Focal mechanisms and aftershock locations of the Songpan earthquakes of August 1976 in Sichuan, China

The precursory swarm, three mainshocks (M = 7.2,6.7, 7.2), and aftershocks of the Songpan earthquakes have been reanalyzed using both local and teleseismic data. The three mainshocks of this sequence occurred on the Huya fault over a 7-day period. Relocations of the aftershocks using local arrival times show that three fault strands were activated during this sequence. Each mainshock occurred on a separate strand, each one south of the strand activated in the previous mainshock, and the aftershock zones of each mainshock appear to abut rather than overlap. Fault plane solutions determined by matching teleseismic P waveforms at World-Wide Standard Seismograph Network stations with synthetic seismograms are consistent with the observed aftershock zones. The first and third mainshocks (M_0 = 1.3 ×10^(19) and 8.4 × 10^(18) N m, respectively) showed almost identical senses of motion, a combination of reverse and left-lateral strike-slip motion, on parallel strands, striking N15°W, that were separated by a large rightstepping en echelon offset. The second mainshock (M_0 = 4.0 × 10^(18) N m), occurred in this offset on a fault at a steep angle (∼125°) to the other two strands and showed almost pure reverse motion. Differences in the orientations of the slip vectors of the three mainshocks show that the first mainshock increased the normal and shear stresses on the fault segment that moved in the second mainshock and that the second mainshock decreased the normal stress on the fault segment activated by the third mainshock. These changes in normal stresses may have given rise to the longer time between the first and second events (5 days) as compared with the time between the second and third events (30 hours). A precursory swarm that preceded the Songpan sequence by 3 years occurred in a volume that surrounded the northernmost part of the planar aftershock zone. The time between the start of the swarm and the mainshocks and the magnitude of the largest event in the swarm are similar to those seen for precursory swarms in Soviet Central Asia

Segmentation and Recent rupture history of the Xianshuihe Fault, Southwestern China

Tibetan Plateau in western Sichuan Province, China, is one of the world's most active faults, having produced at least 8 earthquakes of M ≥ 7 since 1725 on a 350-km-long segment of the fault. In the more limited 150-km long northern segment including Luhuo and Daofu, 5 earthquakes of M ≥ 6.9 have occurred during the past 100 years alone, with well documented overlapping surface ruptures. Some of these events were remarkably similar in magnitude and location to earlier historic events, suggesting that the characteristic earthquake model may apply here. No obvious geometric segmentation characterizes the smoothly curving Luhuo-Daofu sector, although its ends are marked by major left-stepping en echelon offsets that have also been the terminating points of individual earthquake ruptures. Minor en echelon offsets and bends within this segment are associated with local vertical relief, and one 9° bend is located close to the epicenter of the 1973 Luhuo earthquake (M = 7.6). In the southern sector of the Xianshuihe fault, south of Qianning, the faults splits into several branches, one of which broke over its entire 27-km length during the 1955 Kangding earthquake (M = 7.5). The main fault near Kangding is characterized by a series of restraining and releasing bends, associated with corresponding topography, and these bends may be related to the fact that this southern segment of the fault seems to be characterized by infrequent great earthquakes (e.g., M = 7-3/4 in 1786) rather than by more frequent moderate earthquakes (M = ± 7) such as those that characterize the straighter and more continuous Luhuo-Daofu sector to the north. Continuing creep has been documented along some segments of the fault, and this, together with its high degree of activity, superb high-altitude exposures, and other unique attributes, make the Xianshuihe fault one of the most promising sites in the world for earthquake-prediction, hazard-evaluation, and segmentation studies

Segmentation, Geometric Features, and their Seismotectonic Implications for the Holocene Xianshuihe Fault Zone

