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USGS2021

Slip distribution and rupture history of the August 11, 2012, double earthquakes in Ahar – Varzaghan, Iran, using joint inversion of teleseismic broadband and local strong motion data

We use combined teleseismic and strong motion data sets to investigate finite-fault slip models for a double of earthquakes that occurred on August 11, 2012, in northwestern Iran near the cities of Ahar and Varzaghan. The data include teleseismic P-waveforms retrieved from broadband seismic stations located between 30°–94° from the earthquakes and local strong motion data recorded by the Iran Strong Motion Network, installed and operated by the Building and Housing Research Centre. We first invert teleseismic P-waveforms and local strong motion data separately. For the first event (12:23 UTC), the teleseismic broadband inversion yields a somewhat deeper and simpler distribution of slip than the local strong motion inversion. The strong motion inversion results in a more complex distribution because of higher frequency content but can also be influenced by complexities in the propagation path. For the second event (12:34 UTC), the slip distribution from strong motion data is more similar to the teleseismic result and shows a simple slip area with a small relative movement to the west. To resolve the differences between the results of these two data sets and obtain a better constrained slip model, we perform a joint inversion of teleseismic broadband and local strong motion data. The joint inversion for the first event shows two asperities with a maximum slip of 3.9 m up- dip from the hypocenter and extending to the west between depths of 1 and 5 km. A second narrower high-slip area is seen just above the hypocenter from 6 to 10 km depth. The total moment for this earthquake is calculated to be M o = 3.8 × 10 25 dyn-cm (3.8 × 10 18 N.m) (M w 6.4). For the second event, the results of the joint inversion show a simple slip distribution that is mainly confined in a single patch around the hypocenter with a depth range from about 10 to 13 km and maximum slip of 1.9 m. We compute a total seismic moment of M o = 1.6 × 10 25 dyn-cm (1.6 × 10 18 N.m) (M w 6.1) for the second event. The largest stress drops for the first event occur above the hypocenter with an average stress drop over the rupture area of 120 bar (12 Mpa). For the second event, the maximum stress drop occurs at the reported focal depth with an average stress drop over the rupture area of 80 bar (8 Mpa).

East Anatolian Fault↗

USGS2010

Method for calculating self-noise spectra and operating ranges for seismographic inertial sensors and recorders

Understanding the performance of sensors and recorders is prerequisite to making appropriate use of them in seismology and earthquake engineering. This paper explores a critical aspect of instrument performance, the “self” noise level of the device and the amplitude range it can usefully record. Self noise limits the smallest signals, while instrument clipping level creates the upper limit (above which it either cannot produce signals or becomes unacceptably nonlinear). Where these levels fall, and the “operating range” between them, determines much of the instrument's viability and the applications for which it is appropriate. The representation of seismic-instrument self-noise levels and their effective operating ranges (cf., dynamic range) for seismological inertial sensors, recorders (data acquisition units, or DAUs), and integrated systems of sensors and recorders (data acquisition systems, or DASs) forces one to address an unnatural comparison between transient finite-bandwidth signals, such as earthquake records, and the instrument's self noise, an effectively stationary signal of infinite duration. In addition to being transient, earthquakes and other records of interest are characterized by a peak amplitude and generally a narrow, peaked spectral shape. Unfortunately, any power spectrum computed for such transient signals is ill defined, since the maximum of that spectrum depends strongly upon signal and record durations. In contrast, the noise floor of an instrument is approximately stationary and properly described by a power spectral density (PSD) or its root (rPSD). Put another way, earthquake records have units of amplitude ( e.g. , m/s 2 ) while PSDs have units of amplitude-squared per hertz ( e.g. , (m/s 2 ) 2 /Hz) and the rPSD has units of amplitude per root of hertz ( e.g. , (m/s 2 )/Hz 1/2 ). Thus, this incompatability is a conflict between earthquake (amplitude) and PSD (spectral density) units that requires one to make various assumptions before they can be compared. For purposes of instrument operational performance, we provide a means of evaluating signal and noise and the range between them in a manner representative of time-domain instrument performance. We call these “operating range diagrams” (ORDs), plots of instrument self noise and clipping level; the “operating range” is the range between these values. For frequency-domain performance we elect to show self noise as an rPSD that may be compared to another instrument's noise or to ambient Earth noise ( e.g. , Peterson 1993); however, to limit the number of arbitrary choices required to merge transient and stationary signals we do not compare the rPSD to transient signals in the frequency domain. Our solution for a time-domain comparison is not new but rather builds upon the consensus of the first and second Guidelines for Seismometer Testing workshops (Hutt et al. 2009) and long established practice in acoustics. We propose this method as a standard for characterizing seismic instruments, and it has been endorsed by the second workshop (Hutt et al. 2009, 2010) and the Advanced National Seismic System (ANSS) Working Group (2008) and recent ANSS procurement specifications.

