19 April, 2014 - Off Bougainville Island - M7.5

A large earthquake occurred along the northern Solomon Islands subduction zone, just south of Bougainville Island. The earthquake occurred a few 10's of kilometers to the southewest of the M7.1 earthquake that occurred about eight days ago. 

No really cloase stations are in the vicinity. Here are the ground velocity seismograms (proportional to microns per second) observed at station HNR, Honiara, Guadalcanal in the Solomon Islands.

and here is that channel list:

SIGNALPARAMETERS; 2014-04-19 13:27:49; 60; *; 0.005; -0.05
IU; HNR; 00; BHZ; (UP-DOWN) HONIARA, SOLOMON ISLANDS
IU; HNR; 00; BH1; (NORTH-SOUTH) HONIARA, SOLOMON ISLANDS
IU; HNR; 00; BH2; (EAST-WEST) HONIARA, SOLOMON ISLANDS

A closer station is located at Port Moresby, New Guinea, which is about 70 km distant. Here is a channel list for that station.

IU; PMG; 00; BHZ; (Up-Down) Port Moresby, New Guinea
IU; PMG; 00; BH2; (East-West) Port Moresby, New Guinea
IU; PMG; 00; BH1; (North-South) Port Moresby, New Guinea

 The station is pretty close, you should be able to detect many aftershocks at this distance. Here is the mainshock (ground velocity) observed at this station.

18 April, 2014 - Off Coast of Guerrero, Mexico - M7.2

A large earthquake recently occurred off the coast of Guerrero, Mexico, just north of Acapulco. 

The closest real-time station with openly avilable data is about 250 km away near Mexico City. You should be able to see the larger aftershocks monitoring ground motions using this station. Here is a channel list for the station

G; UNM; 00; BHZ; (Up-Down) Mexico City, Mexico
G; UNM; 00; BHE; (East-West) Mexico City, Mexico
G; UNM; 00; BHN; (North-South) Mexico City, Mexico

Follow aftershocks using the broad-band channels (BHZ,N,E). Here is an example

The Mainshock Signals

The strong motion accelerometers are

SIGNALPARAMETERS; 2014-04-18 14:27:16; 5; *; 0.01; -4.0
G; UNM; 00; HNZ; (Up-Down) Mexico City, Mexico
G; UNM; 00; HNE; (East-West) Mexico City, Mexico
G; UNM; 00; HNN; (North-South) Mexico City, Mexico

Use the strong motion instruments to look at the mainshock (it's off-scale on the broad band sensor). Here is a plot of the mainshock ground velocity values (at station UNM):

12-13 April, 2014 - Solomon Islands Doublet - M7.6, M7.4

[Update] A second major earthquake in the region in less than a day. The first earthquake was predominantly a strike-slip earthquake and was consistent with a NW-SE striking fault, which is also consistent with the orientation of some of the aftershocks. The second event appears to be located slightly southwest of the first (this is early, the locations could change). Early results suggest the second event is a reverse fault. 

A major earthquake occurred in the Solomon Islands region. No realt-time stations are very close, but the station an Honiara, Solomon Islands is relatively close and to the west-northwest.

 Here is a channel list for the station and Honiara

IU; HNR; 00; BHZ; (Up-Down) Honiara, Solomon Islands
IU; HNR; 00; BH1; (North-South) Honiara, Solomon Islands
IU; HNR; 00; BH2; (East-West) Honiara, Solomon Islands

Here is a plot of the ground velocity recorded on the strong motion accelerometer at the same station (I integrated the raw records).

 Acceleration Comparison

Here are plots of the accelerations at HNR for both events. The plots are not on the same scale. The peak acceleration values in microns/s^2 are shown to the left of each signal. The peak values for Z, N, E components are (0.08,1.12,1.27 m/s^2) for the first event and (0.10,1.46,1.39 m/s^2). These have the instrument response included, but the instrument gain correction was applied.

The first event:

and here is the second event:

Here is a channel list for the accelerometer. 

