Paleoseismic Studies Research Articles

Recurrence of Large Earthquakes in Magmatic Continental Rifts: Insights from a Paleoseismic Study along the Laikipia-Marmanet Fault, Subukia Valley, Kenya Rift

The seismicity of the Kenya rift is characterized by high-frequency low-magnitude events concentrated along the rift axis. Its seismic character is typical for magmatically active continental rifts, where igneous material at a shallow depth causes extensive grid faulting and geothermal activity. Thermal overprinting and dike intrusion prohibit the buildup of large elastic strains, therefore prohibiting the gen- eration of large-magnitude earthquakes. On 6 January 1928, the MS 6:9 Subukia earthquake occurred on the Laikipia-Marmanet fault, the eastern rift-bounding struc- ture of the central Kenya rift. It is the largest instrumentally recorded seismic event in the Kenya rift, standing in contrast to the current model of the rift's seismic character in which large earthquakes are not anticipated. Furthermore, the proximity of the rup- tured fault and the rift axis is intriguing: The rift-bounding structure that ruptured in 1928 remains seismically active, capable of generating large-magnitude earthquakes, even though thermally weakened crust and better oriented structures are present along the rift axis nearby, prohibiting any significant buildup of elastic strain. We excavated the surface rupture of the 1928 Subukia earthquake to find evidence for preceding ground-rupturing earthquakes. We also made a total station survey of the site topography and mapped the site geology. We show that the Laikipia-Marmanet fault was repeatedly activated during the late Quaternary. We found evidence for six ground-rupturing earthquakes, includ- ing the 1928 earthquake. The topographic survey around the trench site revealed a degraded fault scarp of ≈7:5 m in height, offsetting a small debris slide. Using scarp-diffusion modeling, we estimated an uplift rate of U 0:09-0:15 mm=yr, con- straining the scarp age to 50-85 ka. Assuming an average fault dip of 55°-75°, the preferred uplift rate (0:15 mm=yr) accommodates approximately 10%-20% of the recent rate of extension (0:5 mm=yr) across the Kenya rift.

Are the seismological and geological observations of the Al Hoceima (Morocco, Rif) 2004 earthquake ( M = 6.3) contradictory?

Seismic hazard is associated with recent and present fault activity in mountain ranges. In the Betic-Rif alpine mountain chain, tectonic activity started in the Cretaceous, and topographic uplift continues since Tortonian times as a consequence of the NW–SE oblique convergence between Africa and Eurasia. The deformation is active and produces seismicity that sometimes has catastrophic consequences. The Al Hoceima earthquake (February 24, 2004), considered one of the largest earthquakes ever recorded instrumentally in the westernmost Mediterranean ( M = 6.3), caused great damage in the region. Seismological studies agree that the main shock was situated on land, at the limit between the External and Internal Zones of the Rif, at a depth of 10–14 km. The focal mechanism points to a strike-slip solution with a NW–SE oriented P axis, quite similar to those of the significant 1994 earthquake swarm located to the north. The epicenter aftershocks distribution would signal the presence of a N–S oriented sinistral fault, activated by the NW–SE regional compression associated to plate boundary convergence. In this setting, the seismogenic fault ruptures related to these seismic events are expected to have reached the Earth's surface. However, detailed field work carried out 1 month after the earthquake does not evidence any N–S strike-slip coseismic fault in the epicentral area. The main observed effects were landslides, damages to constructions, and locally open cracks indicating an unexpected NW–SE extension. Scarce N–S faults are normal, the main ones being located several kilometers away from the epicentral area. To explain this apparent contradiction between geological and seismological observations, we propose a decoupled tectonic model with crustal detachments that separate a deep brittle crust from an upper crust undergoing uplift, and the development of large folds and normal faults. This geological setting, common to internal zones of cordilleras, may need to be taken into account in future paleoseismicity studies and in the assessment of seismic hazard.