Seismological Research Letters↗

USGS2010

Earthquake waveform similarity and evolution at Augustine Volcano from 1993 to 2006: Chapter 5 in The 2006 eruption of Augustine Volcano, Alaska

Temporal changes in waveform characteristics and earthquake locations associated with the 2006 Augustine eruption and preeruptive seismicity provide constraints on eruptive processes within the edifice. Volcano-tectonic earthquakes occur within the upper 1 to 2 km at Augustine between and during eruptive cycles, and we use the Alaska Volcano Observatory hypocenter and waveform catalog from 1993 to 2006 to constrain changes in event similarity and location over time. Waveform crosscorrelation with bispectrum verification improves the pick accuracy of the catalog data to yield better locations and allows for identification of families of similar earthquakes. Event waveform similarity is low at Augustine, with ~60 to 70 percent of events failing to form event families of more than 10 events. The remaining earthquakes form event families over multiple time scales. Events prior to the 2006 eruption exhibit a high degree of similarity over multiple years. Earthquakes recorded during the precursory and explosive phases of the 2006 eruption form swarms of similar earthquakes over periods of days or hours. Seismicity rate and event similarity decrease rapidly during the explosive and effusive eruption phases. The largest recorded swarms accompany reports of increased steaming and explosive eruptions at the summit. Relative relocation of some event families indicates upward migration of activity over time, consistent with magma transport by way of an ascending dike. Multiple regions of the edifice generate seismicity simultaneously, however, suggesting the edifice contains a network of fractures and/or dikes.

Alaska↗

USGS1979

Mid-Atlantic Single-channel seismic reflection profiles collected over the Baltimore Canyon Trough area

Available are 377km of mini sparker data collected during the September 1977 cruise of the research vessel Annandale over the Baltimore Canyon Trough area. The prime purpose of the cruise was to obtain vibracores at selected locations across the Continental Shelf. The samples and seismic profiles were collected to study the origin and possible geologic hazards of a broad sedimentary terrace that lies just south of the Hudson Channel. Information about the cruise can be obtained from Dr. Harley J. Knebel, U.S. Geological Survey, Woods Hole, Massachusetts 02543. The original records are on file at the U.S. Geological Survey's Data Library in Woods Hole. Microfilm copies of the seismic profiles may be purchased from the National Geophysical and Solar Terrestrial Data Center (NGSDC), in Boulder, Colorado 80302.

