Evidence For Large Earthquakes Research Articles

Recurrence and Clustering of Large Earthquakes along the Northern Boundary of Ordos Block: Constraining Paleoearthquakes by an Improved Multiple Trench Constraining Method

AbstractTectonic belts along active tectonic block boundaries comprise one or more active faults; along which, large earthquakes recur. Therefore, it is important to establish the recurrence behavior of large earthquakes along such boundary zones for studying their characteristics and developments. Many paleoearthquake studies make it possible to investigate the recurrence behavior of large earthquakes along the northern boundary of the Ordos block (NBOB). Based on the previous studies, data from 52 trenches were collected to reconstruct prehistoric earthquakes using an improved multiple trench constraining method. This method is based on paleoearthquake indicators and trench location distribution to constrain the rupture time and length, thereby reducing the selection bias of fixed rupture length to construct additional rupture scenarios. The results suggest that the NBOB comprises four normal faults (from west to east): the Langshan Piedmont Fault (LPF), Sertengshan Piedmont Fault (SPF), Wulashan Piedmont Fault (WPF), and Daqingshan Piedmont Fault (DPF); along which, six, seven, eight, and six paleoearthquakes have occurred within approximately 15,000 yr, respectively. In addition, recurrence behaviors of the individual faults exhibit remarkable periodicity. The regional fault network along the NBOB reveals clustered characteristics with six clusters propagating either westward or eastward and a recurrence time of approximately 1,300 yr. Large earthquake events have occurred along the LPF, WPF, and DPF according to the most recent cluster; however, earthquakes were absent along the SPF, and no evidence of large earthquakes was observed along the NBOB after the 849 CE earthquake. Thus, we discuss the possibility of occurrence of large earthquakes along the SPF after the 849 CE earthquake based on earthquake recurrence and cluster migration behavior. Additional research is required to assess the potential risk of the occurrence of a large earthquake along the SPF in the future.

Potentially large post-1505 AD earthquakes in western Nepal revealed by a lake sediment record

According to paleoseismological studies, the last earthquake that ruptured the Main Frontal Thrust in western Nepal occurred in 1505 AD. No evidence of large earthquakes has been documented since, giving rise to the concept of a seismic gap in the central Himalaya. Here, we report on a new record of earthquake-triggered turbidites from Lake Rara, western Nepal. Our lake-sediment record contains eight possibly moderate-to-large earthquake-triggered turbidites during the last 800 years, three of which overlap in age with previously reported Mw ≥ 7 events in western Nepal. Shaking intensity modelling, together with instrumental records, suggests that near-field earthquakes (≤15 km) should have a minimum Mw 5.6, and regional earthquakes (≤80 km) a Mw > ~ 6.5, to trigger turbidites. We present a likely scenario that western Nepal may be as seismically active as central Nepal; however, more data are needed to revaluate the seismic risk in the central Himalaya.

New evidence for large earthquakes on the Central Iran plateau: palaeoseismology of the Anar fault

SUMMARY The Central Iran plateau appears aseismic during the last few millenniums based on instrumental and historical seismic records. Nevertheless, it is sliced by several strike-slip faults that are hundreds of kilometres long. These faults display along-strike, horizontal offsets of intermittent gullies that suggest the occurrence of earthquakes in the Holocene. Establishing this is crucial for accurately assessing the regional seismic hazard. The first palaeoseismic study performed on the 200-km long, NS striking Anar fault shows that this right-lateral fault hosted three large (Mw≈ 7) earthquakes during the Holocene or possibly Uppermost Pleistocene for the older one. These three seismic events are recorded within a sedimentary succession, which is not older than 15 ka, suggesting an average recurrence of at most 5 ka. The six optically stimulated luminescence ages available provide additional constraints and allow estimating that the three earthquakes have occurred within the following time intervals: 4.4 ± 0.8, 6.8 ± 1 and 9.8 ± 2 ka. The preferred age of the more recent event, ranging between 3600 and 5200 yr, suggests that the fault is approaching the end of its seismic cycle and the city of Anar could be under the threat of a destructive earthquake in the near future. In addition, our results confirm a previous minimum slip rate estimate of 0.8 ± 0.1 mm yr−1 for the Anar fault indicating that the westernmost prominent right-lateral faults of the Central Iran plateau are characterized by slip rates close to 1 mm yr−1. These faults, which have repeatedly produced destructive earthquakes with large magnitudes and long recurrence interval of several thousands of years during the Holocene, show that the Central Iran plateau does not behave totally as a rigid block and that its moderate internal deformation is nonetheless responsible for a significant seismic hazard.