Rupture history of the Whirinaki fault, an active normal fault in the Taupo rift, new Zealand

Trenching of two fault strands that form part of the 21 km long, and up to 2 km wide Whirinaki Fault, Taupo Rift, New Zealand, shows that surface fault rupture has occurred at least eight times during the past 26.5 kyr. Single‐event displacements on each strand vary from a few decimetres to perhaps >2.5 m. Fault trace mapping, geomorphic analysis, and paleoseismic studies indicate a maximum magnitude for the Whirinaki Fault of c. Mw 6.6, with a recurrence interval of between 1000 and 4000 yr. The variability of single‐event displacement and timing of rupture on the two strands is attributed to various unsegmented and segmented rupture modes. Airfall tephra revealed in the trenches span the past 26.5 kyr, and provide time horizons to enable interpretation of how slip has accumulated on each of the strands of the fault. Temporal variation in slip rate ranges from 0.3 to 1.5 mm/yr and is indicative of spatial and temporal changes in the locus of strain in the Taupo Rift and the interdependence of the displacement rates and paleoearthquakes in the area.

Earthquakes: Recurrence and Interoccurrence Times

The purpose of this paper is to discuss the statistical distributions of recurrence times of earthquakes. Recurrence times are the time intervals between successive earthquakes at a specified location on a specified fault. Although a number of statistical distributions have been proposed for recurrence times, we argue in favor of the Weibull distribution. The Weibull distribution is the only distribution that has a scale-invariant hazard function. We consider three sets of characteristic earthquakes on the San Andreas fault: (1) The Parkfield earthquakes, (2) the sequence of earthquakes identified by paleoseismic studies at the Wrightwood site, and (3) an example of a sequence of micro-repeating earthquakes at a site near San Juan Bautista. In each case we make a comparison with the applicable Weibull distribution. The number of earthquakes in each of these sequences is too small to make definitive conclusions. To overcome this difficulty we consider a sequence of earthquakes obtained from a one million year "Virtual California" simulation of San Andreas earthquakes. Very good agreement with a Weibull distribution is found. We also obtain recurrence statistics for two other model studies. The first is a modified forest-fire model and the second is a slider-block model. In both cases good agreements with Weibull distributions are obtained. Our conclusion is that the Weibull distribution is the preferred distribution for estimating the risk of future earthquakes on the San Andreas fault and elsewhere.

12,000-Year-Long Record of 10 to 13 Paleoearthquakes on the Yammouneh Fault, Levant Fault System, Lebanon

We present results of the first paleoseismic study of the Yammouneh fault, the main on-land segment of the Levant fault system within the Lebanese restraining bend. A trench was excavated in the Yammouneh paleolake, where the fault cuts through finely laminated sequences of marls and clays. First-order variations throughout this outstanding stratigraphic record appear to reflect climate change at centennial and millennial scales. The lake beds are offset and deformed in a 2-m- wide zone coinciding with the mapped fault trace. Ten to thirteen events are identified, extending back more than ~12 kyr. Reliable age bounds on seven of these events constrain the mean seismic return time to 1127 ± 135 yr between ~12 ka and ~6.4 ka, implying that this fault slips in infrequent but large (M ~ 7.5) earthquakes. Our results also provide conclusive evidence that the latest event at this site was the great A.D. 1202 historical earthquake, and suggest that the Yammouneh fault might have been the source of a less well-known event circa A.D. 350. These findings, combined with previous paleoseismic data from the Zebadani valley, imply that the parallel faults bounding the Beqaa release strain in events with comparable recurrence intervals but significantly different magnitudes. Our results contribute to document the clustering of large events on the Levant fault into centennial episodes, such as that during the eleventh through twelfth centuries, separated by millennial periods of quiescence, and raise the possibility of a M > 7 event occurring on the Yammouneh fault in the coming century. Such a scenario should be taken into account in regional seismic-hazard assessments and planned for accordingly.

