BW 3 L m b, 6.3 M w 57 26, 77 11, BW 25 53, 80, 84 SB 4 L m b 337, 64, 27 FM 5 L m b 71, 82, 13 FM 25 65, 83, 87 SB 6 L m b 10, 60, 40 FM 7 L m b 30, 75, 90 FM 22 63, 83, 95 SB 8 L m b, 6.2 M w 29 26, 59 15, BW 35 65, 85, 101 SB 9 L m b 50, 80, 90 FM 30 64, 84, 95 SB 10 L m b 50, 75, 54 FM 11 U m b, 5.8 M w 22 30, 48 9, BW 33 11, 38, 144 SB 12 U m b, 5.8 M w 15 20, 50 10, BW 13 L m b, 6.3 M w 32 17, 70 8, BW 36 52, 84, 98 SB 14 U m b, 6.8 M w , 7 9, BW M s, 6.9 M w 211, 10, 72 TR , 5, 90 SB 15 L m b 40, 58, 173 FM 16 U m b, 6.3 M w , 53 12, BW , 20, 166 SB 17 L m b, 6.2 M w 30 26, 70 13, BW 18 L m b 30, 60, 64 FM 19 L m b, 6.1 M w 263 9, 87 7, BW 20 L m b, 5.6 M w , 63 11, BW 21 U m b 220, 5, 90 FM 22 U m b, 6.7 M w , 45 6, BW 23 L m b 90, 33, 130 FM 24 L m b 10, 62, 142 FM 25 L m b, 6.0 M w , 67 13, BW 26 U m b, 6.1 M w , 88 10, BW 27 L m b, 6.0 M w 36 23, 59 13, BW 28 L m b, 5.9 M w 15 20, 44 10, BW 29 L m b, 6.2 M w 67 20, 81 11, BW 30 L m b, 6.3 M w 70 20, 74 15, BW 31 L m b, 5.7 M w 73 18, 73 9, BW 32 AP m b, 5.3 M w 42, 65, CMT 33 U m b, 5.4 M w 99, 35, CMT(u) 34 L m b, 6.5 M w 233, 6, CMT 35 L m b, 5.7 M w 265, 8, CMT 36 U m b, 6.1 M w 312, 57, CMT 37 U m b, 5.7 M w 65, 56, CMT 38 AP m b, 6.6 M w 102, 30, CMT(u) 39 L m b, 5.4 M w 244, 13, CMT(u) 40 U m b, 5.6 M w 322, 52, CMT 41 L m b, 5.2 M w 241, 46, CMT(u) 42 L m b, 5.8 M w 215, 9, CMT(u) 43 L m b, 6.0 M w 292, 7, CMT 44 L m b, 5.2 M w 85, 59, CMT(u) 45 U m b, 5.5 M w 334, 25, CMT 46 AP m b, 5.3 M w 319, 50, CMT 47 L m b, 5.5 M w 220, 18, CMT(u) 48 U m b, 5.1 M w 89, 64, CMT(u) 49 L m b, 6.3 M w 215, 7, CMT 50 L m b, 6.5 M w 289, 8, CMT

5 Seismicity of the Kodiak Island Region and Its Relation to the 1964 Great Alaska Earthquake L m b, 5.9 M w 241, 11, CMT(u) 52 L m b, 5.3 M w 228, 8, CMT 53 L m b, 5.6 M w 241, 12, CMT 54 U m b, 5.6 M w 323, 56, CMT 55 U m b, 6.4 M w 321, 72, CMT 56 L m b, 5.2 M w 263, 18, CMT(u) 57 L m b, 5.9 M w 208, 13, 60 1 CMT 58 U m b, 5.4 M w 300, 57, CMT(u) 59 L m b, 5.7 M w 319, 27, CMT 60 U m b, 5.6 M w 176, 62, CMT 61 L m b, 6.1 M w 212, 9, CMT 62 U m b, 5.5 M w 67, 61, CMT 63 L m b, 5.0 M w 0, 70, CMT(u) 64 L m b, 5.3 M w 358, 29, CMT 65 L m b, 5.5 M w 251, 13, CMT 66 L m b, 5.0 M w 61, 33, CMT(u) 67 L m b, 5.3 M w 272, 10, CMT 68 U m b, 5.9 M w 67, 57, CMT 69 U m b, 5.2 M w 141, 23, CMT(u) 70 U m b, 5.6 M w 318, 51, CMT 71 U m b, 4.9 M w 91, 42, CMT(u) 72 L m b, 5.4 M w 58, 4, CMT(u) 73 L m b, 5.5 M w 218, 20, CMT(u) 74 L m b, 5.2 M w 268, 11, CMT(u) 75 AP m b, 5.4 M w 82, 77, CMT 76 AP m b, 5.2 M w 165, 74, CMT(u) 77 AP m b, 5.4 M w 347, 71, CMT 78 L m b, 5.7 M w 299, 5, CMT(u) 79 U m b, 5.2 M w 60, 59, CMT 80 U m b, 5.7 M w 98, 41, CMT 81 L m b, 5.0 M w 209, 11, CMT(u) 82 L m b, 6.2 M w 225, 9, CMT 83 L m b, 5.5 M w 165, 78, CMT(u) 84 L M w 29, 67, H 85 L m b, 6.4 M w 234, 89, R 86 OS m b, 5.5 M w 302, 55, CMT(u) 87 L m b, 5.4 M w 350, 10, CMT 88 L m b, 5.1 M w 113, 18, CMT 89 L m b, 5.8 M w 330, 58, CMT 90 U m b, 5.2 M w 82, 63, CMT 91 L m b, 6.5 M w 223, 89, R 92 L m b, 5.8 M w 263, 15, CMT 93 L M w 223, 90, R 94 OS M w 332, 66, CMT 95 U M w 322, 67, CMT 96 L M w 284, 41, CMT *L, Lower Kodiak; U, Upper Kodiak; AP, Alaska Peninsula; OS, offshore. BW, body waveform modeling, this study; CMT, Harvard CMT solution; (u), denotes less reliable CMT solution using the criteria of Frohlich and Davis (1999); FM, first-motion solution, this study; H, Hansen and Ratchkovski (2000); TR, Tichelaar and Ruff (1993); SB, Stauder and Bollinger (1966); R, Ratchkovski and Hansen (2001) to determine the maximum depth of seismic coupling in many subduction zones around the world, including the Alaska Aleutian arc. They determined a focal depth of km for an event occurring in September 1965 beneath Afognak Island (Table 1). Combining this depth with focal depths from the Semidi and Shumagin Islands regions, they concluded that the maximum depth of seismic coupling for this portion of the Alaska Aleutian arc is km. Crustal structure of the Kodiak region has been examined in a series of offshore seismic reflection/refraction experiments, including studies in the Kennedy Entrance region (EDGE) (Moore et al., 1991; Ye et al., von Huene et al., 1998) 1997; and off southeastern Kodiak Island (ALBA- TROSS) (von Huene et al., 1986). Results of these studies will be discussed and compared to cross sections of seismicity in subsequent sections. Savage et al. (1999) used information from a geodetic array stretching from Kodiak Island to the Alaska Peninsula to determine that the plate interface appears to be locked from the trench to km inland. Zweck et al. (2002) inverted a combination of Global Positioning System (GPS) and geodetic data to estimate the current amount of seismic coupling along the 1964 rupture zone. Their results suggest that the plate interface within the PWS region and southwest

