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Linville, LM, Pankow KL, Kilb DL.  2018.  Contour-Based Frequency-Domain Event Detection for Seismic Arrays. SRL.   doi: 10.1785/0220170242   Abstract

We develop a frequency-domain array-based detection algo- rithm that exploits the gridded station configuration (intersta- tion spacing of 70 km) of the EarthScope Transportable Array to detect and locate small magnitude (M < 2:5) earthquakes. Our method uses the geographic extent of frequency–amplitude time-series data to determine epicentral locations for events regardless of their ability to generate impulsive amplitude excur- sions at multiple stations. We apply the method to three sedi- mentary basins in the central United States (CUS), finding a total of 484 new earthquakes. Our results identify new seismicity in all three sedimentary basins but disproportionate increases in seismicity between regions (Williston basin: 50%, Permian: 300%, Denver: 1000%). A majority of the newly detected seis- micity in the Permian and Denver-Julesburg basins may be linked to active wells, while there continues to be a lack of evidence for induced seismicity in the Williston basin. We also apply this method to a dataset from Yellowstone National Park with average interstation distances of ∼80 m. Results from the Yellowstone data demonstrate that when array station spacing remains regular, events from nonearthquake sources such as hydrothermal features can be successfully detected and located without the necessity to tune parameters for specific sources. The capability to generalize across source types makes this algorithm potentially useful for signal exploration when signal characteristics are unknown.

Sahakian, V, Baltay A, Hanks T, Buehler J, Vernon F, Kilb D, Abrahamson N.  2018.  Decomposing Leftovers: Event, Path, and Site Residuals for a Small‐Magnitude Anza Region GMPE. BSSA.   10.1785/0120170376  
Kilb, D, Yang A, Garrett N, Pankow K, Rubinstein J, Linville L.  2018.  Tilt Trivia: A Free Multiplayer App to Learn Geoscience Concepts and Definitions. SRL.   10.1785/0220180049  
Walter, F, Roux P, Roeoesli C, Lecointre A, Kilb D, Roux P-F.  2015.  Using glacier seismicity for phase velocity measurements and Green's function retrieval. Geophysical Journal International. 201(3):1722-1737.   10.1093/gji/ggv069   Abstract

High-melt areas of glaciers and ice sheets foster a rich spectrum of ambient seismicity. These signals not only shed light on source mechanisms (e.g. englacial fracturing, water flow, iceberg detachment, basal motion) but also carry information about seismic wave propagation within glacier ice. Here, we present two approaches to measure and potentially monitor phase velocities of high-frequency seismic waves (≥1 Hz) using naturally occurring glacier seismicity. These two approaches were developed for data recorded by on-ice seasonal seismic networks on the Greenland Ice Sheet and a Swiss Alpine glacier. The Greenland data set consists of continuous seismograms, dominated by long-term tremor-like signals of englacial water flow, whereas the Alpine data were collected in triggered mode producing 1–2 s long records that include fracture events within the ice (‘icequakes’). We use a matched-field processing technique to retrieve frequency-dependent phase velocity measurements for the Greenland data. In principle, this phase dispersion relationship can be inverted for ice sheet thickness and bed properties. For these Greenland data, inversion of the dispersion curve yields a bedrock depth of 541 m, which may be too small by as much as 35 per cent. We suggest that the discrepancy is due to lateral changes in ice sheet depth and bed properties beneath the network, which may cause unaccounted mixing of surface wave modes in the dispersion curve. The Swiss Alpine icequake records, on the other hand, allow for reconstruction of the impulse response between two seismometers. The direct and scattered wave fields from the vast numbers of icequake records (tens of thousands per month) can be used to measure small changes in englacial velocities and thus monitor structural changes within the ice.

