Robert Herrmann - Academia.edu (original) (raw)

Papers by Robert Herrmann

Bulletin of the Seismological Society of America, Apr 1, 1994

The source mechanism of two moderate-size normal-faulting earthquakes located slightly south of t... more The source mechanism of two moderate-size normal-faulting earthquakes located slightly south of the Grand Canyon in Arizona are studied, using several near real-time seismic-wave analysis procedures. The events are found to have source depths near 10 to 15 km, and the smaller of the two events (mb = 4.8) is used to provide Empirical Green's Functions to analyze the rupture directivity of the larger (Mw = 5.3) mainshock. These events indicate complexity in the crustal stress environment on the western margin of the Colorado Plateau.

Journal of Geophysical Research: Solid Earth, 2003

We applied time domain moment tensor inversion of local and regional waveforms to small and moder... more We applied time domain moment tensor inversion of local and regional waveforms to small and moderate (M w = 3.5-5.7) shallow earthquakes from the Iberian Peninsula, northern Morocco, and northern Algeria. For the 6+ years period from November 1995 to March 2002 and the previous Network of Autonomously Recording Seismograms (NARS) experiment, moment tensor solutions were obtained for 58 events, considerably increasing the total number of available solutions in the study area. For each event we performed a moment tensor inversion and a double-couple grid search. For simple faulting events the grid search is valuable as a quality test for its ability to reveal potential ambiguities of the solutions and to assess confidence limits of fault plane parameters or principal axes orientation. The computed mechanisms show regional consistency: A large part of the Iberian Peninsula is characterized by normal faulting mechanisms with SW-NE oriented T axes. Thrusting and SE-NW compression is dominant in Algeria. In the Alboran Sea, the westernmost part of the Mediterranean, and the transition between both regimes, strike-slip mechanisms dominate with approximately N-S oriented P axes. This pattern suggests a regional anomaly characterized by clockwise rotation of the principal horizontal stress orientations.

Circular, 1986

Foreword iii Preface Charleston and the Nation in the 1880's v By Nancy Buxton and G. A. Bollinge... more Foreword iii Preface Charleston and the Nation in the 1880's v By Nancy Buxton and G. A. Bollinger Chapter 1 Introduction 1 Chapter 2 Properties of the earthquake source 5 Chapter 3 The epicentral area immediately following the 9:50 p.m. origin time 15 Chapter 4 Origin time + 30 seconds-50 miles distant 23 Chapter 5 Origin time + 45 seconds-90 miles distant 25 Chapter 6 Origin time+ 1 minute-120 miles distant 28

Bollettino di Geofisica Teorica ed Applicata

The aim of this study is to test the stability of moment tensor solutions for crustal earthquakes... more The aim of this study is to test the stability of moment tensor solutions for crustal earthquakes in the Calabro-Peloritan area (southern Italy). We used waveforms recorded by the Italian National Seismic Network managed by the Istituto Nazionale di Geofisica e Vulcanologia and the CAT-SCAN (Calabria Apennine Tyrrhenian - Subduction Collision Accretion Network) project. We computed the moment tensor solutions using the Cut And Paste (CAP) method. The technique allows the determination of the source depth, moment magnitude and focal mechanisms using a grid search technique. For the earthquakes investigated, we tried different station distributions and different velocity models. Results were also checked by computing the moment tensor solutions using the SLUMT grid-search method. Both methods (CAP and SLUMT) allow time shifts between synthetic and observed data in order to reduce the dependence of the solution on the assumed velocity model and on earthquake location errors. Comparison...