The Rangipo fault, Taupo rift, New Zealand: An example of temporal slip-rate and single-event displacement variability in a volcanic environment

Research Article| May 01, 2007 The Rangipo fault, Taupo rift, New Zealand: An example of temporal slip-rate and single-event displacement variability in a volcanic environment Pilar Villamor; Pilar Villamor 1GNS Science, P.O. Box 30-368, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar Russell Van Dissen; Russell Van Dissen 1GNS Science, P.O. Box 30-368, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar Brent V. Alloway; Brent V. Alloway 1GNS Science, P.O. Box 30-368, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar Alan S. Palmer; Alan S. Palmer 2Institute of Natural Resources, Massey University, Private Bag 11-222, Palmerston North, New Zealand Search for other works by this author on: GSW Google Scholar Nicola Litchfield Nicola Litchfield 3GNS Science, P.O. Box 30-368, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar GSA Bulletin (2007) 119 (5-6): 529–547. https://doi.org/10.1130/B26000.1 Article history received: 12 Mar 2006 rev-recd: 11 Oct 2006 accepted: 15 Nov 2006 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Pilar Villamor, Russell Van Dissen, Brent V. Alloway, Alan S. Palmer, Nicola Litchfield; The Rangipo fault, Taupo rift, New Zealand: An example of temporal slip-rate and single-event displacement variability in a volcanic environment. GSA Bulletin 2007;; 119 (5-6): 529–547. doi: https://doi.org/10.1130/B26000.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Geomorphic mapping and paleoseismic studies reveal that the Rangipo fault, the eastern boundary of the southern section of the Taupo rift, New Zealand, has highly variable single-event displacement and slip rate in time. Variability in single-event displacement (0.1–1.2 m since 14 cal. ka) is possibly the result of different fault rupture modes: primary (unsegmented and segmented) and secondary. Variability in fault-slip rate is attributed to interactions with volcanic activity of the nearby Ruapehu volcano (11 km distant), the southernmost and largest andesitic volcanic edifice within the southern Taupo volcanic zone. The Rangipo fault has had a mean slip rate of ∼1.4 mm/yr since ca. 25 cal. ka (calibrated radiocarbon age in thousands of years before present), and a mean slip rate of only ∼0.2 mm/yr since 14 cal. ka. This implies that during the period between 14 and 17–25 cal. ka, the fault had a slip rate of 2–9 mm/yr. This period of increased slip rate coincided with the most voluminous eruptions from Ruapehu volcano in the past 100 k.y. We infer that there is an interaction between the Rangipo fault and Ruapehu volcano, although we cannot confirm the sense of the interaction (i.e., volcano→fault or fault→volcano). The interaction could be related to pulsed rifting, where major rifting occurs in periods of accelerated faulting that coincide with extensive eruptions. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Evidence for prehistoric coseismic folding along the Tsaotun segment of the Chelungpu fault near Nan‐Tou, Taiwan

Taiwan's 1999 Mw 7.6 earthquake generated over 85 km surface rupture along the Chelungpu thrust fault. Paleoseismic studies at the Shi‐Jia site near Nantou city, reveal folding as the predominant form of deformation. Stratigraphic relations across the 1999 fold scarp show the style and degree of deformation caused by the penultimate event is similar to observed 1999 deformation. A boring transect across the fold scarp provides additional evidence of an earlier earthquake. Investigations at the Shi‐Jia site revealed three prehistoric events; accelerator mass spectrometry (AMS) radiocarbon ages indicate that the penultimate earthquake occurred between 1160 and 1440 A.D. Paleoseismic studies north of the Shi‐Jia site reveal much younger penultimate earthquakes, suggesting a 1999‐type event may not be characteristic along the Tsaotun segment of the Chelungpu fault.

Impact of riser reconstructions on estimation of secular variation in rates of strike–slip faulting: Revisiting the Cherchen River site along the Altyn Tagh Fault, NW China

