Recurrence Interval Of Large Earthquakes Research Articles

Determination of mean recurrence interval of large earthquakes on the Garzê-Yushu Fault (Dengke Segment) on the eastern margin of the Qinghai–Tibetan Plateau

The Garzê-Yushu Fault is a large left-lateral shear zone in the margin of the Tibetan Plateau. It is part of the slip fault system through which the upper crust material on the margin of the Tibetan Plateau moves eastward. At Xiala Village, Luoxu Town, Shiqu County, Sichuan Province, the Garzê-Yushu Fault cuts through the front edge of the alluvial fan, resulting in fault scarps with heights of 0.2–3.7 m. Some fissures and pressure ridges caused by the Dengke AD 1896 earthquake can be found here. The main tributary river and some gullies on the alluvial fan were also left-laterally displaced. Based on active tectonic geomorphologic surveys, detailed topographic mapping, four trench excavations, and test results from radiocarbon (14C) dating and optically stimulated luminescence (OSL), the following conclusions were drawn in this paper. The trenches uncovered five paleoseismic events in the past 5000 BP, including three paleoearthquake events since AD 1100 ± 70, occurring in AD 1100 ± 70, AD 1550 ± 100, and AD 1896. The latest three paleoearthquakes led to an accumulated 8.7 ± 0.5 m sinistral dislocation with a recurrence interval of approximately 400 y. This is consistent with the quasi-periodic earthquake recurrence behavior model.

Comment on “Historical perspective on seismic hazard to Hispaniola and the northeast Caribbean region” by U. ten Brink et al.

[1] The analysis of historical earthquakes in the northeastern Caribbean by ten Brink et al. [2011, hereafter TB11] addresses the occurrence of large and destructive historical earthquakes associated with the North American-Caribbean plate boundary. One conclusion presented in TB11 is that the recurrence interval for large earthquakes on the left-lateral, strike-slip Septentrional Fault (SF) (Figure 1a) is approximately 300 years. Their Figure 7 shows rupture of the SF across the entire island of Hispaniola in CE 1200, 1542, and 1842. Our comment challenges this model for SF earthquake recurrence because it is inconsistent with our published paleoseismic data that show no large historical earthquake is associated with surface rupture along the SF east of Santiago (Figure 1a) [Prentice et al., 1993;Mann et al., 1998; Prentice et al., 2003]. [2] Historical records of earthquakes prior to the twentieth century rarely provide direct observations linking a specific fault with a particular earthquake. Researchers of historical earthquakes typically formulate hypotheses such as those presented in TB11 associating earthquakes with particular faults, based on their modeled isoseismals. In contrast, paleoseismic studies based on direct observations of the fault zone and reliable age analyses provide fault-specific data to constrain when, within the uncertainty of the data, a particular section of a fault did or did not produce large surface-rupturing earthquakes. A model based solely on historical accounts of ground shaking and damage is useful in the absence of paleoseismic data, but interpretations based on historical accounts alone do not have the same resolution as geologic data that constrain the timing and location of fault surface rupture. The ambiguity inherent in using intensity centers alone to assign a historical earthquake to a particular fault is well illustrated by Figure 3 in Bakun et al., [2012], which shows the intensity center for the 1946 earthquake immediately adjacent to the SF (Figure 1a, green triangle). Yet this earthquake is well known to have occurred on the North Hispaniola thrust belt, not on the SF [Dolan and Wald, 1998]. (We note an error in TB11’s Figure 4 showing the intensity center for the 1946 earthquake: the intensity center and all intensity locations are systematically mislocated about 50 km to the north compared to Figure 3 in Bakun et al., [2012].) TB11 dismiss existing paleoseismic data as a constraint when they attribute the 1842 and 1562 Hispaniola earthquakes to the SF. Our published paleoseismic data contradict their interpretation; therefore, we believe their model for SF recurrence is incorrect.

