Robert McCaffrey

Oblique plate convergence, slip vectors, and forearc deformation

Slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis, suggesting that oblique convergence is taken up by partial decoupling. Decoupling means that a component of arc-parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench. Partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike-slip faults that are parallel to the trench and to each other and, starting with a force equilibrium condition, a relationship between the obliquity and the earthquake slip vector orientation is derived. Assuming that either fault slips when shear stress on it reaches a yield stress, oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle. For obliquity at or greater than this angle the stress on the strike-slip fault is large enough to start it slipping, and when both faults are active, the arc-parallel motion of the forearc deflects the slip vector back toward the trench-normal. If we assume that continued slip on either fault occurs at constant stress (but the two faults can be at different stresses), the slip vector will maintain a constant angle relative to the trench-normal even when obliquity is larger than the critical angle. This limiting angle of the slip vector, called ψmax (measured relative to the trench-normal), is simply the arcsine of the ratio of the shear forces resisting slip on the strike-slip and thrust faults. A consequence is that when the obliquity exceeds ψmax the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates. Slip vectors at the Java trench southwest of Sumatra show the relationship clearly with ψmax =20°±5°, while slip vectors at the Aleutian trench show the relationship less clearly with ψmax=25° to 45°. The greater angle at the Aleutian trench suggests that the upper plate is stronger in the Aleutian arc (relative to the thrust fault) than in the Sumatran arc, consistent with the Sumatran arc being continental and having a well-developed strike-slip fault while the Aleutian arc is oceanic and without a clear transcurrent fault. Slip vectors at the Philippine trench which, like Sumatra, has a large strike-slip fault inboard of it, tend to stay within 25° of the trench-normal when obliquity is as large as 50°. If obliquity exceeds ψmax and continues to increase along a subduction zone, the rate of motion of the forearc relative to the upper plate will vary with obliquity, in which case the forearc sliver should extend or contract parallel to the arc. From the geometry of modern island arcs, arc-parallel extension should be the more common and has been hypothesized for both Sumatra and the Aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations. From estimates of ψmax and the arc-parallel gradients in obliquity, arc-parallel strain rates are estimated to be 1 to 3×10−8/yr for the Sumatran forearc, 2 to 6×10−8/yr for the Aleutian forearc, and 0.3 to 3×10−8/yr for the Philippine forearc. Oblique convergence and subsequent arc-parallel extension, if accompanied by crustal thinning, may provide an important yet little appreciated mechanism for bringing high-grade metamorphic rocks to the surface of subduction complexes.

Subduction zone coupling and tectonic block rotations in the North Island, New Zealand

[1] The GPS velocity field in the North Island of New Zealand is dominated by the long-term tectonic rotation of the eastern North Island and elastic strain from stress buildup on the subduction zone thrust fault. We simultaneously invert GPS velocities, earthquake slip vectors, and geological fault slip rates in the North Island for the angular velocities of elastic crustal blocks and the spatially variable degree of coupling on faults separating the blocks. This approach allows us to estimate the distribution of interseismic coupling on the subduction zone interface beneath the North Island and the kinematics of the tectonic block rotations. In agreement with previous studies we find that the subduction zone interface beneath the southern North Island has a high slip rate deficit during the interseismic period, and the slip rate deficit decreases northward along the margin. Much of the North Island is rotating as several, distinct tectonic blocks (clockwise at 0.5-3.8 deg Myr -1 ) about nearby axes relative to the Australian Plate. This rotation accommodates much of the margin-parallel component of motion between the Pacific and Australian plates. On the basis of our estimation of the block kinematics we suggest that rotation of the eastern North Island occurs because of the southward increasing thickness of the subducting Hikurangi Plateau. These results have implications for our understanding of convergent margin plate boundary zones around the world, particularly with regard to our knowledge of mechanisms for rapid tectonic block rotations at convergent margins and the role of block rotations in the slip partitioning process.