Open-File Report↗

USGS2004

Evidence for dike emplacement beneath Iliamna Volcano, Alaska in 1996

Two earthquake swarms, comprising 88 and 2833 locatable events, occurred beneath Iliamna Volcano, Alaska, in May and August of 1996. Swarm earthquakes ranged in magnitude from -0.9 to 3.3. Increases in SO2 and CO2 emissions detected during the fall of 1996 were coincident with the second swarm. No other physical changes were observed in or around the volcano during this time period. No eruption occurred, and seismicity and measured gas emissions have remained at background levels since mid-1997. Earthquake hypocenters recorded during the swarms form a cluster in a previously aseismic volume of crust located to the south of Iliamna's summit at a depth of -1 to 4 km below sea level. This cluster is elongated to the NNW-SSE, parallel to the trend of the summit and southern vents at Iliamna and to the regional axis of maximum compressive stress determined through inversion of fault-plane solutions for regional earthquakes. Fault-plane solutions calculated for 24 swarm earthquakes located at the top of the new cluster suggest a heterogeneous stress field acting during the second swarm, characterized by normal faulting and strike-slip faulting with p-axes parallel to the axis of regional maximum compressive stress. The increase in earthquake rates, the appearance of a new seismic volume, and the elevated gas emissions at Iliamna Volcano indicate that new magma intruded beneath the volcano in 1996. The elongation of the 1996-1997 earthquake cluster parallel to the direction of regional maximum compressive stress and the accelerated occurrence of both normal and strike-slip faulting in a small volume of crust at the top of the new seismic volume may be explained by the emplacement and inflation of a subvertical planar dike beneath the summit of Iliamna and its southern satellite vents. ?? 2003 Elsevier B.V. All rights reserved.

Journal of Volcanology and Geothermal Research↗

USGS1993

Instrumental shaking thresholds for seismically induced landslides and preliminary report on landslides triggered by the October 17, 1989, Loma Prieta, California earthquake

The generation of seismically induced landslide depends on the characteristics of shaking as well as mechanical properties of geologic materials. A very important parameter in the study of seismically induced landslide is the intensity based on a strong-motion accelerogram: it is defined as Arias intensity and is proportional to the duration of the shaking record as well as the amplitude. Having a theoretical relationship between Arias intensity, magnitude and distance it is possible to predict how far away from the seismic source landslides are likely to occur for a given magnitude earthquake. Field investigations have established that the threshold level of Arias intensity depends also on site effects, particularly the fracture characteristics of the outcrops present. -from Author

Geografia Fisica e Dinamica Quaternaria↗

USGS2008

Location of the Green Canyon (Offshore Southern Louisiana) Seismic Event of February 10, 2006

We calculated an epicenter for the Offshore Southern Louisiana seismic event of February 10, 2006 (the 'Green Canyon event') that was adopted as the preferred epicenter for the event by the USGS/NEIC. The event is held at a focal depth of 5 km; the focal depth could not be reliably calculated but was most likely between 1 km and 15 km beneath sea level. The epicenter was calculated with a radially symmetric global Earth model similar to that routinely used at the USGS/NEIC for all earthquakes worldwide. The location was calculated using P-waves recorded by seismographic stations from which the USGS/NEIC routinely obtains seismological data, plus data from two seismic exploration arrays, the Atlantis ocean-bottom node array, operated by BP in partnership with BHP Billiton Limited, and the CGG Green Canyon phase VIII multi-client towed-streamer survey. The preferred epicenter is approximately 26 km north of an epicenter earlier published by the USGS/NEIC, which was obtained without benefit of the seismic exploration arrays. We estimate that the preferred epicenter is accurate to within 15 km. We selected the preferred epicenter from a suite of trial calculations that attempted to fit arrival times of seismic energy associated with the Green Canyon event and that explored the effect of errors in the velocity model used to calculate the preferred epicenter. The various trials were helpful in confirming the approximate correctness of the preferred epicenter and in assessing the accuracy of the preferred epicenter, but none of the trial calculations, including that of the preferred epicenter, was able to reconcile arrival-time observations and assumed velocity model as well as is typical for the vast majority of earthquakes in and near the continental United States. We believe that remaining misfits between the preferred solution and the observations reflect errors in interpreted arrival times of emergent seismic phases that are due partly to a temporally extended source-time function and partly to failure of our travel-time model to account for the extremely complicated velocity structure of the sedimentary section in which the event occurred.