Slip rates are heavily contested for many of the first-order strike–slip faults within the Indo–Asian collision zone. Rates determined geodetically are generally lower than those reported using reconstructions of offset landforms, and it is unclear if this discrepancy reflects true secular variation in slip history, systematic errors in interpretation, or both. Here I examine the methods used to derive slip rates from reconstructions of displaced fluvial risers, and show that such rates are subject to potentially important uncertainties that have largely been overlooked. Review of published data demonstrates that the slip rate can vary by a factor of 1.2 to 5 for the same site, depending on whether the reconstruction uses the age of the upper or lower terrace for the initiation of riser offset. To reduce this epistemic uncertainty, I have developed six geomorphic indices that can be used to identify the most accurate end-member reconstruction: (1) comparison of riser offset with inset channel width; (2) similarity of riser and tread displacements; (3) morphological analysis of the scarp profile; (4) riser deflection in plan view; (5) diachroneity of terrace abandonment; and (6) whether riser crests or bases yield the slip vector. Without the use of these geomorphic observations the epistemic uncertainty associated with current methods for determining slip rate from offset fluvial risers is likely to be so large that there is no resolvable discrepancy between the GPS and geologic rates within the Indo–Asian collision zone. Application of three of these indices to new field mapping of the Cherchen He (River) Site (∼ 86.4°E) along the central Altyn Tagh Fault yields a revised slip rate of 9.4 ± 2.3 mm/yr, equivalent within error to the rates previously derived from GPS and paleoseismic studies. Although reinterpretation of published data from the Sulamu Tagh site (∼ 87.4°) is consistent with this revised rate, additional work is needed to reconcile this result with some of the rates reported near Aksay (∼ 94°E).

Physical Limits on Ground Motion at Yucca Mountain

Abstract Physical limits on possible maximum ground motion at Yucca Mountain, Nevada, the designated site of a high-level radioactive waste repository, are set by the shear stress available in the seismogenic depth of the crust and by limits on stress change that can propagate through the medium. We find in dynamic deterministic 2D calculations that maximum possible horizontal peak ground velocity (PGV) at the underground repository site is 3.6 m/sec, which is smaller than the mean PGV predicted by the probabilistic seismic hazard analysis (PSHA) at annual exceedance probabilities less than 10 -6 per year. The physical limit on vertical PGV, 5.7 m/sec, arises from supershear rupture and is larger than that from the PSHA down to 10 -8 per year. In addition to these physical limits, we also calculate the maximum ground motion subject to the constraint of known fault slip at the surface, as inferred from paleoseismic studies. Using a published probabilistic fault displacement hazard curve, these calculations provide a probabilistic hazard curve for horizontal PGV that is lower than that from the PSHA. In all cases the maximum ground motion at the repository site is found by maximizing constructive interference of signals from the rupture front, for physically realizable rupture velocity, from all parts of the fault. Vertical PGV is maximized for ruptures propagating near the P -wave speed, and horizontal PGV is maximized for ruptures propagating near the Rayleigh-wave speed. Yielding in shear with a Mohr–Coulomb yield condition reduces ground motion only a modest amount in events with supershear rupture velocity, because ground motion consists primarily of P waves in that case. The possibility of compaction of the porous unsaturated tuffs at the higher ground-motion levels is another attenuating mechanism that needs to be investigated.

Rare destructive earthquakes in Europe: The 1904 Bulgaria event case

Seismic hazard is difficult to assess in regions of low strain rates. A major limitation often relates to the absence of large instrumentally recorded events precluding any comparison between seismological data and paleoseismic or morphotectonic informations. We take advantage of the 1904 M s∼7.1 earthquake that struck the southern edge of stable Eurasia and investigate if morphotectonic and paleoseismic observations can provide a reliable estimate of the seismic potential of slow-slipping faults. We have conducted a paleoseismic study of the Krupnik normal fault thought to be responsible for the event. A section of the fault bearing remnants of a 2 m-high scarp has been selected at the base of triangular facets. The trenching site locates where the scarp cuts across colluviums washed from the bedrock facetted slopes. We excavated two neighbouring trenches, one across a well-preserved portion of the scarp, and one across a portion degraded by a landslide. The excavations reveal a set of coarse colluvial units faulted against bedrock and affected by secondary fissures. Faulting appears to have resulted from a single event with normal throw greater than 1.3 m that occurred before the emplacement of the landslide. Accelerator Mass Spectrometry (AMS) radiocarbon dates of charcoal samples are consistent with the interpretation that the Krupnik Fault slipped recently, most probably in 1904, after a long lasting (> 10 ka) period of quiescence. The morphotectonic and paleoseismic observations yield seismic moment estimates compatible with the instrumental magnitude of the event and indicate that destructive and infrequent earthquakes typify the regional seismic behaviour.

