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

Sbar, Marc L.; Barazangi, Muawia; Dorman, James; Scholz, Christopher H.; Smith, Robert B.

doi:10.1130/0016-7606(1972)83[13:totisb]2.0.co;2

Cited by 81 publications

(12 citation statements)

References 28 publications

(35 reference statements)

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“…Sufficient data were available from nearby temporary seismog?aph stations for some of these sequences to determine the faulting mechanism. The mechanisms of the Danville earthquakes were strike slip, whereas shallow thrusting probably dominated the Blue Mountain Lake events and norreal faulting dominated swarms in the Flathead Lake region of northwestern Montana [Sbar et al, 1972a]. Further, seismic data from the Denver [Healy' et al, 1968] and Rangely [Raleigh et al, 1972] sequences are consistent with right lateral strike-slip motion.…”

mentioning

confidence: 79%

“…Swarms in continental areas. Naturally occurring earthquake swarms do occur in continental areas, for example, the swarm in the Santa Barbara Chanhel in 1968 [Sylvester et al, 1970], the Danville, California, earthquakes of 1970 [Lee et al, 1971], the Blue Mountain Lake, New York, swarm of 1971 [Sbar et al, 1972b], and some regions of the intermountain seismic belt of the western United States [Sbar et al, 1972a]. Sufficient data were available from nearby temporary seismog?aph stations for some of these sequences to determine the faulting mechanism.…”

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confidence: 99%

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Faulting mechanisms for two oceanic earthquake swarms

Tatham¹,

Savino²

1974

J. Geophys. Res.

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A study of the relative excitation of long‐period surface waves from earthquake swarms near the Delfin basin in the northern Gulf of California and near the Queen Charlotte Islands fault suggests that a prominent mechanism of faulting for these swarm‐type events was strike slip. Both swarms were apparently located on fracture zones near active spreading centers. Earlier studies have suggested that normal faulting may dominate oceanic earthquake swarms and that this type of faulting may be related to the occurrence of swarms. However, the present observations imply that the mechanisms for generating swarm‐type earthquakes are probably due to factors other than the nature of the faulting involved. The suggested faulting mechanisms were determined by an examination of the relative amplitudes of Love and Rayleigh waves with periods between 10 and 50 s for some 60 events, assumed to be shallow (h < 15 km), that occurred in the northern Gulf of California. These surface waves were recorded at a new very long period seismograph station at Albuquerque, New Mexico. Bathymetry and seismicity of the northern Gulf of California suggest that faulting is occurring at short spreading centers (dip‐slip faulting) and along transform faults (strike‐slip faulting). Theoretical radiation patterns and relative amplitudes of Love and Rayleigh waves determined for both of these fault mechanisms were compared with the observed surface wave amplitudes recorded at Albuquerque. This technique provides a means of discriminating between the two possible types of focal mechanisms. The observations suggest that strike slip, possibly with a small component of dip slip, was the dominant faulting mechanism for many of these swarm‐type events. Further support for this suggestion is provided by the observation of S‐P times at a local (Δ < 100 km) short‐period seismograph station, and thus the epicentral locations of these events are restricted to a portion of a mapped transform fault. This surface wave technique was also applied to an earthquake swarm that occurred near the Queen Charlotte Islands fault, and a similar conclusion was drawn.

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mentioning

confidence: 79%

mentioning

confidence: 99%

Faulting mechanisms for two oceanic earthquake swarms

Tatham¹,

Savino²

1974

J. Geophys. Res.

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“…Moreover, the gradual convergence to greater depths of hypocenters beneath the mountain ridge seems to provide evidence for a crustal root beneath the Ruwenzori Mountain. In contrast, earthquakes occur at shallower depths (<20 km) in other regions of the world experiencing extensional tectonics, such as the Basin and Range Province of the western United States [e.g., Sbar et al, 1972] and Iceland [e.g., Ward et al, 1969].…”