First evidence for large earthquakes on the Deshir Fault, Central Iran Plateau

AbstractAlthough sliced by several strike slip faults, a large part of Central Iran remained aseismic during the period of time covered by the instrumental and historical seismic records. Stating the existence of earthquakes in the Holocene is therefore important for the assessment of the regional seismic hazard. A palaeoseismic study of the Deshir fault demonstrates that Central Iran hosted large earthquakes during latest Pleistocene and Holocene. The last event corresponds to 1 m‐deep fissures, which sandy infilling yielded an optically stimulated luminescence (OSL) age of 2.8 ± 1.4 ka. At least two previous events, outlined by older fissures and/or colluvial wedges, have been recorded over the last 10–30 ka. The magnitudes are difficult to assess because the actual slips per event are unknown. The size of the fissures and the significant vertical displacement associated with a colluvial wedge are nevertheless compatible with M ≈ 7 events along a primary strike‐slip surface break.

Evidence for Large Earthquakes on the San Andreas Fault at the Wrightwood, California, Paleoseismic Site: A.D. 500 to Present

We present structural and stratigraphic evidence from a paleoseismic site near Wrightwood, California, for 14 large earthquakes that occurred on the southern San Andreas fault during the past 1500 years. In a network of 38 trenches and creek-bank exposures, we have exposed a composite section of interbedded debris flow deposits and thin peat layers more than 24 m thick; fluvial deposits occur along the northern margin of the site. The site is a 150-m-wide zone of deformation bounded on the surface by a main fault zone along the northwest margin and a secondary fault zone to the southwest. Evidence for most of the 14 earthquakes occurs along structures within both zones. We identify paleoearthquake horizons using in-filled fissures, scarps, multiple rupture terminations, and widespread folding and tilting of beds. Ages of stratigraphic units and earthquakes are constrained by historic data and 72 14C ages, mostly from samples of peat and some from plant fibers, wood, pine cones, and charcoal. Comparison of the long, well-resolved paleoseimic record at Wrightwood with records at other sites along the fault indicates that rupture lengths of past earthquakes were at least 100 km long. Paleoseismic records at sites in the Coachella Valley suggest that each of the past five large earthquakes recorded there ruptured the fault at least as far northwest as Wrightwood. Comparisons with event chronologies at Pallett Creek and sites to the northwest suggests that approximately the same part of the fault that ruptured in 1857 may also have failed in the early to mid-sixteenth century and several other times during the past 1200 years. Records at Pallett Creek and Pitman Canyon suggest that, in addition to the 14 earthquakes we document, one and possibly two other large earthquakes ruptured the part of the fault including Wrightwood since about a.d. 500. These observations and elapsed times that are significantly longer than mean recurrence intervals at Wrightwood and sites to the southeast suggest that at least the southermost 200 km of the San Andreas fault is near failure.

Evidence for large earthquakes in metropolitan los angeles

The Sierra Madre fault, along the southern flank of the San Gabriel Mountains in the Los Angeles region, has failed in magnitude 7.2 to 7.6 events at least twice in the past 15,000 years. Restoration of slip on the fault indicated a minimum of about 4.0 meters of slip from the most recent earthquake and suggests a total cumulative slip of about 10.5 meters for the past two prehistoric earthquakes. Large surface displacements and strong ground motions resulting from greater than magnitude 7 earthquakes within the Los Angeles region are not yet considered in most seismic hazard and risk assessments.

Evidence for large earthquakes at the Cascadia Subduction Zone

Large, historically unprecedented earthquakes at the Cascadia subduction zone in western North America have left signs of sudden land level change, tsunamis, and strong shaking in coastal sediments. The coastal geological evidence suggests that many of the earthquakes occurred at the boundary between the overriding North American plate and the subducting Juan de Fuca plate. This hypothesis is consistent with geodetic measurements and the results of geophysical modeling, which indicate that part of the plate boundary is locked and accumulating elastic strain that will be released during a future large earthquake. Arguments based on potential amounts of seismic slip and likely rupture areas suggest that most or all of the plate boundary earthquakes were magnitude 8 or larger events. The last earthquake or series of earthquakes, about 300 years ago, ruptured the entire 1000‐km length of the subduction zone; if it was a single quake, it probably exceeded magnitude 9. Other earthquakes may have ruptured one or more segments of the subduction zone or may have occurred on faults in the North American plate. Recurrence intervals are uncertain because of difficulties in identifying and dating earthquakes. In southwestern Washington state, intervals for the seven most recent earthquakes average about 500 years but range from less than 200 years to 700–1300 years. Future research on Cascadian plate boundary earthquakes will probably focus on (1) the relation between plate boundary and crustal earthquakes, (2) earthquake magnitude, (3) the areal extent and severity of seismic ground motions, (4) ages and number of past plate boundary earthquakes, and (5) land level changes preceding earthquakes.