Estimation of Recurrence Interval of Large Earthquakes on the Central Longmen Shan Fault Zone Based on Seismic Moment Accumulation/Release Model

Recurrence interval of large earthquake on an active fault zone is an important parameter in assessing seismic hazard. The 2008 Wenchuan earthquake (Mw 7.9) occurred on the central Longmen Shan fault zone and ruptured the Yingxiu-Beichuan fault (YBF) and the Guanxian-Jiangyou fault (GJF). However, there is a considerable discrepancy among recurrence intervals of large earthquake in preseismic and postseismic estimates based on slip rate and paleoseismologic results. Post-seismic trenches showed that the central Longmen Shan fault zone probably undertakes an event similar to the 2008 quake, suggesting a characteristic earthquake model. In this paper, we use the published seismogenic model of the 2008 earthquake based on Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data and construct a characteristic seismic moment accumulation/release model to estimate recurrence interval of large earthquakes on the central Longmen Shan fault zone. Our results show that the seismogenic zone accommodates a moment rate of (2.7 ± 0.3) × 1017 N m/yr, and a recurrence interval of 3900 ± 400 yrs is necessary for accumulation of strain energy equivalent to the 2008 earthquake. This study provides a preferred interval estimation of large earthquakes for seismic hazard analysis in the Longmen Shan region.

Long-term earthquake prediction in the Marmara region based on the regional time- and magnitude-predictable model

In order to estimate the recurrence intervals for large earthquakes that occurred in the Marmara region, this region, limited with the coordinates of 39°–42°N, 25°–32°E, has been separated into seven seismogenic sources on the basis of certain seismological criteria, and regional time- and magnitude-predictable model has been applied for these sources. Considering the interevent time between successive mainshocks, the following two predictive relations were computed: log Tt = 0.26 Mmin + 0.06 Mp–0.56 log M0 + 13.79 and Mf = 0.63 Mmin − 0.07 Mp + 0.43 log M0 − 7.56. Multiple correlation coefficient and standard deviation have been computed as 0.53 and 0.35 for the first relation and 0.66 and 0.39 for the second relation, respectively. On the basis of these relations and using the occurrence time and magnitude of the last mainshocks in each seismogenic source, the probabilities of occurrence P(Δt) of the next mainshocks during the next five decades and the magnitude of the expected mainshocks were determined.

Onset of aseismic creep on major strike-slip faults

Time series analysis of spaceborne synthetic aperture radar (SAR) data, GPS measurements, and field observations reveal that the central section of the Izmit (Turkey) fault that slipped with a supershear rupture velocity in the A.D. 1999, M w 7.4, Izmit earthquake began creeping aseismically following the earthquake. Rapid initial postseismic afterslip decayed logarithmically with time and appears to have reached a steady rate comparable to the pre-earthquake full fault-crossing rate, suggesting that it may continue for decades and possibly until late in the earthquake cycle. If confirmed by future monitoring, these observations identify postseismic afterslip as a mechanism for initiating creep behavior along strike-slip faults. Long-term afterslip and/or creep has significant implications for earthquake cycle models, recurrence intervals of large earthquakes, and accordingly, seismic hazard estimation along mature strike-slip faults, in particular for Istanbul which is believed to lie adjacent to a seismic gap along the North Anatolian fault in the Sea of Marmara.

Paleoseismic events and recurrence interval along the Beichuan–Yingxiu fault of Longmenshan fault zone, Yingxiu, Sichuan, China

Prior to the 2008 Wenchuan earthquake (Mw 7.9), Sichuan, China, no earthquake of magnitude larger than M 7.0 had been reported in the historical record for the Longmenshan region. To reveal paleoseismic behaviors in this region, we conducted a paleoseismic study along the 2008 Wenchuan earthquake fault scarp at the town of Yingxiu along the Beichuan–Yingxiu fault. Through a trench investigation, geomorphic analysis and radiocarbon dating, we interpret the occurrence of three paleoseismic events (including the Wenchuan earthquake) within the past 6000years. The cumulative vertical displacement resulting from the three seismic events is ~7.6m. The coseismic uplift for the Wenchuan earthquake is ~2.4m. Integrated with previous paleoseismic results along the fault, the age of the penultimate event is constrained between 3300 and 2300cal BP. We suggest the antepenultimate event occurred later than 5920–5730cal BP. An average vertical slip rate of 0.88–0.91mm/year is calculated from the vertical displacement of U2 since 5940–5730cal BP. We estimate that an average recurrence interval of large earthquakes is at most 3000years. Furthermore, the three paleoseismic events revealed at Yingxiu show similar coseismic vertical displacements. We propose that the strain accumulation and deformation across the Longmenshan fault zone is a relatively stable and continuous process since late Quaternary.