6 3274 D. I. Doser, W. A. Brown, and M. Velasquez of Kodiak Island is locked, while the plate interface trenchward of the western Kenai Peninsula and northern Kodiak Island does not appear to be locked. A region of postseismic relaxation also occurs within Cook Inlet and the northern portion of Shelikof Strait. Earthquake Analysis Techniques Earthquakes selected for analysis are listed in Table 1. We generally restricted our study to m b 5.6 earthquakes to obtain adequate body-waveform information at teleseismic distances. We studied both shallow ( 50 km) and intermediate ( 50 km) depth events since previous studies have suggested intermediate depth seismicity tends to cluster down dip of major asperities (Dmowska and Lovison, 1992; Doser et al., 1999). Our waveform modeling and firstmotion analyses have emphasized events occurring prior to 1980 (before operation of digital seismograph networks and routine estimations of seismic moment tensors). Table 1 also lists the focal mechanisms for all events that have occurred since 1980 with M w 4.9 that were found in the Harvard Centroid Moment Tensor (CMT) catalog. In addition, focal mechanisms obtained from several previous studies (e.g., Stauder and Bollinger, 1966; Tichelaar and Ruff, 1993; Hansen and Ratchkovski, 2001; Ratchkovski and Hansen, 2001) are also given in Table 1. We have used the method of Frohlich and Davis (1999) to sort the CMT solutions, with more reliable events distinguished by E rel 0.15, f CLVD 0.20, and n free 6. Frohlich and Davis (1999) suggested that these higher quality events have T-, B- and P-axis azimuth and inclination angle uncertainties of 10 or less. Relocations for 59 events occurring prior to 1990 were obtained using the bootstrap relocation technique of Petroy and Wiens (1989). This technique is well suited for locating events with teleseismic arrival-time data that have poor azimuthal distributions. Ratchkovsky et al. (1997a, 1998) and Hansen and Ratchkovski (2001) have relocated many of the larger events occurring after this date. The relocations of pre events are given in the Appendix (Table A1, Fig. A1). Relocated seismicity is compared to the fault model of Johnson et al. (1996) and to major structures within the Pacific plate in Figure 2b. Note that few events have occurred in the central region of the Kodiak segment, with no events associated with the subfault having the maximum amount of slip (10 15 m) in Numerous events have occurred near the southeastern edge of the Kodiak segment and south toward the 1938 Semidi rupture zone. The Aja fracture zone may control the southern edge of this seismicity. The quiescent central portion of the segment appears to extend from the Kodiak seamount chain to the 58 N fracture zone. Northwest of the Kodiak segment, many events cluster near Iliamna volcano. Focal mechanisms were determined from P-wave firstmotion data for all events occurring prior to 1980 using the grid-search technique of Whitcomb (1973). These mechanisms were used primarily to guide our selection of starting models for the body-waveform modeling analysis. We did, however, determine first-motion mechanisms for a few earthquakes occurring between 1964 and 1969 that were too small (m b 5.7) to obtain adequate body-waveform information. For these events we plotted the ranges of T- and P- axes that would result if up to one impulsive and two emergent arrivals were in error. If these regions covered more than 20% of the focal sphere, the mechanisms were rejected. This left us with 11 mechanisms determined solely from first-motion data (Table 1, Fig. A2). Body-waveform (radial and vertical P, transverse S) modeling was conducted for 20 events using the inversion technique of Baker and Doser (1988). Layered 1D velocity models for the source regions were based on the upper crustal seismic reflection/refraction models of Ye et al. (1997), and von Huene et al. (1986, 1998). Deeper velocity structure was based on the seismic tomography studies