Open-File Report↗

USGS2018

Wetland stratigraphic evidence for variable megathrust earthquake rupture modes at the Cascadia subduction zone

Although widespread agreement that the Cascadia subduction zone produces great earthquakes of magnitude 8 to 9 was reached decades ago, debate continues about the rupture lengths, magnitudes, and frequency of megathrust earthquakes recorded by wetland stratigraphy fringing Cascadia’s estuaries. Correlation of such coastal earthquake evidence along the subduction zone has largely relied on relative position in a stratigraphic sequence and maximum-limiting 14C ages with errors of decades to hundreds of years. Offshore, a 10,000-year record of turbidites in marine cores is interpreted as an archive of strong shaking from great earthquakes, with an average frequency of about 500 years in northern Cascadia versus 200-300 years in southern Oregon and northern California. Onshore, fewer events marked by sharp (<3 mm) peat-mud (mud-over-peat) contacts in tidal wetland stratigraphic sequences have been widely inferred to record sudden relative sea-level rise due to coseismic subsidence during megathrust earthquakes: 4-7 sharp subsidence contacts in 3500 years at estuaries in northern Oregon and southern Washington (500-800 year average recurrence), and 9-12 sharp subsidence contacts in over 6000 years in sequences in central and southern Oregon (500-900 year average recurrence). Improved understanding of the onshore and offshore records is critical to the assessment of earthquake hazard in western North America and of tsunami hazard in the Pacific basin. However, because dating the turbidite record is inherently much less precise than are age models for subsidence events in the most thoroughly studied tidal wetland sequences, accurate reconstruction of the times of Cascadia’s great earthquakes depends on the ages from the onshore record. Although methods to reduce uncertainty in the limits of resolution of tidal stratigraphy for recording earthquakes of a particular magnitude, and ways to distinguish earthquake subsidence stratigraphic contacts from non-seismic contacts, have been discussed for decades (e.g., Nelson, 1992; Atwater, 1992; Darienzo et al., 1994; Nelson et al., 1996a; Atwater and Hemphill-Haley, 1997; Witter et al., 2001; Kelsey et al., 2002; Nelson et al., 2006; Graehl et al., 2014; Milker et al., 2016), consensus about the threshold of resolution (minimum identifiable evidence of an earthquake) of tidal stratigraphy and, therefore, the completeness of Cascadia’s coastal record of great earthquakes, remains elusive. Although the most distinct, widespread contacts likely record close to a meter of coastal subsidence during the greatest megathrust earthquakes (e.g., M8.8-M9), other contacts may record <0.5 m of subsidence onshore of patches of low stress release on the megathrust during great earthquakes, during lesser megathrust earthquakes (e.g., M8.2-8.6), or from localized subsidence near upper-plate faults that slip during or independently of megathrust earthquakes (Nelson et al., 1996b; Wang et al., 2013; Kemp et al., 2018). At the Siuslaw River estuary in central Oregon (lat. 43.97°) a stratigraphy of 9-12 peat-mud contacts, similar to those described from many Cascadia estuaries, may record a greater number of earthquakes during the past 2000 years than at any other of the tens of tidal wetland sites to the north and south. Here, as well as at many tens of other Cascadia tidal wetland sites, peat-mud contacts mark the tops of couplets of tidal flat and low marsh mud gradually shoaling upward into middle and high marsh peat. Along core transects across an 800-m-wide, island marsh in the Siuslaw River, we traced the 9 most continuous of 12-15 peaty beds dating from the past 2000 years for 250-500 m, but we had difficulty correlating the 3-6 intervening beds >50-100 m. We attribute the sharper, more extensive upper contacts on peaty beds—two capped by sandy beds probably deposited by tsunamis—to sudden coseismic subsidence of middle and high marshes, but origins for other upper and lower contacts boundi

Conference Paper↗

USGS2016

Seismic site characterization of an urban dedimentary basin, Livermore Valley, California: Site tesponse, basin-edge-induced surface waves, and 3D simulations