The Mw 8.8 Maule earthquake occurred off the coast of central Chile on 2010 February 27, and was ... more The Mw 8.8 Maule earthquake occurred off the coast of central Chile on 2010 February 27, and was followed by thousands of aftershocks. In this study, we modeled 172 aftershocks recorded by more than 100 temporary broadband stations deployed between March 2010 and January 2011. Each of these earthquakes is characterized by a well-determined hypocentral location and well-constrained focal mechanism and moment magnitudes in the range M 3.7 to 6.2. Most of these earthquakes are characterized by shallow, eastward-dipping, thrust-type focal mechanisms consistent with faulting at or near the plate interface, where the Nazca plate is subducting beneath the South America plate at approximately 74 mm/yr. This study provides a unique opportunity to quantify high-frequency earthquake ground motion in a subduction zone due to the quality and quantity of observations in the frequency and distance range of 0.2-30 Hz and 40-500 km, respectively. The analysis was done using a two-step modeling procedure. A regression is performed to characterize source duration and excitation, source-receiver distance dependence, and station site effects. A point source forward model is then constructed in terms of geometrical spreading, duration, site effects and source scaling to match the regression results. This procedure provides the necessary point source parameters for stochastic finite-fault modeling of peak ground motions for future earthquakes in this subduction zone.

Seismological Research Letters, 1982

Fundamental mode Rayleigh wave phase and group velocities between 15 and 50 seconds are measured ... more Fundamental mode Rayleigh wave phase and group velocities between 15 and 50 seconds are measured along different paths in the northeastern United States (NEUS). Interstation paths are selected that lie mainly in the Precambrian Grenville province or the Appalachian orogenic belt. Although the differences between the two paths are not significant under consideration of measurement errors, separate inversions yield velocity models that are consistent with previous travel-time studies. The Appalachians appear to have a slightly thicker crust with higher velocities in the lower crust relative to the Grenville province. Also, the phase and group velocities in the NEUS appear to lie between those of the Canadian Shield and the central United States. Phase and group velocities between 15 and 40 seconds are also measured across the M.I.T. short-period seismic network in southeastern New England for two large events using frequency-wavenumber analysis. Beamforming indicates that the sporter period Rayleigh waves (T1.15 sec) can arrive as much as 15 degrees offazimuth. The Rayleigh wave phase and group velocities in this region suggest relatively high shear velocities in the mid to lower crust.

Geophysical Research Letters, 2012

Inadequate seismic design codes can be dangerous, particularly when they underestimate the true h... more Inadequate seismic design codes can be dangerous, particularly when they underestimate the true hazard. In this study we use data from a sequence of moderate-sized earthquakes in northeast Italy to validate and test a regional wave propagation model which, in turn, is used to understand some weaknesses of the current design spectra. Our velocity model, while regionalized and somewhat ad hoc, is consistent with geophysical observations and the local geology. In the 0.02-0.1 Hz band, this model is validated by using it to calculate moment tensor solutions of 20 earthquakes (5.6 ≥ M W ≥ 3.2) in the 2012 Ferrara, Italy, seismic sequence. The seismic spectra observed for the relatively small main shock significantly exceeded the design spectra to be used in the area for critical structures. Observations and synthetics reveal that the ground motions are dominated by long-duration surface waves, which, apparently, the design codes do not adequately anticipate. In light of our results, the present seismic hazard assessment in the entire Pianura Padana, including the city of Milan, needs to be re-evaluated.

Seismological Research Letters, 1980

Texas earthquake indicates a focal depth of 3 km and normal faulting on a northeast striking faul... more Texas earthquake indicates a focal depth of 3 km and normal faulting on a northeast striking fault plane. Lg spectra indicate a corner frequency of 0.5 Hz. This corner frequency, together with a surface wave seismic moment of 6.5 E+22 dyne-cm, indicate a stress drop of about 1.5 bars, a value smaller than expected for an earthquake with m b = 4.6-4.7.

