Title: Journal of Geophysical Research-Oceans

Title: Journal of Geophysical Research-Solid Earth

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: Impact Factor

? Matthews, M.V. and Segall, P. (1993), Estimation of depth-dependent fault slip from measured surface deformation with application to the 1906 San Francisco earthquake. Journal of Geophysical Research-Solid Earth, 98 (B7), 12153-12163.

Abstract: We estimate the depth distribution of slip in the 1906 San Francisco earthquake by applying new inversion methods to triangulation data collected near Point Arena, California. Modeling crustal deformation from antiplane slip in a half-space, we define three regularizing functionals to produce minimum-norm inverse problems. These functionals measure components of the coseismic change in elastic strain energy, stress magnitude, and stress variability and are all quadratic in slip. Orthogonality conditions define finite-dimensional representations of slip estimates. Coefficients of basis functions in these representations are estimated by damped least squares with damping parameter and faulting depth chosen by cross validation. The best resulting estimates exhibit right lateral slip to depths of 15 to 20 km with about 6 m of surface slip. This is consistent with directly observed offsets of cultural landmarks at the surface and indicates deeper faulting than has been previously inferred for the 1906 earthquake.

Shaw, J.H. and Suppe, J. (1996), Earthquake hazards of active blind-thrust faults under the central Los Angeles basin, California. Journal of Geophysical Research-Solid Earth, 101 (B4), 8623-8642.

Abstract: We document several blind-thrust faults under the Los Angeles basin that, if active and seismogenic, are capable of generating large earthquakes (M = 6.3 to 7.3). Pliocene to Quaternary growth folds imaged in seismic reflection profiles record the existence, size, and slip rates of these blind faults. The growth structures have shapes characteristic of fault-bend folds above blind thrusts, as demonstrated by balanced kinematic models, geologic cross sections, and axial-surface maps. We interpret the Compton-Los Alamitos trend as a growth fold above the Compton ramp, which extends along strike from west Los Angeles to at least the Santa Ana River. The Compton thrust is part of a larger fault system, including a decollement and ramps beneath the Elysian Park and Pales Verdes trends. The Cienegas and Coyote Hills growth folds overlie additional blind thrusts in the Elysian Park trend that are not closely linked to the Compton ramp. Analysis of folded Pliocene to Quaternary strata yields slip rates of 1.4±0.4 mm/yr on the Compton thrust and 1.7±0.4 mm/yr on a ramp beneath the Elysian Park trend. Assuming that slip is released in large earthquakes, we estimate magnitudes of 6.3 to 6.8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps. Multiple-segment ruptures could yield larger earthquakes (M = 6.9 to 7.3). Relations among magnitude, coseismic displacement, and slip rate yield an average recurrence interval of 380 years for single-segment earthquakes and a range of 400 to 1300 years for multiple-segment events. If these newly documented blind thrust faults are active, they will contribute substantially to the seismic hazards in Los Angeles because of their locations directly beneath the metropolitan area.

Prentice, C.S. and Ponti, D.J. (1997), Coseismic deformation of the Wrights tunnel during the 1906 San Francisco earthquake: A key to understanding 1906 fault slip and 1989 surface ruptures in the southern Santa Cruz Mountains, California. Journal of Geophysical Research-Solid Earth, 102 (B1), 635-648.

Abstract: The Wrights tunnel is an abandoned railroad tunnel that crosses the San Andreas fault in the southern Santa Cruz Mountains in the vicinity of the 1989 Loma Prieta earthquake. The tunnel was damaged and deformed during the 1906 San Francisco earthquake and a plot showing postearthquake measurements made in the tunnel is given by Lawson [1908]. The amount of offset shown on this plot (1.5 m) has been used in several studies as being representative of the amount of fault offset along this segment of the San Andreas fault in 1906. Our historical research shows that different observers reported different amounts of fault offset in the tunnel and that the 1.5 m given on the plot is not a surveyed measurement. In addition, the plot of the tunnel has been interpreted in several previous studies as evidence of a broad (1.5 km) zone of faulting beneath Summit Ridge. Our analysis shows that this plot need not indicate a broad zone of deformation. Our historical research and modeling of the tunnel measurements indicate that faulting was confined to a zone less than 400 m wide and that 60-85% of the coseismic slip occurred across a single fault plane. There is no evidence for offset across a second shear zone beneath Summit Ridge in 1906. This implies that surface fractures reported on Summit Ridge in 1906 were riot associated with significant deformation of the tunnel, implying that they were shallow, surficial features. By analogy, the very similar fractures that occurred on Summit Ridge in 1989 were also probably the result of shallow gravitational, rather than deep-seated tectonic, processes. Our modeling also indicates that total coseismic, near-surface slip across the San Andreas fault zone in the Wrights tunnel in 1906 was at least 1.7-1.8 m.

Thatcher, W., Marshall, G. and Lisowski, M. (1997), Resolution of fault slip along the 470-km-long rupture of the great 1906 San Francisco earthquake and its implications. Journal of Geophysical Research-Solid Earth, 102 (B3), 5353-5367.

