Robert L. Kovach

The Kalapana earthquake of November 29, 1975: An intra-plate earthquake and its relation to geothermal processes☆

Abstract By use of teleseismic and local data, the P-wave source mechanism of the Kalapana, Hawaii, earthquake of November 29, 1975 was found to have a common strike of N64°E for the two nodal planes. One plane dipped 4° to the NW; the other dipped 86° to the SE. After consulting subsurface geological data obtained by the recent Hawaii geothermal exploration program, it was decided that the plane dipping to the NW at 4° was the preferred solution. Seismic moment obtained from body-wave data and surface-wave data averaged 1.2 · 10 27 dyn cm. Fault area from P-wave, surface wave and tsunami data amounted to about 2200 km 2 . Stress drop was on the order of tens of bars. The earthquake appears to be of volcanic origin. When magma pressure in the dike complex of the east rift of Kilauea exceeded the fracture point, the southern flank of the east rift was pushed across the ancient sea floor upon which the volcanic edifice rests. The result was a low-angle overthrust, which also produced a tsunami. The hypothesis of forceful intrusion of magma into the east rift is consistent with the mechanism of the earthquake. The low stress drop (in relation to other intra-plate earthquakes) is probably due to the occurrence of the earthquake in a hot-rock regime.

Seismic data from man-made impacts on the moon

Unusually long reverberations were recorded from two lunar impacts by a seismic station installed on the lunar surface by the Apollo 12 astronauts. Seismic data from these impacts suggest that the lunar mare in the region of the Apollo 12 landing site consists of material with very low seismic velocities near the surface, with velocity increasing with depth to 5 to 6 kilometers per second (for compressional waves) at a depth of 20 kilometers. Absorption of seismic waves in this structure is extremely low relative to typical continental crustal materials on earth. It is unlikely that a major boundary similar to the crustmantle interface on earth exists in the outer 20 kilometers of the moon. A combination of dispersion and scattering of surface waves probably explains the lunar seismic reverberation. Scattering of these waves implies the presence of heterogeneity within the outer zone of the mare on a scale of from several hundred meters (or less) to several kilometers. Seismic signals from 160 events of natural origin have been recorded during the first 7 months of operation of the Apollo 12 seismic station. At least 26 of the natural events are small moonquakes. Many of the natural events are thought to be meteoroid impacts.

Uplift and a possible moho offset across the Dead Sea transform

Abstract The East Africa rifts and the Red Sea spreading center are characterized by uplifted shoulders and sunken median valleys. The Dead Sea transform, on the northern extension of the Arabian-African plate boundary, has a similar morphological character despite its well-documented strike-slip motion. To understand the “rift” morphology and the crustal structure beneath the transform we compiled a 320 km long gravity and topography profile perpendicular to the Dead Sea transform. The gravity field and the topography in the region surrounding the profile are generally parallel to the trend of the Dead Sea transform, justifying the two-dimensional approximation of the analysis. The gravity profile was modeled using constraints from seismic refraction, borehole and surface geology data. The observed gravity anomaly can be explained by the juxtaposition of two different sedimentary and crustal sections which have been offset by a 105 km left-lateral displacement across the transform boundary, suggesting a step in the depth to Moho. The existence of a significant density anomaly under the median valley is not required by the model. The current elevation of the transform shoulders appears not to be compensated locally. Assuming a state of local isostasy prior to the development of the transform, the magnitude of the uplift in the vicinity of the profile is estimated at 700–900 m, with a half width of 100–125 km. This topography (uplift) can be fitted equally well by models which assume either dynamic support or regional compensation. It is suggested that if the source of the uplift is thermal, it is located within the upper mantle, since there is no requirement for a shallow density anomaly under the rift itself or in the crust nearby.

Rate of seismicity of the dead sea region over the past 4000 years

Abstract The results of two millennia of earthquake documentation, a few decades of macroseismic and instrumental routine seismological observations and five months of microearthquake monitoring, are used to estimate the rate of seismic activity of the Dead Sea fault. It is found that these vastly diverse data which combine long- and short-term tectonic processes, are in good accord with the formula: log 10 N=2.54 − 0.86M L where N is the annual number of events of local magnitude M L or greater. If this equation is extrapolated to ca. 2000 B.C., it yields a Richter magnitude M s = 7 for the event of Sodom and Gomorrah which is believed to be associated with the strongest earthquake in the region during historical times. Comparing our findings with the results of other investigators in Turkey, Greece, Aegean Sea and Iran, we note that the b values along the Syrian-African rift zone (0.78–0.86) are smaller than those in Greece and its surrounding seas (0.94–1.16).

