Geophysical Analysis of the 30 July 1972 Sitka, Alaska, Earthquake

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Research Paper THEMED ISSUE: Tectonic, Sedimentary, Volcanic, and Fluid Flow Processes along the Queen Charlotte–Fairweather Fault System and Surrounding Continental Margin

GEOSPHERE Geophysical analysis of the 30 July 1972 Sitka, Alaska, earthquake

GEOSPHERE, v. 16, no. 3 sequence https://doi.org/10.1130/GES02144.1 Juan A. Ochoa Chavez and Diane I. Doser Department of Geological Sciences, The University of Texas at El Paso, El Paso, Texas 79968, USA 8 figures; 1 table; 1 set of supplemental files

CORRESPONDENCE: jaochoachavez@miners nucleation and termination along the northern over an area of ~130,000 km2, causing moderate .utep.edu ABSTRACT Queen Charlotte fault. damage including cracked walls and fallen objects

CITATION: Ochoa Chavez, J.A., and Doser, D.I., 2020, The 1972 Mw 7.6 Sitka earthquake is the larg- (Stover and Coffman, 1993). It ruptured ~180 km Geophysical analysis of the 30 July 1972 Sitka, Alaska, est historical event along the southeastern Alaska (Schell and Ruff, 1989) along part of a seismic gap, earthquake sequence: Geosphere, v. 16, no. 3, p. 712– portion of the strike-slip Queen Charlotte fault, the ■■ INTRODUCTION recognized by Sykes (1971), located between the 722, https://doi.org/10.1130/GES02144.1. transform boundary between the Pacific and North epicenters of 1949 and 1958 earthquakes (Fig. 1). American plates. The fault is one of the fastest The Queen Charlotte fault (QCF) forms the off- Considering that the population of Sitka tripled Science Editor: Shanaka de Silva Guest Associate Editor: Daniel S. Brothers moving transform boundaries in the world, having shore plate boundary between the North American between 1970 and 2010 (State of Alaska, 2018) and accumulated enough slip since 1972 to produce an and Pacific plates from south of Haida Gwaii (Queen that urbanized areas also increased in population

Received 18 March 2019 event of comparable size in the near future. Thus, Charlotte Islands) to the Icy Bay region of south- in other parts of southeastern Alaska, damage Revision received 20 January 2020 understanding the controls on the rupture process eastern Alaska (Fig. 1). With slip rates exceeding caused by anticipated future earthquakes along Accepted 18 February 2020 of the 1972 mainshock is important for seismic haz- 50 mm/yr (Brothers et al., 2018; Brothers et al., the Queen Charlotte fault would be considerably ard assessment in Alaska. Following the mainshock, 2020), it is one of the fastest moving transform higher than in 1972. Published online 2 April 2020 the U.S. Geological Survey installed a network of faults in the world. This high slip rate led to seven Several previous studies (e.g., Schell and Ruff, portable seismographs that recorded over 200 earthquakes of M >7 along the entire offshore fault 1989; Doser and Lomas, 2000) focused on analysis

aftershocks. These locations were never published, system since 1927, including the 2012 (Mw 7.7) Haida of the 1972 mainshock, but only a short abstract

and the original seismograms and digital phase Gwaii, the 2013 (Mw 7.5) Craig, and the great (MS 8.1) (Page and Gawthrop, 1973) was published on data data were misplaced. However, we were able to 1949 Queen Charlotte earthquakes (Fig. 1). In addi- collected by a temporary seismograph deployment scan paper copies of the phase data, convert the tion, the onshore extension of the system, termed by U.S. Geological Survey personnel (Fig. 1); the

data to digital form, and successfully relocate 87 the Fairweather fault, generated the Mw 7.8 (Doser, seismograph recorded aftershocks for about two aftershocks. The relocations show two clusters of 2010) Fairweather earthquake in 1958 (Fig. 1). Unlike months following the 1972 mainshock. By combin- aftershocks along the Queen Charlotte fault, one the San Andreas transform fault system, slip along ing information from the 1972 aftershock survey ~40 km north of the mainshock epicenter and the the Queen Charlotte fault appears localized along as well as recent (1973–2015) and historical seis- other just south of the mainshock, both regions a single fault trace with differences in the material micity (1925–1972), (1) we investigate patterns of adjacent to portions of the fault that experienced properties of the oceanic and continental sides of seismicity along the Queen Charlotte fault system maximum moment release in 1972. Many of the the fault likely controlling the rupture behavior of following the 1972 mainshock, and (2) we assess northern aftershocks locate east of the Queen large earthquakes (e.g., Aderhold and Abercrom- the relation of this seismicity to structural varia- Charlotte fault. This pattern is similar to after- bie, 2015; Walton et al., 2015). Thus, study of large tions and possible segmentation along the Queen

shocks observed in the 2013 Mw = 7.5 Craig, Alaska earthquakes along the QCF allows an opportu- Charlotte fault. earthquake. Recent and pre-1971 (1925–1970) seis- nity to determine the material properties that are Because much of the Queen Charlotte fault micity indicates that the regions where aftershocks most influential in controlling rupture and the per- system is located offshore, recent bathymetric clustered remained active through time. Gravity, sistence of these features over time. studies (e.g., Balster-Gee et al., 2017; Brothers magnetic, and bathymetric anomalies suggest Our study focuses on seismicity associated with et al., 2018; Brothers et al., 2020) help to define that the structural variations in both the Pacific the segment of the Queen Charlotte fault that rup- changes in geometry along the fault system that

