Showing papers on "Hypocenter published in 1975"

Identification of fault planes associated with deep earthquakes

Abstract: Application of the method of Joint Hypocenter Determination (JHD) to carefully determined arrival time data from a small network of sensitive and well-distributed local and teleseismic stations provides very precise locations of deep earthquakes. The precision of locations attained by this method—the relative locations are good to about 5 km—is limited mainly by the accuracy and precision of the measurements of arrival times. Application of the technique to a small and very active part of the deep earthquake zone of the Tonga island arc (depths near 600 km) reveals probable fault planes associated with the occurrence of deep earthquakes. The spatial distribution of precisely relocated hypocenters is closely related to the orientation of the focal mechanism solutions of two deep earthquakes. Hypocenters cluster in a plane parallel to one of the nodal planes for each of the two solutions examined. The planar features are yearly vertical and have linear dimensions of about 40 km. In one case the shocks defining the fault plane also tended to cluster in time near the occurrence of the largest event. These and other data suggest the existence of several distinct fault planes in the area studied. Our study is being extended to examine the structure of the entire Tonga deep seismic zone.

The focal process of the Kita-Mino earthquake of August 19, 1961, and its relationship to a Quaternary fault, the Hatogayu-Koike fault.

Abstract: The focal mechanism of the Kita-Mino earthquake of August 19, 1961, is investigated in relation to an active (Quaternary) fault, the Hatogayu-Koike fault, running in N35°E direction along the Uchinami River near the border region of Fukui and Gifu prefectures in central Japan. The re-located hypocenter is situated at the northeast end of the Hatogayu-Koike fault and is shallow in depth. The re-analysis of the first motion data of Japanese stations yields a fault plane solution of the "quadrant type, " while in a previous work, the P-wave first motion data of this earthquake was explained only as the "cone-type." The spatial distribution. of aftershocks is concentrated on the area along the Hatogayu-Koike fault. The observed trend of the static vertical displacements changes near tbe fault. These facts lead to a conclusion that the Kita-Mino earthquake of 1961 was a result of right-lateral and reverse faulting of the Hatogayu-Koike fault. A comparison of the synthesized seismograms calculated for a semi-infinite medium with the long-period seismograms recorded at Abuyama, about 170 km distant to the southwest from the epicenter, yields rupture velocity and rise time estimates. Focal parameters thus obtained are as follows; origin time 1961, August 19, 5h, 33m, 32.8sec (GMT); epicenter 36°05′N, 136°44′E; focal depth 2 km; fault plane, dip direction N55°W, dip angle 60°; slip direction of the hanging wall 130°(right lateral and reverse); fault length 12km; fault width 10km; average dislocation 2.5m; rise time 2sec; rupture velocity 3km/sec; seismic moment 0.9×1026 dyne·cm; stress drop 160 bars; strain energy released 1.7×1022 ergs.

The focal process of the Kita-Mino earthquake of August 19, 1961, and its relationship to a Quaternary fault, the Hatogayu-Koike fault.

Abstract: The focal mechanism of the Kita-Mino earthquake of August 19, 1961, is investigated in relation to an active (Quaternary) fault, the Hatogayu-Koike fault, running in N35°E direction along the Uchinami River near the border region of Fukui and Gifu prefectures in central Japan. The re-located hypocenter is situated at the northeast end of the Hatogayu-Koike fault and is shallow in depth. The re-analysis of the first motion data of Japanese stations yields a fault plane solution of the "quadrant type, " while in a previous work, the P-wave first motion data of this earthquake was explained only as the "cone-type." The spatial distribution. of aftershocks is concentrated on the area along the Hatogayu-Koike fault. The observed trend of the static vertical displacements changes near tbe fault. These facts lead to a conclusion that the Kita-Mino earthquake of 1961 was a result of right-lateral and reverse faulting of the Hatogayu-Koike fault. A comparison of the synthesized seismograms calculated for a semi-infinite medium with the long-period seismograms recorded at Abuyama, about 170 km distant to the southwest from the epicenter, yields rupture velocity and rise time estimates. Focal parameters thus obtained are as follows; origin time 1961, August 19, 5h, 33m, 32.8sec (GMT); epicenter 36°05′N, 136°44′E; focal depth 2 km; fault plane, dip direction N55°W, dip angle 60°; slip direction of the hanging wall 130°(right lateral and reverse); fault length 12km; fault width 10km; average dislocation 2.5m; rise time 2sec; rupture velocity 3km/sec; seismic moment 0.9×1026 dyne·cm; stress drop 160 bars; strain energy released 1.7×1022 ergs.

