Geology topics
Manuel G. Bonilla
Publications and source records attributed to Manuel G. Bonilla.
Faulting and seismic activity
This chapter traces some of the ideas and concepts leading to the current understanding of the process of faulting and earthquake generation, gives examples of engineering geology investigations contributing to that understanding, describes some engineering projects that have been strongly influenced by the process, and suggests needed research. Each of these topics is discussed in sequence. The understanding of faulting and earthquakes and of the significance of these to engineering has developed over several centuries. John Michell in 1761 was probably the first to publish a cross section of a clearly recognizable fault (Adams, 1938, Fig. 66). Michell did not attribute earthquakes to faulting, but proposed the important idea that seismic vibrations were the result of the propagation of elastic waves in the earth (Adams, 1938). Charles Lyell (1830) emphasized the uplift and depression of land that accompanies earthquakes. He did not attribute earthquakes to faulting, but a contemporary of his evidently did, for the following statement appeared in a review of Lyell’s book (Scrap, 1830, p. 463): The sudden fracture of solid strata by any disruptive force must necessarily produce a violent vibratory jar to a considerable distance along the continuation of these strata. Such vibrations would be propagated in undulations, which may be expected, when influencing a mass of rocks several thousand feet at least in thickness, to produce on the surface exactly the wave-like motion, the opening and shutting of crevices, the tumbling down of cliffs and walls, and other characteristic phenomena of earthquakes. This idea was apparently disregarded, and coseismic faulting, some of which reached the ground surface, was generally considered to be the result rather than the cause of earthquakes until the time of G. K. Gilbert.
The Marina District, San Francisco, California: Geology, history, and earthquake effects
A northwest-trending valley in the bedrock surface is buried by firm Pleistocene bay clay, a dense Pleistocene sand layer, soft Holocene bay sediments, loose to dense Holocene beach and dune sands, and artificial fill that have an aggregate maximum thickness of about 90 m (300 ft). Artificial filling of a cove at the site of The Marina District proceeded gradually from the late 1860s to 1912, when major hydraulic filling was done for the Panama-Pacific International Exposition. The remains of thousands of piles driven for the Exposition very probably still exist and have had unknown effects on long-term ground settlement and earthquake-related ground displacements. Intensity maps of the 1906 earthquake, and seismic recordings and severe building damage in 1989, reported by others, indicate that ground motion was amplified on both natural and artificial ground. This suggests that the configuration of the bedrock surface and the location and thickness of various clay and sand deposits underlying the fill had an important effect on the shaking. However, most of the settlement and liquefaction and the damage to pipelines, building foundations, streets, side-walks, and curbs occurred in areas of artificial fill consisting mainly of loose sand.
Factors affecting the recognition of faults exposed in exploratory trenches
Trenching-a widely used method for evaluating fault activity-has limitations that can mislead investigators. Some segments of fault strands in trench walls may not be visible, and this nonvisibility can lead to incorrect interpretations of time of most recent displacement and recurrence intervals on a fault. We examined the logs of 163 trench exposures and tabulated data on more than 1,200 fault strands to investigate three categories of nonvisibility: (1) strands with obscure (invisible or poorly visible) segments, (2) strands that die out upward, and (3) strands that die out downward. About 14 percent of all the strands have obscure segments. Of the 143 strands on which it is possible to recognize dieout up (limited to strands for which position of ground surface at time of faulting is known), 45 percent do die out upward, and the fraction exceeds 70 percent for strike-slip and reverse faults. Thus a fault strand overlain by an apparently undisturbed deposit is not necessarily older than the deposit. More than 30 percent of all the strands die out downward, providing more evidence that fault strands can end for reasons other than being covered by deposits younger than the fault. Analysis of trench-log data revealed various relations between geologic factors and nonvisibility of fault strands. For example, fault type affects the