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At least 1,711 records · Page 95Linked to original sources
USGS2012

Historic surface slip along the San Andreas Fault near Parkfield, California

The Parkfield Earthquake Prediction Experiment is focusing close attention on the 44-km-long section of the San Andreas fault that last ruptured seismically in 1966 ( M s 6.0). The 20-km-long central segment of the 1966 Parkfield rupture, extending from the mainshock epicenter at Middle Mountain southeastward to Gold Hill, forms a 1- to 2-km salient northeastward away from the dominant N40°W strike. Following the 1966 earthquake afterslip, aseismic slip has been nearly constant. Moderate Parkfield earthquakes have recurred on average every 21 years since 1857, when a great earthquake ( M ≈8) ruptured at least as far north as the southern Parkfield segment. Many measurements of slip have been made near Parkfield since 1966. Nevertheless, much of the history of surface slip remained uncertain, especially the total amount associated with the 1966 event. In 1985 we measured accumulated slip on the four oldest cultural features offset by the fault along the 1966 Parkfield rupture segment. We interpret net slip on each feature as a sum of event slip (sum of coseismic and rapid preseismic and postseismic slip) from Parkfield earthquakes and steady interseismic slip as measured over the last 20 years on nearby alinement arrays, creep meters, and trilateration lines. We assumed for each site that event slip was identical for the 1922, 1934, and 1966 Parkfield events and that long-term average rate of interseismic slip was constant between all events. Two fences on the southern segment, southeast of Gold Hill, indicate event slip of 13 and 15 cm and interseismic slip rate of 0.36 and 0.30 cm/yr since 1959 and 1908, respectively. At these sites, redundant independent data support our assumption that both event and interseismic slip occur uniformly. On the central segment, near Parkfield, both the 1934 and the 1966 ruptures offset a bridge built in 1932. Interseismic slip rate near the bridge has been about 1.1 cm/yr since 1966; thus we deduce an average event slip of 31 cm for the 1934 and 1966 earthquakes. On a parallel fault trace, 1 km to the southwest, slip was about 8 cm in 1966; thus total event slip summed across the entire fault zone near Parkfield was nearly 40 cm. On Middle Mountain, 4 km north of the 1966 mainshock epicenter, an offset fence indicates 17 cm of slip in 1966 and a 2.26-cm/yr interseismic slip rate since circa 1946. Thus the central segment of the 1966 rupture is characterized by much larger event slip (∼40 cm) than both distal segments (∼15 cm). This amount of surface slip per event is about twice what had been previously assumed. Larger 1966 surface slip in the central part of the rupture is geodetically compatible with a coseismic slip of 65±10 cm slip on a narrow, buried asperity between Middle Mountain and Gold Hill that has been inferred from the depth distribution of early aftershocks. Assuming our characteristic surface slip model, one can further deduce a deficit in slip since the great 1857 earthquake. Taking the long-term slip rate as 3.3 cm/yr, the surface slip deficit is 3±0.2 m south of Gold Hill but only 0.3±0.3 m northward from Parkfield.

Journal of Geophysical Research Solid Earth↗
USGS2012

Pb, Sr, and Nd isotopes in seamount basalts from the Juan de Fuca Ridge and Kodiak-Bowie seamount chain, northeast Pacific

Pb, Sr, and Nd isotopic ratios and their parent/daughter element concentrations for 28 basalts from 10 hotspot and nonhotspot seamounts are reported. Nd and Sr isotopic compositions ( 143 Nd/ 144 Nd = 0.51325–0.51304; 87 Sr/ 86 Sr = 0.70237–0.70275) plot in the envelope for Juan de Fuca-Gorda ridge basalts with tholeiitic basalts showing more depleted sources and a better negative correlation than transitional to alkalic basalts. Pb isotopic ratios in tholeiitic and alkalic basalts overlap ( 206 Pb/ 204 Pb = 18.29–19.44) and display a trend toward more radiogenic Pb in alkalic basalts. The isotopic data for hotspot and nonhotspot basalts are indistinguishable and correlate broadly with rock composition, implying that they are controlled by partial melting. The isotopic variation in the seamount basalts is about 60% (Nd-Sr) to 100% (Pb) of that in East Pacific Rise basalts and is interpreted as a lower limit for the magnitude of mantle heterogeneity in the northeast Pacific. The data indicate absence of a chemically distinct plume component in the linear seamount chains and strongly suggest an origin from mid-ocean ridge basalt-like east Pacific mantle.