of Zhou et al. (1995) in the Cook Inlet region. A water layer was included for offshore events. Most seismograms were digitized from computerscanned images using the technique of Liberty and Pelton (1995). A few faint seismograms were digitized by hand. Each seismogram was corrected for mean and trend and resampled at a rate of 0.5 sec. Results of the waveform modeling process are shown in the Appendix (Figs. A3 A22) and given in Table 1. Results Earthquake locations and focal mechanisms determined from this study are given in Tables A1 and 1. The large number of mechanisms for more recent, post-1980, events has precluded illustration of all focal mechanisms; thus, focal mechanisms are shown only for events with M w 6.0. Figures 3 and 7 indicate the estimated locations and tectonic associations of the events (upper plate, lower plate, plate interface). The interpretation of tectonic association was based on crustal structure (Figs. 4, 8), focal depth, focal depth uncertainty, and focal mechanism. For events with unknown focal depth errors (e.g., International Seismological Centre [SC] depths for first-motion mechanisms, CMT solutions with 15- or 33-km default depths) we assumed a 5 km uncertainty. To present our results we have divided the Kodiak region in two subregions (Fig. 2a) that were chosen to highlight structural changes along the Kodiak margin. The Upper Kodiak subregion extends from the southern Kenai Peninsula south to Afognak Island (Fig. 2a). The northern end of the region is located at the transition from shallow ( 4 )to steeper ( 8 ) dip along the plate interface. The southern end of the subregion is located near the point where the Kodiak seamount chain enters the Aleutian trench (Fig. 2b). The Lower Kodiak subregion (Fig. 2a) extends from the Kodiak seamount chain to the northern portion of 1938 Semidi rupture zone, just south of the Aja fracture zone (Fig. 2b). In

7 Figure 3. Earthquakes of the Upper Kodiak region. Numbers followed by a P refer to events studied by Doser et al. (1999) (see their table 1). Other numbers refer to events given in Table 1. (a) Event locations. Diamonds are from this study and circles are events from Doser et al. (1999). Squares are events relocated by Hansen and Ratchkovski (2001) or Ratchkovski et al. (1997a) with M 4.5. Triangles are volcanoes. A, Mt. Augustine; D, Mt. Douglas; I, Mt. Iliamna. AI, Afognak Island; KI, Kodiak Island. Bold lines are offshore faults showing evidence of Quaternary motion from Plafker et al. (1994a). (b) Relation of events to the plate interface. Open symbols denote events occurring above the plate interface, black symbols are events below the plate interface, and gray symbols are events on the plate interface. Events with less reliable CMT solutions are indicated with asterisks. Focal mechanisms shown are for events of M w 6.0 determined from this study, from Doser et al. (1994), or from the CMT catalog. Dashed oval encloses events of the Iliamna region. Bold line is the location of the EDGE seismic line (Moore et al., 1991; Ye et al., 1997). Crustal structure along the EDGE line is shown in Figure

8 3276 D. I. Doser, W. A. Brown, and M. Velasquez Figure 4. Cross section of crustal structure along the EDGE seismic line. Structure from 0 to 400 km modified from Ye et al. (1997). Slab structure northwest of 400 km is based on the Wadati Benioff contours of Plafker et al. (1994a). Earthquakes occurring within 50 km of the EDGE line are shown by solid symbols. Open symbols are selected events from this study located 50 km from the EDGE line. Circles are events with depths obtained from body waveform modeling (error bars denote uncertainties given in Table 1), diamonds are CMT events, with gray diamonds indicating less reliable CMT solutions, squares are events relocated by Ratchkovsky et al. (1997) or Hansen and Ratchkovski (2001). A, Mt. Augustine; CIB, Cook Inlet Basin; LVZ, low-velocity zone, SB, Stevenson Basin; T, Aleutian trench. Arrow labeled 1964 marks down-dip extent of 1964 rupture zone from Johnson et al. (1996). Note that the location for event 86 is shown in Figure 2. addition we analyzed seven events (32, 38, 41, 46, 75, 76, 77) located on or near the Alaska Peninsula and two events within the offshore region (86, 94) that are shown in Figure 2. Upper Kodiak Region This region s most concentrated seismicity lies beneath Iliamna volcano. Other regions of