Kerry Sieh

Holocene activity of the San Andreas fault at Wallace Creek, California

Wallace Creek is an ephemeral stream in central California, the present channel of which displays an offset of 128 m along the San Andreas fault. Geological investigations have elucidated the relatively simple evolution of this channel and related landforms and deposits. This history requires that the average rate of slip along the San Andreas fault has been 33.9 ± 2.9 mm/yr for the past 3,700 yr and 35.8 + 5.4/−4.1 mm/yr for the past 13,250 yr. Small gullies near Wallace Creek record evidence for the amount of dextral slip during the past three great earthquakes. Slip during these great earthquakes ranged from ∼9.5 to 12.3 m. Using these values and the average rate of slip during the late Holocene, we estimate that the period of dormancy preceding each of the past 3 great earthquakes was between 240 and 450 yr. This is in marked contrast to the shorter intervals (∼150 yr) documented at sites 100 to 300 km to the southeast. These lengthy intervals suggest that a major portion of the San Andreas fault represented by the Wallace Creek site will not generate a great earthquake for at least another 100 yr. The slip rate determined at Wallace Creek enables us to argue, however, that rupture of a 90-km-long segment northwest of Wallace Creek, which sustained as much as 3.5 m of slip in 1857, is likely to generate a major earthquake by the turn of the century. In addition, we note that the long-term rates of slip at Wallace Creek are indistinguishable from maximum fault-slip rates estimated from geodetic data along the creeping segment of the fault farther north. These historical rates of slip along the creeping reach thus do represent the long-term—that is, millennial—average, and no appreciable elastic strain is accumulating there. Finally, we note that the Wallace Creek slip rate is appreciably lower than the average rate of slip (56 mm/yr) between the Pacific and North American plates determined for the interval of the past 3 m.y. The discrepancy is due principally to slippage along faults other than the San Andreas, but a slightly lower rate of plate motion during the Holocene epoch cannot be ruled out.

Frictional afterslip following the 2005 Nias-Simeulue earthquake, Sumatra

Continuously recording Global Positioning System stations near the 28 March 2005 rupture of the Sunda megathrust [moment magnitude (Mw) 8.7] show that the earthquake triggered aseismic frictional afterslip on the subduction megathrust, with a major fraction of this slip in the up-dip direction from the main rupture. Eleven months after the main shock, afterslip continues at rates several times the average interseismic rate, resulting in deformation equivalent to at least a Mw 8.2 earthquake. In general, along-strike variations in frictional behavior appear to persist over multiple earthquake cycles. Aftershocks cluster along the boundary between the region of coseismic slip and the up-dip creeping zone. We observe that the cumulative number of aftershocks increases linearly with postseismic displacements; this finding suggests that the temporal evolution of aftershocks is governed by afterslip.

Near-field investigations of the Landers earthquake sequence, April to July 1992.

The Landers earthquake, which had a moment magnitude (Mw) of 7.3, was the largest earthquake to strike the contiguous United States in 40 years. This earthquake resulted from the rupture of five major and many minor right-lateral faults near the southern end of the eastern California shear zone, just north of the San Andreas fault. Its Mw 6.1 preshock and Mw 6.2 aftershock had their own aftershocks and foreshocks. Surficial geological observations are consistent with local and far-field seismologic observations of the earthquake. Large surficial offsets (as great as 6 meters) and a relatively short rupture length (85 kilometers) are consistent with seismological calculations of a high stress drop (200 bars), which is in turn consistent with an apparently long recurrence interval for these faults.

Plate-boundary deformation associated with the great Sumatra-Andaman earthquake

The Sumatra–Andaman earthquake of 26 December 2004 is the first giant earthquake (moment magnitude Mw > 9.0) to have occurred since the advent of modern space-based geodesy and broadband seismology. It therefore provides an unprecedented opportunity to investigate the characteristics of one of these enormous and rare events. Here we report estimates of the ground displacement associated with this event, using near-field Global Positioning System (GPS) surveys in northwestern Sumatra combined with in situ and remote observations of the vertical motion of coral reefs. These data show that the earthquake was generated by rupture of the Sunda subduction megathrust over a distance of >1,500 kilometres and a width of <150 kilometres. Megathrust slip exceeded 20 metres offshore northern Sumatra, mostly at depths shallower than 30 kilometres. Comparison of the geodetically and seismically inferred slip distribution indicates that ∼30 per cent additional fault slip accrued in the 1.5 months following the 500-second-long seismic rupture. Both seismic and aseismic slip before our re-occupation of GPS sites occurred on the shallow portion of the megathrust, where the large Aceh tsunami originated. Slip tapers off abruptly along strike beneath Simeulue Island at the southeastern edge of the rupture, where the earthquake nucleated and where an Mw = 7.2 earthquake occurred in late 2002. This edge also abuts the northern limit of slip in the 28 March 2005 Mw = 8.7 Nias–Simeulue earthquake.