Linville, L, Pankow K, Kilb D, Velasco A.  2014.  Exploring remote earthquake triggering potential across EarthScopes' Transportable Array through frequency domain array visualization. Journal of Geophysical Research: Solid Earth. :2014JB011529.   10.1002/2014JB011529   AbstractWebsite

To better understand earthquake source processes involved in dynamically triggering remote aftershocks, we use data from the EarthScope Transportable Array (TA) that provide uniform station sampling, similar recording capabilities, large spatial coverage, and, in many cases, repeat sampling at each site. To avoid spurious detections, which are an inevitable part of automated time domain amplitude threshold detection methods, we develop a frequency domain earthquake detection algorithm that identifies coherent signal patterns through array visualization. This method is tractable for large data sets, ensures robust catalogs, and delivers higher resolution observations than what are available in current catalogs. We explore seismicity rate changes local to the TA stations following 18 global main shocks (M ≥ 7) that generate median peak dynamic stress amplitudes of 0.001–0.028 MPa across the array. From these main shocks, we find no evidence of prolific or widespread remote dynamic triggering in the continental U.S. within the main shock's wave train or following main shock stress transients within 2 days. However, limited evidence for rate increases exist in localized source regions. These results suggest that for these data, prolific, remote earthquake triggering is a rare phenomenon throughout a wide range of observable magnitudes. We further conclude that within the lower range of previously reported triggering thresholds, surface wave amplitude does not correlate well with observed cases of dynamic triggering. We propose that other characteristics of the triggering wavefield, in addition to specific conditions at the site, will drive the occurrence of triggering at these amplitudes.

Heeszel, DS, Walter F, Kilb DL.  2014.  Humming glaciers. Geology. 42:1099-1102.   10.1130/g35994.1   AbstractWebsite

Mountain glaciers represent one of the largest repositories of fresh water in alpine regions globally. However, little is known about the processes by which water moves through these systems. Analysis of data from a deployment of seismometers located near an ice marginal lake in the Swiss Alps reveals, for the first time, that harmonic tremor occurs within mountain glaciers and that individual icequakes at the glacier base can exhibit harmonic properties. These observations suggest that there is a complex network of fluid-induced fracture processes at the glacier base. Modeling changes in the observed harmonic frequencies indicates that the spectral characteristics of seismic data can provide important information about hydraulic fracture geometry and fluid pressure at depth, leading to important insights into subglacial hydrologic processes. Future modeling of these processes may lead to improved glacial outburst flood hazard predictions.

Lawrence, JF, Cochran ES, Chung A, Kaiser A, Christensen CM, Allen R, Baker JW, Fry B, Heaton T, Kilb D, Kohler MD, Taufer M.  2014.  Rapid earthquake characterization using MEMS accelerometers and volunteer hosts following the M 7.2 Darfield, New Zealand, Earthquake. Bulletin of the Seismological Society of America. 104:184-192.   10.1785/0120120196   AbstractWebsite

We test the feasibility of rapidly detecting and characterizing earthquakes with the Quake-Catcher Network (QCN) that connects low-cost microelectromechanical systems accelerometers to a network of volunteer-owned, Internet-connected computers. Following the 3 September 2010 M 7.2 Darfield, New Zealand, earthquake we installed over 180 QCN sensors in the Christchurch region to record the aftershock sequence. The sensors are monitored continuously by the host computer and send trigger reports to the central server. The central server correlates incoming triggers to detect when an earthquake has occurred. The location and magnitude are then rapidly estimated from a minimal set of received ground-motion parameters. Full seismic time series are typically not retrieved for tens of minutes or even hours after an event. We benchmark the QCN real-time detection performance against the GNS Science GeoNet earthquake catalog. Under normal network operations, QCN detects and characterizes earthquakes within 9.1 s of the earthquake rupture and determines the magnitude within 1 magnitude unit of that reported in the GNS catalog for 90% of the detections.