Slip vectors and stretching of the Sumatran fore arc

Slip vectors from thrust earthquakes at the Java Trench southwest of Sumatra demonstrate that the Sumatran fore arc is not a single rigid plate that is translated to the northwest by oblique plate convergence. Instead they indicate arc-parallel stretching of the fore arc at a uniform strain rate of 3-4 x 10 -8 /yr. The northwestward motion of the fore arc relative to the upper plate (Southeast Asia) increases from near zero at the Sunda Strait to 45-60 mm/yr in northwest Sumatra and should result in variable slip rates on the Sumatran fault.

Crustal motion in Indonesia from Global Positioning System measurements

Terrestrial Reference Frame 2000. We compute poles of rotation for the Australia, Eurasia, and Pacific plates based on our analysis of the global GPS data. We find that regional tectonics is dominated by the interaction of four discrete, rotating blocks spanning significant areas of the Sunda Shelf, the South Banda arc, the Bird’s Head region of New Guinea, and East Sulawesi. The largest, the Sunda Shelf block (SSH), is estimated to be moving 6 ± 3 mm/yr SE relative to Eurasia. The South Banda block (SBB) rotates clockwise relative to both the SSH and Australia plate, resulting in 15 ± 8 mm/yr of motion across the Timor trough and 60 ± 3 mm/yr of shortening across the Flores Sea. Southern New Guinea forms part of the Australia plate from which the Bird’s Head block (BHB) moves rapidly WSW, subducting beneath the Seram trough. The East Sulawesi block rotates clockwise about a nearby axis with respect to the Sunda Shelf, thereby transferring east-west shortening between the Pacific and Eurasia plates into north-south shortening across the North Sulawesi trench. Except for the Sunda Shelf, the crustal blocks are all experiencing significant internal deformation. In this respect, crustal motion in those regions does not fit the microplate tectonics model. INDEX TERMS: 1206 Geodesy and Gravity: Crustal movements—interplate (8155); 1243 Geodesy and Gravity: Space geodetic surveys; 8150 Tectonophysics: Plate boundary—general (3040); 8158 Tectonophysics: Plate motions—present and recent (3040); 9320 Information Related to Geographic Region: Asia; KEYWORDS: crustal motion, Indonesia tectonics, GPS, current plate motions, Southeast Asia

Role of oblique convergence in the active deformation of the Himalayas and southern Tibet plateau

Noting similarities with subduction along curved oceanic trenches and using a simple block model, we show that radial vergence evident in earthquake slip vectors along the Himalayan deformation front, east-west extension on north-trending normal faults in the Himalayas and southern Tibet, and right-lateral strike slip on the Karakorum-Jiali fault zone can all result from basal shear caused by the Indian plate sliding obliquely beneath Tibet along a gently dipping, arcuate plate boundary. Within the framework of this mechanism, the normal faults in the Himalayas and southern Tibet are not proxies for the uplift history of Tibet. The distribution and style of the faults in the Himalayas and southern Tibet suggest that the basal drag from the underthrusting Indian lithosphere extends northward beneath most of southern Tibet.

Strain partitioning during oblique plate convergence in northern Sumatra : Geodetic and seismologic constraints and numerical modeling

Global Positioning System (GPS) measurements along the subduction zone of northern Sumatra (2°S to 3°N) reveal that the strain associated with the oblique convergence of the Australian plate with Eurasia is almost fully partitioned between trench-normal contraction within the forearc and trench-parallel shear strain within a few tens of kilometers of the Sumatran fault Kinematic analyses of interplate earthquake slip vectors provide slip rates on the Sumatran fault within a few millimeters per year of GPS and geologic rates, giving us more confidence in the use of slip vectors for inferring slip partitioning elsewhere. The inferred slip rate on the Sumatran fault is ∼1/3 less than the full margin parallel component of plate motion. An across-forearc rotation in the slip vectors suggests that the missing arc-parallel shear occurs seaward of the geodetic network, between the forearc islands and the trench. Simple finite element models are used to explore the conditions under which the change in the principal strain rate directions between the forearc and the arc region can occur. Modeling suggests that neither a preexisting strike-slip fault nor a zone of thermally induced lithospheric weakness in the overriding plate is needed for strain partitioning to occur. In general, forearc slivers form over the region of interplate coupling and are driven along strike by the basal shear. A volcanic arc can help the partitioning process by localizing the margin-parallel shear strain in the upper plate if its crust and mantle are weaker than its surroundings. Interplate slip vectors and geodetic results from Sumatra together suggest that the highest coupling on the plate boundary occurs beneath and seaward of the forearc islands, consistent with inferences about the rupture zones of great nineteenth century earthquakes there. The Sumatra example suggests that geodetic measurements of interseismic, margin-parallel shear strain at oblique convergent margins can be used to map the landward extent of the relatively high basal stress beneath the overriding plate if one can correct for strain localization caused by weak upper plate strike-slip faults.