activity include southern Cook Inlet and the eastern Kennedy Entrance, where slip in 1964 was 5 10 m (Fig. 2b). Most earthquakes lie below the plate interface (Figs. 3b, 4). The depth to the plate interface in the east is based on crustal structure from the EDGE seismic experiment (bold line, Fig. 3b), and in the west is based on depth contours to the top of Wadati Benioff zone (WBZ) from Plafker et al. (1994a). Ye et al. (1997) suggest that the low velocity zone shown in Figure 4 represents an underplated seamount or oceanic plateau. Depth uncertainties are shown for events that we have analyzed using body-waveform modeling or from the CMT catalog. Depth errors are not known for the first motion solutions (depths from ISC listings). Ratchkovsky et al. (1997a) estimate that depth errors for their locations are less than 5 km. Lower plate events (depths 70 km) in the Kennedy Entrance region have normal and normal-oblique mechanisms (Table 1). Events 11, 22, 69, and 71 appear to have occurred in the upper crust of the Pacific plate, whereas events 12 and 80 occurred within the Pacific plate mantle (Fig. 4). Intermediate depth ( 70 km) events within the lower plate are located beneath Iliamna Volcano, Mt. Augustine, and Mt. Douglas and are characterized by reverseoblique mechanisms. Event 14, located near Afognak Island, was a plate interface event previously studied by Stauder and Bollinger (1966) and Tichelaar and Ruff (1993) (Table 1). This event is the largest interface event to have occurred in the Upper Kodiak region since Our results yield a mechanism similar to that of Stauder and Bollinger and a depth similar to that of Tichelaar and Ruff (1993). Event 14 was followed 3.5 months later by event 16, an event located within the crust of the Pacific plate, down-dip of event 14 (Fig. 4). Event 21 appears to be the only m b 5.6 earthquake to have occurred within the upper plate since 1964, although the error in its focal depth is not known. A focal mechanism obtained from first-motion analysis (Table 1) is consistent

9 Seismicity of the Kodiak Island Region and Its Relation to the 1964 Great Alaska Earthquake 3277 with reverse or thrust faulting. Note that several recent events located to the southwest of event 21 (Ratchkovsky et al., 1997a) also occurred within the upper plate. The relation of hypocenters in the eastern Kennedy Entrance region to the EDGE crustal structure suggests that the earthquakes appear to cluster around the eastern edge of the low velocity zone within the lower crust. The 1964 mainshock rupture (arrow, Fig. 4) extended to the middle of the low-velocity zone, suggesting this zone may have influenced rupture termination. Incomplete moment release along the plate interface below the low-velocity zone in 1964 may have led to further rupture of the plate interface in event 14. P and T axes for all events in the Upper Kodiak region are shown in Figure 5. Figure 5a shows axes for all lower plate events, and 5b shows intermediate depth events near Iliamna. The arrows indicate the strike of the WBZ north ( 20, WBZN) and south of Augustine volcano ( 50, WBZS). Generally, T axes strike 270 to 300 (normal to WBZN), and P axes strike 0 to 20 (parallel to WBZN). Figure 5a suggests down-dip extension and compression parallel to the WBZ for most lower plate events. This is consistent with stress studies by Lu et al. (1997) (for M s 5 earthquakes) and Ratchkovsky et al. (1997b) (for events below km) for earthquakes in the lower Cook Inlet/ Upper Kodiak region. The few events with near-vertical P axes represent normal faulting at depths 50 km occurring within the Kennedy Entrance between 1964 and P and T axes for upper-plate and plate-interface events (Fig. 5c) show a clockwise rotation of T axes and counterclockwise rotation of P axes relative to lower plate events. P axes for the two plate interface events (squares) lie within 10 of the direction of Pacific plate motion (arrow) and are very similar to the axis for the 1964 mainshock (black triangle). Triangle diagram representations of P and T axes (e.g., Frohlich, 1992) (Fig. 6) indicate that most lower plate events represent some mixed combination of strike-slip and reverse motion. All normal-faulting events are from the eastern Kennedy Entrance region, and most mixed strike-slip/reverse events and pure strike-slip events are from the Iliamna region (Fig. 6b). Note that upper plate