Kilb, D, Rohrlick D, Yang A, Choo Y, Ma L, Ruzic R.  2014.  The Game of Curiosity: Using Videogames to Cultivate Future Scientists. Seismological Research Letters. 85(4):923-929.
Kane, DL, Kilb DL, Vernon FL.  2013.  Selecting Empirical Green’s Functions in Regions of Fault Complexity: A Study of Data from the San Jacinto Fault Zone, Southern California. BSSA. 103(2A) Abstract

To constrain an earthquake’s source properties, the path‐ and site‐effect contributions to the seismic waveform can be approximated using another earthquake as an empirical Green’s function (EGF). An ideal EGF earthquake is smaller in magnitude than the mainshock and shares a similar focal mechanism and hypocenter. Here, we quantify how to optimally select EGF events using data from the spatially complex San Jacinto Fault Zone (SJFZ) in southern California. The SJFZ’s high seismicity rate allows us to test the EGF method for 51 target 3200 for each mainshock). We purposefully select a large population of inappropriate EGFs in order to identify thresholds and restrictions that optimize EGF selection criteria. For each mainshock/EGF pair, we compute the spectral ratio, fit the mainshock corner frequency, and measure the variability of these corner frequencies across the network. We assume a suitable EGF event will produce similar corner frequency estimates at every station. We discover that limiting hypocentral separation distances between mainshock and EGF events to <1  km (within ∼1–3 mainshock fault lengths) is an effective criterion in EGF choice. Surprisingly, separation distances of 2–14 km produce negligible changes in corner frequency variability, suggesting that EGF events at a 2‐km distance may be as poor a choice as EGF events at much greater distances. When EGF events within 1 km are not available, we suggest limiting EGF events to those with highly similar waveforms to that of the mainshock to ensure source similarity.

Kilb, D, Biasi G, Anderson J, Brune J, Peng ZG, Vernon FL.  2012.  A Comparison of Spectral Parameter Kappa from Small and Moderate Earthquakes Using Southern California ANZA Seismic Network Data. Bulletin of the Seismological Society of America. 102:284-300.   10.1785/0120100309   AbstractWebsite

Kappa is a one-parameter estimator of the spectral amplitude decay with frequency of a seismogram. Low values (similar to 5 ms) indicate limited attenuation of high-frequency energy whereas higher values (similar to 40 ms) indicate high-frequency energy has been removed. Kappa is often assumed to be a site term and used in seismic designs. We address two key questions about kappa: (1) how to identify source, path, and site contributions to kappa; and (2) can kappa estimates from smaller earthquakes, and more readily accessible weak- motion recordings, be reasonably extrapolated to estimate kappa of larger earthquakes? The use of small earthquakes (M-L < 1) presents many challenges and requires new approaches. We develop estimates of kappa for seismograms from 1137 small earthquakes recorded by the ANZA seismic network in southern California, and compare these to results from the stronger recorded shaking generated by 43 M-L > 3.5 earthquakes inside the network. We find kappa from small earthquakes predicts the relative values of kappa for larger earthquakes (e.g., measurements at stations PFO and KNW are small compared with those at stations TRO and SND). For the SND and TRO data, however, kappa values from small earthquakes overpredict those from moderate and large earthquakes. Site effects are the most important contributor to kappa estimates, but the scatter within kappa measurements at a given station is likely caused by a significant contribution from near the source, perhaps related to near-source scattering. Because of this source-side variability, care is recommended in using individual small events as Green's functions to study source-time effects of moderate and large events.

Kilb, D, Peng Z, Simpson D, Michael A, Fisher M, Rohrlick D.  2012.  Listen, Watch, Learn: SeisSound Video products. Seis. Res. Lett.. :281-286.   10.1785/gssrl.83.2.281   Abstract

The increased popularity of YouTube videos has changed the format of how information is distributed and assimilated, highlighting the importance of including auditory information in videos. Videos that include sound also permeate the research community, as evidenced by their recent increase within online supplements to journal articles. Tapping into this new approach of information exchange, we are creating videos of seismic data that augment visual imagery with auditory counterparts. We term these “SeisSound” video products (Figure 1). We find the richness and complexities of seismic data can more easily be appreciated using these SeisSound products than using just the individual visual or the auditory components independently.

