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Gede is the closest active stratovolcano to the capital of Indonesia, thus it is a potential hazard to millions of people around the volcano and in Jakarta. It is currently being monitored by various geophysical methods in a collaborative project between CVGHM and EOS. However, interpretation of new unrest is fraught with uncertainty unless its geological and petrological history is well understood. I present here a detailed petrochemical study to untangle Gede’s past history since about the last 45 kyr to present that sheds light on its magmatic evolution and reservoir dynamics, and the time-scales of these processes. A key finding is that Gede’s evolutionary path changed in the Holocene: the main magma dynamics has shifted from a deep mafic reservoir (about 24 km below its summit) to a shallow one (at about 4 km) made of silica-rich melts. Mingling and mixing of volatile-rich basaltic and crystal-rich rhyolitic magmas is one of the most important processes that lead to the main erupted compositions (andesites) and which also may lead to eruption. Three-dimensional numerical simulation of diffusion of chemically zoned minerals reveals that these shallow reservoir processes start probably only a month before eruption. These results should guide interpretations of monitoring signals and improve hazard mitigation efforts in a future unrest event. ​Doctor of Philosophy (ASE)

Most volcanoes throughout the world have been monitored with geophysical data (seismology and geodesy) for no more than three decades, a relatively short time compared to their overall life. The consequence is that we lack a long observation of volcanic growth and behavior to get a more complete picture of the interaction between edifice stress state and magma transfer. Here we present the birth and evolution of a 83 x 83 cm analog model, where we reproduce for the first time volcanic growth over 360 successive intrusions (15mL every half hour, at a rate of 3 mL/min) in an analog elasticity-dominated material (pigskin gelatine). By observing the development of this model volcano, we hope to provide insights to the study of long-term volcanic activity. In particular, we are interested in stress accumulation/release cycles and their role in the triggering of distant eruptions. Our model volcano started as a flat topography and ended 3.82 cm in height at the summit. It displayed cyclic eruptive patterns with alternating phases of eruptive and purely intrusive behavior. Alike to many intraplate volcanoes in nature, main dyke swarms produced in the experiment were disposed in a three-branched radial pattern centered above the injection source (“volcanic rift zones”). They were accompanied by two radial sill networks, at source depth and edifice base. Long-term radial compressive stress building during dyke swarming was likely compensated by radial compressive stress release during sill emplacement. Near-surface stresses, deduced from the main orientation eruptive fissures and “dry” fractures, became more localized as the volcano grew. At the end of the experiment, the shallow stress field was interpreted as generally extensional radial at the summit, extensional tangential on the flanks, and compressive radial in distal areas. This experiment showcases the potential of studying long-term stress permutations in volcanic edifices in the understanding of their morphology and successive activity phases. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

Accurate and precise chronologies are essential in understanding the rapid and recurrent climate variations of the Last Glacial – known as Dansgaard-Oeschger (D-O) events – found in the Greenland ice cores and other climate archives. The existing chronological uncertainties during the Last Glacial, however, are still large. Radiometric age data and stable isotopic signals from speleothems are promising to improve the absolute chronology. We present a record of several precisely dated stalagmites from caves located at the northern rim of the Alps (NALPS), a region that favours comparison with the climate in Greenland. The record covers most of the interval from 120 to 60 ka at an average temporal resolution of 2 to 22 yr and 2σ-age uncertainties of ca. 200 to 500 yr. The rapid and large oxygen isotope shifts of 1 to 4.5‰ occurred within decades to centuries and strongly mimic the Greenland D-O pattern. Compared to the updated Greenland ice-core timescale (GICC05modelext) the NALPS record confirms the timing of rapid warming and cooling transitions between 118 and 106 ka, but suggests younger ages for D-O events between 106 and 60 ka. As an exception, the timing of the rapid transitions into and out of the stadial following GI 22 is earlier in NALPS than in the Greenland ice-core timescale. In addition, there is a discrepancy in the duration of this stadial between the ice-core and the stalagmite chronology (ca. 2900 vs. 3650 yr). The short-lived D-O events 18 and 18.1 are not recorded in NALPS, provoking questions with regard to the nature and the regional expression of these events. NALPS resolves recurrent short-lived climate changes within the cold Greenland stadial and warm interstadial successions, i.e. abrupt warming events preceding GI 21 and 23 (precursor-type events) and at the end of GI 21 and 25 (rebound-type events), as well as intermittent cooling events during GI 22 and 24. Such superimposed events have not yet been documented outside Greenland.

Rapid shortening reveals transient creep and the temperature conditions of dislocation creep in the Taiwan lower crust., The strength of the lithosphere controls tectonic evolution and seismic cycles, but how rocks deform under stress in their natural settings is usually unclear. We constrain the rheological properties beneath the Taiwan orogenic belt using the stress perturbation following the 1999 Chi-Chi earthquake and fourteen-year postseismic geodetic observations. The evolution of stress and strain rate in the lower crust is best explained by a power-law Burgers rheology with rapid increases in effective viscosities from ~1017 to ~1019 Pa s within a year. The short-term modulation of the lower-crustal strength during the seismic cycle may alter the energy budget of mountain building. Incorporating the laboratory data and associated uncertainties, inferred thermal gradients suggest an eastward increase from 19.5±2.5°C/km in the Coastal Plain to 32±3°C/km in the Central Range. Geodetic observations may bridge the gap between laboratory and lithospheric scales to investigate crustal rheology and tectonic evolution.

Two earthquake sequences that affected the Mentawai islands offshore of central Sumatra in 2005 (Mw 6.9) and 2009 (Mw 6.7) have been highlighted as evidence for active backthrusting of the Sumatran accretionary wedge. However, the geometry of the activated fault planes is not well resolved due to large uncertainties in the locations of the mainshocks and aftershocks. We refine the locations and focal mechanisms of medium size events (Mw > 4.5) of these two earthquake sequences through broadband waveform modeling. In addition to modeling the depth-phases for accurate centroid depths, we use teleseismic surface wave cross-correlation to precisely relocate the relative horizontal locations of the earthquakes. The refined catalog shows that the 2005 and 2009 “backthrust” sequences in Mentawai region actually occurred on steeply (∼60 degrees) landward-dipping faults (Masilo Fault Zone) that intersect the Sunda megathrust beneath the deepest part of the forearc basin, contradicting previous studies that inferred slip on a shallowly seaward-dipping backthrust. Static slip inversion on the newly-proposed fault fits the coseismic GPS offsets for the 2009 mainshock equally well as previous studies, but with a slip distribution more consistent with the mainshock centroid depth (∼20 km) constrained from teleseismic waveform inversion. Rupture of such steeply dipping reverse faults within the forearc crust is rare along the Sumatra–Java margin. We interpret these earthquakes as ‘unsticking’ of the Sumatran accretionary wedge along a backstop fault separating imbricated material from the stronger Sunda lithosphere. Alternatively, the reverse faults may have originated as pre-Miocene normal faults of the extended continental crust of the western Sunda margin. Our waveform modeling approach can be used to further refine global earthquake catalogs in order to clarify the geometries of active faults. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