incidence of nonvisibility, which is generally most common on strike-slip faults, less common on reverse faults, and least common on normal fau Its. The type of material penetrated by the fault also influences nonvisibility, which tends to be more common in soil horizons and sand, and less common in gravel. Dieout down is weakly influenced by fault displacement, decreasing in frequency with increase in displacement; the frequencies of obscure segments and dieout up do not vary consistently with fault displacement. Frequency of obscure segments generally decreases with increase in length of obscure segments, and frequency of dieout up generally decreases with depth of dieout up. Length of obscure segments and depth of dieout up are typically less than the effective thickness of associated beds. On the basis of few data, obscure segments seem to be more common on faults with younger, rather than older, ages of latest displacement. Our study revealed additional relations not directly related to nonvisibility. For example, the median widths of faults crossed by the trenches vary by fault type, strike-slip faults being narrower than dip-slip faults. In the shallow and mostly unconsolidated materials cut by the trenches, fault widths show only an erratic and, at best, weak relationship to fault displacements. Hanging walls are deformed more frequently than footwalls in dip-slip faults, but both walls are deformed at more than 30 percent of the exposures. We tabulated several phenomena that may indicate faulting or provide evidence of prehistorical earthquakes. Rotation of pebbles was identified in 41 percent of the exposures having gravel in the fault zone; type of fault has no strong influence on the incidence of pebble rotation. Fissures were recorded at 52 percent of the exposures and were more common in strike-slip and normal faults than in reverse fau Its. Gouge was reported at 1 5 percent of the exposures; fault type has no significant influence on its frequency. Slickensides were noted at 10 percent of the exposures, and fault type has an unknown influence on their incidence. Slickensides in unconsolidated materials were restricted to clay, silt, and gouge. Other mechanical or hydrologic effects related to faulting or earthquakesrubble, breccia, mixing, crushing, polishing, water barriers, c;ind probable liquefaction effects-were reported at fewer than 1 0 percent of the exposures.
Geodynamic map of the Circum-Pacific region, Arctic sheet
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Minimum earthquake magnitude associated with coseismic surface faulting
No abstract available.
Statistical relations among earthquake magnitude, surface rupture length, and surface fault displacement
In order to refine correlations of surface-wave magnitude, fault rupture length at the ground surface, and fault displacement at the surface by including the uncertainties in these variables, the existing data were critically reviewed and a new data base was compiled. Earthquake magnitudes were redetermined as necessary to make them as consistent as possible with the Gutenberg methods and results, which make up much of the data base. Measurement errors were estimated for the three variables for 58 moderate to large shallow-focus earthquakes. Regression analyses were then made utilizing the estimated measurement errors. The regression analysis demonstrates that the relations among the variables magnitude, length, and displacement are stochastic in nature. The stochastic variance, introduced in part by incomplete surface expression of seismogenic faulting, variation in shear modulus, and regional factors, dominates the estimated measurement errors. Thus, it is appropriate to use ordinary least squares for the regression models, rather than regression models based upon an underlying deterministic relation in which the variance results primarily from measurement errors. Significant differences exist in correlations of certain combinations of length, displacement, and magnitude when events are grouped by fault type or by region, including attenuation regions delineated by Evernden and others. Estimates of the magnitude and the standard deviation of the magnitude of a prehistoric or future earthquake associated with a fault can be made by correlating M s with the logarithms of rupture length, fault displacement, or the product of length and displacement. Fault rupture area could be reliably estimated for about 20 of the events in the data set. Regression of M s on rupture area did not result in a marked improvement over regressions that did not involve rupture area. Because no subduction-zone earthquakes are included in this study, the reported results do not apply to such zones.