Journal of Geophysical Research Solid Earth↗
USGS2012

Late Cretaceous paleomagnetism and clockwise rotation of the Silver Bell Mountains, south central Arizona

Late Cretaceous ash flow volcanism in the Silver Bell Mountains of southern Arizona (32.3°N, 248.5°E) was associated with caldera formation and porphyry copper mineralization. Oriented samples from 34 sites in volcanic, volcaniclastic, and intrusive units related to this episode of igneous activity (73–69 Ma) yield a mean paleomagnetic direction of I = 61.2°, D = 24.0°, α 95 = 7.6°. Primary remanent magnetizations are indicated by the presence of both normal and reversed polarities and by the significantly improved grouping (95% confidence level) of site-mean directions for a widespread tuff unit after structural correction. A comparison of this direction with a direction at Silver Bell calculated from contemporaneous units for stable North America ( I = 58.8°, D = 342.3°, α 95 = 7.7°) indicates a Late Cretaceous paleolatitude anomaly of −2.4°±7.6° (not significant at the 95% confidence level) and a declination anomaly of 41.7°±14.3° for the Silver Bell Mountains. Previously determined paleomagnetic data for southeastern Arizona suggest that this apparent clockwise rotation in the Silver Bell Mountains is a local phenomenon. Although preliminary, the average paleomagnetic direction for Oligocene and lower Miocene rocks in the Silver Bell area ( I = 43.8°, D = 357.3°, α 95 = 16.5°) is similar to that calculated for stable North America ( I = 50.2°, D = 352.2°, α 95 = 3.9°), implying that the observed rotation in the Silver Bell Mountains occurred before 26 Ma and was most likely associated with dextral strike-slip movement along the Ragged Top and related WNW trending faults bounding the Silver Bell Mountain block. These data, in conjunction with plate reconstructions and other paleomagnetic data from southwestern North America, imply that WNW trending strike-slip faults may have played an important role during Late Cretaceous to early Tertiary deformation in southern Arizona.

Journal of Geophysical Research Solid Earth↗
USGS2012

Age of the Peach Springs Tuff, southeastern California and western Arizona

Sanidine separates from pumice of the early Miocene Peach Springs Tuff are concordantly dated at 18.5±0.2 Ma by two isotopic techniques. The Peach Springs Tuff is the only known unit that can be correlated between isolated outcrops of Miocene strata from the central Mojave Desert of southeastern California to the western Colorado Plateau in Arizona, across five structural provinces, a distance of 350 km. Thus the age of the Peach Springs Tuff is important to structural and paleogeographic reconstructions of a large region. Biotite and sanidine separates from bulk samples of the Peach Springs Tuff from zones of welding and vapor-phase alteration have not produced consistent ages by the K-Ar method. Published ages of mineral separates from 17 localities ranged from 16.2 to 20.5 Ma. Discordant 40 Ar/ 39 Ar incremental release spectra were obtained for one biotite and two of the sanidine separates. Ages that correspond to the last gas increments are as old as 27 Ma. The 40 Ar/ 39 Ar incremental release determinations on sanidine separated from blocks of Peach Springs Tuff pumice yield ages of 18.3±0.3 and 18.6±0.4 Ma. Laser fusion measurements yield a mean age of 18.51±0.10. The results suggest that sanidine and biotite K-Ar ages older than about 18.5 Ma are due to inherited Ar from pre-Tertiary contaminants, which likely were incorporated into the tuff during deposition. Sanidine K-Ar ages younger than 18 Ma probably indicate incomplete extraction of radiogenic 40 Ar, whereas laser fusion dates of biotite and hornblende younger than 18 Ma likely are due to postdepositional alteration. Laser fusion ages as high as 19.01 Ma on biotite grains from pumice suggest that minerals from pre-Tertiary country rocks also were incorporated in the magma chamber.