The September 5, 1970 Sea of Okhotsk Earthquake: A multiple event with evidence of triggering

Abstract: The September 5, 1970 Sea of Okhotsk earthquake consisted of two possibly causally related but dissimilar events: a small (Mag 4.5) event at the hypocenter given by the International Seismologic Centre (52.28 N, 151.49 E, 560 km) followed 5.16±.06 seconds later by a larger (Mag 5.7) event almost at the same epicenter but 23.0±1 km deeper. The fault planes and the principal stress axes of the two events are significantly different; furthermore, the second event does not lie on either of the nodal planes of the first event. We interpret this as a possible triggering of the second event by the smaller first event and not as a change in direction of rupture propagation during a single continuous faulting episode.

Regional variation of travel-time residuals of p waves from nearby deep earthquakes in japan and vicinity

Abstract: The purpose of this study is to obtain an accurate pattern of the geographical distribution of travel-time residuals of P waves from deep earthquakes in and around Japan. The residuals were calculated on the basis of relocated hypocenters. The large-scale lateral heterogeneity of the upper mantle beneath the island arc was taken into account in the hypocenter determination, but the degree of heterogeneity was not assumed. Two kinds of residuals (I and II) were considered. The residual I is the observed travel-time minus the calculated travel-time corrected for the lateral heterogeneity. The residual II is the observed travel-time minus the calculated travel-time without such correction. The residual I is mainly controlled by the local structure of the crust and uppermost mantle beneath the station. It correlates with the gravity anomaly. The residual II is related to the large-scale anomalous upper mantle structure represented by a high-velocity oceanic lithosphere underthrusted beneath the island arc. The average P-velocity contrast between the underthrusted lithosphere and the upper mantle on the continental side of it was estimated to be at least 3%.

The Mt. Rainier earthquake of July 18, 1973, and its tectonic significance

Abstract: On July 18, 1973, a magnitude 3.9 earthquake was strongly felt at Longmire and surrounding areas near Mt. Rainier, Washington. Network analysis permitted an accurate hypocenter to be located at 46°49.29′N and 121°49.86′W at a depth of 10.9 km, about 7 km southwest of the summit of Mt. Rainier. No prolonged aftershock sequence was generated, although two small aftershocks were recorded and located. Aerial photographs of the epicentral region reveal several northwest-trending lineaments which may be related to active faults in the region, although no surface ground breakage was discovered. The focal mechanism obtained for the main shock is well constrained and consistent with right-lateral strike-slip motion along a northwest-trending fracture, in general agreement with northwest-trending surface lineaments. The nature of the relationship of the earthquake occurrence to Mt. Rainier is uncertain. The principal compressive axis direction is in agreement with that found in the central Puget Sound basin. However, the shallow depths, the strike-slip mode of faulting, and the past evidence of earthquakes near Mt. Rainier suggest a direct relationship between faulting, earthquake generation, and the volcano location.

Reconnaissance report of the Antigua, West Indies, earthquake of October 8, 1974

Abstract: A major earthquake ( m b = 6.6; M s = 7.5, U.S. Geological Survey) occurred in the northern Lesser Antilles on October 8, 1974, causing damage of Modified Mercalli intensity VIII on the island of Antigua and lower intensities on the more distant islands. The damage was confined mainly to larger and older buildings, to a petroleum refinery, and to a deep-water harbor. No earthquake-resistant building code exists in most of the Lesser Antilles, and in the majority of cases examined it was clear that structural damage had occurred because the building concerned could not have met the elementary requirements of a typical code. A few people received minor injuries, but no fatality was reported. The hypocenter of the main shock and many of the aftershocks lay about 30 km above the westward-dipping zone defined by activity over the last decade. The earthquake was tectonic in origin since the epicenter was 50 km from the nearest recent volcano.

Numerical investigation of dilatancy biasing of hypocenter locations

Abstract: We assume that a dilatant zone which develops as a precursor to an earthquake may be modeled by a zone of low-velocity rock having the shape of a triaxial ellipsoid. For varying ellipsoid dimensions, velocity contrasts, and hypocenter distributions, we generate a synthetic data set of arrival times at surface stations. The artificial events are relocated using a constant-velocity model to test for systematic hypocenter biasing. For all cases tested, the depth change for the distribution as a whole was not significant although individual events may move substantially. A downward migration of the ensemble of hypocenters was produced by a long, narrow ellipsoidal zone, however, the magnitude of migration for this case was only two percent.