This paper is published under the terms of the and North American plates (e.g., age, density, tured during the Mw 7.6 30 July 1972 earthquake may control its rupture segmentation and moment CC‑BY-NC license. rock type, and thickness) play roles in rupture near the town of Sitka, Alaska (Fig. 1). It was felt release. We used results of these studies along with

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a combination of earthquake seismicity, gravity, and magnetic data to assist in our study of the 1972 rupture zone. 60˚N Icy

Bay 1 9 ■■ TECTONIC SETTING 5 8

M w Queen Charlotte Fault System

7 Yakutat F . 8 F d 1 Terrane n . The Queen Charlotte fault system is located in u 7 o S s s southeastern Alaska and extends offshore from s M T ro 7 2 C 58˚N F C 9 southern Haida Gwaii to the Icy Point area. This

1 h

a mostly dextral strike-slip fault system forms part of

t 1973 Ms 6.7 h the plate boundary between the Pacific and North a N m American plates. Because of these characteristics,

Sitka S it is similar to the San Andreas fault in California. t

r a However, unlike the San Andreas fault, slip along i

1972 Mw 7.6 t the Queen Charlotte fault is localized to a single fault where differences in the material properties between the Pacific and North American plates 56˚N Pacific Plate appear to control rupture behavior (Aderhold and Abercrombie, 2015; Walton et al., 2015). In addi- 2013 Mw 7.5 tion, the Queen Charlotte fault experienced six 137˚W 136˚W 135˚W M>7 earthquakes in the past 100 years, allowing us to better determine the persistence of fault seg-

Q mentation through the earthquake cycle. Chatham Strait Cross Sound C North American GPS studies (e.g., Elliott et al., 2010) indicate 58˚N F Plate that motion along the Queen Charlotte–Fairweather

54˚N FLCN Haida Gwaii fault system north of 56°N is translational. South EVAL of 56°N, varying degrees of transpressional motion IRM 1949 Mw 8.1 are observed in GPS data as well as earthquake MTE SIT Sitka 7˚N focal mechanisms (e.g., Ristau et al., 2007). At BBIKA BLUF 5 1972 2012 Mw 7.8 ~58°N, the Fairweather fault bends and almost par- WHAL 52 mm/yr allels the coast. The Fairweather fault is located at the eastern margin of the Yakutat terrane, which is ALEX QCF subducting beneath the North American plate at 100 km a similar velocity to the Pacific plate (Elliott et al., 52˚N 56˚N 2010). The southern boundary of the Yakutat terrane 140˚W 0˚W 138˚W 136˚W 134˚W 132˚W 13 is the Transition fault (Fig. 1). Geophysical studies (Gulick et al., 2007; Gulick et al., 2013) suggest that Figure 1. Map of study area. Notable earthquakes in the region are represented with stars. The inset map shows the in this region, plate motion is reorganizing, with location of the temporary stations that recorded aftershocks in 1972. Colored areas outlined by dashed lines represent rupture areas for the 1927 (Doser and Rodriguez, 2011), 2012 (Hobbs et al., 2015), 2013 (Holtkamp and Ruppert, 2015), the Transition fault now taking up some strike-slip 1949 (Bostwick, 1984), 1972 (Schell and Ruff, 1989), and 1958 (Doser, 2010) events. Small black triangle near Sitka is motion from the Queen Charlotte fault system. Mount Edgecumbe volcano. Arrow indicates motion of the Pacific plate from Kreemer et al. (2014). Abbreviations: However, focal mechanisms of earthquakes of the FF—Fairweather fault; QCF—Queen Charlotte fault; TF—Transition fault. 1973 Cross Sound sequence (Fig. 1) indicate shortening is also occurring along low-angle structures

located less than 20 km north of the Transition fault (Doser and Lomas, 2000). Glacier Bay National Park Alaska Brothers et al. (2020) analyzed multi-beam

FF bathymetry data to examine geomorphology along the Queen Charlotte fault. Their analysis suggested Icy Point a slip rate of 50–57 mm/yr over the past 12–17 k.y. Icy Gulf of Strait Alaska along a straight, narrow fault trace with few local step-overs. They used the trace of the fault to define Canada a small circle path for plate motion and computed Yakobi its Euler pole. Then they computed along-strike TF Sea Valley Chichagof Island 1927 obliquity variations using this new pole. They con- cluded that obliquity variations appeared to control Washington fault segmentation and development of asperities in M >7 earthquakes along the fault.

