Roger Hansen

Seismicity and seismotectonics of Norway and nearby continental shelf areas

The earthquake activity of Norway and nearby offshore areas is low to intermediate, with few events above magnitude 5. Recent significant improvements in instrumental coverage in parallel with a better utilization of older (including historical) data have shown that the seismicity in the south is predominantly confined to the coastal areas and to the Viking Graben, while from the northern North Sea to Svalbard the earthquakes in a broad sense follow the continental margin. Fifty-one focal mechanism solutions from these areas, about half of them new, reveal stress directions that clearly indicate a connection to the plate tectonic “ridge push” force, at least for the areas at a minimum distance from the continental margin. Along the margin, stress directions also indicate a possible connection to postglacial uplift as well as to lithospheric loading effects. A dominance of normal faulting on the landward side and reverse faulting on the oceanic side agrees with this interpretation. On a regional level, the seismicity in these areas correlate quite well with geologic features such as grabens, fault zones, fault complexes, fracture zones, and the margin itself, indicating that these structures act in a general sense as weakness zones in the presence of a regionally more stable stress field. In the northern North Sea, however, an area with quite anomalous stress orientations, with strike-slip faulting, is found in a region transitional between normal and reverse faulting. Most of the earthquake foci are confined to the presumably brittle parts of the crust, but many events are also located quite close to, and on both sides of, the Moho discontinuity.

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.

Application of wave‐theoretical seismoacoustic models to the interpretation of explosion and eruption tremor signals radiated by Pavlof volcano, Alaska

Tremor and explosion signals recorded on September 29 during the Fall 1996 Pavlof eruption are interpreted using video images, field observations, and seismic data. Waveform analysis of tremor and explosions provided estimates of the melts volcano-acoustic parameters and the magma conduit dimensions. Initial mass fractions of 0.25% water and 0.025% carbon dioxide in the melt can explain the resonance characteristics of the tremor and explosion pulses inferred from seismic data. The magma conduit is modeled as a three-section rectangular crack. We infer that the tremor-radiating region consists of the lowermost two sections, both with cross-sectional areas of ∼10 m2. The deeper section is 43 m long, with magma sound speed of 230 m/s, density of 2600 kg/m3, and viscosity of 1.0×106 Pa s. The section above it, defined by the water nucleation depth, is 64 m long, with magma sound speed of 91 m/s, density of 2000 kg/m3, and viscosity of 1.4×l06 Pa s. An average magma flow velocity of 1.2 m/s, with superposed random oscillations, acts as the tremor source. Explosions are postulated to occur in the uppermost part of the magma conduit after water comes out of solution. The explosion source region consists of a 15 m long section, with cross-sectional area of 20 m2, sound speed of 51 m/s, density of 1000 kg/m3, and viscosity of 1.5×103 Pa s. A burst pressure of 220 MPa at 14 m depth would generate an acoustic pulse whose amplitude and character match the observed signal. Waveform analysis of the explosion pulses shows that the explosive event may be preceded by a long-period fluid transient which may trigger the metastable magma-gas mixture. The modeling procedure illustrates the synergy of fluid dynamic, seismic, and acoustic models and data with geological and visual observations.

Waveform analysis of seismoacoustic signals radiated during the fall 1996 eruption of Pavlof Volcano, Alaska

Theoretical modeling of acoustic and seismic signals associated with the 1996 strombolian eruption of Pavlof volcano suggests that volcanic tremor at Pavlof originates in the deeper part of the magma conduit, and is generated by random fluid oscillations in the magma flow. Explosions are believed to occur in the shallower part of the magma conduit, and to be caused by the rapid and violent expansion of metastable magma-gas mixtures. The effect of increasing the exsolved quantities of H2O and CO2 gas with reduced pressure in the melt is to decrease the sound speed and density of the magma-gas mixture. This causes an acoustic decoupling of the upper and lower parts of the magma conduit. The reduced sound speed and density of the melt at shallow depths present a sharp impedance contrast, which strongly reflects acoustic energy originating at depth and traps it in the lower part of the magma conduit. Alternatively, acoustic energy originating from the upper part of the conduit remains trapped in the low-velocity region formed by the exsolved gas in the melt, and hence shallow explosions may preferentially couple into the atmosphere. Explosion signals may be triggered by an increased flow of melt at depth, and may be preceded and accompanied by vigorous mass flux transients.

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.

A Semiautomatic Calibration Method Applied to a Small-Aperture Alaskan Seismic Array

Small-to-medium aperture (25 km or less) seismic arrays are of great importance for event location and characterization. Effective use of these arrays requires calibration, preferably with large numbers of events, to account for local and distant structural and propagation effects. We implement a cross-correlation method as a semiautomatic procedure applicable to any small array, able to process thousands of events with several days of computer time on a Sun Blade 1000 workstation. We analyze a database of 1228 P and PcP arrivals recorded between 1997 and 1998 at a 19-element Alaskan seismic array. The arrivals are picked by the Prototype International Data Centre for events well located by the United States Geological Survey. Backazimuth and horizontal-velocity residuals are calculated for all events. Complicated geology beneath the elements and elevation differences among the array stations make static corrections necessary. We use 328 core phases (including PcP , PKiKP , PKP , PKKP ) to determine the static corrections. We present first-order structural interpretations of our calibration results, including a Moho discontinuity dipping to the north with a 10.5° dip angle and a strike of 109°. Our method allows for resolution of phase arrivals within 2–3 sec and has potential to be used as an automatic detector of primary and secondary seismic phases.

Active Tectonics of Interior Alaska: Seismicity, Gps Geodesy, and Local Geomorphology

Interior Alaska is an actively deforming landscape that is part of a broad zone of deformation extending hundreds of kilometers inboard of the convergent plate boundary. This chapter provides an overview of recent studies on the seismicity, tectonic geomorphology, and crustal motions from GPS measurements in interior Alaska and the surrounding regions. Although the combined data sets do not point to one single mechanism of deformation for interior Alaska, we suggest that the interactions between North America and two crustal blocks, the Bering and Wrangell, are responsible for the broad diffuse belt of deformation that characterizes this region. We propose that the deformation zone extending along the NNE-striking, left-lateral seismic zones north of the Denali fault and along the Susitna seismic zone south of the fault represents the eastern extent of the distributed boundary between the Bering block and the North American plate. Double-difference earthquake relocations document the fine details of seismic structures and help define the seismogenic part of the crust in interior Alaska. Frequency―magnitude and stress analysis provide insights into faulting parameters and earthquake behavior in the region. Stream channel morphologies and stream profile perturbations help define active surface deformation in interior Alaska. GPS measurements show that the relative motions across the region are different than what would be expected for a stable plate interior, and include the effects of multiple deformation sources. Results from the study show that three dominant types of structures characterize interior Alaska: right-lateral strike-slip faults, left-lateral strike-slip seismic zones, and thrust faults.