Most Recent Event Research Articles

Neotectonics and late Holocene paleoseismic evidence in the Plio-Quaternary Daroca Half-graben, Iberian Chain, NE Spain. Implications for fault source characterization

The official seismic hazard models in Spain used in the seismic building codes do not incorporate Quaternary faults, largely due to insufficient data for their proper characterization. There is an obvious need to conduct investigations in most of the recognised Quaternary faults to unambiguously demonstrate their Quaternary tectonic activity and assess their seismogenic potential. This work illustrates the integration of cartographic, tectonic, geomorphological, paleoseismological and geophysical methods for the characterization of the slow-moving extensional Daroca Fault, related to the negative inversion of the Alpine Daroca Thrust, Iberian Chain, NE Spain. Cartographic data indicate that the 27 km long Daroca Fault and the 17 km long Calamocha Fault, separated by a stepover 1.9 km wide, can be considered as segments of the same structure that might rupture jointly and generate Mw7 earthquakes. A long-term slip rate of 0.06–0.02 mm/yr has been estimated for the 27 km long Daroca Fault using an ESR-dated (Electro Spin Resonance) offset pediment. The work discusses why this slip rate is significantly lower than those estimated in nearby normal faults using OSL ages (Optically Stimulated Luminiscence), but comparable with those derived from offset early Pliocene limestones. A trench excavated across the Daroca Fault exposed evidence of the MRE (most recent event) on the fault, with bracketing ages of 2354–1544 cal yr BP (404 BC – 386 AD). This event likely caused the destruction and abandonment of Roman cities in the vicinity of the fault. Several explanations are proposed for the anomalously low vertical displacement of this surface faulting event recorded in the central sector of a 27 km long fault segment: multi-strand rupture, full-segment rupture, partial segment rupture, spillover rupture, and secondary sympathetic rupture.

Paleoseismic evidence of a major earthquake event(s) along the hinterland faults: Pinjore Garden Fault (PGF) and Jhajra Fault (JF) in northwest Himalaya, India

Despite identification of numerous active faults in hinterland of the Himalaya, most of the paleoseismological studies are confined to the frontal fault – Himalayan Frontal Thrust (HFT). Barring few examples, no attempt has been made to understand the spatial and temporal pattern of earthquakes occurred along the hinterland faults. In this article we present new data on the coseismic slip and timing of the latest faulting event along the two hinterland faults: the Pinjore Garden Fault (PGF) and Jhajra Fault (JF) in northwest Himalaya. Trench investigations suggest at least two earthquakes along PGF and JF, with the most recent event (MRE) subsequent to 1283 CE–1443 CE and preceded 1600 CE, and around 1223 CE–1442 CE respectively, with observed single event and multi-event slip of 4.26 m and 12.7 m on PGF. The timing of the latest event deduced from the previous trenches (Hajipur to Rampur Ganda) indicates striking coherence with those along the hinterland faults (PGF and JF). We propose that the hinterland faults might have been reactivated by an independent earthquake event between 1283 CE and 1600 CE, just like the 1905 Kangra, 2005 Kashmir, 2015 Nepal earthquakes, which failed to rupture the front but produced surface deformation along the hinterland faults. The loading of the un-ruptured not defined portion of MHT with the residual stress along with constant convergence between the plates during the medieval period must have facilitated another earthquake, rupturing the frontal thrust within 100–300 years of time frame. Our results support the evidence that northwest Himalaya must have hosted multiple large magnitude earthquakes between 1200 CE–1600 CE but seismic quiescence now raises an alarm to the society and the government for the earthquake preparedness.

On fault evidence for a large earthquake in the late fifteenth century, Eastern Kunlun fault, China

The EW-trending Kunlun Fault System (KFS) is one of the major left-lateral strike-slip faults on the Tibetan Plateau. It forms the northern boundary of the Bayan Har block. Heretofore, no evidence has been provided for the most recent event (MRE) of the ~70-km-long eastern section of the KFS. The studied area is located in the north of the Zoige Basin (northwest Sichuan province) and was recognized by field mapping. Several trenches were excavated and revealed evidence of repeated events in late Holocene. The fault zone is characterized by a distinct 30–60-cm-thick clay fault gouge layer juxtaposing the hanging wall bedrock over unconsolidated late Holocene footwall colluvium and alluvium. The fault zone, hanging wall, and footwall were conformably overlain by undeformed post-MRE deposits. Samples of charred organic material were obtained from the top of the faulted sediments and the base of the unfaulted sediments. Modeling of the age of samples, earthquake yielded a calibrated 2σ radiocarbon age of A.D. 1489 ± 82. Combined with the historical earthquake record, the MRE is dated at A.D. 1488. Based on the over ~50 km-long surface rupture, the magnitude of this event is nearly M w ~7.0. Our data suggests that a ~200-km-long seismic gap could be further divided into the Luocha and Maqu sections. For the last ~1000 years, the Maqu section has been inactive, and hence, it is likely that the end of its seismic cycle is approaching, and that there is a potentially significant seismic hazard in eastern Tibet.