Water-level fluctuations and earthquakes on the San Andreas fault zone

Observations of the water-level changes in a well drilled into the San Andreas fault zone have been under way since May 1971, with the objective of studying in situ pore-pressure changes in a zone of active tectonic creep and seismicity. Small water-level changes, characterized by a decrease and subsequent rise, have been followed by earthquakes of moderate size on the San Andreas fault zone. Compatibility of these observations with either a dilatancy-type behavior or a dislocation-type behavior for the pre-earthquake process can be demonstrated. Additional water-level observations at other sites in the fault zone are needed to examine the spatial and temporal extent of the actual preseismic process that is responsible for observed creep events, water-level changes, and tilt changes.

The velocity structure of the lunar crust

Seismic refraction data, obtained at the Apollo 14 and 16 sites, when combined with other lunar seismic data, allow a compressional wave velocity profile of the lunar near-surface and crust to be derived. The regolith, although variable in thickness over the lunar surface, possesses surprisingly similar seismic properties. Underlying the regolith at both the Apollo 14 Fra Mauro site and the Apollo 16 Descartes site is low-velocity brecciated material or impact derived debris. Key features of the lunar seismic velocity profile are: (i) velocity increases from 100–300 m s−1 in the upper 100 m to ∼ 4 km s−1 at 5 km depth, (ii) a more gradual increase from ∼ 4 km s−1 to ∼ 6 km s−1 at 25 km depth, (iii) a discontinuity at a depth of 25 km and (iv) a constant value of ∼ 7 km s−1 at depths from 25 km to about 60 km. The exact details of the velocity variation in the upper 5 to 10 km of the Moon cannot yet be resolved but self-compression of rock powders cannot duplicate the observed magnitude of the velocity change and the steep velocity-depth gradient. Other textural or compositional changes must be important in the upper 5 km of the Moon. The only serious candidates for the lower lunar crust are anorthositic or gabbroic rocks.

Shear wave velocities in the Earth's mantle

Direct measurements of the travel time gradient dT/dΔ for S waves over the distance range 14° < Δ < 100°, together with travel time data, are used to derive a shear velocity model for the Earths mantle. A network of seismograph stations in Arizona operated as an array was used for the measurement of dTdΔ. The complex velocity structure in the upper mantle makes the use of multiple arrivals necessary to define the dTdΔ−Δ curve for distances less than 55°. In order to satisfy the data it is necessary to discard the usual assumption of lateral homogeneity below shallow depths and a shear velocity differential of up to 0.1 km/s down to a depth of 1000 km is proposed between western North America and areas of the Pacific Ocean. Distinctive features of the velocity model for the upper mantle beneath western North America are a low-velocity zone centred at 100 km depth and zones of high velocity gradient beginning at 400, 650 and 900 km. Also, the moderate velocity gradient derived for the depth range 150–400 km implies a density reversal in the same area if Love wave dispersion data are to be satisfied. The velocity model derived for the lower mantle beneath oceanic areas has a zone of high velocity gradient at 1200 km depth and a pronounced low-velocity zone at the core-mantle boundary.

The Viking Seismic Experiment

A three-axis short-period seismometer is now operating on Mars in the Utopia Planitia region. The noise background correlates well with wind gusts. Although no quakes have been detected in the first 60 days of observation, it is premature to draw any conclusions about the seismicity of Mars. The instrument is expected to return data for at least 2 years.

Geomagnetic observations and fault creep in California

Abstract To examine the predicted magnitude of magnetic events which might be associated with creep along the San Andreas fault, a finite dislocation model of a ‘locked’ fault is presented. An observable anomaly of 1–2 gammas is obtained for rocks having uniform magnetization I (= 10 −3 e.m.u.) on application of a uniform stress of 10 bars. However, the observed anomalies depend strongly on the local geology. Hence, for a meaningful use of the geomagnetic observations as precursors to earthquakes a dense net of magnetometers is required.

Apollo 17 Seismic Profiling: Probing the Lunar Crust

Apollo 17 seismic data are interpreted to determine the structure of the lunar crust to a depth of several kilometers. Seismic velocity increases in a marked stepwise manner beneath the Taurus-Littrow region at the Apollo 17 site. A thickness of about 1200 meters is indicated for the infilling mare basalts at Taurus-Littrow. The apparent velocity is high (about 4 kilomleters per second) in the material immediately underlying the basalts.