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\begin{document}

\title{On the Origin of Orphan Tremors \& Intraplate Seismicity in Western
Africa}



\author[1,1,1]{Tolulope Morayo Olugboji}%
\author[1,1,1]{Yingping Lu}%
\author[2,2,2]{Adekunle Abraham Adepelumi}%
\author[3,3,3]{Folarin Kolawole}%
\author[4,4]{Manoochehr Shirzaei}%
\affil[1]{University of Rochester}%
\affil[2]{OAU, Ife, Nigeria}%
\affil[3]{University of Oklahoma}%
\affil[4]{Virginia Tech}%


\vspace{-1em}



  
  \date{June 16, 2024}


\begingroup
\let\center\flushleft
\let\endcenter\endflushleft
\maketitle
\endgroup





\selectlanguage{english}
\begin{abstract}
On September 5-7, 2018, a series of tremors were reported in Nigeria's
capital city, Abuja. These events followed a growing list of tremors
felt in the stable intra-plate region, where earthquakes are not
expected. Here, we review available seismological, geological, and
geodetic data that may shed light on the origin of these tremors. First,
we investigate the seismic records for parent location of the orphan
tremors using a technique suitable when a single-seismic station is
available such as the Western Africa region, which has a sparse seismic
network. We find no evidence of the reported tremors within the seismic
record of Western Africa. Next, we consider the possibility of a local
amplification of earthquakes from regional tectonics, reactivation of
local basement fractures by far-field tectonic stresses, landward
continuation of oceanic fracture zones, or induced earthquakes triggered
by groundwater extraction. Our assessments pose important implications
for understanding Western Africa's intraplate seismicity and its
potential connection to tectonic inheritance, active regional tectonics,
and anthropogenic stress perturbation.%
\end{abstract}%



\sloppy


Supporting Information for

\textbf{On the Origin of Orphan Tremors \& Intraplate Seismicity in West
Africa}

\textbf{Olugboji, T.} \textsuperscript{1,*}, Manoochehr
Shirzaei\textsuperscript{2},Yingping Lu.\textsuperscript{\textbf{1}
,2*}, Adepelumi, A.A\textbf{.\textsuperscript{3}} , Kolawole,
F.\textsuperscript{4}

\textsuperscript{1} \emph{Department of Earth and Environmental
Sciences, University of Rochester, NY} \textsuperscript{2} \emph{Goergen
Institute for Data Science, University of Rochester, NY}
\textsuperscript{3} \emph{Department of Geology, OAU, Ife, Nigeria}
\textsuperscript{4}\emph{School of Geology and Geophysics, University of
Oklahoma, Oklahoma}

\textbf{Contents of this file}

Supplementary Tables 1, 2, and 3 (3 Tables) Supplementary Figures 1, 2,
3, 4ab, 5abcd (9 Figures)

\textbf{Additional Supporting Information (File uploaded separately)}

Captions for Datasets S1

\textbf{Introduction}

Benchmarks of the algorithm (\textbf{supplementary figures 1-3} ) are
presented using example events (\textbf{supplementary Tables 1} ) and
with trigger parameters (\textbf{supplementary table 2} ) and properties
(\textbf{supplementary table 3} ). We provide supporting information
showing extended analysis e.g., \textbf{supplementary figure 4a} shows
all other anomalous cases in \textbf{supplementary table 1} (similar to
Figure 4). \textbf{Supplementary figure 5a-d} is a scan for events in
the tremor sequence. We present the manual digitization of the google
earth image of Figure 6 in Dataset S1.

\textbf{Benchmark of Single Station Detector}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image28/image28}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image43/image43}

\end{center}
\end{figure}

When estimating the station-earthquake distance from the \emph{P-SV}
delay time, we extract dominant body wave phases by applying a high-pass
filter (\textgreater{} 0.1 Hz) and using the ak135 1-D model (Kennett et
al., 1995) implemented using the taup tool of (Crotwell et al., 1999). A
first-order approximation for teleseismic distances, , or local/regional
events,

can be used as a quick check for most detections (Havskov \& Ottem\selectlanguage{ngerman}öller,
2010):

\par\null\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image20/image20}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image23/image23}

\end{center}
\end{figure}

When the true epicentral distance is greater than 90 degrees, the core
diffracted, \emph{Pdiff} phase might be hard to detect, especially for
low-magnitude events. In this case, the \emph{PP} phase is often the
clearest arrival and can be wrongly identified as the first arriving
body wave, leading to an under-estimated

(since ). We identify and correct for this error by using the
differential time between the doubly reflected \emph{SS} and the
Rayleigh wave as a second check on the estimated distance. Vertically
polarized shear-waves at large epicentral distances will travel at
mid-lower mantle velocities \textasciitilde{}7 km/s and compared with
typical Rayleigh wave group velocity \textasciitilde{}3.5 km/s provides
a second approximation for the epicentral distance:

\par\null

With the epicentral distance estimated from the pair of body waves or
body-and surface waves, the next step is to estimate the arrival azimuth
of the event. This is easily done using either the P-wave or the
Rayleigh wave. We choose to use the Rayleigh wave in this study because
of their larger amplitudes. To ensure the resistance of surface wave
detection to false triggers: (1) we apply a band-pass filter for
improving surface wave signal to noise ratio (0.006-0.2 Hz (Ekstr\selectlanguage{ngerman}öm,
2006)), and (2) we scan a time window where surface wave are expected to
arrive:\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image36/image36}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image40/image40}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image27/image27}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image17/image17}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image51/image51}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image31/image31}

\end{center}
\end{figure}

where 3.5 km/sec is a typical Rayleigh wave group velocity. All arrival
times: ,

and correspond to the triggered on-condition (binary one) using the
characteristic functions

for the \emph{P} , \emph{SV} , and \emph{SW} phases respectively. For
the definition of the other threshold functions, , and details of how
the appropriate (de-) trigger thresholds are set, see the supplementary
tables 1 and 2. For a benchmark of the algorithm, kindly refer to the
supporting information text.

\textbf{Example Earthquakes}

First, we demonstrate the effectiveness of our single-station approach
by testing on a set of known earthquakes selected from the U.S.
Geological Survey (USGS) earthquake catalog. We use three-component
seismograms recorded at a quiet station in Albuquerque, New Mexico, USA
(\textbf{IU-ANMO} ). We identify six different events
(M\textsubscript{w} \textgreater{}6) with station-earthquake azimuth and
epicentral distance ranging from 30\selectlanguage{ngerman}° to 120° (see s\textbf{upplementary
figure 1 and supplementary table 1} ). For each event, polarization
analysis is applied to an hour-long record: a time duration long enough
to observe first arriving body-waves and fundamental mode surface waves.
A time- series of polarization traces, computed from the three-component
seismogram, is then interpreted, enabling phase detection and allowing
estimation of wave propagation direction. We notice that robust
phase-detection requires the original waveform data to be filtered using
a high-pass frequency of 0.1 Hz and a band-pass frequency of 0.006-0.2
Hz to improve the detection of body waves and surface waves,
respectively. When comparing observed phase-arrival time using our
detector with the predicted arrival time, we compute the predicted
arrival time with a 1-D earth model using event parameters obtained from
the USGS catalog (e.g., earthquake location, origin time, and
magnitude). All seismograms are normalized by the maximum amplitude in
the hour-long record for display purposes only (the vector sum is used
for normalizing horizontal channels).

\textbf{Limits of Seismic Detector}

We test our single-station detector using a few events recorded at
station ANMO (see Table 1). For the M7.6 event located in Nikolskoye,
Russia, we are able to correctly identify the major seismic phases: P, S
and Surface Waves (\textbf{supplementary figure 2} ). Likewise, our
estimate of the event to station azimuth is also correctly calculated
from the largest principal axes. Along with the epicentral distance
evaluated using the relative phase delay between the body wave phases
(S-P time), we correctly locate the epicenter of the test events
(\textbf{supplementary figure 3} ). For teleseismic events to be
correctly located by the single seismometer, the magnitude of all
candidate events must be larger than Mw 6.0.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image30/image30}

\end{center}
\end{figure}

In our test-cases, the azimuth and epicentral distance for shallow
teleseismic events (\textless{}50km) at moderate distances () is well
constrained (\textbf{supplementary figure 3a and 3b} ). However, when
the events are deeper or farther away, the misidentification of the
first arriving phase, weak excitation of the surface wave, or
propagation complexities due to anisotropy (Laske \& Masters, 2000) can
lead to an estimation bias in the source location caused by an incorrect
estimate of azimuth or epicentral distance (\textbf{supplementary figure
3c and 3d} ). One approach to minimizing the error would involve
incorporating information from multiple phases or the joint application
of the coherence technique (Agius \& Galea, 2011; Roberts et al., 1989).
While the accuracy of our results depends on source-depth and
propagation complexity, overall, the algorithm shows good performance
for most of the candidate events. For brevity, we show detailed results
for selected events only. A complete analysis of all test cases,
including 2018 earthquakes recorded on the western African network is
shown \textbf{supplementary figure 4, 5 \& 6} ).

