Natalia A. Ratchkovski

Inverse Kinematic and Forward Dynamic Models of the 2002 Denali Fault Earthquake, Alaska

We perform inverse kinematic and forward dynamic models of the M 7.9 2002 Denali fault, Alaska, earthquake to shed light on the rupture process and dynamics of this event, which took place on a geometrically complex fault system in central Alaska. We use a combination of local seismic and Global Positioning System (gps) data for our kinematic inversion and find that the slip distribution of this event is characterized by three major asperities on the Denali fault. The rupture nucleated on the Susitna Glacier thrust fault, and after a pause, propagated onto the strike-slip Denali fault. Approximately 216 km to the east, the rupture abandoned the Denali fault in favor of the more southwesterly directed Totschunda fault. Three-dimensional dynamic models of this event indicate that the abandonment of the Denali fault for the Totschunda fault can be explained by the Totschunda fault’s more favorable orientation with respect to the local stress field. However, a uniform tectonic stress field cannot explain the complex slip pattern in this event. We also find that our dynamic models predict discontinuous rupture from the Denali to Totschunda fault segments. Such discontinuous rupture helps to qualitatively improve our kinematic inverse models. Two principal implications of our study are (1) a combination of inverse and forward modeling can bring insight into earthquake processes that are not possible with either technique alone, and (2) the stress field on geometrically complex fault systems is most likely not due to a uniform tectonic stress field that is resolved onto fault segments of different orientations; rather, other forms of stress heterogeneity must be invoked to explain the observed slip patterns.

New Constraints on Tectonics of Interior Alaska: Earthquake Locations, Source Mechanisms, and Stress Regime

We used the joint hypocenter determination (JHD) method to relocate 3611 crustal earthquakes that occurred from 1988 to 1999 in central Alaska. The new earthquake locations provide more details on the structure of the Kantishna cluster and better locations for the aftershock sequence of the 29 November 2000 M L 5.6 earthquake in the Minto Flats seismic zone. The JHD locations for the aftershocks of the 1995 M W 6.0 Minto Flats earthquake and 22 October 1996 M W 5.8 earthquake near the Denali fault are also available. A catalog of 196 fault-plane solutions consisting of the moment tensor solutions for the earthquakes with magnitude 4.0 or above and P -wave first-motion solutions for the earthquakes with magnitude 3.4 and above that occurred from 1988 to 2000 was composed. Moment tensor solutions were calculated using regional broadband data. This catalog was used to calculate principal stress orientations in the crust. The stress orientations change across central Alaska. In particular, the maximum principal stress orientation rotates clockwise from SE-NW to SSW-NNE direction as one moves from west to east. These stress orientations are consistent with the stress field transferred from the plate convergence in southern Alaska. In addition, we tested different velocity structures in the moment tensor inversion procedure to identify velocity models for calculating the Greens functions. The moment tensor inversion study shows that it is possible to obtain a reliable moment tensor solution for moderate-sized earthquakes ( M L ≥ 4) using three-component records from a single broadband station when the epicentral distances are between 50 and 300 km. Manuscript received 8 June 2001.

New Evidence for Segmentation of the Alaska Subduction Zone

The purpose of this article is to provide additional evidence for segmentation of the subducted plate in Alaska and to introduce a catalog of the relocated earthquakes for future studies. We used the Joint Hypocenter Determination method to relocate 14,099 subduction-zone earthquakes that occurred from July 1988 to July 1998 and were located between 58° N and 65° N latitude. The earthquake data were taken from the Alaska Earthquake Information Center catalog. The selected earthquakes were divided into 16 blocks on the basis of their hypocentral locations, and each block was relocated separately. Average epicenter shift was 3.8 km and average upward and downward depth shifts were 4.1 and 4.4 km, respectively (roughly the same number of earthquakes shifted upward [47%] and downward [53%]). The overall change with respect to the initial locations is that the seismicity became more compact, revealing details about the fine structure of the Wadati-Benioff zone. In particular, we were able to identify more precisely the boundary between the Kenai and McKinley blocks of the subducting plate. In addition, there is evidence for plate segmentation within the McKinley block. Manuscript received 15 November 2000.