The Mw 6.1 South Napa earthquake occurred near Napa, California, on 24 August 2014 at 10:20:44.03 (UTC) and was the largest inland earthquake in northern California since the 1989 Mw 6.9 Loma Prieta earthquake. The first report of the earthquake from the Northern California Earthquake Data Center (NCEDC) indicates a hypocentral depth of 11.0 km with longitude and latitude of (122.3105° W, 38.217° N). Surface rupture was documented by field observations and Light Detection and Ranging (LiDAR) imaging (Brooks et al., 2014; Hudnut et al., 2014; Brocher et al., 2015), with about 12 km of continuous rupture starting near the epicenter and extending to the northwest. The southern part of the rupture is relatively straight, but the strike changes by about 15° at the northern end over a 6 km segment. The peak dextral offset was observed near the Buhman residence with right‐lateral motion of 46 cm, near the location where the strike of fault begins to rotate clockwise (Hudnut et al., 2014). The earthquake was well recorded by the strong‐motion network operated by the NCEDC, the California Geological Survey and the U.S. Geological Survey (USGS). There are about 12 sites within an epicentral distance of 15 km that had relatively good azimuthal coverage (Fig. 1). The largest peak ground velocity (PGV) of nearly 100 cm/s was observed on station 1765, which is the closest station to the rupture and lies about 3 km east of the northern segment (Fig. 1). The ground deformation associated with the earthquake was also well recorded by the high resolution COSMO–SkyMed (CSK) satellite and Sentinel-1A satellite, providing independent static observations.

USGS has identified certain section of the Manila (Luzon) trench as a high-risk earthquake zone. This zone is where the Eurasian plate is actively subducting eastward underneath the Luzon volcanic arc on the Philippine Sea plate. An earthquake of magnitude 9.0 could be generated based on the worst case scenario rupture of Manila Trench. In this paper, the possible impact of this worst case scenario rupture on Singapore is investigated using a numerical model. It is found that (1) it takes about 12 h for the tsunami waves generated at Manila Trench to arrive at Singapore coastal waters; (2) the wave period of the tsunami wave, i.e., time interval between two peaks, is about 5 h; (3) the maximum water level rise in Singapore water is about 0.8 m; and (4) the maximum velocity associated with the tsunami waves is about 0.5 m/s, which is not likely to have significant impact on the port operations in Singapore.

The Longmen Shan mountain range, which defines the eastern margin of the Himalayas, exhibits greater topographic relief than anywhere else in the Tibetan plateau. It was the site of the devastating 2008 Wenchuan magnitude-7.9 earthquake. Prior to the earthquake, geodetic and geologic surveys measured little shortening across the range front, prompting vigorous debate about the forces producing the topography of this mountain belt. Now Judith Hubbard and John Shaw present evidence from balanced geologic cross-sections that suggests that crustal shortening is a primary driver for uplift of the Longmen Shan and the Tibetan plateau, and that the 2008 Wenchuan earthquake was a product of this shortening process. This paper presents balanced geologic cross-sections showing that crustal shortening, structural relief and topography are strongly correlated in the Longmen Shan mountain range front, suggesting that crustal shortening is a primary driver for uplift and topography of the Longmen Shan on the flanks of the Tibetan plateau. The authors conclude that the 2008 Wenchuan earthquake is an active manifestation of this shortening process. The Longmen Shan mountain range, site of the devastating 12 May 2008 Wenchuan (M = 7.9) earthquake, defines the eastern margin of the Himalayan orogen and exhibits greater topographic relief than anywhere else in the Tibetan plateau. However, before the earthquake, geodetic and geologic surveys measured little shortening across the range front1,2,3, inspiring a vigorous debate about the process by which the topography of the mountain belt is produced and maintained. Two endmember models have been proposed: (1) brittle crustal thickening, in which thrust faults with large amounts of slip that are rooted in the lithosphere cause uplift4, and (2) crustal flow, in which low-viscosity material in the lower crust extrudes outward from the Tibetan plateau and inflates the crust north and east of the Himalayas5,6,7. Here we use balanced geologic cross-sections to show that crustal shortening, structural relief, and topography are strongly correlated in the range front. This suggests that crustal shortening is a primary driver for uplift and topography of the Longmen Shan on the flanks of the plateau. The 2008 Wenchuan (M = 7.9) earthquake, which ruptured a large thrust fault along the range front causing tens of thousands of fatalities and widespread damage, is an active manifestation of this shortening process.

After lying dormant for about a thousand years, sudden slippage of a 1600-km long section of the Sunda megathrust fault caused uplift of the seafloor between Aceh and Myanmar, resulting in a great earthquake and the horrific Indian Ocean tsunami of 2004. Three months later and just to the south, sudden slippage of a 350-km length of the megathrust beneath Simeulue and Nias islands caused another destructive great earthquake and lesser tsunami. Because it takes centuries for tectonic strains to build up again after such big earthquakes, these two events are unlikely to recur within the next hundred years. Farther south, however, offshore West Sumatra and Bengkulu provinces, another great earthquake and tsunami will likely occur within the next few decades. We are trying to characterize that future earthquake and tsunami, to encourage and to focus preparations for and mitigation of the coming disaster. Similar efforts need to be initiated throughout much of south and southeast Asia, if the disastrous effects of future large earthquakes and tsunamis are to be mitigated.