Journal of Geophysical Research Solid Earth↗
USGS2012

Tertiary basin development and tectonic implications, Whipple Detachment System, Colorado River Extensional Corridor, California and Arizona

Colorado River extensional corridor. In the Mohave Mountains and Aubrey Hills of Arizona and the eastern Whipple Mountains of California near Parker Dam, these deposits comprise four unconformity-bounded sequences composed of locally derived epiclastic and volcanic rocks and the Peach Springs Tuff. The three older sequences represent syntectonic units that were deposited coeval with detachment faulting, and the fourth is interpreted to be postextensional. The sequences are correlated between four fault-bounded regions, which are the remnants of four different depositional basins. Similar sequences can be correlated over broad areas of the extensional corridor despite the general lack of widespread units. The basins developed in about the same positions, relative to each other and to volcanic sources, as they occupy at present. This is shown by gradational changes of pre-Tertiary rock types between regions, systematic variations in the abundance of magmatic units, and correlative volcanic units that occur in two adjacent regions. The basins formed in the early Miocene from segmentation of the upper crust into blocks bounded by high-angle faults that trended both parallel and perpendicular to the direction of extension and which were terminated at middle crustal depths by a low-angle detachment fault. Extreme rotation of one large crustal block, which constitutes the central Mohave Mountains, is recorded by a major unconformity in the lower Miocene section of one basin. Because coeval sections that formed in adjoining basins do not record this rotation, the underlying crustal blocks must have been separated by transfer faults that allowed them to rotate independently. These proposed transfer faults are represented at present by major faults with trends that parallel the direction of extension on the Whipple detachment system.

Arizona, California↗
USGS2012

Lithology and evolution of the crust-mantle boundary region in the southwestern Basin and Range Province

Mantle and crustal xenoliths from volcanic rocks in the southwestern Basin and Range province and Colorado Plateau Transition Zone reveal histories of episodic magmatism and deformation that have profoundly influenced the crustal structure of this region. Seismic transects in this area show a strongly reflective Moho of generally low relief, which, in the area of modern transects, consists of a thin zone (<2 km thick) of short reflectors. The upper mantle is transparent and has a P n of 7.8–8.0 km/s similar to much of the western United States. A lower crustal zone, 2–13 km thick, has variable internal reflectivity and a relatively low velocity of 6.6–6.8 km/s. Upper mantle peridotite xenoliths show both ductile and brittle deformational features and have structures and compositions affected by magmatic intrusion; intrusions form complex dike systems and extensive zones of grain boundary infiltration in peridotite xenoliths. Whereas melt infiltration preceded and followed ductile deformation, brittle deformation, represented by closely spaced joint systems and faults, followed ductile deformation and is related to the youngest magmatic episodes. These structural characteristics and high uppermost mantle temperature (∼1000°C) may combine to explain the relatively low P n . Alternating layers of ductily deformed and undeformed peridotites, with or without igneous intrusions, may contribute to the reflectivity of the Moho. Lower crustal xenoliths are dominantly igneous-textured pyroxenites and mafic to intermediate gabbros identical to the dikes in peridotite xenoliths. The crustal xenoliths also commonly are jointed, and in addition many show partial melting and have abundant cavities that probably were filled with CO 2 -rich fluids. These rocks are interpreted as products of underplated magmas that were fed through the mantle dike systems and may represent the lowest crustal unit identified in the seismic records. The mafic compositions and high densities of the crustal xenoliths indicate that the low velocity of the lower crust may be caused in part by fracture systems, partial melts, and high temperatures. Garnet granulite xenoliths from a locality with no mantle peridotite xenoliths probably represent crust of the region before late Miocene extension. Felsic granulite xenoliths from two localities have velocities like those of the two lower crustal units identified seismically and could be present in the modern crust as unequilibrated remnants of old crust. The preferred model for the evolution of the lower lithosphere is one in which extension affects the upper mantle as well as the crust and is overlapped in time by multiple magmatic episodes. The earliest magmatic events preceded extension, and later events accompanied and followed extension.