Thirty‐two accelerometers were deployed in the Livermore Valley, California, for approximately one year to study sedimentary basin effects. Many local and near‐regional earthquakes were recorded, including the 24 August 2014 M w 6.0 Napa, California, earthquake. The resulting ground‐motion data set is used to quantify the seismic response of the Livermore basin, a major structural depression in the California Coast Range Province bounded by active faults. Site response is calculated by two methods: the reference‐site spectral ratio method and a source‐site spectral inversion method. Longer‐period (≥1  s) amplification factors follow the same general pattern as Bouguer gravity anomaly contours. Site response spectra are inverted for shallow shear‐wave velocity profiles, which are consistent with independent information. Frequency–wavenumber analysis is used to analyze plane‐wave propagation across the Livermore Valley and to identify basin‐edge‐induced surface waves with back azimuths different from the source back azimuth. Finite‐element simulations in a 3D velocity model of the region illustrate the generation of basin‐edge‐induced surface waves and point out strips of elevated ground velocities along the margins of the basin.

Bulletin of the Seismological Society of America↗

USGS1985

Description and preliminary testing of the CDSN Seismic Sensor Systems

The China Digital Seismograph Network (CDSN) is being designed and installed to provide the People's Republic of China with the facilities needed to create a national digital database for earthquake research. The CDSN, which is being developed jointly by the PRC State Seismological Bureau and the U.S. Geological Survey, will consist initially of nine digitally-recording seismograph stations, a data management system to be used for compiling network-day tapes, and a depot maintenance center. Data produced by the network will be shared with research scientists throughout the world. A national seismograph network must be designed to support a variety of research objectives. From this standpoint, the choices and tradeoffs involved in specifying signal bandwidth, resolution, and dynamic range are the most important decisions in system design. As in the case of the CDSN, these decisions are made during the selection and design of the seismic sensor system and encoder components. The purpose of this report is to describe the CDSN sensor systems, their important signal characteristics, and the results of preliminary tests that have been performed on the instruments. Four overlapping data bands will be recorded at each station: short period (SP), broadband (BB), long period (LP), and very long period (VLP). Amplitude response curves are illustrated in Figure I. Vertical and horizontal components will be recorded for each data band. The SP and LP channels will be recorded with sufficient sensitivities to resolve earth background noise at seismically quiet sites. The BB channels will have a lower sensitivity and are intended for broadband recording of moderate-to-large body-wave signals and for increasing the effective amplitude range in the short- and long-period bands. The VLP channel does not provide additional spectral coverage at long periods; its purpose is to make use of on-site filtration and decimation to reduce post processing requirements for VLP studies. Early plans also included a triaxial set of low-sensitivity accelerometers for recording strong signals from large local and regional earthquakes. The accelerometers are not being installed; however, they may be added in the future. The short-period signals will be derived from a three-component set of PRC-supplied Model DJ-I SP seismometers and US-supplied SP amplifiers. The seismometers will be installed in surface or shallow subsurface vaults, except at two of the stations where they will be installed in boreholes. The BB, LP, and VLP signals will be derived from Streckeisen STS-1 broadband sensor systems installed in vaults, except at one site where the LP signals only will-be derived from a KS-36000 borehole seismometer installed at a depth of 100 meters. Analog signals will be sampled and quantized by an analog-to-digital converter (ADC) that is part of the recording system. Sampling rates chosen for the CDSN are as follows: * SP 40 samples/second * BB 20 samples/second * LP 1 sample/second * VLP 6 samples/minute The ADC 16-bit data word format makes use of 14 bits to quantize the signal and 2 bits to specify an automatically ranged gain of 1, 8, 32, or 128. This will provide 84 dB of resolution and up to 42 dB of gain ranging for a total opera- ting range of 126 dB peak to peak. Magnetic tape cartridges, each having a capacity of 67 megabytes, will be used for recording the digital data. LP and VLP data will be recorded continu- ously. SP and BB data will be processed through an automatic signal detector of the type described by Murdock and Hutt (1983), and only detected events will be stored on tape. Detection parameters, such as turn-on sensitivity and mini- mum recording duration for the SP and BB channels, will be fully programmable and easily changed. One or more of the data channels may also be recorded on analog recorders. A CDSN recording system was not available at the time that the preliminary tests were performed on the CDSN sensor systems. This did not interfere with the principal goals of the testing which were to determine the best sensor installation techniques and to demonstrate the operational performance of the sensor systems, especially with regard to instrument noise and detection capability. When the CDSN recording system is available, additional tests will be performed to measure distortion levels in the full system and components. Distortion tests have been performed on the Streckeisen STS-1 broadband seismometers, the most critical component from the standpoint of linearity, and the results of the tests are reported by Wielandt and Streckeisen (1982). Calibration accuracy and stability are also important sensor system characteristics that cannot be defined without additional testing. These tests will be performed as the instruments are installed at the stations and periodically during operation.

