USGS scientists discuss 7.0 magnitude California earthquake

right now speaking to information and how it's powered the USGS is having a briefing on Zoom let's go ahead and listen into that up and one in the chat so I just want to ask you to ask a question and then um then we'll come back to you for more questions if you have one but next up is Hannah Hagman name and affiliation hi um I'm Hannah I'm with the San Francisco Chronicle I have a um a clarifying question um at the last press conference which was super helpful um I asked a question about the earthquake that occurred near Cobb California which was initially estimated in the five to six range and then um Christine ghoul um mentioned that it was a much smaller event and then quote reassessed to a 2. 5 magnitude um my colleague brought up that on the USGS site I think that's still a higher number so I just wanted to follow up is that accurate that it was reassessed at a 2. 5 magnitude it was I I perhaps to refresh the page but the number was was adjusted uh to magnitude two and a half um okay great okay could I ask one more question Paul or can you want to come back to me a quick question okay um just uh Stephen I was really interested in the science aspect you're talking about on how this earthquake occurred um can you just also like clarify um so it sounds like this we understand this to be there was right lateral motion on the fault and there was more can you talk about the horizontal versus vertical and and the difference in damage I'm really interested in that sure and I'll just say this is all a little bit preliminary we do continue to do analyses on the data to make sure we um know exactly how these faults move but you know based on the history of how earthquakes in this area occur and our current understanding of the data this is a fault that transs east to west offshore Northern California just south of Eureka area uh petroia is the nearest populated place which is a little town just south of Eureka in the in the mountains um and there's uh two sort of different parts of the Pacific Plate so the oceanic crust underwater and uh the the plate to the north the Gorda plate is moving to the to the east underneath North America and it's moving faster than the plate to the South and so because of that this difference in in velocity is accommodated on this mesino fault and uh this case it appears that the um the fault the earthquake was what we call right lateral so the plate to the north moves farther to the right um if you're standing on the fault that's what it would look like if you're down there on the on the floor of the of the ocean and so you know this is largely horizontal motion and this is similar to like you the San Andreas fault which is the most famous fault in California has earthquakes and those largely have horizontal motion so they generate a lot of shaking but it's not like one piece of land goes way up higher than the other and there are other faults for example the Cascadia subduction zone to the north that's the long fault that traces offshore the entire uh distance up up into offshore Canada and that one tends to have a lot of vertical motion because it's actually one plate diving under another so in this case two plates sliding back uh against one another and a largely horizontal motion and so that is less likely to lift up the water and and create a tsunami then if if the ground actually move vertically quite a bit yeah imagine taking a like a cup of water in a knife and S of slicing through the cup of water and it nothing really moves knife just sort of goes through and that that the waters moved really y great thank you for that analogy um and that explanation um next up is uh Monica Woods affiliation please hi there Monica Woods abc1 in sacrmento thanks again for doing this follow-up presser uh I want to put into into context in perspective perhaps that this was an offshore earthquake 7.