Arizona, California, Nevada↗
USGS2012

Crustal strain near the Big Bend of the San Andreas Fault: Analysis of the Los Padres-Tehachapi Trilateration Networks, California

In the region of the Los Padres-Tehachapi geodetic network, the San Andreas fault (SAF) changes its orientation by over 30° from N40°W, close to that predicted by plate motion for a transform boundary, to N73°W. The strain orientation near the SAF is consistent with right-lateral shear along the fault, with maximum shear rate of 0.38±0.01 μrad/yr at N63°W. In contrast, away from the SAF the strain orientations on both sides of the fault are consistent with the plate motion direction, with maximum shear rate of 0.19±0.01 μrad/yr at N44°W. The strain rate does not drop off rapidly away from the fault, and thus the area is fit by either a broad shear zone below the SAF or a single fault with a relatively deep locking depth. The fit to the line length data is poor for locking depth d less than 25 km. For d of 25 km a buried slip rate of 30 ± 6 mm/yr is estimated. We also estimated buried slip for models that included the Garlock and Big Pine faults, in addition to the SAF. Slip rates on other faults are poorly constrained by the Los Padres-Tehachapi network. The best fitting Garlock fault model had computed left-lateral slip rate of 11±2 mm/yr below 10 km. Buried left-lateral slip of 15±6 mm/yr on the Big Pine fault, within the Western Transverse Ranges, provides significant reduction in line length residuals; however, deformation there may be more complicated than a single vertical fault. A subhorizontal detachment on the southern side of the SAF cannot be well constrained by these data. We investigated the location of the SAF and found that a vertical fault below the surface trace fits the data much better than either a dipping fault or a fault zone located south of the surface trace.

Journal of Geophysical Research Solid Earth↗
USGS2012

H2O grain size and the amount of dust in Mars' residual North polar cap

In Mars' north polar cap the probable composition of material residual from the annual condensation cycle is a mixture of fine dust and H 2 O grains of comparable size and abundance. However, metamorphism of such material will gradually lower its albedo by increasing the size of the H 2 O grains only. If the cap is undergoing net annual sublimation (as inferred from water vapor observations), late summer observations should be of old ice with H 2 O grain sizes of 100 μm or more. Ice of this granularity containing 30% fine dust has a reflectivity similar to that of dust alone; the observed albedo and computed ice grain size imply dust concentrations of 1 part per 1000 or less. The brightness of the icy areas conflicts with what would be expected for a residual cap deposited by an annual cycle similar to that observed by Viking and aged for thousands of years. The residual cap surface cannot be “old dirty” ice. It could be old, coarse, and clean; or it could be young, fine, and dirty. This brings into question both the source of the late summer water vapor and the formation rate of laminated terrain.

Journal of Geophysical Research↗
USGS2012

Variations in fault slip and strain accumulation at Parkfield, California: Initial results using two-color geodimeter measurements 1984-1988

