Geology topics

Keith D. Koper

Publications and source records attributed to Keith D. Koper.

6 recordsLinked to original sources

USGS2021

Preface to the Focus Section on the 2020 Intermountain West earthquakes

The Intermountain West region of the United States extends from the eastern margin of the Sierra Nevada and Cascade Mountains in the west to the Rocky Mountains in the east. The region is characterized by dextral shear along the eastern margin of the Sierra Nevada and nearly east-west extension in the Basin and Range. This region experienced four significant earthquake sequences in the first half of 2020. The most significant mainshocks were the 18 March 2020 Mw 5.7 earthquake north of Magna, Utah (a suburb of Salt Lake City), the 31 March 2020 Mw 6.5 earthquake northwest of Stanley, Idaho, the 15 May 2020 Mw 6.5 earthquake in the Monte Cristo Range, northwest of Tonopah, Nevada, and the 24 June 2020 Mw 5.8 earthquake near Lone Pine, California. The 15 articles appearing in this focus section explore timely and important topics associated with these sequences, including kinematic rupture models, near-field ground motions, aftershock statistics, geologic observations, seismic hazard implications, and seismotectonics. It is noteworthy that the efforts to record and characterize these earthquake sequences took place during travel and work restrictions necessitated by the COVID-19 pandemic.

Seismological Research Letters↗

USGS2020

Seismic analysis of the 2020 Magna, Utah, earthquake sequence: Evidence for a listric Wasatch fault

The 18 March 2020 M w 5.7 Magna earthquake near Salt Lake City, Utah, offers a rare glimpse into the subsurface geometry of the Wasatch fault system—one of the world's longest active normal faults and a major source of seismic hazard in northern Utah. We analyze the Magna earthquake sequence and resolve oblique-normal slip on a shallow (30–35°) west-dipping fault at ~9- to 12-km depth. Combined with near-surface geological observations of steep dip (~70°), our results support a curved, or listric, fault shape. High-precision aftershock locations show the activation of multiple, low-angle (<30–35°) structures, indicating the existence of a complicated fault system. Our observations constrain the deep structure of the Wasatch fault system and suggest that ground shaking in the Salt Lake City region in future Wasatch fault earthquakes may be higher than previously estimated.

Utah↗

USGS2020

Spatiotemporal seismic structure variations associated with the 2018 Kīlauea eruption based on temporary dense geophone arrays

During the 2018 Kīlauea volcanic eruption, lava erupted from a series of new fissures in the lower East Rift Zone more than 30 km away from the summit through a dike intrusion. Between late May and early August, variations in the effusion rate at the persistent eruptive vent (Fissure 8) were observed following near‐daily summit caldera collapse events. Targeting the ongoing eruptive activity and the subsurface magma movement, we deployed a temporary dense seismic array. The observed time‐lapse changes in seismic velocity associated with the response of the summit collapse in three areas are presented in this study. The results show (1) clear spatially dependent co‐collapse velocity reductions across the newly‐intruded dike structure, (2) a gradual post‐collapse velocity increase near Fissure 8 correlated with the surge of magma supply, and (3) a gradual post‐collapse velocity increase on the summit likely associated with reservoir pressurization and crustal welding.

Hawaii↗

USGS2020

How processing methodologies can distort and bias power spectral density estimates of seismic background noise

Power spectral density (PSD) estimates are widely used in seismological studies to characterize background noise conditions, assess instrument performance, and study quasi‐stationary signals that are difficult to observe in the time domain. However, these studies often utilize different processing techniques, each of which can inherently bias the resulting PSD estimates. The level of smoothing, the size of the data window, and the method used for actually estimating the spectral content can all have strong influences on PSD estimates and background noise statistics. We show that although smoothing reduces the variance of the PSD estimate, the corresponding decrease in frequency resolution can eliminate or distort features of interest. For instance, popular software packages such as Incorporated Research Institutions for Seismology Modular Utility for STAatistical kNowledge Gathering (MUSTANG) and earlier versions of Portable Array Seismic Studies of the Continental Lithosphere Quick Look eXtended (PQLX), which were designed for data quality control and are effective in that regard, are less suitable for scientific studies that require accurate resolution of spectral peaks, even for peaks as broad as the primary ( ⁠ ∼ 14    s "> ∼ 14 s period) and secondary ( ⁠ ∼ 7    s "> ∼ 7 s period) microseisms. We also demonstrate how the 1 and 3 hr data windows used in MUSTANG and PQLX can be strongly influenced by energy generated from moderate‐size ( ⁠ M > ∼ 4.8 "> M > ∼ 4.8 ⁠ ) teleseismic earthquakes. The ubiquity of these events is likely skewing median ambient‐noise estimates by as much as 5 dB upward, for periods of 10–50 s at high‐quality broadband stations. Finally, we illustrate that many of the discrepancies between global low‐noise models are attributable to processing methodologies rather than fundamental differences in the underlying seismic data.

