scholarly journals The 1918 and 1957 Vancouver Island earthquakes

1988 ◽  
Vol 78 (2) ◽  
pp. 617-635
Author(s):  
John F. Cassidy ◽  
Robert M. Ellis ◽  
Garry C. Rogers
Keyword(s):  
Surface Wave ◽  
Fault Zone ◽  
Relative Motion ◽  
Stress Drop ◽  
Seismic Moment ◽  
Focal Depth ◽  
Vancouver Island ◽  
Strike Slip ◽  
Lateral Motion ◽  
Juan De Fuca

Abstract Since 1918, six significant earthquakes (5.2 < M < 7.2) have occurred in the region of central Vancouver Island where the Juan de Fuca, Explorer, and America plates interact. In this study, two of the largest earthquakes are examined in detail: the 1918 (MS ≃ 7) and the 1957 (MS ≃ 6) events. The preferred location of the 1918 earthquake is on Vancouver Island at 49.44°N, 126.22°W, with a focal depth of 15 km. Magnitudes determined are MS = 6.9 ± 0.3 and mb = 7.2 ± 0.4. Analysis of surface waves suggests this is a predominantly strike-slip earthquake occurring along either a NNW- or an ENE-striking fault. The seismic moment is estimated as 7.40 × 1025 dyne-cm and the stress drop to be 122 bars. The 1957 earthquake has been relocated on the continental shelf west of Vancouver Island at 49.64°N, 127.00°W, with a focal depth of 30 km. Magnitudes determined are MS = 5.9 ± 0.2 and mb = 6.3 ± 0.3. Surface wave and P-nodal analyses indicate that this is a predominantly strike-slip earthquake; either dextral along an NNW-striking fault, or sinistral along a ENE-striking fault. The seismic moment is estimated as 8.14 × 1024 dyne-cm and the stress drop to be 36 bars. The 1918 earthquake appears to be a crustal intraplate event occurring in the lithosphere of the America plate, resulting from the complicated interaction of the Explorer, Juan de Fuca, and America plates. The preferred epicenter, depth, focal mechanism, and stress drop for the 1957 earthquake are consistent with the left-lateral motion between the Juan de Fuca and Explorer plates along the subducted extension of the Nootka fault zone. This earthquake is identical, within uncertainties, to events occurring in 1972 and 1986. We believe that these three earthquakes provide the best definition to date of both the position of the subducting portion of the Nootka fault zone west of Vancouver Island and the direction of relative motion along this fault.

The 4.6 mb,Lg Northeastern Kentucky Earthquake of September 7, 1988

1993 ◽  
Vol 64 (3-4) ◽  
pp. 187-199 ◽  
Author(s):  
R. Street ◽  
K. Taylor ◽  
D. Jones ◽  
J. Harris ◽  
G. Steiner ◽  
...  
Keyword(s):  
Surface Wave ◽  
Seismic Moment ◽  
Wave Amplitude ◽  
Linear Trend ◽  
Source Parameters ◽  
Strike Slip ◽  
Source Depth ◽  
Nodal Plane ◽  
Amplitude Spectra ◽  
Conjugate Fault

Abstract Source parameters for the September 7, 1988 northeastern Kentucky earthquake have been estimated from the analysis of surface-wave amplitude spectra. The source that best fits the observed data had a seismic moment of 2.0 × 1022 dyne-cm, a mechanism with strike = 198° ± 10°, dip = 51° ± 11°, and slip = −178° ± 17°, (T) trend = 160°, plunge = 25°, (P) trend = 55°, plunge = 28°, and source depth of 4 to 7 km. Thirty-two aftershocks were recorded during 2 weeks of monitoring following the mainshock; 23 of the aftershocks were locatable and fall on a roughly NW-SE linear trend. This trend is subparallel with the NW-SE nodal plane of the mainshock. Our analysis shows the 1988 event to be different from the July 27, 1980 mb,Lg = 5.3 earthquake located 11 km to the northwest. First, the 1988 event is considerably shallower (4 to 7 km) than the 1980 event (14 to 22 km). Second, data from the 1988 event suggest the motion is on a conjugate fault and is in contrast with the 1980 event, which had right-lateral strike-slip on a southeast-dipping plane.

Source parameters of the June 13, 1967 earthquake near Lake Ontario, New York State

1977 ◽  
Vol 14 (11) ◽  
pp. 2651-2657 ◽  
Author(s):  
Michio Hashizume ◽  
Nagakoto Tange
Keyword(s):  
New York ◽  
Surface Wave ◽  
Epicentral Distance ◽  
Source Parameters ◽  
New York State ◽  
Focal Depth ◽  
Lake Ontario ◽  
Strike Slip ◽  
York State ◽  
Long Period

Source parameters of an earthquake with magnitude mb = 4.4 were determined by using surface waves. Small but clear surface wave signals were observed on long period records gathered from seismograph stations within an epicentral distance of about 2000 km. The focal mechanism was determined to be of strike-slip type with the maximum and the minimum compression axes trending NNW–SSE and ENE–WSW, respectively. The focal depth was determined to be near either 3 or 20 km.