Open-File Report↗

USGS2015

A robust calibration technique for acoustic emission systems based on momentum transfer from a ball drop

We describe a technique to estimate the seismic moment of acoustic emissions and other extremely small seismic events. Unlike previous calibration techniques, it does not require modeling of the wave propagation, sensor response, or signal conditioning. Rather, this technique calibrates the recording system as a whole and uses a ball impact as a reference source or empirical Green’s function. To correctly apply this technique, we develop mathematical expressions that link the seismic moment $M_{0}$ of internal seismic sources (i.e., earthquakes and acoustic emissions) to the impulse, or change in momentum $\Delta p $, of externally applied seismic sources (i.e., meteor impacts or, in this case, ball impact). We find that, at low frequencies, moment and impulse are linked by a constant, which we call the force‐moment‐rate scale factor $C_{F\dot{M}} = M_{0}/\Delta p$. This constant is equal to twice the speed of sound in the material from which the seismic sources were generated. Next, we demonstrate the calibration technique on two different experimental rock mechanics facilities. The first example is a saw‐cut cylindrical granite sample that is loaded in a triaxial apparatus at 40 MPa confining pressure. The second example is a 2 m long fault cut in a granite sample and deformed in a large biaxial apparatus at lower stress levels. Using the empirical calibration technique, we are able to determine absolute source parameters including the seismic moment, corner frequency, stress drop, and radiated energy of these magnitude −2.5 to −7 seismic events.

Bulletin of the Seismological Society of America↗

USGS1983

Seismicity at Fuego, Pacaya, Izalco, and San Cristobal Volcanoes, Central America, 1973-1974

Seismic data collected at four volcanoes in Central America during 1973 and 1974 indicate three sources of seismicity: regional earthquakes with hypocentral distances greater than 80 km, earthquakes within 40 km of each volcano, and seismic activity originating at the volcanoes due to eruptive processes. Regional earthquakes generated by the underthrusting and subduction of the Cocos Plate beneath the Caribbean Plate are the most prominent seismic feature in Central America. Earthquakes in the vicinity of the volcanoes occur on faults that appear to be related to volcano formation. Faulting near Fuego and Pacaya volcanoes in Guatemala is more complex due to motion on a major E-W striking transform plate boundary 40 km north of the volcanoes. Volcanic activity produces different kinds of seismic signatures. Shallow tectonic or A-type events originate on nearby faults and occur both singly and in swarms. There are typically from 0 to 6 A-type events per day with b value of about 1.3. At very shallow depths beneath Pacaya, Izalco, and San Cristobal large numbers of low-frequency or B-type events are recorded with predominant frequencies between 2.5 and 4.5 Hz and with b values of 1.7 to 2.9. The relative number of B-type events appears to be related to the eruptive states of the volcanoes; the more active volcanoes have higher levels of seismicity. At Fuego Volcano, however, low-frequency events have unusually long codas and appear to be similar to tremor. High-amplitude volcanic tremor is recorded at Fuego, Pacaya, and San Cristobal during eruptive periods. Large explosion earthquakes at Fuego are well recorded at five stations and yield information on near-surface seismic wave velocities (??=3.0??0.2 km/sec.). ?? 1983 Intern. Association of Volcanology and Chemistry of the Earth's Interior.

