Robert C. Jachens

Geophysical observations of Kilauea volcano, Hawaii, 1. temporal gravity variations related to the 29 November, 1975, M = 7.2 earthquake and associated summit collapse

Abstract Repeated high-precision gravity measurements made near the summit of Kilauea volcano, Hawaii, have revealed systematic temporal variations in the gravity field associated with a major deflation of the volcano that followed the 29 November, 1975, earthquake and eruption. Changes in the gravity field with respect to a stable reference station on the south flank of neighboring Mauna Loa volcano were measured at 18 sites in the summit region of Kilauea and at 4 sites far removed from its summit. The original survey, conducted 10–23 November, 1975, was repeated during a two-week period after the earthquake. The results indicate that sometime between the first survey and the latter part of the second survey the gravity field at sites near the summit increased with respect to that at sites far removed from the summit. The pattern of gravity increase is essentially radially symmetrical, with a half-width slightly less than 3 km, about the point of maximum change 1.5 km southeast of Halemaumau pit crater. Gravity changes at sites near the summit correlate closely with elevation decreases that occurred sometime between leveling surveys conducted in late September 1975 and early January 1976. The systematic relation between gravity and elevation change (−1.71 ± 0.05 ( s . e .) μgal / cm ) shows that deflation was accompanied by a loss of mass from beneath the summit region. Mass balance calculations indicate that for all reasonable magma densities, the volume of magma withdrawn from beneath the summit region exceeded the volume of summit collapse. Analysis suggests that magma drained from at least two distinct areas south of Kilauea caldera that coincide roughly with two reservoir areas active during inflation before the 1967–1968 Kilauea eruption.

Regional extent of Great Valley basement west of the Great Valley, California: Implications for extensive tectonic wedging in the California Coast Ranges

Interpretation and modeling of the magnetic field of central California indicate that the magnetic basement of the forearc deposits of the Great Valley sequence extends westward beneath the coeval subduction-related rocks of the Franciscan Complex. The basement surface slopes gently to the west, reaching midcrustal depths (15-19 km) at distances of 50-100 km west of the Great Valley. This magnetic basement is disrupted by the Hayward-Rodgers Creek Fault system and is cut by the San Andreas Fault at the south end of the Great Valley and possibly throughout much of central California. The widespread presence of the Great Valley basement beneath rocks of the Franciscan Complex implies that the basement is more extensive than proposed in earlier interpretations based on seismic studies near the Franciscan Complex-Great Valley sequence contact. This result forces major modifications to ideas concerning this fossil subduction complex and other subduction zones. The eastern boundary fault of the Franciscan Complex (Coast Range Fault) is not (and never was) a subduction zone thrust fault but rather was originally a roof thrust (wedge-roof fault) formed above the eastward wedging mass of Franciscan Complex intruded along the top of the basement beneath the Great Valley deposits. This tectonic interpretation offers a solution for the question of how high-pressure metamorphic rocks of the Franciscan Complex were juxtaposed at the Coast Range Fault against low-pressure metamorphic rocks of the Great Valley sequence. This interpretation also implies an older flat-lying thrust fault (wedge-floor fault) that forms the top of magnetic basement between the active San Andreas and Hayward Faults at depths of 15-17 km. This older thrust fault may today transfer strain between the two young strike-slip faults, possibly explaining the apparent coupling of major nineteenth century earthquakes on these two faults. The former east dipping subduction zone along which the rocks of the Franciscan Complex accumulated must lie west of the western limit of the Great Valley magnetic basement.

Regional study of mineral resources in Nevada: Insights from three-dimensional analysis of gravity and magnetic anomalies

A three-dimensional interpretation of the basins of Nevada was developed based on statewide data bases of digital-gravity, magnetic, geologic, well, and topographic information. An iterative technique was applied to isostatic residual gravity anomalies in Nevada in order to define the location and shape of pre-Tertiary basement and to produce a gravity map that reflects variations of density within the pre-Tertiary basement. The dominant feature of the basement gravity of Nevada is an enormous area of low gravity that spans the entire state between latitudes 37°N and 40.5°N. This regional low strongly correlates with the distribution of middle and late Tertiary volcanic rocks and may reflect silicic intrusions within the mid-crust and upper crust that are the counterparts of volcanic rocks at the surface. Although 80% of Nevada is covered by Cenozoic deposits, these deposits are thicker than 1 km over only about 20% of the state. The remaining 60% of Nevada may have pre-Tertiary basement rocks within reach of current mineral-exploration techniques. Aeromagnetic profiles from the National Uranium Resource Evaluation (NURE) were analyzed in order to produce a map showing the location of shallow magnetic sources in Nevada. This analysis shows that 46% of the state has magnetic sources, generally Mesozoic and Cenozoic igneous rocks, within 1 km of the surface. A linear magnetic anomaly in north-central Nevada has been interpreted by others as a rift zone active during middle Miocene time. The rift also is evident in NURE magnetic profiles, but our interpretation suggests that the magnetic expression of the rift continues south-southeast with similar strike to at least 38°N and perhaps to the amagmatic zone (lat. 37°N). The survival since the middle Miocene of this narrow crustal feature, essentially linear over a distance of 500 km, is difficult to interpret in light of later Basin and Range deformation. Our analysis of gravity anomalies shows that many deep Cenozoic basins are located near the rift, yet only two basins cut across it, and at least five others change strike near the rift, as if to avoid it. The rift may have remained linear because it is associated with crustal structures that acted to resist subsequent deformation.