We mapped and analyzed two vertical exposures—exposed on the walls of a 3- to 5-m-deep, 70-m-long excavation and a smaller 3-m-deep, 10-m-long excavation—across the 1999 rupture of the Chelungpu fault. The primary exposure revealed a broad anticlinal fold with a 2.5-m-high west-facing principal thrust scarp contained in fluvial cobbly gravel beds and overlying fine-grained overbank deposits. Sequential restoration of the principal rupture requires initial failure on the basal, east-dipping thrust plane, followed by wedge thrusting and pop-up of an overlying symmetrical anticline between two opposing secondary thrust faults. Net vertical offset is about 2.2 m across the principal fault zone. From line-length changes, we estimate about 3.3 m of horizontal shortening normal to fault strike. The ratio of these values yields a total slip of 4.0 m and an estimate of about 34 for the dip of the fault plane below the excavation. This value is nearly the same as the 35 average dip of the fault plane from the surface to the hypocenter. Restoration of the exposed gravelly strata and adjacent overbank sediments deposited prior to the 1999 event around the principal rupture suggests the possible existence of a prior event. A buried 30-m-wide anticlinal warp beneath the uplifted crest of the 1999 event is associated with three buried reverse faults that we interpret as evidence for an earlier episode of folding and faulting in the site. The prior event is also recorded in the smaller excavation, which is located 40 m south and is oriented parallel to the larger excavation. Radiocarbon dating of samples within the exposed section did not place tight constraints on the date of the previous event. Available data are interpreted as indicating that the previous event occurred before the deposition of the less than 200 14 C yr B.P. overbank sands and after the deposition of the much older fluvial gravels. We interpret the previous event as the penultimate event relative to the 1999 ChiChi earthquake. We estimated the long-term slip rate of the Chelungpu fault to be 10–15 mm/yr during the last 1 Ma, based on previously published retrodeformable cross sections. This rate is, however, significantly higher than geodetic rates of shortening across the Chelungpu thrust where two pairs of permanent Global Positioning System stations suggest 7–10 mm/yr of shortening across the fault. Given the 4 m of average slip, the long-term slip rate yields an interseismic interval of between 267 and 400 yr for the Chelungpu fault.

A vertical exposure across the principal thrust scarp of the 1999 Mw 7.6 earthquake allows quantification offault slip. The exposure is located on the active Chelungpu fault near Wufeng, along the range front of the fold-and-thrust belt in western Taiwan. The 1999 surface ruptures at the Wufeng site are characterized by a west-facing 2 to 3 m high principal thrust scarp and an east-facing lesser backthrust scarp. We mapped a 15 mlong, 5 m-deep exposure across the principal thrust scarp and characterized complex deformation structures, which include a main basal thrust fault, a wedge thrust, and a pop-up anticlinal fold with two secondary opposing thrust faults. The vertical displacement across the principal thrust scarp is measured directly from the offsets of the same sedimentary horizons between the hangingwall and the footwall. The average vertical displacement is 2.2 ^ 0.1 m, and the maximum displacement is 2.5 m, at the crest of the small pop-up fold. Horizontal displacement estimates were determined using line- and area-balancing methods. With line-length methods we estimated a horizontal displacement of 3.3 ^ 0.3 m across the principal scarp for four sedimentary horizons. For area balancing, first we selected three horizontal soil/sand deposits with a total thickness of about 0.5 m. The estimate yields an average horizontal displacement of 4.8 ^ 1.0 m. Using these individual and relatively thin stratigraphic layers yielded significant standard deviations in displacement estimates as a result of thickness variations. Second, we used the 3 mthick overbank soil/sand and the lower part offluvial pebble/cobble to calculate a horizontal displacement of 2.6 ^ 0.2 m with the area-balancing technique. According to the geometry of the dip angle (35‐ 408) of the basal thrust, the line-length measurement and the 3 m-thick package area balancing both providedreasonable results of horizontal displacement. By comparing thedifferent deposits applied to theline- and area-balancing methods, we interpret that decoupling of deformation occurred between the lower fluvial gravels and the upper overbank sand and mud deposits. Due to lesser confining pressure at the surface, additional deformation occurred in the upper 1 ‐ 2 m thick overbank deposits. This additional deformation yielded further vertical uplift of 0.3‐ 0.5 m and horizontal displacement of 0.2 ‐ 0.8 m around the core of the pop-up fold. Our work suggests that determination of slip across surface thrust ruptures varies as a function of the mechanical behavior of young late Quaternary deposits. q 2003 Elsevier Ltd. All rights reserved.

The Ventura Avenue anticline is one of the fastest uplifting structures in southern California, rising at ∼5 mm/yr. We use well data and seismic reflection profiles to show that the anticline is underlain by the Ventura fault, which extends to seismogenic depth. Fault offset increases with depth, implying that the Ventura Avenue anticline is a fault‐propagation fold. A decrease in the uplift rate since ∼30±10 ka is consistent with the Ventura fault breaking through to the surface at that time and implies that the fault has a recent dip‐slip rate of ∼4.4–6.9 mm/yr. To the west, the Ventura fault and fold trend continues offshore as the Pitas Point fault and its associated hanging wall anticline. The Ventura–Pitas Point fault appears to flatten at about 7.5 km depth to a detachment, called the Sisar decollement, then step down on a blind thrust fault to the north. Other regional faults, including the San Cayetano and Red Mountain faults, link with this system at depth. We suggest that below 7.5 km, these faults may form a nearly continuous surface, posing the threat of large, multisegment earthquakes. Holocene marine terraces on the Ventura Avenue anticline suggest that it grows in discrete events with 5–10 m of uplift, with the latest event having occurred ∼800 years ago (Rockwell, 2011). Uplift this large would require large earthquakes (Mw 7.7–8.1) involving the entire Ventura/Pitas Point system and possibly more structures along strike, such as the San Cayetano fault. Because of the local geography and geology, such events would be associated with significant ground shaking amplification and regional tsunamis. Published Version

The giant Sumatran subduction earthquake of 1833 appears as a large emergence event in fossil coral microatolls on the reefs of Sumatra's outer-arc ridge. Stratigraphic analysis of these and living microatolls nearby allow us to estimate that 1833 emergence increased trenchward from about 1 to 2 m. This pattern and magnitude of uplift are consistent with about 13 m of slip on the subduction interface and suggest a magnitude (Mw ) of 8.8–9.2 for the earthquake. The fossil microatolls also record rapid submergence in the decades prior to the earthquake, with rates increasing trenchward from 5 to 11 mm/yr. Living microatolls show similar rates and a similar pattern. The fossil microatolls also record at least two less extensive emergence events in the decades prior to 1833. These observations show that coral microatolls can be useful paleoseismic and paleogeodetic instruments in convergent tectonic environments. Published version

Data from geotechnical boreholes and trenches, in combination with geomorphologic mapping, indicate that the Hollywood fault is an oblique, reverse–left-lateral fault that has undergone at least one surface-rupturing earthquake during latest Pleistocene to middle or late Holocene time. Geomorphologic observations show that the fault extends for 14 km along the southern edge of the eastern Santa Monica Mountains, from the Los Angeles River westward through downtown Hollywood to northwestern Beverly Hills, where the locus of active deformation steps 1.2 km southward along the West Beverly Hills lineament to the Santa Monica fault. Rupture of the entire Hollywood fault, by itself, could produce a M w ∼ 6.6 earthquake, similar in size to the highly destructive, 1994 Northridge earthquake, but even closer to more densely urbanized areas. Assuming a 0.35 mm/yr minimum fault-slip rate consistent with available geologic data, we calculate an average maximum recurrence interval for such moderate events of ≤∼4000 yr. Although occurrence of such moderate events is consistent with the elapsed time since the poorly constrained age of the most recent surface rupture, the data do not preclude a longer quiescent interval suggestive of larger earthquakes. If earthquakes much larger than M w ∼6.6 occurred in the past, we speculate that they may have been generated by the Hollywood fault together with other faults in the Transverse Ranges Southern Boundary fault system.