Peng, Z, Aiken C, Kilb D, Shelly DR, Enescu B.  2012.  Listening to the 2011 magnitude 9.0 Tohoku-Oki, Japan earthquake. Seis. Res. Lett.,. :287-293.   10.1785/gssrl.83.2.287  
Brothers, D, Kilb D, Luttrell K, Driscoll N, Kent G.  2011.  Loading of the San Andreas fault by flood-induced rupture of faults beneath the Salton Sea. Nature Geoscience. 4:486-492.   10.1038/ngeo1184   AbstractWebsite

The southern San Andreas fault has not experienced a large earthquake for approximately 300 years, yet the previous five earthquakes occurred at similar to 180-year intervals. Large strike-slip faults are often segmented by lateral stepover zones. Movement on smaller faults within a stepover zone could perturb the main fault segments and potentially trigger a large earthquake. The southern San Andreas fault terminates in an extensional stepover zone beneath the Salton Sea-a lake that has experienced periodic flooding and desiccation since the late Holocene. Here we reconstruct the magnitude and timing of fault activity beneath the Salton Sea over several earthquake cycles. We observe coincident timing between flooding events, stepover fault displacement and ruptures on the San Andreas fault. Using Coulomb stress models, we show that the combined effect of lake loading, stepover fault movement and increased pore pressure could increase stress on the southern San Andreas fault to levels sufficient to induce failure. We conclude that rupture of the stepover faults, caused by periodic flooding of the palaeo-Salton Sea and by tectonic forcing, had the potential to trigger earthquake rupture on the southern San Andreas fault. Extensional stepover zones are highly susceptible to rapid stress loading and thus the Salton Sea may be a nucleation point for large ruptures on the southern San Andreas fault.

Felzer, KR, Kilb D.  2009.  A Case Study of Two M similar to 5 Mainshocks in Anza, California: Is the Footprint of an Aftershock Sequence Larger Than We Think? Bulletin of the Seismological Society of America. 99:2721-2735.   10.1785/0120080268   AbstractWebsite

It has been traditionally held that aftershocks occur within one to two fault lengths of the mainshock. Here we demonstrate that this perception has been shaped by the sensitivity of seismic networks. The 31 October 2001 M(w) 5.0 and 12 June 2005 M(w) 5.2 Anza mainshocks in southern California occurred in the middle of the densely instrumented ANZA seismic network and thus were unusually well recorded. For the June 2005 event, aftershocks as small as M 0.0 could be observed stretching for at least 50 km along the San Jacinto fault even though the mainshock fault was only similar to 4.5 km long. It was hypothesized that an observed aseismic slipping patch produced a spatially extended aftershock-triggering source, presumably slowing the decay of aftershock density with distance and leading to a broader aftershock zone. We find, however, the decay of aftershock density with distance for both Anza sequences to be similar to that observed elsewhere in California. This indicates there is no need for an additional triggering mechanism and suggests that given widespread dense instrumentation, aftershock sequences would routinely have footprints much larger than currently expected. Despite the large 2005 aftershock zone, we find that the probability that the 2005 Anza mainshock triggered the M 4.9 Yucaipa mainshock, which occurred 4.2 days later and 72 km away, to be only 14% +/- 1%. This probability is a strong function of the time delay; had the earthquakes been separated by only an hour, the probability of triggering would have been 89%.

Kilb, D.  2008.  Volcanology - Throwing mud. Nature Geoscience. 1:572-573.   10.1038/ngeo299   AbstractWebsite

The causes of the catastrophic eruption of the Lusi mud volcano in Indonesia are hotly debated. Data from a nearby exploration well and a new look at the stress regime suggest that drilling operations, and not an earthquake set the eruption off.

Jacobs, AM, Kilb D, Kent G.  2008.  3-D Interdisciplinary Visualization: Tools for Scientific Analysis and Communication. Seismological Research Letters. 79:867-876.   10.1785/gssrl.79.6.867   Website
Kane, DL, Kilb D, Berg AS, Martynov VG.  2007.  Quantifying the remote triggering capabilities of large earthquakes using data from the ANZA Seismic network catalog (Southern California). Journal of Geophysical Research-Solid Earth. 112   10.1029/2006jb004714   AbstractWebsite

