Sumatran Megathrust Earthquakes: From Science to Saving Lives

Most of the loss of life, property and well-being stemming from the great Sumatran earthquake and tsunami of 2004 could have been avoided and losses from similar future events can be largely prevented. However, achieving this goal requires forging a chain linking basic science—the study of why, when and where these events occur—to people's everyday lives. The intermediate links in this chain are emergency response preparedness, warning capability, education and infrastructural changes. In this article, I first describe our research on the Sumatran subduction zone. This research has allowed us to understand the basis of the earthquake cycle on the Sumatran megathrust and to reconstruct the sequence of great earthquakes that have occurred there in historic and prehistoric times. On the basis of our findings, we expect that one or two more great earthquakes and tsunamis, nearly as devastating as the 2004 event, are to be expected within the next few decades in a region of coastal Sumatra to the south of the zone affected in 2004. I go on to argue that preventing future tragedies does not necessarily involve hugely expensive or high-tech solutions such as the construction of coastal defences or sensor-based tsunami warning systems. More valuable and practical steps include extending the scientific research, educating the at-risk populations as to what to do in the event of a long-lasting earthquake (i.e. one that might be followed by a tsunami), taking simple measures to strengthen buildings against shaking, providing adequate escape routes and helping the residents of the vulnerable low-lying coastal strips to relocate their homes and businesses to land that is higher or farther from the coast. Such steps could save hundreds and thousands of lives in the coastal cities and offshore islands of western Sumatra, and have general applicability to strategies for helping the developing nations to deal with natural hazards

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

Lateral Offsets and Revised Dates of Large Prehistoric Earthquakes at Pallett Creek, Southern California

Recent excavation and new radiocarbon dates of sediments at Pallett Creek are the basis for new conclusions regarding the late Holocene history of the San Andreas fault. Systematic dissection of a 50-m-long, 15-m-wide, 5-m-deep volume of earth, centered on the fault, enables documentation in three dimensions of fault patterns, lateral offsets, and vertical deformation associated with large earthquakes of the past. The excavations expose evidence for 12 earthquakes that occurred between about 260 and 1857 A.D., with an average recurrence interval of about 145 years. Prehistoric slip events that occurred in 1720±50, 1550±70, 1350±50, 1080±65, and 845±75 A.D. have lateral offsets that are comparable to those of the most recent great earthquake of 1857. Thus all of these events represent earthquakes of large magnitude. The lateral offsets of two other events, in 935±85 and 1015±100 A.D., are an order of magnitude smaller and may be interpreted in several ways with regard to the size of these events. The new data constrain the average recurrence interval for large earthquakes at this site to between 145 and 200 years but suggest a monotonic decrease in individual intervals to below this range during the past 900 years. On the basis of these data, the probability of a large earthquake with surficial fault rupture at this site is between 0.2 and 5% during 1984 and 7 and 60% by the year 2000

Slip along the San Andreas fault associated with the great 1857 earthquake

Historical records indicate that several meters of lateral slip along the San Andreas fault accompanied the great 1857 earthquake in central and southern California. These records, together with dendrochronological evidence, suggest that the rupture occurred along 360 to 400+ km of the fault, including several tens of kilometers of the currently creeping reach in central California. Geomorphic expressions of late Holocene right-lateral offsets are abundant along the 1857 rupture. Along 300 kilometers of the 1857 rupture, between Cholame and Wrightwood, the youngest discernible offset ranges from 3 to 9 1/2 meters. Dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857. Fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault. It is relatively uniform along each of several long segments, but changes rather abruptly in value between these segments. This nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long-term slip rate than others. The 1857 offsets indicate a seismic moment, m_o, between 5.3 and 8.7 × 10^(27) dyne-cm, assuming a 10- to 15-km depth of rupture and relatively uniform slip as a function of depth. A comparison with the rupture length, average slip value, and tectonic setting of the California earthquake of 1906 (M_s = 8 1/4) indicates a value of M = 8 1/4 + for the 1857 event