Bulletin Volcanologique↗

USGS2016

A possible source mechanism of the 1946 Unimak Alaska far-field tsunami, uplift of the mid-slope terrace above a splay fault zone

In 1946, megathrust seismicity along the Unimak segment of the Alaska subduction zone generated the largest ever recorded Alaska/Aleutian tsunami. The tsunami severely damaged Pacific islands and coastal areas from Alaska to Antarctica. It is the charter member of “tsunami” earthquakes that produce outsized far-field tsunamis for the recorded magnitude. Its source mechanisms were unconstrained by observations because geophysical data for the Unimak segment were sparse and of low resolution. Reprocessing of legacy geophysical data reveals a deep water, high-angle reverse or splay thrust fault zone that leads megathrust slip upward to the mid-slope terrace seafloor rather than along the plate boundary toward the trench axis. Splay fault uplift elevates the outer mid-slope terrace and its inner area subsides. Multibeam bathymetry along the splay fault zone shows recent but undated seafloor disruption. The structural configuration of the nearby Semidi segment is similar to that of the Unimak segment, portending generation of a future large tsunami directed toward the US West coast.

Alaska↗

USGS2006

Character and distribution of exposed glaciodeltaic deposits off outer Cape Cod, Massachusetts, and their effects on hydrogeology and benthic habitats

Seabed outcrops of glaciodeltaic sediments were identified in four places east of Cape Cod, Massachusetts, during seismic-reflection, multibeam bathymetric and backscatter, bottom photographic, and sediment sampling surveys. These strata record coarser-grained ice-proximal glaciofluvial topset to finer-grained distal glaciolacustrine bottomset deposition within deltaic systems that prograded southwestward into glacial lakes from the South Channel lobe about 18 ka B.P. These beds are important because they (1) influence the outer Cape's hydrogeologic framework, and (2) constitute relatively stable, locally rough habitats within an area of seafloor dominated by mobile sand and gravelly sediment, and benefit the benthic fauna by providing shelter and a substrate amenable to burrow construction. ?? Springer-Verlag 2006.

Massachusetts↗

USGS2003

Infrasonic tremor observed at Kilauea Volcano, Hawai'i

[1] Infrasonic array data collected at Kīlauea Volcano, Hawai'i, during November 12–21, 2002 indicate that the active vents and lava tube system near the Pu'u 'Ō'ō vent complex emit almost continuous infrasound in the 0.3–10 Hz frequency band. The spectral content of these infrasonic signals matches well that of synchronous seismic tremor. In sites protected from wind noise, significant signal to noise ratios were recorded as far as ∼13 km from the crater of Pu'u 'Ō'ō. The infrasonic recordings suggest that one or more tremor sources may be close to the surface. In addition, these results demonstrate that adequate site and instrument selections for infrasonic arrays are essential in order to obtain consistent and reliable infrasonic detections.

Hawai'i↗

USGS2019

The 12 November 2017 Mw 7.3 Ezgeleh–Sarpolzahab (Iran) earthquake and active tectonics of the Lurestan arc

