Normal Fault Mechanism Research Articles

The miocene Great Basin of Western North America as an extending back-arc region

A narrow continental volcanic arc has lain near the western margin of the North American continent since mid-Miocene time. Its south end began retreating northward 18 m.y. ago. The active remnant of this andesitic arc today is the Cascade Range of Oregon, Washington, and British Columbia. A large part of the Great Basin and region to its north was situated throughout this period first wholly, then partially, in a back-arc position relative to the arc. This region had, and still has, most of the characteristics of back-arc regions elsewhere in the world—a thinned, faulted and extended crust and lithosphere, limited development of a geophysical bilateral symmetry (a mid-to-late-Miocene symmetry axis is marked by narrow swarms of basaltic dikes with associated positive linear magnetic anomalies that are parallel to the contemporary trench), high heat flow, low Bouguer gravity and seismic wave velocity values, notable seismic attenuation in the mantle, and a fault plane solution record dominated by normal-fault mechanisms. This back-arc region differs somewhat from most others (those composed of oceanic crust) because of its continental setting and recent geologic history. Despite its name, the Great Basin stands high, nearly 2 km above sea level. This high elevation owes, in part, to the fact that extension and heating of the continental lithosphere are still underway, though not as a back-arc phenomenon. The present lithospheric stretching results from oblique extension near the transform boundary of the North American plate, the San Andreas fault system. The latest stretching has probably maintained the physical state of the lithosphere first developed during back-arc spreading.

Source processes of large earthquakes along the Xianshuihe fault in southwestern China

abstractThe Xianshuihe fault is one of the most active faults in southwestern China. Recently, three large earthquakes occurred along it in 1967 (Ms = 6.1), 1973 (Ms = 7.5), and 1981 (Ms = 6.8).The 1981 event occurred near the central portion of the fault zone. Modeling of the body and surface waves indicates pure left-lateral strike-slip motion on a vertical fault striking N40°W consistent with the surface trend of the Xianshuihe fault. Two major ruptures are suggested for this source, with a total moment of 1.3 ×1026 dyne-cm. The 1973 event occurred about 65 km northwest of the 1981 event and ruptured about 90 km bilaterally along the fault. The body-wave synthetics indicate three main ruptures during faulting within 43 sec, with a total moment of 1.8 ×1027 dyne-cm. The mechanisms are similar to the 1981 event, and the average slip is determined to be 3.8 m. The largest aftershock (Ms = 5.9) occurred 1 day after the main event with a normal-fault mechanism striking almost perpendicular to the surface breakage. The 1967 event occurred at the northwestern end of the fault zone, with a strike of N65°E. It had a nearly normalfault mechanism with a seismic moment of 4.5 ×1025 dyne-cm. The largest aftershock (Ms = 5.1) occurred 7 hr later with a similar focal mechanism.The primary faulting along the Xianshuihe fault is left-lateral strike-slip, but the normal faulting with strike direction about perpendicular to the Xianshuihe fault trace is common, especially in the northwestern segment. The faulting pattern in this region is consistent with the regional stress field caused by the India-Tibet collision. The normal event which is not on the major fault seems to have more frequent foreshocks and aftershocks than those on the main fault.

Thrust and extensional faulting under the Chilean coast: 1965, 1971 Aconcagua earthquakes

Summary. Earthquakes under coastal northern Chile, from 18° S to 33° S occur at two very different depths. The shallower interplate events have the typical thrust mechanisms associated with subduction of the Nazca Plate under South America. The deeper (close to 70 km) events have normal fault mechanisms with a tension axis approximately parallel to the dip of the downgoing slab. We study in detail a. couple of Ms∼ 7.5 events that took place about 60 km north of Valparaiso, Chile. The 1971 July 9 event was a shallow thrust interplate event at 40 km depth, while the 1965 March 28 earthquake had a normal fault mechanism and a depth of 72 km. We carry out a detailed analysis of long period P-and surface waves observed in the WWSSN network of stations in order to improve fault plane solutions and depth determinations. For the 1971 event we find a seismic moment of 5.6 ∼ 1027 dyne cm from both P-and surface waves. The source area for this event is relatively small compared to that of other underthrust events with similar moments. Assuming a circular fault shape and a rigidity of 7 ∼ 1011 dyne cm−2 we find a stress drop of 38 bar. The 1965 event is more complex; seismic moments of 1.8 and 1.0 ∼ 1027 dyne cm are found from surface and body waves respectively. The stress drop estimated from P-waves and a circular fault model is close to 91 bar. The existence of earthquakes at two different depths near the Chilean coast may be interpreted in terms of a double layered Benioff zone, as has been found in Honshu, Kuriles or central Peru. Unfortunately the hypo-central determinations of events in this region are not precise enough to separate the two layers. An alternative method to demonstrate the existence of a double Benioff zone is to look for compressional events associated with the upper layer of the down-going slab. A search among available fault plane solutions is ambiguous; three shallow coastal events are found in Stauder's catalogue with horizontal pressure axes normal to the trench, but they might be intra-continental plate events.

