Douglas H. Christensen

High resolution image of the subducted Pacific (?) plate beneath central Alaska, 50–150 km depth

Abstract A receiver function transect across the Alaska Range images the subducting Pacific plate at 50–150 km depth. Across a 200 km long array of 30 receivers, the largest observed P-to-S conversions come from the top of the subducting slab. This signal is coherent across the array and is strongly asymmetric, requiring a complicated interface at the top of the slab. Waveform inversion shows that the conversion is generated by a 11–22 km thick low velocity zone at the top of the slab, as much as 20% slower than the surrounding mantle. The velocity of this zone increases with increasing depth of the slab, approaching velocities of the mantle near 150 km depth. All intermediate depth earthquakes occur within the zone, along a plane dipping 5° steeper. The layer is too thick to represent metamorphosed oceanic crust, as proposed for other subduction zones. It may represent a thick serpentinized zone or, more likely, a thick exotic terrane subducting along with the Pacific plate.

Temporal variation of large intraplate earthquakes in coupled subduction zones

The focal mechanisms of intraplate earthquakes within subducting lithosphere are frequently used to infer large-scale stress regimes induced by slab-pull, bending or unbending, and lateral segmentation and undulations of the slab. Numerous studies have further postulated that the intraplate activity is influenced by transitory regional stress regimes such as those associated with interplate thrust events. Temporal variations of the latter type may potentially play an important role in assessing regions of uncertain seismic potential, and possibly even in earthquake forecasting. A systematic analysis of 1130 focal mechanisms for intraplate earthquakes with m_b ≥ 5.0 in the depth range 0–300 km is conducted for nine circum-Pacific subduction zones, all of which are known to have large interplate thrust events. The spatial and temporal relationships of the earthquakes within the subducting slab to the large thrust events in each region are appraised. The earthquake catalog assembled contains all published focal mechanisms, and is probably complete for m_b ≥ 6.5 for the years 1963–1986. For many of the localized regions considered in detail the catalog is complete to lower thresholds of m_b ≥ 6.0 or m_b ≥ 5.5. This analysis provides compelling evidence for a temporal link between large interplate thrust activity and intraplate seismicity. For the seismically coupled regions considered here, outer rise compressional events have occurred prior to several large thrust events or are associated with seismic gaps, while outer rise tensional events generally only follow interplate ruptures. In the intermediate depth range, large down-dip tensional events generally precede interplate thrusts, and are often concentrated at the down-dip edge of the coupled zone. A transition to down-dip compressional stress or diminished tensional activity at intermediate depth is observed after several large thrust events (e.g., 1960 Chile, 1974 Peru, 1957 Aleutian, 1971 New Britain). These examples support the notion that the intraplate stress environment responds viscoelastically to the temporally varying interplate stress regime. Assuming that this concept is correct, the seismic potential of several seismic gaps is considered on the basis of both outer rise and intermediate depth earthquake activity.

Tomographic imaging of the Alaska subduction zone

The Alaska subduction zone is characterized by the Pacific plate descending beneath the North American plate, causing abundant seismic activity in the crust and along the Wadati-Benioff zone down to a depth of approximately 200 km, We have used 142,908 P wave arrival times from 12,237 shallow- and intermediate-depth earthquakes recorded by the Alaska Earthquake Information Center jointly run by the Geophysical Institute, University of Alaska Fairbanks and U.S. Geological Survey in the period from January 1977 to November 1991, to investigate the three-dimensional (3-D) P wave velocity structure beneath central and southern Alaska. Travel times and ray paths are accurately calculated by using an efficient 3-D ray-tracing technique. The nonlinear tomographic problem is solved by iteratively conducting linear inversions, and the velocity structure and hypocentral locations are simultaneously determined. We conducted two types of inversions. One is an inversion with a laterally homogeneous starting model. The others are what we call slab inversions in which we introduce into the initial model the high-velocity subducting Pacific plate as a priori information. We found that the slab inversions gave a seismologically more plausible result and a final root-mean-square travel time residual significantly smaller than that of the inversion with the homogeneous starting model. Detailed P wave tomographic images are obtained for the crust and upper mantle down to a depth of 200 km with spatial resolutions of 30–60 km. The tomographic image of the upper crust correlates well with the major surface geological features, such as slow sedimentary basins and fast ultramafic bodies. Prominent low-velocity anomalies exist in the crust and upper mantle beneath active volcanoes. In the mantle wedge the low-velocity anomalies dip toward the continental side and extend to a depth of about 150 km, which are considered to be associated with the active volcanism in the Alaska subduction zone. The results suggest that the subducting Pacific plate has a thickness of 45–55 km and a P wave velocity 3–6% higher than that of the surrounding mantle.