Airborne thermal images of the Chaitén rhyolite lava dome were obtained on three occasions between January 2009 and January 2010. These images were useful for understanding the nature and pace of growth of the newly extruded lava, which formed a complex of lobes and a spine. The images also revealed contrasting growth styles affecting different parts of the lava dome complex. Observed synchronous endogenous and exogenous growth was likely the result of multiple flow paths within the lava dome. We suggest that contrasts in morphology and surface texture between various lava lobes are the result of different extrusion rates. Published version

Detailed mapping of the fresh surficial ruptures in the right step between the Homestead Valley Fault and the Emerson Fault, two right-lateral en echelon faults that broke in the 1992 Landers earthquake, indicates that the transfer of dextral slip between the faults is accommodated primarily by a series of obliquely striking right-lateral strike-slip cross faults. Right-lateral slip on the cross faults and counterclockwise rotation of the interior blocks are sufficient to transfer virtually 100% of the dextral slip and to accommodate the extension across the jog. Asymmetry of the dextral slip curves along the cross faults indicates that some of them may have been induced to fail by slip on the Homestead Valley Fault, while others were induced to fail by slip on the Emerson Fault. Comparison of the magnitude of slip in 1992 to bedrock and geomorphic offsets suggests that the Homestead Valley Fault and several secondary faults have experienced about 100 nominal Landers events and that the stepover structure originated at the same time as the Homestead Valley Fault, about 1 m.y. ago. Because the Emerson Fault has an order of magnitude greater total offset but only slightly greater surficial slip in 1992, we conclude that it is a significantly older fault. Repetition of 1992-like events should eventually lead to a merging of the Homestead Valley Fault with the Emerson. Published version

The oblique convergence between North American and Caribbean plates is accommodated in a bookshelf faulting manner by active, oblique-normal faults in the northern part of the Lesser Antilles arc. In the last 20 years, two M > 6 earthquakes occurred along a large, arc parallel, en echelon fault system, the 16 March 1985 in Redonda and 21 November 2004 in Les Saintes. A better understanding of active faulting in this region permit us to review the location and magnitude of historical earthquakes by using a regional seismic attenuation law. Several others moderate earthquakes may have occurred along the en echelon fault system implying a strong seismic hazard along the arc. These faults control the effusion of volcanic products and some earthquakes seem to be correlated in time with volcanic unrest. Shallow earthquakes on intraplate faults induced normal stress and pressure changes around neighboring volcano and may have triggered volcanic activity. The Redonda earthquake could have initiated the 1995 eruption of Montserrat's Soufrière Hills by compressing its plumbing system. Conversely, pressure changes under the volcano increased Coulomb stress changes and brought some faults closer to failure, promoting seismicity. We also discuss the magnitude of the largest 11 January 1839 and 8 February 1843 megathrust interplate earthquakes. We calculate that they have increased the stress on some overriding intraplate faults and the extensional strain beneath several volcanoes. This may explain an increase of volcanic and seismic activity in the second half of the 19th century culminating with the devastating, 1902 Mount Pelée eruption. Published version

In August 1989, an earthquake sequence including ten events with 6:3 ! M ! 5:5 in the first two days produced widespread ground deformation in the Dobi graben of central Afar. Numerous surface breaks with complex geometry, including fresh scarplets with vertical throws up to 30 cm high and open fissures up to 30 cm wide, were observed. Coseismic slip incremented the deformation (normal faulting, block tilting, and counterclockwise rotation of basaltic slices) accumulated in the last 2 m.y. in the transfer zone between the Dobi and Hanle grabens. By combining maps of surface ruptures, relative event relocations with the local Djibouti network, published focal mechanisms, and source sizes, we tentatively relate most of the main- shocks of the sequence to slip on individual faults. The largest shocks at 11h16 on 20 August 1989 (MS 6:2) and at 1h09 on 21 August 1989 (MS 6:3) ruptured southern segments of the southwestern bounding fault of the graben. A dozen other faults also slipped along the edges of, and inside, the graben. On average, triggered seismic fault- ing propagated about 35 km northwestward along the graben in about 20 hr. Slip on the main faults was coupled with slip on secondary antithetic faults branching from them at depth. Although the Dobi earthquakes ruptured part of the fault array between the Asal rift (1978 sequence) and the Serdo region (1969 sequence), an approximately 50-km-long gap subsists along the Der'ela half-graben. We infer the patterns of sur- face faulting in the Dobi sequence, which coinvolved bookshelf-faulting about both horizontal and vertical axes, to typify the complexity of coseismic stress release in central Afar and in other active zones of distributed extension (e.g., Iceland, Abruzzi, Basin and Range). Online Material: Additional photos and descriptions of surface effects of the Dobi earthquake sequence.

Volcanic lateral blasts are among the most spectacular and devastating of natural phenomena, but their dynamics are still poorly understood. Here we investigate the best documented and most controversial blast at Mount St. Helens (Washington State, United States), on 18 May 1980. By means of three-dimensional multiphase numerical simulations we demonstrate that the blast front propagation, final runout, and damage can be explained by the emplacement of an unsteady, stratified pyroclastic density current, controlled by gravity and terrain morphology. Such an interpretation is quantitatively supported by large-scale observations at Mount St. Helens and will influence the definition and predictive mapping of hazards on blast-dangerous volcanoes worldwide. Published Version