Solid Earth Discussions, 2019

During September-October of 2013 an intense swarm of earthquakes occurred off the east coast of S... more During September-October of 2013 an intense swarm of earthquakes occurred off the east coast of Spain associated with the injection of the base gas in an offshore underground gas storage. Two weeks after the end of the injection operations, three moderate-sized earthquakes (Mw 4.0-4.1) occurred near the storage. These events were widely felt by the nearby population, leading to the indefinite shutdown of the facility. Here we investigate the source parameters (focal depth and mechanism) of the largest earthquakes in the sequence in order to identify the faults reactivated by the gas injection, and to help understand the processes that caused the earthquakes. Our waveform modeling results indicate that the largest earthquakes occurred at depths of 6-8 km beneath the sea floor, significantly deeper than the injection depth (~ 1800 m). Although we cannot undoubtedly discriminate the fault plane from the two nodal planes of the mechanisms, most evidence seems to favor a NW-SE striking fault plane. We propose that the gas injection reactivated unmapped faults in the Paleozoic basement, with regional orientation possibly inherited from the opening of the Valencia Trough.

Geophysical Journal International, 2016

The Leading Edge, 2015

The sharp increase in seismicity over a broad region of central Oklahoma has raised concerns rega... more The sharp increase in seismicity over a broad region of central Oklahoma has raised concerns regarding the source of the activity and its potential hazard to local communities and energy-industry infrastructure. Efforts to monitor and characterize the earthquake sequences in central Oklahoma are reviewed. Since early 2010, numerous organizations have deployed temporary portable seismic stations in central Oklahoma to record the evolving seismicity. A multiple-event relocation method is applied to produce a catalog of central Oklahoma earthquakes from late 2009 into early 2015. Regional moment tensor (RMT) source parameters were determined for the largest and best-recorded earthquakes. Combining RMT results with relocated seismicity enabled determination of the length, depth, and style of faulting occurring on reactivated subsurface fault systems. It was found that the majority of earthquakes occur on near-vertical, optimally oriented (northeast-southwest and northwest-southeast) str...

Russell (2003) developed a time-domain method for measuring surface waves with minimum digital pr... more Russell (2003) developed a time-domain method for measuring surface waves with minimum digital processing, using zero-phase Butterworth filters. For applications over typical continental crusts, the proposed magnitude equation for zero-to-peak measurements in millimicrons is In the second and third applications of the method, we demonstrate that measurements of Ms(VMAX) versus mb provide adequate separation of the explosion and earthquake populations at the Nevada and Lop Nor Test Sites. At the Nevada Test Site, our technique resulted in the misclassification of two earthquakes. We also determined that the new technique reduces the scatter in the magnitude estimates by 25% when compared to our previous studies using a calibrated regional magnitude formula. For the Lop Nor Test Site, we had no misclassified explosions or earthquakes; however, the data were less comprehensive. A preliminary analysis of Eurasian earthquake and explosion data suggests that similar slopes are obtained fo...

Seismological Research Letters, 2008

Seismological Research Letters, 1986

Geophysical Journal International, 2006

Predictive relationships for the ground motion in the Marmara region (northwestern Turkey) are pa... more Predictive relationships for the ground motion in the Marmara region (northwestern Turkey) are parametrized after regressing three-component waveforms from regional earthquakes, in the frequency range: 0.4-15.0 Hz, and in the distance range: 10-200 km. The data set consists of 2400 three-component recordings from 462 earthquakes, recorded at 53 stations. Moment magnitudes, M w , range between 2.5 and 7.2. The largest event for which we have waveforms available (M w 7.2) occurred in Duzce on 1999 November 12. The aftershocks of that earthquake, together with the aftershocks of the 1999 August 17 Izmit event (M w = 7.4), are included in the dataset. Regressions are performed, independently, on Fourier velocity spectra and on peak ground velocities, for a large number of sampling frequencies. A simple model is used to relate the logarithm of the measured ground motion to excitation, site, and propagation terms. Results obtained for peak velocities are used to define a piecewise continuous geometrical spreading function, g(r), a frequency-dependent Q(f), and a distance-dependent duration function. The latter is used, through random vibration theory (RVT), in order to predict time-domain characteristics (i.e. peak values) of the ground motion. The complete model obtained for the peak ground motion was used to match the results of the regressions on the Fourier amplitudes. Fourier velocity spectra for the combined horizontal motion are best fit by a hinged quadrilinear geometrical spreading function for observations in the 10-200 km hypocentral distance ranges as a function of frequency: f < 1.