A Pragmatic Approach to Seismic Parameters in a region with Low Seismicity: The Case of Eastern Iberia

In intra-plate regions with low-seismic activity, seismic cycles last between 103 and 105 years and, consequently, quiescent faults may be capable of producing catastrophic earthquakes. Paleoseismic studies, which are necessary to define capable faults in these regions and to establish their seismic parameters, are scarce and not always possible. In order to overcome the scarcity of paleoseismic data, this paper proposes an approach to evaluate the capability and seismic parameters of faults. This approach takes into account instrumental, historical, paleoseismological and geological data. Assuming that in a given structural region with definite climatic and geological characteristics similar geomorphic features reflect a similar structural-seismic evolution, the capability of faults is established by comparing the fault related geomorphic features of the few proven seismogenic faults with those of the rest of the faults. The seismic parameters were estimated using some mathematical relationships from geological and geomorphological data, where the slip rate of the faults was determined from geological and geomorphological criteria. The case of eastern Iberia is presented to illustrate this approach. This area is characterized by low seismicity, few historical destructive earthquakes, and only one fault with good paleoseismological data. Of the 249 potential faults (longer than 10 km and adequately oriented with respect to the current stress field), 23 were regarded as capable faults. Some of these were located in seismic quiescent areas, and their main seismic parameters were estimated.

Paleoseismic and Postseismic Observations of Surface Slip along the Parkfield Segment of the San Andreas Fault

Parkfield is considered a transitional segment along the San Andreas fault (saf) between continuous fault creep to the northwest and segments to the southeast that last slipped in the great 1857 Fort Tejon earthquake. Historically, creep and recurring M 6.0 earthquakes have been observed at Parkfield, California, but the segment’s relevance in great saf earthquakes has remained uncertain. This paleoseismic study of the central Parkfield segment provides a >2000-year record of tectonically deformed fluvial and sag stratigraphy. Two fault-perpendicular excavations across a pressure ridge and a sag pond ∼200 m north of Carr Hill exposed five primary fault zones displaying apparent vertical offsets, upward splaying clay shear bands, and warped stratigraphy. Four of five fault zones extended into the uppermost stratigraphy, suggesting recent surface offset and fault creep. Several antithetic fault splays and one primary fault zone displayed upward terminations, but strong indicators of large-magnitude earthquakes with meter-scale surface offset and rupture such as filled fissures and colluvial scarp deposits were not observed. The absence of evidence for large-magnitude earthquakes does not preclude the possibility of 1857-style earthquakes extending into the Parkfield segment. However, all deformation exposed within these trenches is consistent with repeated small ground rupture and fault creep. The 2004 M 6.0 Parkfield earthquake ruptured through the site and activated at least three of the five fault zones exposed in our excavations. Comparison between 2004 vertical offset and vertical offsets within the exposed stratigraphy suggests a recurrence interval between 8 and 188 years for M 6.0 earthquakes at Parkfield. Online material : Supplemental unit descriptions, unanalyzed radiocarbon samples, and trench logs and photos.

Holocene and Latest Pleistocene Activity on the Mesquite Lake Fault near Twentynine Palms, Eastern California Shear Zone: Implications for Fault Interaction

