Review of crustal seismicity in the Aleutian Arc and implications for arc deformation

Tectonophysics 522–523 (2012) 150–157

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Review Article

Review of crustal seismicity in the Aleutian Arc and implications for arc deformation Natalia A. Ruppert ⁎, Natalia P. Kozyreva, Roger A. Hansen Geophysical Institute, University of Alaska Fairbanks, United States

a r t i c l e

i n f o

Article history: Received 6 April 2011 Received in revised form 21 October 2011 Accepted 26 November 2011 Available online 8 December 2011 Keywords: Aleutian Arc Crustal earthquakes Arc deformation Crustal blocks

a b s t r a c t Central and eastern Aleutian Arc is characterized by oblique convergence between the subducting Pacific and overriding Bering Plates. This results in westward arc translation and formation of rotating crustal blocks in the forearc. In 2006–2010 several moderate, shallow crustal earthquakes (up to magnitude 6.7) occurred in the region. These events are located about 150 km away from the trench, on the volcanic axis, and have either strike–slip (west of 174°W) or normal (east of 174°W) faulting mechanisms. We improve aftershock locations by applying precise relocation methods to aid in identifying preferred fault planes. We also review similar earthquakes that occurred prior to 2006. For the central Aleutian Arc we conclude that, while some of these events occurred along the boundaries of the rotating blocks, the majority are left-lateral strike–slip events on NW- to N-oriented fault planes in the unrotated Bering massif. These manifest Riedel shearing in response slip partitioning due to the oblique convergence. Normal faulting events in eastern Aleutian Arc reflect along-arc extension. © 2011 Elsevier B.V. All rights reserved.

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2. Tectonic and seismic background . . . . . . . . . . . . . . 3. Review of pre-2006 crustal earthquakes . . . . . . . . . . . 4. 14 June 2006 MW 6.4 earthquake in Rat Islands . . . . . . . 5. 27 June 2006 MW 6.2 earthquake in Rat Islands . . . . . . . 6. 14 and 15 April 2008 MW 6.4 and 6.6 earthquakes in Andreanof 7. 2 May 2008 MW 6.6 earthquake in Andreanof Islands . . . . . 8. 17 July 2010 MW 6.7 earthquake in Fox Islands . . . . . . . . 9. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . Islands . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The Aleutian Arc lies along a 2000-km long section of the so-called “Pacific Ring of Fire”. It is best known for generating great earthquakes and associated tsunamis, and for producing numerous volcanic eruptions. Between 2006 and 2010, several strong (magnitude 6–6.7) crustal earthquakes occurred in the western and central Aleutian Arc (Fig. 1). The series started in the Rat Islands region with a moment magnitude MW 6.5 event on June 14, 2006, that occurred immediately west of Kiska Island. Two weeks later, a MW 6.2

⁎ Corresponding author. Tel.: + 1 907 474 7472. E-mail address: [email protected] (N.A. Ruppert). 0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2011.11.024

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event occurred near Buldir Island. On April 14 and 15, 2008, MW 6.4 and MW 6.6 earthquakes occurred in the Amchitka Pass region of the Andreanof Islands. Subsequently on May 2, 2008, a MW 6.6 earthquake occurred between the Kanaga and Tanaga Islands. Finally, on July 17, 2010, a MW 6.7 earthquake occurred southwest of Umnak Island in the Fox Islands region. While it is not unusual to have magnitude 6+ earthquakes in the Aleutian Islands, these strong events normally originate on the interface between the subducting Pacific and overriding Bering Plates. However, all before-mentioned earthquakes occurred about 150 km away from the trench along the island axis, had shallow crustal depths, and were either strike–slip or normal faulting events. These events and their aftershocks were recorded by the regional seismic network, which was greatly expanded between the end of the 1990s and the mid-2000s.