On May 20 2012, an event of Ml 5.9 (Mw 5.6) stuck the southem edge of the Po river plain (Pianura... more On May 20 2012, an event of Ml 5.9 (Mw 5.6) stuck the southem edge of the Po river plain (Pianura Padana). The earthquake was preceded by a foreshock of Ml 4.1 (Mw 3.8), less than 3 hours before the Mw 5.6 main. Hypocentral depths were 6.3 km for both events. Centroid depths were 5 and 6 km, respectively. The activated fault was a reverse one, dipping to the south. Then a complex seismic sequence started, in which more than six earthquakes with Ml greater than 5 stuck the area, the last one on June 3, 2012. Aftershocks delineated a 50 km long and 10-15 km wide zone, approximately elongated in the WE direction. More than 2100 events were located between May 19 and June 25 2012 by the INGV National Seismic Network, 80 of them with Ml greater than 3.5. The damage due to the Ml 5+ earthquakes was widespread, as they severely hit historical towns and industrial infrastructures. However, a striking inconsistency exists between the relatively small moment magnitudes and the corrisponding high level of damage. In order to define a velocity structure for the crust beneath the Pianura Padana, to be used for waveform inversion of moment tensors, we gathered all the geophysical and geological information available for the area. The model is characterized by very thick and shallow Quaternary sediments, to be used for the inversion of broadband waveforms for moment tensor (MT) solutions, in the frequency band between 0.02-0.1 Hz. We calculated moment tensors for 20 events down to Mw~3.2. We demonstrate how surface waves dominate the seismograms in the region, which may have played a major role in enhancing the damage to industrial structures observed in the epicentral area. Synthetic seismograms computed using the developed model well reproduced the anomalous durations of the ground motion observed in Pianura Padana, also highlighting important implications for the seismic hazard in the entire area. The present seismic hazard assessment as well as the size of the historical earthquakes in the region (and so their recurrence times), may need to be re-evaluated in the light of this new results.

●Mainshock moment release was not at hypocenter, but much shallower and to SE ●Mainshock is compl... more ●Mainshock moment release was not at hypocenter, but much shallower and to SE ●Mainshock is complex with two zones of moment release about 13 km apart. The shallow Mw 5's at upper left coincide with secondary fault ●Velocity model provides detailed fit to high frequency ground velocity throughout Apennines and can be used for future large earthquake inversions until superceded ●Finite fault specification can be used for forward modeling of teleseismic observations ●Forward modeling of velocity in 0.01 – 0.10 Hz band matches observed amplitude and waveform shapes, but could not be used for inversion because of time shifts due to slightly imperfect Green's functions ●Grid search MT inversion was efficient Objective Although the 2009 L'Aquila, Italy, and 2008 Wells, Nevada, earthquakes have similar moment magnitudes, the L'Aquila sequence had over 230 events with ML ≥ 3.0. In addition the L'Aquila events were within a dense broadband seismic network that was augment...

In this study, the composite source model is applied to generate three-component rock motions at ... more In this study, the composite source model is applied to generate three-component rock motions at three bridge sites within the New Madrid Seismic Zone for various combinations of fault mechanism, moment magnitude, and distance. To take into account the uncertainties associated with the earthquake rupturing process, an equal-weight logic tree of all parameters in the composite source model is developed so that every parameter is within physical, geological, and seismological constraints. The results are validated by comparing the average of peak rock accelerations of the 100 simulations of each combination with those derived from an attenuation relation that is representative to the Central and Eastern United States. The characteristics of near-field motions are also examined.