SIGNALPARAMETERS; 2014-04-13 12:36:08; 4; *; 0.01; -1
IU; HNR; 20; HNZ; (UP-DOWN) HONIARA, SOLOMON ISLANDS
IU; HNR; 20; HN1; (NORTH-SOUTH) HONIARA, SOLOMON ISLANDS
IU; HNR; 20; HN2; (EAST-WEST) HONIARA, SOLOMON ISLANDS

[Update] The western Solomon's are an interesting area in terms of earthquakes. The region is quite active and often has events that trigger other events (as in the two events in this sequence). Here is an animation of the seismicity of the region to the west of these events that includes events from 1973 to 2014. Click here to view the animation.

01 April, 2014 - Near Iquique, Northern Chile - M 8.2

A M8.2 earthquake occurred near the northern end of the earthquake activity from the last few weeks that began with a M6.7 earthquake. All of this activity is occurring in the region that last ruptured in a large earthquake in 1877. The event is significant because of the damage and harm it has done. The lengthy foreshock sequence makes sequence event scintifically interesting, add to that the fact that it is located inside the 1877 gap, and that the observations suggest that the mainshock ruptured back towards (and perhaps downdip) of the foreshocks, makes it scientifically intriguing. The rupture size is somewhat compact, roughly 100 km by 50 km. Much of the 1877 gap still remains unbroken. 

A few hours later...

You can monitor aftershocks using the same station that I listed in the post from a few days ago (see below). The mainshock records are off scale because the event was close and larger. However, other channels can provide on-scale records for such strong motions.

C;GO01; --;HNZ;VERTICAL ACCELERATION,CHUSMIZA, CHILE
C;GO01; --;HNE;EAST-WEST ACCELERATION, CHUSMIZA, CHILE
C;GO01; --;HNN;NORTH-SOUTH ACCELERATION, CHUSMIZA, CHILE

Use an asterisk in the Sample Rate field to insure you get the highest value for these channels. Here is a plot of signals proportional to ground velocity during the mainshock and an early aftershock (I integrated the accleration signals). 

For aftershock monitoring, I added repeat the usual channels to provide two different bandwidths to observe the short-period signals from small events and the long-period signals from the larger aftershocks.

C;GO01; --;BHZ;VERTICAL,CHUSMIZA, CHILE
C;GO01; --;BHE;EAST-WEST, CHUSMIZA, CHILE
C;GO01; --;BHN;NORTH-SOUTH, CHUSMIZA, CHILE
C;GO01; --;BHZ;VERTICAL,CHUSMIZA, CHILE;0.004; 0.0333
C;GO01; --;BHE;EAST-WEST, CHUSMIZA, CHILE;0.004; 0.0333
C;GO01; --;BHN;NORTH-SOUTH, CHUSMIZA, CHILE;0.004; 0.0333

Here is a plot of the aftershocks occurring between one and three hours after the mainshock. The mainshock drove these sensors off scale (clipped).

The smaller events were occurring nearly continuously, the longer-period signals are waves from the mainshcok still propagating around the planet an hour later (they will be visible for weeks at the longest periods).

Here is a recent, magnitude six or so aftershock.

Update on Friday, April 4, 2014 at 11:26AM by epicentral

I made a map for research purposes and thought that I would post it on this site. The map was made using the generic mapping tools package. Earthquakes are represented by circles centered on their epicenters, and the area of the circle is roughly the area expected to rupture in the corresponding magnitude. Of course, earthquake ruptures are not circles, so the symbols are conceptual. This approach is the only way to put the relative size of events in a realisitic context. The arrow shows the direction of motion between the Nazca and South American plates (the rate is roughly 65 mm/yr).

Yellowstone National Park, WY - 29 March, 2014 - M 4.8

A magnitude 4.8 earthquake occurred in north-central Yellowstone this morning.

The area is heavily monitored, so there are plenty of real-time seismic stations from which to choose. However, many only record the higher frequency motions. Here is a channel list with a select number of three-component broad-band stations. 