and plate interface are mixed events falling between reverse and normal faulting. A worldwide study of shallow (depth 30 km) earthquake focal mechanisms by Frohlich (2001) indicates that mixed events are unusual ( 11% of total). Mixed events generally occur along plate boundaries where crustal thickness is highly variable or in regions of oblique convergence; both of these factors are present in the Upper Kodiak region. Lower Kodiak Region The Lower Kodiak region has a higher level of seismicity than the Upper Kodiak region (Figs. 1, 2). A band of offshore seismicity, parallels the trench, with several clusters of activity that have persisted throughout the more than 35 years following the 1964 mainshock. This region of seismicity corresponds to portions of the Kodiak rupture zone Figure 5. P (solid symbols) and T (open symbols) axes of Upper Kodiak region earthquakes. (a) All lower plate earthquakes. Squares are from body-waveform and firstmotion analyses of this study. Circles are high-quality CMT solutions, diamonds are less reliable solutions. Arrows denote strike of WBZ north (WBZN) and south (WBZS) of Mt. Augustine. (b) Lower plate earthquakes of the Iliamna region (see Fig. 3b for location) (c) Earthquakes of the upper plate (circles) and plate interface (squares). Triangles are axes for the 1964 mainshock. Arrow denotes motion of the Pacific plate relative to North America from DeMets et al. (1990).

10 3278 D. I. Doser, W. A. Brown, and M. Velasquez Figure 6. Triangle diagram representations. (a) All earthquakes of the Upper Kodiak region. Square is upper plate event from first-motion data; triangles are lower plate events; circles are plate interface events. Black symbols are mechanisms from body waveform modeling, gray symbols are higher quality CMT solutions, and open symbols are less reliable CMT solutions. (b) Earthquakes of the Iliamna region. Gray symbols are higher quality CMT solutions. Open symbols are less reliable CMT solutions and black symbols are from body waveform modeling. having slip less than 5 m in 1964 (Fig. 2b). The January 2001 M w 7.0 Albatross Basin earthquake (event 93, Figs. 7, 8a) is one of the two largest events to have occurred within the Kodiak PWS region since The northeastern end of the seismicity corresponds to the location of the Kodiak seamount chain (Fig. 2b). Few events are located south of the northern edge of the 1938 Semidi rupture zone, which has been modeled by Johnson and Satake (1994) to have slipped 3.3 m during the 1938 mainshock (Fig. 2b). The southern portion of Kodiak Island is also seismically active and is the site of the December 1999 (M w 7.0) Karluk Lake earthquake (event 84, Figs. 7, 8b). Scattered seismicity occurs throughout the Alaska Peninsula (Fig. 2). The majority of earthquakes ( 70%) in the Lower Kodiak region also occur below the plate interface (Figs. 7b, 8). However, unlike the Upper Kodiak region, at least 20% of the events appear to have occurred along the plate interface (Fig. 7b). Several additional CMT solutions with default depths of 33 km (e.g., events 34, 42, 78) also have focal mechanisms consistent with slip along the interface. A few events ( 10%) occur above the plate interface, most notably a sequence in 1974 (events 29, 30, 31) with reverse-oblique mechanisms (Fig. 7b). Ratchkovski and Hansen (2001) have suggested that the 2001 Albatross Basin earthquake involved near-vertical faulting within the Pacific plate, based in part on a 30-km focal depth determined from JHD relocations (event 93, square, Fig. 8b). We feel, however, that this earthquake occurred along the plate interface. The CMT depth for the event was 21 km (open diamond, Fig. 8b), relocated aftershocks of the sequence (Ratchkovski and Hansen, 2001) extended laterally from the mainshock (rather than vertically), and its mechanism is similar to that of smaller magnitude interface events that have occurred within the region in the past 20 years. Earthquakes in south-central Kodiak Island are associated primarily with the 1999 Karluk Lake sequence. Hansen and Ratchkovski (2001) interpreted the 1999 mainshock (event 84, Fig. 8b) as rupturing from the top of the subducted Pacific plate into the mantle. Aftershocks of this sequence extended the mainshock rupture zone laterally and with depth. Smaller magnitude aftershocks also occurred in the upper plate (Hansen and Ratchkovski, 2001). The mainshock