A Comparative Study of the Sumatran Subduction-Zone Earthquakes of 1935 and 1984

A M_s 7.7 earthquake struck the western, equatorial coast of Sumatra in December 1935. It was the largest event in the region since the two devastating giant earthquakes of 1833 and 1861. Historical seismograms of this event from several observatories around the world provide precious information that constrains the source parameters of the earthquake. To more precisely quantify the location, geometry, and mechanism of the 1935 event and to estimate the coseismic deformation, we analyze the best of the available teleseismic historical seismograms by comparing systematically the records of the 1935 earthquake with those of a smaller event that occurred in the same region in 1984. First we constrain the source parameters of the 1984 event using teleseismic records. Then, we compare the records of the 1935 event with those of 1984 from the same sites and instruments. To do this, we choose several time windows in the corresponding seismograms that contain clearly identifiable phases and deconvolve the modern event from the older one. The deconvolutions result in very narrow pulses with similar sizes, thus confirming similar locations and mechanisms for the events. The initiation of the 1984 event was on the subduction interface at a depth of 27 ± 2 km; its M_0 is 6.5 x 10^(19) N m (M_w is 7.2). The sense of slip was nearly pure thrust, on a plane dipping 12°. The 1935 event also involved rupture of the shallow subduction interface, but was about five times larger (M_0 3.3 x 10;^(20) N m, M_w 7.7) and initiated a few kilometers to the southeast, along strike. The 1935 rupture propagated unilaterally toward the southeast. The along-strike rupture length was about 65 km. From these source parameters, we calculate the surface deformations, assuming an elastic multilayered medium. These deformations compare favorably with those actually recovered from paleoseismic data in the form of coral microatolls

Non-Cyanide Electrodeposited Ag–PTFE Composite Coating Using Direct or Pulsed Current Deposition

The effects of FC-4 cationic surfactant on electrodeposited Ag–PTFE composite coating using direct or pulsed currents were studied using scanning electron microscope (SEM), energy dispersive X-ray (EDS), optical microscope, and a linear tribometer. FC-4:PTFE in various ratios were added to a non-cyanide succinimide silver complex bath. Direct or pulsed current method was used at a constant current density to enable comparison between both methods. A high incorporation rate of PTFE was successfully achieved, with pulsed current being highly useful in increasing the amount of PTFE in the composite coating. The study of coating wear under sliding showed that a large majority of the electrodeposited coatings still managed to adhere to the substrate, even after 10 wear cycles of sliding tests. Performance improvements were achieved on all the samples with a coefficient of friction (CoF) between 0.06 and 0.12

Slip Along the San Andreas Fault Associated with the Earthquake

Some of the fault slip associated with the 1979 Imperial Valley earthquake occurred along other than the Imperial fault and the Brawley fault zone. More than 90 km to the north of the seismogenic fault, a 39-km-long section of the San Andreas fault developed a discontinuous set of surficial fractures soon after the earthquake. This set of fractures consisted of small left-stepping echelon cracks displaying extensional and dextral components of movement. Average dextral slip was about 4 mm, and slip reached 10 mm at one point along the fault. In one locality the cracks formed between Va and 4Vfe days after the main shock, although slippage at depth may have been nearly simultaneous with the earthquake. In general, this set of breaks duplicates the location, style, and slip magnitude of the set that was mapped in 1968 after the Borrego Mountain, Calif, earthquake. Such near-duplication indicates that this section of the San Andreas fault, in particular, is susceptible to small amounts of triggered slip. Although the reasons for such behavior are far from clear, similar behavior of the Imperial fault before 1979 suggests that this section of the San Andreas fault may generate a moderate earthquake within the next few decades

Central California foreshocks of the great 1857 earthquake

Analysis of contemporary accounts indicates that several small to moderate central California earthquakes preceded the great 1857 earthquake by 1 to 9 hr. The earliest events apparently were felt only in the San Francisco area or the Sacramento and Sierran Foothills region. Two later and much more widely felt foreshocks were experienced within the region bounded by San Francisco, Visalia, Fort Tejon, and Santa Barbara. A comparison with felt areas and intensity distributions of modern events of known source and magnitude indicates that these later two shocks were 5 ≦M ≲ 6 and probably originated at some point within an area of radius ≈60 km that includes the southeastern 100 km of the historically creeping segment of the San Andreas fault. The northwestern terminus of the 1857 rupture is probably located along this segment. If the location of these foreshocks is indicative of the epicenter of the main event, then the several-hundred-kilometer main-event rupture propagated principally in a unilateral fashion toward the southeast. This implies that, like many great earthquakes, the 1857 rupture originated on a fault segment historically characterized by moderate activity and propagated into an historically quiet segment. There is a strong possibility that the foreshock activity represents a moderate Parkfield-Cholame sequence similar to those of 1901, 1922, 1934, and 1966. To the extent that such premonitory activity is characteristic of the failure of the 1857 segment of the fault, studies of the creeping segment of the fault may be relevant to the prediction of large earthquakes in central and southern California