Coral records of relative sea level change provide a history of vertical interseismic and coseismic deformation along the coast of northern Simeulue Island, Sumatra, and reveal details about earthquakes in the 10th and 14th–15th centuries A.D. along the southern end of the December 2004 Mw 9.2 Sunda megathrust rupture. Over a 56 year period between A.D. 1390 and 1455, northern Simeulue experienced a cluster of megathrust ruptures, associated with total uplift that was considerably more than in 2004. Uplifted corals at two sites constrain the first event of the cluster to A.D. 1393 ± 3 and 1394 ± 2 (2σ). A smaller but well-substantiated uplift occurred in northern Simeulue in 1430 ± 3. An inferred third uplift, in A.D. 1450 ± 3, killed all corals on the reef flats of northern Simeulue. The amount of uplift during this third event, though confirmed only to have exceeded 28 and 41 cm at two sites, probably surpassed the 100 and 44 cm that occurred, respectively, at those sites in 2004, and it was likely more than in 2004 over all of northern Simeulue. The evidence for past earthquake clustering combined with the inference of considerably greater uplift in A.D. 1390–1455 than in 2004 suggests that strain may still be stored along the southernmost part of the 2004 rupture. Interseismic subsidence rates recorded by northern Simeulue coral microatolls have varied by up to a factor of 4 at some sites from one earthquake cycle to another. Published version

Characterizing internal structures of active volcanoes remains an enigmatic issue in geosciences. Yet studies of such structures can greatly improve hazard assessments, helping scientists to better monitor seismic signatures, geodetic deformation, and gas emissions, data that can be used to improve models and forecasts of future eruptions. Several passive seismic tomography experiments—which use travel times of seismic waves from natural earthquakes to image underground structures—have been conducted at active volcanoes (Hawaii's Kilauea, Washington's Mount St. Helens, Italy's Etna, and Japan's Unzen), but an inhomogeneous distribution of earthquakes compromises resolution. Further, if volcanic earthquakes are dominantly shallow at a given location, passive methods are limited to studying only shallow features. Thus, active source experiments—where seismic waves from the explosion of deliberately set charges are used to image below the surface—hold great potential to illuminate structures not readily seen through passive measures. Published version

International audience; We provide a new set of complementary geodetic data for the 2005 rifting event of Afar (Ethiopia). Interferometric synthetic aperture radar and subpixel correlations of synthetic aperture radar and SPOT images allow us to deduce 3-D surface displacement unambiguously. We determine the geometry of the dike and neighboring magma chambers and invert for the distribution of opening of the dike, as well as slip on rift border faults. The volume of the 2005 dike (1.5–2.0 km3) is not balanced by sufficient volume loss at Dabbahu and Gabho volcanoes (0.42 and 0.12 km3, respectively). Taking into account the deflation of a suspected deep midsegment magma chamber simultaneously to dike intrusion produces a smoother opening distribution along the southern segment. Above the dike, faults slipped by an average 3 m, yielding an estimated geodetic moment of 3.5 × 1019 Nm, one order of magnitude larger than the cumulative seismic moment released during the earthquake swarm. Between Dabbahu and Ado'Ale volcanic complexes, significant opening occurred on the western side of the dike. The anomalous location of the dike at this latitude, offset to the east of the axial depression, may explain this phenomenon. A two-stage intrusion scenario is proposed, whereby rifting in the northern Manda Hararo Rift was triggered by magma upwelling in the Dabbahu area, at the northern extremity of the magmatic segment. Although vigorous dike injection occurred during the September 2005 event, the tectonic stress deficit since the previous rifting episode was not fully released, leading to further intrusions in 2006–2009.

The Mw 7.9 Wenchuan, China, earthquake ruptured two large thrust faults along the Longmenshan thrust belt at the eastern margin of the Tibetan Plateau. This earthquake generated a 240-km-long surface rupture zone along the Beichuan fault and an additional 72-km-long surface rupture zone along the Pengguan fault. Maximum vertical and horizontal offsets of 6.5 m and 4.9 m, respectively, were measured along the Bei chuan fault. A maximum vertical offset of 3.5 m was measured along the Pengguan fault. Coseismic surface ruptures, integrated with aftershocks and industry seismic profiles, show that two imbricate structures have ruptured simultaneously, resulting in the largest continental thrust event ever documented. Large oblique thrusting observed during this earthquake indicates that crustal shortening is an important process responsible for the high topography in the region, as everywhere along the edge of Tibetan Plateau.

Long-term slip rates for the Denali Fault in southern Alaska are derived using 10Be cosmogenic radionuclide (CRN) dating of offset glacial moraines at two sites. Correction of 10Be CRN model ages for the effect of snow shielding uses historical, regional snow cover data scaled to the site altitudes. To integrate the time variation of snow cover, we included the relative changes in effective wetness over the last 11 ka, derived from lake-level records and δ 18O variations from Alaskan lakes. The moraine CRN model ages are normally distributed around an average of 12.1 ± 1.0 ka (n = 22, ± 1σ). The slip rate decreases westward from ∼13 mm/a at 144°49′W to about 7 mm/a at 149°26′W. The data are consistent with a kinematic model in which southern Alaska translates northwestward at a rate of ∼14 mm/a relative to a stable northern Alaska with no rotation. This suggests progressive slip partitioning between the Denali Fault and the active fold and thrust belt at the northern front of the Alaska range, with convergence rates increasing westward from ∼4 mm/a to 11 mm/a between ∼149°W and 145°W. As the two moraines sampled for this study were emplaced synchronously, our suggestion of a westward decrease in the slip rate of the Denali Fault relies largely upon the measured offsets at both sites, regardless of any potential systematic uncertainty in the CRN model ages. Published version

We quantify fore-arc deformation using fossil reefs to test the assumption commonly made in seismic cycle models that anelastic deformation of the fore arc is negligible. Elevated coral microatolls, paleoreef flats, and chenier plains show that the Sumatran outer arc island of Nias has experienced a complex pattern of relatively slow long-term uplift and subsidence during the Holocene epoch. This same island rose up to 2.9 m during the Mw 8.7 Sunda megathrust rupture in 2005. The mismatch between the 2005 and Holocene uplift patterns, along with the overall low rates of Holocene deformation, reflects the dominance of elastic strain accumulation and release along this section of the Sunda outer arc high and the relatively subordinate role of upper plate deformation in accommodating long-term plate convergence. The fraction of 2005 uplift that will be retained permanently is generally

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.