Section: Discussionmentioning

confidence: 99%

The seismicity of the Ruwenzori Region in Uganda

Maasha¹

1975

J. Geophys. Res.

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The Ruwenzori Mountain is a fault‐bounded, uplifted block of Precambrian rocks situated immediately north of the equator within the western branch of the rift system of East Africa. Understanding the structure and development of the Ruwenzori Mountain may yield additional insight into the evolution of the East African rift system as a whole. During June through mid‐September 1973 four portable seismographs were used in a microearthquake survey of the area around the Ruwenzori Mountain. A microearthquake reconnaissance of various districts of Uganda during the latter part of September 1973 showed the Ruwenzori region to be the most seismically active area in Uganda. In the Ruwenzori region the spatial trends of the microearthquakes are correlated with the major rift faults and volcanic zones. The seismicity is shown to extend beneath and transverse to the Ruwenzori Mountain block, where there is an indication of subcrustal seismicity (25–40 km). Elsewhere in the region studied, hypocentral depths ranged between 0 and 25 km. Composite focal mechanism solutions for events associated with the Bwamba, Ruimi‐Wasa, and Nyamwamba faults show dip slip motion along steeply dipping planes. The extensional axes trend east‐west. The motion tends to raise the mountain block relative to the surrounding country. A focal mechanism determined at the junction of the northern and southern Ruwenzori indicates that the northern portion is being uplifted with respect to the southern block.

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“…The Centennial tectonic belt and the Intermountain seismic belt (Sbar et al, 1972;Smith and Sbar, 1974), from Yellowstone southward, define the flanks of the parabolic wake of latest Quaternary faulting, uplift, and seismicity (Anders et al, 1989) associated with the eastward migration of the Yellowstone hotspot (e.g., Morgan, 1972;Iyer, 1984;Rodgers et al, 1990). The stress field associated with the migrating hotspot is superimposed upon the stress field associated with development of the Basin and Range province (Stickney and Bartholomew, 1987b;Rodgers et al, 1990;Parsons et al, 1998).…”

Section: Introductionmentioning

confidence: 98%

Late Quaternary paleoseismites: Syndepositional features and section restoration used to indicate paleoseismicity and stress-field orientations during faulting along the main Lima Reservoir Fault, southwestern Montana

Bartholomew¹,

Stickney²,

Wilde³

et al. 2002

Ancient Seismites

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Syndepositional features, interpreted as paleoseismites indicative of six late Quaternary surface ruptures, were exposed in a trench across the Lima Reservoir fault in the Centennial Valley of southwestern Montana. Younger events progressively displaced four loess deposits capped by paleosol, and overstepping unconformities. Older events are marked by paleo-sandblow vents, ejected sand, and a deformed stream-channel deposit. Fossils of horse (Equus) and northern pocket gopher (Thomomys talpoides), place a 120-10 ka range on the oldest event. Stratigraphy and soils suggest that it occurred during the last Pinedale interstade (ca. 20 ka).Line-length balancing and progressive retro-deformation of faults show that > 8.8 m of cumulative fault displacement with ~2 m of horizontal extension occurred since then. Orientation of faults, clastic dikes, and slip vectors, along with earthquake focal mechanism solutions, were used to determine stress-field orientations for the six events. Normal faulting associated with northeast-southwest Basin and Range extension produced three events with 4.7 m offset, a displacement rate of ~23.5 cm/k.y., and a recurrence interval of 6.7 k.y. Oblique-reverse faulting associated with east-northeast-west-southwest compression produced two events with 3.7 m offset, clastic dike injection along joints or joint-reactivated faults in arched areas above the oblique-reverse faults, a displacement rate of 18.5 cm/k.y., and a recurrence interval of ~10 k.y. Reverse faulting, associated with northsouth compression, produced clastic dike injection during one event with 0.4 m offset, a displacement rate of <2 cm/k.y., and a recurrence interval of >20 k.y. Bartholomew, M.J., Stickney, M.C., Wilde, E.M., and Dundas, R.G., 2002, Late Quaternary paleoseismites: Syndepositional features and section restoration used to indicate paleoseismicity and stress-field orientations during faulting along the main Lima Reservoir fault, southwestern Montana, in Ettensohn, F.R., Rast, N., and Brett, C.E., eds.,

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

Abstract: Between the above two areas of uplift and extension lies a region of complicated geology and seismicity.

Cited by 81 publications

References 28 publications

Faulting mechanisms for two oceanic earthquake swarms

Faulting mechanisms for two oceanic earthquake swarms

The seismicity of the Ruwenzori Region in Uganda

Late Quaternary paleoseismites: Syndepositional features and section restoration used to indicate paleoseismicity and stress-field orientations during faulting along the main Lima Reservoir Fault, southwestern Montana

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