N.A. Ruppert et al. / Tectonophysics 522–523 (2012) 150–157

151

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Fig. 1. Location and tectonic setting of the study region. Black arrows show direction of convergence between the Pacific and Bering plates. Dashed arrows show westward translation of the Aleutian Arc. White stars are locations of the recent moderate crustal earthquakes discussed in the text. Locations of seismic stations are shown as: inverted triangles — AVO; squares — WC/ATWC; triangles — AEIC; circles — GSN; yellow — short period and red — broad band instruments.

Currently, the Aleutian seismic network is comprised of about 100 sites; most of which are operated by the Alaska Volcano Observatory (AVO) to monitor seismic activity at the volcanoes (Fig. 1). The remaining stations are operated by the Alaska Earthquake Information Center (AEIC) and the West Coast and Alaska Tsunami Warning Center (WC/ATWC). While the majority of the stations are equipped with short period analog instruments, several of the regional and volcanic sites have broadband digital instrumentation. All seismic data is processed jointly by AEIC, which produces regional catalog of earthquakes in Alaska and the Aleutians. The purpose of this paper is to describe the before-mentioned crustal earthquakes and to interpret them within the framework of Aleutian Arc tectonics. For this we use statistical analysis of the aftershock sequences, double-difference relocations and waveform crosscorrelations. 2. Tectonic and seismic background The Aleutian Arc extends from Unimak Pass in the east (~165°W) to west of Attu Island (173°E). It is believed to be formed in the early Eocene (Scholl et al., 1987). The arc is comprised of the summits of the Aleutian Ridge. A wide forearc basin, reaching a water depth of 7 km, occupies the area between the Arc and the Aleutian Trench. The geologic framework and evolution of the Aleutian Ridge can be found in Scholl et al. (1983, 1987). Active volcanoes are arranged along the north edge of the Aleutian Ridge summit platform and extend as far west as ~175°E. Along the Aleutian Arc, the Pacific Plate is moving with respect to the North American Plate at a rate of about 48 mm/year eastward and 78 mm/year westward (DeMets et al., 1990; Freymueller et al., 2008). In the eastern Aleutians, the direction of convergence is orthogonal to the trench, while in the central Aleutians the direction becomes oblique. West of 175°E, the direction of the relative plate motion parallels the trench (Fig. 1). Oblique subduction results in westward transport of the arc (Cross and Freymueller, 2007, 2008; Lallemant and Oldow, 2000), and causes portions of the overriding plate between the trench and the island arc to break off into rotating blocks (Fig. 2). Geist et al. (1988) proposed a block rotation model for the forearc along the central and western Aleutians. Five blocks of various sizes are identified based on geomorphic evidence (Fig. 2). Boundaries

between the blocks are delineated based on the submarine faultcontrolled canyons. Northern boundaries are defined as the southern edge of the corresponding summit basins, which formed as the result of rotation of the blocks. Southern boundaries are described along the seaward edge of the arc massif. The block boundaries delineate regions of cohesive movement, and are bounded by zones significantly disrupted by normal and strike–slip faults. See also Krutikov et al. (2008) for additional constraints on the blocks' rotations. The Aleutian region produces thousands of earthquakes each year that manifest the subduction zone processes. The earthquakes are associated with faulting along the Aleutian megathrust, as well as faulting within both the subducting and overriding plates. Additionally, hundreds of earthquakes associated with Aleutian volcanic activity occur every year. Many of the regional earthquakes have identified source models, which makes it possible to differentiate between interplate and intraplate events (Fig. 2). Most of the events have thrust mechanisms indicating that they occur on the plate interface. However, some shallow (depths less than 30 km) events have either strike–slip or normal faulting mechanisms. Most of the normal faulting events occur in the Aleutian outer rise region and are caused by bending of the plate as it enters the trench, while most of the shallow strike–slip events are concentrated along the island axis. Lu and Wyss (1996) studied tectonic stresses along the Aleutian subduction zone based on associated earthquakes' focal mechanisms. They concluded that directions of maximum compressive stress change along the Arc, and identified several segments with homogeneous stress conditions. The boundaries between the identified blocks are most strongly correlated with the fracture zones and ends of rupture zones of great earthquakes. In the Aleutian region, the maximum horizontal stress direction is not perpendicular to the trench, but parallel to plate motion (Cross and Freymueller, 2007, 2008; Lu and Wyss, 1996). Ruppert et al. (2011) computed stress directions from source parameters of crustal earthquakes in the central Aleutian Arc. Stress directions in the overriding plate differ from those computed for the subduction interface. 3. Review of pre-2006 crustal earthquakes Prior to expansion of the Aleutian seismic network at the end of the 1990s, earthquakes in the region were located and reported by