Seven years of postseismic deformation following the 2003 Mw = 6.8 Zemmouri earthquake (Algeria) from InSAR time series

We study the postseismic surface deformation of the Mw 6.8, 2003 Zemmouri earthquake (northern Algeria) using the Multi‐Temporal Small Baseline InSAR technique. InSAR time series obtained from 31 Envisat ASAR images from 2003 to 2010 reveal sub‐cm coastline ground movements between Cap Matifou and Dellys. Two regions display subsidence at a maximum rate of 2 mm/yr in Cap Djenet and 3.5 mm/yr in Boumerdes. These regions correlate well with areas of maximum coseismic uplifts, and their association with two rupture segments. Inverse modeling suggest that subsidence in the areas of high coseismic uplift can be explained by afterslip on shallow sections (<5 km) of the fault above the areas of coseismic slip, in agreement with previous GPS observations. The earthquake impact on soft sediments and the ground water table southwest of the earthquake area, characterizes ground deformation of non‐tectonic origin. The cumulative postseismic moment due to 7 years afterslip is equivalent to an Mw 6.3 earthquake. Therefore, the postseismic deformation and stress buildup has significant implications on the earthquake cycle models and recurrence intervals of large earthquakes in the Algiers area.

Under the Hood of the Earthquake Machine: Toward Predictive Modeling of the Seismic Cycle

Advances in observational, laboratory, and modeling techniques open the way to the development of physical models of the seismic cycle with potentially predictive power. To explore that possibility, we developed an integrative and fully dynamic model of the Parkfield segment of the San Andreas Fault. The model succeeds in reproducing a realistic earthquake sequence of irregular moment magnitude (M(w)) 6.0 main shocks--including events similar to the ones in 1966 and 2004--and provides an excellent match for the detailed interseismic, coseismic, and postseismic observations collected along this fault during the most recent earthquake cycle. Such calibrated physical models provide new ways to assess seismic hazards and forecast seismicity response to perturbations of natural or anthropogenic origins.

Paleoseismology of the North Anatolian Fault at Güzelköy (Ganos segment, Turkey): Size and recurrence time of earthquake ruptures west of the Sea of Marmara

The Ganos fault is the westernmost segment of the North Anatolian Fault that experienced the Mw = 7.4 earthquake of 9 August 1912. The earthquake revealed 45‐km‐long of surface ruptures inland, trending N70°E, and 5.5 m of maximum right lateral offset near Güzelköy. The long‐term deformation of the fault is clearly expressed by several pull‐apart basins and sag ponds, pressure and shutter ridges and offset streams. In parallel with detailed geomorphologic investigations, we measured co‐seismic and cumulative displacements along the fault, and selected the Güzelköy site for paleoseismology. A microtopographic survey at the site yields 10.5 ± 0.5 m and 35.4 ± 1.5 m cumulative lateral offsets of stream channels and geomorphologic features. Seven paleoseismic parallel and cross‐fault trenches document successive faulting events and provide the timing of past earthquakes on the Ganos fault segment. Radiocarbon dating of successive colluvial wedges in trench T1, and the fresh scarplet above (probably 1912 surface rupture) indicate the occurrence of three faulting events since the 14th century. Parallel trenches (3, 5, 6 and 7) expose paleo‐channels and show a cumulative right‐lateral offset of 16.5 ± 1.5 m next to the fault, and 21.3 ± 1.5 m total channel deflection. Radiocarbon dating of past channel units and fault scarp‐related colluvial deposits imply an average 17 +/− 5 mm/year slip rate and 323 ± 142 years recurrence interval of large earthquakes during the last 1000 years on the Ganos fault. The succession of past faulting events and inferred slip rate west of the Marmara Sea provide more constraint on the long‐term faulting behavior in the seismic gap of the North Anatolian Fault and may contribute to a better seismic hazard assessment in the Istanbul region.