1972 Sitka Earthquake

Only a few studies focus on the 1972 earth-

1972 quake sequence. Schell and Ruff (1989) determined Baranof the source characteristics of the mainshock and Island its rupture process using accelerograms of the mainshock and teleseismic data. They calculated a

QCF moment magnitude of 7.6, depth extent of 0–10 km, average slip of 6 m, and an average stress drop of 100 bars. They estimated two zones of maxi- Chatham mum slip release, one located 0–40 km northwest Strait of the epicenter near Mount Edgecumbe (dashed

Baranof rectangles, Figs. 2 and 3), which is a Holocene vol- Fan cano (Brew, 1994), and the second at 60–90 km southeast of the epicenter. In addition, a strong pulse of moment release clearly observed on accelerograms of the earthquake (see Schell and Ruff, 1989) occurred near the northern terminus of rup- 2013 ture (dashed oval, Figs. 2 and 3). Doser and Lomas (2000) observed that a focal mechanism of a larger

aftershock of the 1972 sequence (4 August 1972, Mw = 5.8; strike = 167 ± 13, dip = 78 ± 8, rake = 178 ± 8) Baker Noyes suggested a small change in fault orientation may Canyon Fan occur at the southern end of the 1972 mainshock rupture zone. In a later study, Doser and Rodriguez (2011) relocated 16 historical events (occurring between 1919 and 1971) within the rupture zone of the 1972 earthquake. They also noted a lack of Figure 2. Bathymetry (modified from Balster-Gee et al., 2017) and fault rupture zones of the northern Queen Charlotte post-1972 seismicity within the regions of highest fault (indicated by solid lines east of fault). Dashed rectangles indicate regions of maximum moment release in 1972; dashed oval is portion of fault zone where a strong pulse of energy release occurred in 1972 (Schell and Ruff, 1989). The slip that could be real or could be related to poor open star is the 1972 epicenter. Abbreviations: FF—Fairweather fault; QCF—Queen Charlotte fault; TF—Transition fault. station coverage.

59˚N 59˚N

FF FF

1958 1958

1958 1958 58˚N 58˚N Figure 3. 1972 Sitka earthquake aftershocks 1973 1973 1927 1927 (A) before and (B) after relocation. Black cir- 1927 1927 cles in (A) indicate aftershocks that could FLCN FLCN not be relocated. Green circles in (A) indi- TF TF cate aftershocks that were relocated. Yellow EVAL EVAL triangles indicate seismograph stations. Red star is the 1972 mainshock. Dashed IRM IRM rectangles and oval as in Figure 2. Abbrevi- MTE SIT MTE SIT ations: FF—Fairweather fault; QCF—Queen Charlotte fault; TF—Transition fault. 57˚N 57˚N BIOBIKA BLUF BIOBIKA BLUF

1972 1972 WHAL WHAL

N QCF N QCF

ALEX ALEX

50 km 50 km 56˚N 1972 56˚N 1972 137˚W 136˚W 135˚W 134˚W 137˚W 136˚W 135˚W 134˚W

Other Significant (M >7) Earthquakes of the extend from northern Haida Gwaii to the southern during the earthquake; they attributed this rup- Queen Charlotte Fault Zone end of the 1972 rupture zone (Fig. 1). On the other ture to the differing mechanical properties of the hand, a surface-wave directivity study by Bostwick two sides of the fault. Aderhold and Abercrombie

The 1927 (MS 7.1) earthquake, located ~100 km (1984) suggests an ~265-km-long rupture zone that (2015) indicated that rupture in the mainshock was to the northwest of the Sitka earthquake, is the first would extend to only ~54.5°N. faster along the northern portion of the fault and large earthquake that was instrumentally recorded The 1958 Fairweather earthquake occurred that super-shear rupture could have occurred. In in this region. It had a strike-slip focal mechanism, ~200 km to the north-northwest of the 1972 Sitka addition, they suggested that the rheology of the and its bilateral rupture length was ~35 km (Doser event. Doser (2010) estimated its rupture length at North American plate appears to be more mafic and Lomas, 2000). This rupture length corresponds 260–370 km using body waveform modeling and than average continental crust, leading to main- to most of the gap between the extent of the 1958 the distribution of relocated aftershocks. This event shock rupture behavior more characteristic of an and the 1972 ruptures (Doser and Lomas, 2000). ruptured unilaterally from the mainshock epicenter oceanic transform fault. A tomographic study of The next large earthquake along the system northwards. Rupture to the south may have been the rupture zone by Walton et al. (2019) shows

