Showing papers on "Hypocenter published in 1985"

Planar high-angle faulting in the basin and range: Geodetic analysis of the 1983 Borah Peak, Idaho, earthquake

Abstract: Geodetic elevation changes record the broad-scale deformation associated with the M -- 7.0 October 28, 1983, Borah Peak, Idaho, earthquake on the Lost River fault. The crest of the Lost River Range rose 0.2 m, and adjacent Thousand Springs Valley subsided 1.0 m, in relation to reference points 45 km from the main shock epicenter. The deformation was modeled by dislocations in an elastic half-space. A planar fault with uniform dip slip of 2.05 + 0.10 m, dipping 47 o + 2oSW and extending to a vertical depth of 13.3 + 1.2 km, fits the geodetic data best and is also consistent with the main shock hypocenter and fault plane solution. The geodetic moment is 2.6 +_ 0.5 x 1019 N m (2.6 _+ 0.5 x 1026 dyn cm), and the estimated static stress drop is 2.9 _+ 0.4 MPa (29 _+ 4 bars). Tests for coseismic slip on listric faults (which flatten with depth) and on detachments (horizontal faults or shear zones) showed fits to the geodetic data that are inferior to those for planar high-angle faults. No detectable coseismic slip occurred on the Mesozoic White Knob thrust fault, although the low-angle thrust sheet intersects the south end of Lost River fault near the 1983 mainshock epicenter. If the high-angle Lost River fault abuts a flat-lying detachment fault or shear zone, such a structure must lie at depths of > 12 km, near the brittle-ductile transition, where stick-slip behavior gives way to creep. The depth and geometry of faulting at Borah Peak is similar to that inferred from seismic and geodetic evidence for the 1954 M = 7.2 Fairview Peak, Nevada, and the 1959 M = 7.3 Hebgen Lake, Montana, events, suggesting that if detachments are active at these localities, they are deep and most likely slip by creep.

Changes in Vp/Vs with depth: Implications for appropriate velocity models, improved earthquake locations, and material properties of the upper crust

Abstract: Properties associated with the arrival times of shallow microearthquakes are determined for three tectonically different sites: Soviet Central Asia; the Central United States; and southern California. Simple graphical presentations of the earthquake data, including Wadati and Riznichenko diagrams, are used to resolve the trade-off in origin time and focal depth. Estimates of the average half-space velocities for P and S waves, and of the travel-time ratio ( ts/tp ) are also obtained that are relatively free of any model constraints used to initially locate the earthquake. In all cases, P and S arrivals exhibit a systematic decrease in ts/tp with depth in the upper few kilometers. This decrease is significant and matches similar variations observed in situ for the velocity ratio ( Vp/Vs ). Based on laboratory and theoretical studies, such observations are consistent with the closing of saturated microcracks with increasing confining pressure. Independent determinations of velocity structure and rock properties from both refraction studies and borehole experiments show analogous results. By specifying separate velocity models for both P and S waves that reflect this variation, rather than assuming a constant value of Vp/Vs , we find improved stability in hypocenter determination and increased resolution of shallow seismicity patterns.

Crustal structure in the northern part of the philippine sea plate as derived from seismic observations of hatoyama-off izu peninsula explosions

Abstract: In August 1980, observations of seismic waves generated by 35 explosions, one on land and the others at sea, were conducted at about 60 sites on land and four sites at sea bottom in a profile from Hatoyama to off Izu Peninsula to obtain the crustal structure in the northern part of the Philippine Sea plate. The profile, approximately parallel to the Suruga trough, crosses the Kan'nawa fault area, Hakone volcanoes, Izu Peninsula, and the Zenisu ridge.By the time term analysis, a velocity of 5.9 km/s was obtained for the granitic layer and a velocity of 6.8 km/s for the basaltic layer. The Pn velocity of 7.7 km/s was assumed for the time term analysis on the basis of travel time data in this experiment as well as previous results. The crustal structure thus obtained shows an interesting transition from the continental to oceanic type towards the Shikoku basin. It is possible that there are offsets in the upper boundaries of the granitic and the basaltic layers and in the Moho. Since these offsets are located in a narrow zone, this zone might be the boundary between the Philippine Sea plate and the Eurasian plate. Comparison of hypocenter distribution along the profile with the crustal structure supports the above idea since the pattern of hypocenter distribution between the two sides of the offset zone differs. This comparison also shows that earthquakes take place in the granitic layer beneath the Izu Peninsula. The distribution of observed Bouguer gravity anomaly along the profile is consistent with that expected from the crustal structure.