Repeated length measurements of several geodetic baselines near Parkfield, California, have revealed significant variations in the local rates of shallow fault slip and strain. This network of baselines, surveyed several times each week, straddles the San Andreas fault in the transition zone between the creeping section to the northwest and the locked section to the southeast. The length measurements, characterized by a precision approaching 0.1 ppm, reveal large fluctuations in the rates of baseline extension. Principal mode analysis of the length change data indicates that the two largest components of the signal are (1) secular extensions and contractions consistent with surface slip on the main strand of the San Andreas fault, and (2) a large seasonal oscillation with no obvious spatial coherence. On most of these baselines, the second component appears to be in phase with seasonal rainfall. When data from the baselines with the largest amplitude of the seasonal signal are excluded, the remaining data can be modeled in terms of both spatial and temporal variations in surface slip, variations in the components of the changes in uniform strain, and the possible displacement of the central monument in this radial network. In parameterizing this model, the spatial variation of slip beneath the near surface is reflected by changes in shear strain. Although the computed secular shear is highly dependent upon the specified parameterization of surface slip, the data are consistent with the hypothesis that slip at intermediate depths lags behind the surface slip rate. However, the range in models that fit the data does not necessarily imply that there is a deficit in slip at depth relative to the surface. Comparison of the inferred values of surface slip from the model with the observed fault slip measured by very short baseline creep meters indicates close agreement in secular rates, but the short-term variations observed with the creep meters are either highly attenuated or nonexistent in the modeled slip since the modeled slip is a spatial average which smooths out possible short-wavelength variations in the surface slip for which the creep instruments are most sensitive. An interesting conclusion from the two-color data is that surface slip on the San Andreas fault appears to be spread over a 2-km-wide zone on the south flank of Middle Mountain but is confined to a very narrow zone to the south as the fault passes through the center of the network. This conclusion is dependent upon the assumption that a few critical monuments are stable and track tectonic displacements in the long term. Finally, the largest observed strain change is an extensional strain coincident with the Kettleman Hills earthquake M 5.5 in August 1985.

Journal of Geophysical Research Solid Earth↗
USGS2012

Order and diversity in the modes of circum-Pacific earthquake recurrence

Recurrence characteristics of great circum-Pacific earthquakes and determinations of spatial distribution of seismic moment release are surveyed in order to delineate their general features and relate earthquake slip distribution to models of recurrent rupture. As noted by others, the pattern of moment release is typically very irregular, with strong concentrations in a few isolated regions of a much larger aftershock zone. Despite this complexity, rupture nucleation is notably systematic, with mainshock epicenter showing a strong tendency to locate in or immediately adjacent to identified regions of high moment release. This generalization suggests that earthquake recurrence is more likely to be controlled by maximum rather than average fault slip. Well-documented case histories from twelve plate boundary regions demonstrate that seismic strain release tends to be temporally well-ordered, while source dimensions, slip, and cumulative moment release vary considerably from cycle to cycle. On none of the segments studied do earthquakes consistently recur in nearly identical events. Instead, individual great earthquakes differ significantly from cycle to cycle or rerupture takes place in a sequence of two or more smaller events. Despite these differences the duration of the seismic cycle is approximately uniform. Furthermore, when strain release occurs in a sequence of large earthquakes, these events take place towards the end of the cycle and occur with increasing rupture length and magnitude. These ordered and irregular features of earthquake recurrence argue for the existence of corresponding elements on plate boundary faults. The ordered characteristics are identified with zones of concentrated moment release that slip comparable amounts in each cycle and have high shear strength. The irregular features are associated with intervening weaker regions that move in response to the stress concentration of dynamic rupture and slip by differing amounts in each event. Although these mechanistic associations are indirect and tentative, the features of recurrent behavior documented here have implications for long-term earthquake hazard assessment that are not dependent on the models proposed to explain them. Chief among these are the cycle-to-cycle differences among gap-filling events and the absence of shocks that fill major slip-deficient regions early in the seismic cycle.

Journal of Geophysical Research Solid Earth↗
USGS2012

Cooling rate and thermal structure determined from progressive magnetization of the dacite dome at Mount St. Helens, Washington