was the 1949 Queen Charlotte earthquake (MS = impeded by the 1927 rupture zone (Fig. 1). that the velocities of the oceanic (Pacific) crust 8.1). Pure strike-slip motion was likely involved, but The last large event in the northern Queen Char- and mantle of the fault zone are 3%–11% slower

details of its rupture process are not well known. lotte fault region was the Mw 7.5 Craig earthquake than those of the continental (North American) side. Seismic radiation studies by Ben-Menahem (1978) on 5 January 2013. Analysis of GPS and regional They suggested that factors other than large dif- and aftershock location studies by Bostwick (1984) waveform data for the mainshock by Yue at al. ferences in the velocities of materials along the estimate a rupture length of ~495 km that would (2013) determined super-shear rupture occurred fault zone, such as fault zone damage or fault

smoothness, could play important roles in super- and 0.05 s for telemetered data recorded on micro- southeastern Alaska, in our relocation process. Due shear rupture. fiche (R. Page, 2015, written commun.). Over 250 to the reduced number of stations, the geometry of Holtkamp and Ruppert (2015) relocated the events were located during the nearly two months the station array with respect to the aftershocks and aftershocks of the Craig sequence, and their results of network operation. A summary of this prelimi- the fact that we only had the tabulated first motion revealed that a complex fault network was activated nary analysis is found in an abstract by Page and information, we were not able to determine focal in response to the mainshock. The distribution of Gawthrop (1973). The original seismograms and mechanisms or reliable focal depths. the aftershocks also shows that northern extent of digital phase data were then misplaced, and further rupture along the Queen Charlotte fault was near analysis remained unpublished. the southern limit of the 1972 rupture (Fig. 1). In 2015, we obtained paper copies of the ■■ RESULTS Walton et al. (2015) suggested that the northern original computer printouts of the phase data, extent of rupture in 2013 was controlled by several including P and S arrival times, phase amplitudes, Figure 3 shows the events before and after factors including the intersection of the Aja frac- and durations, as well as information on original the relocation process. From the data set of over ture zone with the Queen Charlotte fault, causing a locations (W. Ellsworth, 2015, written commun.). 285 earthquakes, we only successfully relocated 3 m.y. offset in the age of the Pacific plate along the We converted the arrival times to digital form by 87 events (see Fig. 3 and Supplemental Material fault and the end of flexure of the Pacific plate as scanning paper copies, saving them as images, and Item 1 [footnote 1]). This small number of relo- observed to the south. Bathymetry data (Brothers transforming the images to text files using optical cations is mainly due to the fact that many phase et al., 2018; Brothers et al., 2020) indicate that the character recognition software. We manually cor- residuals were higher than 0.5 s and consequently Queen Charlotte fault takes a series of three to five rected the text files for errors by comparing them were not used. steps and bends that define a series of pull-apart to the original computer printout and then refor- Before interpreting our relocations, we per- basins at the northern end of the 2013 rupture (see matted the files for use in relocation algorithms formed a number of tests to estimate our location Fig. 2), again suggesting structural complexity. with the help of a MATLAB script. errors (Fig. 4). We did this by perturbing the top four To the south of our study area, transpression We then relocated the earthquakes using a layers of the 1D velocity model by ± 0.1 km/s and across the Queen Charlotte fault increases. In 2012, double-difference algorithm (HYPODD, Waldhau- then comparing the relocations derived from these

this change in plate motion led to a Mw 7.8 thrust ser and Ellsworth, 2000) with the parameters and eight perturbed models to the relocations from the earthquake (e.g., Lay et al., 2013) that occurred two relocations given in Supplemental Material 21 and original model. For latitude, 98% of the epicenters months before the Craig mainshock near the west Supplemental Material 1, respectively. Because fall within 5 km (~0.05°); and for longitude, 94% fall coast of the Haida Gwaii (Fig. 1). It generated a we could not access the original seismograms, we within 5 km (0.08°). tsunami that affected at least 170 km of the coast could only use P and S arrival times in the algo- After the first five days of network operation line with maximum run-ups of over 7 m (Leonard rithm. To obtain more reliable relocations, we used (Fig. 5), few aftershocks were recorded south of and Bednarski, 2014). phases with an initial residual smaller than 0.5 s. We 57°N. This effect does not appear to be related used the one-dimensional (1D) velocity model of to the magnitude of the events or the number of Matumoto and Page (1969) (Table 1), the standard stations used in the locations. Written notes from ■■ DATA AND METHODS model used by the Alaska Earthquake Center for R. Page (2015) indicate the weather grew progres- sively worse over the time of the deployment, and About a week after the 1972 Sitka mainshock, access to seismographs in the southern portion the U.S. Geological Survey deployed a temporary TABE 1. EOCITY MODE of the network grew difficult. Figure 5 illustrates FOR SOTHERN AASA seismic network of eight portable, smoked-paper how the number of events decreased with time, seismographs on the landward side of the rupture Depth P‑ave velocity as is expected, over the first ten days of network km km/s zone (R. Page, 2015, written commun.). A week later, operation. After ten days, the number of relocated three additional semi-permanent seismographs 0–4 .3 events remained almost constant. 4–10 .6 were added to the network with signals teleme- Before relocation (Fig. 3A), aftershocks were 10–1 6.2 tered to Palmer, Alaska, for recording on microfilm mostly distributed in two regions, with the remain- 1 Supplemental Material. Supplemental Material 1 con- 1–20 6.9 tains relocated aftershocks of 30 July 1972 sequence. (R. Page, 2015, written commun.). Phase data for the 20–2 7.4 der scattered across the area. The first group of Supplemental Material 2 contains relocation param- aftershocks were handpicked from paper seismo- 2–33 7.7 events was located to the north of the mainshock eters used in double-difference algorithm (HYPODD). grams and microfiche, entered into digital form, and 33–47 7.9 epicenter and east of the Queen Charlotte fault near Please visit https://doi.org/10.1130/GEOS.S.12041772 used to determine locations. Picking accuracy was 47–6 8.1 its intersection with the Transition fault. The second or access the full-text article on www.gsapubs.org to Belo 6 8.3 view the Supplemental Material. estimated at 0.2 s for smoked-paper seismograms group was distributed along the Queen Charlotte