Paleoseismic data from the Mesquite Lake fault reveal evidence for up to four prehistoric earthquakes since the latest Pleistocene, with three large surface- rupturing events in about the past 10.2 ka. Three events were recorded in trenches excavated for this investigation on Mesquite Lake playa. A buried fault scarp brack- eted by accelerator mass spectrometer radiocarbon ages of detrital charcoal indicate that a mid to late Holocene event occurred between about 2.7 and 7.4 ka, most likely between about 3.9 and 4.6 ka. An early Holocene rupture defined by a subtle buried scarp and upward-terminating fissures is bracketed between about 7.7 and 10.2 ka. Evidence for an inferred late Pleistocene event prior to about 10.2 ka includes a subtle scarp, upward termination of faults, folding, and in-filled craters and fissures. A previous paleoseismic investigation just south (2 km) of the playa identified at least one event in the past 1.3 ka, although this event was not identified in this study at the playa site. Apparent vertical offsets of about 1.0 m and 1.2 m for the last two events at the playa site suggest a sequence of similar serial ruptures. These separa- tions are comparable to the height of surficial scarps just south of the playa. Because the vertical separations produced by each event are similar, the Mesquite Lake fault, at least locally, experiences similar slip-per-event. Since the initiation of Holocene playa development just over 10.2 ka ago, we estimate a horizontal slip rate of 0.7- 0.9 mm yr 1 that is consistent with slip rates for other faults in the eastern California shear zone. Despite large uncertainties, event stratigraphy at the playa site shows that periods of quiescence up to thousands of years separated large ruptures, similar to other faults in the eastern California shear zone. Comparison of paleoseismic studies in the eastern California shear zone suggest that the Mesquite Lake, and possibly the Bullion and Lavic Lake faults in the eastern part of the shear zone may have failed during periods of seismic quiescence in the western part of the zone, at about 1- 5 ka. This suggests that seismic strain release may vary spatially across the shear zone and that temporal clusters of earthquakes are not necessarily spatially limited to specific regions.

Liquefaction and fluidization of lacustrine deposits from Lahaul-Spiti and Ladakh Himalaya: Geological evidences of paleoseismicity along active fault zone

Soft-sediment deformational structures are important diagnostic features of paleoseismic studies. These seismites result from local vigorous fluidization and hydroplastic deformation formed during liquefaction as a direct consequence of permanent deformation of ground surface due to earthquakes of large magnitude. The region of eastern Ladakh, Spiti Basin and the upper Sutlej River basin record a concentration of earthquakes in a well defined NE-trending seismic zone sub-parallel to the strike of Leo-Pargil Horst and the Kaurik-Chango Fault Zone. Soft-sediment deformational studies of the fluvial and fluvio-lacustrine sequences near Sumdo village, along Sangla valley of Himachal Pradesh and near Saspol village of Ladakh indicate that the Kaurik-Chango Fault Zone is a seismogenic fault causing liquefaction and fluidization as the consequence of earthquake activity in the past. Thermal Luminescence (TL) date of the deformational structures near the Sumdo village constrains the paleoseismic activities of large magnitude between 90,000 years (third activity) to 26,000 years (last geological evidence) with a recurrence interval of approximately 10,000 years during the Quaternary period.

Submarine earthquake geology along the North Anatolia Fault in the Marmara Sea, Turkey: A model for transform basin sedimentation

The submerged portions of the North Anatolia Fault system beneath the Marmara Sea were studied with high-resolution multibeam bathymetry, subbottom profiling and sediment cores. The major objectives were to learn about the seismic and tectonic history of the fault from the stratigraphic record at a scale similar to paleoseismic studies on land and to develop tools for submarine earthquake geology that can be applied to fault-controlled basins in general. We focused on Holocene sediment in several Marmara Sea basins of different sizes. The approach was to test whether: (1) the depocenters of the larger basins contain a record of all historic Ms > 7 earthquakes within the Marmara Sea region; (2) the small transform basins record earthquakes that rupture through them; (3) vertical and strike-slip Holocene deformation can be quantified; and (4) the effects of an earthquake generally includes both primary structural features due to rupture of the sea floor, such as strata offset, scarps and tilting, as well as secondary effects due to shaking, such as mass-wasting and gravitational flows. We found evidence of earthquakes that we correlate with historic events in 181 AD, 740 AD, 1063 AD, 1343 AD, 1509 AD, 1766 AD, 1894 AD and 1912 AD. The geologic evidence is primarily from those basins adjacent to the rupture as inferred from historic data. This suggests that coseismic deformation of the sea floor along the rupture is a critical factor in the sedimentary record. We propose a qualitative sedimentation model that relates this coseismic deformation to mass-wasting of the slope, scour of the basin floor, seiche motions and homogenite deposition. Frequent earthquake activity sheds sediments from the flanks, contributes to the much thicker sediment on the basin floor and decreases the likelihood of sediment failures in response to normal marine gravity-driven processes. The surveying techniques and approaches used have therefore the potential of documenting earthquake ruptures of fault segments and to extend the earthquake record far before the known history, thus improving hazard evaluations and the fundamental understanding of earthquake process.

