Strong Ground-Motion Measurements during the 2003 Bam, Iran, Earthquake

2005 ◽  
Vol 21 (1_suppl) ◽  
pp. 165-179 ◽  
Author(s):  
Mehdi Zaré ◽  
Hossein Hamzehloo
Keyword(s):  
Ground Motion ◽  
Local Time ◽  
Strong Ground Motion ◽  
Strike Slip ◽  
Bam Earthquake ◽  
Lateral Strike Slip ◽  
Slip Mechanism ◽  
Motion Measurements ◽  
The City ◽  
Strong Ground

The Bam earthquake of 26 December 2003 ( Mw 6.5) occurred at 01:56:56 (GMT, 05:26:56 local time) near the city of Bam in the southeast of Iran. Two strong phases of energy are seen on the accelerograms. The first comprises a starting subevent with right-lateral strike-slip mechanism located south of Bam. The mechanism of the second subevent was a reverse mechanism.

scholarly journals Inversion of strong ground motion and teleseismic waveform data for the fault rupture history of the 1979 Imperial Valley, California, earthquake

1983 ◽  
Vol 73 (6A) ◽  
pp. 1553-1583
Author(s):  
Stephen H. Hartzell ◽  
Thomas H. Heaton
Keyword(s):  
Ground Motion ◽  
Strong Ground Motion ◽  
The United States ◽  
Strike Slip ◽  
Body Waves ◽  
Rupture Velocity ◽  
Fault Rupture ◽  
Imperial Valley ◽  
History Of ◽  
Strong Ground

Abstract A least-squares point-by-point inversion of strong ground motion and teleseismic body waves is used to infer the fault rupture history of the 1979 Imperial Valley, California, earthquake. The Imperial fault is represented by a plane embedded in a half-space where the elastic properties vary with depth. The inversion yields both the spatial and temporal variations in dislocation on the fault plane for both right-lateral strike-slip and normal dip-slip components of motion. Inversions are run for different fault dips and for both constant and variable rupture velocity models. Effects of different data sets are also investigated. Inversions are compared which use the strong ground motions alone, the teleseismic body waves alone, and simultaneously the strong ground motion and teleseismic records. The inversions are stabilized by adding both smoothing and positivity constraints. The moment is estimated to be 5.0 × 1025 dyne-cm and the fault dip 90° ± 5°. Dislocation in the hypocentral region south of the United States-Mexican border is relatively small and almost dies out near the border. Dislocation then increases sharply north of the border to a maximum of about 2 m under Interstate 8. Dipslip motion is minor compared to strike-slip motion and is concentrated in the sediments. The best-fitting constant rupture velocity is 80 per cent of the local shear-wave velocity. However, there is a suggestion that the rupture front accelerated from the hypocenter northward. The 1979 Imperial Valley earthquake can be characterized as a magnitude 5 earthquake at the hypocenter which then grew into or triggered a magnitude 6 earthquake north of the border.

scholarly journals A physical model of the effect of a shallow weak layer on strong ground motion for strike-slip ruptures

1998 ◽  
Vol 88 (4) ◽  
pp. 1070-1078 ◽  
Author(s):  
James N. Brune ◽  
Abdolrasool Anooshehpoor
Keyword(s):  
Ground Motion ◽  
Rise Time ◽  
High Frequency ◽  
Strong Ground Motion ◽  
Strike Slip ◽  
Weak Zone ◽  
Weak Layer ◽  
Strong Ground ◽  
Frequency Radiation ◽  
Shallow Part

Abstract We report results of foam rubber modeling of the effect of a shallow weak layer on ground motion from strike-slip ruptures. Computer modeling of strong ground motion from strike-slip earthquakes has in some cases involved somewhat arbitrary assumptions about the nature of slip along the shallow part of the fault (e.g., fixing the slip to be zero along the upper 2 km of the fault plane). Fault-slip inversion studies indicate that the high-frequency radiation from the shallow part of strike-slip faults is typically less than that for the deeper parts of the fault. In many cases, faults (1) may be weak along the upper few kilometers of the fault zone and may not be able to maintain high levels of shear strain required for high dynamic energy release during earthquakes and (2) may have different constitutive relations for fault slip, for example, slip strengthening. The object of this article is to present results of physical modeling using a shallow weak layer, in order to support the physical basis for assuming a long rise time and a reduced high-frequency pulse for the slip on the shallow part of faults. A weak zone was modeled by inserting weak plastic layers of a few inches in width into the foam rubber model. The long-term strength of the weak layer is about an order of magnitude less than the rest of the model. The transient strength is velocity strengthening with the strength estimated to be about three times higher at slip velocities typical of dynamic slip events. It appears a 2-km-deep, weak zone along strike-slip faults could indeed reduce the high-frequency energy radiated from shallow slip and that this effect can best be represented by superimposing a small-amplitude, short rise-time pulse at the onset of a much longer rise-time slip. For the 15-cm weak zone, the average pulse amplitude is reduced by a factor of about 0.4. The reduction factor for the 20-cm case is about 0.2. For the 30-cm case, it is about 0.1. From these results, we can see that the thicker the weak layer, the more difficult it is for a short rise-time acceleration pulse to push its way through the weak layer to the surface. The velocity strengthening property of the weak layer further damps the slip motion and increases the rise time. These results support reducing the high-frequency radiation from shallower parts of strike-slip faults in modeling studies if it is known that the shallow part of the fault is weak or has not stored up a large shear stress.

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