fault south of the mainshock epicenter. After the 400 00 relocation process, the general patterns of epicenters remain (Fig. 3B). The group of events north of 400 the mainshock epicenter still appears to be primar- 300 s t n

ily located east of the Queen Charlotte fault and to e 300 v E

form several linear bands (arrows, Fig. 6) with a f 200 o

strike similar to the Transition fault. r

e 200 b m u 100 N ■■ DISCUSSION 100

0 0 Seismicity ‑20 ‑10 0 10 20 ‑20 ‑10 0 10 20 atitude Residual km ongitude Residual km Relocated aftershocks appear to primarily Figure 4. Relocation errors. Residual histograms for (A) latitude (north-south) and (B) longitude (east-west) in km. occur outside of the regions of maximum moment release (Schell and Ruff, 1989) observed during the 1972 mainshock (dashed rectangles, Figs. 2 and 3). Note that the strongest pulse of energy release in the fault plane relative to the main Queen Charlotte the other near the mainshock epicenter, with an 1972 (Schell and Ruff, 1989) (dashed oval in Figs. 2 fault trace. ~25 km gap between the mainshock and the north- and 3) occurred at the southern end of the north- The total length of the 1972 aftershock zone is ern cluster of seismicity. Post-1972 seismicity is also ern cluster of aftershocks. The northern cluster ~160 km; this is slightly smaller than the rupture found at the southernmost end of the 1972 rupture. extends to the intersection of the Queen Charlotte length of 180 km estimated by Schell and Ruff Seismicity within most of the 1927 rupture zone fault with the Transition fault, suggesting that this (1989). It is possible that aftershocks continued to is limited to M ≤2 events, while there is abundant intersection may serve as a barrier to rupture. The the south but were not detected by enough stations seismicity within the 1958 Fairweather rupture zone. northwest-southeast lineations of aftershocks to be adequately located. The mainshock epicenter With the exception of the southernmost part within the north cluster (arrows, Fig. 6) also suggest also is located near the midpoint of the aftershock of the 1972 rupture zone, only events with M ≤5 activation of faults en echelon to the Transition fault, zone, consistent with bilateral rupture. occurred within <25 km of the Queen Charlotte consistent with the idea of Gulick et al. (2007) and In order to determine if the regions where Gulick et al. (2013) that the Transition fault may now maximum slip occurred in 1972 were seismi- be taking up a portion of strike-slip motion as plate cally quiescent before and after the mainshock, 7. motion reorganization occurs within this region. we examined background seismicity from 1919 Holtkamp and Ruppert (2015) also observed to 1972 relocated by Doser and Rodriguez (2010) off-fault aftershock clusters in the 2013 Craig using regional and teleseismic phase data (blue 7 aftershocks, including thrust events, which are circles, Fig. 6) and from 1973 to 2015 using net- not common along this part of the Queen Char- work phase data from the Alaska Earthquake Center lotte fault system, and strike-slip events with nodal (2016) (green circles, Fig. 6). We also examined the planes rotated by ~45° from the Queen Charlotte aftershock distribution of the 2013 Craig earthquake 6. atitude N fault. Unfortunately, we do not have enough (Holtkamp and Ruppert, 2015) (yellow circles, Fig. 6). first-motion data or any waveform information that Note that instrumental coverage of the region was

would be required to compute reliable focal mech- very poor prior to installation of the local network 6 anisms. This prevents us from determining whether in 1973, and coverage continues to be hampered 210 220 230 240 20 some of the north off-fault aftershocks could have by the lack of offshore seismographs. Julian Day, 1972