Tests of the precursory accelerating moment release model using a synthetic seismicity model for Wellington, New Zealand

We have constructed a synthetic seismicity model of the Wellington region, New Zealand, including seven superfaults and 500 subfaults which are randomly positioned. From this model, a synthetic catalogue of 2000 years duration, containing events of magnitude 5.0 or more, has been generated. The properties of the catalogue, such as the long‐term slip rates, b value, average activity rate, and hypocenter distribution, are in accord with paleoseismic studies and the real seismicity over the last 40 years. Such a synthetic catalogue can replace the short, incomplete, and inhomogeneous historic and instrumental records in research which needs a long time duration and many strong shocks. We have used our catalogue to examine tests for the existence of accelerating moment release (AMR) before large events and compared the results with those from random (Poisson) catalogues. We find that (1) the apparent success rate is very dependent on the rules used to define the test window; (2) when appropriately defined, the AMR pattern occurs before about 20% of the strong (M ≥ 7.0) shocks with a typical precursor time of about 22 years; (3) the AMR pattern is found almost equally frequently before large events in random catalogues; and (4) there are some false alarms (AMR pattern without a large event). This extended study reinforces the conclusion in our preliminary report: that in synthetic catalogues of the kind we have constructed, the AMR pattern is essentially an artefact of the method of sampling.

New Constraints on Recent Large Earthquakes along the Xidatan-Dongdatan Segment of the Kunlun Fault, Western China

Field investigation and paleoseismic study constrain the offset and timing of recent large earthquakes on the Xidatan–Dongdatan segment of the strike-slip Kunlun fault, northern Tibet. Displaced terrace risers and gullies preserved on the lowest and youngest terrace in the central part of the Xidatan–Dongdatan valley indicate that the offset produced by the most recent earthquake is 3–6 m, which is much smaller than what has been previously estimated; however, it is consistent with the slip of historical earthquakes along the Kunlun fault and recent well-recorded large earthquakes on other strike-slip faults. Accumulated offsets on higher terraces indicate repeated seismic activity of the Kunlun fault. We assume a 150 ± 20-km- long surface rupture for the most recent event based on roughly continuous linear features on remote-sensing images and distinct mole tracks in the field. The lack of seismicity records in the region earlier than 100 years ago led us to carry out paleoseismic investigation. Samples collected from the faulted upper layer show that the most recent large event on the Xidatan–Dongdatan segment is not older than 663 yr b.p. At least three prehistoric events are identified on the trench wall. Due to a lower constraint on the offset produced by the most recent earthquake, it would be necessary to reevaluate the seismic behavior of the Xidatan–Dongdatan segment.

Paleoseismology in Venezuela: Objectives, methods, applications, limitations and perspectives

The privileged location of Venezuela along an active interplate deformation belt, despite of being a “so-called” developing country, has led to a long paleoseismic tradition as attested by 45 trench assessments since 1968. Since then, a first 2-trench study was carried out by the American Woodward–Clyde company across the Oca fault at Sinamaica. Since 1980, all further paleoseismic studies have been performed by FUNVISIS and the Uribante–Caparo hydroelectric project (southern Mérida Andes) became their first assessment where 22 huge trenches were bulldozer-dug. Except for these Compañía Anónima de Administración y Fomento Eléctrico (CADAFE) financed trenches and two others, all other assessments were for Petróleos de Venezuela S. A. —PDVSA–.In this paper, geographic and geologic factors, as well as logistic limitations, conditioning success in paleoseismic studies by trenching, shall be discussed based on the Venezuelan experience developed over the years. The scientific contribution of this approach refer to: confirmation of Holocene fault activity, slip-per-event and average slip rate of a given fault (or segment), seismic potential (repeat of maximum credible earthquakes) of known faults, fault segmentation, fault interaction as consequence of stress loading by stick-slip on contiguous faults, time-space distribution of seismic activity along a given tectonic feature, seismotectonic association of historical earthquakes and landscape evolution on the short term and its implications on the long-term evolution (poorly discussed since this is really part of the field of Neotectonics). In recent years (since 1999), a new approach has been introduced in Venezuela consisting in complementing the seismic history derived from trenching studies with the evaluation of seismically induced perturbations in the continuous Quaternary sedimentary record of (either active or fossil) lakes. The future of this discipline in Venezuela heads to more trenching and lake coring in order to gather more data on the previously mentioned aspects. Other paleoseismic approaches have been developed very little in Venezuela since either climate or the geodynamic setting do not favor their application.

