Figure 2 - uploaded by Tina Niemi
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(a) Oblique aerial photograph view looking northwest of the San Andreas Fault zone in Marin County from Bolinas Lagoon in the foreground to Tomales Bay. Dashed lines show the approximate limits of the fault zone. D represents the Dogtown trench site; V represents the Vedanta trench site. (Niemi and Hall, 1992). Photography by R. E. Wallace (USGS). (b) Drainage map of the NSAF showing Olema, Bear Valley, and Pine Gulch Creeks. The stippled pattern indicates the approximate limits of fault zone morphology. The dashed lines show the crests of the bounding ridges. (This figure is modified from Lawson [1908].)
Source publication
Our research describes historical observations of the 1906 earthquake rupture trace in Marin County, documents the style of subsurface deformation, and summarizes our interpretation of the number and timing of pre-1906 ground-rupturing earthquakes exposed in 18 trenches excavated across the northern San Andreas fault (NSAF) at the Dogtown site. Dog...
Contexts in source publication
Context 1
... valley or trough 21 km long and 0.8-1.6 km wide cre- ated by the NSAF lies between Inverness and Bolinas ridges. The drowned ends of this trough form Bolinas Lagoon on the southeast and Tomales Bay on the northwest (Fig. 2). The narrowest part of the NSAF valley in Marin County is a struc- tural high at Five Brooks (Fig. 1b) where the Franciscan basement is exposed at the surface. South of this high, the Franciscan basement is overlain by the Plio-Pleistocene Merced Formation, a sequence of fine-grained shallow marine sandstones and siltstones (Blake et ...
Context 2
... fol- low linear valleys between the ridges but locally also cut across the valleys and occasionally cut across the ridges to create water gaps and wind gaps. The two major subsequent streams draining the fault trough are Olema Creek, which flows northwestward into Tomales Bay, and Pine Gulch Creek, which flows southeastward into Bolinas Lagoon (Fig. 2b). For a distance of 3 km, these axial streams overlap each other and flow in opposite directions, and they are separated only a few tens of meters by low tectonic ridges (Fig. ...
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Citations
... 。另外， 可以通过标志地层跨断层的位错变化 获取同震位移。虽然中国古地震研究始于 20 世纪 七八十年代 [1] ， 但能引起国际关注的标杆性古地震 图 1 不同变异系数的 50 个古地震事件分布图 [8] (变异系数越大， 地震周期性越弱， 灰色矩形框内平均为 5 次事件， 其中系数越大， 矩形框中的事件重复特征越不具有代表性) 断裂 SAF [9][10][11][12][13] SAF [3,5,14] SAF [6][7] SAF [15~16] HWF [17][18][19] MCF [20] MCF [21] SAF [22] SAF [23] SMF [24] SAF [25] RCF [26] SAF [27] SAF [28] SAF [29] SAF [30][31] SAF [32] EF [33] SAF [34] SAF [35] SAF [36] PVF [37] GF [38][39] GF [40] CCF [41] SJF [42] MVFZ [43] MVFZ ① LLF [44] RF [45] CF [46] NAF [47] EAF [48] NAF [49] NAF [50] NAF [51] NAF [52] NAF [53] NAF [54] NAF [55] NAF [56] NAF [57] NAF [58] DSF [59] SF [60] DSF [61] DSF [62] DSF [63] YF [64][65] AWF [66] AF [67] HF [69] WF [70] AAF [71] NBF [72] ZMHF [73] ZMHF [74] ZMHF [75] ANHF [76] HYF [77] HYF [78] HYF [79] HYF [80] ATF [81] ATF [82] ATF [83] ATF [83] ...
Ideal trenching sites, which directly determine the quality of the paleoseismic research, are very rare. In this paper, based on 82 research papers of paleoseismic investigations published both in Chinese and English, the authors analyzed the common geomorphological characteristics of trenching sites that recorded long series of paleoearthquakes and coseismic slips. The sites with long paeloearthquake records are usually located at the sag pond, pull-apart basin, small closed basin, or upstream of a shutter ridge, a gentle fan or the toe of a large-drainage fan, which have common depositional environment with few or no depositional hiatuses, high rates of sedimentation, and abundant carbon dating materials. We should not only analyze the modern depositional environment but also its paleoenvironment during the choice of trenching sites. On the other hand, trenching sites for the purpose of recovering coseismic slips are often located at places where frequent incision events are recorded, such as channels, which provide offset piercing markers.
In these studies, trenches are opened in an array with orientations both parallel and perpendicular to fault strike. Due to the fact that good trenching sites are rare, decade-long persistent efforts are usually invested at these sites for detailed studies, to uncover as many as possible the events back in time.
... They were felt in San Juan Bautista (sometimes six in a day), buildings were damaged, and ground cracks were reported near rivers. Therefore, 1776 has become the accepted date for the beginning of the historical period for large (M ∼ 7 or larger) earthquakes in the SFBR and has been used in a series of studies (i.e., Hecker et al., 2005;Hall and Niemi, 2008;Kelson et al., 2008) to trim radiocarbon probability density functions (PDFs) in dating models of recent SFBR paleoearthquakes. There is probably some softness in this date, for although the Mission Dolores was founded in 1776, construction of the permanent mission building was started in 1782 and completed in 1791. ...