Various studies have examined remote earthquake triggering in geothermal areas, but few studies have investigated triggering in nongeothermal areas. We search the ANZA ( southern California) network catalog for evidence of remote triggering. Using three statistical tests ( binomial, Kolmogorov-Smirnov, and Wilcoxon rank sum), we determine the significance of the rates and timing of earthquakes in southern California following large teleseismic events. To validate our statistical tests, we identify 20 local main shocks (M-L >= 3.1) with obvious aftershock sequences and 22 local main shocks (ML >= 3.0) that lack obvious aftershock sequences. Our statistical tests quantify the ability of these local main shocks to trigger aftershocks. Assuming that the same triggering characteristic (i.e., a particular seismic wave amplitude, perhaps in a specific frequency band) is evident for both local and remote main shocks, we apply the same tests to 60 remote main shocks (m(b) >= 7.0) and assess the ability of these events to trigger seismicity in southern California. We find no obvious signature of remote triggering. We find minimal differences between the spectral amplitudes and maximum ground velocities of the local triggering and nontriggering earthquakes. Similar analysis of a select few of our remote earthquakes shows that the related ground motion regularly exceeds that of local earthquakes both at low frequencies and in maximum velocity. This evidence weakly suggests that triggering requires larger amplitudes at high frequencies and that a maximum ground velocity alone is not the primary factor in remote triggering. Our results are complex, suggesting that a triggering threshold, if it exists, may depend on several factors.

Kilb, D, Martynov VG, Vernon FL.  2007.  Aftershock detection thresholds as a function of time: Results from the ANZA seismic network following the 31 October 2001 M-L 5.1 ANZA, California, earthquake. Bulletin of the Seismological Society of America. 97:780-792.   10.1785/0120060116   AbstractWebsite

We examine aftershock detectability thresholds for events in the initial part of the 31 October 2001, M-L 5.1 sequence in southern California. This sequence occurred directly below the broadband ANZA seismic network, which recorded continuous waveform data at 13 azimuthally well-distributed stations within the study region (seven had epicentral distances < 20 km). Of the 608 aftershocks (0 < ML < similar to 2.8) in the initial 2 hr of this sequence, the first five aftershocks recorded were only identifiable at stations within 30 km after applying a high-pass filter. Using a cluster (radius <= 1.1 km) of 200 representative aftershocks, we track the maximum seismogram amplitude versus earthquake magnitude. This relationship helps us quantify the visibility of aftershocks within the mainshock coda and assess our detection capabilities. We estimate that detectable aftershocks within the mainshock coda include (1) those over magnitude similar to 3 that are within 15 km of the network centroid that occur 12 sec or more into the sequence, and (2) those over magnitude similar to 2 that are within 30 km of the centroid of the network that occur 60 see or more into the sequence. We find a lack of large aftershocks in this sequence. The largest aftershock (M-L similar to 2.8) is substantially smaller than the mainshock (M-L 5.1). We suggest this relatively large-magnitude differential is dictated by a combination of factors that includes complexity of the San Jacinto fault system and the lack of large earthquakes in the region in the past similar to 20 years.

Mellors, R, Kilb D, Aliyev A, Gasanov A, Yetirmishli G.  2007.  Correlations between earthquakes and large mud volcano eruptions. Journal of Geophysical Research-Solid Earth. 112   10.1029/2006jb004489   AbstractWebsite

[ 1] We examine the potential triggering relationship between large earthquakes and methane mud volcano eruptions. Our data set consists of a 191-year catalog ( 1810 - 2001) of eruptions from 77 volcanoes in Azerbaijan, central Asia, supplemented with reports from mud volcano eruptions in Japan, Romania, Pakistan, and the Andaman Islands. We compare the occurrence of historical regional earthquakes ( M > 5) with the occurrence of Azerbaijan mud volcano eruptions and find that the number of same-day earthquake/ eruption pairs is significantly higher than expected if the eruptions and earthquakes are independent Poisson processes. The temporal correlation between earthquakes and eruptions is most pronounced for nearby earthquakes ( within similar to 100 km) that produce seismic intensities of Mercalli 6 or greater at the location of the mud volcano. This assumed magnitude/distance relationship for triggering observed in the Azerbaijan data is consistent with documented earthquake-induced mud volcano eruptions elsewhere. We also find a weak correlation that heightened numbers of mud volcano eruptions occur within 1 year after large earthquakes. The distribution of yearly eruptions roughly approximates a Poisson process, although the repose times somewhat favor a nonhomogenous failure rate, which implies that the volcanoes require some time after eruption to recharge. The volcanic triggering likely results from some aspect of the seismic wave's passage, but the precise mechanism remains unclear.