The Holocene Xianshuihe fault zone, which consists of five main branches with left-lateral strike-slip, can be divided into two segments of different structural styles, jointing at the pull-apart area of Huiyuan Monastery. The northwestern segment has a relatively simple structure. While the southeastern segment exhibits a complex structure composed of several branches. The segmentation of fault structure is the main cause of the different historic strong-earthquake activity, and perhaps, the spatial variation of recent fault slip-rates in different segments. The "multiple-order en echelon" pattern is one of the important geometric characteristics of the fault zone. Based on sizes of stepovers, orders of en echelon discontinuities have been relatively classified. Of which, a left-stepping discontinuity in the order A en echelon, which is apparently accompanied with topographic effect of pull-apart structure, has been taken as the dividing point of the segmentation of the fault zone. This discontinuity is also reflected in stopping surficial ruptures during historic earthquakes with approximate magnitude 7. Stepping discontinuities in en echelon faults of the order B and C also have topographic effects to a certain degree, but do not appear to have been significant in stopping large-earthquake ruptures. The earthquake ground-fissures developed within the Quaternary cover are mainly in forms of even lower order of en echelons. Bending is another important geometric characteristics of the fault zone. Along-strike bends at different degrees occur along the entire fault zone and some parts of faults. Local bends along a fault is the main geometric causes for unsymmetrical rupture-spreading and intensity-attenuating during large earthquakes, and perhaps, one of the tectonic backgrounds for recurrences of large or strong earthquakes at the same localities. Finally, three-dimensional models for the source faults of two large historic earthquakes have been analysed and discussed

中国大陆和香港高程基准面重力位差的测定

Author name used in this publication: 王建华Author name used in this publication: 罗志才Author name used in this publication: 卓力格图Title in Traditional Chinese: 中國大陸和香港高程基準面重力位差的測定Journal title in Traditional Chinese: 大地測量與地球動力學2003-2004 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Segmentation, Geometric Features, and their Seismotectonic Implications for the Holocene Xianshuihe Fault Zone

The Holocene Xianshuihe fault zone, which consists of five main branches with left-lateral strike-slip, can be divided into two segments of different structural styles, jointing at the pull-apart area of Huiyuan Monastery. The northwestern segment has a relatively simple structure. While the southeastern segment exhibits a complex structure composed of several branches. The segmentation of fault structure is the main cause of the different historic strong-earthquake activity, and perhaps, the spatial variation of recent fault slip-rates in different segments. The "multiple-order en echelon" pattern is one of the important geometric characteristics of the fault zone. Based on sizes of stepovers, orders of en echelon discontinuities have been relatively classified. Of which, a left-stepping discontinuity in the order A en echelon, which is apparently accompanied with topographic effect of pull-apart structure, has been taken as the dividing point of the segmentation of the fault zone. This discontinuity is also reflected in stopping surficial ruptures during historic earthquakes with approximate magnitude 7. Stepping discontinuities in en echelon faults of the order B and C also have topographic effects to a certain degree, but do not appear to have been significant in stopping large-earthquake ruptures. The earthquake ground-fissures developed within the Quaternary cover are mainly in forms of even lower order of en echelons. Bending is another important geometric characteristics of the fault zone. Along-strike bends at different degrees occur along the entire fault zone and some parts of faults. Local bends along a fault is the main geometric causes for unsymmetrical rupture-spreading and intensity-attenuating during large earthquakes, and perhaps, one of the tectonic backgrounds for recurrences of large or strong earthquakes at the same localities. Finally, three-dimensional models for the source faults of two large historic earthquakes have been analysed and discussed

Field study of a highly active fault zone: The Xianshuihe fault of southwestern China

The Xianshuihe fault of western Sichuan Province, China, is one of the world's most active faults, having produced 4 earthquakes during this century of magnitude ≥7 along a 350-km length of the fault. At least 8 such events have occurred since 1725. In the more limited 150-km-long segment including Luhuo and Daofu, major earthquakes in 1904, 1923, 1973, and 1981 (M = 7, 7½, 7.6, 6.9) were associated with overlapping surficial fault ruptures and with individual left-lateral displacements as large as 3.6 m. Field studies indicate that this high degree of activity is typical of the fault's longer-term history. The Holocene left-lateral slip rate on the north-western segment of the fault has been 15 ± 5 mm/yr, decreasing to about 5 mm/yr on its southeastern segment, based on radiometrically dated offset stream-channel and terrace deposits and on offset glacial moraines. Physiographic features of active faulting are fully as diagrammatic as those of California's San Andreas fault, mainly because of high-altitude preservation and the absence of cultural modification on this eastern margin of the Tibetan Plateau. Detailed en echelon tensional and pushup features resulting from surface ruptures in 1973, 1955, 1923, and 1893 can still be recognized today, and new data have been collected bearing on the offsets and fault-rupture lengths during these and other events. The locations and magnitudes of historic earthquakes suggest that the characteristic earthquake model may apply to the Xianshuihe fault. Obvious geometric segmentation of the fault has controlled the initiation and termination of ruptures in some events, whereas segmentation control for others remains obscure. Based on the historic record, repeat times estimated from slip rates, and current seismic gaps, two segments are particularly likely sites for M = 7+ earthquakes in the near future: the 65-km-long segment between Daofu and Qianning, and the 135-km-long segment bracketing Kangding. Continuing creep has been documented along some segments of the fault, and this, together with the high degree of activity and other unique attributes, makes the Xianshuihe fault one of the most promising sites in the world for earthquake prediction and hazard-evaluation studies