Our study of a magnetic anomaly associated with the recently active dacite dome at Mount St. Helens suggests that the dome consists of a hot, nonmagnetized core surrounded by a cool, magnetized carapace and flanking talus. The talus does not contribute to the anomaly because its constituent blocks are randomly oriented. Temporal changes in the magnetic anomaly indicate that the magnetized carapace thickened at an average rate of 0.03±0.01 m/d from 1984 to 1986. Petrographic and rock magnetic properties of dome samples indicate that the dominant process responsible for these changes is magnetization of extensively oxidized rock at progressively deeper levels within the dome as the rock cools through its blocking temperature, rather than subsequent changes in magnetization caused by further oxidation. Newly extruded material cools rapidly for a short period as heat is conducted outward in response to convective heat loss from its surface. The cooling rate gradually declines for several weeks, and thereafter the material cools at a relatively constant rate by convective heat loss from its interior along fractures that propagate inward. The rate of internal convective heat loss through fractures varies with rainfall, snowmelt, and large-scale fracturing during subsequent eruptive episodes. In accordance with a model for solidification of the 1959 lava lake at Kilauea Iki, Hawaii, we picture the dome's magnetized carapace as being a two-phase, porous, convective zone separated from the nonmagnetized core of the dome by a thin, single-phase conductive zone. As a consequence of the heat balance between the conductive and convective zones, the blocking-temperature isotherm migrates inward at a relatively constant rate. If the dome remains inactive, the time scale for its complete magnetization is estimated to be 18–36 years, a forecast which can be refined by shallow drilling into the dome and by continuing studies of its growing magnetic anomaly.

Journal of Geophysical Research Solid Earth↗
USGS2012

Earthquake depths and the relation to strain accumulation and stress near strike-slip faults in southern California

In this paper I report and attempt to interpret several observations about the depth distribution of earthquakes in the region of southern California containing the southern San Andreas and San Jacinto fault zones. These observations are as follows: (1) Earthquakes in the major fault zones are predominantly deep (maximum, 22 km; characteristic, 11–18 km). (2) Earthquakes in the crustal blocks bounding the fault zones are predominantly shallow (maximum, 17 km; characteristic, 1–6 km). (3) In the San Jacinto fault zone, maximum earthquake depths correlate with surface heat flow. These relations together with focal mechanisms, geodetic strain measurements, and fault zone models are consistent with the following ideas: (1) Interseismic plate motion is accommodated by aseismic slip along an extension of the major fault zone below a brittle zone that is locked between large earthquakes. (2) The aseismic slip in a narrow fault zone in the brittle-plastic transition region concentrates strain at the base of the brittle fault zone. (3) Deep earthquakes occur in the lower part of the brittle fault zone due to stick-slip failure of highly stressed patches. (4) Background earthquakes and aftershocks that occur several kilometers deeper than large earthquake hypocenters suggest that a zone of mixed slip behavior may exist between the stable sliding (deep) and stick-slip (shallow) regions of the fault zone. This zone of mixed slip may fail predominantly by stable sliding, analogous to the central San Andreas and Calaveras faults of California. Seismic precursors to large earthquakes may occur in this zone, and large earthquakes may nucleate near the upper edge of this zone. Furthermore, the difference in seismicity between the San Jacinto and southern San Andreas faults suggests that the nature of this mixed zone may evolve as total displacement in the fault zone increases. (5) Shear stress may be less in the crustal blocks than in the deep brittle fault zones and generally at a level sufficient to cause brittle failure only shallow in the crustal blocks. (6) In the stress field produced by plate motion and slip in the deep fault zone, the upper brittle fault zone is not oriented favorably for shear failure. Lack of shallow earthquakes in the fault zones and the predominance of shallow earthquakes on favorably oriented fractures in the adjacent crustal blocks suggest that either stress in the upper brittle fault zone is relatively low or the upper fault zone is effectively strong due to its orientation.

California↗
USGS2012

Equivalent strike-slip earthquake cycles in half-space and lithosphere-asthenosphere earth models