A palaeoseismic trench investigation of early Holocene neotectonic faulting along the Kango Fault, southern Cape Fold Belt, South Africa – Part I: stratigraphic and structural features

Who would have thought that the southern part of ‘stable, old South Africa’ was in fact ‘not-so-stable’, and that there has been spectacular tectonic activity in the Cape Fold Belt as recent as the early Holocene, about 10,327 ± 755 years ago? Large-scale surface rupture, with at least 2 m of vertical geomorphic offset, has been recorded along the Kango Fault over a lateral distance of at least 84 km, from near the town of De Rust, east of Oudtshoorn, towards the western end of the Baviaanskloof in the Eastern Cape. Although the Kango Fault scarp was reported in the mid-1970’s and late-1980’s, detailed palaeoseismic investigation was not undertaken until recently, when the chief energy supplier for South Africa, Eskom, rekindled investigations to establish a number of nuclear power plants along the South African coastline. This paper reports the results of the first palaeoseismic trench excavated across the Kango Fault during the initial phase of the nations’ current New Build programme. The location and date of the most recent event (MRE) is reported, as well as the style of faulting and local stress direction. These palaeoseismic data are derived from carefully-mapped logs of an 82 m long, 5 m deep trench, supported by 12 optically-stimulated luminescence (OSL) dates of stratigraphic horizons exposed in the trench. Various features of the stratigraphy, pedology and structure in the trench sidewall are described in detail, particularly those leading to the interpretation of the MRE. Development of a shear fabric in the cobble- to small boulder-sized alluvium at the main fault zone indicates that interpretation of local neotectonic fault style and stress direction should be made at least 4 to 5 m below the free face of the fault scarp. Retro-deformation of the trench log clearly defines the MRE and distinguishes apparent vertical displacement from the actual displacement of 2.0 ± 0.06 m. It also reveals that 1.9 m of surface extension occurred during the rupture, as well as the depositional processes that subsequently buried the 32 m wide graben formed at the site.

Paleoseismology of the southern Panamint Valley fault: Implications for regional earthquake occurrence and seismic hazard in southern California

AbstractPaleoseismologic data from the southern Panamint Valley fault (PVF) reveal evidence of at least four surface ruptures during late Holocene time (0.33–0.48 ka, 0.9–3.0 ka, 3.3–3.6 ka, and >4.1 ka). These paleo‐earthquake ages indicate that the southern PVF has ruptured at least once and possibly twice during the ongoing (≤1.5 ka) seismic cluster in the Mojave section of the eastern California shear zone (ECSZ). The most recent event (MRE) on the PVF is also similar in age to the 1872 Owens Valley earthquake and the geomorphically youthful MRE on the Death Valley fault. The timing of the three oldest events at our site shows that the PVF ruptured at least once and possibly thrice during the well‐defined 2–5 ka seismic lull in the Mojave section of the ECSZ. Interestingly, the 3.3–3.6 ka age of Event 3 overlaps with the 3.3–3.8 ka age of the penultimate (i.e., pre‐1872) rupture on the central Owens Valley fault. These new PVF data support the notion that earthquake occurrence in the ECSZ may be spatially and temporally complex, with earthquake clusters occurring in different regions at different times. Coulomb failure function modeling of the Panamint Valley and Garlock faults reveals significant stress interactions between these two faults that may influence future earthquake occurrence. Specifically, our models suggest a possible rupture sequence whereby an event on the southern Panamint Valley fault can lead to the potential triggering of an event on the Garlock fault, which in turn could trigger the Mojave section of the San Andreas Fault.