Historical perspective on seismic hazard to Hispaniola and the northeast Caribbean region

[1] We evaluate the long-term seismic activity of the North-American/Caribbean plate boundary from 500 years of historical earthquake damage reports. The 2010 Haiti earthquakes and other earthquakes were used to derive regional attenuation relationships between earthquake intensity, magnitude, and distance from the reported damage to the epicenter, for Hispaniola and for Puerto Rico and the Virgin Islands. The attenuation relationship for Hispaniola earthquakes and northern Lesser Antilles earthquakes is similar to that for California earthquakes, indicating a relatively rapid attenuation of damage intensity with distance. Intensities in Puerto Rico and the Virgin Islands decrease less rapidly with distance. We use the intensity-magnitude relationships to systematically search for the location and intensity magnitude MI which best fit all the reported damage for historical earthquakes. Many events occurred in the 20th-century along the plate-boundary segment from central Hispaniola to the NW tip of Puerto Rico, but earlier events from this segment were not identified. The remaining plate boundary to the east to Guadeloupe is probably not associated with M > 8 historical subduction-zone earthquakes. The May 2, 1787 earthquake, previously assigned an M 8–8.25, is probably only MI 6.9 and could be located north, west or SW of Puerto Rico. An MI 6.9 earthquake on July 11, 1785 was probably located north or east of the Virgin Islands. We located MI < 8 historical earthquakes on April 5, 1690, February 8, 1843, and October 8, 1974 in the northern Lesser Antilles within the arc. We speculate that the December 2, 1562 (MI 7.7) and May 7, 1842 (MI 7.6) earthquakes ruptured the Septentrional Fault in northern Hispaniola. If so, the recurrence interval on the central Septentrional Fault is ∼300 years, and only 170 years has elapsed since the last event. The recurrence interval of large earthquakes along the Hispaniola subduction segment is likely longer than the historical record. Intra-arc M ≥ 7.0 earthquakes may occur every 75–100 years in the 410-km-long segment between the Virgin Islands and Guadeloupe.

A record of stable continental region earthquakes from Western Australia spanning the late Pleistocene: Insights for contemporary seismicity

In plate boundary regions moderate to large earthquakes are often sufficiently frequent that fundamental seismic parameters such as the recurrence intervals of large earthquakes and maximum credible earthquake (Mmax) can be estimated with some degree of confidence. The same is not true for the Stable Continental Regions (SCRs) of the world. Large earthquakes are so infrequent that the data distributions upon which recurrence and Mmax estimates are based are heavily skewed towards magnitudes below M w5.0, and so require significant extrapolation up to magnitudes for which the most damaging ground-shaking might be expected. The rarity of validating evidence from surface rupturing palaeo-earthquakes typically limits the confidence with which these extrapolated statistical parameters may be applied. Herein we present a new earthquake catalogue containing, in addition to the historic record of seismicity, 150 palaeo-earthquakes derived from 60 palaeo-earthquake features spanning the last > 100 ka of the history of the Precambrian shield and fringing extended margin of southwest Western Australia. From this combined dataset we show that Mmax in non-extended-SCR is M7.25 ± 0.1 and in extended-SCR is M7.65 ± 0.1. We also demonstrate that in the 230,000 km 2 area of non-extended-SCR crust, the rate of seismic activity required to build these scarps is one tenth of the contemporary seismicity in the area, consistent with episodic or clustered models describing SCR earthquake recurrence. A dominance in the landscape of earthquake scarps reflecting multiple events suggests that the largest earthquakes are likely to occur on pre-existing faults. We expect these results might apply to most areas of non-extended-SCR worldwide.

The M S7.1 Yushu earthquake surface rupture and large historical earthquakes on the Garzê-Yushu Fault

As revealed by field investigations, the co-seismic surface rupture zone of the 2010 M S7.1 Yushu earthquake, Qinghai is a characteristic sinistral strike-slip feature consisting of three distinct sinistral primary ruptures, with an overall strike of 310°–320° and a total length of 31 km. In addition, an approximately 2-km-long en-echelon tensile fissure zone was found east of Longbao Town; if this site is taken as the north end of the rupture zone, then the rupture had a total length of ∼51 km. The surface rupture zone is composed of a series of fissures arranged in an en-echelon or alternating relationship between compressive bulges and tensile fissures, with a measured maximum horizontal displacement of 1.8 m. The surface rupture zone extends along the mapped Garze-Yushu Fault, which implicates it as the seismogenic fault for this earthquake. Historically, a few earthquakes with a magnitude of about 7 have occurred along the fault, and additionally traces of paleoearthquakes are evident that characterize the short-period recurrence interval of large earthquakes here. Similar to the seismogenic process of the 2008 Wenchuan earthquake, the Yushu earthquake is also due to the stress accumulation and release on the block boundaries resulting from the eastward expansion of Qinghai-Tibet Plateau. However, in contrast with the Wenchuan earthquake, the Yushu earthquake had a sinistral strike-slip mechanism resulting from the uneven eastward extrusion of the Baryan Har and Sichuan-Yunnan fault blocks.