Seismological Research Letters↗

USGS2019

On the portability of ML-MC as a depth discriminant for small seismic events recorded at local distances

In this paper we show that M L -M C is a viable and regionally portable depth discriminant and therefore may contribute in nuclear test ban treaty verification. A recent study found that the difference between local magnitude (M L ) and coda duration magnitude (M C ) discriminates shallow seismic events (mining blasts, mining-induced earthquakes, and shallow tectonic earthquakes) from deeper tectonic earthquakes in the Utah region. The shallow seismic events had anomalously high M C values, with increasingly negative M L -M C values as depth decreased. Here we evaluate the performance of M L -M C as a depth discriminant in three new regions, finding that M L -M C increases between 0–9 km depth in all cases. Initially, we investigated M L -M C as a function of depth for naturally occurring earthquakes in the region around Yellowstone National Park, as recorded by the University of Utah Seismograph Stations. For 3,358 Yellowstone earthquakes with well-constrained depths, we found M L -M C increased 0.030 ± 0.007 magnitude units (m.u.) for each 1 km increase in depth up to 10 km depth. Next, we examined M L -M C values for anthropogenic seismicity in northern Oklahoma and southern Kansas, as recorded by the National Earthquake Information Center. For 1,628 events with well-constrained depths, we computed a slope for M L -M C of 0.022 ± 0.010 m.u./km. Finally, we analyzed M L -M C for 28,722 well-located earthquakes in Italy, as recorded by the National Institute of Geophysics and Volcanology, and found an M L -M C slope of 0.018 ± 0.001 m.u./km. In each case, the quoted error bounds represent 95% confidence regions which exclude zero, implying that the depth-dependence of M L -M C is statistically significant. We performed several robustness tests in which we varied the criterion used to define a well-constrained depth and the depth range used in the linear fit. In nearly all cases, we found a positive slope for M L -M C vs. depth at a confidence level above 95%.

Utah, Montana, Idaho, Wyoming, Kansas, Oklahoma↗

USGS2018

Seismicity in the Challis, Idaho region, January 2014 - May 2017: Late aftershocks of the 1983 Ms 7.3 Borah Peak earthquake

In April 2014, after about 20 yrs of relatively low seismicity, an energetic earthquake sequence (maximum M L "> M L 4.8) began 25–30 km northwest of the 1983 M s "> M s 7.3 Borah Peak earthquake rupture area near the town of Challis, Idaho. This sequence ended in the fall of 2014, but in January 2015, a second energetic sequence (maximum M L "> M L 5.0) began about 20 km to the southeast. Modest seismicity has continued in both regions with ∼ 1000 "> ∼ 1000 earthquakes detected and located through May 2017. To better characterize the seismicity in the area, we deployed a seven‐station local seismometer network during April–October 2014; one of the stations remained active until July 2015. Here, we report updated locations for earthquakes in the Challis area for 1 January 2014–31 May 2017. Using a combination of absolute and differential arrival times, we generated a catalog of high‐accuracy relocations. The earthquakes clustered into four primary groups, three of them with strikes similar to the Lost River fault—the fault responsible for the 1983 Borah Peak event. We used a modified cut‐and‐paste method to determine moment tensors for 15 of the largest events. All of the moment tensors showed normal faulting with nodal plane strikes consistent with the trend of the relocated seismicity and the regional stress field. We suggest that the recent seismicity near Challis is best interpreted as a continuation of the 1983 M s "> M s Ms 7.3 Borah Peak aftershock sequence, which is unusually long compared to plate boundary aftershock sequences because of the lower regional strain rate.

Idaho↗