One hundred eighty U–Th data, including 23 isochrons on 24 pristine modern and Holocene corals and 33 seawater samples, were analyzed using sector-field mass spectrometry to understand the variability of initial 230Th/232Th (230Th/232Th0). This dataset allows us to further assess the accuracy and precision of coral 230Th dating method. By applying quality control, including careful sampling and subsampling protocols and the use of contamination-free storage and workbench spaces, the resulting low procedural blanks give an equivalent uncertainty in age of only ±0.2–0.3 yr for 1–2 g of coral sample. Using site-specific 230Th/232Th0 values or isochron techniques, our study demonstrates that corals with an age less than 100 yrs can be 230Th-dated with precisions of ±1 yr. Six living subtidal coral samples were collected from two continental shelf sites, Nanwan off southern Taiwan in the western Pacific and Son Tra off central Vietnam in the South China Sea; one coral core was drilled from an open-ocean site, Santo Island, Vanuatu, in the western tropical Pacific; and modern and fossil intertidal coral slabs, 17 in total, were cut from six sites around the islands of Simeulue, Lago, North Pagai and South Pagai of Sumatra in the eastern Indian Ocean. The results indicate that the main source of thorium is the dissolved phase of seawater, with variation of 230Th/232Th0 depending on local hydrology. With intense input of terrestrial material, low 230Th/232Th0 atomic ratios of 4.9 × 10−6 and 3.2 × 10−6 with a 10% variation are observed in Nanwan and Son Tra, respectively. At the Santo site, we find a value of 5.6 × 10−6 at 4 horizons and one high value of 24 × 10−6 in a sample from AD 1974.6 ± 0.5, likely due to the upwelling of cold water during a La Nina event between AD 1973 and 1976. The natural dynamics of 230Th/232Th0 recorded in the intertidal corals at sites in the Sumatran islands are complicated so that this value varies significantly from 3.0 to 9.4 × 10−6. Three of the 141 modern coral 230Th ages differ from their true ages by −23 to +4, indicating the presence of detrital material with anomalous 230Th/232Th values. Duplicate measurement of coeval subsamples is therefore recommended to verify the age accuracy. This improved high precision coral 230Th dating method raises the prospects of refining the age models for band-counted and tracer-tuned chronologies and of advancing coral paleoclimate research.

We use SPOT image pairs to determine horizontal offsets associated with the Mw 7.9 November 2002 Denali earthquake in the vicinity of Slate Creek, AK. Field measurements and aerial photographs are used to further characterize the geometry of the surface rupture. Aerial photographs show that shear localization occurs where the rupture trace is linear, and distributed off-fault deformation is common at fault bends and stepovers, or at geologic contacts between rock, glacial sediments, and ice. The displacement field is generated using a sub-pixel cross correlation technique between SPOT images taken before and after the earthquake. We identify the effects of glacier motion in order to isolate the tectonic displacements associated with the Denali earthquake. The resulting horizontal displacement field shows an along-strike variation in dextral shear, with a maximum of approximately 7.5 m in the east near 144° 52′W, which decreases to about 5 m to the west near 145° 45′W. If the November 2002 earthquake represents the long-term behavior of the Denali fault, it implies a westward decrease in the long-term dextral slip rate. A possible mechanism to accommodate the westward decreasing slip on the Denali fault is to transfer fault slip to adjacent easttrending contractional structures in the western region of the central Alaskan Range. Accepted version

Numerous landforms along the Longitudinal Valley suture of eastern Taiwan indicate that two opposing reverse faults currently dominate the suturing process between the Luzon volcanic arc and the Central Range of Taiwan. The east-dipping Longitudinal Valley fault, on the eastern flank of the valley, is well known. The west-dipping Central Range reverse fault, on the western flank of the valley, is more obscure. Nonetheless, it has produced many uplifted lateritic fluvial terraces along the eastern flank of the Central Range in the central reach of the valley, from just north of the Wuhe Tableland to near Chihshang. The fault appears to be active but blind south of Chihshang and inactive along the northern part of the Longitudinal Valley. The late Quaternary slip rate of the fault is less than 12.8 mm/yr. This constraint means that the fault is absorbing far less than half of the horizontal shortening across the Longitudinal Valley suture. However, the late Quaternary slip rate along the fault may be comparable to the uplift and exhumation rate of the Central Range. This suggests that localized brittle slip along the Central Range fault is an important component of crustal thickening and uplift of the range, even though additional shortening and crustal thickening may be occurring because of pervasive deformation beneath the range.

The earthquakes of November 1951 constitute the most destructive seismic episode in the recorded history of the Longitudinal Valley, eastern Taiwan. However, information about their source parameters is sparse. To understand the relationship between the 1951 ruptures and new interpretations of the regional neotectonic architecture of the Longitudinal Valley, we re-evaluated the November 1951 ruptures by analyzing old documents, reports and photographs, and by interviewing local residents who experienced the earthquake. As a result, we have revised significantly the rupture map previously published. We divide the surface ruptures from south to north into the Chihshang, Yuli, and Rueisuei sections. The first shock of the 1951 series probably resulted from the Chihshang rupture, and the second shock probably resulted from the Yuli and Rueisuei ruptures. The lengths of these ruptures indicate that the two shocks had similar magnitudes. The Chihshang and Rueisuei ruptures are along segments of the Longitudinal Valley fault, a left-lateral oblique fault along which the Coastal Range thrusts westward over the Longitudinal Valley. The Yuli rupture, on the other hand, appears to be part of a separate, left-lateral strike-slip Yuli fault, which traverses the middle of the Longitudinal Valley. The complex behavior of these structures and interaction between them are important in understanding the future seismic hazard of the area. Accepted version

The Longitudinal Valley fault is a key element in the active tectonics of Taiwan. It is the principal structure accommodating convergence across one of the two active sutures of the Taiwan orogeny. To understand more precisely its role in the suturing process, we analyzed fluvial terraces along the Hsiukuluan River, which cuts across the Coastal Range in eastern Taiwan in the fault's hanging wall block. This allowed us to determine both its subsurface geometry and its long-term slip rate. The uplift pattern of the terraces is consistent with a fault-bend fold model. Our analysis yields a listric geometry, with dips decreasing downdip from about 50° to about 30° in the shallowest 2.5 km. The Holocene rate of dip slip of the fault is about 22.7 mm/yr. This rate is less than the 40 mm/yr rate of shortening across the Longitudinal Valley derived from GPS measurements. The discrepancy may reflect an actual difference in millennial and decadal rates of convergence. An alternative explanation is that the discrepancy is accommodated by a combination of slip on the Central Range fault and subsidence of the Longitudinal Valley floor. The shallow, listric geometry of the Longitudinal Valley fault at the Hsiukuluan River valley differs markedly from the deep listric geometry illuminated by earthquake hypocenters near Chihshang, 45 km to the south. We hypothesize that this fundamental along-strike difference in geometry of the fault is a manifestation of the northward maturation of the suturing of the Luzon volcanic arc to the Central Range continental sliver. Published version

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.