occurred near the western edge of the 1964 rupture zone (arrow, Fig. 8b), leading Hansen and Ratchkovski to suggest that the 1999 mainshock was consistent with slabpull concentrating stress at the edge of the 1964 rupture zone. Two M w events in 1970 and 1974 (events 25 and 28) occurred northeast of the 1999 sequence at similar depths to events of the 1999 sequence (Figs. 7, 8b). This suggests that the northern end of the aftershock zone may abut the southern end of rupture during 1970 and Larger earthquakes located on the Alaska Peninsula fall into two groups. Events 75, 76, and 77 (Fig. 2) occurred within the lower crust of the North American plate (30- to 40 km depth), while events 32, 46, and 83 (Figs. 2, 7, and 8c) occurred within the Pacific plate at depths of at least 70 km. Two events with poorer quality CMT solutions are located offshore west of the down-dip edge of the 1938 Semidi rupture (38, 41, Fig. 2). Both events have normal-oblique mechanisms (Table 1) and depths suggesting rupture within the Pacific plate. Only two earthquakes (events 65 and 67, Fig. 7) appear to fall within the 1938 rupture zone (Fig. 1). Event 65 ap-

11 Figure 7. Lower Kodiak region earthquakes. Diamonds are events from this study. Event numbers are from Table 1. (a) Event locations. Squares indicate events relocated by Hansen and Ratchkovski (2001) or Ratchkovski and Hansen (2001). Triangles are volcanoes. K, Katmai; S, Snowy; AI, Afognak Island. Other events of the Lower Kodiak region located on or near the Alaska Peninsula or far offshore are shown in Figure 2. Bold lines show offshore faults with suspected Quaternary movement from Plafker et al. (1994a). (b) Positions of earthquakes relative to the plate interface. Asterisks denote events with less reliable CMT solutions. Dark symbols are events below the plate interface, open symbols are above the plate interface, and gray symbols are events on the plate interface. The text discusses the reliability of focal depths for plate interface events. Focal mechanisms (M w 6.0) are from this study, CMT solutions, Hansen and Ratchkovski (2001), or Ratchkovski and Hansen (2001) (see Table 1). The location of the ALBATROSS seismic line is indicated by bold line. A cross section along the ALBATROSS line is shown in Figure 8a. 3279

12 3280 D. I. Doser, W. A. Brown, and M. Velasquez Figure 8. Cross sections of crustal structure of the Lower Kodiak region. Earthquakes occurring within 65 km of these cross sections have been projected onto the cross sections. Circles are events with depths determined in this study from body waveform modeling. Diamonds are from CMT solutions. Squares are from Hansen and Ratchkovski (2001) and Ratchkovski and Hansen (2001). (a) Crustal structure along the ALBATROSS seismic line is based on von Huene et al. (1996, 1998). AB, Albatross Basin; T, Aleutian trench. Arrow labeled L denotes down-dip limit of currently locked zone along the plate interface from Savage et al. (1994). Open diamond indicates CMT focal depth for the 2001 Albatross Basin earthquake (event 93) discussed in text. Gray diamonds in this and part (b) denote less reliable CMT solutions. (b) Structure to the northwest of the ALBATROSS line is based on Hansen and Ratchkovski (2001). Arrow labeled 1964 denotes the down-dip limit of the 1964 rupture zone from Johnson et al. (1996). (c) Crustal structure of entire Lower Kodiak region is shown at same scale as structure in the Upper Kodiak region (Fig. 4b). The structure is a composite of Figure 8a and b, with slab structure northwest of 300 km based on the Wadati Benioff contours of Plafker et al. (1994a). AB, Albatross Basin; K, Katmai; T, trench. Gray diamond shows CMT depth for the 2001 Albatross Basin earthquake. Figure 9. P (solid symbols) and T axes (open symbols) of Lower Kodiak region earthquakes Squares are from body waveform analysis, and triangles are from first-motion analysis. Circles indicate high-quality CMT solutions. Ovals are less reliable CMT solutions. Diamonds are from Hansen and Ratchkovski (2001) and Ratchkovski and Hansen (2001). (a) All lower plate earthquakes. Arrow denotes strike of the WBZ in this region. (b) Earthquakes on the plate interface. (c) Earthquakes in the upper plate. Axes for events on the Alaska Peninsula are labeled AP. Arrow indicates the direction of motion of the Pacific plate relative to North America from DeMets et al. (1990).

