Diane I. Doser

The transition from strike–slip to oblique subduction in southeastern Alaska from seismological studies

Abstract Body waveform modeling of 11 moderate to large earthquakes within southeastern Alaska has been incorporated with earthquake relocations and the results of previous seismicity studies to examine the transition from strike–slip to oblique subduction in southeastern Alaska. In the Sitka region of extreme southeastern Alaska, earthquakes indicate seismic slip is parallel to the direction of motion between North America and the Pacific plate. As the plate margin begins to bend to the west near Cross Sound and encounters the southeastern edge of the Yakutat Block, partitioning of seismic slip is evident both onshore and offshore. Although the largest earthquakes (magnitude>6.0) in this region have slip parallel to plate motion, most moderate sized events have slip vectors rotated clockwise from plate motion. Offshore, the 1973 Cross Sound sequence indicates that the southern end of the Transition zone is seismogenic, with the Pacific plate being thrust beneath the Yakutat block, while onshore, strike–slip faulting has occurred along the Fairweather, Border Ranges and Denali faults. In the St. Elias region, thrust faulting is occurring along and above the plate interface. Moderate sized events in the St. Elias show a mix of slip vector orientations. In the Pamplona zone slip vectors of offshore earthquakes and deformation directions determined onshore from GPS studies show a counter-clockwise rotation relative to plate motion, suggesting that a change in strain field occurs just west of the St. Elias region.

Seismicity in the Kingdom of Bhutan (1937-2003) : Evidence for crustal transcurrent deformation

We investigate moderate-sized historic to recent earthquakes in the Bhutan Himalaya spanning the years from 1937 to 2003. We find that few moderate-sized earthquakes occurred in the region during this time period. In order to better characterize the seismicity, we relocate all earthquakes and estimate focal mechanisms for events for which we have adequate data. We use first motion data for older events and waveform model digital seismograms for three earthquakes that occurred in 1980 (M w = 6.4), 1995 (M w = 5.4), and 2003 (M w = 5.4). For the modern events, we utilize three techniques for focal mechanism and depth determination: regional full waveform time domain modeling of the two moderate (M w = 5.4) events, and surface wave spectral method and body wave depth modeling approach for the M w = 6.4 event. We find that the first motion and digital data focal mechanisms are mostly strike-slip, with midcrustal to deep crustal depths. Although no recent great event has occurred along the main Himalayan thrust, the Indian plate is undergoing significant, midcrustal to deep crustal transcurrent deformation, likely due to oblique convergence of the Indian-Asian collision in this region. Unlike oceanic-continent oblique subduction, strike-slip and thrust partitioning occurs throughout the crust, not in distinct zones. The geological and geophysical nature of the Bhutan Himalaya appears significantly different than other Himalayan regions due to the strike-slip nature of many of the events, an out of sequence thrust fault, and the implications of deformation in the Shillong Plateau.

A methodology for probabilistic fault displacement hazard analysis (PFDHA)

We present a methodology for conducting a site-specific probabilistic analysis of fault displacement hazard. Two approaches are outlined. The first relates the occurrence of fault displacement at or near the ground surface to the occurrence of earthquakes in the same manner as is done in a standard probabilistic seismic hazard analysis (PSHA) for ground shaking. The methodology for this approach is taken directly from PSHA methodology with the ground-motion attenuation function replaced by a fault displacement attenuation function. In the second approach, the rate of displacement events and the distribution for fault displacement are derived directly from the characteristics of the faults or geologic features at the site of interest. The methodology for probabilistic fault displacement hazard analysis (PFDHA) was developed for a normal faulting environment and the probability distributions we present may have general application in similar tectonic regions. In addition, the general methodology is applicable to any region and we indicate the type of data needed to apply the methodology elsewhere.

Rift localization in suture-thickened crust: Evidence from bouguer gravity anomalies in northeastern Tanzania, East Africa

Abstract The tectonic framework of northeastern Tanzania consists of three major elements, the Archean Tanzania Craton, the Proterozoic Mozambique Belt, and a portion of the Eastern arm of the Cenozoic East African rift system, which developed within the Mozambique Belt adjacent to the craton margin. We present some 1500 new point gravity measurements from northeastern Tanzania that, when combined with existing gravity data, characterize further the gravity field over the craton-mobile belt suture, as well as over the rift structures. The primary new observation which can be drawn from our data compilation and analysis is that a short wavelength gravity low exists over the craton-mobile belt suture. In a previous study, it was shown that a gravity low also exists over the Tanzania Craton-Mozambique Belt suture to the north in Kenya, and thus there appears to be a consistent gravity low over this suture from at least 6°S to the northern termination of the craton at ∼3°N. Based on the observation that gravity lows over Precambrian sutures elsewhere commonly arise from thickened crust, the gravity low over the Tanzania Craton-Mozambique Belt suture can be attributed to a few (2–5) kilometers of crustal thickening within an area 100–200 km wide, an interpretation that is also supported by seismic studies of crustal structure in Kenya. The development of Cenozoic rift structures within proximity to suture-thickened crust in northeastern Tanzania and Kenya suggests that rifting in the Eastern arm of the East African rift system may have been localized by the presence of the suture-thickened crust.