Late Pleistocene–Holocene activity of the Zemuhe Fault on the southeastern margin of the Tibetan Plateau

A N–S-trending active fault zone, which includes the Anninghe, Zemuhe, and Xiaojiang faults, forms the tectonic boundary of the Tibetan Plateau on its southeastern margin, marking the limit of the eastward extrusion of the Tibetan Plateau. In this paper, we document the Late Pleistocene–Holocene activity of the Zemuhe Fault, which is the central segment of the fault zone. We have been able to determine slip rate and the average recurrence interval of large earthquakes upon the fault, based on aerial photograph interpretations, field investigations, trench excavations, and radiocarbon age data. The Zemuhe Fault is mainly characterized by left-lateral offsets with a horizontal slip rate of ~ 10–12 mm/yr, and a vertical slip component of ~ 2.0–2.4 mm/yr. Trench investigations and 14 C ages reveal that at least four large earthquakes occurred during the last 1100–1500 years, indicating an average recurrence interval of ~ 300–400 years for M 7.0–7.5 earthquakes, consistent with the documented history of the region. Our results confirm that the Zehume Fault plays an important role as a tectonic boundary in releasing the strain energy caused by the southeastwards motion of the Tibetan Plateau. The energy is released in the form of repeated large earthquakes.

An application of regional time and magnitude predictable model for long-term earthquake prediction in the vicinity of October 8, 2005 Kashmir Himalaya earthquake

A regional time and magnitude predictable model has been applied to estimate the recurrence intervals for large earthquakes in the vicinity of 8 October 2005 Kashmir Himalaya earthquake (25°–40°N and 65°–85°E), which includes India, Pakistan, Afghanistan, Hindukush, Pamirs, Mangolia and Tien-Shan. This region has been divided into 17 seismogenic sources on the basis of certain seismotectonics and geomorphological criteria. A complete earthquake catalogue (historical and instrumental) of magnitude Ms ≥ 5.5 during the period 1853–2005 has been used in the analysis. According to this model, the magnitude of preceding earthquake governs the time of occurrence and magnitude of future mainshock in the sequence. The interevent time between successive mainshocks with magnitude equal to or greater than a minimum magnitude threshold were considered and used for long-term earthquake prediction in each of seismogenic sources. The interevent times and magnitudes of mainshocks have been used to determine the following predictive relations: logTt = 0.05 Mmin + 0.09 Mp − 0.01 log M0 + 01.14; and Mf = 0.21 Mmin − 0.01 Mp + 0.03 log M0 + 7.21 where, Tt is the interevent time of successive mainshocks, Mmin is minimum magnitude threshold considered, Mp is magnitude of preceding mainshock, Mf is magnitude of following mainshock and M0 is the seismic moment released per year in each seismogenic source. It was found that the magnitude of following mainshock (Mf) does not depend on the interevent time (Tt), which indicates the ability to predict the time of occurrence of future mainshock. A negative correlation between magnitude of following mainshock (Mf) and preceding mainshock (Mp) indicates that the larger earthquake is followed by smaller one and vice versa. The above equations have been used for the seismic hazard assessment in the considered region. Based on the model applicability in the studied region and taking into account the occurrence time and magnitude of last mainshock in each seismogenic source, the time-dependent conditional probabilities (PC) for the occurrence of next shallow large mainshocks (Ms ≥ 6.5), during next 20 years as well as the expected magnitudes have been estimated.