13 Seismicity of the Kodiak Island Region and Its Relation to the 1964 Great Alaska Earthquake 3281 pears to have occurred along the plate interface, whereas event 67 involved normal-oblique faulting above the plate interface. Although there is considerable scatter in P and T axes for lower plate events (Fig. 9a), the majority of T axes, especially those of the most recent earthquakes (circles, ovals, and diamonds), strike northwest (down dip), in contrast to the west-northwest strike of T axes seen in the Upper Kodiak region (Fig. 5a). This likely reflects the change in the strike of the WBZ between the two regions. Unlike the Upper Kodiak region (especially the Iliamna region), few P axes of the Lower Kodiak region are parallel to the WBZ. This difference could indicate a decrease in margin parallel compressional stress with distance from the Yakutat block or a decrease due to changes in the amount of plate coupling between the Upper and Lower Kodiak regions (Zweck et al., 2002). The difference may also reflect the greater number of intermediate depth earthquakes within the Upper Kodiak region that sample a different stress regime within the slab. P and T axes for plate interface events (Fig. 9b) are similar to those observed for the interface events in the Upper Kodiak region and the 1964 mainshock (Fig. 5b). P and T axis orientations for upper plate events (Fig. 9c) show considerable variability, reflecting stress changes between the offshore and Alaska Peninsula (symbols labeled AP in Fig. 9c) regions. Most events in the Lower Kodiak region fall outside of the pure normal, strike-slip, and reverse-faulting regions of the triangle diagram (Fig. 10). Like the Upper Kodiak region, some lower plate events are a mix of reverse and strikeslip faulting. However, more appear to lie between reverse and normal faulting, as do plate interface and upper plate events in the Upper Kodiak region. Figure 10. Triangle diagram representation of earthquakes of the Lower Kodiak region. Squares are upper plate events, triangles are lower plate events, and circles are plate interface events. Black symbols are mechanisms from body waveform modeling (this study; Hansen and Ratchkovski, 2001; Ratchkovski and Hansen, 2001), symbols highlighted by bold lines are from first-motion studies, gray symbols are highquality CMT solutions, and open symbols are less reliable CMT solutions. Discussion Seismic slip along the plate interface in the Upper Kodiak region since the 1964 mainshock has been minimal. The most notable interface event (event 14, M w 6.8) occurred in 1965 and was located down dip of the portion of the Kodiak rupture zone that slipped 10 m during the 1964 mainshock (Fig. 2b). In the Lower Kodiak region, however, about 20% of the earthquakes (including the 2001 M w 7.0 Albatross Basin event) appear to be occurring along the plate interface in a region that experienced low slip ( 5 m) during the 1964 earthquake. Seismicity within the North American plate is low. Offshore upper plate events are located primarily beneath arched regions of the crust between sedimentary basins (Stevenson and Cook Inlet Basins, Fig. 4; Albatross Basins and trench, Fig. 8c), or beneath the Alaska Peninsula. The majority of seismicity since the 1964 mainshock occurs within the subducting Pacific plate. The focal mechanisms in the lower plate suggest a change of stress orientation may have occurred over time. Between 1964 and 1974 lower plate seismicity was characterized by normal to Figure 11. Comparison of post-1964 mainshock seismicity (this study, Doser et al., 1999) to the locked zones (coupling coefficient greater than 0.6, bold dashed lines) and postseismic relaxation region (coupling coefficient less than 0.6, bold solid line) of Zweck et al. (2002).