International audience; [1] We determine the long-term slip rate of the southern San Andreas Fault in the southeastern Indio Hills using 10 Be and 26 Al isotopes to date an offset alluvial fan surface. Field mapping complemented with topographic data, air photos and satellite images allows precise determination of piercing points across the fault zone that are used to measure an offset of 565 ± 80 m. A total of 26 quartz-rich cobbles from three different fan surfaces were collected and dated. The tight cluster of nuclide concentrations from 19 samples out of 20 from the offset fan surface implies a simple exposure history, negligible prior exposure and erosion, and yields an age of 35.5 ± 2.5 ka. The long-term slip rate of the San Andreas Fault south of Biskra Palms is thus 15.9 ± 3.4 mm/yr. This rate is about 10 mm/yr slower than geological (0–14 ka) and short-term geodetic estimates for this part of the San Andreas Fault, implying changes in slip rate or in faulting behavior. This result puts new constraints on the slip rate of the San Jacinto and on the Eastern California Shear Zone for the last 35 kyr. Our study shows that more sites along the major faults of southern California need to be targeted to better constrain the slip rates over different timescales.

The disastrous effects of the 1999 Chi-Chi earthquake in Taiwan demonstrated an urgent need for better knowledge of the island's potential earthquake sources. Toward this end, we have prepared a neotectonic map of Taiwan. The map and related cross sections are based upon structural and geomorphic expression of active faults and folds both in the field and on shaded relief maps prepared from a 40-m resolution digital elevation model, augmented by geodetic and seismologic data. The active tandem suturing and tandem disengagement of a volcanic arc and a continental sliver to and from the Eurasian continental margin have created two neotectonic belts in Taiwan. In the southern part of the orogen both belts are in the final stage of consuming oceanic crust. Collision and suturing occur in the middle part of both belts, and postcollisional collapse and extension dominate the island's northern and northeastern flanks. Both belts consist of several distinct neotectonic domains. Seven domains (Kaoping, Chiayi, Taichung, Miaoli, Hsinchu, Ilan, and Taipei) constitute the western belt, and four domains (Lutao-Lanyu, Taitung, Hualien, and Ryukyu) make up the eastern belt. Each domain is defined by a distinct suite of active structures. For example, the Chelungpu fault (source of the 1999 earthquake) and its western neighbor, the Changhua fault, are the principal components of the Taichung Domain, whereas both its neighboring domains, the Chiayi and Miaoli Domains, are dominated by major blind faults. In most of the domains the size of the principal active fault is large enough to produce future earthquakes with magnitudes in the mid-7 values. Published version

Taiwan’s numerous active faults and folds demarcate distinct eastern and western neotectonic belts. The western belt results from the attachment and subsequent detachment of a sliver of continental lithosphere to the Eurasian continental margin. The eastern belt is the product of the same continental sliver docking with and then separating from the Luzon volcanic arc. Thus, the active Taiwan orogen is a tandem suturing and tandem disengagement of a volcanic arc and a continental sliver to and from the Eurasian continental margin. This progressive suturing and separation is a superb, living demonstration of the fundamental weakness of lithospheric sutures. Furthermore, this neotectonic architecture provides the basis for understanding the Taiwan’s seismic sources. Accepted version

Geologic relationships and patterns of crustal seismicity constrain the three-dimensional geometry of the active portions of San Andreas fault zone near San Gorgonio Pass, southern California. Within a 20-km-wide contractional stepover between two segments of the fault zone, the San Bernardino and Coachella Valley segments, folds, and dextral-reverse and dextral-normal faults form an east-west belt of active structures. The dominant active structure within the stepover is the San Gorgonio Pass-Garnet Hill faults, a dextral-reverse fault system that dips moderately northward. Within the hanging wall block of the San Gorgonio Pass-Garnet Hill fault system are subsidiary active dextral and dextral-normal faults. These faults relate in complex but understandable ways to the strike-slip faults that bound the stepover. The pattern of crustal seismicity beneath these structures includes a 5–8 km high east-west striking step in the base of crustal seismicity, which corresponds to the downdip limit of rupture of the 1986 North Palm Springs earthquake. We infer that this step has been produced by slip on the linked San Gorgonio Pass-Garnet Hill-Coachella Valley Banning (SGP-GH-CVB) fault. This association enables us to construct a structure contour map of the fault plane. The large step in the base of seismicity downdip from the SGP-GH-CVB fault system probably reflects a several kilometers offset of the midcrustal brittle-plastic transition. (U/Th)/He thermochronometry supports our interpretation that this south-under-north thickening of the crust has created the region's 3 km of topographic relief. We conclude that future large earthquakes generated along the San Andreas fault in this region will have a multiplicity of mostly specifiable sources having dimensions of 1–20 km. Two tasks in seismic hazard evaluation may now be attempted with greater confidence: first, the construction of synthetic seismograms that make useful predictions of ground shaking, and second, theoretical investigations of the role of this complexity in retarding the propagation of future seismic ruptures. Published version

We analyze the spatial relationship of relocated aftershocks to the principal rupture planes of the M w 7.3 1992 Landers mainshock from a structural point of view. We find that the aftershocks constitute primarily a several-kilometer-wide damage zone centered on the mainshock rupture planes. The intensity of damage decreases away from the principal faults. Less than half of the aftershocks occurred within 1 km of the mainshock planes, and perhaps only 5% of the aftershocks are candidates for rerupture of the mainshock faults. Moreover, it seems that aftershocks along the Landers rupture have b -values that correlate well with the complexity of the mainshock rupture. Low b -values occur along segments that are simple, whereas higher b -values correlate with sections that are more complex. Thus, structural complexity appears to correlate with a greater relative abundance of small earthquakes. These observations imply that aftershock populations reflect fault populations in the medium surrounding the principal faults rather than the behavior of the mainshock planes themselves. Online material : Arcview information about the surface rupture of the Landers mainshock.

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 (Mw9) earthquake of 1833 suggest that a geometrically simple subducting slab results in both simple strike-slip faulting and unusually large subduction earthquakes. Published version

On 28 March 2005 the Sunda megathrust in Indonesia ruptured again, producing another great earthquake three months after the previous one. The rupture was contiguous with that of the December 2004 Sumatra–Andaman earthquake, and is likely to have been sparked by local stress, although the triggering stresses at its hypocentre were very small — of the order of just 0.1 bar. Calculations show that stresses imposed by the second rupture have brought closer to failure the megathrust immediately to the south, under the Batu and Mentawai islands, and have expanded the area of increased stress on the Sumatra fault. Palaeoseismologic studies show that the Mentawai segment of the Sunda megathrust is well advanced in its seismic cycle and is therefore a good candidate for triggered failure.