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56 N

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utia

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Fig. 2. Focal mechanisms for shallow (depths less than 30 km) earthquakes from Global CMT catalog: grey — thrust, red — normal, and yellow — strike–slip faulting type. Forearc crustal blocks are shown by dashed polygons: A — Near block; B — Buldir block; C — Rat block; D — Delarof block; E — Andreanof block (Geist et al., 1988).

the National Earthquake Information Center (NEIC). Only larger events (magnitude 4 or greater) were located, and all had large location uncertainties due to the absence of recordings at regional distances. Some of these events were large enough (magnitude 5.5 or greater) to be identified by the Global CMT project (Dziewonski et al., 1981). This location record allows us to identify several crustal events in the region that occurred before the regional network expansion. The most notable are the earthquakes that occurred after the great megathrust events in the region, such as the magnitude MS 7.2 near Semisopochnoi Island in 1966 that followed the magnitude 8.7 1965 Rat Islands earthquake (Fig. 3), and a series of strong crustal shocks (up to magnitude 6.6) near Atka Island following the magnitude 8.0 1986 Adak earthquake (Fig. 4). The 4 July 1966 MS 7.2 earthquake was studied by Spence (1977) as part of the 1965 aftershock sequence. Stauder (1968) identified its focal mechanism as almost purely strike–slip with NS and EW striking nodal planes (Fig. 3). Based on the Rayleigh wave spectra Udias (1971) concluded that the event ruptured from north to south, parallel to the NS-striking fault plane. While Spence (1977) attributed this event to the faulting along the Rat/Delarof block boundary, later definition of the boundary by Geist et al. (1988) lies quite distant to the event location. Therefore, its location is along the volcanic axis, within the unrotated massif of the Bering plate. Spence (1977) concluded that this event occurred along a pre-existing zone of weakness in response to the Rat Island mainshock's underthrusting motion.

In 1986 several strike–slip events occurred as part of the MW 8.0 aftershock sequence, and were located about 150 km from the trench, near the volcanic axis and at shallow depths (Fig. 4; Boyd and Nabelek, 1988; Ekstrom and Engdahl, 1989). The largest events, magnitudes 6.4 and 6.5, occurred two days apart, on May 15 and 17, respectively. Boyd and Nabelek (1988) studied source parameters of the 1986 mainshock and its aftershocks. Based on the waveform inversion of body and surface waves they concluded that the MW 6.4 and 6.5 events occurred at shallow depths (10–14 km) on either NNW- or ENE-striking vertical planes. Due to complexity of the P-wave coda, directivity generated by the rupture propagation could not be unambiguously resolved. The authors relocated eight events in the source region using joint epicenter determination method, with the May 15 earthquake as the calibration event (PDE location). The relocated epicenters do not line up along one trend, but rather occupy a volume. Based on this weak evidence the authors concluded that these events occurred on an arc-parallel fault plane. Our interpretation of the May 1986 events differs from that of Boyd and Nabelek (1988). While the authors inverted the focal mechanism and centroid depth, they did not search for the centroid epicenter. The epicenter location was fixed to that determined by the single event location method using teleseismic arrivals (PDE catalog). Therefore, Boyd and Nabelek (1988) determined the location of initiation of the event. Global CMT, however, searches for the best centroid location over all three spatial parameters — latitude, longitude and depth — meaning the Global CMT solution reflects the location

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Fig. 3. Location map of the 4 July 1966 magnitude 7.2 earthquake (yellow star). Outline of the 1965 Rat Islands earthquake rupture is shown by red contour (Haeussler and Plafker, 1995). Crustal blocks of Geist et al. (1988) are shown by dashed lines. Red diamonds are locations of active volcanoes.