A Record of Large Earthquakes during the Past Two Millennia on the Southern Green Valley Fault, California

We document evidence for surface‐rupturing earthquakes (events) at two trench sites on the southern Green Valley fault, California (SGVF). The 75–80 km long dextral SGVF creeps ∼1–4 mm/yr. We identify stratigraphic horizons disrupted by upward‐flowering shears and infilled fissures unlikely to have formed from creep alone. The Mason Rd site exhibits four events from ∼1013 CE to the present. The Lopes Ranch site (LR, 12 km to the south) exhibits three events from 18 BCE to present including the most recent event (MRE), 1610±52 yr CE (1 σ ) and a two‐event interval (18 BCE–238 CE) isolated by a millennium of low deposition. Using OxCal to model the timing of the four‐event earthquake sequence from radiocarbon data and the LR MRE yields a mean recurrence interval (RI or μ ) of 199±82 yr (1 σ ) and ±35 yr (standard error of the mean), the first based on geologic data. The time since the most recent earthquake (open window since MRE) is 402 yr±52 yr, well past μ ∼200 yr. The shape of the probability density function (PDF) of the average RI from OxCal resembles a Brownian passage time (BPT) PDF (i.e., rather than normal) that permits rarer longer ruptures potentially involving the Berryessa and Hunting Creek sections of the northernmost GVF. The model coefficient of variation (cv, σ / μ ) is 0.41, but a larger value (cv∼0.6) fits better when using BPT. A BPT PDF with μ of 250 and cv of 0.6 yields 30 yr rupture probabilities of 20%–25% versus a Poisson probability of 11%–17%. Online Material: OxCal model for ages of paleoearthquakes, Lopes site 1998 logs, and description of a model for estimation of rupture frequency.

Holocene subaqueous paleoseismology of Lake Tahoe

Gravity-flow deposits recovered in a suite of sediment cores in Lake Tahoe were examined to determine if the event deposits were triggered by strong shaking from earthquakes on active faults within and in close proximity to the Lake Tahoe Basin. The acoustic character and distribution of individual lacustrine deposits as well as potential source regions were constrained by high-resolution seismic Chirp reflection and multibeam bathymetric data. Between 14 and 17 Holocene event deposits have been identified in Lake Tahoe, and examination of their source areas suggests they originated from different initiation points along the steep margin, with some being synchronous around the basin, as opposed to flood-related deposits. Lithologic characteristics, magnetic susceptibility, carbon and nitrogen isotopic signatures, opal content, and 14 C dating indicate that these event deposits are reworked lacustrine material. Radiocarbon dates indicate that the emplacement of these event deposit sediments correlates well with the late Holocene paleoseismic earthquake record developed for the Tahoe Basin. When taken alone, the causality of these events may appear ambiguous, but when the evidence is examined comprehensively, it suggests that strong shaking may likely have been the primary trigger for many of the event deposits observed in the lake throughout the Holocene. For example, four event deposits are assigned to Tahoe Basin faults. The most recent earthquakes occurred on the Incline Village fault (between 630 and 120 cal. yr B.P.); the southern segment of the West Tahoe fault (between 4510 and 4070 cal. yr B.P.); on the central and northern segments of the West Tahoe fault (5600–5330 cal. yr B.P.); and on the West Tahoe fault (between 7890 and 7190 cal. yr B.P.). The oldest of the four associated Tahoe Basin events coincides with the beginning of an extended period when Lake Tahoe was likely not spilling or spilling intermittently, and this suggests that active faulting and footwall uplift cut off the outlet at this time, exaggerating drought conditions downstream. Likewise, the event between 5600 and 5330 cal. yr B.P. on the West Tahoe fault may have exaggerated a smaller drought reflected downstream in Pyramid Lake. This event may also be the most recent event (MRE) on the largest segment of the West Tahoe fault. If correct, the period since the last rupture is approximately twice the estimated average recurrence interval for the Rubicon segment of the West Tahoe fault. A more complete Holocene record of strong shaking greatly extends the paleoseismic record in the region and indicates a combined recurrence interval of between 750 and 800 yr for all faults in the region.