Tectonic setting of the southern Cascade Range as interpreted from its magnetic and gravity fields

We have compiled and analyzed aeromagnetic data from the southern Cascade Range and compared them with residual gravity data from the same region in order to investigate regional aspects of these young volcanic rocks and of basement structures beneath them. Various constant-level aeromagnetic surveys were mathematically continued upward to 4,571 m and numerically mosaicked into a single compilation extending from lat. 40°10′N to lat. 44°20′N. These data were reduced to the pole, upward continued an additional 10 km, and compared with a magnetic topographic model and with residual gravity data upward continued to the same level. Several intriguing regional features are suggested by these data. (1) The Trinity ophiolite complex that is exposed west of Mount Shasta probably dips at a shallow angle to the east and continues in the subsurface at least 10 km east of Mount Shasta. (2) Mount Shasta, Lassen Peak, and Medicine Lake volcanoes are located in a widespread magnetic low possibly caused by an upwarp of the Curie-temperature isotherm. (3) Crater Lake caldera is located at the intersection of various linear anomalies interpreted to be related to structure in basement rocks below the Cascade Range. (4) Three Sisters volcanoes and Newberry Crater are connected to each other by an arcuate magnetic source. (5) The High Cascades, from lat. 40°10′N to at least lat. 44°30′N, are marked by a residual gravity low which includes the Three Sisters volcanoes, Mount Shasta, Medicine Lake volcano, Mount McLoughlin, and Crater Lake. (We believe this gravity feature represents a major structural depression beneath the High Cascades.) (6) Except for Newberry Crater, every major volcano of the study area is located on the perimeter of a local gravitational low. We suggest that the gravity lows reflect subsidence of low-density volcanic material relative to denser country rock and that the major volcanoes have developed over structures at the perimeters of their respective depressions.

Modeling and validation of a 3D velocity structure for the Santa Clara Valley, California, for seismic-wave simulations

A 3D seismic velocity and attenuation model is developed for Santa Clara Valley, California, and its surrounding uplands to predict ground motions from scenario earthquakes. The model is developed using a variety of geologic and geophysical data. Our starting point is a 3D geologic model developed primarily from geologic mapping and gravity and magnetic surveys. An initial velocity model is constructed by using seismic velocities from boreholes, reflection/refraction lines, and spatial autocorrelation microtremor surveys. This model is further refined and the seismic attenuation is estimated through waveform modeling of weak motions from small local events and strong-ground motion from the 1989 Loma Prieta earthquake. Waveforms are calculated to an upper frequency of 1 Hz using a parallelized finite-difference code that utilizes two regions with a factor of 3 difference in grid spacing to reduce memory requirements. Cenozoic basins trap and strongly amplify ground motions. This effect is particularly strong in the Evergreen Basin on the northeastern side of the Santa Clara Valley, where the steeply dipping Silver Creek fault forms the southwestern boundary of the basin. In comparison, the Cupertino Basin on the southwestern side of the valley has a more moderate response, which is attributed to a greater age and velocity of the Cenozoic fill. Surface waves play a major role in the ground motion of sedimentary basins, and they are seen to strongly develop along the western margins of the Santa Clara Valley for our simulation of the Loma Prieta earthquake.

Correlation of Changes in Gravity, Elevation, and Strain in Southern California

Measurements made once or twice a year from 1977 through 1982 show large correlated changes in gravity, elevation, and strain in several southern California networks. Precise gravity surveys indicate changes of as much as 25 microgals between surveys 6 months apart. Repeated surveys show that annual elevation changes as large as 100 millimeters occur along baselines 40 to 100 kilometers long. Laser-ranging surveys reveal coherent changes in areal strain of 1 to 2 parts per million occurred over much of southern California during 1978 and 1979. Although the precision of these measuring systems has been questioned, the rather good agreement among them suggests that the observed changes reflect true crustal deformation.

