During the summer of 1969, six portable seismographs were operated at 82 sites along the Intermountain Seismic Belt from southwestern Utah to northwestern Montana. This survey followed a well-known seismic belt along the eastern physiographic boundary of the Basin and Range province, and within the middle and northern Rocky Mountains. In general, the 120 microearthquakes located in this study follow the same spatial trend as the macroseismic earthquakes reported by the NOS (formerly USCGS). Most of the micro-earthquakes clustered in time and space along well-known fault zones on which late Tertiary or younger movements have occurred. All of the accurately located hypocenters occurred between the surface and a 20 km depth. Composite fault plane solutions along the Hurricane and Sevier fault zones (southwestern Utah), Tushar and Sevier fault zones (Marysvale area, Utah), and Wasatch and East Cache fault zones in central and northern Utah indicate vertical-motion on steeply dipping fault planes. These motions may be indicative of differential movements between the Basin and Range province and the Colorado Plateau-Rocky Mountains. Composite fault plane solutions (CFPS) in the Caribou Mountains, southeastern Idaho, and Flathead Lake area, northwestern Montana, show normal faulting on less steeply dipping planes and have west-northwest trending extensional axes. Swarm activity was also observed in the above two regions. Between the above two areas of uplift and extension lies a region of complicated geology and seismicity.

Tectonics of the Intermountain Seismic Belt, Western United States: Microearthquake Seismicity and Composite Fault Plane Solutions

This paper demonstrates that high-gain, high-frequency portable seismographs operated for short intervals can provide unique data on the details of the current tectonic activity in a very small area. Five high-frequency, high-gain seismographs were operated at 25 sites along the coast of northern California during the summer of 1968. Eighty per cent of 160 microearthquakes located in the Cape Mendocino area occurred at depths between 15 and 35 km in a well-defined, horizontal seismic layer. These depths are significantly greater than those reported for other areas along the San Andreas fault system in California. Many of the earthquakes of the Cape Mendocino area occurred in sequences that have approximately the same magnitude versus length of faulting characteristics as other California earthquakes. 

Consistent first-motion directions are recorded from microearthquakes located within suitably chosen subdivisions of the active area. Composite fault plane solutions indicate that right-lateral movement prevails on strike-slip faults that radiate from Cape Mendocino northwest toward the Gorda basin. This is evidence that the Gorda basin is undergoing internal deformation. Inland, east of Cape Mendocino, a significant component of thrust faulting prevails for all the composite fault plane solutions. Thrusting is predominant in the fault plane solution of the June 26 1968 earthquake located along the Gorda escarpement. In general, the pattern of slip is consistent with a north-south crustal shortening. 

The Gorda escarpment, the Mattole River Valley, and the 1906 fault break northwest of Shelter Cove define a sharp bend that forms a possible connection between the Mendocino escarpment and the San Andreas fault. The distribution of hypocenters, relative travel times of P waves, and focal mechanisms strongly indicate that the above three features are surface expressions of an important structural boundary. The sharp bend in this boundary, which is concave toward the southwest, would tend to lock the dextral slip along the San Andreas fault and thus cause the regional north-south compression observed at Cape Mendocino. 

The above conclusions support the hypothesis that dextral strike-slip motion along the San Andreas fault is currently being taken up by slip along the Mendocino escarpment as well as by slip along northwest trending faults in the Gorda basin.

Microearthquake seismicity and tectonics of Coastal Northern California

Low-level monitoring in the St. Elias region between September 1978 and March 1981 has identified two principal zones of seismicity that are consistent with the historical pattern of seismicity at higher magnitude levels. The most active zone follows the plate boundary along the coast of southeast Alaska. A recurrence relation based on completeness over varying time intervals is log( N ≧ M ) = −0.74 M + 3.77. A less active zone is observed generally following the Duke River and Dalton segments of the Denali fault system through the southwest Yukon. Instead of continuing into Chatham Strait, this trend of seismicity turns south into the Glacier Bay region to connect with the Fairweather fault north of Cross Sound. The seismicity and high rate of uplift observed in the Glacier Bay region is probably the result of convergence across the Fairweather fault north of Cross Sound. The recurrence relation for the Denali zone is log( N ≧ M ) = −0.90 M + 3.77. A similiar recurrence curve is found for activity with magnitudes down to about 2.0 observed over the 39-month low-level monitoring program. The rate of activity in the Denali zone appeared to decrease for a period of about 1 yr following the 28 February 1979, M 7.2 St. Elias earthquake. The rate of aftershock activity was also found to vary, with the b value decreasing throughout the sequence. Only minor seismicity is observed northeast of the Denali fault system in the southwest Yukon and east of Chatham Strait in northwest British Columbia. A prominent cluster east of Juneau near the British Columbia border may be related to volcanic activity. A 5-week microearthquake survey over a 40-km segment of the Denali fault system found activity in a highly faulted zone about 15 km wide at depths of less than about 15 km. A composite P -nodal solution indicates motion on either east-west or north-south striking planes, neither of which are consistent with the general trend of the Denali system in the area. An average slip rate of no more than about 1 mm/yr is estimated from the seismicity for the Denali fault system through the southwest Yukon. An estimate of the minimum present accumulated elastic strain suggests the potential for a magnitude 6 1 2 earthquake.

Seismicity in the St. Elias region of northwestern Canada and southeastern Alaska

Microseismicity, composite focal-mechanism solutions, and previously-published focal parameter data are used to determine the current tectonic activity of the prominent zone of seismicity in western Nevada and eastern California, termed the Nevada Seismic Zone. The microseismicity substantially agrees with the historic seismicity and delineates a narrow, major zone of activity that extends from Owens Valley, California, north past Dixie Valley, Nevada. Focal parameters indicate that a regional pattern of NW-SE tension exists for the western Basin and Range and is now producing crustal extension within the Nevada Seismic Zone. An eastward shift of the seismic zone along the Excelsior Mountains and left-lateral strike-slip faulting determined from a composite focal mechanism indicate transform-type faulting between Mono Lake and Pilot Mountain. Based on these results and other data, it is suggested that the Nevada Seismic Zone is caused by the interaction of a westward flow of mantle material beneath the Basin and Range Province with the boundary of the Sierra Nevada batholith.

Microseismicity and tectonics of the Nevada Seismic Zone

The first detailed microearthquake survey of a major part of the Denali fault was carried out during July and August of 1967. The microseismicity of the fault was sampled by high-gain portable seismographs at 22 sites between Mt. McKinley National Park and Haines, Alaska. The rates of very local microearthquake activity (S-P times less than 2 sec) varied from 0 to about 40 events per day. The average microseismicity was found to be about the same as that recently found for the San Andreas fault. This result is surprising because in the last 60 years only one small-magnitude teleseism has been located near the Denali fault east of Mt. McKinley National Park. The significant microearthquake activity on this section of the fault could not have been predicted on the basis of recent major earthquake activity. A new fault is suggested in the Dezadeash Lake region of the Denali fault zone based on a concentration of microearthquake activity as well as on geological and topographic evidence. The new fault provides a mechanically more acceptable trend for large transcurrent displacements by straightening the Denali fault zone near the junction of the Shakwak Valley and the Haines fault segments.

Gary Boucher

Papers

Microearthquake seismicity of the Denali Fault

Localized microearthquakes in the Denali Fault Zone