Precise chronology of the last interglacial period: ^(234)U-^(230)Th data from fossil coral reefs in the Bahamas

A detailed study of ^(238)U-^(234)U-^(230)Th ages was made for different coral species from two Bahamian reefs to determine the time scale of the sea-level high during the last interglacial period, using recently developed mass spectrometric techniques for the measurement of U and Th isotopes. Thirty-seven coral samples were analyzed in replicate. Typical errors at 125 ky are ±1.5 ky (2σ), and 3-ky time intervals appear to be well resolved. This high precision permits detailed chronologic study with time resolution adequate to define stages of the reef's history. These data demonstrate that the time between transgression and regression of the ocean in the last interglacial was about 12 ky, after which sea level fell rapidly at more than ∼2 m/ky. The high sea-level stand began possibly by 132 ky and certainly by 129 ky ago, when sea level reached ∼6 m above present mean low sea level. High sea level was sustained until 120 ky and then fell rapidly. This time period covers the Milankovitch insolation peak at 65°N but is not sharply defined, and the sea-level high lasted from about 4 ky before the insolation peak to 8 ky after it. We find no evidence for a double peak of sea-level rise in the last interglacial episode (that is, high stands at both ∼125 and 142 ky). There appears to be some discrepancy between this precise chronology for a coral reef on a tectonically stable platform and the chronology assigned to deep-sea sediments. The present results show that the duration of the last interglacial led to a 12-ky period of high sea-level stand which is much longer than the insolation peak. The study area consists of two sizeable bank/barrier coral reefs of Sangamon age that crop out along the coast northwest of Cockburn Town on San Salvador Island and at Devil's Point on Great Inagua Island. The initial stage of reef development is not revealed at either site, but the main parts of both reefs are well preserved. At Cockburn Town, the beginning of the reef was marked by colonization of Acropora palmata on hardground areas and possibly other corals at 132 ky ago. Patch reefs at the southeastern flank of the developing reef were growing by 129 to 126 ky ago. By 126 to 123 ky ago, the crest of the reef was formed by the frame-building coral A. palmata, and associated patch reefs flourished along its flanks. A. palmata coral heads grew upward to a mean low sea level ∼6 m above the present one. The reef began to decline as sea level dropped at about 123 to 120 ky ago. Near two measured profiles at Devil's Point, the top of a rubblestone, dominated by A. cervicornis, and patch-reef corals preserved in growth position have been beveled off and represent an ancient wave-cut surface. ^(230)Th ages on corals below and above this surface indicate that it was cut at about 125 ± 1 ky ago

The Active Characteristics of Xianshuihe Fault in Holocene

The Xianshuihe fault is an important strike - slip and strong seismogenic zone in West China. The active fault traces are controlled by existing Xianshuihe fault in bed rock and indicate the feature of arrangement in en echelon. Fault produced landforms such as offset streams, offset terraces, fault scarps, side - hell ridges and sag ponds are indicators of violent displacements on the fault and strong seismic activities since Holocene. Some pieces of information indicate that the long term slip rates of the Xianshuihe fault are remarkably different among the north and the south segments of the fault. It is 15 ± 5mm/y for the north of Qianning and 5.5mm/y for the Kangding fault south of Qianning. The discrepancy of slip rates along the Xianshuihe fault might be the basic reason which cause the difference in seismic activity of the fault. At last, the large earthquake trend on the Xianshuihe fault in the coming future has been discussed in this paper based on slip rates and co-seismic displacements and we believe that the segment of 40km long between Daofu and Qianning might be the most probably candidate one for an earthquake with magnitude about 7