Seismotectonics of the Central Denali Fault, Alaska, and the 2002 Denali Fault Earthquake Sequence

In this article we analyze the spatial and temporal variations in the seismicity and stress state within the central Denali fault system, Alaska, before and during the 2002 Denali fault earthquake sequence. Seismicity for 30 years prior to the 2002 earthquake sequence along the Denali fault was very light, with an average of four events with magnitude M L ≥ 3 per year. We observe a significant increase in the seismicity rate prior to the M W 7.9 event of 3 November 2002 within its epicentral region, starting about 8 months before its occurrence. The majority of the aftershocks of the M W 7.9 event are located within the upper 11 km of the crust and form several persistent clusters with a few aseismic patches along the ruptured fault. The most active aftershock source is associated with the epicentral region of the earthquake. The overall b -value of the aftershock sequence is 0.96 with the highest b -values within the epicentral region. We estimate that it will take 14 years for the seismicity rate to drop back to the background level. The stress regime across the region varies in space and time. The inferred stress regime prior to the 2002 sequence is predominately strike slip. Along the central part of the rupture zone, the orientations of the least- and intermediate-stress axes are reversed after the 2002 earthquake sequence. The maximum compressive stresses along the Denali fault rotate clockwise by up to 35°; the greatest rotations occur in the area of the rupture step-over from the Denali to the Totschunda fault. The inferred stress regime after the 2002 sequence reflects an interchanging thrusting and strike-slip faulting along the ruptured fault. The thrust faulting is concentrated in the epicentral region of the M W 7.9 event and along the rupture segments showing the largest surface offsets.

Seismic Velocity Models for the Denali Fault Zone along the Richardson Highway, Alaska

Crustal-scale seismic-velocity models across the Denali fault zone along the Richardson Highway show a 50-km-thick crust, a near vertical fault trace, and a 5-km-wide damage zone associated with the fault near Trans-Alaska Pipeline Pump Station 10, which provided the closest strong ground motion recordings of the 2002 Denali fault earthquake. We compare models, derived from seismic reflection and refraction surveys acquired in 1986 and 1987, to laboratory measurements of seismic velocities for typical metamorphic rocks exposed along the profiles. Our model for the 1986 seismic reflection profile indicates a 5-km-wide low-velocity zone in the upper 1 km of the Denali fault zone, which we interpret as fault gouge. Deeper refractions from our 1987 line image a 40-km wide, 5-km-deep low-velocity zone along the Denali fault and nearby associated fault strands, which we attribute to a composite damage zone along several strands of the Denali fault zone and to the obliquity of the seismic line to the fault zone. Our velocity model and other geophysical data indicate a nearly vertical Denali fault zone to a depth of 30 km. Aftershocks of the 2002 Denali fault earthquake and our velocity model provide evidence for a flower structure along the fault zone consisting of faults dipping toward and truncated by the Denali fault. Wide-angle reflections indicate that the crustal thickness beneath the Denali fault is transitional between the 60-km-thick crust beneath the Alaska Range to the south, and the extended, 30-km-thick crust of the Yukon–Tanana terrane to the north. Online Material: Tables of locations for the tact 1986 and tact 1987 lines.

Geophysical Data Reveal the Crustal Structure of the Alaska Range Orogen within the Aftershock Zone of the Mw 7.9 Denali Fault Earthquake

Geophysical information, including deep-crustal seismic reflection, magnetotelluric (MT), gravity, and magnetic data, cross the aftershock zone of the 3 November 2002 Mw 7.9 Denali fault earthquake. These data and aftershock seis- micity, jointly interpreted, reveal the crustal structure of the right-lateral-slip Denali fault and the eastern Alaska Range orogen, as well as the relationship between this structure and seismicity. North of the Denali fault, strong seismic reflections from within the Alaska Range orogen show features that dip as steeply as 25� north and extend downward to depths between 20 and 25 km. These reflections reveal crustal structures, probably ductile shear zones, that most likely formed during the Late Cretaceous, but these structures appear to be inactive, having produced little seis- micity during the past 20 years. Furthermore, seismic reflections mainly dip north, whereas alignments in aftershock hypocenters dip south. The Denali fault is nonre- flective, but modeling of MT, gravity, and magnetic data suggests that the Denali fault dips steeply to vertically. However, in an alternative structural model, the Denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the Alaska Range orogen. Modeling of MT data indicates a rock body, having low electrical resistivity (� 10 Xm), that lies mainly at depths greater than 10 km, directly beneath aftershocks of the Denali fault earthquake. The maxi- mum depth of aftershocks along the Denali fault is 10 km. This shallow depth may arise from a higher-than-normal geothermal gradient. Alternatively, the low electrical resistivity of deep rocks along the Denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the after- shock zone.