Neotectonics of the Sumatran fault, Indonesia

The 1900-km-long, trench-parallel Sumatran fault accommodates a significant amount of the right-lateral component of oblique convergence between the Eurasian and Indian/Australian plates from 10°N to 7°S. Our detailed map of the fault, compiled from topographic maps and stereographic aerial photographs, shows that unlike many other great strike-slip faults, the Sumatran fault is highly segmented. Cross-strike width of step overs between the 19 major subaerial segments is commonly many kilometers. The influence of these step overs on historical seismic source dimensions suggests that the dimensions of future events will also be influenced by fault geometry. Geomorphic offsets along the fault range as high as ~20 km and may represent the total offset across the fault. If this is so, other structures must have accommodated much of the dextral component of oblique convergence during the past few million years. Our analysis of stretching of the forearc region, near the southern tip of Sumatra, constrains the combined dextral slip on the Sumatran and Mentawai faults to be no more than 100 km in the past few million years. The shape and location of the Sumatran fault and the active volcanic arc are highly correlated with the shape and character of the underlying subducting oceanic lithosphere. Nonetheless, active volcanic centers of the Sumatran volcanic arc have not influenced noticeably the geometry of the active Sumatran fault. On the basis of its geologic history and pattern of deformation, we divide the Sumatran plate margin into northern, central and southern domains. We support previous proposals that the geometry and character of the subducting Investigator fracture zone are affecting the shape and evolution of the Sumatran fault system within the central domain. The southern domain is the most regular. The Sumatran fault there comprises six right-stepping segments. This pattern indicates that the overall trend of the fault deviates 4° clockwise from the slip vector between the two blocks it separates. The regularity of this section and its association with the portion of the subduction zone that generated the giant (M_w 9) earthquake of 1833 suggest that a geometrically simple subducting slab results in both simple strike-slip faulting and unusually large subduction earthquakes.

Partial rupture of a locked patch of the Sumatra megathrust during the 2007 earthquake sequence

The great Sumatra–Andaman earthquake and tsunami of 2004 was a dramatic reminder of the importance of understanding the seismic and tsunami hazards of subduction zones. In March 2005, the Sunda megathrust ruptured again, producing an event of moment magnitude (Mw) 8.6 south of the 2004 rupture area, which was the site of a similar event in 1861 (ref. 6). Concern was then focused on the Mentawai area, where large earthquakes had occurred in 1797 (Mw = 8.8) and 1833 (Mw = 9.0). Two earthquakes, one of Mw = 8.4 and, twelve hours later, one of Mw = 7.9, indeed occurred there on 12 September 2007. Here we show that these earthquakes ruptured only a fraction of the area ruptured in 1833 and consist of distinct asperities within a patch of the megathrust that had remained locked in the interseismic period. This indicates that the same portion of a megathrust can rupture in different patterns depending on whether asperities break as isolated seismic events or cooperate to produce a larger rupture. This variability probably arises from the influence of non-permanent barriers, zones with locally lower pre-stress due to the past earthquakes. The stress state of the portion of the Sunda megathrust that had ruptured in 1833 and 1797 was probably not adequate for the development of a single large rupture in 2007. The moment released in 2007 amounts to only a fraction both of that released in 1833 and of the deficit of moment that had accumulated as a result of interseismic strain since 1833. The potential for a large megathrust event in the Mentawai area thus remains large.

Red River and associated faults, Yunnan Province, China: Quaternary geology, slip rates, and seismic hazard