A Late Pleistocene shoreline at the overflow level of Searles Lake has been displaced 82 to 106 m (preferred value is 90 m) in a left-lateral sense and 2.5 m (net) north side up along the Garlock fault, at the southeastern corner of Searles Valley. Previously published radiocarbon dates from both surface and subsurface strata indicate that the most recent highstand of Searles Lake ended sometime between 10,000 and 13,800 14C years ago. The maximum slip rate of the Garlock fault in southeastern Searles Valley is thus 11 mm/14C yr. If part of the offset of the shoreline occurred during older lakestands, then the slip rate may be somewhat less. A channel incised after the most recent highstand, however, is offset about 68 m, indicating that the minimum slip rate is 5 mm/14C yr. Subjective evaluation of the constraints on the offset and on the age of the shoreline yields a preferred rate of 6–8 mm/14C yr at this site. Assuming Bard et al.'s (1990) recent calibration of the radiocarbon time scale, the calibrated slip rate of the Garlock fault is between 4 and 9 mm/yr with a preferred value of 5–7 mm/yr. This estimate is similar to a previous estimate of the Holocene slip rate and is slightly less than an estimate derived from modelling of geodetic data. Extension north of the Garlock fault in Indian Wells and Searles valleys contributes no more than 3 mm/yr left slip to the Garlock fault. Published version

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. Published version

The three-dimensional geometry of a buried channel-fill deposit that crosses, and is offset by, a strand of the San Jacinto fault zone in San Bernardino, California, is reconstructed from an extensive suite of trench logs. The reconstruction shows that the original course of the channel was deflected for a short distance along strike of the fault before it was offset. As a result, apparent offset of the channel deposit across the fault zone is about 12 m in a right-lateral sense, but only 3.5 to 6 m of the apparent offset is due to fault slip. The age of the deposit is ≤1931 ± 109 yr B.P. based on radiometric dating of detrital charcoal collected from the unit. Our excavations do not encompass all active strands of the fault at this site. Hence, our study points to a minimum slip rate of 1.7 to 3.3 mm/yr along the northern San Jacinto fault zone. Historical records show that it has been at least 64 to 90 yr since the last large earthquake occurred along the northern San Jacinto fault. It can thus be inferred from our estimate of fault slip rate that at least 10 to 30 cm of slip is currently stored along the northern segment of the San Jacinto fault. Because prior studies along the same fault zone to the southeast indicate much higher slip rates (on the order of 10 mm/yr), and coseismic offsets for the largest historical earthquakes along the San Jacinto fault zone have been on the order of 30 cm, it is inferred that the northern reach of the San Jacinto fault is now in the later stages of the strain accumulation cycle prior to a moderate to large M 6-7 earthquake. Additionally, the complex geometry of the reconstructed channel serves to illustrate the caution that must be exercised when determining the true offset of buried channel deposits.

Old trees growing along the San Andreas fault near Wrightwood, California, record in their annual ring-width patterns the effects of a major earthquake in the fall or winter of 1812 to 1813. Paleoseismic data and historical information indicate that this event was the "San Juan Capistrano" earthquake of 8 December 1812, with a magnitude of 7.5. The discovery that at least 12 kilometers of the Mojave segment of the San Andreas fault ruptured in 1812, only 44 years before the great January 1857 rupture, demonstrates that intervals between large earthquakes on this part of the fault are highly variable. This variability increases the uncertainty of forecasting destructive earthquakes on the basis of past behavior and accentuates the need for a more fundamental knowledge of San Andreas fault dynamics.

The spatial and temporal pattern of range front normal faulting and volcanism in the Mono Basin of eastern California suggests that dikes are being intruded underneath the Mono Craters in response to crustal stretching and are now accommodating strain that was once taken up by range front faulting. The section of the Sierra Nevadan range front near the craters accommodated as much as 1 mm/yr of extension as recently as about 40,000 years ago. For the past 40,000 years, this section of range front has been inactive, even though range front extension to the north and south has continued at up to 0.9 mm/yr. For the past 40,000 years, dikes, intruding underneath the Mono Craters, seem to have been accommodating the 1 mm/yr of extension that was previously taken up by faulting. Since the basin is extending obliquely to the trend of the frontal faults, there is a component of dextral shear to their motion, so that the Mono Craters may be forming on an extensional boundary of a pull-apart basin. If the craters represent incipient caldera formation, then calderas such as Long Valley may also have formed in pull-apart zones. Published version

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.

Late Holocene marsh deposits composing a terrace about 55 km northeast of Los Angeles, California, contain geologic evidence of many large seismic events produced by slip on the San Andreas fault since the sixth century A .D. I excavated several trenches into the deposits in order to study this evidence. The principal indicators of past events are (1) sandblows and other effects of liquefaction, (2) the termination of secondary faults at distinct levels within the stratigraphic section, and (3) sedimentary deposits and faulted relationships along the main fault. The effects upon the marsh deposits of six of the eight prehistoric events are comparable to those of the great (Me = 8 1/4 +) 1857event, which is the youngest of the nine events disturbing the strata and is associated with about 4 1/2 m of right lateral slip nearby. Two large events may be smaller than this. Radiocarbon dates indicate that the events occurred in the nineteenth,eighteenth,fifteenth, thirteenth, late twelfth, tenth, ninth, seventh,and sixth centuries A .D. Recurrence intervals average 160 years but vary from 1/2 century to about 3 centuries. The dates may indicate a fairly systematic pattern of occurrence of large earthquakes. Published version

Trees may suffer damage during major earthquakes due to shaking or faulting of their substrate. Damage may result in temporarily asymmetric growth and/or reduction in width of annual growth rings. To determine whether trees contain useful records of prehistoric earthquakes in southern California, we cored eight conifers along the 1857 trace of the San Andreas fault near Wrightwood and Frazier Park, California. Annual ring widths were measured and plotted against the growth year determined by ring counting.We examined significant departures from normal growth trends and interpreted them in light of the 1857 earthquake and other possible environmental factors. Of eight trees sampled, five showed damage or growth anomalies attributablet o the 1857 event. One 120-year-old t ree straddling the fault is undamaged, suggesting no substantial slip since about 1870. To evaluate asymmetry, ratios of correlative rings widths from opposite sides of three seismically damaged trees were calculated and plotted against growth year. Two types of ratio anomalies can be recognized:(1) short-term,unilateral suppression of growth resulting from damage and (2) long-term,unilateral enhancement of growth attributable to tilt or changes in environmental factors. Further study of ring ratio anomalies may facilitate recognition of seismically damaged trees. Success in recognizing the 1857 event in five out of eight trees suggests that a larger suite of even older trees may contain a valuable 'dendroseismological' record of large prehistoric earthquakes in southern California. Published version

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.