The Rupture Process and Tectonic Implications of the Great 1964 Prince William Sound Earthquake

We have determined the rupture history of the March 28, 1964, Prince Williams Sound earthquake (Mw=9.2) from long-period WWSSNP-wave seismograms. Source time functions determined from the long-periodP waves indicate two major pulses of moment release. The first and largest moment pulse has a duration of approximately 100 seconds with a relatively smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture. The second smaller pulse of moment release starts at approximately 160 seconds after the origin time and has a duration of roughly 40 seconds. Because of the large size of this event and thus a deficiency of on-scale, digitizableP-wave seismograms, it is impossible to uniquely invert for the location of moment release. However, if we assume a rupture direction based on the aftershock distribution and the results of surface wave directivity studies we are able to locate the spatial distribution of moment along the length of the fault. The first moment pulse most likely initiated near the epicenter at the northeastern down-dip edge of the aftershock area and then spread over the fault surface in a semi-circular fashion until the full width of the fault was activated. The rupture then extended toward the southwest approximately 300 km (Ruff andKanamori, 1983). The second moment pulse was located in the vicinity of Kodiak Island, starting at ∼500 km southwest of the epicenter and extending to about 600 km. Although the aftershock area extends southwest past the second moment pulse by at least 100 km, the moment release remained low. We interpret the 1964 Prince William Sound earthquake as a multiple asperity rupture with a very large dominant asperity in the epicentral region and a second major, but smaller, asperity in the Kodiak Island region.The zone that ruptured in the 1964 earthquake is segmented into two regions corresponding to the two regions of concentrated moment release. Historical earthquake data suggest that these segments behaved independently during previous events. The Kodiak Island region appears to rupture more frequently with previous events occurring in 1900, 1854, 1844, and 1792. In contrast, the Prince William Sound region has much longer recurrence intervals on the order of 400–1000 years.

Rupture process of the February 4, 1965, Rat Islands Earthquake

The great Rat Islands underthrusting earthquake (Mw = 8.7), of February 4, 1965, represents subduction of the Pacific plate beneath the North American plate along a 600-km segment of the western end of the Aleutian Islands. Body wave inversion techniques are used to determine the spatial and temporal heterogeneities associated with the Rat Islands earthquake. We have deconvolved World-Wide Standard Seismograph Network long-period teleseismic P wave seismograms to obtain source time functions. Directivity associated with the three major pulses of moment release in the source time functions indicates a total source duration of 160 s, unilateral rupture in the direction 300°, fault length of 420 km, and average rupture velocity of 2.5 km/s. The three pulses of moment release are located along the fault, and these regions of high moment release are interpreted as asperities. The first asperity extends from the epicenter to 100 km to the WNW. This is the largest asperity and corresponds to a smooth pulse of moment release in the source time function with a duration of 50 s. The second pulse of moment release is very jagged, is less coherent between stations, and is centered ∼200 km WNW of the epicenter. The third pulse of moment release extends from 360 to 420 km WNW of the epicenter. Although the aftershock area is ∼600 km in length, we can not resolve any moment release from the P waves beyond ∼420 km WNW of the epicenter. The Rat Islands event was closely followed by a large tensional outer-rise event on March 30, 1965, (Ms = 7.5), which is located oceanward of the largest moment release associated with the Rat Islands mainshock rupture. Detailed analysis of the P waves for this large outer-rise event confine the depth extent to the upper 30–5 km of the crust. The spatial and temporal association between the February 4 mainshock and the March 30 tensional outer-rise event suggests the tensional event may have been triggered by the large displacement near the mainshock epicenter. The overriding plate along the western Aleutian subduction zone is laterally segmented into a series of rigid tectonic blocks separated by fault controlled canyons and extensional basins (Geist et al., 1988). We suggest that the central undeformed parts of the blocks have the strongest coupling with the down-going plate and hence are the sites of the largest moment release during an underthrusting earthquake. The three asperities determined from the P waves correspond to the Rat, Buldir, and Near tectonic blocks respectively. Hence the P wave seismic moment release of the Rat Islands earthquake is controlled by the lateral segmentation of the overriding plate.