Kilb, D, Hardebeck JL.  2006.  Fault parameter constraints using relocated earthquakes: A validation of first-motion focal-mechanism data. Bulletin of the Seismological Society of America. 96:1140-1158.   10.1785/0120040239   AbstractWebsite

We estimate the strike and dip of three California fault segments (Calaveras, Sargent, and a portion of the San Andreas near San Jaun Bautistia) based on principle component analysis of accurately located microearthquakes. We compare these fault orientations with two different first-motion focal mechanism catalogs: the Northern California Earthquake Data Center (NCEDC) catalog, calculated using the FPFIT algorithm (Reasenberg and Oppenheimer, 1985), and a catalog created using the HASH algorithm that tests mechanism stability relative to seismic velocity model variations and earthquake location (Hardebeck and Shearer, 2002). We assume any disagreement (misfit > 30 degrees in strike, dip, or rake) indicates inaccurate focal mechanisms in the catalogs. With this assumption, we can quantify the parameters that identify the most optimally constrained focal mechanisms. For the NCEDC/FPFIT catalogs, we find that the best quantitative discriminator of quality focal mechanisms is the station distribution ratio (STDR) parameter, an indicator of how the stations are distributed about the focal sphere. Requiring STDR > 0.65 increases the acceptable mechanisms from 34%-37% to 63%-68%. This suggests stations should be uniformly distributed surrounding, rather than aligning, known fault traces. For the HASH catalogs, the fault plane uncertainty (FPU) parameter is the best discriminator, increasing the percent of acceptable mechanisms from 63%-78% to 81%-83% when FPU <= 35 degrees. The overall higher percentage of acceptable mechanisms and the usefulness of the formal uncertainty in identifying quality mechanisms validate the HASH approach of testing for mechanism stability.

Taesombut, N, Wu XR, Chien AA, Nayak A, Smith B, Kilb D, Im T, Samilo D, Kent G, Orcutt J.  2006.  Collaborative data visualization for Earth Sciences with the OptIPuter. Future Generation Computer Systems. 22:955-963.   doi:10.1016/j.future.2006.03.023  
Prieto, GA, Shearer PM, Vernon FL, Kilb D.  2004.  Earthquake source scaling and self-similarity estimation from stacking P and S spectra. Journal of Geophysical Research-Solid Earth. 109   10.1029/2004jb003084   AbstractWebsite

[1] We study the scaling relationships of source parameters and the self-similarity of earthquake spectra by analyzing a cluster of over 400 small earthquakes (M-L = 0.5 to 3.4) recorded by the Anza seismic network in southern California. We compute P, S, and preevent noise spectra from each seismogram using a multitaper technique and approximate source and receiver terms by iteratively stacking the spectra. To estimate scaling relationships, we average the spectra in size bins based on their relative moment. We correct for attenuation by using the smallest moment bin as an empirical Green's function (EGF) for the stacked spectra in the larger moment bins. The shapes of the log spectra agree within their estimated uncertainties after shifting along the omega(-3) line expected for self-similarity of the source spectra. We also estimate corner frequencies and radiated energy from the relative source spectra using a simple source model. The ratio between radiated seismic energy and seismic moment ( proportional to apparent stress) is nearly constant with increasing moment over the magnitude range of our EGF-corrected data (M-L = 1.8 to 3.4). Corner frequencies vary inversely as the cube root of moment, as expected from the observed self- similarity in the spectra. The ratio between P and S corner frequencies is observed to be 1.6 +/- 0.2. We obtain values for absolute moment and energy by calibrating our results to local magnitudes for these earthquakes. This yields a S to P energy ratio of 9 +/- 1.5 and a value of apparent stress of about 1 MPa.