Exploratory trench across the Pleasant Valley Fault, Nevada
An exploratory trench was excavated across the 1915 trace of the Pleasant Valley fault 60 km south of Winnemucca, Nevada, to get information on the history of recent displacements on a fault that had produced a major earthquake in historic time, and on the appearance of such a fault in a trench cut in gravels, sands and silts of an alluvial fan. The trench exposed 16 mappable sedimentary units and four soils, including three buried paleosols. The ages of the mapped units could not be narrowly defined but they are of late Quaternary age. Some rodent bones suggest a possible age of about 5,000 years for one of the higher stratigraphic units. The fault zone is very clearly represented in the trench, and, to the full 4-m depth of the trench, consists of a zone of fault rubble as much as 1.5 m wide. Two fractures outside the fault rubble show no vertical displacement. In addition to the fault rubble, the fault is conspicuous because several of the mapped units terminate abruptly against the rubble zone, and because the sediments southeast of the zone are coarser-grained than the sediments northwest of the zone. Maximum vertical component of the 1915 displacement was estimated to be 0.4–0.6 m based on topography and 0.5–0.6 m based on displacement of stratigraphic units including soils. Two or more episodes of vertical displacement, one of about 0.3 m and another totaling at least 1.15 m prior to 1915 are recorded and may have occurred in the last 5,000 years. These and other displacement events prior to 1915 are poorly dated, but that several did occur in late Pleistocene and Holocene time is certain. Lack of wedge-shaped deposits or concentrations of large clasts adjacent to the fault suggest that all displacements were produced in small increments of probably less than one meter each.
Surface faulting near Livermore, California, associated with the January 1980 earthquakes
The earthquakes of 24 January (M s 5.8) 1980 north of Livermore, California, and 26 January (M s 5.2), were accompanied by surface faulting in the Greenville fault zone and apparently in the Las Positas fault zone also. The surface faulting was discontinuous and of small displacement. The main rupture within the Greenville fault zone trended about N.38°W. It was at least 4.2 km long and may have extended southward to Interstate Highway 580, giving a possible length of 6.2 km; both of these lengths included more gaps than observed surface rupture. Maximum displacements measured by us were about 25 mm of right slip (including afterslip through 28 January); vertical components of as much as 50 mm were seen locally, but these included gravity effects of unknown amount. The main break within the Greenville fault zones is very close to a fault strand mapped by Herd (1977, and unpublished data). A subsidiary break within the Greenville fault zone was about 0.5 km. long, had a general trend of N.46°W., and lay 0.12 to 0.25 km east of the main break. It was characterized by extension of as much as 40 mm and right slip of as much as 20 mm. This break was no more than 25 m from a fault mapped by Herd (unpublished data). Another break within the Greenville fault zone lay about 0.3 km southwest of the projection of the main break and trended about N33°W. It was at least 0.3 km long and showed mostly extension, but at several places a right-lateral component (up to 5 mm) was seen. This break was 80 to 100 m from a strand of the Greenville fault mapped by Herd (1977). Extensional fractures within the Greenville fault zone on the frontage roads north and south of Interstate Highway 580 may be related to regional extension or other processes, but do not seem to have resulted from faulting of the usual kind. One exception in this group is a fracture at the east side of Livermore valley which showed progressive increase in right-lateral displacement in February and March, 1980, and is directly on the projection of a fault in the Greenville fault zone mapped by Herd (1977). A group of more than 20 extensional fractures in Laughlin Road 1 km north of Interstate 580 probably are related to small tectonic displacements on faults in the Greenville fault zone. They are adjacent and parallel to two faults mapped by Herd (1977), are diagonal to the road, and most of them developed between 25 and 29 January, a period that included the M s 5.2 shock of 26 January. Observations at two locations indicate tectonic displacement on the Las Positas fault zone as mapped by Herd (1977). At Vasco Road a prominent break on a strand of the fault showed about 0.5 mm of left-lateral strike slip on 7 February. An alinement array across this and other fractures at the locality indicates about 6 mm of left-lateral displacement occurred between 21 February and 26 March. On Tesla Road several right-stepping fractures, one of which showed 1.5 mm of left-lateral strike slip, lie on or close tp previously mapped strands of the Las Positas fault zone. The evidence at these two localities indicates that tectonic surface displacement occurred along at least 1.1 km of the Las Positas fault zone.
Regression analysis of earthquake magnitude and surface fault length using the 1970 data of Bonilla and Buchanan
No abstract available.
Trench exposures across surface fault ruptures associated with the San Fernando earthquake
No abstract available.
Landslide susceptibility in San Mateo County, California
Information on areas susceptible to failure by landsliding is available in many general geologic maps and reports. However, it is commonly presented in complex, technical language, together with data not connected with slope stability. This derivative map is intended to convey such information in nontechnical form so that it will be intelligible to the non-geologist. It portrays only those geologic factors that bear directly on the susceptibility of specific areas to landsliding.