Crustal thickness variation in south-central Alaska

Crustal thicknesses have been determined by receiver function analysis of broadband teleseismic waveforms recorded during the Broadband Experiment Across the Alaska Range (BEAAR). Typical crust beneath the northern lowlands is 26 km thick, while beneath the mountains it is 35–45 km thick. The transition from thick to thin crust coincides with the location of the Hines Creek fault, a major tectonostratigraphic boundary. Crustal thicknesses determined by receiver functions agree with those predicted from topography assuming Airy type isostasy, suggesting that the Alaska Range is compensated by its crustal root. North of the range, however, the crust is systematically thinner than predicted by simple Airy isostasy. A crustal density contrast of 4.6% across the Hines Creek fault, 2700 kg m−3 to the north and 2830 kg m−3 to the south, explains the observed difference between the crustal thicknesses predicted by simple Airy isostasy, and the crustal thicknesses determined by receiver function analysis.

Alaska Megathrust 2: Imaging the megathrust zone and Yakutat/Pacific plate interface in the Alaska subduction zone

We image the slab underneath a 450 km long transect of the Alaska subduction zone to investigate (1) the geometry and velocity structure of the downgoing plate and their relationship to slab seismicity and (2) the interplate coupled zone where the great 1964 earthquake (Mw 9.2) exhibited the largest amount of rupture. The joint teleseismic migration of two array data sets based on receiver functions (RFs) reveals a prominent, shallow-dipping low-velocity layer at ~25–30 km depth in southern Alaska. Modeling of RF amplitudes suggests the existence of a thin layer (Vs of ~2.1–2.6 km/s) that is ~20–40% slower than underlying oceanic crustal velocities, and is sandwiched between the subducted slab and the overriding plate. The observed megathrust layer (with Vp/Vs of 1.9–2.3) may be due to a thick sediment input from the trench in combination with elevated pore fluid pressure in the channel. Our image also includes an unusually thick low-velocity crust subducting with a ~20° dip down to 130 km depth at ~200 km inland beneath central Alaska. The unusual nature of this subducted segment results from the subduction of the Yakutat terrane crust. Our imaged western edge of the Yakutat terrane aligns with the western end of a geodetically locked patch with high slip deficit, and coincides with the boundary of aftershock events from the 1964 earthquake. It appears that this sharp change in the nature of the downgoing plate could control the slip distribution of great earthquakes on this plate interface.