WY; YHB; 01; HHZ; Up-Down - Horse Butte YNP, MT, USA
WY; YHB; 01; HHN; North-South - Horse Butte YNP, MT, USA
WY; YHB; 01; HHE; East-West - Horse Butte YNP, MT, USA
WY; YNR; 01; HHZ; Up-Down -  Norris Junction, YNP, WY, USA
WY; YNR; 01; HHN; North-South - Norris Junction, YNP, WY, USA
WY; YNR; 01; HHE; East-West - Norris Junction, YNP, WY, USA
WY; YPP; 01; HHZ; Up-Down - Pitchstone Plateau, YNP, WY, USA
WY; YPP; 01; HHN; North-South - Pitchstone Plateau, YNP, WY, USA
WY; YPP; 01; HHE; East-West - Pitchstone Plateau, YNP, WY, USA

Norris Junction is close to the earthquake, the seismograms went off scale during the main shock, but is the best station to look at for aftershocks. Here are the signals proportional to the displacement at the other stations.

Many small aftershocks are still occuring (about 7 hrs after the mainshock). Here's the last fifteen minutes.

Oklahoma - 29 March, 2014 - M 4.3, 4.3, 4.4

The seismicity of Oklahoma is an interesting and unfolding story - in the last few years the number of earthquakes with magnitudes greater than 3.0 has increased significantly. Although Oklahoma has a history of moderate-size earthquakes dating back many decades (an estimated magnitude 5.5 occurred on 09 April, 1952 near El Reno). The rate of small-magnitude earthquake activity in the central and eastern US increased first around 2003 (USGS), and then again in late 2009. Much of the 2009 increase may be associated with activity in Oklahoma, which increased sharply in 2009:

The M≥3.0 activity rate for Oklahoma from about 1975 to 2008 was roughly constant. The recent activity is believed to be related to fluid injection into the subsurface, which is known to trigger small-to-moderate size earthquakes (USGS page on induced earthquakes), and believed to have at least indirectly played a role in the largest earthquake of the recent OK activity, an Mw 5.6 earthquake that occurred near Prague, OK in 2011. Like the Prague earthquake, most of the smaller activity seems to be caused by the triggered release of tectonic stress that includes a roughly east-west oriented horizontal compression (see the earthquake information on the Saint Louis University Earthquake site).

Two M4.3 earthquakes and numerous small events occurred last night in Oklahoma, just south of the town of Marshall, located north of Oklahoma City, and a M4.4 occurred this morning (these are USGS mbLg values, those of the Oklahoma Geological Survey are slightly different). Here's a map of earthquakes from the last 30 days (as of 12:30PM EDT). The red circles show the more recent events, then the orange, the yellow are more than a week old. The circles in the northwest occurred last night.

Here's a close up of the recent events on a satellite view - not many people in the area, but the town of Marshall is only a few kilometers distant.

Seismograms

The seismic station Liberty Lake, which is located just south of the town of Guthrie, is the closest station (about 60 km to the south-southeast). Here is a channel list for the station: 

GS; OK029; 00; HHZ; Up-Down, Liberty Lake, OK, USA 
GS; OK029; 00; HH1; North-South, Liberty Lake, OK, USA 
GS; OK029; 00; HH2; East-West, Liberty Lake, OK, USA

A list of other stations was posted a few days ago (see the previous post).

A plot of the displacement seismograms for the first M 4.3 is shown below (you can right-click on the image to open a larger version in a new window so that you can see the details better). Note that the times in the display are UTC, the local time reference is my local time - eastern US. At the Liberty Lake Station, the signals were largest for the first event, then the third event, and the middle event signals were slightly smaller. This may not be the same at other stations, and the magnitudes are computed using many more that one station.

Not surprisingly, the displacement seismograms from the later event look similar (but not exactly the same).

Here are the seismograms from the most recent, M4.4 event. Again, the shape is very similar, but a careful look shows subtle differences.

If you look closely, you'll see that the signals from the M4.4 event are a little smoother than those of the earlier events, which is consistent with it being slightly larger and having a slightly longer duration, which subtlely smooths the signals. Of course a subtle observations such as this could also be a result of a slightly different propagation paths of azimuths from the source to the receiver.

Near-Station Site Resonances

The displacement seismograms are obtained by integrating the ground velocity seismograms returned from the waveform server (press and hold on the seismogram display for a menu with this option). A number of resonances, probably associated with the near-surface geology beneath the seismic station are obvious in the signals. Here are the seismograms for the later event returned from the seismogram server (these are proportional to ground velocity).