Deep crustal earthquakes associated with continental rifts

Abstract Deep (> 20 km) crustal earthquakes have occurred within or along the margins of at least four continental rift zones. The largest of these deep crustal earthquakes (M ⩾ 5.0) have strike-slip or oblique-slip mechanisms with T-axes oriented similarly to those associated with shallow normal faulting within the rift zones. The majority of deep crustal earthquakes occur along the rift margins in regions that have cooler, thicker crust. Several deep crustal events, however, occur in regions of high heat flow. These regions also appear to be regions of high strain, a factor that could account for the observed depths. We believe the deep crustal earthquakes represent either the relative motion of rift zones with respect to adjacent stable regions or the propagation of rifting into stable regions.

A Study of Historic Earthquakes of the Prince William Sound, Alaska, Region

We have investigated the character of seismicity that occurred in the Prince William Sound/Cook Inlet region during ∼40 years prior to the 1964 Great Alaskan earthquake. We have relocated all events of magnitude > 5.5. Focal mechanisms have been determined from body-waveform modeling and first-motion analysis for the larger (generally magnitude > 6.3) events. The results suggest a number of similarities and differences between pre- and post-1964 mainshock seismicity. Similarities include the persistence of normal faulting at depths of 40 to 60 km within the subducted slab in both the Tazlina Glacier and Columbia Bay regions north of Prince William Sound and among deeper events within northern Cook Inlet. Differences include a higher level of shallow seismicity (<40 km depth) in the Upper Cook Inlet region and north of Cook Inlet prior to 1964, higher levels of seismicity within the subducting plate in the south-central and eastern Kenai Peninsula prior to 1964, and a lower level of seismicity in the offshore Prince William Sound region prior to 1964. Much of the lower plate seismicity for the past 70 years appears to cluster immediately downdip of the 1964 asperity. Two of the events in this study caused notable damage (Modified Mercalli intensity VII to VIII) in the Anchorage area. The first, in 1933 ( M w 6.9) appears to have occurred at ∼9 km depth along a strike-slip fault within northern Cook Inlet. The second, in 1954 ( M w 6.7) occurred within the subducting plate at ∼60 km depth beneath the northern Kenai Peninsula. Analysis of these events, as well as smaller events we have studied, will greatly improve models of seismic hazard for the Anchorage region. Manuscript received 18 September 2000.

Earthquakes in the Pamplona zone, Yakutat block, south central Alaska

The Pamplona zone is a region of complex deformation and moderate seismicity located within the Yakutat block, a region that has been relatively aseismic since a series of large (M>7.8) earthquakes in 1899. In 1970 a sequence of moderate to large sized earthquakes occurred within the Pamplona zone (largest event of Mw=6.7). Together with a Mw=6.1 event in 1958, these events are the only M≥5.5 events known to have occurred in the Pamplona region since 1900. Thus these events give important information on internal deformational processes within the Yakutat block. Waveform modeling of three earthquakes in April 1970, showed rupture complexity along low angle, thrust faults. Focal depths indicate that two of the events occurred above the Wrangell-Aleutian megathrust, while the largest event may have occurred on the megathrust. Events in 1958 and February 1970 indicate that deformation within the western Pamplona zone is occurring along high angle (>60°) faults with reverse-oblique motion. We believe the Pamplona spur, the easternmost part of the Pamplona zone, may have behaved as an asperity during the 1899 sequence. The location of the spur may be influenced by a north-south trending fault zone in the subducting Pacific plate that appears to be responsible for the 1987–1992 Gulf of Alaska sequence, occurring 50 to 200 km south of the Pamplona zone.

Strike-slip faulting in continental rifts: examples from Sabukia, East Africa (1928), and other regions

Abstract Information on the nature of faulting associated with large (magnitude ⩾ 5.5) earthquakes in continental rift zones is important to an understanding of rift evolution and development. A study of available earthquake source parameter information obtained from seismic waveform modeling suggests that many large earthquakes have large amounts of strike-slip motion, although they occur within extensional regimes. We illustrate this observation with an examination of the seismic waveforms of the 1928 Sabukia earthquake. Large vertical offsets were observed at the surface after this earthquake, but body waveform modeling shows that the earthquake was a double event and that nearly pure strike-slip faulting (rakes of −6°) occurred at depth in both events. A comparison of faulting during large earthquakes in east Africa, the Cordillera of the western United States, Tibet, and Lake Baikal shows that over a third of the earthquakes of magnitudes ⩾ 5.5 have large components of strike-slip motion (rakes of −150° to −180°, −30° to 30°, and 150° to 180°). This suggests that many of these earthquakes occur on faults that are oblique to the present-day extension direction.

Estimating Uncertainties for Geophysical Tomography

We present statistical and interval techniques for evaluating the uncertainties associated with geophysical tomographic inversion problems, including estimation of data errors, model errors, and total solution uncertainties. These techniques are applied to the inversion of traveltime data collected in a cross well seismic experiment. The inversion method uses the conjugate gradient technique, incorporating expert knowledge of data and model uncertainty to stabilize the solution. The technique produced smaller uncertainty than previous tomographic inversion of the data.

Extensional tectonics in northern Utah-southern Idaho, U.S.A., and the 1934 Hansel Valley sequence

Abstract The Hansel Valley-Pocatello Valley region of northern Utah and southern Idaho is one of the most seismically active areas of the eastern Basin and Range. Study of the seismicity of this region provides insight into the crustal processes taking place near the physiographic boundary between the Basin and Range and the Middle Rocky Mountains. This study presents source parameters determined for the March 12, 1934 Hansel Valley, Utah, mainshock and largest aftershock. These results are then integrated with studies of regional and local seismicity to determine better the tectonic processes currently active in the region.