A determination of source properties of large intraplate earthquakes in Alaska

Historically, large and potentially hazardous earthquakes have occurred within the interior of Alaska. However, most have not been adequately studied using modern methods of waveform modeling. The 22 July 1937, 16 October 1947, and 7 April 1958 earthquakes are three of the largest events known to have occurred within central Alaska (Ms=7.3,Ms=7.2 andMs=7.3, respectively). We analyzed teleseismic body waves to gain information about the focal parameters of these events. In order to deconvolve the source time functions from teleseismic records, we first attempted to improve upon the published focal mechanisms for each event. Synthetic seismograms were computed for different source parameters, using the reflectivity method. A search was completed which compared the hand-digitized data with a suite of synthetic traces covering the complete parameter space of strike, dip, and slip direction. In this way, the focal mechanism showing the maximum correlation between the observed and calculated traces was found. Source time functions, i.e., the moment release as a function of time, were then deconvolved from teleseismic records for the three historical earthquakes, using the focal mechanisms which best fit the data. From these deconvolutions, we also recovered the depth of the events and their seismic moments. The earthquakes were all found to have a shallow foci, with depths of less than 10 km.The 1937 earthquake occurred within a northeast-southwest band of seismicity termed the Salcha seismic zone (SSZ). We confirm the previously published focal mechanism, indicating strike-slip faulting, with one focal plane parallel to the SSZ which was interpreted as the fault plane. Assuming a unilateral fault model and a reasonable rupture velocity of between 2 and 3 km/s, the 21 second rupture duration for this event indicates that all of the 65 km long SSZ may have ruptured during this event. The 1947 event, located to the south of the northwest-southeast trending Fairbanks seismic zone, was found to have a duration of about 11 seconds, thus indicating a rupture length of up to 30 km. The rupture duration of the 1958 earthquake, which occurred near the town of Huslia, approximately 400 km ENE of Fairbanks, was found to be about 9 seconds. This gives a rupture length consistent with the observed damage, an area of 16 km by 64 km.

Composite distribution inversion applied to crosshole tomography

Crosshole tomography requires solution of a mixed‐determined inverse problem and addition of a priori information in the form of auxiliary constraints to achieve a stable solution. Composite distribution inversion (CDI) constraints are developed by assuming parameters are drawn from a composite distribution consisting of both normally and uniformly distributed parameters. Nonanomalous parameter estimates are assumed to be Gaussian while anomalous parameters are assumed uniform. The resulting constraints are sensitive to anomaly volume and are an alternative to the usual constraints of minimizing L2 solution length or some measure of roughness. Damped least‐squares inversion, which minimizes solution length, distributes anomalous signal through poorly resolved areas to produce in attenuated and smoothed anomalies. Similar regularization methods, such as smoothness or flatness constraints, also degrade small spatial wavelength features and produce diffuse images of distinct anomalies. CDI constraints preser...

Preface to the Issue Dedicated to the 2002 Denali Fault Earthquake Sequence

On 3 November 2002, a M w 7.9 earthquake, the largest continental strike-slip earthquake in North America since the 1857 Fort Tejon, California, event, occurred in central Alaska. The earthquake began with reverse faulting on a ∼40-km extent of the previously unknown Susitna Glacier fault, but rupture transferred eastward to the right-lateral Denali fault and continued for over 200 km, finally transferring to rupture ∼70 km of the Totschunda fault. This large, complex event we term the Denali fault earthquake (dfe), after the major crustal fault that carried most of the displacement. The initiation of the rupture, the Susitna Glacier fault, is in a remote region of central Alaska that under normal circumstances is sparsely instrumented. On 23 October of that year, however, a large earthquake of M w 6.7, referred to as the Nenana Mountain earthquake (nme), occurred only 22 km to the west of the dfe epicenter. The nme, in hindsight recognized as a foreshock to the dfe, prompted deployment of a temporary network by the Alaska Earthquake Information Center (aeic). Hence, the area was under significantly enhanced seismic surveillance at the time of the dfe, 10 days later, which was further augmented by the addition of 19 more stations following the dfe mainshock. As a result, high-quality data were available in the near field, providing enhanced coverage for aftershock activity from the Susitna Glacier fault initiation point, along the Denali fault as far as the western portion of the Totschunda fault, to augment regional and teleseismic data for this sequence. As the rupture proceeded eastward, the Richardson Highway, one of the two north–south roads connecting the central and southern parts of the state, was disrupted where it crosses the Denali fault trace. Also significantly displaced was the Trans-Alaska Pipeline, operated by the Alyeska Pipeline …