Paleomagnetic measurement of nonbrittle coseismic deformation across the San Andreas Fault at Pallett Creek

Paleomagnetic data have been obtained to address a problem at the Pallett Creek paleoseismological site: the 9 mm/yr slip rate determined from three-dimensional mapping of late Holocene offsets across discrete faults is only a quarter of the expected value. We suspected that nonbrittle deformation adjacent to the faults might account for the 26 mm/yr discrepancy. In our search for the missing slip we collected and analyzed 264 paleomagnetic samples from a 53-m-wide transect across the fault zone. Half the samples came from a unit deposited immediately after a large earthquake of about A.D. 1480; these samples were affected by two large earthquakes that involved rupture at the site in 1812 and 1857. We collected the other half of the samples from a slightly older bed, one that was deposited before the earthquake of about A.D. 1480. Relative to “control” groups composed of 10 samples and collected 50 m from the fault, samples closer to the fault display clockwise rotations of 30° or less. If interpreted as block rotations, the data from the older unit imply that it has sustained a total of 14.0 ± 2.9 m of dextral warp during the past three major earthquakes and that the younger unit has experienced a total of 8.5 ± 1.0 m of warp during the most recent two. Combining these values with the amounts of dextral slip across the mapped fault planes yields dextral offsets of 5.5, 6.25, and 6.25 m for the events of A.D. 1480, 1812, and 1857 and a slip rate of 35.6 ± 6.7 mm/yr. This slip rate, averaged over the past three complete seismic cycles, is consistent with published rates from other sites. Offsets associated with the past three events are remarkably similar. These amounts, however, appear independent of the length of interseismic cycles. These observations suggest (1) that this part of the San Andreas fault has a characteristic strength and (2) that conventional concepts of strain accumulation and relief (for example, time- and slip-predictable models of earthquake occurrence) are unrealistic

Source parameters of the great Sumatran megathrust earthquakes of 1797 and 1833 inferred from coral microatolls

Large uplifts and tilts occurred on the Sumatran outer arc islands between 0.5° and 3.3°S during great historical earthquakes in 1797 and 1833, as judged from relative sea level changes recorded by annually banded coral heads. Coral data for these two earthquakes are most complete along a 160-km length of the Mentawai islands between 3.2° and 2°S. Uplift there was as great as 0.8 m in 1797 and 2.8 m in 1833. Uplift in 1797 extended 370 km, between 3.2° and 0.5°S. The pattern and magnitude of uplift imply megathrust ruptures corresponding to moment magnitudes (M_w) in the range 8.5 to 8.7. The region of uplift in 1833 ranges from 2° to at least 3.2°S and, judging from historical reports of shaking and tsunamis, perhaps as far as 5°S. The patterns and magnitude of uplift and tilt in 1833 are similar to those experienced farther north, between 0.5° and 3°N, during the giant Nias-Simeulue megathrust earthquake of 2005; the outer arc islands rose as much as 3 m and tilted toward the mainland. Elastic dislocation forward modeling of the coral data yields megathrust ruptures with moment magnitudes ranging from 8.6 to 8.9. Sparse accounts at Padang, along the mainland west coast at latitude 1°S, imply tsunami runups of at least 5 m in 1797 and 3–4 m in 1833. Tsunamis simulated from the pattern of coral uplift are roughly consistent with these reports. The tsunami modeling further indicates that the Indian Ocean tsunamis of both 1797 and 1833, unlike that of 2004, were directed mainly south of the Indian subcontinent. Between about 0.7° and 2.1°S, the lack of vintage 1797 and 1833 coral heads in the intertidal zone demonstrates that interseismic submergence has now nearly equals coseismic emergence that accompanied those earthquakes. The interseismic strains accumulated along this reach of the megathrust have thus approached or exceeded the levels relieved in 1797 and 1833