... Kelson et al. (2006) summarize event ages at six of these in their discussion of the Fort Ross Orchard site (see below). Subsequently, additional timing of paleoearthquakes has been reported at the Vedanta Marsh (Zhang, 2005) and Dogtown (Hall and Niemi, 2008) sites. The present discussion focuses on the results at three of the sites on land (Fort Ross Orchard, Vedanta Marsh, and Dogtown). ...
... If the rupture extended only to the middle of the horizon, the event would have occurred between 1510 and 1640. Fifteen kilometers to the south of Vedanta, at the Dogtown site, Hall and Niemi (2008) combined observations from earlier paleoseismic investigations (Hall, 1981;Cotton et al., 1982) with more recent trenching. They interpret the occurrence of the penultimate northern San Andreas rupture based on the presence of fissure infills and upward-terminating faults in deposits immediately below a gravel that was only offset by the 1906 event. ...
Stress changes produced by the 1906 San Francisco earthquake had a profound effect on the seismicity of the San Francisco Bay region (SFBR), dramati-cally reducing it in the twentieth century. Whether the SFBR is still within or has emerged from this seismic quiescence is an issue of debate with implications for earth-quake mechanics and seismic hazards. Historically, the SFBR has not experienced one complete earthquake cycle (i.e., the accumulation of stress, its release primarily as coseismic slip during surface-faulting earthquakes, its re-accumulation in the interval following, and its subsequent rerelease). The historical record of earthquake occur-rence in the SFBR appears to be complete at about M 5.5 back to 1850 (Bakun, 1999). For large events, the record may be complete back to 1776, which represents about half a cycle. Paleoseismic data provide a more complete view of the most recent pre-1906 SFBR earthquake cycle, extending it back to about 1600. Using these, we have developed estimates of magnitude and seismic moment for alternative sequences of surface-faulting paleoearthquakes occurring between 1600 and 1776 on the region's major faults. From these we calculate seismic moment and moment release rates for different time intervals between 1600 and 2012. These show the variability in moment release and suggest that, in the SFBR regional plate boundary, stress can be released on a single fault in great earthquakes such as that in 1906 and in multiple ruptures dis-tributed on the regional plate boundary fault system on a decadal time scale.
... However, a rupture (past or future) initiating northwest of the intersection and propagating to it would encounter a branching geometry with the San Gregorio fault on the extensional side. For the segment of the San Andreas fault north of the intersection radiocarbon age ranges indicate the penultimate rupture occurred no earlier than AD 1660 and suggest a more likely occurrence in the 1700 s [Hall and Niemi, 2008;Kelson et al., 2006]. For the San Gregorio fault the most recent large event occurred between AD 1700 and 1776 Pollitz and Schwartz, 2008]. ...
The propagation of the rupture of the Mw7.9 Denali fault
earthquake from the central Denali fault onto the Totschunda fault has
provided a basis for dynamic models of fault branching in which the
angle of the regional or local prestress relative to the orientation of
the main fault and branch plays a principal role in determining which
fault branch is taken. GeoEarthScope LiDAR and paleoseismic data allow
us to map the structure of the Denali-Totschunda fault intersection and
evaluate controls of fault branching from a geological perspective.
LiDAR data reveal the Denali-Totschunda fault intersection is
structurally simple with the two faults directly connected. At the
branch point, 227.2 km east of the 2002 epicenter, the 2002 rupture
diverges southeast to become the Totschunda fault. We use paleoseismic
data to propose that differences in the accumulated strain on each fault
segment, which express differences in the elapsed time since the most
recent event, was one important control of the branching direction. We
suggest that data on event history, slip rate, paleo offsets, fault
geometry and structure, and connectivity, especially on high slip
rate-short recurrence interval faults, can be used to assess the
likelihood of branching and its direction. Analysis of the
Denali-Totschunda fault intersection has implications for evaluating the
potential for a rupture to propagate across other types of fault
intersections and for characterizing sources of future large
earthquakes.
This report details investigations along the Northern San Andreas fault at lake Merced, California, and the offshore North Coast section of the fault. It investigates possible linkages between NSAF ruptures and Cascadia Subduction earthquakes.
The unproven assumption that the Gutenberg–Richter (GR) relationship can be extrapolated to estimate the return time, Tr (1/probability of occurrence), of major and large earthquakes has been shown to be incorrect along 196 faults, so far. Here, two more examples of great, well-known faults that do not produce enough earthquakes to fulfill the hypothesis are analyzed. The 300 km section of the San Andreas fault, California, United States, that ruptured in 1906 in the M 8 San Francisco earthquake, produced 200 earthquakes with M≥2 in the last 52 yr, when about 250,000 such events are expected according to the hypothesis. Along a 250 km section that broke in an M 7.9 earthquake in 1717 along the Alpine fault, New Zealand, the number of reported M≥3.6 earthquakes during the last 34 yr was 100, when about 6000 would be expected, based on the hypothesis. Extrapolating the GR relationships for these two fault segments, one estimates Tr of mainshocks of M 8 to be about 10,000 and 100,000 for the 1717 and 1906 ruptures, respectively. Regardless of choice of analysis parameters, this is by factors of 10–400 larger than estimates based on paleogeology, tectonics, and geodesy. In addition, second catalogs for each case yield estimates of probabilities for M 8 earthquakes along the 1717 and 1906 rupture segments that differ by factors of about 2 and 80 (between 5000 and 98,000 yr) from the first respective catalogs. It follows that the probability of large earthquakes cannot be estimated correctly based on local seismicity rates along major faults.