The Santiago Landslide and Associated Ridge-top Graben (Sackungen): Implications for Paleoseismic Landslide Studies

Some recent paleoseismic studies have focused on dating ridge-top graben deposits to evaluate the timing of paleoseismic events. By contrast, our study of the Santiago landslide demonstrates that ridge-top grabens also can be associated with aseismic, deep-seated landsliding. The Santiago landslide in Anaheim Hills, California, failed during the winter of 1992–1993 in response to elevated groundwater conditions associated with intense rainfall. The head of the active landslide included a zone of extensional deformation along the bounding ridgeline. Interpretation of historical, aerial photographs indicates that the active landslide is a re-activated, ancient, deep-seated, translational landslide and an associated ridge-top graben. Large-diameter borings within the ridge-top graben encountered thick colluvium, steeply dipping colluvium-filled fractures, and shears with normal offsets. In contrast to the rupture surface within the central part of the landslide, the basal rupture surfaces in the graben area had significantly less gouge. We interpret this contrast in gouge development as an indication that the ridge-top graben developed later than the original landslide by upslope progression of the deformation. Our limit-equilibrium, slope-stability analyses indicate that either high groundwater or seismic ground motion could have previously activated the ancient landslide and ridge-top graben. Because colluvial deposits preserved within the ridge-top graben and produced by these two different types of triggering events could be misinterpreted as representing the late Quaternary paleoseismic record, these features are not useful for paleoseismic studies unless aseismic activation can be clearly precluded.

Historical Seismic Activity in the Central United States

The expected recurrence of the 1811-1812 New Madrid sequence about every 500 years (Tuttle et al., 2002) dominates the U.S. Geological Survey (USGS) estimates of seismic hazard over much of the central United States (CUS) (Harmsen et al., 1999; Frankel et al., 2000, 2002). While earthquakes in the New Madrid seismic zone (NMSZ) apparently dominated CUS seismicity in the first two decades of the 19th century, more moment magnitude M 5.0 and larger earthquakes have occurred in the Illinois basin in southern Illinois and Indiana in the past 150 years than in the NMSZ (Bakun and Hopper, 2004a). Paleoseismic studies ( e.g., Munson et al. , 1997) have shown, moreover, that large Holocene earthquakes have occurred in the Illinois basin where 5.0≤ M ≤6.0 historical earthquakes have occurred. M ≥ 5.0 historical earthquakes have also occurred in Kansas, Kentucky, Nebraska, and Oklahoma. Thus, a consistent evaluation of historical seismic activity occurring throughout the CUS is important in estimating the seismic hazard. Bakun et al. (2003) developed a model specifically designed to estimate the location and M for historical earthquakes in eastern North America (ENA) and analyzed the 5 January 1843 and 31 October 1895 M 6 CUS earthquakes. Bakun and Hopper (2004b) used this model to estimate the location and M for the three largest events in the 1811-1812 NMSZ earthquake sequence. Bakun et al. 's (2003) model can be used to analyze historical CUS earthquakes with only a few intensity assignments and includes empirical site corrections to account for anomalous site effects, such as the enhanced shaking effects in 1811-1812 in communities built on weak ground along waterways (Hough et al., 2000). Bakun and Hopper (2004a) applied Bakun et al. 's (2003) model to M < 6 historical CUS earthquakes and compiled a catalog of historical seismicity wherein …