An isostatic residual gravity map of california: a residual map for interpretation of anomalies from intracrustal sources

The Lake salt mass (10, Figure 2) causes a spectacular circular 25-mgal gravity low 15 miles west-northwest of Phoenix. This body measures 5 X 8 miles in horizontal extent and is 8000 ft thick. Hydrocarbon shows occur in the southwest and northeast parts of Arizona, but so far production is not significant. The search continues, however, and exploration for energy sources, including geothermal occurrences, will be guided by regional geophysical and geologic maps.

Three-dimensional geologic map of the Hayward fault, northern California: Correlation of rock units with variations in seismicity, creep rate, and fault dip

In order to better understand mechanisms of active faults, we studied relationships between fault behavior and rock units along the Hayward fault using a three-dimensional geologic map. The three-dimensional map—constructed from hypocenters, potential field data, and surface map data—provided a geologic map of each fault surface, showing rock units on either side of the fault truncated by the fault. The two fault-surface maps were superimposed to create a rock-rock juxtaposition map. The three maps were compared with seismicity, including aseismic patches, surface creep, and fault dip along the fault, by using visualization software to explore three-dimensional relationships. Fault behavior appears to be correlated to the fault-surface maps, but not to the rock-rock juxtaposition map, suggesting that properties of individual wall-rock units, including rock strength, play an important role in fault behavior. Although preliminary, these results suggest that any attempt to understand the detailed distribution of earthquakes or creep along a fault should include consideration of the rock types that abut the fault surface, including the incorporation of observations of physical properties of the rock bodies that intersect the fault at depth.

Geophysical and isotopic mapping of preexisting crustal structures that influenced the location and development of the San Jacinto fault zone, southern California

We examine the role of preexisting crustal structure within the Peninsular Ranges batholith on determining the location of the San Jacinto fault zone by analysis of geophysical anomalies and initial strontium ratio data. A 1000-km-long boundary within the Peninsular Ranges batholith, separating relatively mafic, dense, and magnetic rocks of the western Peninsular Ranges batholith from the more felsic, less dense, and weakly magnetic rocks of the eastern Peninsular Ranges batholith, strikes north-northwest toward the San Jacinto fault zone. Modeling of the gravity and magnetic field anomalies caused by this boundary indicates that it extends to depths of at least 20 km. The anomalies do not cross the San Jacinto fault zone, but instead trend northwesterly and coincide with the fault zone. A 75-km-long gradient in initial strontium ratios (Sr i ) in the eastern Peninsular Ranges batholith coincides with the San Jacinto fault zone. Here rocks east of the fault are characterized by Sr i greater than 0.706, indicating a source of largely continental crust, sedimentary materials, or different lithosphere. We argue that the physical property contrast produced by the Peninsular Ranges batholith boundary provided a mechanically favorable path for the San Jacinto fault zone, bypassing the San Gorgonio structural knot as slip was transferred from the San Andreas fault 1.0-1.5 Ma. Two historical M6.7 earthquakes may have nucleated along the Peninsular Ranges batholith discontinuity in San Jacinto Valley, suggesting that Peninsular Ranges batholith crustal structure may continue to affect how strain is accommodated along the San Jacinto fault zone.

Relationship of the 1999 Hector Mine and 1992 Landers Fault Ruptures to Offsets on Neogene Faults and Distribution of Late Cenozoic Basins in the Eastern California Shear Zone

This report examines the Hector Mine and Landers earthquakes in the broader context of faults and fault-related basins of the eastern California shear zone (ECSZ). We compile new estimates of total strike-slip offset (horizontal separation) at nearly 30 fault sites based on offset magnetic anomaly pairs. We also present a map of the depth to pre-Cenozoic basement rock (thickness of basin-filling late Cenozoic deposits) for the region, based on an inversion of gravity and geologic data. Our estimates of total long-term strike-slip offsets on faults that slipped during the 1999 Hector Mine (3.4 km), and the 1992 Landers earthquakes (3.1? to 4.6 km) fall within the 3- to 5-km range of total strike-slip offset proposed for most faults of the western ECSZ. Faults having offsets as great as 20 km are present in the eastern part of the ECSZ. Although the Landers rupture followed sections of a number of faults that had been mapped as independent structures, the similarity in total strike-slip offset associated with these faults is compatible with one of the following hypotheses: (1) the Landers multistrand rupture is a typical event for this linked fault system or (2) this complex rupture path has acted as a coherent entity when viewed over some characteristic multiearthquake cycle. The second hypothesis implies that, for each cycle, slip associated with smaller earthquakes on individual fault segments integrates to a uniform slip over the length of the linked faults. With one exception, the region surrounding the Hector Mine and Landers ruptures is devoid of deep late Cenozoic basins. In particular, no deep basins are found immediately north of the Pinto Mountain fault, a place where a number of kinematic models for development of the ECSZ have predicted basins. In contrast, some basins exist near Barstow and along the eastern part of the ECSZ, where the model of Dokka et al. (1998) predicts basins.