Structure of the Benioff Zone beneath the Shumagin Islands, Alaska: Relocation of local earthquakes using three-dimensional ray tracing

Abstract: Seismic rays are traced through a prescribed three-dimensional inhomogeneity that simulates the subducted slab below the Shumagin Islands region to calculate local station delays for a given hypocenter and a slab model. Hypocenters determined by the Shumagin seismic network are then relocated using the station delays, a flat-layered velocity structure and a standard earthquake location computer program. Station delays are calculated for 12 hypocenters with respect to six different slab models to identify the slab model that is most consistent with the available arrival time and waveform data. A set of path corrections that is calculated for each grid point-station pair on a preliminary grid of 36 points in the depth range from 60 to 300 km is used to recalculate the hypocenters for all of the 1982 earthquakes with depths greater than 50 km. Application of this method to data from 1982 for the digitally recording Shumagin seismic network shows the following results: (1) a previously observed apparent increase in dip of the subducted slab at depths of ≈ 100 km disappears, (2) the subducted slab can be modeled as a dipping structure that dips at a constant angle of 45° toward north-northwest at depths between 80 and 250–300 km and has a 7% higher velocity than the surrounding mantle, (3) hypocenters determined from Shumagin network data are located only 10–20 km south of high-quality hypocenters determined from teleseismic data alone, (4) qualitative comparison of digitally recorded seismograms with calculated ray paths shows enrichment of high-frequency coda, possible converted phases, and low amplitudes of first P arrivals for rays that travel mostly along the slab. Conversely, for rays that travel almost vertically through the upper plate the seismograms show a high amplitude of first P arrivals that are followed by an insignificant coda and low-amplitude S waves.

Anomalous hydrogen emissions from the San Andreas fault observed at the Cienega Winery, central California

Abstract: We began continuous monitoring of H2 concentration in soil along the San Andreas and Calaveras faults in central California in December 1980, using small H2/O2 fuel-cell sensors. Ten monitoring stations deployed to date have shown that anomalous H2 emissions take place occasionally in addition to diurnal changes. Among the ten sites, the Cienega Winery site has produced data that are characterized by very small diurnal changes, a stable baseline, and remarkably distinct spike-like H2 anomalies since its installation in July 1982. A major peak appeared on 1–10 November 1982, and another on 3 April 1983, and a medium peak on 1 November 1983. The occurrences of these peaks coincided with periods of very low seismicity within a radius of 50 km from the site. In order to methodically assess how these peaks are related to earthquakes, three H2 degassing models were examined. A plausible correlational pattern was obtained by using a model that (1) adopts a hemicircular spreading pattern of H2 along an incipient fracture plane from the hypocenter of an earthquake, (2) relies on the FeO−H2O reaction for H2 generation, and (3) relates the accumulated amount of H2 to the mass of serpentinization of underlying ophiolitic rocks; the mass was tentatively assumed to be proportional to the seismic energy of the earthquake.

Precursors of the 1983 Japan Sea Earthquake

Abstract: On the basis of the space-time distribution of shallow earthquakes of M4 or above, it is pointed out that a seismic gap of the second kind (seismic quiescence) appeared in a region including the focal region of the 1983 Japan Sea earthquake, off the west coast of northern Honshu. Data are from JMA and ISC. This seismic gap appeared in about mid-1978 and continued for five years until the large earthquake of M7.7. About twelve days before the large earthquake, foreshocks including a M5.0 shock occurred near the epicenter of the main shock. Data concerning smaller earthquakes (M≥ 3.0) observed by Tohoku University also show that the seismic activity in the hypocentral region decreased from about mid-1978. Other geophysical data in this region were reexamined, and the following results were obtained. Remarkable earthquake swarms and volcanic eruptions occurred in the land areas adjacent to the seismic gap after about 1978; ground uplift at Iwasaki and the Oga Peninsula located near the eastern boundary of the seismic gap region seems to have accelerated in the past several years; tide gauge data is consistent with the recent uplift at the same places; and water-tube tiltmeter data from Tohoku University shows that a general tendency of the eastward tilt at Oga started in 1978. All these remarkable phenomena started around 1978 in the surrounding region of the seismic gap. These phenomena seem to be long-term precursory phenomena of the 1983 Japan Sea earthquake.