The 12 November 2017 M w 7.3 Ezgeleh‐Sarpolzahab earthquake is the largest instrumentally recorded earthquake in the Zagros Simply Folded Belt by a factor of ∼10 in seismic moment. Exploiting local, regional, and teleseismic data and synthetic aperture radar interferometry imagery, we characterize the rupture, its aftershock sequence, background seismicity, and regional tectonics. The mainshock ruptured slowly (∼2 km/s), unilaterally southward, for ∼40 km along an oblique (dextral‐thrust) fault that dips ∼14°E beneath the northwestern Lurestan arc. Slip is confined to basement depths of ∼12–18 km, resolvably beneath the sedimentary cover which is ∼8 km thick in this area. The gentle dip angle and basement location allow for a broad slip area, explaining the large magnitude relative to earthquakes in the main Fars arc of the Zagros, where shallower, steeper faults are limited in rupture extent by weak sedimentary layers. Early aftershocks concentrate around the southern and western edges of the mainshock slip area and therefore cluster in the direction of rupture propagation, implying a contribution from dynamic triggering. A cluster of events ∼100 km to the south near Mandali (Iraq) reactivated the ∼50° dipping Zagros Foredeep Fault. The basement fault responsible for the Ezgeleh‐Sarpolzahab earthquake probably accounts for the ∼1 km elevation contrast between the Lurestan arc and the Kirkuk embayment but is distinct from sections of the Mountain Front Fault that define frontal escarpments elsewhere in the Zagros. It may be related to a seismic interface underlying the central and southern Lurestan arc, and a key concern is whether or not the more extensive regional structure is also seismogenic.

Journal of Geophysical Research: Solid Earth↗

USGS2012

Reﬁnements to the method of epicentral location based on surface waves from ambient seismic noise: introducing Love waves

The purpose of this study is to develop and test a modiﬁcation to a previous method of regional seismic event location based on Empirical Green’s Functions (EGFs) produced from ambient seismic noise. Elastic EGFs between pairs of seismic stations are determined by cross-correlating long ambient noise time-series recorded at the two stations. The EGFs principally contain Rayleigh- and Love-wave energy on the vertical and transverse components, respectively, and we utilize these signals between about 5 and 12 s period. The previous method, based exclusively on Rayleigh waves, may yield biased epicentral locations for certain event types with hypocentral depths between 2 and 5 km. Here we present theoretical arguments that show how Love waves can be introduced to reduce or potentially eliminate the bias. We also present applications of Rayleigh- and Love-wave EGFs to locate 10 reference events in the western United States. The separate Rayleigh and Love epicentral locations and the joint locations using a combination of the two waves agree to within 1 km distance, on average, but conﬁdence ellipses are smallest when both types of waves are used.

Geophysical Journal International↗

USGS2017

Influence of lithostatic stress on earthquake stress drops in North America

We estimate stress drops for earthquakes in and near the continental United States using the method of spectral ratios. The ratio of acceleration spectra between collocated earthquakes recorded at a given station removes the effects of path and recording site and yields source parameters including corner frequency for, and the ratio of seismic moment between, the two earthquakes. We determine stress drop from these parameters for 1121 earthquakes greater than M ∼3 in 60 earthquake clusters. We find that the average Brune stress drop for the few eastern United States (EUS) tectonic mainshocks studied (2.6–36 MPa) is about three times greater than that of tectonic mainshocks in the western United States (WUS, 1.0–7.9 MPa) and five times greater than mainshocks potentially induced by wastewater injection in the central United States (CUS, 0.6–5.6 MPa). EUS events tend to be deeper thrusting events, whereas WUS events tend to be shallower but have a wide range of focal mechanisms. CUS events tend to be shallow with strike‐slip to normal‐faulting mechanisms. With the possible exception of CUS aftershocks, we find that differences in stress drop among all events can be taken into account, within one standard deviation of significance, by differences in the shear failure stress as outlined by Mohr–Coulomb theory. The shear failure stress is a function of vertical stress (or depth), the fault style (normal, strike slip, or reverse), and coefficient of friction (estimated here to be, on average, 0.64). After accounting for faulting style and depth dependence, we find that the average Brune stress drop is about 3% of the failure stress. These results suggest that high‐frequency shaking hazard (>∼1 Hz) from shallow induced events and aftershocks is reduced to some extent by lower stress drop. However, the shallow hypocenters will increase hazard within several kilometers of the source.

Bulletin of the Seismological Society of America↗