Supplementary Table 1: Earthquake parameters (7 exemplary events) used
to test the algorithm at station IU-ANMO and testing results.\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
\begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
Event No.\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
Mag.\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
Time (UTC)\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
Location\strut
\end{minipage} & \begin{minipage}[b]{0.19\columnwidth}\raggedright\strut
Coordinates (Lat, Lon)\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
1\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{7.3}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{2018-12-20}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{Nikol'skoye, Russia}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{55.099, -106.457}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{7.2}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{2004-09-05}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{Shingu, Japan}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{33.070, 136.735}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
3\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{7.3}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{2019-07-14}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{Laiwui, Indonesia}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{-0.586, 128.034}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
4\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
7.9\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2018-09-06\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Suva, Fiji\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
-18.474,~179.350\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
5\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
7.5\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2018-01-14\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Acari, Peru\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
-15.768,~-74.709\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
6\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{7.1}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{2018-11-30}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{Anchorage, Alaska}\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
\textbf{61.346, -149.955}\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
7\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
6.5\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
2016-10-30\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Norcia Italy\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
42.862, 13.096\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{longtable}[]{@{}lllllllll@{}}
\toprule
& Distance & Distance & Azimuth & Azimuth & Error & Error & Error &
Error\tabularnewline
\midrule
\endhead
& & & & & \textbf{Distance} & \textbf{Distance} & \textbf{Distance} &
\textbf{Azimuth}\tabularnewline
Event & \selectlanguage{greek}\textbf{Δ\textsuperscript{a}}\selectlanguage{english} & \selectlanguage{greek}\textbf{Δ\textsuperscript{0,1}}\selectlanguage{english}
& \selectlanguage{greek}\textbf{Φ\textsuperscript{a}}\selectlanguage{english} & \selectlanguage{greek}\textbf{Φ\textsuperscript{0} (mode)}\selectlanguage{english} &
\selectlanguage{greek}\textbf{εΔ\textsuperscript{0}}\selectlanguage{english} & \selectlanguage{greek}\textbf{εΔ\textsuperscript{0}}\selectlanguage{english} &
\selectlanguage{greek}\textbf{εΔ\textsuperscript{1}}\selectlanguage{english} & \selectlanguage{greek}\textbf{ε Φ\textsuperscript{0}
(mode)}\selectlanguage{english}\tabularnewline
1 & \selectlanguage{ngerman}\textbf{62°} & 61\selectlanguage{ngerman}° & \selectlanguage{ngerman}\textbf{69°} & 68\selectlanguage{ngerman}° (68°) & 1\% & 1\% & 46\% &
2\% (1\%)\tabularnewline
2 & \selectlanguage{ngerman}\textbf{90°} & 95\selectlanguage{ngerman}° & \selectlanguage{ngerman}\textbf{47°} & 62\selectlanguage{ngerman}° (64°) & 5\% & 5\% & 50\% &
32\% (36\%)\tabularnewline
3 & \selectlanguage{ngerman}\textbf{119°} & 88\selectlanguage{ngerman}° & \selectlanguage{ngerman}\textbf{50°} & 37\selectlanguage{ngerman}° (44°) & 64\% & 64\% & 26\%
& 24\% (11\%)\tabularnewline
6 & \selectlanguage{ngerman}\textbf{38°} & 48\selectlanguage{ngerman}° & \selectlanguage{ngerman}\textbf{114°} & 63\selectlanguage{ngerman}° (76°) & 25\% & 25\% & 62\%
& 44\% (33\%)\tabularnewline
\bottomrule
\end{longtable}

\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{a} : Actual distance in degrees between event and
station.

\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{0,1}: Observed distance in degrees between station and
event using S-P time (0) or SW-S time (1).

\(\epsilon^{0,1}=\frac{|^{0,1\ }-\ ^{a}|}{{}^{a}}\): The normalized error between observed distance and
true distance between station and event using S-P time (0) and SW-S (1)
time respectively.

\selectlanguage{greek}Φ\selectlanguage{english}\textsuperscript{a}: Actual angle from events to the station.

\selectlanguage{greek}Φ\selectlanguage{english}\textsuperscript{0}: Observed azimuth from events to station from the
distribution of azimuth using mean (node).

Supplementary Table 2: (De-) Trigger thresholds used to translate
polarization attributes to binary characteristic functions for phase
detection (e.g. Russia M 7.3).\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
\begin{minipage}[b]{0.24\columnwidth}\raggedright\strut
Function\strut
\end{minipage} & \begin{minipage}[b]{0.24\columnwidth}\raggedright\strut
Description\strut
\end{minipage} & \begin{minipage}[b]{0.24\columnwidth}\raggedright\strut
Trigger (\textgreater{}Percentile)\strut
\end{minipage} & \begin{minipage}[b]{0.24\columnwidth}\raggedright\strut
Detrigger (\textless{}Percentile)\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Strength of Linearity\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
80\%\\
e.g. 0.63\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
75\%\\
0.56\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
P Energy\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
70\%\\
0.62\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
65\%\\
0.55\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
S Energy\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
65\%\\
0.83\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
60\%\\
0.76\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
P Angle (\textless{})\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
20\%\\
46.86\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
25\%\\
55.61\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
S Angle~ (\textless{})\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
10\%\\
58.43\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
15\%\\
65.65\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
Surface wave (\textless{})\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
80\%\strut
\end{minipage} & \begin{minipage}[t]{0.22\columnwidth}\raggedright\strut
85\%\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