The P-wave reverberations, observable on the up-down component of motion just after the initiation of motion, has a repetition time of about 1.0 seconds, the shear-wave reverberations, observable with the three peaks on the east-west component of motion starting just before 08:42:50, repeat about every 1.8 seconds.

Los Angeles, CA Area - 29 March, 2014 - M 5.1

A moderate-magnitude earthquake occurred in the Los Angeles area (about 1 km south of Brea) on the evening of the 28th of March (29th in UTC time).

Here's a closer view of the mainshock (larger circle) and the aftershocks (as of about 02:00 AM Pacific time). The aftershocks appear to be occurring roughly northeast of the mainshock, over a distance of a few kilometers (keep in mind, these are early location estimates).

Here is a channel list that includes three stations roughly the same distance from the earthquake.

CI; PASC; 00; BHZ; Pasadena, CA, USA (UP-DOWN)
CI; PASC; 00; BHE; Pasadena CA, USA (EAST-WEST)
CI; PASC; 00; BHN; Pasadena, CA, USA (NORTH-SOUTH)
CI; USC; --; BHZ; USC, Los Angeles, CA, USA (UP-DOWN)
CI; USC; --; BHE; USC, Los Angeles, CA, USA (EAST-WEST)
CI; USC; --; BHN; USC, Los Angeles, CA, USA (NORTH-SOUTH)
CI; MWC; --; BHZ; MOUNT WILSON OBSERVATORY, ALTADENA, CA, USA (UP-DOWN)
CI; MWC; --; BHE; MOUNT WILSON OBSERVATORY, ALTADENA, CA, USA (EAST-WEST)
CI; MWC; --; BHN; MOUNT WILSON OBSERVATORY, ALTADENA, CA, USA (NORTH-SOUTH)

Here is a plot of the mainshock as observed at these stations. From the look of the signals, it appears that the S and Love waves were relatively large in the direction of USC. 

The quietest station is Mount Wilson, so if you want to monitor for aftershocks, that is probably the best choice. Select the "Near" filter settings and then change the "High Corner" value to be a negative number (this disables the filter). Use a "Sample Rate" of 10, 20, or 40 sps - the smaller value will download smaller seismogram files, the higher values will include higher frequencies.

CI; MWC; --; BHZ; MOUNT WILSON OBSERVATORY, ALTADENA, CA, USA (UP-DOWN)
CI; MWC; --; BHE; MOUNT WILSON OBSERVATORY, ALTADENA, CA, USA (EAST-WEST)
CI; MWC; --; BHN; MOUNT WILSON OBSERVATORY, ALTADENA, CA, USA (NORTH-SOUTH)

Amplitudes of an earlier M3.6 afteshock were roughly 25-30 miconrs/second (the numbers to the left of the signals) in the Near-filter frequency band. Very roughly, a mid-magnitude 2 would be 10 times smaller, and a mid-magnitude four would be 10 times larger (expect at least factors of two uncertainty in these amplitude values).

You could watch all three vertical component channels to identify the larger aftershocks (the waves for each will show up at about the same time on all three channels). Here's a display of the most recent aftershocks using all three components from Mount Wilson and the verticals from Pasdena and USC.

You can see that the signals from downtown near USC are noisier than the other two locations. 

Oklahoma City, OK to Dallas/Fort Worth, TX

The seismicity of Oklahoma is at a very high level given it's tectonic environment. Much of the activity is believed to be related to well-water injection. Here is a suite of stations arranged from north to south, that includes vertical component signals.

N4; T35B; --; BHZ; Up-Down, Sooner Cattle Company, Foraker, OK, USA
OK; CROK; --; HHZ; Up-Down, Carrier, OK, USA 
GS; OK029; 00; HHZ; Up-Down, Liberty Lake, OK, USA 
GS; OK026; 00; HHZ; Up-Down, New Dominion Farley field, Oklahoma City, OK, USA 
OK; X34A; --; HHZ; Up-Down, Smith Ranch, Marlow, OK, USA 
N4; Z35B; --; BHZ; Up-Down, Perchaven, Sanger, TX, USA

You can set the filters to have corners of 1 to 10 hertz, or use the "near" settings. I am using 20 samples for second for 1 to 10 Hz, but you can use 10 samples per second for the near setting. For a select set of three-component channels, use:

N4; T35B; --; BHZ; Up-Down, Sooner Cattle Company, Foraker, OK, USA
N4; T35B; --; BHN; North-South, Sooner Cattle Company, Foraker, OK, USA
N4; T35B; --; BHE; East-West, Sooner Cattle Company, Foraker, OK, USA
GS; OK029; 00; HHZ; Up-Down, Liberty Lake, OK, USA 
GS; OK029; 00; HH1; North-South, Liberty Lake, OK, USA 
GS; OK029; 00; HH2; East-West, Liberty Lake, OK, USA
OK; X34A; --; HHZ; Up-Down, Smith Ranch, Marlow, OK, USA 
OK; X34A; --; HHN; North-South, Smith Ranch, Marlow, OK, USA 
OK; X34A; --; HHE; East-West, Smith Ranch, Marlow, OK, USA
N4; Z35B; --; BHZ; Up-Down, Perchaven, Sanger, TX, USA
N4; Z35B; --; BHN; North-South, Perchaven, Sanger, TX, USA
N4; Z35B; --; BHE; East-West, Perchaven, Sanger, TX, USA

 Here's a map of the station locations.

The Great 1964 Alaska Earthquake

Fifty years ago today the largest earthquake in US history struck Alaska. The Great 1964 Alaska Earthquake had a moment magnitude of 9.2  and generated a large tsunami that devasted parts of coastal Alaska and impacted other areas around the northern Pacific.

The U.S. Geological Survey produced this 11 minute video on the quake, which was scientifically important for a number of reasons, including it's role in demonstrating the validity of plate tectonics. 

    https://www.youtube.com/watch?noredirect=1&v=lE2j10xyOgI&app=desktop

You can find more links, videos, articles, and information at

    http://earthquake.usgs.gov/earthquakes/events/alaska1964/

Here's a vertical component seismogram from station SCP, State College, PA, USA from the day of the earthquake (Friday, 27 March, 1964). Open the image in a new tab or window to see it clearly.

Off The Coast of Northern Chile - 16 March, 2014 - M 6.7

This sequence is interesting because it is located within the region of the chilean subduction zone that ruptured in a large earthquake in 1877. The magnitude 6.7 event has been followed by a vigorous aftershock sequence that has included three events with magnitudes of 6.0 and larger and just over 20 other events larger than magnitude 5.0. 

Although the event is 10's of kilometers offshore, a high-quality station is located nearby.  Here is a channel set for the location 

C;GO01; --;BHZ;VERTICAL,CHUSMIZA, CHILE
C;GO01; --;BHE;EAST-WEST, CHUSMIZA, CHILE
C;GO01; --;BHN;NORTH-SOUTH, CHUSMIZA, CHILE

Here is a shot from an hour or so ago, showing the continuing aftershock sequence.

You can monitor the activity by using the above channel list and applying the "near" filters.  Based on the vertical-component amplitudes of earlier events with estimated magnitudes (USGS), maximum vertical ground motion values in microns/second for a mid-magnitude five earthquake are in the range of 800, those for a mid-magnitude four are in the range 50. So the numbers that are on the order of a few microns per second are likely mid-magnitude three earthquakes.

Off The Coast of Northern California - 10 March, 2014 - M 6.9

A large earthquake occurred off the coast of northern California last night. The magnitude is 6.9 and although the event was felt along the coast, no serious damage and no injuries have been reported. The motion of the rocks across the fault is such that the sea-floor uplift or down-drop will be minor, so there was no tsunami of any significance (and this event is small for generating large tsunami).

Here is a channel list for a nearby (but not too close) station roughly east of the mainshock.

TA; N02D; --; BHZ; Trinity Center, CA, USA, Up-Down
TA; N02D; --; BHE; Trinity Center, CA, USA, East-West
TA; N02D; --; BHN; Trinity Center, CA, USA, North-South

This is a little far from the event, but you should be able to see the larger aftershocks at this station. Here's a plot of the broadband displacement signals from the mainshock and a high-frequency plot of the mainshock and early aftershocks.