By virtue of the images used in the dislocation solution, the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike-slip fault in an Earth model consisting of an elastic plate (lithosphere) overlying a viscoelastic half-space (asthenosphere) can be duplicated by prescribed slip on a vertical fault embedded in an elastic half-space. For the case in which each earthquake ruptures the entire lithosphere (thickness H ), the half-space equivalent slip rate is as follows: Depth interval 0- H , slip identical to that in lithosphere-asthenosphere model (i.e., abrupt coseismic slip and no subsequent slip); depth interval (2 n −1) H to (2 n +1)H ( n = 1,2,…), slip rate uniform in space and dependent upon time as F n ( t ) exp (− t /τ a ) where F n is a ( n - 1) degree polynomial in t , τ a is twice the asthenosphere relaxation time (η/μ), and t is measured from the instant after the preceding earthquake. The slip rate averaged over the seismic cycle in each depth interval equals the secular rate of relative plate motion. For reasonable values of τ a , slip rates below 5 H do not vary much from that mean value and can be treated as constant. Thus the surface deformation due to the earthquake cycle in the lithosphere-asthenosphere model can be calculated very simply from the half-space model with time-dependent slip in the two depth intervals H −3 H and 3 H −5 H , and uniform slip at a rate equal to the secular relative plate velocity below depth 5 H . Inversion of 1973–1988 geodetic measurements of deformation across the segment of the San Andreas fault in the Transverse Ranges north of Los Angeles for the half-space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm/yr below 30 km. One equivalent lithosphere-asthenosphere model would have a 30-km thick lithosphere and an asthenosphere relaxation time greater than 33 years, but other models are possible.

Journal of Geophysical Research↗
USGS2012

Age estimates and uplift rates for late Pleistocene marine terraces: Southern Oregon portion of the Cascadia forearc

Interest in the Cascadia subduction zone has increased because recent investigations have suggested that slip along plates at certain types of convergent margins is characteristically accompanied by large earthquakes. In addition, other investigations have suggested that convergent margins can be broadly classified by the magnitude of their uplift rates. The authors generated new uranium series, amino acid, and stable isotope data for southern Oregon marine terrace fossils. These data, along with terrace elevations and two alternative estimates of sea level at the time of terrace formation, allow one to determine terrace ages and uplift rates. Uranium series analysis of fossil coral yields an age of 83 {plus minus} 5 ka for the Whisky Run terrace at Coquille Point in Bandon, Oregon. A combination of amino acid and oxygen isotope data suggest ages of about 80 and 105 ka for the lowest two terraces at Cape Blanco. These ages indicate uplift rates of 0.45-1.05 and 0.81-1.49 m/kyr for Coquille Point and Cape Blanco, respectively. In order to assess the utility of the southern Oregon uplift rates for predicting the behavior of the Cascadia subduction zone, the authors compared late Quaternary uplift rates derived from terrace data from subduction zones around the world. On the basis of this comparison the southern Oregon rates of vertical deformation are not usually high or low. Furthermore, late Quaternary uplift rates show little relationship to the type of convergent margin. In the case of the southern Oregon coast, variability in uplift rate probably reflects local structures in the overriding plate, and the rate of uplift cannot be used as a simple index of the potential for great earthquakes along the southern Cascadia subduction zone.

California, Oregon↗
USGS2012

Rupture process of a multiple main shock sequence: analysis of teleseismic, local and field observations of the Tennant Creek, Australia, earthquakes of January 22, 1988