Evidence for a Tang-Song Dynasty great earthquake along the Longmen Shan Thrust Belt prior to the 2008 M w 7.9 Wenchuan earthquake, China

The magnitude (M w) 7.9 Wenchuan earthquake occurred on 12 May 2008 in the Longmen Shan region of China, the transition zone between the Tibetan Plateau and the Sichuan Basin, resulting in widespread damage throughout central and western China. The steep, high-relief eastern margin of the Tibetan Plateau has undergone rapid Cenozoic uplift and denudation accompanied by folding and thrusting, yet no large thrust earthquakes are known prior to the 2008 M w 7.9 Wenchuan earthquake. Field and excavation investigations reveal that a great historical earthquake occurred in the Sichuan region that ruptured a >200-km-long thrust fault within the Longmen Shan Thrust Belt, China, which also triggered the 2008 M w 7.9 Wenchuan earthquake. The average co-seismic slip amount produced by this historical earthquake is estimated to be 2–3 m, comparable with that caused by the 2008 Wenchuan earthquake. Paleoseismic and archaeological evidence and radiocarbon dating results show that the penultimate great earthquake occurred in the Sichuan region during the late Tang-Song Dynasty, between AD 800 and 1000, suggesting a recurrence interval of ~1,000–1,200 years for Wenchuan-magnitude (M = ~8) earthquakes in the late Holocene within the Longmen Shan Thrust Belt. This finding is in contrast with previous estimates of 2,000–10,000 years for the recurrence interval of large earthquakes within the Longmen Shan Thrust Belt, as obtained from long-term slip rates based on the Global Positioning System and geological data, thereby necessitating substantial modifications to existing seismic-hazard models for the densely populated region at the eastern marginal zone of the Tibetan Plateau.

Landslides associated with the May 12, 2008 Wenchuan earthquake: Implications for the erosion and tectonic evolution of the Longmen Shan

Satellite images and aerial-photography following the Wenchuan earthquake show that a broad area experienced severe landsliding in association with the main earthquake and aftershocks. A zone of high density landsliding (> 10% landslide surface area affected) stretches for ~ 240 km along the earthquake region, in proximity to the Yingxiu, Beichuan and Pennguan faults, which ruptured during the earthquake. The width of this severe landsliding zone varies along the strike, from ~ 25–30 km wide in the southwestern section of the earthquake region, to ~ 3–5 in the northeastern section. These variations coincide with differences in hillslope gradients, bedrock lithologies, co-seismic slip and ground shaking observed in these sections of the earthquake region. In general, there is good correlation between the magnitude and distribution of ground shaking experienced during the earthquake and mapped landslide density, though limited data and varying fault models prevent any in-depth comparison with high-resolution maps of ground shaking experienced. Landslides associated with Wenchuan earthquake highlight interesting aspects of erosion and tectonic evolution of the Longmen Shan. Short-term erosion rates in the Longmen Shan region measured from cosmogenic nuclides prior to the earthquake were ~ 0.2–0.3 mm/yr, reflecting the last 2000–3000 years — a time period shorter than the estimated recurrence interval of large earthquakes along the fault that ruptured, while long-term erosion rates measured by low-temperature thermochronology were higher, 0.5–0.7 mm/yr for the last 8–10 Ma. Integrating the total amount of erosion associated the earthquake into the short-term erosion rate estimates pushes the rate much higher. This suggests that the earthquake and associated landslide erosion was necessary for the landscape to ‘catch-up’ and balance the long-term tectonic growth of the Longmen Shan, particularly in the southwestern section of the earthquake region. Taken together, the earthquake and associated landslide erosion can be seen as the most recent expression of the rapid deformation/uplift and erosion that previous geomorphic evidence and geodynamic models had argued for along Longmen Shan front.

Prehistoric seismicity-induced liquefaction along the western segment of the strike-slip Kunlun fault, northern Tibet

Abstract The 2001 M w 7.8 Kunlun earthquake occurred in northern Tibet, and produced a 450-km-long surface rupture zone along the western segment of the strike-slip Kunlun fault. There are, however, no historic or instrumental records of large earthquakes in this fault segment. Field investigations of liquefaction structures and radiocarbon dating results reveal that at least three large earthquakes, including the 2001 earthquake, occurred in the western segment of the Kunlun fault during the past seven to nine centuries. Liquefaction structures formed in alluvial deposits composed of sand-gravel yielding 14 C ages of 679–901 yr bp are observed on the current stream channel which is sinistrally offset 75–82 m, including 3–6 m displacement produced by the 2001 event. On the basis of the field investigations and 14 C dating results, we conclude that the liquefaction structures and subsequent faulting events were caused by at least two large earthquakes of M&gt;7 prior to the 2001 earthquake and the average recurrence interval of large earthquakes is estimated to be about 400 years in the late Holocene.