New Paleoearthquake Ages from the Western Garlock Fault: Implications for Regional Earthquake Occurrence in Southern California

Abstract New paleoseismic investigations on the western segment of the Garlock fault at Twin Lakes, California, reveal evidence for up to six surface ruptures in the past ∼5600 years. Calibrated radiocarbon dates from accelerator mass spectrometer analysis of detrital charcoal constrain the timing of three well‐defined events at Twin Lakes to post‐A.D. 1450 (event A), 720–395 B.C. (event G), and 3425–2200 B.C. (event I); two probable events are also constrained to 625–1525 A.D. (event C), and 155 B.C.–A.D. 615 (event E), and a possible additional event to 3425–2200 B.C. and prior to event I. Our findings offer new insights into mid‐ to late‐Holocene behavior of the western and central segments of the Garlock fault, and regional fault interactions. The timing of the most recent event (MRE) on the western segment likely correlates with the MRE at paleoseismic sites on the central segment, suggesting that both segments do sometimes rupture together during large earthquakes. Evidence for events during periods of seismic quiescence on adjacent segments demonstrates that the western and central segments also sometimes rupture independently of one another. The occurrence of event G during a lull in seismic strain release at 2–5 ka on faults in the eastern California shear zone (ECSZ), contrasts with other studies that suggest the Garlock fault ruptures in phase with ECSZ faults. Our data suggest that seismic strain release on the Garlock fault may actually be more in phase with moment release on the Mojave section of the San Andreas fault and the Transverse Ranges faults of the Los Angeles region. Online Material: Two input files written for the computer program OxCal 4.1.7 and two output files from the OxCal analyses. The first input file calibrates and re‐weights probability density functions (PDFs) for radiocarbon ages based on stratigraphic ordering, and the second input file calculates PDFs for earthquake ages based on radiocarbon ages and stratigraphy bracketing the events.

Technology Focus: Artificial Lift (July 2012)

Technology Focus Calling all technology champions! A few years ago, I ran across the seven steps to stagnation, which was a list originally compiled by Erwin M. Soukup. I got a feeling of déjavu reading through this list because I had heard these same words spoken from many managers and peers over my career. If you search for these seven steps on the Inter-net, you will find different variations; however, the message is the same. The seven steps are We have never done it that way. We are not ready for that yet. We are doing all right without it. We tried it once, and it did not work out. It costs too much. That is not our responsibility. It will not work. Great ideas for technology improvement or development can have an early demise when faced with feedback similar to what is on this list. Even with a patent, a product may never be commercialized without someone to be its champion. While we are fortunate to have many technology champions in the area of artificial lift, we need more. The best way to meet and learn from our industry’s best artificial-lift champions is by attending some of the artificial-lift forums, workshops, and conferences coming up in 2012 and 2013. Please check out the global events calendar on www.spe.org. One major SPE artificial-lift event you will not see on the global calendar, however, is the 2013 Electric Submersible Pump (ESP) Workshop. This is still a section-sponsored event; however, it has grown to be the primary conference for the ESP industry (the most-recent event had 560 attendees from 24 countries). Please go to this address for more information: www.spegcs.org/en/committees/display.asp?committeeid=5. The first paper highlighted features the use of a downhole linear motor to drive a reciprocating-pump system. This is a new technology that is also featured in two papers to be presented at the 2012 Annual Technical Conference and Exhibition in San Antonio, Texas, this October. The two other highlighted papers focus on offshore artificial-lift systems and discuss the unique challenges and concepts being applied. Recommended additional reading at OnePetro: www.onepetro.org. OTC 23079 - Mobile Gas Lift Compressor and Well-Unloading System for Enhancing Oil Production and Reserves, While Reducing Greenhouse-Gas Emissions in Offshore Environments by Jarrad Rexilius, Chevron, et al. OTC 22579 - ESP-Assisted-Production Allocation in Peregrino Field by H. Olsen, Statoil Brazil, et al. SPE 152229 - Calculation of Gearbox Torques of Rotaflex Pumping Units Considering the Elasticity of the Load Belt by G. Takacs, University of Miskolc, et al.

New on-fault evidence for a great earthquake in A.D. 1717, central Alpine fault, New Zealand

The dextral-reverse Alpine fault is the major onshore plate-boundary structure between the Australian and Pacific plates in New Zealand. No previous study of the central portion of the 200-km-long central segment has provided on-fault evidence for the most recent event (MRE). Using lidar (light detection and ranging) data coupled with field mapping, we recognized the main trace of the Alpine fault north of Gaunt Creek (South Island) as a north-striking fault scarp. We enhanced a natural exposure that revealed evidence for repeated late Holocene thrust fault movement. The north-northwest–striking fault zone is characterized by a distinct 5–50-cm-thick clay fault-gouge layer juxtaposing hanging-wall bedrock (mylonites and cataclasites) over unconsolidated late Holocene footwall colluvium. The bedrock is cut by a strath terrace and overlain by mid-Holocene (ca. 5400 calibrated 14 C yr B.P.) alluvial terrace, which has been faulted repeatedly and is conformably overlain by undeformed late Holocene colluvium and alluvium. An unfaulted peat at the base of the scarp is buried by post-MRE alluvium and yields a calibrated 2σ radiocarbon age of A.D. 1710–1930, which dates the MRE as post-1709. Our data are consistent with sparse on-fault data, and validate earlier off-fault records that suggest an A.D. 1717 MRE. The 1717 event had a moment magnitude of M w 8.1 ± 0.1, based on the 380-km-long surface rupture. Because the fault has not ruptured for ∼300 yr, it is likely approaching the end of its seismic cycle and poses a significant seismic hazard to New Zealand.