Crustal structure of the Cascadia subduction zone, southwestern British Columbia, from potential field and seismic studies

1997 ◽  
Vol 34 (3) ◽  
pp. 317-335 ◽  
Author(s):  
Ron M. Clowes ◽  
David J. Baird ◽  
Sonya A. Dehler
Keyword(s):  
Subduction Zone ◽  
Potential Field ◽  
Vancouver Island ◽  
Strike Slip ◽  
High Density ◽  
Pacific Rim ◽  
Cascadia Subduction Zone ◽  
Crustal Material ◽  
Juan De Fuca Plate ◽  
Juan De Fuca

The northern Cascadia subduction zone is a region of convergence between the oceanic Explorer and northern Juan de Fuca plates and the continental North American plate. Potential field and new seismic reflection data coupled with previous seismic results and geology enable derivation of a series of density – magnetic susceptibility cross sections and a block density model from the ocean basin to the volcanic arc from 2.5- and 3-dimensional interpretations. The lateral extent and thickness of the accreted wedge vary significantly along the zone. The narrow, metasedimentary Pacific Rim terrane lies immediately west of Wrangellia and extends the length of Vancouver Island, consistent with its emplacement by strike-slip faulting following the accretion of Wrangellia. The ophiolitic Crescent terrane is a narrow slice lying seaward of the Pacific Rim terrane but not extending northward beyond the Juan de Fuca plate. In this region, the Crescent terrane was emplaced in a strike-slip or obliquely convergent style during the latter stages of emplacement of Pacific Rim terrane. Below the accreted terranes, the Explorer plate is shallower than Juan de Fuca plate, resulting in a thinner crust. High-density lower crustal material lies beneath the western edge of Vancouver Island, supporting interpretations of wide-scale underplating of Wrangellia. The shape of the boundary region between Wrangellia and the Coast belt to the east varies along strike and may be controlled by properties of preexisting plutonic rocks. The low-density Coast belt plutons extend throughout most of the crust and are underlain by a lowermost crustal high-density layer, which may be indicative of fractionation accompanying magma generation.

Microearthquakes and the nature of the creeping-to-locked transition of the San Andreas fault zone near San Juan Bautista, California

1984 ◽  
Vol 74 (1) ◽  
pp. 235-254
Author(s):  
William H. Bakun ◽  
Marcia McLaren
Keyword(s):  
Fault Zone ◽  
Stress Drop ◽  
Seismic Moment ◽  
Dynamic Stress ◽  
San Andreas Fault ◽  
Static Stress ◽  
P Wave ◽  
Apparent Stress ◽  
San Andreas ◽  
Static Stress Drop

Abstract Eighteen digital event recorders were deployed during May-June 1981 along the creeping-to-locked transition of the San Andreas fault zone near San Juan Bautista, California, as a supplement to the U.S. Geological Survey's central California seismic network. Analysis of 18 well-recorded microearthquakes (0.7 ≦ ML ≦ 2.8) located along the transition confirms the complexity of the crust and fault-zone structure of the transition region. Seismic-wave site amplification is 2 to 10 times greater at sites between the San Andreas and Sargent fault traces, consistent with other evidence for lower velocities in the upper 3 km of crust there. Routine mislocation of epicenters 2 to 4 km northeast of the Sargent fault are consistent with greater P-wave velocity northeast of the Sargent fault. Microearthquake source parameters are consistent with a more segmented and splayed fault geometry toward the northwest locked end of the transition. P-wave nodal planes for 10 microearthquakes located to the northwest, 9 on the Sargent fault, and 1 near the San Andreas, are oriented more westerly than nodal planes commonly obtained for the frequent moderate-size earthquakes on the creeping section of the San Andreas fault to the southeast. Static stress drop, dynamic stress drop, and apparent stress estimates all increase with seismic moment, with the apparent stress and dynamic stress drops equal to about 3 and 20 per cent, respectively of the static stress drop. Average fracture energies, calculated assuming complete stress drop, generally increase with source size (seismic moment, rupture area, seismic slip, etc.) from 7 to 3000 J/m2; the two events with anomalously low fracture energies occurred on the creeping section of the San Andreas fault.

Improved Scaling Relationships for Seismic Moment and Average Slip of Strike-Slip Earthquakes Incorporating Fault-Slip Rate, Fault Width, and Stress Drop

2021 ◽  
Author(s):  
John G. Anderson ◽  
Glenn P. Biasi ◽  
Stephen Angster ◽  
Steven G. Wesnousky
Keyword(s):  
Stress Drop ◽  
Seismic Moment ◽  
Likelihood Function ◽  
Surface Displacement ◽  
Slip Rate ◽  
Strike Slip ◽  
Fault Slip ◽  
Constant Stress ◽  
Rupture Length ◽  
Fault Slip Rate