1986 rupture (Mw 8.0)

Andreanof Block

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Fig. 4. Location map of the May 1986 earthquakes (yellow stars — PDE locations of May aftershocks north of the 1986 rupture, white stars — Global CMT locations of the mainshocks and larger aftershocks). Outline of the 1986 magnitude 8.0 Adak earthquake rupture is shown by red contour (Haeussler and Plafker, 1995). Crustal blocks of Geist et al. (1988) are shown by dashed lines. Red diamonds are locations of active volcanoes.

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of the largest moment release. Following the methodology of Smith and Ekstrom (1997) we assume that the two points defining high frequency hypocenter and centroid hypocenter define the preferred fault plane. Therefore, we interpret NNW-striking nodal planes as the preferred faults of the magnitudes 6.4 and 6.5 May 1986 earthquakes west of Atka Island. On 18 March and 28 May 2002, two magnitude 4.3 earthquakes occurred in the vicinity of Great Sitkin volcano. Both events had characteristics of a typical tectonic mainshock–aftershock sequence (Pesicek et al., 2008). The P-wave first motion focal mechanisms for both events show strike–slip faulting, with SW and NW-trending focal planes. The May event and its aftershocks are located on the eastern flank of the volcano, 10–20 km away from the summit. Precise relocations reveal a seismicity trend consistent with the leftlateral strike–slip faulting on a NW-trending fault plane (Pesicek et al., 2008). While enough circumstantial evidence is presented to suggest that this sequence is associated with dike intrusion, without additional supporting data the authors note that “it is difficult to attribute this swarm conclusively to an intrusion”. This opens a door to interpreting this event as a regional tectonic earthquake that could have been triggered on a pre-existing fault or zone of weakness due to magmatic intrusion. The March event and its aftershocks are located offshore and west of the island, and suffer from larger location uncertainties. The relocations, however, indicate that the faulting occurred on a NW-trending focal plane (Pesicek et al., 2008). Another notable source of shallow crustal earthquakes on the volcanic axis can be found farther east in Unalaska Island region (Fig. 5). This area produced a magnitude MW 6.9 earthquake on 27 February 1987. Several magnitude 5–6 events were registered in this source volume between 1986 and 2008. Focal mechanisms of these events show normal faulting with EW-oriented extension. While this source region is located between two active volcanic centers — Okmok to the southwest and Makushin to the northeast — it appears that these earthquakes are not related to volcanic activity, being about 50 km distant from either volcanic center. Farther east in the Unimak Island region, a magnitude 6.4 earthquake occurred on 11 November 1993 (Fig. 6). It is located at a shallow depth and has a normal faulting mechanism similar to the Unalaska events. Thus, there is a history of magnitude 6+ crustal earthquakes located away from the Aleutian trench, on a volcanic axis, that are characterized by either strike–slip or normal faulting. Earlier interpretations of the strike–slip events that followed the 1965 and 1986 megathrust earthquakes indicate that these events occurred in response to “relaxation” after the megathrust earthquakes. However, similar crustal events in 2006 and 2008 did not follow any major megathrust earthquakes. No 170 W 54 30 N