Paleoseismology of the Yangsan Fault, southeastern part of the Korean Peninsula

The NNE-trending Yangsan Fault (YSF) is the most prominent fault in the southeastern part of the Korean Peninsula and has a continuous trace about 200 km long. Activity on this fault was recently investigated using aerial photographs, topographic analysis, and trenching. The geomorphologic evidence of Late Quaternary faulting is clearly recognized on both the northern (Yugyeri and Tosung-ri areas) and southern parts (Eonyang to Tongdosa area) of the fault. The main YSF is marked by a zone of shattered rock that is tens of meters wide and zone of fault gouge. During the Late Quaternary, right-lateral movement occurred mainly on the southern part, as shown by lowangle striations on the fault plane, elongation of deformation features in the fault gouge. The estimated vertical slip rate is about 0.02-0.07 mm/yr, and the lateral slip rate may be several times larger than the vertical rate. The most recent event occurred prior to deposition of Holocene alluvium. In the northern part, the fault locally changes trend to almost N-S, dips to the east and has reverse movement. The average vertical slip rate is estimated less than 0.1 mm/yr. The most recent event probably occurred after 1314 cal. years BP (A.D. 536).

New Constraints on Deformation, Slip Rate, and Timing of the Most Recent Earthquake on the West Tahoe-Dollar Point Fault, Lake Tahoe Basin, California

High-resolution seismic compressed high intensity Radar pulse (CHIRP) data and piston cores acquired in Fallen Leaf Lake (FLL) and Lake Tahoe provide new paleoseismic constraints on the West Tahoe-Dollar Point fault (WTDPF), the western- most normal fault in the Lake Tahoe Basin, California. Paleoearthquake records along three sections of the WTDPF are investigated to determine the magnitude and recency of coseismic slip. CHIRP profiles image vertically offset and folded strata along the southern and central sections that record deformation associated with the most recent event (MRE) on the WTDPF. Three faults are imaged beneath FLL, and the maximum vertical offset observed across the primary trace of the WTDPF is ∼3:7 m. Coregis- tered piston cores in FLL recovered sediment and organic material above and below the MRE horizon. Radiocarbon dating of organic material constrained the age of the MRE to be between 3.6 and 4.9 k.y. B.P., with a preferred age of 4.1-4.5 k.y. B.P. In Lake Tahoe near Rubicon Point, approximately 2.0 m of vertical offset is observed across the WTDPF. Based on nearby core data, the timing of this offset occurred be- tween ∼3-10 k:y: B.P., which is consistent with the MRE age in FLL. Offset of Tioga- aged glacial deposits provides a long-term record of vertical deformation on the WTDPF since ∼13-14 k:y: B.P., yielding a slip rate of 0:4-0:8 mm=yr. In summary, the slip rate and earthquake potential along the WTDPF is comparable to the nearby Genoa fault, making it the most active and potentially hazardous fault in the Lake Tahoe Basin.

Late Quaternary episodic displacement on a sackung scarp in the central Spanish Pyrenees. Secondary paleoseismic evidence?

A sackung scarp has been investigated by trenching in the central Spanish Pyrenees. This feature is located 18 km to the SW of the North Maladeta Fault, which is the most probable source of the Mw 5.3 Vielha earthquake of 1923. Three displacement events have been inferred for the trenched sackung based on colluvial wedge stratigraphy and fault truncation. The increasing amount of deformation in each successive faulting event may be related to the progressive weakening of the slope through time. A minimum vertical slip rate of 0.19 mm/yr has been calculated for the sackung scarp. Several arguments suggest that the episodic displacement of the analysed sackung is controlled by strong seismic shaking: (a) Spatial association of the sackung features with the North Maladeta Fault; (b) Episodic displacement with a millennial recurrence (5.6 kyr) consistent with the expectable earthquake recurrence interval for a low slip rate fault, like the neighbouring North Maladeta Fault. Demonstrating in future investigations that the sackung features in the area constitute archives of large paleoearthquakes would be of great interest for seismic hazard assessments. They might help to improve the catalogue of paleoearthquakes and might provide information on earthquake recurrence intervals and the age of the most recent event (MRE).