involved reverse or strike-slip faulting similar to Seismicity following the 1972 Sitka sequence Figure 5. Change in locations over time. The asterisk represents that observed in 2013. Body waveform modeling (Fig. 6, green circles) is similar to the pattern of the mainshock; open circles are original aftershock locations; and the plus symbols represent the relocated aftershocks. Note of a Mw 5.8 aftershock by Doser and Lomas (2000) aftershocks in 1972 (squares). Background seis- that it took approximately ten days after the mainshock for located at the southernmost end of the 1972 rup- micity continues to occur in the vicinity of two the temporary network to be installed and aftershocks to be ture zone also indicates a slight rotation (~7°) of aftershock clusters, one north of 57°N latitude and detected.

9N fault between 1919 and 1972 (blue circles, Fig. 6). The pre-1972 events of M >5 at the southernmost end of the 1972 rupture zone are located west of the Queen Charlotte fault in the region where Wal- FF ton et al. (2015) suggest the Aja fracture zone may serve as a barrier to rupture. Aftershocks of the 2013 Craig earthquake (yellow circles) suggest little overlap between the ruptures of the 1972 and 2013 earthquakes. In summary, these observations suggest that the regions of maximum slip in the 1972 mainshock were seismically quiescent both 198 before and after the mainshock. 8N

Free-Air Gravity 1927 We high-pass filtered free-air gravity resid- TF ual data (Sandwell and Smith, 2009) with a pass band starting at wavelengths of 120 km to com- pare the free-air anomalies to seismicity and fault structure. The resulting map (Fig. 7) shows that the Queen Charlotte fault is associated with high free-air anomaly values (red and pink), but these 7N highs are discontinuous. 1972 egend The 2013 Craig aftershock sequence (X symbols, Fig. 7) occurs along a free-air anomaly high M 2 that is 40–60 km wide and extends from 55.1°N to M 4 56.1°N. The anomaly extends along both sides of M 6 the fault zone but is two to three times as wide on CF Seismicity the eastern side of the fault. The termination of the 11/1972‑12/201 free-air anomaly high at 56.1°N occurs at the point where the Aja fracture zone intersects the Queen Seismicity before the 1972 mainshock Charlotte fault, a feature that Walton et al. (2015) 0 km indicate was a major barrier to northward rupture 6N N Seismicity 01/2013‑03/2013 in 2013. The free-air anomaly high associated with the 2013 aftershocks is paired with an anomaly low 1972 Relocations (~60 mGal) to the west of the Queen Charlotte fault. 2013 Using a combination of seismic, magnetic, sonar, 1972 Mainshock and gravity data (processed differently than in our study), Walton et al. (2015) noted that the flexure 138 137 136 13 134 133 of the Pacific plate south of 56°N produces the thicker sediment deposits that correlate with the Figure 6. Comparison of the 1972 aftershock relocations with background seismicity. The red star represents the mainshock; black squares are the relocated 1972 aftershocks; green circles are the background seismicity from November 1972 to Decem- offshore low. ber 2015 (not including the events of the 1972 aftershock sequence); and yellow circles are Craig earthquake aftershocks from The spatial distribution of the aftershocks of Holtkamp and Ruppert (2015). November 1972 to December 2015 seismicity was obtained from the Alaska Earthquake Center the 2013 Craig earthquake correlates well with the database (2016). Blue circles are relocations for events prior to 1972 (Doser and Rodriguez, 2011). Focal mechanism is for 4 August 1972 aftershock from Doser and Lomas (2000). Arrows indicate northwest-southeast alignment of some aftershock width of the free-air anomaly high, although a small clusters. Abbreviations: FF—Fairweather fault; QCF—Queen Charlotte fault; TF—Transition fault. cluster of aftershocks at 54°N is located to the west

F Legend F Mt. Edgecumbe Background seismicity (post 1972) near the QCF 1958 2013 Craig relocations

1972 Sitka mainshock 1973 58° N Large (M > 6) historical events 1927 1972 Sitka aftershock relocations TF mGal

Figure 7. Residual free-air gravity anomalies. Symbols indicate different groups of seis- 57° N micity as noted in the legend, and white lines indicate known fault traces. Dashed dark-blue lines indicate discontinuities in 1972 magnetic anomalies as shown in Figure 8. Solid black lines to the east of Queen Char- lotte–Fairweather fault system indicate fault rupture zones. Abbreviations: AFZ— Aja fracture zone; FF—Fairweather fault; QCF—Queen Charlotte fault; TF—Transi- tion fault. 56° N