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4. 14 June 2006 MW 6.4 earthquake in Rat Islands The 14 June 2006 magnitude MW 6.4 earthquake occurred just west of Kiska Island (Fig. 7) in the Rat Islands region. The largest aftershock, MW 6.0, occurred one half-hour after the mainshock. AEIC located nearly 1500 aftershocks in the first month of the sequence. Magnitude of completeness of the recorded sequence is estimated to be 2.5, with the frequency–magnitude b-value of 1.1. The nearest recording stations are the Little Sitkin network, located about 120 km east of the source region. The nearest station to the west is about 290 km away on Shemya Island. Given this station distribution, the aftershock locations have large uncertainties (average horizontal error is about 9 km with larger errors in the arc-normal direction). Double-difference relocation method (Waldhauser and Ellsworth, 2000) and waveform cross-correlation are used to improve aftershock locations. The velocity model is that used by AEIC for locating earthquakes in the Aleutian region, and is developed for the Shumagin Islands region of the eastern Aleutians (Hauksson, 1985, 52 30 N

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detailed reports are available for the normal faulting crustal earthquakes (such as in the Unimak Island and Unalaska) in the eastern Aleutian Arc.

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Fig. 6. Location map of the Unimak Island earthquakes (yellow stars — PDE locations of events with source parameters from Global CMT catalog, white circles — aftershocks of the 11 November 1993 magnitude 6.4 earthquake). Red diamonds are locations of active volcanoes.

AKUTAN

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ULIAGA KAGAMIL CLEVELAND

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y ra

Fig. 5. Location map of the Unalaska Island earthquakes (yellow stars — PDE locations of events with source parameters from Global CMT catalog, white circles — aftershocks of the 27 February 1987 magnitude 6.9 earthquake). Red diamonds are locations of active volcanoes.

Rat Block

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on ny a C

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Fig. 7. Location map of the 14 June 2006 earthquakes. Yellow stars — PDE locations, white stars — Global CMT locations, and red stars — double-difference locations of the mainshock and largest aftershock. Light grey circles are original AEIC catalog aftershock locations; black circles are the magnitude 3.0 and greater relocated events. Crustal blocks of Geist et al. (1988) are shown by dashed lines. Red diamonds are locations of active volcanoes. Yellow triangles are seismic stations.

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Table 1). Waveform cross-correlation is performed using dbcorrelate program written using Antelope, BRTT Inc., libraries and Datascope relational databases (D. Vonseggern, pers. comm., 2008). It performs time domain cross-correlation with a specified time window and lag time. Refinement of peak time is done via interpolation of 2nd degree polynomial. It has been shown (e.g., Menke and Schaff, 2004) that not only do the double-difference algorithms produce much better relative event locations, but that the absolute location errors are comparable, or even less, than those obtained with traditional methods. Ruppert et al. (2011) performed extensive tests of earthquake locations in the central Aleutian Islands while working on relocations of the 2008 Kasatochi volcano seismic swarm. They showed that even under limitations of station coverage in the region, the arc-parallel and arc-normal seismicity trends are preserved after double-difference relocations. We first relocate all events with magnitudes 3.0 or greater that occurred within one month after the mainshock. A magnitude 3.0 cutoff ensures reliable phase picks, while still providing enough events to delineate the fault plane. Next, we use these new locations to cross-correlate waveforms of events located within 5-km radius of each other based on station/channel pairs with picked P and S arrivals and using a 2-second time window. We calculate lag times for each station/event pair with cross-correlation coefficients of 0.6 and greater. Lastly, we relocate events once again using cross-correlated travel time lags. The final relocated dataset contains approximately 200 events (Fig. 7). The new locations have less EW-direction spread and are in better agreement with the NE striking fault plane, defining a 40-km long linear trend. Relocated depths vary between 3 and 17 km, with the mainshock located at 4 km. Therefore, we interpret this event as a left-lateral strike–slip event on a NE-trending fault plane. Proximity of this earthquake to the boundary between the Buldir crustal block to the west and Rat block to the east suggests a possible relation to the block's rotation, with the Buldir block moving to the southwest and the Rat block moving to the northeast.