Evidence for ground-rupturing earthquakes on the Northern Wadi Araba fault at the archaeological site of Qasr Tilah, Dead Sea Transform fault system, Jordan

The archaeological site of Qasr Tilah, in the Wadi Araba, Jordan is located on the northern Wadi Araba fault segment of the Dead Sea Transform. The site contains a Roman-period fort, a late Byzantine–Early Umayyad birkeh (water reservoir) and aqueduct, and agricultural fields. The birkeh and aqueduct are left-laterally offset by coseismic slip across the northern Wadi Araba fault. Using paleoseismic and archaeological evidence collected from a trench excavated across the fault zone, we identified evidence for four ground-rupturing earthquakes. Radiocarbon dating from key stratigraphic horizons and relative dating using potsherds constrains the dates of the four earthquakes from the sixth to the nineteenth centuries. Individual earthquakes were dated to the seventh, ninth and eleventh centuries. The fault strand that slipped during the most recent event (MRE) extends to just below the modern ground surface and juxtaposes alluvial-fan sediments that lack in datable material with the modern ground surface, thus preventing us from dating the MRE except to constrain the event to post-eleventh century. These data suggest that the historical earthquakes of 634 or 659/660, 873, 1068, and 1546 probably ruptured this fault segment.

Paleoseismology and Neotectonics of the Shivwits Section of the Hurricane Fault, Northwestern Arizona

The Shivwits section of the Hurricane Fault in northwestern Arizona has been largely ignored in evaluating the seismic hazard posed to the rapidly growing populations of southwestern Utah. To assess this hazard, we conducted studies along the Shivwits section using field observations and geomorphic modeling to understand the Quaternary tectonism of this portion of the Hurricane Fault. We have found evidence that it ruptured with up to 2 to 3 m of vertical displacement per event and likely produced ∼ M 7 earthquakes. Our results suggest that the slip rate along the southern Hurricane Fault has not decreased during the Quaternary. The Moriah Knoll basalt, offset 150 to 200 m along the Hurricane Fault, yielded a maximum long-term slip rate of 0.15 to 0.25 mm/yr, estimated using a new 40Ar/39Ar age of 0.85 ± 0.06 Ma. The late Quaternary slip rates on alluvial fan surfaces offset 2 to 7.4 m were estimated using pedogenic carbonate rind thickness as a calibrated proxy for age. These observations yielded slip rates of ∼0.05 to 0.3 mm/yr. Paleoseismic trench investigations showed that two surface-rupturing events occurred in the past 15-78 k.y. A radiocarbon sample from a most-recent event (MRE) fissure yielded a calibrated age of 8900-10,400 years B.P.; the penultimate event was likely ≥10 k.y. before the most recent event. Slip-rate estimates using the Moriah Knoll basalt (∼0.15-0.24 mm/yr; 850 ka), surface offset (∼0.05-0.3 mm/yr; <100 ka), morphologic modeling (∼0.06-0.21 mm/yr; <100 ka), and observations from the trench (∼0.06-0.34 mm/yr; 15-75 ka) suggest that there has been no detectable change in slip rate in the past 1 million years or so. This implies a constant deformation rate for this portion of the Colorado Plateau Margin; therefore, Basin and Range extension is actively encroaching. Manuscript received 24 November 2003.

The Holocene paleoseismicity of the Aremogna-Cinque Miglia Fault (Central Italy)