The Aleutian Arc: Tectonic blocks, episodic subduction, strain diffusion, and magma generation

A relocated earthquake sequence in the western Aleutian arc lasting from February 1965 through December 1968 is shown to map an episode of subduction of the Pacific plate. The February 4, 1965, Rat Island main shock (M/sub s/ + 8.1) was a complex rupture involving greatest underthrusting at lock zones beneath the Aleutian arc transverse canyons. The primary tectonic consequences of the main shock rupture were (1) the downslab diffusion of a compressional pulse, (2) the oceanward diffusion of an extensional pulse in the oceanic lithosphere, and (3) the slow rebound of the continental plate. The velocities of these strain pulses (actual plate motions) are determined by the viscosity of the constraining mantle material; observed strain velocities indicate a Newtonian mantle viscosity of 6 x 10/sup 19/ P. The extensional pulse that propagated oceanward is shown to determine the locations and normal-faulting mechanisms of the trench earthquakes of this sequence. It is inferred that the entire subducting oceanic lithosphere contains a sequence of near-vertical faults that strike parallel to the trench axis, reflecting former trench-related normal-faulting earthquakes. This condition is supported by considerable data and suggests a slabbing, escalatorlike descent mechanism for the upper 100 km of the subducting Pacificmore » plate. A working hypothesis is presented that relates the primaryfeatures of the typical volcanic arc and interarc basin to episodic downslab compressional pulses. This hypothesis involves a sequence of high-loading-rate mantle compression, viscoelastic rebound, pressure reduction, increased partial melt concentrations, and counterflow in a thin tabular zone above the subducting plate.« less

Seismicity and contemporary tectonics of the Helena, Montana area

abstract An earthquake survey conducted during 1973 in the Helena, Montana area indicates that there may be a possible relationship between the local seismicity and the nearby Marysville high heat-flow area. Earthquakes were located in two clusters northwest of Helena along a N50°W trend that is parallel to the Intermountain Seismic Belt. Focal depths were generally shallow with maximums of 17 km. The Marysville high heat-flow anomaly 30 km northwest of Helena lacked detectable earthquakes, but corresponded to an area of low P -wave velocity. Composite fault-plane solutions for the Helena region showed both strike-slip and normal fault mechanisms, but with consistent northeast-trending T axes. The occurrence of both types of faulting could be interpreted as a result of stress reorientation with depth or the result of a simple shear acting along a northwest-trending zone of pre-existing weakness. The trends of the T axes at Helena are intermediate to north-south trends at the Hebgen Lake-Yellowstone Park area, 200 km to the southeast, and east-west trending T axes at Flathead Lake, 150 km to the northwest. The rotation of T axes across western Montana probably reflects a reorientation of the regional stress field along the Intermountain Seismic Belt produced by the relative motion of the Northern Rocky Mountain subplate with respect to the North American plate.

Earthquake mechanisms and island arc tectonics in the Indonesian-Philippine region

abstract Forty-four new focal mechanisms from shallow-focus earthquakes in the Indonesian-Philippine region have been determined from P and S waves recorded on long-period instruments. The dominant modes of deformation are thrust and normal faulting rather than strike-slip faulting. Along the Sunda and Philippine arcs the most active zone of shallow focus activity occurs between the ocean trench and the line of active volcanoes. Mechanism solutions from earthquakes in this zone are all of the thrust type with the sense of motion on the shallower dipping of the two nodal planes consistent with underthrusting beneath the island arc. Seismic slip vectors strike in a northeasterly direction along the convex side of the western Sunda arc and strike in a westerly direction along the Philippine arc and its extension along the eastern margin of the Celebes Sea. Normal faulting mechanisms from one earthquake under the Java Trench and another under the Philippine Trench may result from extension in the upper surface of the lithosphere as it bends beneath the island arc. The curvature of the Sunda arc may have caused the inclined seismic zone beneath the arc to shoal toward the northwest. There is a correlation between negative isostatic gravity anomalies, maximum water depth in the trench and the length and shape of the seismic zone beneath the Sunda arc. Shoaling of the inclined seismic zone beneath the nearly linear Phillippine arc can be explained by a decrease in the slip rates from south to north along the arc. Complex regions such as western New Guinea, the Sulu Spur and lesser Philippine islands may include a number of small plates of lithosphere.