On January 22, 1988, three large intraplate earthquakes (with M S 6.3, 6.4, and 6.7) occurred within a 12-hour period near Tennant Creek, Australia. These earthquakes, which occurred over a small interval of time and within a small volume of space, present a unique opportunity to study the rupture process of the class of intraplate earthquakes that occur as multiple main shocks. Broadband displacement and velocity records of body waves from teleseismically recorded data are analyzed to determine source mechanisms, depths, and complexity of rupture of each of the three main shocks. Hypocenters of an additional 150 foreshocks and aftershocks constrained by local arrival time data and field observations of surface rupture are used to complement the source characteristics of the main shocks in order to derive as complete a description of the rupture process as possible. The interpretation of the combined data sets suggests that the overall rupture process involved unusually complicated stress release. As locations of the main shock hypocenters progressively moved from west to east, we infer that the first and third main shocks, denoted as MS 1 and MS 3 , produced the southeast-northwest trending scarps observed at the western end (the Kunayungku fault) and at the eastern end (the east end of the Lake Surprise fault), respectively, of the rupture zone. The epicenter of the only immediate foreshock was located in the gap between these two fault scarps. MS 1 nucleated near this epicenter and ruptured upward and to the northwest from a depth of 6.5 km. MS 3 ruptured predominantly to the SE at a depth of 4.5 km. The second main shock, MS 2 , is inferred to have produced the deformation of the southwest trending central scarp segment (the western end of the Lake Surprise fault). From the sense of thrusting seen at the surface and from the distribution of aftershock hypocenters, the south dipping nodal planes derived from waveform modeling are identified as the fault planes for earthquakes MS1 and MS 3 . In contrast, the dip of the central fault scarp is reversed relative to the dips of the western and eastern fault scarps. The rupture process Of MS 2 turns out to be commensurately complex and sufficiently explains the geological complexity. MS 2 consisted of three subevents. The southeast dipping nodal plane of the first two subevents is coplanar with a southeast dipping plane implied by locations of aftershocks which did not break the surface. Choice of the north dipping plane as the rupture plane of the third subevent, consistent with the surface deformation and coplanar with a second plane delineated by aftershocks, would imply conjugate faulting. The majority of the aftershocks are concentrated near the edges of the fault planes, and there is an absence of activity in the center of the planes. The areas of absent activity may represent the failed asperities of the main shocks in which substantial stress relief occurred. The rupture process of each main shock is characterized by the rapid release of energy followed by a much slower release of moment and by aftershock zones whose dimensions exceed the inferred dimensions of the rupture. These characteristics suggest that substantial slow slip occurred on each of the three fault interfaces that was not accompanied by major energy release. The first main shock nucleated at the deepest part of its aftershock zone and ruptured upward. In contrast, MS 2 and MS 3 nucleated at depths that were one third to one half of the maximum depth of their aftershock zones. This variation of focal depth and the strong increase of moment and radiated energy with each main shock imply that lateral variations of strength were more important than vertical gradients of shear stress in controlling the progression of rupture.

Journal of Geophysical Research Solid Earth↗
USGS2012

Reflected and mode-converted seismic waves within the shallow Aleutian subduction zone, southern Kenai Peninsula, Alaska

Pronounced secondary phases observed in local recordings of quarry shots and earthquakes on the southern Kenai Peninsula, Alaska, are identified as reflected P and S and converted S-to-P phases originating within four depth ranges: in the upper few kilometers of the Cook Inlet Tertiary basin, at midcrustal depths within the overthrust North American plate, at about 35 km depth near the top of the Wadati-Benioff seismic zone in proximity to the inferred interplate megathrust, and at about 5–10 km below the megathrust in the subducted Pacific plate. Beneath the landward margin of the accreted Chugach terrane, the mid-upper plate (MUP) discontinuity dips as steeply as 20°–30° to the west-northwest over the depth range 12–18 km. At shallower depths it flattens and possibly arches with the crest at about 10 km depth. Similar midcrustal reflectors have been imaged about 125 km to the southwest and about 350 km to the northeast along the structural trend of the Chugach terrane. The extensive reflectors may have a common origin, possible caused by the presence of underplated rocks. Relative amplitudes and polarities of the secondary phases originating at MUP depths provide few constraints on the nature of the discontinuity. It appears that the MUP discontinuity is seismically inactive and does not represent a brittle-ductile transition zone within the upper plate. The two converted S -to- P phases generated near the top of the subducted plate could indicate a low-velocity zone associated with subducted oceanic crust.

Journal of Geophysical Research Solid Earth↗
USGS2012

Slip partitioning of the Calaveras Fault, California, and prospects for future earthquakes

Examination of main shock and microearthquake data from the Calaveras Fault during the last 20 years reveals that main shock hypocenters occur at depths of 8-9 km near the base of the zone of microearthquakes. Microseismicity extends between depths of 4 and 10 km and defines zones of concentrated microseismicity and aseismic zones. Estimates of the fault regions which slipped during the Coyote Lake and Morgan Hill earthquakes as derived from seismic radiation coincide with zones which are otherwise aseismic. We propose that these persistent aseismic zones represent stuck patches which slip only during moderate earthquakes. From the pattern of microearthquake locations we recognize six aseismic zones where we expect future main shocks will rupture the Calaveras Fault. -from Authors

Journal of Geophysical Research Solid Earth↗