Seismic environment of the Burro Flats site, Ventura County, California: a brief, limited literature review
A limited review of available literature suggests that the maximum horizontal ground acceleration at the Burro Flats site from earthquakes in the region could range from less than 0.1 to 0.49 g. A magnitude 8 earthquake on the nearby San Andreas fault could produce ground acceleration in the range 0.18 to 0.31 g, and an expectable larger earthquake on that fault could produce larger accelerations. Ground motion from possible smaller but closer earthquakes ranges up to 0.49 g for an earthquake of magnitude 6.5 on the adjacent "Burro Flats fault". Estimation of these accelerations is dependent on determining the geologic environment of the site, the appropriate earthquake magnitudes to be assigned significant faults in that environment, and the attenuation of shaking between the earthquake epicenters and the site. The site lies within a tectonically active region--the historically active San Andreas fault is only 34 miles to the northeast, and lesser faults showing evidence of late Quaternary displacement are located closer to the site. Evidence for youthfulness of these lesser faults varies, and except for the active Newport-Inglewood zone and the Santa Ynez fault, they qualify as possible but as yet-unproven active faults. All known faults with appropriate length to site-distance ratios that are reasonably classed as late Quaternary faults are discussed, and are included as potential earthquake generators. Earthquakes of appropriate magnitude to be assigned to each fault are determined by assuming rupture in one event of half the map length of the fault, and applying relations (determined by several authors) between earthquake magnitude and rupture length in historic events to determine magnitudes. These magnitudes are, for the purposes of this brief review, probably reasonable estimates of the capabilities of each fault, although earthquakes of larger magnitude are possible. Accelerations are then determined by assuming earthquakes of the above determined magnitude placed at the closest point to the site on the fault trace, and applying attenuation curves of three different authors. Considerable uncertainty is inherent in the rough estimates of seismic accelerations made herein, for they are dependent on a chain of judgments, each of which, in itself, is uncertain. Present knowledge of the geology of the region is incomplete, so that geometry and structural relations of the faults are in part uncertain, and much evidence bearing on the youth of the faults has yet to be gathered and evaluated. Estimation of earthquake magnitude is also uncertain, and even assuming that approximate magnitude is known rather than estimated from fault length, estimates of maximum ground acceleration may differ greatly depending on the authority used. Further consideration of ground acceleration at the site might refine the estimates made herein and resolve the apparent contradictions between the authorities cited. Attention to frequency and duration of strong shaking would also be appropriate. This study was undertaken at the request of A. J. Pressesky, Assistant Director for Nuclear Safety, Division of Reactor-Development and Technology, U.S. Atomic Energy Commission, in March, 1969. It is based on a brief review of pertinent literature to which the authors had immediate access during the few weeks (April-May, 1969) available for report preparation. Because the report is limited both in scope and thoroughness, it must be considered no more than a first estimate of the tectonic and seismic environment of the Burro Flats site, and should not be considered sufficient, in itself, as a basis for design. The report is intended, however, to indicate the breadth of inquiry that is necessary in the consideration of ground acceleration at sites in California, and to indicate the incomplete status of geologic mapping and other geologic studies in the region. The report describes the tectonic environment of the Burro Flats site, discusses 10 pertinent faults individually, and presents possible earthquake magnitudes for those faults and resultant potential ground accelerations at the site.
The Alaska earthquake, March 27, 1964: effects on transportation, communications, and utilities
This is the forth in a series of six reports that the U.S. Geological Survey published on the results of a comprehensive geologic study that began, as a reconnaissance survey, within 24 hours after the March 27, 1964, Magnitude 9.2 Great Alaska Earthquake and extended, as detailed investigations, through several field seasons. The 1964 Great Alaska earthquake was the largest earthquake in the U.S. since 1700. Professional Paper 545, in 4 parts, describes the effects on transportation, communications, and utilities.
Geologic and seismic investigations of a proposed nuclear power plant site on Bodega Head, Sonoma County, California : Part I--Geologic investigations. Part II--Seismic hazards evaluations
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