Aftershocks of an M = 4.2 earthquake in Hawaii and comparison with long-term studies of the same volume

Abstract: Study of the aftershocks recorded in a 3-hr period after a 4.2 magnitude event on the East Rift Zone of Kilauea volcano, Hawaii, on 12 April 1982 shows that the aftershocks occurred on different planes than the main shock, probably as a result of stress redistribution; the aftershock locations are probably controlled by preexisting structures. This study also suggests that these relatively small aftershocks occurred in the same seismicity patterns as larger events recorded in the same volume over a period of 10 yr. Slips on most of the aftershocks and the main shock are in the same direction, perpendicular to the East Rift Zone, as has been found in studies of other, larger earthquakes. However, fault-plane solutions varied more, as did the tensional axes, and several of the smaller events showed movement in the opposite direction from the main shock and the rest of the aftershocks, suggesting some rebound was occurring near the edges of the aftershock zone. Because ten times as much energy was released in the aftershocks in a narrow linear region as elsewhere, and since the main shock epicenter was oceanward of all the aftershocks, we suggest that rupture began at the main shock hypocenter and propagated landward, implying an almost “one-dimensional” fault. For the aftershocks, the relationship between moment and magnitude was: log M 0 = (1.18 ± 0.17) M L + (17.3 ± 0.17). Differences in amplification lead to site differences of up to 0.8 units in local magnitude and 1.5 orders of magnitude in energy release. These correlated somewhat with station time corrections in that the stations with the longest delay times also had greatest amplification.

The Source Characteristics of the Liège Earthquake on November 8, 1983, From Digital Recordings in West Germany

Abstract: The instrumental hypocenters of the Liege mainshock on November, 8, 1983, at 0:49 GMT (with macroseismic intensity VII MSK-scale and local magnitude 5.1) and its largest aftershock at 2:13 GMT (magnitude 3.0) have been calculated using weighted arrival times of direct P and S waves from 21 seismic stations in the distance range up to 144 km. A comparison of hypocenter results calculated with various velocity models shows the possibility of determining the epicentral coordinates with a precision of up to ± 1 to 2 km for the best fitting crustal model. The favoured solution for the mainshock is: Origin time 00:49:34.4 GMT, Longitude 5°30.8′ E, Latitude 50°37.9′ N, Depth 6 km. The instrumental epicenter is less than 1 km from the macroseismic epicenter at St.Nicolas.

Subduction zone geometry and stress patterns in the tyrrhenian sea

Abstract: Joint hypocenter determination is performed for intermediate and deep earthquakes of the Tyrrhenian Sea region. This analysis allowed us to obtain a catalogue of 70 well-located events in this peculiar Benioff zone, which is characterized by quite low seismic activity, compared to the Pacific deep earthquake regions. The method used for the analysis is that ofFrohlich (1979), a variant of the successive approximation technique, which allows use of a great number of events and stations but saves computer memory. The results show a spoon-shaped Benioff zone, dipping NW in the Tyrrhenian Sea to 500km depth. 32 reliable fault-plane solutions have been determined using these new earthquake locations, confirming the predominance of down-dip compression in the central part of the slab and more complex motion along the borders of the zone, as previously suggested byGasparini et al. (1982).

Geology and Geomorphology of the Ogasawara Arc and the Sofugan Tectonic Line

Abstract: The Ogasawara Arc consists of four ridges; they are the Izu, Shichito (volcanic front), Ogasawara and Shinkurose Ridges. Geology and geomorphology of the arc are reviewed b sed on dredged submarine rocks and subaerial geology of islands on the arc.On the basis of submarine topographic features the Shichito Ridge is divided into three parts (segments 1, 2 and 3). Topographic gaps are found between segments 1 and 2, and 3. The Shinkurose Ridge appears to be displaced at the gaps. The gaps are situated between Smith Rock (Sumisujima) and Torishima Is., and between Sofugan and Nishinoshima Is. The latter is correlated with the Sofugan Tectonic Line (YUASA 1983). Volcanic rock bulk chemistry of the volcanic front, and hypocenter distribution pattern are different between northern and southern parts of the Ogasawara Arc separated by the Sofugan Tectonic Line. Distribution of back-arc depressions and frontal arcs are also different between them.In 1983, BANDY and HILDE proposed three right-lateral transverse faults on the Ogasawara Arc based on the displacement of outer ridge structure, and the existence of gaps on free-air gravity anomaly map. The right-lateral displacement proposed by them is apparently different from left-lateral displacement of the Sofugan Tectonic Line and dislocations within the Shichito Ridge.The Sofugan Tectonic Line may not be the only transverse fault on the Ogasawara Arc. There seem to be other faults between segments 1 and 2 of the Shichito Ridge and elsewhere. The gap, however, affecting the geological and geophysical phenomena such as rock chemistry, and hypocenter distribution is only the Sofugan Tectonic Line.