The 900-km-long right-slip Red River fault of southernmost China and northern Vietnam is a profound structural discontinuity that is mechanically associated with the collision of the Indian and Eurasian plates. Although history records no large earthquakes resulting from slippage along at least the principal segment of the fault in China, youthful landforms and disruptions of young sedimentary rocks indicate that it has generated large earthquakes during the Pleistocene and Holocene epochs. The historic quiescence thus must be regarded as being indicative of a current seismic gap, although the recurrence interval between major earthquakes is evidently much longer than for many other major active fault systems. That recent displacement has been primarily right lateral is indicated by consistently displaced drainages, ranging in offset from 9 m to 6 km, and the freshness of the smallest and most recent offsets implies repeated Holocene movements. Although physiographic features typical of active faulting such as scarps and drainage diversions are present throughout, the general absence of sag ponds reflects both the high rate of dissection of the fault by the Red River and its tributaries and the lower degree of activity as compared to highly active faults such as the San Andreas fault of California. In its middle 170 km, the fault zone is made up of two branches. The range-front branch demarcates the northeastern base of the Ailao Mountains and, at least locally, has an appreciable component of dip slip. The mid-valley branch, in large part previously unrecognized, traverses principally deeply dissected Cenozoic valley fill northeast of the range-front fault and has undergone almost pure lateral slip. Lateral postfill offsets along the range-front branch diminish toward the southeast, whereas those along the mid-valley branch diminish northwestward; the net effect is that the total postfill offset across both branches is almost uniform. The Red River and its major tributaries appear to have experienced about 5.5 km of right slip since the beginning of a major episode of incision that continues to the present day. Restoration of this offset provides a remarkable alignment of most large tributaries as well as removing a major kink in the course of the Red River itself. Using maximum credible rates of incision, we estimate an average fault-slip rate of 2 to perhaps 5 mm/yr. At this long-term rate of slip, the smallest offsets observed along the fault (9 m) would occur no more frequently than every 1,800 to 4,500 yr on the average. This is consistent with the historical record of fault dormancy for the past 300 yr. North of the Red River fault, there is a large seismically active region laced with numerous faults of north and northwesterly trends. Several of these faults display clear and even spectacular evidence of youthful normal faulting, and some appear to have left-lateral components as well. These faults, as well as the Red River fault itself, are accommodating regional east-west crustal extension and north-south shortening.

Deformation and Slip Along the Sunda Megathrust in the Great 2005 Nias-Simeulue Earthquake

Seismic rupture produced spectacular tectonic deformation above a 400-kilometer strip of the Sunda megathrust, offshore northern Sumatra, in March 2005. Measurements from coral microatolls and Global Positioning System stations reveal trench-parallel belts of uplift up to 3 meters high on the outer-arc islands above the rupture and a 1-meter-deep subsidence trough farther from the trench. Surface deformation reflects more than 11 meters of fault slip under the islands and a pronounced lessening of slip trenchward. A saddle in megathrust slip separates the northwestern edge of the 2005 rupture from the great 2004 Sumatra-Andaman rupture. The southeastern edge abuts a predominantly aseismic section of the megathrust near the equator.

Earthquake Supercycles Inferred from Sea-Level Changes Recorded in the Corals of West Sumatra

Records of relative sea-level change extracted from corals of the Mentawai islands, Sumatra, imply that this 700-kilometer-long section of the Sunda megathrust has generated broadly similar sequences of great earthquakes about every two centuries for at least the past 700 years. The moment magnitude 8.4 earthquake of September 2007 represents the first in a series of large partial failures of the Mentawai section that will probably be completed within the next several decades.

Holocene rate of slip and tentative recurrence interval for large earthquakes on the San Andreas fault, Cajon Pass, southern California

Abstract Detailed mapping of the San Andreas fault zone where it crosses Cajon Creek, southern California, has revealed a number of late Quaternary deposits and geomorphological features offset by the fault. Radiocarbon dates from alluvial and swamp deposits provide a detailed chronology with which to characterize the activity of the San Andreas fault. Four independent determinations of the slip rate on the San Andreas fault yield an average rate of 24.5 ± 3.5 mm/yr for the past 14,400 years. The similarity of the four values, which span different intervals of time from 5900 to 14,400 years ago, suggest that the slip rate has been constant during this period. The slip rate confirms that the San Andreas fault is accumulating slip faster than any other fault in the plate boundary, throughout California. Also, the sum of the slip rates on the San Andreas (24.5 mm/yr) and the San Jacinto (10 mm/yr) faults south of their junction is the same as the San Andreas north of their junction (35 mm/yr). An excavation provided evidence for at least two and up to four earthquakes that caused rupture on the fault between 1290 and 1805 A.D., and tentative evidence for six earthquakes in about the last 1000 years. Both lines of evidence imply an average recurrence interval for large earthquakes of about 1 to 2 centuries. Combined with the historic record, this investigation indicates that the last major earthquake at Cajon Creek was probably at the beginning or middle of the 18th century. While the record at Cajon Creek may suggest that the next major earthquake on the San Andreas fault is overdue, the data can be interpreted to indicate that the recurrence interval for earthquakes on the San Andreas south of the 1857 rupture is 3 to 4 centuries and that the anticipated event should not be expected until the next century.