Earthquake Occurrence Patterns - 04 March, 2014

The last post to this site was 161 days ago. That's long (but a good thing, since that means there wasn't any large earthquakes recently), but I decided to share some calculations that I did for my Earthquakes and Society course. We begin each class meeting with a discussion of recent earthquakes activity or earthquake news. Interesting earthquakes occur all the time, but we certainly have noticed the that no major or great earthquakes have occurred so far this semester (and the mid-semester break is upon us). I list recent activity below, and follow the list with some simple statistics using data from the US Geological Survey's earthquake catalog. The last M ≥ 7.0 earthquake occurred 99 days ago. How unusual is that?

Recent Earthquake History

Here are the time intervals between Tue Mar 4 12:40:02 EST 2014 and recent large earthquakes.

Great Earthquakes (last three years)
8.3 Mw  2013-05-24 05:44:49 /  284 days, 11 hrs ago
8.0 Mw  2013-02-06 01:12:27 /  391 days, 16 hrs ago
8.2 mwc 2012-04-11 10:43:10 /  692 days,  6 hrs ago
8.6 mww 2012-04-11 08:38:36 /  692 days,  9 hrs ago
9.0 mww 2011-03-11 05:46:24 / 1089 days, 11 hrs ago

Major and Great Earthquakes (last year)
7.0 mww 2013-11-25 06:27:33 /   99 days, 11 hrs ago
7.7 mww 2013-11-17 09:04:55 /  107 days,  8 hrs ago
7.1 mww 2013-10-25 17:10:19 /  130 days,  0 hrs ago
7.1 mww 2013-10-15 00:12:32 /  140 days, 17 hrs ago
7.1 mww 2013-09-25 16:42:43 /  160 days,  0 hrs ago
7.7 mww 2013-09-24 11:29:47 /  161 days,  6 hrs ago
7.0 mww 2013-08-30 16:25:02 /  186 days,  1 hrs ago
7.3 Mww 2013-07-15 14:03:43 /  232 days,  3 hrs ago
7.3 Mww 2013-07-07 18:35:30 /  239 days, 23 hrs ago
8.3 Mw  2013-05-24 05:44:49 /  284 days, 11 hrs ago
7.4 Mw  2013-05-23 17:19:04 /  285 days,  0 hrs ago
7.2 Mw  2013-04-19 03:05:52 /  319 days, 14 hrs ago
7.7 Mw  2013-04-16 10:44:20 /  322 days,  6 hrs ago
7.0 Mw  2013-04-06 04:42:35 /  332 days, 12 hrs ago

A Longer Look Back and Some Statistics

I downloaded information on all major and great earthquakes in the US Geological Survey's earthquake catalog, which contains data on earthquakes that have occurred since 1973. Using a particular magnitude (7.0) as a cutoff for classifying earthquakes is somewhat arbitrary, there's not much difference between a magnitude 6.9 and 7.0. But we often use this criteria for simple earthquake descriptive statistic summaries. Here is a plot of the 612 earthquakes returned for this roughly 41-year (plus a couple of months) time span.

Time Intervals Between Large Earthquakes

The key parameter for us to know how unusual the time since the last large earthquake is the time between earthquakes. You would expect that there will be many events with short time intervals since earthquakes cluster in time and space (mainshock-aftershock sequences), and that's what you'll see. I computed the time between each successive pair of earthquakes. With this simple approach, we have an average of about 15 major and great earthquakes occur each year; the average time interval between major and/or great earthquakes is about 24 days and the standard deviation is close to that, 26. The distribution of time intervals is shown below.

Empirical Likelihoods

We can use this empirical distribution to compute the likelihoods of particular time intervals between large earthquakes. Here is a table of the likelihood of a time interval, delta-t, with the given number of days.

Days	P(δt ≤ Days)
1	0.04
2	0.07
3	0.11
4	0.14
5	0.18
6	0.22
7	0.25
8	0.29
9	0.33
10	0.36
20	0.56
30	0.70
40	0.79
50	0.86
60	0.90
70	0.93
80	0.95
90	0.97
100	0.98
110	0.99

As you can see from the figure and the table, the current interval of 99 days is out in the tail of the distribution. So, the answer to the original question, how unusual is the current time since the last large earthquake? This isn't the longest interval in the last 40 years (that was 159 days), but it among the longest 1-2% of all intervals that we've see in the last four decades.