Geomorphic and trench investigations are used toanalyze the seismic potential of the Aremogna-CinqueMiglia fault, an active N- to NW-trending, W-facingnormal fault located in Central Apennines. Wereconstructed a complex 16 km-long, as much as 6m-high, fault scarp that displaces late Holocenesediments in the Aremogna and Cinque Miglia basins.The complex surface expression of the fault, withdouble sub-parallel scarp sections, a change in strikeof about 40° and local complexity showingimportant horizontal component, appears to becontrolled by the presence of older tectoniclineaments. We opened two trenches across the faultscarp, used a quarry exposure, and reinterpreted atrench opened by Frezzotti and Giraudi (1989), to findthe geological evidence for three Holocene surfacefaulting earthquakes on the Aremogna-Cinque Migliafault. Based on radiocarbon dating and stratigraphicand climatic considerations timing of the events isconstrained between 800 B.C. and 1030 A.D., between3735 and 2940 B.C., and between 3540 and 5000 B.C.. The most recent event is not reported in the twomillennia-long Italian Catalogues of HistoricalSeismicity. We suggest that the most recent eventcould be one of the Middle Age earthquakes of unknownorigin for which several felt reports exist in Rome.Moreover, we also consider the hypothesis that one ofthe shocks of the ambiguous September 1349 earthquakesequence could be the Aremogna-Cinque Miglia mostrecent event. Anyway, based on historicalconsideration we indicate A.D. 1349 as the youngestpossible age for this event. Finally, we suggest theAremogna-Cinque Miglia fault is part of the easternsecondary Apennines seismogenic belt. The faultparameters we obtain for this fault (i.e., recurrence interval longer than 2000 yr, verticallong-term slip rate of 0.3–0.5 mm/yr and m 6.5–6.8 forthe event) can be used as a first hand reference tocharacterize the seismic behavior of other faultsalong this section of the Apennines.

The northern San Gregorio fault zone: Evidence for the timing of late Holocene earthquakes near Seal Cove, California

AbstractThe San Gregorio fault is the principal active fault west of the San Andreas fault in central coastal California, yet it remains the largest known fault in the region whose seismogenic potential is not known. In this study, we integrate traditional paleoseismic and archaeologic investigations to define the location, style, and timing of slip events on the northern San Gregorio fault at a site near Seal Cove in Moss Beach, California. The on-land portion of the San Gregorio fault at Seal Cove is a late-Holocene active dextral slip fault. Trench excavations revealed a broad zone of faulting, at least 22 m wide, consisting of five Holocene-active strands. These include a single mid-Holocene east-vergent reverse fault and four late-Holocene near-vertical strike-slip faults. The most recent event occurred after the deposition of a native Californian cooking hearth dated A.D. 1270 to A.D. 1400, but prior to the arrival of Spanish missionaries ca. 1775. The penultimate event at the site is less well constrained but appears to have occurred between A.D. 620 and A.D. 1400. The penultimate event was associated with horizontal displacement on the order of 3 m, based on reconstruction of a thrust wedge within the fault zone. The geometry of midden deposits shows a 5 (−2, +6) m deflection along the projection of faults associated with the most recent event (MRE). All or part of this deflection may be associated with the MRE. These displacements are consistent with Mw 7 to 7¼ earthquakes and show that the San Gregorio fault is an active seismogenic source that should be considered in seismic hazard assessments in the San Francisco Bay area.

Large mid-Holocene and late Pleistocene earthquakes on the Oquirrh fault zone, Utah