Rotation and plate locking at the Southern Cascadia Subduction Zone

Global Positioning System vectors and surface tiltratesareinvertedsimultaneouslyfortherotationofwest- ernOregonandplate lockingonthesouthernCascadia sub- duction thrust fault. Plate locking appears to be largely oshore, consistent with earlier studies, and is sucient to allow occasional great earthquakes inferred from geology. Clockwise rotation ofmostofOregonaboutanearbypoleis likely driven by collapse of the Basin and Range and results in shortening in NW Washington State. The rotation pole liesalongtheOlympic-Wallowalineamentandexplainsthe predominanceofextensionsouthofthepoleandcontraction north of it.

Global frequency of magnitude 9 earthquakes

For decades seismologists have sought causal relationships between maximum earthquake sizes and other properties of subduction zones, with the underlying notion that some subduction zones may never produce a magnitude ∼9 or larger event. The 2004 Andaman M w - 9.2 earthquake called into question such ideas. Given multicentury return times of the greatest earthquakes, ignorance of those return times and our very limited observation span, I suggest that we cannot yet make such determinations. Present evidence cannot rule out that any subduction zone may produce a magnitude 9 or larger earthquake. Based on theoretical recurrence times, I estimate that one to three M9 earthquakes should occur globally per century, and the past half century with five M9 events reflects temporal clustering and not the long-term average.

Geodetic Observations of Interseismic Strain Segmentation at the Sumatra Subduction Zone

Deformation above the Sumatra subduction zone, revealed by Global Positioning System (GPS) geodetic surveys, shows nearly complete coupling of the forearc to the subducting plate south of 0.5°S and half as much to the north. The abrupt change in plate coupling coincides with the boundary between the rupture zones of the 1833 and 1861 (Mw>8) thrust earthquakes. The rupture boundary appears as an abrupt change in strain accumulation well into the interseismic cycle, suggesting that seismic segmentation is controlled by properties of the plate interface that persist through more than one earthquake cycle. Structural evidence indicates that differences in basal shear stress may be related to elevated pore pressure in the north.

Estimates of modern arc-parallel strain rates in fore arcs

Deflections of slip vectors of interplate thrust earthquakes from expected directions are used to estimate arc-parallel strain rates within the overriding plates at the world9s major convergent plate margins. Arc-parallel extension strain rates, between 10 −8 /yr and 10 −7 /yr and significant at 2 standard deviations, are observed in the fore arcs of the Aegean, Aleutian, Mariana, Sumatran, southern Kuriles, New Hebrides, Scotia, and southern Central American (lat 8° to 12°N) subduction zones and the Himalayas. Northern Chile (lat 17° to 31°S) and Central America (lat 11° to 18°N) show arc-parallel compression. Available geologic and geodetic estimates of fore-arc slip and strain rates agree within a factor of two with slip-vector estimates. Arc-parallel strain in fore arcs is rapid enough to produce geologically significant effects, such as unroofing of high-grade metamorphic rocks and disruption of transported fore-arc terranes. Fore arcs deform even where convergence is perpendicular to curved margins, demonstrating that head-on subduction can produce a three-dimensional strain field.