farther south than the CMT and PDE locations. Taking location errors into account (~ 17 km in horizontal error for PDE, ~10 km for both the CMT and double-difference location), all events fall within close proximity to the northern boundary of the Buldir block. The relocated seismicity trend is in better agreement with the WNW–ESE striking focal plane. The location and focal mechanism are consistent with this event occurring along the northern boundary of the Buldir block and unrotated Bering massif. 6. 14 and 15 April 2008 MW 6.4 and 6.6 earthquakes in Andreanof Islands The 14–15 April 2008 magnitude MW 6.4 and 6.6 earthquakes occurred in close proximity to the Gareloi volcano (Fig. 9). The AEIC located nearly 600 aftershocks in the first month after the mainshocks. Estimated magnitude of completeness of the aftershock catalog is 2.1, with frequency–magnitude b-value of 0.7. While this sequence occurred close to the Gareloi seismic network, only one station from the network was working at the time. Semisopochnoi and Little Sitkin networks located to the west were out for most of the time as well, recording only a handful of aftershocks, thus leaving Tanaga (~50 km away) and Kanaga (~ 100 km away) to the east as the primary contributors to the aftershock locations. As a result, raw catalogs aftershock locations occupy a large volume and have high location uncertainties (mean horizontal error ~12 km), making it impossible to determine the preferred fault plane. We use double-difference and waveform cross-correlation to relocate magnitude 3.0 and greater events in this sequence, using the previously described procedure. We thus obtained new locations for about 100 aftershocks, with aftershock depths ranging down to 15 km and the mainshocks' depths located at about 5 km. The relocated seismicity forms a 35-km long NS trend, which is consistent with orientation of the SSE–NNW trending focal plane (Fig. 9). We interpret these earthquakes as a left-lateral strike–slip events located somewhat north of the defined boundary of the Delarof crustal block.

5. 27 June 2006 MW 6.2 earthquake in Rat Islands 7. 2 May 2008 MW 6.6 earthquake in Andreanof Islands The June 27, 2006, magnitude MW 6.2 earthquake occurred southeast of Buldir Island, and about 100 km west of the MW 6.4 June 14, 2006 earthquake's epicenter (Fig. 8). It is located at a shallow depth and has a strike–slip focal mechanism. The nearest recording stations are located 150 km to the west on Shemya Island and 170 km to the east on Little Sitkin volcano. AEIC located only 20 aftershocks, magnitudes 2.1 to 4.1, all with large location uncertainties (up to 20 km horizontal and up to 30 km vertical errors). We relocated all recorded aftershocks and the mainshock using the procedure described in the previous section. Fifteen events were successfully relocated, with the depths varying between 16 and 20 km. The relocated events are 52 30 N

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The 2 May 2008 magnitude MW 6.6 earthquake occurred between Tanaga Volcano to the west and Kanaga Volcano to the east, with the nearest seismic stations located just 20 km away (Fig. 10). It is once again a shallow earthquake with a strike–slip focal mechanism located on the volcanic axis. This is the best recorded sequence among the 2006 and 2008 crustal strike–slip events. AEIC located over 1500 aftershocks during one month after the mainshock. Magnitude of completeness of the aftershock catalog is 1.4, with b-value of 0.8. Two off-fault

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ra y M

ur

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Fig. 8. Location map of the 27 June 2006 earthquake. Yellow star — PDE location, white star — Global CMT location, and red star — double-difference location of the mainshock. Light grey circles are original AEIC catalog aftershock locations; black circles are relocated events. Crustal blocks of Geist et al. (1988) are shown by dashed lines.

180

Delarof Block 179 W

178 W

Fig. 9. Location map of the April 2008 earthquakes. Yellow stars — PDE locations, white stars — Global CMT locations, and red stars — double-difference locations of the magnitude 6.4 and 6.6 events. Light grey circles are original AEIC catalog aftershock locations; black circles are magnitude 3.0 and greater relocated events. Crustal blocks of Geist et al. (1988) are shown by dashed lines. Red diamonds are locations of active volcanoes. Yellow triangles are seismic stations.