ABSTRACT We develop a self-consistent scaling model relating magnitude Mw to surface rupture length (LE), surface displacement DE, and rupture width WE, for strike-slip faults. Knowledge of the long-term fault-slip rate SF improves magnitude estimates. Data are collected for 55 ground-rupturing strike-slip earthquakes that have geological estimates of LE, DE, and SF, and geophysical estimates of WE. We begin with the model of Anderson et al. (2017), which uses a closed form equation for the seismic moment of a surface-rupturing strike-slip fault of arbitrary aspect ratio and given stress drop, ΔτC. Using WE estimates does not improve Mw estimates. However, measurements of DE plus the relationship between ΔτC and surface slip provide an alternate approach to study WE. A grid of plausible stress drop and width pairs were used to predict displacement and earthquake magnitude. A likelihood function was computed from within the uncertainty ranges of the corresponding observed Mw and DE values. After maximizing likelihoods over earthquakes in length bins, we found the most likely values of WE for constant stress drop; these depend on the rupture length. The best-fitting model has the surprising form WE∝logLE—a gentle increase in width with rupture length. Residuals from this model are convincingly correlated to the fault-slip rate and also show a weak correlation with the crustal thickness. The resulting model thus supports a constant stress drop for ruptures of all lengths, consistent with teleseismic observation. The approach can be extended to test other observable factors that might improve the predictability of magnitude from a mapped fault for seismic hazard analyses.

The Southeastern Illinois Earthquake of 10 June 1987

1989 ◽  
Vol 60 (3) ◽  
pp. 101-110 ◽  
Author(s):  
K. B. Taylor ◽  
R. B. Herrmann ◽  
M. W. Hamburger ◽  
G. L. Pavlis ◽  
A. Johnston ◽  
...  
Keyword(s):  
Spatial Distribution ◽  
Surface Wave ◽  
Seismic Moment ◽  
Wave Amplitude ◽  
Main Shock ◽  
Source Parameters ◽  
Focal Depth ◽  
Precambrian Basement ◽  
Amplitude Spectra ◽  
Best Fit

Abstract The June 10 southeastern Illinois earthquake was the 11th largest earthquake felt in the central U.S. during this century. Source parameters of the main shock were estimated from an analysis of surface-wave amplitude spectra. The source that best fit the observed data has focal depth of 10 ± 1 km; mechanism with strike= 40.6°± 5.9°, dip= 76.2° ± 5.6°, slip= 159.7° ± 6.0°; tension and pressure axes of (T) trend= 357°, plunge= 24°, (P) trend= 89°, plunge= 4°; and a seismic moment of 3.1 * 1023 dyne-cm. With the combined efforts of six institutions, a 24-station analog microearthquake network was deployed around the main shock epicenter. One hundred eighty-five aftershocks were recorded in the first week of monitoring, providing 144 hypocenter determinations. A subset of 51 well recorded events was used for joint relocation and calculation of station corrections for the stations within 100 km of the main shock epicenter. Joint hypocenter locations differ only slightly from the original locations. The spatial distribution of well located aftershocks indicates that the rupture was confined to a pencil-like zone within the Precambrian basement, extending from 7 to 11 km depth.

The Sharpsburg, Kentucky, earthquake of 27 July 1980

1982 ◽  
Vol 72 (4) ◽  
pp. 1219-1239 ◽  
Author(s):  
Robert B. Herrmann ◽  
Charles A. Langston ◽  
James E. Zollweg
Keyword(s):  
Surface Wave ◽  
Focal Mechanism ◽  
Seismic Moment ◽  
Wave Solution ◽  
Focal Depth ◽  
The United States ◽  
P Wave ◽  
Nodal Plane ◽  
Epicentral Zone ◽  
Short Period

abstract The Sharpsburg, Kentucky, earthquake was the second largest earthquake to have occurred in the United States, east of the Continental Divide, in the past 20 yr, having a seismic moment of 4.1 × 1023 dyne-cm. A surface-wave focal mechanism study defines a nodal plane striking N30°E, dipping 50°SE, and a nearly vertical nodal plane striking N60°W. P-wave first motion data indicate right-lateral motion on the nodal plane striking N30°E, with the pressure axes oriented east-west. These angles can be varied by ±10° without affecting the fit to the surface-wave data. The surface-wave solution is reinforced by a modeling of long-period seismograms at regional distances. The P, pP, and sP polarities and amplitudes from the short-period vertical component array stack at NORSAR are used together with six unambiguous short-period P-wave first motions recorded in North America to test whether it is possible to constrain focal mechanism solutions with such data. These solutions are compatible with the surface-wave solution. Waveform modeling of the NORSAR data suggests a source pulse duration of 1.0 sec and constrains the depth to 12.0 km. To match mb estimates from NORSAR and Canadian stations, t*, for teleseismic P, must be 0.7 and 0.5, respectively, when the synthetics are scaled using the surface-wave seismic moment. In spite of extensive coverage of the epicentral zone, fewer than 70 aftershocks were recorded. The largest aftershock and an mbLg = 2.2. Aftershock locations suggest that the nodal plane striking N30°E is the fault plane. An aftershock area of 30 to 50 km2 implies a stress drop of 2.8 to 6 bars and a dislocation of 2.0 to 3.4 cm. Because of the variety of studies performed, this earthquake is presently the best-studied eastern North American seismic event with well-constrained estimates of focal depth, focal mechanism and seismic moment, and indications of the duration of the source time function and upper mantle P-wave t*.

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