Sequence of strong intraplate earthquakes in the Kodiak Island Region, Alaska in 1999–2001

A sequence of strong earthquakes was registered in 1999–2001 in the Kodiak Island region of the Alaska-Aleutian subduction zone. Two Mw 7 earthquakes occurred in December, 1999 and January, 2001 and an Mw 6.5 event occurred in July, 2000. These events and their aftershocks recorded by the regional seismograph network were relocated using a Joint Hypocenter Determination (JHD) method. Regional broadband data were used to obtain seismic moment tensors for the main shocks and their largest aftershocks. Relocation and moment tensor inversion results indicate that these events originated inside the subducting Pacific plate. The focal mechanisms indicate down-dip tension with the fault planes being nearly vertical and parallel to the strike direction of the subducting plate. The 1999 and 2000 events were located down-dip of the locked portion of the megathrust, while the 2001 event was located directly beneath it.

Geophysical investigation of the Denali fault and Alaska Range orogen within the aftershock zone of the October-November 2002, M = 7.9 Denali fault earthquake

The aftershock zone of the 3 November 2002, M = 7.9 earthquake that ruptured along the right-slip Denali fault in south-central Alaska has been investigated by using gravity and magnetic, magnetotelluric, and deep-crustal, seismic reflection data as well as outcrop geology and earthquake seismology. Strong seismic reflections from within the Alaska Range orogen north of the Denali fault dip as steeply as 25°N and extend to depths as great as 20 km. These reflections outline a relict crustal architecture that in the past 20 yr has produced little seismicity. The Denali fault is nonreflective, probably because this fault dips steeply to vertical. The most intriguing finding from geophysical data is that earthquake aftershocks occurred above a rock body, with low electrical resistivity (>10 Ω·m), that is at depths below ∼10 km. Aftershocks of the Denali fault earthquake have mainly occurred shallower than 10 km. A high geothermal gradient may cause the shallow seismicity. Another possibility is that the low resistivity results from fluids, which could have played a role in locating the aftershock zone by reducing rock friction within the middle and lower crust.

Changes in Crustal Seismic Deformation Rates Associated with the 1964 Great Alaska Earthquake

We calculated seismic moment rates from crustal earthquake information for the upper Cook Inlet region, including Anchorage, Alaska, for the 30 yr prior to and 36 yr following the 1964 Great Alaska earthquake. Our results suggest over a factor of 1000 decrease in seismic moment rate (in units of dyne centimeters per year) following the 1964 mainshock. We used geologic information on structures within the Cook Inlet basin to estimate a regional geologic moment rate, assuming the structures extend to 30 km depth and have near-vertical dips. The geologic moment rates could underestimate the true rates by up to 70% since it is difficult determine the amount of horizontal offset that has occurred along many structures within the basin. Nevertheless, the geologic moment rate is only 3-7 times lower than the pre-1964 seismic moment rate, suggesting the 1964 mainshock has significantly slowed regional crustal deformation. If we compare the geologic moment rate to the post-1964 seismic moment rate, the moment rate deficit over the past 36 yr is equivalent to a moment magnitude 6.6-7.0 earthquake. These observed differences in moment rates highlight the difficulty in using seismicity in the decades following a large megathrust earthquake to adequately characterize long-term crustal deformation.

Seismological Aspects of the 2002 Denali Fault, Alaska, Earthquake

The M7.9 Denali fault earthquake occurred on 3 November 2002 with an epicenter located 135 km south of Fairbanks and 283 km north of Anchorage. This epicenter is 22–25 km east of the M6.7 Nenana Mountain earthquake that occurred eleven days earlier, October 23, awakening some inhabitants of central Alaska at about 3:30 that morning. Like most earthquakes of its size, the M7.9 earthquake was a complex event. The rupture began with vertical slip along a 40-km segment of the previously unrecognized Susitna Glacier thrust fault, which is connected to the Denali fault. The rupture continued with right-lateral horizontal slip along the main trace of the Denali fault. It eventually split off the Denali fault onto the more southeast-trending Totschunda fault. The total rupture length was 330–340 kilometers, with at least three areas of high slip, or high energy release.