Supplementary Table 3. Attributes used for Phase Detection\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
Method & Attribute & Description & Equation\tabularnewline
\midrule
\endhead
Energy Analysis & & Total energy &\tabularnewline
& & Vertical energy &\tabularnewline
& & Horizontal energy &\tabularnewline
Polarization Analysis & & Eigenvector of the dominant particle motion
&\tabularnewline
& , & Strength of linear and elliptical polarization &\tabularnewline
& & Linearity of polarization &\tabularnewline
& & Azimuth &\tabularnewline
& & Incidence angle &\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image50/image50}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
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Supplementary Figure 1. ~Orientation of seismic channels at station ANMO
(triangle) in cartesian coordinates and the exemplary earthquake (stars)
used to demonstrate the single-station locator, showing the horizontal
(b1) and vertical planes (b2) of wave polarization for surface waves
(SW) and body waves (BW: P and S)~respectively. (b1) The Azimuth

of the source-earthquake great circle path in plain view, measured
relative to the two horizontal channels (H1 \& H2)~ of the seismic
station, with the H1 channel oriented at an angle, , measured clockwise
from north, and H2 perpendicular to H1. In our reference frame,
coordinates N and~ E are oriented at respectively. (b2) Incidence
angle,, in elevation view, measured clockwise from vertical (here a P or
S wave arriving perfectly vertical from below the station would have .
In single-station source location, arrival direction can be determined
by using P (BW) or Rayleigh (SW) to determine and the station-earthquake
distance is determined by the P-S delay time , following the successful
phase identification (see supplementary figure 3 for E1).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image16/image16}

\end{center}
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\includegraphics[width=0.70\columnwidth]{figures/image14/image14}

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\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
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\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image29/image29}

\end{center}
\end{figure}

Supplementary Figure 2. (a) Three-component seismogram recorded at
station ANMO, showing the unfiltered teleseismic P-wave arrival from a
magnitude 7.3 earthquake with origin time Dec 20, 2018, in Nikolskoye,
Russia with (E1 on Figure 1). The dashed line indicates the exact
arrival time of the first arriving P (blue) and S (red) waves. (b)
Application of polarization analysis on the filtered signal (BW using
high-pass and SW using a bandpass). Each trace (light grey) represents
an attribute used for phase detection with all triggered periods marked
in black. Specifically, f and

represent the linearity of polarization and azimuth, with

and

representing the incidence angle and the stability of the calculated
angle.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image26/image26}

\end{center}
\end{figure}

Supplementary Figure 3: Successful application of the multi-attribute
polarization technique to the source location of four earthquakes (see
Figure 3). The actual (red) and estimated distribution (polar histogram)
of azimuth and epicentral distance demonstrate that a single-station
event location is possible.~ For events within 100\selectlanguage{ngerman}°, the epicentral
distance is computed using the relative P-S delay time comparing the
relative P-S delay time with the 1-D Earth model of (Snoke, 2009) using
the\emph{taup} software tool (Crotwell et al., 1999). For teleseismic
events with an epicentral distance greater than 100° (Event 3), the
relative S-SW delay time is used instead (the initial result using P-S
delay time marked with a grey dashed line). See Fig. S1 for the result
of anomalous events listed in Table 1.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image52/image52}

\end{center}
\end{figure}

\textbf{Supplementary Figure 4a.} Single-station location using
polarization attributes. Events with large bias showing types of errors
for 3 test cases (similar to Figure 4).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image33/image33}

\end{center}
\end{figure}

\textbf{Supplementary Figure 4b.} Demonstration of single-station
location for three earthquakes in 2018 using stations: GT.DBIC and G.MBO
(inset map). (Left -1) Guinea event (Middle-2) Central Mid Atlantic, and
(Right-3) Ascension Island event. Location and phase identification for
events 2 and 3 are better than for event 1. Only a single phase is
identified for the Guinea event (compare with Fig. 4 for details).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image56/image56}

\end{center}
\end{figure}

\textbf{Supplementary Figure 5a.} Scan anecdotal event 1: Sept 5. 13:30
UTC.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image55/image55}

\end{center}
\end{figure}

\textbf{Supplementary Figure 5b.} Scan anecdotal event 2: Sept 5. 16:30
UTC.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image54/image54}

\end{center}
\end{figure}

\textbf{Supplementary Figure 5c.} Scan anecdotal event 3. Sept 6. 01:30
UTC\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image53/image53}

\end{center}
\end{figure}

\textbf{Supplementary Figure 5d.} Scan anecdotal event 4. Sept 7. 05:30
UTC

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