The Oquirrh fault zone is a range-front normal fault that bounds the east side of Tooele Valley and it has long been recognized as a potential source for large earthquakes that pose a significant hazard to population centers along the Wasatch Front in central Utah. Scarps of the Oquirrh fault zone offset the Provo shoreline of Lake Bonneville and previous studies of scarp morphology suggested that the most recent surface-faulting earthquake occurred between 9000 and 13,500 years ago. Based on a potential rupture length of 12 to 21 km from previous mapping, moment magnitude ( M w) estimates for this event range from 6.3 to 6.6 In contrast, our results from detailed mapping and trench excavations at two sites indicate that the most-recent event actually occurred between 4300 and 6900 yr B.P. (4800 and 7900 cal B.P.) and net vertical displacements were 2.2 to 2.7 m, much larger than expected considering estimated rupture lengths for this event. Empirical relations between magnitude and displacement yield M w 7.0 to 7.2. A few, short discontinuous fault scarps as far south as Stockton, Utah have been identified in a recent mapping investigation and our results suggest that they may be part of the Oquirrh fault zone, increasing the total fault length to 32 km. These results emphasize the importance of integrating stratigraphic and geomorphic information in fault investigations for earthquake hazard evaluations. At both the Big Canyon and Pole Canyon sites, trenches exposed faulted Lake Bonneville sediments and thick wedges of fault-scarp derived colluvium associated with the most-recent event. Bulk sediment samples from a faulted debris-flow deposit at the Big Canyon site yield radiocarbon ages of 7650 ± 90 yr B.P. and 6840 ± 100 yr B.P. (all lab errors are ±1 α). A bulk sediment sample from unfaulted fluvial deposits that bury the fault scarp yield a radiocarbon age estimate of 4340 ± 60 yr B.P. Stratigraphic evidence for a pre-Bonneville lake cycle penultimate earthquake was exposed at the Pole Canyon site, and although displacement is not well constrained, the penultimate event colluvial wedge is comparable in size to the most-recent event wedges. Charcoal from a marsh deposit, which overlies the penultimate event colluvium and was deposited during the Bonneville lake cycle transgression, yields an AMS radiocarbon age of 20,370 ± 120 yr B.P. Multiple charcoal fragments from fluvial deposits faulted during the penultimate event yield an AMS radiocarbon age of 26,200 ± 200 yr B.P. Indirect stratigraphic evidence for an antepenultimate event was also exposed at Pole Canyon. Charcoal from fluvial sediments overlying the eroded free-face for this event yields an AMS age of 33,950 ± 1160 yr B.P., providing a minimum limiting age on the antepenultimate event. Ages for the past two events on the Oquirrh fault zone yield a recurrence interval of 13,300 to 22,100 radiocarbon years and estimated slip rates of 0.1 to 0.2 mm/yr. Temporal clustering of earthquakes on the nearby Wasatch fault zone in the late Holocene does not appear to have influenced activity on the Oquirrh fault zone. However, consistent with findings on the Wasatch fault zone and with some other Quaternary faults within the Bonneville basin, we found evidence for higher rates of activity during interpluvial periods than during the Bonneville lake cycle. If a causal relation between rates of strain release along faults and changes in loads imposed by the lake does exist, it may have implications for fault dips and mechanics. However, our data are only complete for one deep-lake cycle (the past 32,000 radiocarbon years), and whether this pattern persisted during the previous Cutler Dam and Little Valley deep-lake cycles is unknown.

Assessment of the Style and Timing of Surficial Deformation Along the Central Reelfoot Scarp, Lake County, Tennessee

Abstract Stratigraphic and structural relations exposed in a 90-m-long trench across the Reelfoot scarp in the central New Madrid Seismic Zone (NMSZ) provide data to assess the style and timing of late Holocene tectonic surficial deformation in the central NMSZ. Near-surface deposits exposed in the trench include natural levee, overbank, colluvial, and liquefaction-related deposits. The levee deposits consist of fine-grained, cross-bedded sands and silty sands and are overlain by clayey overbank and scarp-derived colluvial deposits. Liquefaction-related features include sand dikes and sills that intrude into the levee and overbank deposits, and a possible older extrusive sand deposit. Charcoal and archaeological artifacts from a deposit inset into and overlying the levee deposits suggest that the levee deposits are older than about A.D. 800 to 900. Charcoal from the overbank deposits yielded an age estimate of cal. A.D. 1310 ± 90; charcoal from the overlying colluvial deposits yielded an age estimate of cal. A.D. 1540 ± 90. Distinct marker beds within the levee deposits define a broad monocline that parallels the ground surface and exhibits more than 5 m of down-to-the-east vertical separation. This fold consists, in part, of four smaller-scale flexures each having amplitudes of about 1 m. Associated with these flexures are numerous west-dipping normal faults that have a total net vertical separation of about 0.4 m in a down-to-the-west sense, which is opposite in sense to that exhibited by the scarp and the silty marker beds. We interpret that these faults are related to extension in the crest of the monocline, and that the monocline represents deformation above a west-dipping reverse fault that reaches or approaches the ground surface east of the trench and the base of the scarp. At the trench site, this interpretation places the surface projection of the fault near the western margin of Reelfoot Lake. Stratigraphic relations exposed in the trench and several shallow boreholes permit identification of at least one and possibly three late Holocene earthquakes. Stratigraphic relations and age-estimates best support the interpretation that the unweathered liquefaction related features exposed in the trench are a result of the 1811–12 earthquakes, and that the scarp-derived colluvial deposits are a result of a prior event. Radiocarbon analyses show that this penultimate event occurred between about A.D. 1310 and A.D. 1540. Stratigraphic evidence of a third event prior to about A.D. 900 is present but equivocal. Given that the most-recent event occurred in A.D. 1812, we estimate that the time between the two most-recent earthquakes large enough to produce liquefaction and/or surface deformation was about 200 to 600 years.