14 3282 D. I. Doser, W. A. Brown, and M. Velasquez normal-oblique faulting with moderately dipping (40 60 ) nodal planes (Table 1). Since 1974, reverse-oblique faulting has become more common (Table 1). Modeling of GPS/geodetic data by Zweck et al. (2002) suggests that two locked zones along the plate interface (Fig. 11) can best fit the data. These locked zones (coupling coefficient 0.6, dashed ovals), as well as an area of postseismic relaxation (coupling coefficient 0.6, bold solid line), are compared to M w 4.5 events of the PWS and Kodiak regions in Figure 11. The along-strike position of the locked zone south of Kodiak Island is poorly resolved (J. Freymueller, personal comm., 2001). Lower plate and plate interface seismicity clusters at the eastern and northern edges of the Kodiak locked zone. In the PWS locked zone, the southern cluster of earthquakes represents upper-plate events that occurred within 10 years of the mainshock, whereas the northern cluster is lower plate seismicity that has continued for over 37 years. The concentrated seismicity seen in the Pacific plate in the northern PWS region has been persistent since at least 1928 (Doser and Brown, 2001). Preliminary results for the Kodiak region (Doser et al., 2001) also indicate persistent seismicity near the locked region south of Kodiak Island since Thus the locked zones appear to repeat from one earthquake cycle to another. The lack of seismicity within much of central and northern onshore and offshore Kodiak Island corresponds well with a region of the plate interface undergoing aseismic slip (Zweck et al., 2002); (coupling coefficient 0). The reduced coupling could explain the lack of observed down-dip migration of seismicity through time in the Kodiak region, in contrast to the PWS region where plate coupling is stronger. The eastern Kennedy Entrance region has moderate seismicity, with the southwestern portion of the entrance experiencing the largest plate interface event since the 1964 mainshock. This suggests that an intermediate degree of plate coupling may exist in the Kennedy Entrance region that was not resolvable in the GPS/geodetic data. It is also possible that the regional seismicity may be related to stress changes associated with the deformation of a suspected underplated seamount/oceanic plateau (Moore et al., 1991; Ye et al., 1997) observed on the EDGE seismic line. The northeastern and west-central edges of the region of postseismic relaxation (Fig. 11) appear to be defined by increased seismicity within the subducting plate. The western Kenai Peninsula has had very few earthquakes since 1964, in contrast to pre-1964 seismicity patterns (Doser and Brown, 2001). Thus as postseismic relaxation continues to decrease; we might expect to observe an increase in seismicity within the Kenai region. Seismic moment release is significantly greater ( 6.5 times) within the Kodiak region than the PWS region. Although 48% of the seismic moment release in the Kodiak region is associated with events occurring between 1999 and 2001, seismic moment release in the region between 1964 and 1974 still exceeds that of the PWS region over the past 37 years by a factor of 2. The lack of a zone of postseismic relaxation in the Kodiak region, the lesser amount of slip along the Kodiak rupture zone during the 1964 mainshock, and the difference in the structure/composition of the lower plate beneath the Kodiak region all may influence the observed difference in seismic moment release between the regions. Conclusions Studies of the source processes of M w 5.0 earthquakes occurring in the Kodiak Island region since the 1964 great Alaska mainshock show that a majority of the seismicity has occurred within the Pacific plate. The region of greatest lower plate and plate interface seismicity lies southeast of Kodiak Island and corresponds closely to the northern and eastern edges of a locked zone along the plate interface that has been defined by GPS/geodesy studies. This suggests that the high degree of plate coupling is responsible for continued earthquakes within the region. Lower levels of seismicity are associated with regions of low plate coupling, suggesting that aseismic slip may be the dominant mode of deformation in much of the area offshore of central and northern Kodiak Island. Unlike the PWS region, down-dip migration of seismicity within the lower plate was not observed following the 1964 mainshock. This may be related to weaker seismic coupling along the plate interface in much of the central and northern Kodiak region. Within the Upper Kodiak region, intermediate depth seismicity is concentrated near Iliamna Volcano. A moderate level of upper and lower plate seismicity within the eastern Kennedy Entrance may be related to the presence of an underplated seamount/oceanic plateau observed in seismic reflection/refraction studies. Within the Lower Kodiak region, offshore seismicity southwest of Kodiak Island has been a persistent feature throughout the 37-year study period. Deeper, lower plate events ( 35 km) beneath Kodiak Island also occurred throughout the study period, culminating in the recent ( ) Karluk Lake sequence (M w 7.0 mainshock). The 2001 M w 7.0 Albatross Basin event most likely occurred along the plate interface, representing part of a continued sequence of plate interface events over the past 25 years. Although earthquakes occurring between 1999 and 2001 represent nearly half the moment release within the Kodiak region since 1964, moment release throughout the 37 year period exceeds (by 6.5 times) that of the PWS region. Acknowledgments We thank N. Ratchkovski for providing her catalogs of relocated earthquakes and helpful comments. Discussions and information from J. Freymueller, R. Page, and C. Stephens are also appreciated. The manuscript has benefited from reviews by C. Rowe, C. Frohlich, P. Haeussler, R. Page, and an anonymous reviewer. R. von Huene helped to steer us toward important references early in our investigations. J. Hincapie assisted in the relocation of several events. Funding from the U.S. Geological Survey s Earthquake Hazards Reduction Program (Grants 01HQGR0159 and

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