Locations and spectral properties of earthquakes accompanying an eruption of Mount St. Helens

Abstract: A joint hypocenter determination routine has been developed to accurately determine the locations of earthquakes occurring under Mount St. Helens. The method has a special application to Mount St. Helens, where most of the earthquakes are suspected to occur at shallow depth in a rather limited volume under the volcano. Differences in elevation among stations are allowed for in this routine. The location procedure determines, a best half-space velocity and one station correction for each station simultaneously with earthquake hypocenters. Arrival time data from an array of portable digital seismic event recorders are supplemented with data from stations of the permanent University of Washington seismic network operating at the volcano. Earthquakes are found to occur along a line trending south-east of the dome inside the crater. The shallowest events occur near the dome with hypocentral depth increasing with increasing distance from the dome. Station corrections can be interpreted to imply the existence of a high-velocity body beneath the crater which extends at least between depths of .5 and 2.1 km beneath the crater floor. Spectra are calculated from earthquake waveforms recorded at three stations. The spectra show very little evidence for local site or path effects on frequency content of waveforms. Stress drops of earthquakes are calculated from measurements of corner frequency and low-frequency spectral amplitude. Stress drops of earthquakes vary between .5 and 5 bars. There is no evidence for a limiting corner frequency for small earthquakes as has been found for earthquakes occurring in Mammoth Lakes, California. Data fit well a linear trend between log seismic moment and log source radius fit to data for earthquakes ranging in size from 1014 to 1027 dyn cm, suggesting that there is no limiting source radius observed for earthquakes in this range of size.

Determination of elastic wave velocity and relative hypocenter locations using refracted waves. II. Application to the Haicheng, China, aftershock sequence

Abstract: We located the aftershocks of the 4 February 1975 Haicheng, China, aftershock sequence using an arrival time difference (ATD) simultaneous inversion method for determining the near-source (in situ) velocity and the location of the aftershocks with respect to a master event. The aftershocks define a diffuse zone, 70 km × 25 km, trending west-northwest, perpendicular to the major structural trend of the region. The main shock and most of the large aftershocks have strike-slip fault plane solutions. The preferred fault plane strikes west-northwest, and the inferred sense of motion is left-lateral. The entire Haicheng earthquake sequence appears to have been the response of an intensely faulted range boundary to a primarily east-west crustal compression and/or north-south extension. The calculated upper mantle P-wave velocity is 7.6 ± 0.09 km/sec, and the inferred crustal thickness is between 31 and 32.5 km. The low upper mantle velocity and thin crust may be indicative of local lithospheric extension.

Determination of elastic wave velocity and relative hypocenter locations using refracted waves. I. Methodology

Abstract: An arrival time difference method utilizing refracted arrivals from earthquakes in a homogeneous, layered earth model has been developed for the simultaneous determination of near-source ( in situ ) velocity and relative locations of earthquakes. The method is particularly applicable when analyzing data from arrays in which most of the recording stations are far (i.e., several focal depths) from a group of events. This iterative scheme locates earthquakes relative to a master event and performs an inversion for in situ velocity using a generalized inverseleast squares estimation procedure. Direct arrivals, when available, may be included to stabilize the inversion and increase the accuracy of the event locations. We tested this scheme on artificial data contaminated by random and systematic arrival time errors, gaps in azimuthal coverage, and inaccuracies in the assumed velocity model. As usual, depth is the least well-resolved hypocenter coordinate, but this scheme yielded accurate locations of most events while converging to the correct velocity model.

The Morgan Hill Earthquake of April 24, 1984—The 1.29g Acceleration at Coyote Lake Dam: Due to Directivity, a Double Event, or Both?:

Abstract: The 1984 Halls Valley (Morgan Hill, California) earthquake had a complex seismic source. Velocities of the major seismic phases measured from continuous broadband seismograms at Berkeley Seismographic Station (BKS) and Richmond Field Station (RFS) show unambiguously that the earthquake is predominantly a double event with the second source hypocenter located approximately 17 km southeast of the mainshock hypocenter given by Bolt, Uhrhammer and Darragh (1985). The southeasterly fault rupture of the first source and the location of the focus of the second source have critical implications for the observed spatial variation of the recorded accelerograms. Of particular engineering interest, the high frequency 1.29g pulse of horizontal ground acceleration measured at Coyote Lake dam can be explained primarily as due to the second source and constructive interference of the principal S waves from the two sources.

The Earthquakes of Liege of November 8, 1983 and December 21, 1965

Abstract: The hypocenter of the earthquake of Liege of november 8, 1983 has been calculated using the data from 40 stations at an epicentral distance lower than 500 km: Latitute 50.63°N Longitude 5.50°E Depth 4 km Origin time 0h49m35.4s The epicenter is close (2 km) to the more damaged area and to the epicenter of the ML = 4.3 earthquake of december 21, 1965.