0 a lot of people reaching out to us that they were in the 1989 San Francisco one which was smaller so what are the similarities and the differences in an offshore earthquake or a landmass earthquake so the the things that really um affect how much shaking people feel is how far they are from where they earthquake has generated and how deep that earthquake is in the crust so in this case the earthquake is not particularly deep we're still working on a good estimate but it's on the order of you know 10 to 20 kilometers deep and it but it's fairly far offshore so if the earthquake happens farther away there's less shaking and in the case of the 1989 earthquake you know it occurred underneath the Santa Cruz mountains in the Bay Area and the shaking radiated under a lot of urban areas and you know a lot of the the damage was due to things like weak soils and uh liquefaction and so um this because it's farther offshore farther from populated places we just don't anticipate um the sort of humanitarian effects that we that we see from these earthquakes that occur under the land great thank you Steve um next up Tara or Tara hi there it's Tara Dugen from the San Francisco Chronicle um another chronical person I appreciate it just wanted to get a little more information about the um tsunami warning and how why it was you know first announced and then changed it sounds like you kind of addressed that a little bit with the science earlier but could you go into that a little more in detail I think it's important to emphasize that uh Noah is the agency that handles the tsunami warnings so we're not really privy to to that that system we uh we see that as they are they are issued um but certainly an earthquake that's this energetic offshore underwater is something that is going to be a cause for concern for tsunamis and it's possible that some of the initial estimates of motion um made it look like there was a possibility of a tsunami and once those were refined the chances of a tsunami actually went down and so that's a little bit speculative but that's our current understanding of things and that could be updated as we continue to look at these data okay thank you so you don't think it had perhaps anything to do with the type of earthquake it might have been or you know which fault it was exactly coming from yeah you know we try to agencies and we you like I said Noah is a different agency than USGS they try to react quickly and uh send out warnings based on the best available information at that time and so the initial information to them obviously looked like there was a probability of a tsunami that was high enough to issue a warning and that was uh refined later more information came in so this is this is how these systems work sometimes okay thanks so much great thank you um cron TV you're on okay we're not hearing I saw your mic came undone but I'm not hearing you yet okay uh you might want to check the mic settings in I'm going to go ahead and move to Rena right now Kon but check your mic settings in the in the um in the uh settings box there Rena and affiliation please yeah hi Rena kenik NPR um I joined about two minutes late so apologies if you answered this but you had said that you know in places there was intensity 8 there could be slight damage to welld Designed structures common structures receiving considerable damage how widespread would you expect the impacts to be and where you know would you expect to see them uh Steve you want to take that question or is that go ahead okay so yes so one thing that we know think of an earthquake is like dropping a rock into a lake and you have the waves sort of moving away and typically we see the highest levels of chiking closest to the epicenter of that earthquake of course the epicenter of this event was off the coast like 40 miles off the coast to California we we see by observation on the event page as well the heaviest levels of shaking the intensity 8 where you're getting damage to structures closest to the basically the coastline so you're talking Eureka Arcada sort of those cities that are sitting you know humble Bay Region where people are are right along the coast and of course the the um intensities decrease the farther you get away uh from from that event and there are some exceptions to that and Steve was mentioned mentioning this earlier in the 1989 LMA Prieta earthquake even though it occurred in the Santa Cruz mountains we still saw some really intense shaking in places like West Oakland um especially the freeway the Cypress structure that that that collapsed and also in the Marina District um there are so there sort of sight conditions loose soils that actually make seismic waves slow down and get bigger when they hit those softer soils so you see some of those those differences but generally you can say and on the event page you can see the highest levels of shaking that are closest to where the epicenter of the earthquake is off the coast the the one Nuance I would add to that is in this area um you know Eureka is the main population Center that's going to be that and some some towns around there and some of those towns sit on like you know the Eel River has a fairly large Delta there's some marshy sediments you know there are some L SS so the uh the effects of what actually a structure is sitting on top of in Terms of the soil conditions can be very important so when we see an event like this the first thing we think about are those people in low-lying areas maybe on um River or Coastal sediments that sort of thing and places that are up on hard Bedrock are going to be less effective but again it's pretty early for us to really have a good comment on the the damage that occurred up there thanks Steve that's great yeah thanks you guys okay cron looks like you guys fixed your mic you want to try again ah we're not hearing you um you know sometimes if you're wearing a headset you know take it on or maybe put the headset on if you're using that okay we're still just not hearing you yet I'm going to go ahead and uh Rena I'm gonna assume that your hand is a legacy so let's go ahead and move to um Amy Isaac but Rena take your hand down if uh or leave it up if you are indeed still up okay Amy okay um all right we'll just go ahead um let's go ahead to Alise Cox Hannah will'll come to you in a second Elise Cox yeah hi it's Alise Cox and I had two questions in the chat but I was also curious about the relationship of um this Quake or Quakes which I'm asking you you to clarify to the um you know the big one that's expected off of point mesino sry does that make sense to you guys or a I didn't totally catch that did you catch that enough B have you did you hear me at all are you able to hear me at all there might be some yeah are hear you maybe the question yes please okay so first of all was were there one was it were there one was it one Quake or two there was one large Quake this magnitude 7. 0 was the large quake and that was Far larger than any of the aftershocks that we've seen so far most of the shocks I haven't looked at the latest but are no greater than 4. 3 is something like that so one large 7.

0 followed by much smaller aftershocks okay I did see that it had been measured a 7. 3 by some of the um some of the equipment uh it is possible there was an early estimate of 7. 3 as more data has come in it's been refined to 7.

0 and one of our size M just uh made a comment recently that they expect the magnitude to hold at magnitude 7. 0 okay and how does this compare with other large Quakes in the past you take that one well in in the case of of earthquakes in this particular region I mean in the last five years we've seen uh events upward of magnitude six so magnitude 6.