Abstract: Data from all available triangulation networks affected by the 1906 earthquake have been combined to assess the trade-off between slip resolution and its uncertainty and to construct a conservative image of coseismic slip along the rupture. Because of varying network aperture and station density, slip resolution is very uneven. Although slip is determined within uncertainties of±1.0 m along 60% of the fault, constraints are poor on the remaining, mostly offshore portions of the rupture. Slip decreases from maxima of 8.6 and 7.5 m at Shelter Cove and Tomales Bay to 4.5 m near Mount Tamalpais and 2.7 m at Loma Prieta. The geodetically derived slip distribution is in poor agreement with estimates based on analysis of S wave seismograms, probably because these waves register only 20-30% of the total seismic moment obtained from longer-period surface waves. Consideration of a range of fault geometries for 1906 slip near Loma Prieta indicates right-lateral motions lie between 2.3 and 3.1 m, These values are considerably greater than the 1.5 m of measured surface slip on which several assessments of high earthquake hazard for this fault segment were based. This factor, along with the absence of 1989 slippage where 1906 surface slip was used to make the forecasts, casts doubt on some claims of success in predicting the 1989 M = 6.9 Loma Prieta earthquake.

Collier, R.E.L., Pantosti, D., DAddezio, G., DeMartini, P.M. and Masana, E. and Sakellariou, D. (1998), Paleoseismicity of the 1981 Corinth earthquake fault: Seismic contribution to extensional strain in central Greece and implications for seismic hazard. Journal of Geophysical Research-Solid Earth, 103 (B12), 30001-30019.

Abstract: We present paleoseismological trenching results for the active Skinos Fault, which ruptured the surface on the Alkyonides Gulf basin margin in the 1981 Gulf of Corinth earthquake sequence. Three trenches expose evidence of up to six previous events which are comparable to the 1981 deformation in terms of size and geometry. Vertical displacement produced by the 1981 earthquake ranged from 0.45 to 1.3 m at the Bambakies Fan trench sites, decreasing towards the eastern fault tip east of the trenches. Trench 1 reveals two previous events with vertical displacements between 0.5 and 1.2 m since 390 A.D. Trench 2 reveals five or six previous events, but these are not dated. Trench 3 reveals four previous events since 670 A.D. Vertical displacements associated with interpreted paleoearthquakes at the trench sites are less than or equal to 1.2 m. The recurrence interval on the Skinos Fault is estimated to average 330 years. However, significant variation in recurrence interval is allowed by the available radiometric dates. Average vertical displacement rates derived from the trenches are in the range 0.7-2.5 mm/yr. A similar long-term average vertical displacement rate of 1.2-2.3 mm/yr is estimated fur the lifespan of the basin-bounding fault. This equates to st horizontal seismic strain contribution of less than or equal to 2.5 mm/yr from the Skinos Fault. This local seismic strain rate overlaps, within error, with geodetically determined velocities across the Alkyonides Gulf assumed to represent uniform deep-crustal strain. Thus seismic deformation on the basin-bounding fault system may take up the major part of extension across the basin, and aseismic strain is not necessitated by the data, if correct, this would imply that geodetically determined strain rates may be used as a proxy for potential seismic moment release in seismic hazard analyses for this region.

King, C.Y., Azuma, S., Igarashi, G., Ohno, M., Saito, H. and Wakita, H. (1999), Earthquake-related water-level changes at 16 closely clustered wells in Tono, central Japan. Journal of Geophysical Research-Solid Earth, 104 (B6), 13073-13082.

Abstract: Water-level/pressure data recorded at a dense network nf 16 wells of depths ranging from 23 to 201 m within 400 m of the Tone Mine in Gifu prefecture, central Japan, have been studied in search of possible earthquake-related changes. The study area is traversed by the east-west trending Tsukiyoshi fault, which is ordinarily impermeable, sustaining a significantly higher groundwater pressure on the north side of the fault than the south side. On March 16, 1997, a local earthquake of magnitude 5.8 occurred about 50 km south of Tone. The related water-level/pressure Changes recorded at different sites showed different features. At three deeper wells on the north side of the fault the water level and pressure showed large coseismic drops (up to 29 cm) followed by larger rises (of as much as 1.8 m). Two days before the earthquake the water level at another deeper well began to show a total drop of about 3 m during the next 2 months, Most of the shallow wells on the north side of the fault showed water-level drops after the earthquake. On the south side, however, only a small delayed surge and a subsequent gradual increase were recorded at a deeper well. Long-term data since 1989 at one of the deeper wells on the north side showed coseismic drops and recoveries for 25 moderate Local and large distant earthquakes up to about 1000 km away. The different observed earthquake-related water-level/pressure changes cannot be explained by the static strain field calculated on the basis of poroelastic dislocation models; the changes require earthquake-related permeability changes at the monitoring sites. The high sensitivity of some of the water wells to seismic shaking may be due to the near-critical condition of the tapped aquifer/barrier system beginning several years prior to the March 16 earthquake, such that the local seismic shaking caused by each of the above mentioned earthquakes was sufficient to produce some quickly recoverable fissures in the system, resulting in the observed water-level changes. The sensitivity appears to be variable, showing an apparent decrease during a 1-year period after the earthquake.