Slip Along the San Andreas Fault Associated with the Earthquake

Some of the fault slip associated with the 1979 Imperial Valley earthquake occurred along other than the Imperial fault and the Brawley fault zone. More than 90 km to the north of the seismogenic fault, a 39-km-long section of the San Andreas fault developed a discontinuous set of surficial fractures soon after the earthquake. This set of fractures consisted of small left-stepping echelon cracks displaying extensional and dextral components of movement. Average dextral slip was about 4 mm, and slip reached 10 mm at one point along the fault. In one locality the cracks formed between Va and 4Vfe days after the main shock, although slippage at depth may have been nearly simultaneous with the earthquake. In general, this set of breaks duplicates the location, style, and slip magnitude of the set that was mapped in 1968 after the Borrego Mountain, Calif, earthquake. Such near-duplication indicates that this section of the San Andreas fault, in particular, is susceptible to small amounts of triggered slip. Although the reasons for such behavior are far from clear, similar behavior of the Imperial fault before 1979 suggests that this section of the San Andreas fault may generate a moderate earthquake within the next few decades

Central California foreshocks of the great 1857 earthquake

Analysis of contemporary accounts indicates that several small to moderate central California earthquakes preceded the great 1857 earthquake by 1 to 9 hr. The earliest events apparently were felt only in the San Francisco area or the Sacramento and Sierran Foothills region. Two later and much more widely felt foreshocks were experienced within the region bounded by San Francisco, Visalia, Fort Tejon, and Santa Barbara. A comparison with felt areas and intensity distributions of modern events of known source and magnitude indicates that these later two shocks were 5 ≦M ≲ 6 and probably originated at some point within an area of radius ≈60 km that includes the southeastern 100 km of the historically creeping segment of the San Andreas fault. The northwestern terminus of the 1857 rupture is probably located along this segment. If the location of these foreshocks is indicative of the epicenter of the main event, then the several-hundred-kilometer main-event rupture propagated principally in a unilateral fashion toward the southeast. This implies that, like many great earthquakes, the 1857 rupture originated on a fault segment historically characterized by moderate activity and propagated into an historically quiet segment. There is a strong possibility that the foreshock activity represents a moderate Parkfield-Cholame sequence similar to those of 1901, 1922, 1934, and 1966. To the extent that such premonitory activity is characteristic of the failure of the 1857 segment of the fault, studies of the creeping segment of the fault may be relevant to the prediction of large earthquakes in central and southern California

Lateral Offsets and Revised Dates of Large Prehistoric Earthquakes at Pallett Creek, Southern California

Recent excavation and new radiocarbon dates of sediments at Pallett Creek are the basis for new conclusions regarding the late Holocene history of the San Andreas fault. Systematic dissection of a 50-m-long, 15-m-wide, 5-m-deep volume of earth, centered on the fault, enables documentation in three dimensions of fault patterns, lateral offsets, and vertical deformation associated with large earthquakes of the past. The excavations expose evidence for 12 earthquakes that occurred between about 260 and 1857 A.D., with an average recurrence interval of about 145 years. Prehistoric slip events that occurred in 1720±50, 1550±70, 1350±50, 1080±65, and 845±75 A.D. have lateral offsets that are comparable to those of the most recent great earthquake of 1857. Thus all of these events represent earthquakes of large magnitude. The lateral offsets of two other events, in 935±85 and 1015±100 A.D., are an order of magnitude smaller and may be interpreted in several ways with regard to the size of these events. The new data constrain the average recurrence interval for large earthquakes at this site to between 145 and 200 years but suggest a monotonic decrease in individual intervals to below this range during the past 900 years. On the basis of these data, the probability of a large earthquake with surficial fault rupture at this site is between 0.2 and 5% during 1984 and 7 and 60% by the year 2000

The Effect of the 1857 Fort Tejon Earthquake on Trees near Wrightwood, California

Trees may suffer damage during major earthquakes due to shaking and faulting of the ground beneath them. External effects, such as topping, root and limb damage, and scars may result in a temporary reduction in the width of their annual growth rings. Tilting and changes in environmental factors, such as light, space and water availability may initiate asymmetric growth. Dendrochronologic techniques enable dating of such growth anomalies, and hence earthquakes

A documentary study of the felt effects of the great California earthquake of 1857

We have collected over 60 hitherto unpublished accounts of the California earthquake of January 9, 1857. We have used them, together with those already known, to estimate felt intensities and prepare an isoseismal map which roughly indicates the level of short-period ground motion experienced during this earthquake. Modified Mercalli intensities of VI to VII occurred in the modern metropolitan areas of southern California, and VI to VIII in the southern San Joaquin Valley. The intensity along the fault was IX or more. Instances of seiching, fissuring, sandblows, and hydrologic changes were reported from Sacramento to the Colorado River delta. Most reports say that shaking lasted between one and three minutes. At least two large aftershocks occurred within a week of the main event

The Effect of the 1857 Fort Tejon Earthquake on Trees near Wrightwood, California

Trees may suffer damage during major earthquakes due to shaking and faulting of the ground beneath them. External effects, such as topping, root and limb damage, and scars may result in a temporary reduction in the width of their annual growth rings. Tilting and changes in environmental factors, such as light, space and water availability may initiate asymmetric growth. Dendrochronologic techniques enable dating of such growth anomalies, and hence earthquakes

