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J.G. Armbruster

Publications and source records attributed to J.G. Armbruster.

4 recordsLinked to original sources
USGS2003

A shallow fault-zone structure illuminated by trapped waves in the Karadere-Duzce branch of the North Anatolian Fault, western Turkey

We discuss the subsurface structure of the Karadere-Duzce branch of the North Anatolian Fault based on analysis of a large seismic data set recorded by a local PASSCAL network in the 6 months following the Mw = 7.4 1999 Izmit earthquake. Seismograms observed at stations located in the immediate vicinity of the rupture zone show motion amplification and long-period oscillations in both P- and S-wave trains that do not exist in nearby off-fault stations. Examination of thousands of waveforms reveals that these characteristics are commonly generated by events that are well outside the fault zone. The anomalous features in fault-zone seismograms produced by events not necessarily in the fault may be referred to generally as fault-zone-related site effects. The oscillatory shear wave trains after the direct S arrival in these seismograms are analysed as trapped waves propagating in a low-velocity fault-zone layer. The time difference between the S arrival and trapped waves group does not grow systematically with increasing source-receiver separation along the fault. These observations imply that the trapping of seismic energy in the Karadere-Duzce rupture zone is generated by a shallow fault-zone layer. Traveltime analysis and synthetic waveform modelling indicate that the depth of the trapping structure is approximately 3-4 km. The synthetic waveform modelling indicates further that the shallow trapping structure has effective waveguide properties consisting of thickness of the order of 100 m, a velocity decrease relative to the surrounding rock of approximately 50 per cent and an S-wave quality factor of 10-15. The results are supported by large 2-D and 3-D parameter space studies and are compatible with recent analyses of trapped waves in a number of other faults and rupture zones. The inferred shallow trapping structure is likely to be a common structural element of fault zones and may correspond to the top part of a flower-type structure. The motion amplification associated with fault-zone-related site effects increases the seismic shaking hazard near fault-zone structures. The effect may be significant since the volume of sources capable of generating motion amplification in shallow trapping structures is large.

Geophysical Journal International↗
USGS2003

Intraplate triggered earthquakes: Observations and interpretation

We present evidence that at least two of the three 1811-1812 New Madrid, central United States, mainshocks and the 1886 Charleston, South Carolina, earthquake triggered earthquakes at regional distances. In addition to previously published evidence for triggered earthquakes in the northern Kentucky/southern Ohio region in 1812, we present evidence suggesting that triggered events might have occurred in the Wabash Valley, to the south of the New Madrid Seismic Zone, and near Charleston, South Carolina. We also discuss evidence that earthquakes might have been triggered in northern Kentucky within seconds of the passage of surface waves from the 23 January 1812 New Madrid mainshock. After the 1886 Charleston earthquake, accounts suggest that triggered events occurred near Moodus, Connecticut, and in southern Indiana. Notwithstanding the uncertainty associated with analysis of historical accounts, there is evidence that at least three out of the four known Mw 7 earthquakes in the central and eastern United States seem to have triggered earthquakes at distances beyond the typically assumed aftershock zone of 1-2 mainshock fault lengths. We explore the possibility that remotely triggered earthquakes might be common in low-strain-rate regions. We suggest that in a low-strain-rate environment, permanent, nonelastic deformation might play a more important role in stress accumulation than it does in interplate crust. Using a simple model incorporating elastic and anelastic strain release, we show that, for realistic parameter values, faults in intraplate crust remain close to their failure stress for a longer part of the earthquake cycle than do faults in high-strain-rate regions. Our results further suggest that remotely triggered earthquakes occur preferentially in regions of recent and/or future seismic activity, which suggests that faults are at a critical stress state in only some areas. Remotely triggered earthquakes may thus serve as beacons that identify regions of long-lived stress concentration.

South Carolina↗
USGS2000

On the modified Mercalli intensities and magnitudes of the 1811-1812 New Madrid earthquakes

We reexamine original felt reports from the 1811–1812 New Madrid earthquakes and determine revised isoseismal maps for the three principal mainshocks. In many cases we interpret lower values than those assigned by earlier studies. In some cases the revisions result from an interpretation of original felt reports with an appreciation for site response issues. Additionally, earlier studies had assigned modified Mercalli intensity (MMI) values of V-VII to a substantial number of reports that we conclude do not describe damage commensurate with intensities this high. We investigate several approaches to contouring the MMI values using both analytical and subjective methods. For the first mainshock on 02∶15 LT December 16, 1811, our preferred contouring yields M ω7.2–7.3 using the area-moment regressions of Johnston [1996]. For the 08∶00 LT on January 23, 1812, and 03∶45 LT on February 7, 1812, mainshocks, we obtain M ω7.0 and M ω7.4–7.5, respectively. Our magnitude for the February mainshock is consistent with the established geometry of the Reelfoot fault, which all evidence suggests to have been the causative structure for this event. We note that the inference of lower magnitudes for the New Madrid events implies that site response plays a significant role in controlling seismic hazard at alluvial sites in the central and eastern United States. We also note that our results suggest that thrusting may have been the dominant mechanism of faulting associated with the 1811–1812 sequence.

Arkansas, Illinois, Kentucky, Missouri, Tennessee↗