AFZ

2013

0 30 60 Q 55° N C km F

140° W 138° W 136° W 134° W

of the Queen Charlotte fault on the negative free- At 56.5°N, the free-air anomaly high changes energy pulse in the 1972 mainshock (Figs. 2 and air anomaly. Holtkamp and Ruppert (2015) indicate strike, broadens (~50 km), and begins to span both 3). In addition, this bend lies just east of Mount thrust mechanisms were associated with this small sides of the fault (Fig. 7). This portion of the fault Edgecumbe (Fig. 7, black triangle). Brew (1994) cluster. Post-1972 background seismicity (plus sym- contains the 1972 epicenter and a cluster of after- suggested a localized perturbation in the stress bols) appears to surround the free-air anomaly low. shocks (asterisk symbols). Post-1972 background field must be occurring near Mount Edgecumbe in The southern 30 km of the 1972 rupture extends seismicity also occurs within this zone (Fig. 7). order to produce the observed Holocene volcanism. along a narrow (<10-km-wide) free-air anomaly There is a slight westward bend in the free- Multi-beam bathymetry data (Brothers et al., 2020) high along the Queen Charlotte fault. This area was air anomaly high at ~57°N, with a corresponding show a small (~10-km-long) pull-apart basin along one of the two regions of the fault that experienced ~5° change in its strike. This bend occurs near the Queen Charlotte fault at this latitude, indicating maximum moment release during the mainshock the northern edge of a zone maximum moment divergence is occurring. There is little change in (see Figs. 2 and 3). release and the region associated with the strong the strike or width of the free-air gravity anomaly

high from the northern end of the 1972 rupture zone indicates another region of similar water depth flysch and basalt assemblage locally intruded by (Fig. 7) through the northern end of the 1927 rup- occurs on both sides of the fault at this point. lower Miocene to upper Paleocene granites (Plafker ture zone to the termination of the high near Cross Brothers et al. (2020) suggest that fault obliq- et al., 1994). With the exception of the granitic Sound. Two free-air anomaly lows observed west of uity controls fault geometry, rupture segmentation, intrusions, this material may also be expected to the 1972 rupture zone (Fig. 7) indicate thicker sed- and asperity development along the Queen Char- have higher than average crustal densities giving iment is being deposited offshore in regions that lotte fault. They compute divergence (maximum rise to the free-air anomaly highs. The variation in appear to parallel the strike of the free-air anomaly of 5 mm/yr) along the southernmost portion of rock strength between the granitic intrusions and highs located to the east. Background seismicity the 1972 rupture zone; this divergence decreases surrounding terrane is also likely to influence fault occurs between the southern low (−30 mGal) that to near zero ~30 km south of the 1972 mainshock rupture (i.e., persistent fault asperities are not as strikes north-northwest and the smaller magnitude epicenter. This region of divergence corresponds likely to form in the weaker granitic material). (−20 mGal) northern low that strikes north-south well with the southern zone of maximum moment Regions where free-air anomaly highs occur on (Fig. 7). The large free-air anomaly high associated release (Fig. 2) in 1972. Immediately north of the both sides of the fault (Fig. 7) appear to correlate with the Yakutat terrane appears to intersect the 1972 mainshock, they compute an ~17-km-long with areas of shallower bathymetry on both sides Queen Charlotte fault near the northern end of the zone of convergence (maximum of 3 mm/yr). This of the fault (Fig. 2) and often with higher levels of 1927 rupture zone. This high is likely due to the roughly corresponds to the southern half of the aftershock and background seismicity. This sug- shallow depth of water over the Yakutat terrane. northern zone of maximum moment release in 1972 gests the rock types along both sides of the fault Free-air anomaly lows indicate sediment accu- (Fig. 2). Few aftershocks or background seismicity in these locations may be similar, but how this mulation on the Pacific plate may be related to fault have occurred in this region (Fig. 6). The north- influences observed seismicity patterns requires segmentation. Walton et al. (2015) suggested the ern 60 km of the 1972 rupture lie within a zone further investigation. free-air low associated with 2013 aftershock region of divergence (maximum of 3 mm/yr) (Brothers records Pacific plate flexure caused by oblique con- et al., 2020); this zone spans the northern region vergence along the western side of fault before it of aftershocks (Fig. 6). Obliquity decreases to near Magnetic Data moved northward into the strike-slip regime of the zero in the 1927 rupture zone (Brothers et al., 2020). northern fault. It is possible the lows associated An increase in convergence (maximum of 3 mm/ Magnetic data (Fig. 8) also indicate variations in with the 1972 and 1927 rupture zones record past yr) then occurs along the northernmost portion of structure within the Pacific plate that likely influence periods of plate flexure that would cause variations the fault between the 1927 and 1958 rupture zones. fault segmentation. The reduced to pole magnetic in the sediment thickness on the Pacific plate along data were provided by R. Saltus (2017, written the fault zone leading to variations in coupling commun.). The most distinct change in magnetic across the fault. Rock Type patterns is related to the Aja fracture zone, which juxtaposes two portions of the Pacific plate that dif- The association of free-air anomaly highs with fer in age by 3 m.y. Walton et al. (2015) previously Bathymetry and Fault Obliquity fault segmentation also indicates that density vari- noted this feature as controlling the end of the 1972 ations, related to changes in plate thickness or rock and 2013 rupture zones. Although Colpron and Nel- We observe a number of correlations between type, control rupture behavior along the fault zone. son (2011) do not indicate any major variation in the seismicity and fault morphology as revealed by A major change in North American plate geology age of the Pacific plate that forms the west side of the multi-beam bathymetry (Fig. 2). There are sim- occurs at 56°N where the Alexander terrane (to the Queen Charlotte fault between the Aja fracture ilar water depths on either side of the fault trace south) abuts the Chugach terrane (Plafker et al., 1994). zone and the Transition fault, several variations in extending ~20 km from the southern end of the Based on seismic velocity studies, the Alexander ter- the intensity of magnetic patterns within the Pacific 1972 rupture zone, roughly corresponding to one rane appears to be more mafic in composition than plate appear to align (Fig. 8, dark-blue dashed lines). of the zones of maximum moment release (dashed average continental crust and has a faster velocity One change in intensity patterns aligns with the epi- rectangle) in 1972. The 1972 mainshock epicentral than the young Pacific plate crust in the Craig after- center of the 1972 mainshock, and the other aligns region and a portion of the second zone of maxi- shock region (e.g., Walton et al., 2019). This faster, with the northern end of the 1972 rupture. In addi- mum moment release also are located in a region denser terrane could explain the wideness of the free- tion, these changes in magnetic field also appear to where similar water depths occur on either side air anomaly high associated with the east side of the correlate with edges of the free-air anomaly highs of the fault. Although there is little change in the Queen Charlotte fault in the 2013 aftershock zone. (Fig. 7). These variations could be related to age free-air gravity anomaly at the northern end of the The Chugach terrane, an accretionary complex in or compositional changes within the Pacific plate 1972 rupture zone (Fig. 7), the bathymetry (Fig. 2) southeastern Alaska, is characterized by a Cretaceous that affect both its density and magnetic properties.