The 3 December 1988 Pasadena, California earthquake: Evidence for strike-slip motion on the Raymond fault

The Pasadena earthquake (M_L = 4.9) occurred on 3 December 1988, at a depth of 16 km. The hypocenters of the earthquake and its aftershocks define a east-northeast striking, steeply northwest-dipping surface that projects up to the active surficial trace of the Raymond fault. One of the nodal planes of the focal mechanism of the earthquake parallels the Raymond fault with left-lateral strike-slip movement on that plane, and is consistent with geomorphic and paleoseismic evidence that the Raymond fault is dominantly a left-lateral strike-slip fault. The existence of a component of sinistral slip along the Raymond fault had been suspected prior to the earthquake, but the northward dip of the fault and the prominent scarp along the western portion of its trace had led most workers to conclude that slip along the fault was dominantly reverse

Paleomagnetic measurement of nonbrittle coseismic deformation across the San Andreas Fault at Pallett Creek

Paleomagnetic data have been obtained to address a problem at the Pallett Creek paleoseismological site: the 9 mm/yr slip rate determined from three-dimensional mapping of late Holocene offsets across discrete faults is only a quarter of the expected value. We suspected that nonbrittle deformation adjacent to the faults might account for the 26 mm/yr discrepancy. In our search for the missing slip we collected and analyzed 264 paleomagnetic samples from a 53-m-wide transect across the fault zone. Half the samples came from a unit deposited immediately after a large earthquake of about A.D. 1480; these samples were affected by two large earthquakes that involved rupture at the site in 1812 and 1857. We collected the other half of the samples from a slightly older bed, one that was deposited before the earthquake of about A.D. 1480. Relative to “control” groups composed of 10 samples and collected 50 m from the fault, samples closer to the fault display clockwise rotations of 30° or less. If interpreted as block rotations, the data from the older unit imply that it has sustained a total of 14.0 ± 2.9 m of dextral warp during the past three major earthquakes and that the younger unit has experienced a total of 8.5 ± 1.0 m of warp during the most recent two. Combining these values with the amounts of dextral slip across the mapped fault planes yields dextral offsets of 5.5, 6.25, and 6.25 m for the events of A.D. 1480, 1812, and 1857 and a slip rate of 35.6 ± 6.7 mm/yr. This slip rate, averaged over the past three complete seismic cycles, is consistent with published rates from other sites. Offsets associated with the past three events are remarkably similar. These amounts, however, appear independent of the length of interseismic cycles. These observations suggest (1) that this part of the San Andreas fault has a characteristic strength and (2) that conventional concepts of strain accumulation and relief (for example, time- and slip-predictable models of earthquake occurrence) are unrealistic

Triggered slip along the San Andreas fault after the 8 July 1986 North Palm Springs earthquake

In addition to minor surface cracks in the region of the 8 July 1986 North Palm Springs earthquake, minor aseismic surficial rupture occurred along three segments of the San Andreas fault, 44 to 86 km southeast of the epicenter. Data from a creepmeter and a tiltmeter at one locality suggest that triggered slip occurred coseismically beneath the instruments but took 33 hr to propagate to the surface. That slippage occurred coseismically at depth favors mechanisms for triggered slip that involve dynamic or static strain changes rather than creep migrating from the source region. The distribution of slip along the San Andreas fault associated with the North Palm Springs earthquake differed significantly from that recorded after the moderate 1968 Borrego Mountain, California and 1979 Imperial Valley, California, earthquakes. During these earthquakes, triggered slip occurred along the San Andreas fault in the Durmid Hill area and in the Mecca Hills. Triggered slip associated with the North Palm Springs earthquake occurred in these two areas again, but also extended farther northwest into the Indio Hills, where as much as 9 mm of dextral slip occurred. In the Mecca Hills, surface cracks in 1986 appeared over a shorter fault length than in previous events, and the dextral displacement was smaller, with maximum values of only 2 to 3 mm. On Durmid Hill, surface cracks in 1986 were localized along a 200-m-long stretch of the fault spanning the Mecca Beach creepmeter and extending about 150 m to the southeast. Right-lateral displacements on surface cracks in this area were 1.4 to 2.0 mm, smaller than those observed in previous events. Although the mechanism of triggered aseismic slip is poorly understood, examination of displacement rates for the past several decades to centuries may indicate whether the aseismic slip rate is constant or represents accelerating premonitory failiure of the southernmost San Andreas fault

Highly Variable Recurrence of Tsunamis In the 7,400 Years Before the 2004 Indian Ocean Tsunami

The devastating 2004 Indian Ocean tsunami caught millions of coastal residents and the scientific community off-guard. Subsequent research in the Indian Ocean basin has identified prehistoric tsunamis, but the timing and recurrence intervals of such events are uncertain. Here we present an extraordinary 7,400 year stratigraphic sequence of prehistoric tsunami deposits from a coastal cave in Aceh, Indonesia. This record demonstrates that at least 11 prehistoric tsunamis struck the Aceh coast between 7,400 and 2,900 years ago. The average time period between tsunamis is about 450 years with intervals ranging from a long, dormant period of over 2,000 years, to multiple tsunamis within the span of a century. Although there is evidence that the likelihood of another tsunamigenic earthquake in Aceh province is high, these variable recurrence intervals suggest that long dormant periods may follow Sunda megathrust ruptures as large as that of the 2004 Indian Ocean tsunami