Nonuniform Slip Rate and Millennial Recurrence Interval of Large Earthquakes along the Eastern Segment of the Kunlun Fault, Northern Tibet

The Kunlun fault is a major, active, left-lateral strike-slip fault upon the Tibet Plateau. Analysis of the long-term behavior of the fault is important in understanding the tectonic behavior of strike-slip faults and in assessing the mode of continental deformation and nature of seismic hazards upon the Tibet Plateau. However, our knowledge of the paleoseismicity and seismic behavior of the Kunlun fault remains limited and controversial because of difficult access, lack of field data, and the low spatial density of modern monitoring instruments. This study involved field investigations, fault-trench excavations, and an analysis of remote sensing images of the eastern (Maqen-Maqu) segment of the Kunlun fault near the town of Maqu in Gansu Province, China, revealing systematic deflections of rivers and gullies and mole track structures along the fault. Trench excavations revealed horizons of fluvial sand–gravel and silt–soil deposits, containing organic soil and peat, deformed by and offset along the fault. Based on the distribution of faulted and unfaulted sediments, fault-related structures, and ![Graphic][1] age data, nine paleoseismic surface-faulting events are identified over the past 9000–10,000 yr, of which the youngest event is constrained to the past 1000–1500 yr. A millennial recurrence interval of large earthquakes is estimated with an average slip of ∼3 m. An average Holocene slip rate of 3 mm/yr is estimated for this portion of the Kunlun fault. These results, together with those of previous studies, demonstrate that the along strike-slip rate is nonuniform, diminishing toward the east at an average gradient of 1 mm/100 km. This is in contrast to previous findings that the slip rate along the Kunlun fault is uniform and on the order of 11.5±2 mm/yr. [1]: /embed/inline-graphic-1.gif

Surface Ruptures Associated with the 1937 M 7.5 Tuosuo Lake and the 1963 M 7.0 Alake Lake Earthquakes and the Paleoseismicity along the Tuosuo Lake Segment of the Kunlun Fault, Northern Tibet

Examination of aerial photos taken in the late 1960s and field mapping allowed us to re-evaluate the surface ruptures associated with the 1937 M 7.5 Tuosuo Lake and 1963 M 7.0 Alake Lake earthquakes along the Tuosuo Lake and Alake Lake segments, central part of the Kunlun fault. Individual ruptures can be distinguished by their freshness of surface expression, slip distributions, and surficial geometry; they are separated by the geometrical bend that probably acts as an effective barrier to rupture propagation of these earthquakes. The rupture (∼40 km long with 1–2 m left-lateral slip) of the 1963 Alake Lake earthquake occurred on the west of the bend, whereas that of the 1937 Tuosuo Lake earthquake (about 150 km long with 4.1 m mean left-lateral offset) occurred on the east of the bend. The change of dip- slip sense reflects the characteristics of jogs and bends along the segment boundaries. Systematically and progressively displaced terrace risers and gullies indicate repeated activity of the fault during the late Quaternary. Five trenches excavated across the surface ruptures along the Tuosuo Lake segment reveal distinct indicators of paleoseismic events. At least nine earthquakes are constrained in the Holocene deposits, and a 630 ± 130 yr, upper-bound recurrence interval of large earthquakes in the past 2000 years is determined.

Geophysical surveys of earthquake-induced liquefaction deposits in the New Madrid seismic zone

We explore the effectiveness and limitations of electrical and electromagnetic methods in imaging buried, earthquake-induced liquefaction deposits. Geophysical surveys conducted at liquefaction sites in the New Madrid seismic zone (NMSZ) in the central United States demonstrate that these subsurface-imaging techniques can be useful tools in paleoseismology. Paleoseismological studies of liquefaction features provide one of the few means for estimating recurrence intervals of large earthquakes in the NMSZ, a region with widespread evidence of strong ground shaking but short instrumental record. Noninvasive geophysical methods minimize ground disturbance during these studies, an attribute of particular importance when the studies are conducted at federally protected archaeological sites. Surveys such as those described here can be used to locate buried liquefaction deposits and to site trenches for detailed geologic studies.