PSN-L Email List Message

Subject: RE: 6.1M Guatemala
From: "Kareem Lanier" system98765@.............
Date: Tue, 5 May 2009 10:29:47 -0700

This description of these quakes made me think about the various sensations
experienced during different earthquakes I have felt over the years. I have
often wondered whether one could distinguish between the arrival of the P
and S waves as they arrive at your location during an event. I've spent a
great deal of time listening to eyewitness accounts of quakes and usually
determined how large and the location by having listened to them. I am also
very aware that the structure and soil conditions can determine the
intensity, sharpness, etc. I will recall several quakes I have felt in the

A) 1989, Loma Prieta quake, ~70 miles away, M7.1: Very long, rolling,
gentle, swaying motion. It seemed it moved in the floor in 1.5-2' feet in
circular directions which was astonishing to me. There was absolutely no
shaking, no sharpness nor hardness about it. It was very long but in all,
this earthquake really did not shake at all. Also to note, there seemed to
be no noise (rumbling, roaring, etc.) other than car alarms I heard before
the floor began to move. 20-30 seconds or so.

B) 1991, moderate event, ~15 miles away, M4.5: This felt like a "true"
quake. House suddenly began quivering, built slightly in the side-to-side
movement than slowly faded off. Probably a few inches of horizontal motion.
No rolling, no swaying recalled. Also, there was no initial jolt or motion
often described to a Mack truck hitting the building. The only noise I heard
was the sound of the house. 5-10 seconds

C) Moderate/strong event, ~25-30 miles away, M5.0: Awakened to the quivering
motion. There may have been an initial boom or "sudden Mack truck hitting"
motion since my quake alarm was sounding. The quake alarm will only be
triggered by a sudden shock motion and not by shaking. The quivering seemed
to be happening simultaneously with the gentle swaying or rolling (as if my
apartment unit were on a turntable and that turntable was moving about half
a foot or so in various directions but slowly, all while the turntable was
being shaken.) Seemed more horizontal motion than anything else. First
quake, where I could actually hear slight rumble plus the structure noise.
10-20 seconds duration. First quake, I actually watched occur on the
seismograph as I was feeling it. The pen was maxing out for about two
minutes via L4, vertical geophone.

D) Small event, half a mile or less away from epicenter, M2.8: This was
interesting. There seemed to be two parts to this event. First, there was
one, single, quick motion of which the direction couldn't be determined
because of its "quickness". This was just like a Mack truck hitting the
house or as if everyone slammed the doors all at once. This part of the
quake was the same duration of a sneeze. It was followed by the
rolling/swaying motion that slowly faded off. The only noise heard was when
the first "boom" part of the quake occurred. This quake actually spooked me
in that it was all of a sudden and out of the blue.

E) Small event, half a mile or less, M3.0: This was as if my neighbor
slammed something big against the wall. In fact, I thought it came from my
closet door, so in a sense I could ascertain a direction. And that was it.
Just one single motion and noise. 

I just wanted to get your comments on the event descriptions and see if P,
S, or any of the surface waves are illustrated here. Any other additional
comments welcomed...


-----Original Message-----
From: psn-l-request@.............. [mailto:psn-l-request@............... On
Behalf Of tchannel
Sent: Monday, May 04, 2009 1:04 PM
To: psn-l@..............
Subject: Re: 6.1M Guatemala

Rolando, Thank you.

----- Original Message -----
Sent: Monday, May 04, 2009 12:50 PM
Subject: Re: 6.1M Guatemala

>> Hello Rolando Benitez,  If you have time, would you tell the group about
>> your experience.
>> Thanks, Ted
> Hello Ted,
> Here is a fragment of an email I sent few hours after the quake. I am just
> reading your msg today monday, after several days out of the country.
> We have had several strong quakes in the recent weeks, sometimes a lesser
> magnitude event is felt stronger when it is closer and shallower that the
> one of sunday morning, I say this because 6 hours before the M6.1 we had a
> M4 but at 12km deep and at about  6 miles away... it rattled everything
> but for a shorter time. Here is the edited comment:
> "The story begun last midnight. I just came home from a hospital check in
> Florida at about 0300 UTC. At exactly 0600 UTC we had a very strong
> movement but short in duration. I was not in the mood to check it in the
> computer at that very time, but it called every one's attention. This
> morning while I was checking the data logger pc and calculating the P and
> S locations in the trace of the midnight event, all of the sudden I felt a
> very strong vertical movement that lasted some seconds followed by an even
> stronger rocking movement. At this time every piece of glass at home was
> rattling, some books fell down from the shelves and the alarm in the cars
> went off. This event felt like it lasted forever.
> As I was in front of the datalogger, I were able to "watch" the event in
> real time. It begun with no previous warning or precursors and had just
> one small aftershock. The P and S arrivals were very clearly felt and
> differentiated. Rayleigh and Love waves also were felt, they had a slower
> rocking movement. The event was sensible for over 5 minutes, the sensors
> kept on moving for over 20 minutes. The traces in the pc were totally
> saturated, the sensors were literally moving from stop to stop for a
> while. An interesting and scary experience to say the least.
> So far there has been reports of some damages in the area near the
> epicenter, some injured but no casualties so far.
> An active volcano (Santiaguito), near the epicenter started to erupt
> violently just at the time of the quake and it still is abnormally
> exacerbating erupting. Others Like the Pacaya volcano and Fuego volcano
> start erupting also but with lesser intensity. They all are in the
> epicenter vicinity.
> If we could predict the earthquakes, I am sure these could be a very
> interesting ones to call for a seismologists meeting, I can bet nobody
> would get boring."
> Well hope this help to spoil your curiosity on learning more about living
> in quake territory, or to mitigate your wanting to know.
> Best regards,
> Rolando
> PS
> My postings are in the PSN web page. We have had several dozens of
> aftershocks since the major one...
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