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52 00 N

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176 W

Fig. 10. Location map of the May 2006 earthquakes. Yellow stars — PDE locations, white stars — Global CMT locations, and red stars — double-difference locations of the mainshock and largest aftershock. Light grey circles are original AEIC catalog aftershock locations; black circles are magnitude 2.5 and greater relocated events. Crustal blocks of Geist et al. (1988) are shown by dashed lines. Red diamonds are locations of active volcanoes. Yellow triangles are seismic stations.

earthquake clusters appeared at about a 20-km distance both east and west of the main rupture immediately following the mainshock. To improve the event's location, we perform double-difference relocations and waveform correlations on a subset of events with magnitudes 2.5 and greater. About 90 events were relocated, defining a clear SSE– NNW seismicity trend down to 16-km depth, with the mainshock located at ~6 km. Some of the relocated events belong to the off-fault clusters. This sequence is located north of the Delarof crustal block. 8. 17 July 2010 MW 6.7 earthquake in Fox Islands The 17 July 2010 magnitude MW 6.7 earthquake occurred southeast of Umnak Island in the Fox Islands region. It produced a very energetic aftershock sequence, with over 4000 aftershocks located within the first month of the sequence, including 65 aftershocks with magnitudes 4.0 or greater. The largest aftershock was a magnitude MW 6.0 and occurred 14 h after the mainshock. A magnitude 4.0 foreshock occurred about 18 h prior to the MW 6.7 event. The mainshock and larger aftershocks are all characterized by normal faulting. Numerous aftershocks were felt in the community of Nikolski, located within 20–50 km of the source region, where broadband seismic site NIKO is located. Further northeast, Okmok and Makushin volcano seismic networks are located at about 100-km and 150-km distances, respectively. The nearest stations to the west are positioned about 300 km distant on Atka Island: broadband station ATKA and the Korovin volcano network. Only events with magnitudes of about 3 or greater were recorded with relatively clear phases on westerly stations. Magnitude of completeness of the recorded aftershock sequence is 2.1, with b-value of 0.9. We use only magnitude 3.0 and greater events occurring within the first month of the sequence for the double-difference relocation and waveform cross-correlation. The relocated events (nearly 900) define a SE–NW trending zone about 50-km long and 30-km wide (Fig. 11). From the cross-section we define a WSW-dipping nodal plane as the preferred fault plane. The mainshock and the largest aftershock are located on this plane, while the majority of the aftershocks are located in an above hanging wall, mostly in the upper 20 km of the crust. 9. Discussion Slip partitioning in response to oblique subduction has been recognized as an important component of subduction tectonics for some time (e.g., Fitch, 1972). These forces may play a more significant role in the regions of rapid subduction and high obliquity, especially where coupling between the overriding and subducting plates is high (Beck, 1991; McCaffrey, 1992). The partitioning may manifest

52 30 N M6.7

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distance, km Fig. 11. Location map of the 18 July 2010 earthquake. Yellow stars — PDE locations, white stars — Global CMT locations, and red stars — double-difference locations of the mainshock and the largest aftershock. White circles are original AEIC catalog aftershock locations; dark grey circles are magnitude 3.0 and greater relocated events. Red diamonds are locations of active volcanoes. Yellow triangles are seismic stations.