■■ CONCLUSIONS

Legend We converted paper copies of P and S phase arrival times for aftershocks of the 30 July 1972 Mt. Edgecumbe Background seismicity Mw 7.6 Sitka earthquake in Alaska to digital format 1958 (post 1972) near the QCF and successfully relocated 87 aftershocks using a 2013 Craig relocations double-difference relocation algorithm. Relocated aftershocks concentrate in two clusters, one at the 1972 Sitka mainshock 58° N northern end of the rupture zone where significant 1973 Large (M > 6) historical moment release occurred during the mainshock events 1927 and one near the mainshock epicenter. Many after- 1972 Sitka aftershock T shocks of the northern cluster appear to be occurring F relocations east of the Queen Charlotte fault on features that Magnetic anomalies have a strike similar to that of the Transition fault. (nT) 851 The aftershock distribution suggests a rupture length of ~160 km compared to a rupture length of -834 180 km from waveform modeling studies by Schell 57° N and Ruff (1989). Background seismicity prior to and after 1972 indicates the regions where aftershocks occurred are seismically active at a low level, while 1972 very few earthquakes are located in regions experi- encing maximum moment release in 1972. Changes in the width and strike of free-air anomaly highs within the North American plate appear to correlate with the 1972 epicenter region, the ends of maximum moment release in 1972, and the southern end of the 1972 rupture. Regions where anomaly 56° N AFZ highs are found on both sides of the fault tend to be associated with higher levels of aftershock and background seismic activity. Three free-air anomaly lows west of the 1927 and 1972 rupture zones indicate regions of thicker sediment deposition. These may represent shallow basins created when the 2013 Pacific plate was in flexure due to shortening along the plate margin before it was translated northwest- ward along the Queen Charlotte fault to the region 55° N 0 30 60 Q where plate motion becomes translational. Walton C et al. (2015) have suggested this mechanism for km F the creation of the basin associated with an anomaly low located west of the 2013 Craig sequence. 138° W 136° W 134° W Anomaly highs likely reflect differences in rock types Figure 8. Reduced to pole magnetic anomalies (data from R. Saltus, 2017, written commun.). Discontinuities in magnetic within the North American plate, which is composed anomalies on the Pacific plate are indicated by dashed lines. Position of Aja fracture zone (AFZ) is from Walton et al. of a complex mixture of flysch and basalt intruded (2015). Unlabeled discontinuities are features we believe might be related to Pacific plate structures. Solid black lines to the east of Queen Charlotte–Fairweather fault system indicate fault rupture zones. Abbreviations: QCF—Queen Charlotte by younger granites. Regions where anomaly highs fault; TF—Transition fault. are found along both sides of the fault also appear to be associated with fewer changes in bathymetry across the fault and higher levels of aftershocks and

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