itself as a well-developed strike–slip fault or series of faults parallel to the trench (e.g., in Sumatra McCaffrey, 1992; Sieh and Natawidjaja, 2000; in Philippines Barrier et al., 1991; in Central America Carr, 1976; Manton, 1987; Marshall et al., 2000; in New Zealand Cashman et al., 1992) or as a series of strike–slip or normal faults that are orthogonal or near-orthogonal to the trench (e.g., in Nicaragua — La Femina et al., 2002; in southern Banda Arc — McCaffrey, 1988). In most subduction zones only a narrow zone of anastomosing or en echelon faults is present, usually occurring along the arc. The magmatic arc is characterized by high thermal gradients, decreased lithospheric thickness and increased crustal thickness. Therefore, it is the weakest portion of the overriding plate and is most likely to accommodate crustal faulting due to slip partitioning (Beck, 1980; Jarrard, 1986). The Aleutian Arc represents different scenarios of slip partitioning. In the west, well-developed arc-parallel strike–slip faults are present (Geist et al., 1988; Scholl et al., 1987). No arc-parallel faults have been identified in the central Aleutians that could be accommodating the strike–slip component of the slip partitioning. Instead, as we showed in this study based on earthquake activity, in the central and eastern Aleutians arc-normal crustal faults develop along the volcanic axis about 150 km away from the trench. These faults are predominately strike–slip west of and normal east of ~ 174°W, respectively. The change in faulting between eastern and central Aleutians is not likely to be controlled by the interface coupling. These crustal earthquakes occur both in the regions of weak (e.g., Unalaska Island, Unimak Island region, off Atka Island, andoff Kiska Island) and strong coupling (2010 Fox Island event, 2008 earthquakes) (see Freymueller et al., 2008, for review of plate coupling variations along the arc). More likely, the nature of faulting is controlled by the change in degree of obliquity of the plate convergence. McCaffrey (1992) estimates that in the Aleutians, the strain partitioning between the subduction zone and crustal fault structures does not occur for obliquity less

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5/28/02 M4.3

Unrotated Massif (Bering Plate) 7/4/66 Ms7.2 6/14/06 M6.4

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Pacific plate Fig. 12. Schematic representation of tectonic context of crustal strike–slip earthquakes in the central Aleutian Arc. Locations of crustal blocks, sedimentary basins and selected volcanoes are indicated. The features are not to scale.

than about 20° to 40°. This distinction would place the central and western Aleutian Arc into a category of partitioned slip between the subduction and crustal faults. The crustal faults are more likely to form along the volcanic arc (Beck, 1980). Therefore, it is logical to assume that arc-normal left-lateral strike–slip faults in the central Aleutians accommodate slip partitioning. No slip partitioning is expected in the eastern Aleutian Arc. Instead, normal faulting there along the volcanic arc is indicative of arc extension. Arc-parallel extension of the Aleutians has been recognized previously based on geodetic deformation studies (Lallemant and Oldow, 2000). Strike–slip faulting in the central Aleutians resembles the nature of faulting in Nicaragua (La Femina et al., 2002). The crustal strike– slip earthquakes in Nicaragua are also located about 150 km from the trench and can reach M6+ in size. The authors suggest that these earthquakes accommodate bookshelf faulting (Mandl, 1987) on northeast-striking left-lateral faults. Bookshelf faulting is a type of Riedel shears. These slip surfaces are typically arranged en echelon and may have varying orientations depending on the stress conditions, material properties and stage of progressive shear zone formation (Drezen, 1991). It has been noted that in subduction zones these faults form at inclinations of between 10° and 30° to the direction of relative motion (Jackson, 1997). Therefore arc-normal strike–slip crustal faults in central Aleutian Arc may indicate early stages of shearing that accommodates slip partitioning and may later transform into a single arc-parallel through-going fault, such as in the western Aleutian Arc.

10. Conclusions We studied recent magnitude 6+ crustal earthquakes in the central and eastern Aleutian Arc. These earthquakes are located about 150 km away from the trench and have either strike–slip or normal faulting parameters. Two of the earthquakes are associated with faulting along the boundaries of rotating crustal blocks (Fig. 12). The 27 June 2006 magnitude 6.2 earthquake occurred along the northern boundary of the Buldir block and the 14 June 2006 magnitude 6.4 earthquake occurred along the boundary between the Buldir and Rat blocks. The remaining strike–slip crustal earthquakes (e.g., 1966, 1986, and 2008) occurred on NNW-striking faults in the unrotated part of the Bering massif. They may be manifestations of Riedel shearing in the region north of the blocks. Such shears are usually arranged en echelon, at inclinations between 10 and 30° to the direction of relative plate motion (Jackson, 1997). They represent early stages of shear zone formation and accommodate slip partitioning in the central Aleutian region. Normal faulting crustal earthquakes east of 174°W manifest extension of the arc in response to the arc

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